Shaking the Ebola Tree

Genetic analysis offers insights into the workings of a notorious virus
If one were to rank the world's most gruesome ways to die, Ebola infection would surely sit near the top of the list. It begins with a sudden fever and then kills by liquefying peoples' insides. Fifty to 90 percent of those who become ill, die, making Ebola among the most lethal viruses known. Other diseases kill far larger numbers of people, but Ebola's mystery and ferocity has come to symbolize the growing risk from emerging and re-emerging pathogens.

The two worst Ebola outbreaks occurred in Zaire in 1976 and 1995, with almost no cases reported in between. (Some reserchers have even speculated that the "Plague of Athens" that ended in 425 B.C. was an isolated Ebola epidemic.) No one knows where the virus hides during its respites.

In the April 16, issue of the Proceedings of the National Academy of Sciences, U.S.A., Anthony Sanchez and his colleagues at the Centers for Disease Control and Prevention in Atlanta report some notable progress in uncovering the genetic mechanisms that make this microscopic monster tick.

Virologists have classified the four known variants of the Ebola virus (the Zaire, Sudan, Ivory Coast and Reston strains) along with Marburg, a genetically related monkey virus, into the genus Filovirus. Sanchez's team analyzed certain key viral genes; using that information, the researchers placed the filoviruses along a phylogenetic tree, a branching diagram that shows the evolutionary relationship between organisms. Each of the five viruses occupies its own, genetically distinct branch. Sprouting from these are evolutionary "twigs"--mini-strains into which the Ebola variants are further divided.

Such minor divergences are an inevitable result of mutations, natural errors in the replication process. The big surprise revealed in the Sanchez paper is just how tiny those twigs are. Scientists had expected to see far more genetic heterogeneity in each of the Ebola strains, given the way that the viruses reproduce.

In most higher organisms, the genetic material (DNA) proofreads each replica to ensure that the genetic material in daughter cells is exactly the same as in the parents. The story is somewhat different for viruses. Take for example HIV, the virus that causes AIDS. HIV is a retrovirus: the genetic material within the virus is RNA, a simpler molecule that lacks the double-helix structure of DNA. Once inside a host cell, HIV's RNA is converted into DNA before the virus reproduces. In this way, retroviruses too have proofreading abilities, but not as effective as those in higher organisms.

Filoviruses, however, reproduce as RNA; unlike HIV, they do not have the ability to check each copy. Consequently, "the error rate is one million fold greater than that of DNA based systems," says Timothy Nichol, one of Sanchez's co-authors. To find out how Ebola's penchant for mutation plays out in the real world, the CDC researchers compared the Ebola strain captured from the 1976 outbreak in Kikwit, Zaire, to one taken from the 1995 outbreak in Yambuku, Zaire. Even though, the two epidemics occurred more than 1,000 kilometers apart, and the virus had 18 years to mutate, the genetic sequences of the two isolates of Ebola-Zaire are virtually identical. In addition, Bernard LeGuenno at the Pasteur Institute found that the recent 1996 outbreak of the Ebola-Zaire strain in Gabon is also nearly identical to the 1976 isolate. Something seems to be restraining the natural tendency of filoviruses toward genetic divergence.

Sanchez and his colleagues posit that, for the past 20 years, Ebola-Zaire has essentially been most comfortable in its original form--in other words, natural selection pressures have favored the survival of the original strain over any mutants. This penchant for the status quo lies in marked contrast to the behavior of HIV. HIV also mutates as it reproduces (proofreading is not foolproof), though not as readily as do filoviruses. But because humans apply a selective force by attacking HIV with drugs, the mutants that are, by chance, resistant will proliferate.

To determine the genetic distance between strains, Sanchez's team focused their attention on the genes in which the viruses differ the most. Since all Ebola strains are known to have jumped species--from their presumed natural hosts to monkeys and humans--the researchers predicted that the greatest variations would occur among the genes which give the virus its ability to recognize different cell species. In the case of Ebola, the most relevant gene turned out to be the glycoprotein gene, which produces proteins that sit on the virus's surface and are thought to shuttle the virus inside the host cell. The CDC group has since looked at other parts of the Ebola genome; so far, the glycoprotein gene does in fact seem the most variable, Nichol reports.

Choosing to analyze the glycoprotein gene allowed an added insight: it revealed that filoviruses, in addition to killing their victims by destroying the cells they infect, might work by suppressing the immune system. This may be one of the reasons they are so deadly. Sanchez's team found that a section of the filovirus glycoprotein gene is very similar to the corresponding sections of the glycoprotein genes in other viruses whose function is to subdue the immune system. Indeed, Ebola victims usually die without evidence of an effective immune response.

A related line of research concerns the search for an animal "reservoir," the hiding place where the Ebola resides during the long stretches between human outbreaks. Scientists don't yet know in which creature or creatures Ebola lies dormant, but groups from the CDC, the World Health Organization and the U.S. Army are all currently screening hundreds of African animal species in search of the reservoir. Whatever it is, the high degree of similarity between the 1976 and 1995 Ebola strains in Zaire (less than 1.6 percent change in the studied RNA segments) suggests to the CDC team that the reservoir is the same in both locations and that the creature is either widespread in Zaire, or else is a migratory species.

On the other hand, the four Ebola strains, along with the Marburg virus, show genetic divergences as great as 45 percent. Such marked differences hint that the various filoviruses are carried by more than one species. Each strain may have slowly co-evolved to live comfortably with their own as yet unknown hosts.

Compared to other less spectacular but more widely distributed illnesses such as tuberculosis and hepatitis, Ebola actually poses a rather limited threat: precisely because it kills its victims so quickly, it cannot easily spread. Yet all pathogens mutate, sometimes leading to the appearance of more pernicious versions (as demonstrated by the emergence of drug-resistant strains of TB). Virologists are continuing their studies of the genetics of filoviruses, hoping to find a treatment or a vaccine. But even if they ever do, the threat that Ebola will reappear, like so many other pathogens, will always remain.

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