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Cell_Biology_Alberts. that the virus had picked up on its travels. v-Src was unmistakably similar, but not identical, to a gene—c-Src—that was discovered in the normal vertebrate genome. c-Src had evidently been caught up accidentally by the retrovirus from the genome of a previously infected host cell, and it had undergone mutation in the process to become an oncogene (v-Src).


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Cell_Biology_Alberts. This Nobel Prize-winning finding was followed by a flood of discoveries of other viral oncogenes carried by retroviruses that cause cancer in nonhuman animals. Each such oncogene turned out to have a counterpart proto-oncogene in the normal vertebrate genome. As was the case for Src, these other oncogenes generally differed from their normal counterparts, either in structure or in level of expression. But how did this relate to typical human cancers, most of which are not infectious and in which retroviruses play no part? Figure 20–17 Cancer-critical mutations fall into two readily distinguishable categories, dominant and recessive.


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Cell_Biology_Alberts. Figure 20–17 Cancer-critical mutations fall into two readily distinguishable categories, dominant and recessive. In this diagram, activating mutations are represented by solid red boxes, inactivating mutations by hollow red boxes. (A) Oncogenes act in a dominant manner: a gain-of-function mutation in a single copy of the cancer-critical gene can drive a cell toward cancer. (B) Mutations in tumor suppressor genes, on the other hand, generally act in a recessive manner: the function of both alleles of the cancer-critical gene must be lost to drive a cell toward cancer. Although in this diagram the second allele of the tumor suppressor gene is inactivated by mutation, it is often inactivated instead by loss of the second chromosome. Not shown is the fact that mutation of some tumor suppressor genes can have an effect even when only one of the two gene copies is damaged. Different Searches for Oncogenes Converged on the Same Gene—Ras


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Cell_Biology_Alberts. Different Searches for Oncogenes Converged on the Same Gene—Ras In an attempt to answer the above question, other researchers searched directly for oncogenes in the genomes of human cancer cells. They did this by searching for DNA fragments from cancer cells that could provoke uncontrolled proliferation when introduced into noncancerous cell lines. As tester cells for the assay, cell lines derived from mouse fibroblasts were used. These cells had been previously selected for their ability to proliferate indefinitely in culture, and they are thought to already contain alterations that take them part of the way toward malignancy. For this reason, the addition of a single oncogene can sometimes be enough to produce a dramatic effect.


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Cell_Biology_Alberts. When DNA was extracted from the human tumor cells, broken into fragments, and introduced into the cultured cells, occasional colonies of abnormally proliferating cells began to appear in the culture dish. These cells showed a transformed phenotype, outgrowing the untransformed cells in the culture and piling up in layer upon layer (see Figure 20–11). Each colony was a clone originating from a single cell that had incorporated a DNA fragment that drove cancerous behavior. This fragment, which carried markers of its human origin, could be isolated from the transformed cultured mouse cells. And once isolated and sequenced, it could be recognized: it contained a human version of a gene already known from study of a retrovirus that caused tumors in rats—an oncogene called v-Ras.


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Cell_Biology_Alberts. The newly discovered oncogene was clearly derived by mutation from a normal human gene, one of a small family of proto-oncogenes called Ras. This discovery in the early 1980s of the same oncogene in human tumor cells and in an animal tumor virus was electrifying. The implication that cancers are caused by mutations in a limited number of cancer-critical genes transformed our understanding of the molecular biology of cancer.


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Cell_Biology_Alberts. As discussed in Chapter 15, normal Ras proteins are monomeric GTPases that help transmit signals from cell-surface receptors to the cell interior (see Movie 15.7). The Ras oncogenes isolated from human tumors contain point mutations that create a hyperactive Ras protein that cannot shut itself off by hydrolyzing its bound GTP to GDP. Because this makes the protein hyperactive, its effect is dominant—that is, only one of the cell’s two gene copies needs to change to have an effect. One or another of the three human Ras family members is mutated in perhaps 30% of all human cancers. Ras genes are thus among the most important of all cancer-critical genes. Figure 20–18 summarizes the types of accidents that can convert a proto-oncogene into an oncogene. (1) A small change in DNA sequence such as a point Figure 20–18 The types of accidents that can convert a proto-oncogene into an oncogene.


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Cell_Biology_Alberts. Figure 20–18 The types of accidents that can convert a proto-oncogene into an oncogene. extracellular domain binding of growth factor of receptor triggers intracellular signaling mutation or deletion may produce a hyperactive protein when it occurs within a protein-coding sequence, or lead to protein overproduction when it occurs within a regulatory region for that gene. (2) Gene amplification events, such as those that can be caused by errors in DNA replication, may produce extra gene copies; this can lead to overproduction of the protein. (3) A chromosomal rearrangement— involving the breakage and rejoining of the DNA helix—may either change the protein-coding region, resulting in a hyperactive fusion protein, or alter the control regions for a gene so that a normal protein is overproduced.


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Cell_Biology_Alberts. As one example, the receptor for the extracellular signal protein epidermal growth factor (EGF) can be activated by a deletion that removes part of its extracellular domain, causing it to be active even in the absence of EGF (Figure 20–19). It thus produces an inappropriate stimulatory signal, like a faulty doorbell that rings even when nobody is pressing the button. Mutations of this type are frequently found in the most common type of human brain tumor, called glioblastoma.


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Cell_Biology_Alberts. As another example, the Myc protein, which acts in the nucleus to stimulate cell growth and division (see Chapter 17), generally contributes to cancer by being overproduced in its normal form. In some cases, the gene is amplified— that is, errors of DNA replication lead to the creation of large numbers of gene copies in a single cell. Or a point mutation can stabilize the protein, which normally turns over very rapidly. More commonly, the overproduction appears to be due to a change in a regulatory element that acts on the gene. For example, a chromosomal translocation can inappropriately bring powerful gene regulatory sequences next to the Myc protein-coding sequence, so as to produce unusually large amounts of Myc mRNA. Thus, in Burkitt’s lymphoma, a translocation brings the Myc gene under the control of sequences that normally drive the expression of antibody genes in B lymphocytes. As a result, the mutant B cells tend to proliferate excessively and form a tumor. Different specific


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Cell_Biology_Alberts. the control of sequences that normally drive the expression of antibody genes in B lymphocytes. As a result, the mutant B cells tend to proliferate excessively and form a tumor. Different specific chromosome translocations are common in other cancers.


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Cell_Biology_Alberts. Studies of Rare hereditary Cancer Syndromes First Identified Tumor Suppressor Genes Identifying a gene that has been inactivated in the genome of a cancer cell requires a different strategy from finding a gene that has become hyperactive: one cannot, for example, use a cell transformation assay to identify something that simply is not there. The key insight that led to the discovery of the first tumor suppressor gene came from studies of a rare type of human cancer, retinoblastoma, which arises from cells in the retina of the eye that are converted to a cancerous state by an unusually small number of mutations. As often happens in biology, the discovery arose from examination of a special case, but it turned out to reveal a gene of widespread importance. Retinoblastoma occurs in childhood, and tumors develop from neural precursor cells in the immature retina. About one child in 20,000 is afflicted. One form of the disease is hereditary, and the other is not. In the hereditary form,


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Cell_Biology_Alberts. Figure 20–19 Mutation of the epidermal growth factor (EGF) receptor can make it active even in the absence of EGF, and consequently oncogenic. Only one of the possible types of activating mutations is illustrated here. NORMAL, HEALTHY INDIVIDUAL HEREDITARY RETINOBLASTOMA NONHEREDITARY RETINOBLASTOMA occasional cell inactivates one of its two good Rb genes excessive cell proliferation leading to retinoblastoma excessive cell proliferation leading to retinoblastoma


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Cell_Biology_Alberts. RESULT: MOST PEOPLE WITH INHERITED RESULT: ONLY ABOUT 1 IN 30,000 RESULT: NO TUMOR multiple tumors usually arise independently, affecting both eyes; in the nonhereditary form, only one eye is affected, and by only one tumor. A few individuals with retinoblastoma have a visibly abnormal karyotype, with a deletion of a specific band on chromosome 13 that, if inherited, predisposes an individual to the disease. Deletions of this same region are also encountered in tumor cells from some patients with the nonhereditary disease, which suggested that the cancer was caused by loss of a critical gene in that location.


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Cell_Biology_Alberts. Using the location of this chromosomal deletion, it was possible to clone and sequence the Rb gene. It was then discovered that those who suffer from the hereditary form of the disease have a deletion or loss-of-function mutation present in one copy of the Rb gene in every somatic cell. These cells are predisposed to becoming cancerous, but do not do so if they retain one good copy of the gene. The retinal cells that are cancerous are defective in both copies of Rb because of a somatic event that has eliminated the function of the previously good copy. In patients with the nonhereditary form of the disease, by contrast, the noncancerous cells show no defect in either copy of Rb, while the cancerous cells have become defective in both copies. These nonhereditary retinoblastomas are very rare because they require two independent events that inactivate the same gene on two chromosomes in a single retinal cell lineage (Figure 20–20).


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Cell_Biology_Alberts. The Rb gene is also missing in several common types of sporadic cancer, including carcinomas of lung, breast, and bladder. These more common cancers arise by a more complex series of genetic changes than does retinoblastoma, and they make their appearance much later in life. But in all of them, it seems, loss of Rb function is frequently a major step in the progression toward malignancy. The Rb gene encodes the Rb protein, which is a universal regulator of the cell cycle present in almost all cells of the body (see Figure 17–61). It acts as one of the main brakes on progress through the cell-division cycle, and its loss can allow cells to enter the cell cycle inappropriately, as we discuss later.


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Cell_Biology_Alberts. For tumor suppressor genes, it is their inactivation that is dangerous. This inactivation can occur in many ways, with different combinations of mishaps serving to eliminate or cripple both gene copies. The first copy may, for example, be lost by a small chromosomal deletion or inactivated by a point mutation. The second copy is commonly eliminated by a less specific and more probable mechanism: Figure 20–20 The genetic mechanisms that cause retinoblastoma. In the hereditary form, all cells in the body lack one of the normal two functional copies of the Rb tumor suppressor gene, and tumors occur where the remaining copy is lost or inactivated by a somatic event (either mutation or epigenetic silencing). In the nonhereditary form, all cells initially contain two functional copies of the gene, and the tumor arises because both copies are lost or inactivated through the coincidence of two somatic events in a single line of cells.


