The ABCs of DNA: Should You Consider Genetic Testing?

Lauren J. Massingham, MD

As humans, we each have 46 chromosomes, containing about 20,000 genes. These sequences of letters serve as the body’s instructions. Genetic tests allow us to look at each sequence to see if there are variants, known as mutations. When present, mutations increase an individual’s risk for developing a disease, such as cancer.  

There are actually three different types of genetic evaluations: prenatal evaluation, diagnostic evaluation of a hereditary condition, and hereditary cancer syndrome evaluation.

Let’s focus on genetic testing for hereditary cancer syndromes. Although this testing started only about 20 years ago with what’s known as the “BRCA genes” for hereditary breast and ovarian cancer, more options became available to test for multiple genes in 2010. These tests focus on panels of between seven and 100 genes that have been associated with hereditary cancer predisposition syndromes.

When is genetic testing recommended?

About five to 10 percent of cancer is considered genetic. Certain patterns of cancer or family history may be suggestive of a hereditary cancer syndrome. Some indicators include:

  • Cancer diagnosed at an unusually young age, such as breast or colon cancer before age 50
  • Two or more close family members on the same side of the family who have a history of cancer
  • Two or more primary cancer diagnoses in the same individual, such as two primary breast cancers or a primary colon cancer and primary stomach cancer
  • A known genetic mutation in the family
  • Individuals of Ashkenazi Jewish descent
  • Other physical signs associated with hereditary cancer predisposition, such as multiple colon polyps or specific skin findings

If your family has any of these indicators, genetic testing may be right for you.

I’ve inherited a cancer predisposition gene. Will I definitely get cancer?

No. Even if a person inherits a cancer-predisposing mutation, he or she might not develop cancer. For each gene, we have two copies, one from Mom and one from Dad. Typically, when we are born, both copies are functioning. In an individual with a hereditary cancer syndrome, one copy is working and the other copy is mutated and nonfunctional.  Our cells continuously divide over the years. It’s not until a “second hit” or a second mutation occurs that a cancer can develop.

A sporadic cancer is one that develops for unknown reasons, not due to a hereditary cancer syndrome. Looking at the diagram below, it takes two mutations to develop over the years to cause a sporadic cancer. In a hereditary cancer syndrome, it only takes one hit for the cancer to develop, since this individual is already born with the first mutation. 

This is why we consider early development of cancer and multiple cancers in one individual as risk factors for hereditary cancer predisposition syndromes.

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What are the risks?

The lifetime risks for cancer have been studied extensively with varying results. In general:.

The lifetime risk for cancer in an individual with BRCA1:
  • Breast cancer is 57 to 87 percent
  • Ovarian cancer is 24 to 54 percent
The lifetime risk for cancer in an individual with BRCA2:
  • Breast cancer is 41 to 84 percent
  • Ovarian cancer is 11 to 27 percent
  • Prostate cancer is 20 to 34 percent
The lifetime risk for cancer in an individual with Lynch syndrome:
  • Colon cancer can be up to 82 percent
  • Uterine cancer can be up to 54 percent
  • Ovarian cancer can be up to 24 percent

Why is genetic testing performed on individuals already affected by cancer?

While it may seem like too little too late, there are several benefits for this testing. Identification of the gene mutation can be beneficial for family members. Genetic testing can also yield risk reduction options. For example, a woman with a BRCA mutation may choose preventive measures, such as a bilateral mastectomy over a lumpectomy due to the risk of a second new breast cancer.

Testing can also offer an opportunity for early detection and cancer risk reduction in the other organs. Other treatment options can also be offered. For example, there are several treatment options that are available to patients with BRCA and other mutations. Knowing if those mutations are present can change the recommended treatment options for an individual.

What are the benefits of genetic testing for family members?

If an individual tests positive for a hereditary cancer predisposition syndrome, first degree relatives, such as their children, siblings, and parents, are typically at a 50 percent risk of also carrying this gene. Second degree family members like grandchildren, cousins, or grandparents are at a 25 percent risk. These family members could consider genetic testing.

If family members are also found to carry the cancer-predisposition mutation, they have options:

  • More frequent screening, such as frequent breast mammograms and breast MRI, colonoscopies, endometrial sampling, or screening for genitourinary cancers
  • Risk-reducing interventions that can lower the risk of cancer, including bilateral mastectomy for breast cancer, or salpingo-oophorectomy (surgical removal of the ovaries), which reduces the risk for ovarian cancer by up to 96 percent 

Which family member should be tested first?

To give us the best chance of detecting the mutation, we typically prefer to begin testing with the individual affected with cancer. If that’s not possible, we can still test other family members.

Who should I speak to about genetic testing? 

Primary care doctors and oncologists can make a referral to see a genetics provider. There are many nuances to what testing to send to be inclusive and what lab should be used based on family history and health insurance.  It can be challenging to obtain insurance approval for testing if a smaller gene panel has already been completed, even if it doesn’t include all the genes that we are looking for. 

Should I consider home testing?

There are genetic tests for individuals to perform in their home, then send for results, such as 23andMe and Ancestry.com. In 2013, the Food and Drug Administration stopped these direct-to-consumer test kits from including hereditary cancer genes from their products. These products are helpful for individuals who are interested in their heritage or to find out where their ancestors are from, or to help connect with new family members. It is not particularly helpful for medical management at this time. 

For more information visit the Cancer Genetics Program online.