Cancer Genetic Testing: Tumor vs. Healthy Tissue

Genetic testing is not an easy topic to understand, especially if you’ve also recently been diagnosed with cancer. In general, genetic testing looks at our DNA or genetic information. Essentially, there are two main groups of genetic testing–testing of the cancer tissue/tumor and testing of healthy tissue. Learning the basic differences between these two can help you understand the different options that may be discussed with you as you navigate your cancer treatment.

Germline Genetic Testing—Genetic Tests for the DNA You Are Born With

The first type of genetic testing is germline genetic testing. This testing looks at healthy tissue (blood, saliva, cheek swab) to evaluate the DNA a person was born with. This DNA would have most often come from either the person’s mother or father. Therefore, this type of genetic information is heritable – or able to be passed from generation to generation.

With germline genetic testing, we look for a risk factor being passed through the family that makes a person more susceptible to developing specific types of cancer. An example would be a BRCA1 mutation – a person can be born with this and it would make them at a higher than average risk to develop breast and ovarian cancer.

Tumor Somatic Genetic Testing—Genetic Tests for the DNA Make-Up of a Tumor

If you’ve unfortunately had a cancer diagnosis, your physician may have discussed the option of sending your tumor (either through a biopsy or resection sample) to a laboratory for genetic testing. This second type of genetic testing is somatic genetic testing. It is looking at the DNA or the genetic make-up of the cancer itself.

Cancer by nature is caused by DNA mutations that occur after birth. A cancer starts as an abnormal cell that divides without control. When somatic tests are done, it is not unusual to find many gene mutations as cancer is caused by acquired DNA mutations in a cell. Sometimes learning what makes your cancer tick can help a Medical Oncologist determine what therapeutic agents are most likely to get a response. This type of testing is becoming more common as there are more medications and therapies that are designed to work on cancers with specific genetic mutations or changes.

When Germline and Somatic Genetics Overlap

Sometimes there is overlap between information you can get from germline and somatic genetic tests. For example, testing on the tumor can sometimes reveal germline mutations as well. However, additional testing would be needed to tell for sure. If you’ve had a tumor genetic test, and your provider feels that some of the findings could be clinically significant if those findings were also found in your germline, they may refer to genetic counselors. Genetic counselors will review these types of reports to determine if germline (blood) genetic testing is indicated.

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For example, say a woman has colorectal cancer, and her tumor is sent for somatic or tumor genetic testing, and a BRCA1 mutation is identified in her cancer. This specific mutation could be of clinical significance if this mutation were identified in the rest of her DNA or germline DNA. If a woman is found to carry a BRCA1 mutation in her germline DNA, she is at high risk for breast and ovarian cancers. This could be of significance for this person’s future medical management, but also the medical management of the person’s family. In this circumstance, this woman could see a genetic counselor, and the genetic counselor may discuss the option of ordering genetic testing from a blood sample to determine if this genetic mutation is solely found in the cancer or if it is also within the woman’s germline DNA.

This is just a brief example of how technology is quickly evolving in the field of Genetics. Information and technology will continue to evolve, but we are going to be continually challenged by the quest to interpret the information we learn from genetic testing. Genetic counselors will be a vital piece of this interpretation for the patients and providers requesting this somatic genetic testing.

If you have questions regarding genetic counseling or genetic testing, please call ProMedica Cancer Institute’s Cancer Genetics Program at 419-824-5073.

Sarah AdelspergerSarah Adelsperger, MS, LCGC, is a board-certified genetic counselor specializing in cancer. Sarah joined Kelly Morse, MS, LCGC, in the Cancer Genetics Program in May 2015 at ProMedica Cancer Institute–the only program in Northwest Ohio and Southeast Michigan with two licensed and board-certified genetics counselors specializing in cancer genetics.