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Cell_Biology_Alberts. the chromosome carrying the remaining normal copy may be lost from the cell through errors in chromosome segregation; or the normal gene, along with neighboring genetic material, may be replaced by a mutant version through either a mitotic recombination event or a gene conversion that accompanies it (see p. 286). Figure 20–21 summarizes the range of ways in which the remaining good copy of a tumor suppressor gene can be lost through a DNA sequence change, using the Rb gene as an example. It is important to note that, except for the point mutation mechanism illustrated at the far right, these pathways all produce cells that carry only a single type of DNA sequence in the chromosomal region containing their Rb genes—a sequence that is identical to the sequence in the original mutant chromosome.


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Cell_Biology_Alberts. Epigenetic changes provide another important way to permanently inactivate a tumor suppressor gene. Most commonly, the gene may become packaged into heterochromatin and/or the C nucleotides in CG sequences in its promoter may become methylated in a heritable manner (see pp. 404–405). These mechanisms can irreversibly silence the gene in a cell and in all of its progeny. Analysis of methylation patterns in cancer genomes shows that epigenetic gene silencing is a frequent event in tumor progression, and epigenetic mechanisms are now thought to help inactivate several different tumor suppressor genes in most human cancers (Figure 20–22). Systematic Sequencing of Cancer Cell Genomes has Transformed Our Understanding of the Disease


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Cell_Biology_Alberts. Systematic Sequencing of Cancer Cell Genomes has Transformed Our Understanding of the Disease Methods such as those we have described above shone a spotlight on a set of can-cer-critical genes that were identified in a piecemeal fashion. Meanwhile, the rest of the cancer cell genome remained in darkness: it was a mystery how many other mutations might lurk there, of what types, in which varieties of cancer, at what frequencies, with what variations from patient to patient, and with what consequences. With the sequencing of the human genome and the dramatic advances in DNA sequencing technology (see Panel 8–1, pp. 478–481), it has become possible to see the whole picture—to view cancer cell genomes in their entirety. This transforms our understanding of the disease.


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Cell_Biology_Alberts. Cancer cell genomes can be scanned systematically in several different ways. At one extreme—the most costly, but no longer prohibitively so—one can determine a tumor’s complete genome sequence. More cheaply, one can focus just on the 21,000 or so genes in the human genome that code for protein (the so-called exome), looking for mutations in the cancer cell DNA that alter the amino acid sequence of the product or prevent its synthesis (Figure 20–23). There are also efficient techniques to survey the genome for regions that have undergone


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Cell_Biology_Alberts. Figure 20–21 Six ways of losing the remaining good copy of a tumor suppressor gene through a change in DNA sequences. A cell that is defective in only one of its two copies of a tumor suppressor gene—for example, the Rb gene—usually behaves as a normal, healthy cell; the diagrams below show how this cell may lose the function of the other gene copy as well and thereby progress toward cancer. A seventh possibility, frequently encountered with some tumor suppressors, is that the gene may be silenced by an epigenetic change, without alteration of the DNA sequence, as illustrated in Figure 20–22. (After


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Cell_Biology_Alberts. W.K. Cavenee et al., Nature 305:779–784, 1983. With permission from Macmillan Publishers Ltd.) deletion or duplication, without the need for complete sequence information. The genome can be scanned for epigenetic changes. And finally, alterations in levels of gene expression can be systematically determined by analysis of mRNAs (see Figure 7–3). These approaches generally involve comparing cancer cells with normal controls—ideally, noncancerous cells originating in the same tissue and from the same patient. Figure 20–22 The pathways leading to loss of tumor suppressor gene function in cancer involve both genetic and epigenetic changes. (A) As indicated, the changes that silence tumor suppressor genes can occur in any order. Both DNA methylation and the packaging of a gene into condensed chromatin can prevent its expression in a way that is inherited when a cell divides (see Figure 4–44).


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Cell_Biology_Alberts. (B) The frequency of gene silencing by hypermethylation observed in four different types of cancer. The five genes listed at the top can all function as tumor suppressor genes; BRCA1 and hMLH1 affect genome stability and are in the subclass known as genome maintenance genes. ND, no data. (Adapted from M. Esteller et al., Cancer Res. 61:3225–3229, 2001.)


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Cell_Biology_Alberts. Figure 20–23 The distinct types of DNA sequence changes found in oncogenes compared to tumor suppressor genes. In this diagram, mutations that change an amino acid are denoted by blue arrowheads, whereas mutations that truncate the polypeptide chain are marked by yellow arrowheads. (A) As in this example, oncogene mutations can be detected by the fact that the same nucleotide change is repeatedly found among the missense mutations in a gene. (B) For tumor suppressor genes, by contrast, missense mutations that abort protein synthesis by creating stop codons predominate. (Adapted from B. Vogelstein et al., Science 339:1546–1558, 2013.) chromosome alterations found in the particular primary tumor. As indicated, purple lines connect sites at which two different chromosomes have become joined to create an interchromosomal rearrangment, while green lines connect the sites of rearrangements found within a single chromosome. The intrachromosomal rearrangements can be seen to predominate, and


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Cell_Biology_Alberts. to create an interchromosomal rearrangment, while green lines connect the sites of rearrangements found within a single chromosome. The intrachromosomal rearrangements can be seen to predominate, and most join neighboring sections of DNA that were originally located within 2 million nucleotide pairs of each other. The increases in copy number,


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Cell_Biology_Alberts. CANCER-CRITICAL GENES: hOW ThEY ARE FOUND AND WhAT ThEY DO


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Cell_Biology_Alberts. Cancer genome analysis reveals, first of all, the scale of gross genetic disruption in cancer cells. This varies greatly from one type of cancer and one cancer patient to another, both in severity and in character. In some cases, the karyotype—the set of chromosomes as they appear at mitosis—is normal or nearly so, but many point mutations are detected in individual genes, suggesting a failure of the repair mechanisms that normally correct local errors in the replication or maintenance of DNA sequences. Often, however, the karyotype is severely disordered, with many chromosome breaks and rearrangements. In some breast cancers, for example, genome sequencing reveals an astonishing scene of genetic chaos (Figure 20–24), with hundreds of chromosome breaks and translocations, resulting in many deletions, duplications, and amplifications of parts of the genome. In such cells, the normal machinery for avoidance or repair of DNA double-strand breaks is evidently somehow defective,


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Cell_Biology_Alberts. in many deletions, duplications, and amplifications of parts of the genome. In such cells, the normal machinery for avoidance or repair of DNA double-strand breaks is evidently somehow defective, destabilizing the genome by giving rise to broken chromosomes whose fragments then rejoin in random combinations. From the pattern of changes, one can infer that this disruptive process has occurred repeatedly during the evolution of the tumor, with a progressive increase of genetic disorder. Breast cancers showing the most extreme chromosome disorder are usually hard to treat and have a gloomy prognosis.


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Cell_Biology_Alberts. One survey of more than 3000 individual cancer specimens showed that on average 24 separate blocks of genetic material were duplicated in each tumor, amounting to 17% of the normal genome, and 18 blocks were deleted, amounting to 16% of the normal genome. Many of these changes were found repeatedly, suggesting that they contain cancer-critical genes whose loss (tumor suppressor genes) or gain (oncogenes) confers a selective advantage.


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Cell_Biology_Alberts. Whole-genome analysis also helps to explain some cancers that seem, at first sight, to be exceptions to the general rules. An example is retinoblastoma, with its early onset during childhood. If cancers in general require an accumulation of many genetic changes and are thus diseases of old age, what makes retinoblastoma different? Whole-genome sequencing confirms that in retinoblastoma, the tumor cells contain loss-of-function mutations in the Rb gene; but, astonishingly, they contain practically no mutations or genome rearrangements that affect any other oncogene or tumor suppressor gene. Instead, they contain many epigenetic modifications, which alter the level of expression of many known cancer-critical genes—as many as 15 in one well-analyzed case. Many Mutations in Tumor Cells are Merely Passengers


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Cell_Biology_Alberts. Many Mutations in Tumor Cells are Merely Passengers Cancer cells generally contain many mutations in addition to gross chromosome abnormalities: point mutations can be scattered over the genome as a whole at a rate of about one per million nucleotide pairs, in addition to the abnormalities Figure 20–24 The chromosomal rearrangements in breast cancer cells. The results of an extensive DNA sequencing analysis performed on two different primary tumors are displayed as “Circos plots.” In each plot, the reference DNA sequences of the 22 autosomes and single sex chromosome (X) of a normal human female (3.2 billion nucleotide pairs) are aligned end-to-end to form a circle. Colored lines within the circle are then used to indicate the shown in blue, reveal the amplified DNA sequences (see the highly amplified regions indicated). (Adapted from P.J. Stephens et


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Cell_Biology_Alberts. BREAST CANCER 1 BREAST CANCER 2 al., Nature 462:1005–1010, 2009.) attributed to chromosome breakage and rejoining. Systematic surveys of the pro-tein-coding genes in common solid tumors—such as those of the breast, colon, brain, or pancreas—have revealed that an average of 33 to 66 genes have undergone somatic mutation affecting the sequence of their protein product. Mutations in noncoding regions of the genome are much more numerous, as one would expect from the much larger fraction of the genome that noncoding DNA represents. But they are considerably more difficult to interpret.


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Cell_Biology_Alberts. The high frequency of mutations testifies to the genetic instability of many cancer cells, but it leaves us with a difficult problem. How can we discover which of the mutations are drivers of cancer—that is, causal factors in the development of the disease—and which are merely passengers—mutations that happen to have occurred in the same cell as the driver mutations, thanks to genetic instability, but are irrelevant to the development of the disease? A simple criterion is based on frequency of occurrence. Driver mutations affecting a gene that plays a part in the disease will be seen repeatedly, in many different patients. In contrast, passenger mutations, occurring at more-or-less random locations in the genome and conferring no selective advantage on the cancer cell, are unlikely to be found in the same genes in different patients.


