Genes are instruction manuals for your cells. What can testing your genes tell you about your cancer?
First off, let’s talk about the words we use to describe testing. Tumor genomic testing and genetic testing are often used interchangably, but they actually refer to two different things.
As you grow and age, your cells have to replicate trillions of times — and sometimes, mistakes are made. An error in replication is known as a somatic mutation, as it happens to just one cell in your body.
Most of the time, these somatic mutations are completely harmless. Other times, like when mutations happen in oncogenes — important genes that contain instructions for cell division, programmed death, and other essential functions — you might end up with cancer.
Studying the specific mutations in your tumor’s DNA is known as genomic testing. This is also referred to as molecular testing.
If you were born with a specific gene variation (such as a variant that makes cilantro taste like soap!), this variation can be found in the DNA of every cell in your body.
This is what we call a germline mutation.
These mutations are inherited from your parents, and can be passed down to your children. Checking for such heritible mutations in your DNA is called germline testing. In the context of CRC, this would be done with the help of a genetic counselor. Your genetic counselor can explain how the test works, interpret the results, and tell you what you need to know about your family’s cancer risk. This information can help your oncologist and primary care physician decide what cancer screening recommendations are right for you and your family.
This is confusing, right? Let’s go over all that one more time.
Genomic testing refers to testing for somatic mutations in your tumor’s DNA. This is also called molecular testing.
Genetic testing refers to studying germline mutations you’ve had since birth.
Not everyone needs to be concerned about their tumor’s genomic profile.
For most people in stages I and II, the most important and successful therapy is surgery. For people who have stage III CRC, a combination of surgery and chemotherapy can be curative.
Getting extensive genomic testing may be a good idea for some stage III patients, especially if you have a family history of CRC (or any other kinds of cancers), are young (under 45 years old at diagnosis), or have a particularly aggressive cancer. Tumor genomic testing may not be covered by insurance for stage III patients.
About 30% of people with stage III CRC will progress to stage IV. For those with stage IV cancer, it’s very important to understand the genomic profile of your tumor. This helps you and your doctors figure out the nature of your tumor and see what else — in addition to first and second line chemotherapy — might be effective for you.
Around 95% of colorectal cancers come from spontaneous somatic mutations, and around 5% come from inherited germline mutations. This means that for the majority of patients, germline testing is unlikely to turn up anything significant.
Your oncologist might order a germline test if you were diagnosed with cancer at a young age, or if you have a family history of colorectal cancer.
It’s important to know that about 30% of colorectal cancers have hereditary components — which means that your family may have an increased risk of CRC. However, the specific gene mutations responsible for your increased risk might not yet be known — and therefore not included in commonly available germline testing panels. So you may test negative for germline mutations even though there is a history of colorectal cancer in your family.
In these cases, it’s very important to bring up your family history of CRC with your care team and discuss how you can stay updated on research in this area. Newly discovered gene mutations are likely to be identified and added to the list of known inherited mutations, so germline panels will likely get updated as time passes.
It’s a good idea to save your germline testing results and reach out to your genetic counselor periodically to see if your panel has been updated to include new mutations. This information may be important for your kids and siblings.
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Last updated: May 18, 2022