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Understanding Genetic Testing for Cancer

Cancer is a disease of the genes. Most cancers develop as a result of genetic damage or a mutation that happens sometime during a person's lifetime. These are called sporadic cancers (occurs by chance). These types of mutations only affect the cells that grow from the original mutated cell. Cancers that result from DNA damage that is passed on is called hereditary cancers. These cancers tend to run in families. They cause hereditary cancer syndromes, but overall are linked to only about 5% to 10% of all cancer cases. Genetic testing can be used to look for the gene changes that are linked to these hereditary cancer syndromes in people thought to be at risk for having one of them.

Young woman

Cancer and genes

The risk of developing cancer is higher in people who are born with certain genetic defects that are passed on from their parents and found in every cell in their body. These defects occur in areas of the DNA that affect cell division, cell death, and the repair of damaged DNA.

The National Cancer Institute describes genes as "nature's blueprints for every living thing." Genes determine your body's potential for function and growth. Environmental factors influence the outcome. For example, a woman might have the genetic potential to be 6 feet tall. However, if she has poor nutrition as a child, she may not reach that potential. Similarly, a woman may have a genetic risk of developing breast cancer. Whether she develops it or not may be influenced by environmental exposures she has during her lifetime.

Genes come in pairs. One set is passed down (inherited) from your mother and the other set from your father. Genetic risks and potential can come from either parent.

A number of genetic defects can result in an increased risk for cancer. For instance, for breast cancer, the most common defects are found on genes named BRCA1 (breast cancer gene 1) and BRCA2 (breast cancer gene 2). A woman who has inherited a harmful mutation in BRCA1 or BRCA2 is at higher risk of developing breast cancer and some other types of cancer than a woman who does not have such a mutation.

Researchers at the National Human Genome Research Institute have developed tests using DNA microchip technology that can distinguish between hereditary and sporadic forms of cancer. Today, more than 50 different hereditary cancer syndromes can be identified with genetic testing. 

Common cancer syndromes that can be identified with genetic testing

These are some of the more common gene mutations that are linked to cancer syndromes and can be found with genetic testing.

APC gene mutations can cause familial adenomatous polyposis or FAP. This syndrome causes colon polyps, colon and small intestine cancer, and cancers of the stomach, bone, skin, brain, and other tissues. 

BRCA 1 and BRCA 2 gene mutations are linked to hereditary breast and ovarian cancer syndrome. These mutations can put a person at an increased risk for breast (in both men and women), ovarian, pancreatic, prostate cancer.

MSH2, MLH1, MSH6, PMS2, and EPCAM gene changes can cause hereditary nonpolyposis colorectal cancer, also called Lynch Syndrome.This is linked to many different kinds of cancer, including those that start in the colon, rectum, endometrium, ovary, kidney, pancreas, small intestine, liver and biliary tract, stomach, brain, and breast

PTEN gene changes cause Cowden syndrome, which is linked to breast, thyroid, endometrium, and other cancers.  

TP53 gene mutations cause Li-Fraumeni syndrome, and are linked to an increased risk of many cancer types, including breast cancer, sarcoma, leukemia, and brain tumors. 

Deciding on genetic testing for cancer

Genetic testing for cancer should strongly be considered in people with all of the following:

  • A personal or family history that suggests a hereditary cancer syndrome is present, such as:

    • Cancers have been found at unusually young ages

    • Several close relatives who have had the same kind of cancer

    • One person has had multiple types of cancer

    • Cancer has been diagnosed in both organs when organs are found in pairs, such as both breasts or both kidneys 

    • There's evidence of other birth defects that are linked to certain cancer syndromes

    • Other family members have been tested and have genetic mutations that are linked to cancer

  • A genetic test is available and has been proven to be accurate

  • The results of testing can help the person make decisions about medical care

It's strongly recommended that anyone considering genetic testing first talk to a professional about it. It's important to understand the limitations of genetic testing and how the test results might be used before getting tested. You also need to discuss the costs and whether your health insurance will help pay for testing.

If you are considering genetic testing for cancer risks, you face many complex issues. If you test positive, you may face difficult decisions about treatments to prevent cancer. You may have increased fear and anxiety about developing cancer. You may have concerns about losing your health insurance coverage. If you test negative, you may also face difficult challenges, such as survivor guilt, if other family members have tested positive.

However, if you are anxious or considering treatments such as preventive surgery based on your family history, having the results may be very helpful. Because of these issues, you should seek genetic counseling before, during, and after any genetic testing. Remember, genetic testing cannot tell you everything about inherited cancers. A positive result does not always mean you will develop cancer, in most cases other factors are involved. In the same way, a negative result does not mean you won't get cancer. Genetic counselors can talk to you about what a particular test will or will not tell you, and can help you decide whether to get tested.

How is genetic testing done?

In most cases, a blood sample is needed to do genetic testing, but sometimes saliva, skin cells, or cells swabbed from the inside of the cheek can be used. The sample is sent to a lab where the actual testing is done. It usually takes a few weeks to get the results.

You will meet with the genetic counselor to go over the results and figure out what they mean for you and your family.