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Cell_Biology_Alberts. Figure 20–25 shows the results of an analysis of this sort for a large sample of colorectal cancers. The different sites in the genome are laid out on a two-dimensional array, with chromosome serial number along one axis and position within each chromosome along the other. The frequency with which mutations are encountered is shown by height above this plane, creating a mutation “landscape” with mountains (sites where mutations are found in a large proportion of the tumors in the sample), hills (where mutations are found less frequently but still more often than would be expected for a random scattering over the genome), and hillocks (sites of occasional mutations, occurring at a frequency no higher than would be expected for mutations scattered at random in each individual tumor). The mountains and the hills are strong candidates to be the sites of driver mutations—in other words, sites of cancer-critical genes; the hillocks are likely to correspond to passengers. Indeed, many of the


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Cell_Biology_Alberts. and the hills are strong candidates to be the sites of driver mutations—in other words, sites of cancer-critical genes; the hillocks are likely to correspond to passengers. Indeed, many of the mountains and hills turn out to be sites of known oncogenes or tumor suppressor genes, whereas the hillocks mostly correspond to genes that have no known or probable role in causation of cancer. Of course, some hillocks may correspond to genes that are mutated in only a few rare patients but are nevertheless cancer-critical for them.


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Cell_Biology_Alberts. About One Percent of the Genes in the human Genome Are Cancer-Critical From studies such as the one just described, it is estimated that the number of driver mutations for an individual case of cancer (the sum of meaningful epigenetic and genetic changes in both coding sequences and regulatory regions) is typically on the order of 10, explaining why cancer progression generally involves an increase in genetic and/or epigenetic instability that enhances the rate of such changes.


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Cell_Biology_Alberts. Figure 20–25 The mutation landscape in colorectal cancer. In this two-dimensional representation of the human genome, the green surface depicts the 22 human autosomes plus the X sex chromosome as being laid out side-by-side in numerical order from left to right, with the DNA sequence of each chromosome running from back to front. The mountains represent the locations of genes mutated with high frequency in different, independent tumors. As indicated, these are suspected driver mutations in the adenomatous polyposis coli (APC), K-Ras, p53, phosphoinositide 3-kinase (PIK3CA), and ubiquitin ligase (FBXW7) proteins. (Adapted from L.D. Wood et al., Science 318:1108–1113, 2007.)


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Cell_Biology_Alberts. By compiling the data for different types of cancer, each with its own range of identified driver mutations, we can develop a comprehensive catalog of genes that are strongly suspected to be cancer-critical. Current estimates put the total number of such genes at about 300, about 1% of the genes in the human genome. These cancer-critical genes are amazingly diverse. Their products include secreted signal proteins, transmembrane receptors, GTP-binding proteins, protein kinases, transcription regulators, chromatin modifiers, DNA repair enzymes, cell–cell adhesion molecules, cell-cycle controllers, apoptosis regulators, scaffold proteins, metabolic enzymes, components of the RNA splicing machinery, and more besides. All these are susceptible to mutations that can contribute, in one way or another, in one tissue or another, to the evolution of cells with the cancerous properties that we listed earlier on page 1103.


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Cell_Biology_Alberts. Clearly, the molecular changes that cause cancer are complex. As we now explain, however, the complexity is not quite as daunting as it may initially seem. Disruptions in a handful of Key Pathways Are Common to Many Cancers


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Cell_Biology_Alberts. Disruptions in a handful of Key Pathways Are Common to Many Cancers Some genes, like Rb and Ras, are mutated in many cases of cancer and in cancers of many different types. The involvement of genes such as Rb and Ras in cancer is no surprise, now that we understand their normal functions: they control fundamental processes of cell division and growth. But even these common culprits feature in considerably less than half of individual cases. What is happening to the control of these processes in the many cases of cancer where, for example, Rb is intact or Ras is not mutated? What part do mutations in the hundreds of other cancer-critical genes play in the development of the disease? With our increasing knowledge of the normal functions of the genes in the human genome, it is becoming easier to see patterns in the cataloged driver mutations and to give some simplifying answers to these questions.


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Cell_Biology_Alberts. Glioblastoma—the commonest type of human brain tumor—provides a good example. Analysis of the genomes of tumor cells from 91 patients identified a total of at least 79 genes that were mutated in more than one individual. The normal functions of most of these genes were known or could be guessed, allowing them to be assigned to specific biochemical or regulatory pathways. Three functional groupings stood out, accounting for a total of 21 of the recurrently mutated genes. One of these groupings consisted of genes in the Rb pathway (that is, Rb itself, along with genes that directly regulate Rb); this pathway governs initiation of the cell-division cycle. Another consisted of genes in the same regulatory subnetwork as Ras—a more loosely defined system of genes referred to as the RTK/Ras/PI3K pathway, after three of its core components; this pathway serves to transmit signals for cell growth and cell division from the cell exterior into the heart of the cell. The third grouping consisted


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Cell_Biology_Alberts. pathway, after three of its core components; this pathway serves to transmit signals for cell growth and cell division from the cell exterior into the heart of the cell. The third grouping consisted of genes in a pathway regulating responses to stress and DNA damage—the p53 pathway. We shall have more to say about each of these pathways below.


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Cell_Biology_Alberts. Out of all tumors, 74% had identifiable mutations in all three pathways. If one were to trace these three pathways further upstream and include all the components, known and unknown, on which they depend, this percentage would almost certainly be even higher. In other words, in almost every case of glioblastoma, there are mutations that disrupt each of three fundamental controls: the control of cell growth, the control of cell division, and the control of responses to stress and DNA damage. Strikingly, in any given tumor-cell clone, there is a strong tendency for no more than one gene to be mutated in each pathway. Evidently, what matters for tumor evolution is the disruption of the control mechanism, and not the genetic means by which that is achieved. Thus, for example, in a patient whose tumor cells have no mutation in Rb itself, there is generally a mutation in some other component of the Rb pathway, producing a similar biological effect.


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Cell_Biology_Alberts. Similar patterns are seen in other types of cancers. A survey of many specimens of the major variety of ovarian cancer, for example, identified 67% of patients as having mutations in the Rb pathway, 45% in the Ras/PI3K pathway (defined more narrowly than in the glioblastoma study), and more than 96% in the p53 pathway. Allowing for additional pathway components not included in the analysis, it seems that most cases of this type of cancer, too, have mutations disrupting the same three controls, leading to misregulated cell growth, misregulated cell proliferation, and abnormal disregard of stress and DNA damage. It seems that these three fundamental controls are subverted in one way or another in virtually every type of cancer. We have devoted an entire chapter to the cell cycle and growth controls (Chapter 17). Some important details of the other two control pathways are reviewed next. Mutations in the PI3K/Akt/mTOR Pathway Drive Cancer Cells to Grow


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Cell_Biology_Alberts. Mutations in the PI3K/Akt/mTOR Pathway Drive Cancer Cells to Grow Cell proliferation is not simply a matter of progression through the cell cycle; it also requires cell growth, which involves complex anabolic processes through which the cell synthesizes all the necessary macromolecules from small-molecule precursors. If a cell divides inappropriately without growing first, it will get smaller at each division and will ultimately die or become too small to divide. Cells appear to require two separate signals to grow and divide (Figure 20–26). Cancer depends, therefore, not only on a loss of restraints on cell-cycle progression, but also on disrupted control of cell growth. The phosphoinositide 3-kinase (PI 3-kinase)/Akt/mTOR intracellular signaling pathway is critical for cell growth control. As described in Chapter 15, various extracellular signal proteins, including insulin and insulin-like growth factors,


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Cell_Biology_Alberts. Figure 20–26 Cells seem to require two types of signals to proliferate. (A) In order to multiply successfully, most normal cells are suspected to require both extracellular signals that drive cell-cycle progression (shown here as blue mitogen) and extracellular signals that drive cell growth (shown here as red growth factor). how mitogens activate the Rb pathway to drive entry into the cell cycle is described in Figure 17–61. (B) Diagram of the signaling system containing Akt that drives cell growth through greatly stimulating glucose uptake and utilization, including a conversion of the excess citric acid produced from sugar intermediates in mitochondria into the acetyl CoA that is needed in the cytosol for lipid synthesis and new membrane production. As indicated, protein synthesis is also increased. This system becomes abnormally activated early in tumor progression. TCA cycle indicates the tricarboxylic acid cycle (citric acid cycle).


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Cell_Biology_Alberts. normally activate this pathway. In cancer cells, however, the pathway is activated by mutation so that the cell can grow in the absence of such signals. The resulting abnormal activation of the protein kinases Akt and mTOR not only stimulates protein synthesis (see Figure 17–64), but also greatly increases both glucose uptake and the production of the acetyl CoA in the cytosol required for cell lipid synthesis, as outlined in Figure 20–26B.


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Cell_Biology_Alberts. The abnormal activation of the PI 3-kinase/Akt/mTOR pathway, which normally occurs early in the process of tumor progression, helps to explain the excessive rate of glycolysis that is observed in tumor cells, known as the Warburg effect, as discussed earlier (see Figure 20–12). As expected from our previous discussion, cancers can activate this pathway in many different ways. Thus, for example, a growth factor receptor can become abnormally activated, as in Figure 20–19. Also very common in cancers is the loss of the PTEN phosphatase, an enzyme that normally suppresses the PI 3-kinase/Akt/mTOR pathway by dephosphorylating the PI (3,4,5) P3 molecules that the PI 3-kinase forms (see pp. 859–861). PTEN is thus a common tumor-suppressor gene.


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Cell_Biology_Alberts. Of course, mutation is not the only way to overactivate the pathway: high levels of insulin in the circulation can have a similar effect. This may explain why the risk of cancer is significantly increased, by a factor of two or more, in people who are obese or have type 2 diabetes. Their insulin levels are abnormally high, driving cancer cell growth without need of mutation in the PI 3-kinase/Akt/mTOR pathway. Mutations in the p53 Pathway Enable Cancer Cells to Survive and Proliferate Despite Stress and DNA Damage


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Cell_Biology_Alberts. Mutations in the p53 Pathway Enable Cancer Cells to Survive and Proliferate Despite Stress and DNA Damage That cancer cells must break the normal rules governing cell growth and cell division is obvious: that is part of the definition of cancer. It is not so obvious why cancer cells should also be abnormal in their response to stress and DNA damage, and yet this too is an almost universal feature. The gene that lies at the center of this response, the p53 gene, is mutated in about 50% of all cases of cancer— a higher proportion than for any other known cancer-critical gene. When we include with p53 the other genes that are closely involved in its function, we find that most cases of cancer harbor mutations in the p53 pathway. Why should this be? To answer, we must first consider the normal function of this pathway.


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Cell_Biology_Alberts. In contrast to Rb, most cells in the body have very little p53 protein under normal conditions: although the protein is synthesized, it is rapidly degraded. Moreover, p53 is not essential for normal development. Mice in which both copies of the gene have been deleted or inactivated typically appear normal in all respects except one—they universally develop cancer before 10 months of age. These observations suggest that p53 has a function that is required only in special circumstances. In fact, cells raise their concentration of p53 protein in response to a whole range of conditions that have only one obvious thing in common: they are, from the cell’s point of view, pathological, putting the cell in danger of death or serious injury. These conditions include DNA damage, putting the cell at risk from a faulty genome; telomere loss or shortening (see p. 1016), also dangerous to the integrity of the genome; hypoxia, depriving the cell of the oxygen it needs to keep its metabolism going;


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Cell_Biology_Alberts. risk from a faulty genome; telomere loss or shortening (see p. 1016), also dangerous to the integrity of the genome; hypoxia, depriving the cell of the oxygen it needs to keep its metabolism going; osmotic stress, causing the cell to swell or shrivel; and oxidative stress, generating dangerous levels of highly reactive free radicals.


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Cell_Biology_Alberts. Yet another form of stress that can activate the p53 pathway arises, it seems, when regulatory signals are so intense or uncoordinated as to drive the cell beyond its normal limits and into a danger zone where its mechanisms of control and coordination break down, as in an engine driven badly or too fast. The p53 concentration rises, for example, when Myc is overexpressed to oncogenic levels. All these circumstances call for desperate action, which may take either of two forms: the cell can block any further progress through the division cycle in order to take time out to repair or recover from the pathological condition; or it can accept that it must die, and do so in a way that minimizes damage to the organism. A good death, from this point of view, is a death by apoptosis. In apoptosis, stable, active p53


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Cell_Biology_Alberts. Figure 20–27 Modes of action of the p53 tumor suppressor. The p53 protein is a cellular stress sensor. In response to hyperproliferative signals, DNA damage, hypoxia, telomere shortening, and various other stresses, the p53 levels in the cell rise. As indicated, this may either arrest cell cycling in a way that allows the cell to adjust and survive, trigger cell suicide by apoptosis, or cause cell “senescence”—an irreversible cell-cycle arrest that stops damaged cells from dividing. the cell is phagocytosed by its neighbors and its contents are efficiently recycled. A bad death is a death by necrosis. In necrosis, the cell bursts or disintegrates and its contents are spilled into the extracellular space, inducing inflammation.


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Cell_Biology_Alberts. The p53 pathway, therefore, behaves as a sort of antenna, sensing the presence of a wide range of dangerous conditions, and when any are detected, triggering appropriate action—either a temporary or permanent arrest of cell cycling (senescence), or suicide by apoptosis (Figure 20–27). These responses serve to prevent deranged cells from proliferating. Cancer cells are indeed generally deranged, and their survival and proliferation thus depend on inactivation of the p53 pathway. If the p53 pathway were active in them, they would be halted in their tracks or die (Movie 20.4).


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Cell_Biology_Alberts. The p53 protein performs its job mainly by acting as a transcription regulator (see Movie 17.8). Indeed, the most common mutations observed in p53 in human tumors are in its DNA-binding domain, where they cripple the ability of p53 to bind to its DNA target sequences. Because p53 binds to DNA as a tetramer, a single mutant subunit within a tetrameric complex can be enough to block its function. Thus, mutations in p53 can have a dominant negative effect, causing loss of p53 function even when the cell also contains a wild-type version of the gene. For this reason, in contrast with other tumor suppressor genes such as Rb, the development of cancer does not always require that both copies of p53 be knocked out.


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Cell_Biology_Alberts. As discussed in Chapter 17, the p53 protein exerts its inhibitory effects on the cell cycle, in part at least, by inducing the transcription of p21, which encodes a protein that binds to and inhibits the cyclin-dependent kinase (Cdk) complexes required for progression through the cell cycle. By blocking the kinase activity of these Cdk complexes, the p21 protein prevents the cell from progressing through S phase and replicating its DNA. The mechanism by which p53 induces apoptosis includes stimulation of the expression of many pro-apoptotic genes, and it will be described in Chapter 18. If the p53 pathway is functional, a cell with unrepaired DNA damage will stop dividing or die; it cannot proliferate. Mutations in the p53 pathway are, therefore, generally present in cancer cells showing genome instability—which is to say, the majority. But how does this genome instability originate? Here too, cancer genome studies are illuminating.


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Cell_Biology_Alberts. In ovarian cancers, for example, chromosome breaks, translocations, and deletions are very common, and these aberrations correlate with a high frequency of mutations and epigenetic silencing in the genes needed for repair of DNA double-strand breaks by homologous recombination, especially Brca1 and Brca2 (see pp. 281–282). In a subset of colorectal cancers with DNA mismatch repair defects, on the other hand, one instead finds many point mutations scattered throughout the genome (see pp. 250–251). In both kinds of cancer, the genome is commonly destabilized, but different types of mutations can bring this about. Cancers of Specialized Tissues Use Many Different Routes to Target the Common Core Pathways of Cancer


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Cell_Biology_Alberts. Cancers of Specialized Tissues Use Many Different Routes to Target the Common Core Pathways of Cancer Mutations in core components of the machinery that regulates cell growth, division, and survival, such as Rb, Ras, PTEN, or p53, are not the only way to pervert the control of these processes. Specialized tissues depend on a variety of pathways, as discussed in Chapter 15, to relay environmental signals to the core control machinery, and each pathway lays the cells open to subversion in a different set of ways. Thus, in different cancers, we can find examples of driver mutations in practically all the major signaling pathways through which cells communicate during development and tissue maintenance (discussed in Chapters 21 and 22).


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Cell_Biology_Alberts. In glioblastoma, for example, most patients have mutations in one or other of a set of cell-surface receptor tyrosine kinases, especially the EGF receptor mentioned earlier (linking into the Ras/PI3K pathway), suggesting that the cells from which the cancer originates are normally controlled by this route. The cells of the prostate gland, on the other hand, respond to the androgen hormone testosterone, and in prostate cancer, components of the androgen receptor signaling pathway (a variety of nuclear hormone receptor signaling; see Chapter 15) are often mutated. In the normal gut lining, Wnt signaling is critical, and Wnt pathway mutations are present in most colorectal cancers. Pancreatic cancers generally have mutations in the transforming growth factor-β (TGFβ) signaling pathway. Activating mutations in the Notch pathway are present in more than 50% of T cell acute lymphocytic leukemias, and so on.


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Cell_Biology_Alberts. Cells are generally regulated by several different types of external signals that must act in combination, representing a “fail-safe” control mechanism that protects the organism as a whole from cancer. These signals are different in different tissues. As expected, therefore, the corresponding cancers often have mutations in several signaling pathways concurrently. This is true of the examples we have just listed, which commonly have mutations in other signaling pathways in addition to the ones that we have singled out. Studies Using Mice help to Define the Functions of Cancer-Critical Genes


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Cell_Biology_Alberts. The ultimate test of a gene’s role in cancer has to come from investigations in the intact, mature organism. The most favored organism for such studies, apart from humans themselves, is the mouse. To explore the function of a candidate oncogene or tumor suppressor gene, one can make a transgenic mouse that overexpresses it or a knockout mouse that lacks it. Using the techniques described in Chapter 8, one can engineer mice in which the misexpression or deletion of the gene is restricted to a specific set of cells, or in which expression of the gene can be switched on at will at a chosen point in time, or both, to see whether and how tumors develop. Moreover, to follow the growth of tumors from day to day in the living organism, the cells of interest can be genetically marked and made visible by expression of a fluorescent or luminescent reporter (Figure 20–28). In these ways, one can begin to clarify the part that each cancer-critical gene plays in cancer initiation or progression.


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Cell_Biology_Alberts. Figure 20–28 Monitoring tumor growth and metastasis in a mouse with a luminescent reporter. A mouse was genetically engineered in a way that allows both copies of its PTEN tumor suppressor gene to be inactivated in the prostate gland, simultaneously with the prostate-specific activation of a gene engineered to produce the enzyme luciferase (derived from fireflies). After an injection of luciferin (the substrate molecule for luciferase) into the mouse’s bloodstream, the cells in the prostate emit light and can be detected by their bioluminescense in a live mouse, as seen in the 67-day-old animal at the left. Cells lacking the PTEN phosphatase enzyme contain elevated amounts of the Akt activator, PI(3,4,5)P3, and this causes the prostate cells to proliferate abnormally, progressing over time to form a cancer. In this way, the process of metastasis could be followed in the same animal over the course of a year. The light intensity in these experiments is proportional to the number of


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Cell_Biology_Alberts. time to form a cancer. In this way, the process of metastasis could be followed in the same animal over the course of a year. The light intensity in these experiments is proportional to the number of prostate-cell descendants, increasing from light blue to green, to yellow, to red in this representation. (Adapted from C.-P. Liao et al., Cancer Res. 67:7525– 7533, 2007.)


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Cell_Biology_Alberts. Figure 20–29 Oncogene collaboration in transgenic mice. The graphs show the incidence of tumors in three types of transgenic mouse strains, one carrying a Myc oncogene, one carrying a Ras oncogene, and one carrying both oncogenes. For these experiments, two lines of transgenic mice were first generated. One carries an inserted copy of an oncogene created by fusing the proto-oncogene Myc with the mouse mammary tumor virus regulatory DNA (which then drives Myc overexpression in the mammary gland). The other line carries an inserted copy of the Ras oncogene under 50 control of the same regulatory element. Both strains of mice develop tumors much more frequently than normal, most often in the mammary or salivary glands. Mice that carry both oncogenes together are obtained by crossing the two strains. These hybrids develop tumors at a far higher rate still, much greater than the sum of the rates for the two oncogenes separately. 0 Nevertheless, the tumors arise only after a delay and only


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Cell_Biology_Alberts. strains. These hybrids develop tumors at a far higher rate still, much greater than the sum of the rates for the two oncogenes separately. 0 Nevertheless, the tumors arise only after a delay and only from a small percentage of tumor-free mice proportion of the cells in the tissues where the two genes are expressed. Further accidental changes, in addition to the two oncogenes, are apparently required for the development of cancer. (After E. Sinn et al., Cell 49:465–475, 1987. With permission from Elsevier.)


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Cell_Biology_Alberts. Transgenic mouse studies confirm, for example, that a single oncogene is generally not enough to turn a normal cell into a cancer cell. Thus, in mice engineered to express a Myc or Ras oncogenic transgene, some of the tissues that express the oncogene may show enhanced cell proliferation, and, over time, occasional cells will undergo further changes to give rise to cancers. Most cells expressing the oncogene, however, do not give rise to cancers. Nevertheless, from the point of view of the whole animal, the inherited oncogene is a serious menace because it creates a high risk that a cancer will arise somewhere in the body. Mice that express both Myc and Ras oncogenes (bred by mating a transgenic mouse carrying a Myc oncogene with one carrying a Ras oncogene) develop cancers earlier and at a much higher rate than either parental strain (Figure 20–29); but, again, the cancers originate as scattered, isolated tumors among noncancerous cells. Thus, even cells expressing these two


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Cell_Biology_Alberts. and at a much higher rate than either parental strain (Figure 20–29); but, again, the cancers originate as scattered, isolated tumors among noncancerous cells. Thus, even cells expressing these two oncogenes must undergo further, randomly generated changes to become cancerous. This strongly suggests that multiple mutations are required for tumorigenesis, as supported by a great deal of other evidence discussed earlier. Experiments using mice with deletions of tumor suppressor genes lead to similar conclusions.


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Cell_Biology_Alberts. Cancers Become More and More heterogeneous as They Progress From simple histology, looking at stained tissue sections, it is clear that some tumors contain distinct sectors, all clearly cancerous, but differing in appearance because they differ genetically: the cancer cell population is heterogeneous. Evidently, within the initial clone of cancerous cells, additional mutations have arisen and thrived, creating diverse subclones. Today, the ability to analyze cancer genomes lets us look much deeper into the process. One approach involves taking samples from different regions of a primary tumor and from the metastases that it has spawned. With modern methods, it is even possible to take representative single cells and analyze their genomes. Such studies reveal a classic picture of Darwinian evolution, occurring on a time scale of months or years rather than millions of years, but governed by the same rules of natural selection (Figure 20–30).


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Cell_Biology_Alberts. One such investigation compared the genomes of 100 individual cells from different regions of a primary tumor of the breast. A large fraction—just over half—of the chosen cells was genetically normal or nearly so: these were connective-tissue cells and other cell types, such as those of the immune system, that were mixed up with the cancer cells. The cancer cells themselves were distinguished by their severely disrupted genomes. The detailed pattern of gene deletions and amplifications in each such cell revealed how closely it was related to the others, and from this data one could draw up a family tree (Figure 20–30B). In this case, three main branches of the tree were seen; that is, the cancer consisted of three major mammary appearance of most epithelial recent common ancestor development number of cells subclones. From the shared abnormalities, one could deduce that their last common ancestor—the presumed founder of the cancer—was already very different from a normal cell, but


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Cell_Biology_Alberts. number of cells subclones. From the shared abnormalities, one could deduce that their last common ancestor—the presumed founder of the cancer—was already very different from a normal cell, but that the first split between branches occurred early, when the tumor was small. This was followed by a large amount of additional change within each branch. A hint of the future could be seen in the smallest of the three major subclones: its cells were distinguished by a massive amplification of a Ras oncogene. Given more time, perhaps they would have out-competed the other cancer cells and taken over the whole tumor.


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Cell_Biology_Alberts. Similar results have been obtained with other cancers. Clearly, cancer cells are constantly mutating, multiplying, competing, evolving, and diversifying as they exploit new ecological niches and react to the treatments that are used against them (Figure 20–30C). Diversification accelerates as they metastasize and colonize new territories, where they encounter new selection pressures. The longer the evolutionary process continues, the harder it becomes to catch them all in the same net and kill them. The Changes in Tumor Cells That Lead to Metastasis Are Still Largely a Mystery


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Cell_Biology_Alberts. The Changes in Tumor Cells That Lead to Metastasis Are Still Largely a Mystery Perhaps the most significant gap in our understanding of cancer concerns invasiveness and metastasis. For a start, it is not clear exactly what new properties a cancer cell must acquire to become metastatic. In some cases, it is possible that invasion and metastasis require no further genetic changes beyond those needed to violate the normal controls on cell growth, cell division, and cell death. On the other hand, it may be that, for some cancers, metastasis requires a large number of additional mutations and epigenetic changes. Clues are coming from comparisons of the genomes of cells of primary tumors with the cells of metastases that they have spawned. The results appear complex and variable from one cancer to another. Nevertheless, some general principles have emerged.


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Cell_Biology_Alberts. As we discussed earlier, it is helpful to distinguish three phases of tumor progression required for a carcinoma to metastasize (see Figure 20–16). First, the cells Figure 20–30 How cancers progress as a series of subclones. (A) Schematic illustration of the pattern of mutation and natural selection in a clone of tumor cells. A family tree of cancer cells sampled from different regions of a single breast tumor, showing how the cells have evolved and diversified from a common ancestor, the cancer founder cell. The genome of each of the indicated 100 cells from a human breast tumor was sequenced to produce an evolutionary tree. About half of these cells were normal cells from the stroma (blue cells). The red cells have greatly amplified their K-Ras gene. Note that many subclones appear to have died out, including the one that contained the founder cells for the three subclones that survive.


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Cell_Biology_Alberts. A depiction of how driver mutations are thought to cause cancer progression over long periods of time, before producing a large enough clone of proliferating cells to be detected as a tumor. The data indicate that driver mutations occur only rarely in a background of long-lived subclones of cells that continually accumulate passenger mutations without gaining a growth advantage. (A, adapted from M. Greaves, Semin. Cancer Biol. 20:65–70, 2010; B, adapted from N. Navin et al., Nature 472:90–94, 2011; C, adapted from S.


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Cell_Biology_Alberts. S. Nik-Zainal et al., Cell 149:994–1007, 2012.) invasiveness survival in the arrest in exit into survival of initial growth persistence causes circulation capillary or remote tissue cells in of cells in of growth entry into other small or organ foreign foreign vessel vessel tissue tissue must escape the normal confines of their parent epithelium and begin to invade the tissue immediately beneath. Second, they must travel via the blood or lymph to lodge in distant sites. Third, they must survive there and multiply. It is the first and last steps in this sequence that are the most difficult to accomplish for most cancers (Figure 20–31).


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Cell_Biology_Alberts. The first step, local invasiveness, requires a relaxation of the mechanisms that normally hold epithelial cells together. As mentioned earlier, this step resembles the normal developmental process known as the epithelial–mesenchymal transition (EMT ), in which epithelial cells undergo a shift in character, becoming less adhesive and more migratory (discussed in Chapter 19). A key part of the EMT process involves switching off expression of the E-cadherin gene. The primary function of the transmembrane E-cadherin protein is in cell–cell adhesion, binding epithelial cells together through adherens junctions (see Figure 19–13). In some carcinomas of the stomach and of the breast, E-cadherin has been identified as a tumor suppressor gene, and a loss of E-cadherin may promote cancer development by facilitating local invasiveness.


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Cell_Biology_Alberts. The initial entry of tumor cells into the circulation is helped by the presence of a dense supply of blood vessels and sometimes lymphatic vessels, which tumors attract to themselves as they grow larger and become hypoxic in their interior. This process, called angiogenesis, is caused by the secretion of angiogenic factors that promote the growth of blood vessels, such as vascular endothelial growth factor (VEGF; see Figure 22–26). An abnormal fragility and leakiness of the new vessels that form may help the cells that have become invasive to enter and then move through the circulation with relative ease.


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Cell_Biology_Alberts. The remaining steps in metastasis, involving exit from a blood or lymphatic vessel and the effective colonization of remote sites, are much harder to study. To discover which of the later steps in metastasis present cancer cells with the greatest difficulties, one can label the cells with a fluorescent dye or green fluorescent protein (GFP), inject them into the bloodstream of a mouse, and then monitor their fate (Movie 20.5). In such experiments, one observes that many cells survive in the circulation, lodge in small vessels, and exit into the surrounding tissue, regardless of whether they come from a tumor that metastasizes or one that does not. Some cells die immediately after they enter foreign tissue; others survive entry into the foreign tissue but fail to proliferate. Still others divide a few times and then stop, forming micrometastases containing ten to several thousand cells. Very few establish full-blown metastases.


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Cell_Biology_Alberts. What, if anything, distinguishes the survivors from the failures? A clue may come from the fact that in many types of tumors, the cancer cells show a kind of heterogeneity that resembles the heterogeneity seen among the cells of those normal tissues that renew themselves continually by a stem-cell strategy, as we discuss next. A Small Population of Cancer Stem Cells May Maintain Many Tumors Self-renewing tissues, where cell division continues throughout life, are the breeding ground for the great majority of human cancers. They include the epidermis Figure 20–31 The barriers to metastasis.


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Cell_Biology_Alberts. Studies of labeled tumor cells leaving a tumor site, entering the circulation, and establishing metastases show which steps in the metastatic process, outlined in Figure 20–16, are difficult or “inefficient,” in the sense that they are steps in which large numbers of cells fail and are lost. It is in these difficult steps that cells from highly metastatic tumors are observed to have much greater success than cells from a nonmetastatic source. It seems that the ability to escape from the parent tissue, and an ability to survive and grow in the foreign tissue, are key properties that cells must acquire to become metastatic. (Adapted from A.F. Chambers et al., Breast Cancer Res. 2:400–407, 2000. With permission from BioMed Central Ltd.) (the outer epithelial layer of the skin), the lining of the digestive and reproductive tracts, and the bone marrow, where blood cells are generated (see Chapter 22). In almost all these tissues, renewal depends on the presence of stem cells, which divide


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Cell_Biology_Alberts. the digestive and reproductive tracts, and the bone marrow, where blood cells are generated (see Chapter 22). In almost all these tissues, renewal depends on the presence of stem cells, which divide to give rise to terminally differentiated cells, which do not divide. This creates a mixture of cells that are genetically identical and closely related by lineage, but are in different states of differentiation. Many tumors seem likewise to consist of cells in varied states of differentiation, with different capacities for cell division and self-renewal.


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Cell_Biology_Alberts. To see the implications, it is helpful to consider how normal stem-cell systems operate. When a normal stem cell divides, each daughter cell has a choice—it can remain a stem cell, or it can commit to a pathway leading to differentiation. A stem-cell daughter remains in place to generate more cells in the future. A committed daughter typically undergoes some rounds of cell proliferation (as a so-called transit amplifying cell) but then stops dividing, terminally differentiates, and eventually is discarded and replaced (it may die by apoptosis, with recycling of its materials, or be shed from the body). On average, the two fates—stem cell or differentiating cell—normally occur with equal probability, so that half the daughters of stem-cell divisions take the one path and half take the other. In a healthy body, feedback controls regulate the process, adjusting this balance of cell-fate choices to correct for any departure from the proper cell population numbers. Thus, the number of stem


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Cell_Biology_Alberts. In a healthy body, feedback controls regulate the process, adjusting this balance of cell-fate choices to correct for any departure from the proper cell population numbers. Thus, the number of stem cells remains approximately constant, and the terminally differentiated cells are continually replaced at a steady rate. Because of the divisions undergone by the transit amplifying cells, the stem cells may be vastly outnumbered by the cells that are committed to terminal differentiation and have lost the capacity for self-renewal. But the stem cells, though few and far between and often relatively slowly dividing, carry the whole responsibility for maintenance of the tissue in the long term.


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Cell_Biology_Alberts. Some cancers seem to be organized in a similar way: they consist of rare cancer stem cells capable of dividing indefinitely, together with much larger numbers of dividing transit amplifying cells that are derived from the cancer stem cells but have a limited capacity for self-renewal (Figure 20–32). These non-stem cells appear to constitute the great majority of the cell population in some tumors. The Cancer Stem-Cell Phenomenon Adds to the Difficulty of Curing Cancer Evidence for the cancer stem-cell phenomenon comes chiefly from experiments in which individual cells from a cancer are tested for their ability to give rise to fresh tumors: a standard assay is to implant the cells into an immunodeficient mouse (Figure 20–33). It has been known for half a century that there is usually only a small chance—typically much less than 1%—that a tumor cell chosen at random and tested in this way will generate a new tumor. This by itself does not prove that


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Cell_Biology_Alberts. Figure 20–32 Cancer stem cells can be responsible for tumor growth and yet remain only a small part of the tumor-cell population. (A) how stem cells produce transit amplifying cells.


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Cell_Biology_Alberts. (B) how a small proportion of cancer stem cells can maintain a tumor. Suppose, for example, that each daughter of a cancer stem cell has a probability slightly greater than 50% of retaining stem-cell potential and a probability slightly less than 50% of becoming a transit amplifying cell that is committed to a program of cell divisions that stops after 10 division cycles. While the number of cancer stem cells will increase slowly but steadily to give a growing tumor, the non-stem cells that they give rise to will always outnumber the stem cells by a large factor—in this example, by a factor of about 1000. (If the cell-divisioncycle and survival times for the two classes of cells are equal.) the tumor cells are heterogeneous: like seeds scattered on difficult ground, each of them may have only a small chance of finding a spot where it can survive and grow. Modern technologies for sorting cells have shown, however, that in some cancers at least, the rate of success in founding new


Cell_Biology_Alberts

Cell_Biology_Alberts. have only a small chance of finding a spot where it can survive and grow. Modern technologies for sorting cells have shown, however, that in some cancers at least, the rate of success in founding new tumors is even lower than it would otherwise be because the cancer cells are heterogeneous in their state of differentiation, and only a small subset of them—the cancer stem cells—have the special properties needed for tumor propagation. For example, in several types of cancer, including breast cancers and leukemias, one can fractionate the tumor cells using monoclonal antibodies that recognize a particular cell-surface marker that is present on the normal stem cells in the tissue of origin of the cancer. The purified cancer cells expressing this marker are found to have a greatly enhanced ability to found new tumors. And the new tumors consist of mixtures of cells that express the marker and cells that do not, all generated from the same founder cell that expressed the marker.


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Cell_Biology_Alberts. Experiments with breast cancer cells have revealed that, instead of following a rigid program from stem cell to transit amplifying cell to terminally differentiated cell, these cancer cells can randomly switch to and fro—with a certain low transition probability—between different states of differentiation that express different molecular markers. In one state, they behave like stem cells, dividing slowly but capable of founding new tumors; in other states, they behave like transit amplifying cells, dividing rapidly but unable to found new tumors in a standard transplant assay. But a single cell in any of these states—given time in culture, or a congenial environment in the body—will give rise to a mixed population that includes all the other states as well.


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Cell_Biology_Alberts. The cancer stem-cell phenomenon, whatever its basis, implies that even when the tumor cells are genetically similar, they are phenotypically diverse. A treatment that wipes out those in one state is likely to allow survival of others that remain a danger. Radiotherapy or a cytotoxic drug, for example, may selectively kill off the rapidly dividing cells, reducing the tumor volume to almost nothing, and yet spare a few slowly dividing cells that go on to resurrect the disease. This greatly adds to the difficulty of cancer therapy, and it is part of the reason why treatments that seem at first to succeed often end in relapse and disappointment. Colorectal Cancers Evolve Slowly Via a Succession of Visible Changes


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Cell_Biology_Alberts. Colorectal Cancers Evolve Slowly Via a Succession of Visible Changes At the beginning of this chapter, we saw that most cancers develop gradually from a single aberrant cell, progressing from benign to malignant tumors by the accumulation of a number of independent genetic and epigenetic changes. We have discussed what some of these changes are in molecular terms and seen how they contribute to cancerous behavior. We now examine one of the common human cancers more closely, using it to illustrate and enlarge upon some of the general principles and molecular mechanisms we have introduced. We take colorectal cancer as our example.


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Cell_Biology_Alberts. Colorectal cancers arise from the epithelium lining the colon (the large intestine) and rectum (the terminal segment of the gut). The organization of this tissue is broadly similar to that of the small intestine, discussed in detail in Chapter 22 (pp. 1217–1221). For both the small and large intestine, the epithelium is renewed at an extraordinarily rapid rate, taking about a week to completely replace most of the epithelial sheet. In both regions, the renewal depends on stem cells that lie in deep pockets of the epithelium, called intestinal crypts. The signals that maintain the stem cells and control the normal organization and renewal of the epithelium are beginning to be quite well understood, as explained in Chapter 22. Mutations that disrupt these signals begin the process of tumor progression for most colorectal cancers (Movie 20.6).


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Cell_Biology_Alberts. Colorectal cancers are common, currently causing nearly 60,000 deaths a year in the United States, or about 10% of total deaths from cancer. Like most cancers, they are not usually diagnosed until late in life (90% occur after the age of 55). However, routine examination of normal adults with a colonoscope (a fiber


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Cell_Biology_Alberts. Figure 20–33 An immunodeficient mouse, as used in transplantation assays to test human cancer cells for their ability to found new tumors. This nude mouse has a mutation that blocks development of the thymus and, as a side effect, robs it of hair. Because it has practically no T cells, it tolerates grafts of cells even from other species. (Courtesy of harlan Sprague Dawley.) optic device for viewing the interior of the colon and rectum) often reveals a small benign tumor, or adenoma, of the gut epithelium in the form of a protruding mass of tissue called a polyp (see Figure 22–4). These adenomatous polyps are believed to be the precursors of a large proportion of colorectal cancers. Because the progression of the disease is usually very slow, there is typically a period of about 10 years in which the slowly growing tumor is detectable but has not yet turned malignant. Thus, when people are screened by colonoscopy in their fifties and the polyps are removed through the colonoscope—a


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Cell_Biology_Alberts. in which the slowly growing tumor is detectable but has not yet turned malignant. Thus, when people are screened by colonoscopy in their fifties and the polyps are removed through the colonoscope—a quick and easy surgical procedure—the subsequent incidence of colorectal cancer is much lower: according to some studies, less than a quarter of what it would be otherwise.


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Cell_Biology_Alberts. In microscopic sections of polyps smaller than 1 cm in diameter, the cells and their arrangement in the epithelium usually appear almost normal. The larger the polyp, the more likely it is to contain cells that look abnormally undifferentiated and form abnormally organized structures. Sometimes, two or more distinct areas can be distinguished within a single polyp, with the cells in one area appearing relatively normal and those in the other appearing clearly cancerous, as though they have arisen as a mutant subclone within the original clone of adenomatous cells. At later stages in the disease, some tumor cells become invasive in a small fraction of the polyps, first breaking through the epithelial basal lamina, then spreading through the layer of muscle that surrounds the gut, and finally metastasizing to lymph nodes via lymphatic vessels and to liver, lung, and other organs via blood vessels. A Few Key Genetic Lesions Are Common to a Large Fraction of Colorectal Cancers


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Cell_Biology_Alberts. A Few Key Genetic Lesions Are Common to a Large Fraction of Colorectal Cancers What are the mutations that accumulate with time to produce this chain of events? Of those genes so far discovered to be involved in colorectal cancer, three stand out as most frequently mutated: the proto-oncogene K-Ras (a member of the Ras gene family), in about 40% of cases; p53, in about 60% of cases; and the tumor suppressor gene Apc (discussed below), in more than 80% of cases. Others are involved in smaller numbers of colon cancers, and some of these are listed in Table 20–1. The role of Apc first came to light through study of certain families showing a rare type of hereditary predisposition to colorectal cancer, called familial


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Cell_Biology_Alberts. Figure 20–34 Colon of familial adenomatous polyposis coli patient compared with normal colon. (A) The normal colon wall is a gently undulating but smooth surface. (B) The polyposis colon is completely covered by hundreds of projecting polyps, each resembling a tiny cauliflower when viewed with the naked eye. (Courtesy of Andrew Wyllie and Mark Arends.) adenomatous polyposis coli (FAP). In this syndrome, hundreds or thousands of polyps develop along the length of the colon (Figure 20–34). These polyps start to appear in early adult life, and if they are not removed, one or more will almost always progress to become malignant; the average time from the first detection of polyps to the diagnosis of cancer is 12 years. The disease can be traced to a deletion or inactivation of the tumor suppressor gene Apc, named after the syndrome. Individuals with FAP have inactivating mutations or deletions of one copy of the Apc gene in all their cells and show loss of heterozygosity in tumors, even


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Cell_Biology_Alberts. gene Apc, named after the syndrome. Individuals with FAP have inactivating mutations or deletions of one copy of the Apc gene in all their cells and show loss of heterozygosity in tumors, even in the benign polyps. Most patients with colorectal cancer do not have the hereditary condition. Nevertheless, in more than 80% of the cases, their cancer cells (but not their normal cells) have inactivated both copies of the Apc gene through mutations acquired during the patient’s lifetime. Thus, by a route similar to that which we discussed for retinoblastoma, mutation of the Apc gene was identified as one of the central ingredients of colorectal cancer.


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Cell_Biology_Alberts. The Apc protein, as we now know, is an inhibitory component of the Wnt signaling pathway (discussed in Chapter 15). It binds to the β-catenin protein, another component of the Wnt pathway, and helps to induce the protein’s degradation. By inhibiting β-catenin in this way, Apc prevents the β-catenin from migrating to the nucleus, where it would act as a transcriptional regulator to drive cell proliferation and maintain the stem-cell state (see Figure 15–60). Loss of Apc results in an excess of free β-catenin and thus leads to an uncontrolled expansion of the stem-cell population. This causes massive increase in the number and size of the intestinal crypts (see Figure 22–4).


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Cell_Biology_Alberts. When the β-catenin gene was sequenced in a collection of colorectal tumors, it was discovered that, many of the tumors that did not have Apc mutations had activating mutations in β-catenin instead. Thus, it is excessive activity in the Wnt signaling pathway that is critical for the initiation of this cancer, rather than any single oncogene or tumor suppressor gene that the pathway contains. This being so, why is the Apc gene in particular so often the most common culprit in colorectal cancer? The Apc protein is large and it interacts not only with β-catenin but also with various other cell components, including microtubules. Loss of Apc appears to increase the frequency of mitotic spindle defects, leading to chromosome abnormalities when cells divide. This additional, independent cancer-promoting effect could explain why Apc mutations feature so prominently in the causation of colorectal cancer.


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Cell_Biology_Alberts. In addition to the hereditary disease (FAP) associated with Apc mutations, there is a second, more common kind of hereditary predisposition to colon carcinoma in which the course of events differs from the one we have described for FAP. In this more common condition, called hereditary nonpolyposis colorectal cancer (HNPCC), the probability of colon cancer is increased without any increase in the number of colorectal polyps (adenomas). Moreover, the cancer cells are unusual, in that they have a normal (or almost normal) karyotype. The majority of colorectal tumors in non-HNPCC patients, in contrast, have gross chromosomal abnormalities, with multiple translocations, deletions, and other aberrations, as well as having many more chromosomes than normal (Figure 20–35).


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Cell_Biology_Alberts. The mutations that predispose HNPCC individuals to colorectal cancer occur in one of several genes that code for central components of the DNA mismatch repair system. These genes are homologous in structure and function to the MutL and MutS genes in bacteria and yeast (see Figure 5–19). Only one of the two copies of the involved gene is defective, so the repair system is still able to remove the inevitable DNA replication errors that occur in the patient’s cells. However, as discussed previously, these individuals are at risk, because the accidental loss or inactivation of the remaining good gene copy will immediately elevate the spontaneous mutation rate by a hundredfold or more (discussed in Chapter 5). These genetically unstable cells then can presumably speed through the standard processes of mutation and natural selection that allow clones of cells to progress to malignancy.


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Cell_Biology_Alberts. This particular type of genetic instability produces invisible changes in the chromosomes—most notably changes in individual nucleotides and short expansions and contractions of monoand dinucleotide repeats such as AAAA… or CACACA…. Once the defect in HNPCC patients was recognized, the epigenetic silencing or mutation of mismatch repair genes was found in about 15% of the colorectal cancers occurring in people with no inherited predisposing mutation.


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Cell_Biology_Alberts. Thus, the genetic instability found in many colorectal cancers can be acquired in at least two ways. The majority of the cancers display a form of chromosomal instability that leads to visibly altered chromosomes, whereas in the others the instability occurs on a much smaller scale and reflects a defect in DNA mismatch repair. Indeed, many carcinomas show either chromosomal instability or defective mismatch repair—but rarely both. These findings clearly demonstrate that genetic instability is not an accidental by-product of malignant behavior but a contributory cause—and that cancer cells can acquire this instability in multiple ways. The Steps of Tumor Progression Can Often Be Correlated with Specific Mutations


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Cell_Biology_Alberts. The Steps of Tumor Progression Can Often Be Correlated with Specific Mutations In what order do K-Ras, p53, Apc, and the other identified colorectal cancer-critical genes mutate, and what contribution does each of them make to the asocial behavior of the cancer cell? There is no single answer, because colorectal cancer can arise by more than one route: thus, we know that in some cases, the first mutation can be in a DNA mismatch repair gene; in others, it can be in a gene regulating cell proliferation. Moreover, as previously discussed, a general feature such as genetic instability or a tendency to proliferate abnormally can arise in a variety of ways, through mutations in different genes.


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Cell_Biology_Alberts. Nevertheless, certain sets of mutations are particularly common in colorectal cancer, and they occur in a characteristic order. Thus, in most cases, mutations inactivating the Apc gene appear to be the first, or at least a very early step, as they are detected at the same high frequency in small benign polyps as in large malignant tumors. Changes that lead to genetic and epigenetic instability are likely also to arise early in tumor progression, since they are needed to drive the later steps. Activating mutations in the K-Ras gene occur later, as they are rare in small polyps but common in larger ones that show disturbances in cell differentiation and histological pattern.


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Cell_Biology_Alberts. Activating mutations in the K-Ras gene occur later, as they are rare in small polyps but common in larger ones that show disturbances in cell differentiation and histological pattern. Inactivating mutations in p53 are thought to come later still, as they are rare in polyps but common in carcinomas (Figure 20–36). We have seen that loss of p53 function allows cancer cells to endure stress and to avoid apoptosis and cell-cycle arrest. Additionally, loss of p53 is related to the heightened activation Figure 20–35 Chromosome complements (karyotypes) of colon cancers showing different kinds of genetic instability. (A) The karyotype of a typical cancer shows many gross abnormalities in chromosome number and structure. Considerable variation can also exist from cell to cell (not shown).


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Cell_Biology_Alberts. (B) The karyotype of a tumor that has a stable chromosome complement with few chromosomal anomalies; the genetic abnormalities are mostly invisible, having been created by defects in DNA mismatch repair. All of the chromosomes in this figure were stained as in Figure 4–10, the DNA of each human chromosome being marked with a different combination of fluorescent dyes. (Courtesy Wael Abdel-Rahman and Paul Edwards.) acquisition of increased genetic and epigenetic instability of oncogenes such as Ras. Experiments in mice show that an initial low level of oncogene activation can give rise to a slowly growing tumor even while p53 is functional: genes such as Ras are, after all, part of the normal machinery of growth control, and moderate activation is not stressful for a cell and does not call the p53 protein into play. Progression of a tumor from slow to rapid, malignant growth, however, involves activation of oncogenes beyond normal physiological limits to a higher, stressful level. If


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Cell_Biology_Alberts. the p53 protein into play. Progression of a tumor from slow to rapid, malignant growth, however, involves activation of oncogenes beyond normal physiological limits to a higher, stressful level. If the p53 protein is present and functional, this should lead to cell-cycle arrest or death. Only by losing p53 function can the cancer cells with hyperactive oncogenes survive and progress.


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Cell_Biology_Alberts. The steps we have just described are only part of the picture. It is important to emphasize that each case of colorectal cancer is different, with its own detailed combination of mutations, and that even for the mutations that are commonly shared, the sequence of occurrence may vary. The same is true for cancers in general. Advances in molecular biology have recently provided the tools to find out precisely which genes are amplified, deleted, mutated, or misregulated by epigenetic mechanisms in the tumor cells of any given patient. As we discuss in the next section, such information promises to become as important for the diagnosis and treatment of cancer as was the breakthrough of being able to identify microorganisms for the treatment of infectious diseases.


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Cell_Biology_Alberts. The molecular analysis of cancer cells reveals two classes of cancer-critical genes: oncogenes and tumor suppressor genes. A set of these genes becomes altered by a combination of genetic and epigenetic accidents to drive tumor progression. Many cancer-critical genes code for components of the social control pathways that regulate when cells grow, divide, differentiate, or die. In addition, a subclass of tumor suppressors can be categorized as “genome maintenance genes,” because their normal role is to help maintain genome integrity. The inactivation of the p53 pathway, which occurs in nearly all human cancers, allows genetically damaged cells to escape apoptosis and continue to proliferate. Inactivation of the Rb pathway also occurs in most human cancers, illustrating how fundamental each of these pathways is for protecting us against cancer.


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Cell_Biology_Alberts. The sequencing of cancer cell genomes reveals that—except for the cancers of childhood—many cancers acquire 10 or so driver mutations over the long course of tumor progression, along with a considerably larger number of passenger mutations of no consequence. The same methods reveal how subclones of cells arise and die out as a tumor ages. Tumors thus contain a heterogeneous mixture of cells, some—the so-called cancer stem cells—being much more dangerous than others.


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Cell_Biology_Alberts. We can often correlate the steps of tumor progression with mutations that activate specific oncogenes and inactivate specific tumor suppressor genes, with colon cancer providing a good example. But different combinations of mutations and epigenetic changes are found in different types of cancer, and even in different patients with the same type of cancer, reflecting the random way in which these inherited changes arise. Nevertheless, many of the same changes are encountered repeatedly, suggesting that there are a limited number of ways to breach our defenses against cancer. Figure 20–36 Suggested typical sequence of genetic changes underlying the development of a colorectal carcinoma. This oversimplified diagram provides a general idea of the way mutation and tumor development are related. But many other mutations are generally involved, and different colon cancers can progress through different sequences of mutations (and/or epigenetic changes).


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Cell_Biology_Alberts. CANCER PREVENTION AND TREATMENT: PRESENT AND FUTURE We can apply the growing understanding of the molecular biology of cancer to sharpen our attack on the disease at three levels: prevention, diagnosis, and treatment. Prevention is always better than cure, and indeed many cancers can be prevented, especially by avoiding smoking. Highly sensitive molecular assays promise new opportunities for earlier and more precise diagnosis, with the aim of detecting primary tumors while they are still small and have not yet metastasized. Cancers caught at these early stages can often be nipped in the bud by surgery or radiotherapy, as we saw for colorectal polyps. Nevertheless, full-blown malignant disease will continue to be common for many years to come, and cancer treatments will continue to be needed.


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Cell_Biology_Alberts. In this section, we first examine the preventable causes of cancer and then consider how advances in our understanding at a molecular level are beginning to transform the treatment of the disease. Epidemiology Reveals That Many Cases of Cancer Are Preventable A certain irreducible background incidence of cancer is to be expected regardless of circumstances. As discussed in Chapter 5, mutations can never be absolutely avoided because they are an inescapable consequence of fundamental limitations on the accuracy of DNA replication and repair. If a person could live long enough, it is inevitable that at least one of his or her cells would eventually accumulate a set of mutations sufficient for cancer to develop.


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Cell_Biology_Alberts. Nevertheless, environmental factors seem to play a large part in determining the risk for cancer. This is demonstrated most clearly by a comparison of cancer incidence in different countries: for almost every cancer that is common in one country, there is another country where the incidence is much lower. Because migrant populations tend to adopt the pattern of cancer incidence typical of their new host country, the differences are thought to be due mostly to environmental, not genetic, factors. From such findings, it has been suggested that 80–90% of cancers should be avoidable, or at least postponable (Figure 20–37).


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Cell_Biology_Alberts. Unfortunately, different cancers have different environmental risk factors, and a population that escapes one such danger is usually exposed to another. This is not, however, inevitable. There are some human subgroups whose way of life substantially reduces the total cancer death rate among individuals of a given age. Under the current conditions in the United States and Europe, approximately one in five people will die of cancer. But the incidence of cancer among strict Mormons in Utah—who avoid alcohol, coffee, cigarettes, drugs, and casual sex—is only about half the incidence for non-practicing members of the same family or for Americans in general. Cancer incidence is also low in certain relatively affluent populations in Africa.


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Cell_Biology_Alberts. Although such observations on human populations indicate that cancer can often be avoided, it has been difficult in most cases—with tobacco as a striking exception—to pinpoint the specific environmental factors responsible for these large population differences or to establish how they act. Nevertheless, several important classes of environmental cancer risk factors have been identified (Figure 20–37B). One thinks first of mutagens. But there are also many other influences—including the amount of food we eat, the hormones that circulate in our bodies, and the irritations, infections, and damage to which we expose our tis-sues—that are no less important and favor development of the disease in other ways. Many quite disparate chemicals are carcinogenic when they are fed to experimental animals or painted repeatedly on their skin. Examples include a range


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Cell_Biology_Alberts. Many quite disparate chemicals are carcinogenic when they are fed to experimental animals or painted repeatedly on their skin. Examples include a range SOUTH AMERICA NORTH AMERICA HAWAII JAPAN CHINA AFRICA prostate, colon, breast prostate, colon, breast stomach naso-pharyngeal Burkitt’s lymphoma Burkitt’s lymphoma (A) EASTERN EUROPE Hodgkin’s disease Figure 20–37 Cancer incidence is related to environmental influences. This map of the world shows the rates of cancer increasing (red arrows) or decreasing (blue arrows) when specific populations move from one location to another. Such observations suggest the importance of environmental factors, including diet, in dictating cancer risk. (B) Some estimated effects of environment and lifestyle on cancer in the United States (US). The table shows both the yearly deaths in the US attributable to each cancer and the estimated percentage of that cancer that could be eliminated through prevention. (B, data from G.A. Colditz, K.Y. Wolin and


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Cell_Biology_Alberts. S. Gehlert, Sci. Transl. Med. 4:127rv4, 2012.) of aromatic hydrocarbons and derivatives of them such as aromatic amines, nitrosamines, and alkylating agents such as mustard gas. Although these chemical carcinogens are diverse in structure, a large proportion of them have at least one shared property—they cause mutations. In one common test for mutagenicity (the Ames test), the carcinogen is mixed with an activating extract prepared from rat liver cells (to mimic the biochemical processing that occurs in an intact animal). The mixture is then added to a culture of specially designed test bacteria and the bacterial mutation rate measured. Most of the compounds scored as mutagenic by this rapid and convenient assay in bacteria also cause mutations or chromosome aberrations when tested on mammalian cells.


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Cell_Biology_Alberts. A few of these carcinogens act directly on DNA. But generally the more potent ones are relatively inert chemically; these chemicals become damaging only after they have been converted to a more reactive molecule by metabolic processes in the liver, catalyzed by a set of intracellular enzymes known as the cytochrome P-450 oxidases. These enzymes normally help to convert ingested toxins into harmless and easily excreted compounds. Unhappily, their activity on certain chemicals generates products that are highly mutagenic. Examples of carcinogens activated in this way include benzo[a]pyrene, a cancer-causing chemical present in coal tar and tobacco smoke and the fungal toxin aflatoxin B1 (Figure 20–38). Fifty Percent of Cancers Could Be Prevented by Changes in Lifestyle


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Cell_Biology_Alberts. Tobacco smoke is the most important carcinogen in the world today. Even though many other chemical carcinogens have been identified, none of these appear to be responsible for anything like the same numbers of human cancer deaths. It is sometimes thought that the main environmental causes of cancer are the products of a highly industrialized way of life—the rise in pollution, the enhanced use of food additives, and so on—but there is little evidence to support this view. The idea may have come in part from the identification of some highly carcinogenic materials used in industry, such as 2-naphthylamine and asbestos. Except for the increase in cancers caused by smoking, however, age-adjusted death rates for most common human cancers have stayed much the same over the past half-century, or, in some cases, have declined significantly (Figure 20–39). Survival rates, moreover, have improved. Thirty years ago, less than 50% of patients lived more than five years from the time of diagnosis;


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Cell_Biology_Alberts. in some cases, have declined significantly (Figure 20–39). Survival rates, moreover, have improved. Thirty years ago, less than 50% of patients lived more than five years from the time of diagnosis; now, more than two-thirds do so.


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Cell_Biology_Alberts. (A) AFLATOXIN AFLATOXIN-2,3-EPOXIDE CARCINOGEN BOUND TO GUANINE IN DNA Figure 20–38 Some known carcinogens. (A) Carcinogen activation. A metabolic transformation must activate many chemical carcinogens before they will cause mutations by reacting with DNA. The compound illustrated here is aflatoxin B1, a toxin from a mold (Aspergillus flavus oryzae) that grows on grain and peanuts when they are stored under humid tropical conditions. Aflatoxin is an important cause of liver cancer in the tropics. (B) Different carcinogens cause different types of cancer. (B, data from Cancer and the Environment: Gene Environment Interactions, National Academies Press, 2002.)


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Cell_Biology_Alberts. Most of the carcinogenic factors that are known to be significant are by no means specific to the modern world. The most potent known carcinogen, by certain assays at least, is aflatoxin B1 (see Figure 20–38). It is produced by fungi that naturally contaminate foods such as tropical peanuts and is an important cause of liver cancer in Africa and Asia. Except for tobacco, chemical toxins and mutagens are of lesser importance as contributory causes of cancer than other factors that are more a matter of personal choice. One important factor is the quantity of food we eat: as mentioned earlier, the risk of cancer is greatly increased in people who are obese. In fact, it is estimated that as many as 50% of all cancers could be avoided by simple, identifiable changes in lifestyle (see Figure 20–37B). Viruses and Other Infections Contribute to a Significant Proportion of human Cancers


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Cell_Biology_Alberts. Viruses and Other Infections Contribute to a Significant Proportion of human Cancers Cancer in humans is not an infectious disease, and most human cancers do not have any infectious cause. However, a small but significant proportion of human cancers, perhaps 15% in the world as a whole, are thought to arise by mechanisms that involve viruses, bacteria, or parasites. Evidence for their involvement comes partly from the detection of viruses in cancer patients and partly from epidemiology. Thus, cancer of the uterine cervix is associated with infection with a papillomavirus, while liver cancer is very common in parts of the world (Africa and Southeast Asia) where hepatitis-B viral infections are common. Chronic infection  VINYL CHLORIDE:  BENZENE:  ARSENIC: skin carcinomas, bladder cancer  ASBESTOS:  RADIUM: Figure 20–39 Age-adjusted cancer death rates, United States, 1930–2008.