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What can 23andMe results tell you about your colorectal cancer risk?

Last week, 23andMe announced that it had received FDA approval to report on the two most common variants found in the MUTYH (aka MYH) gene.

If you’ve ordered 23andMe’s Health + Ancestry test, what will these results tell you about your colorectal cancer risk?

For most people–the answer is (once again) not too much!

Colorectal cancer is one of the more common cancers diagnosed in the United States. Most colorectal cancer is considered “sporadic” and is not related to hereditary risk. These are two important reasons that screening for colorectal cancer is recommended for everyone (after a certain age), regardless of family history.

If you have a family history of colorectal cancer, will this test be helpful for you?

Probably not. Most familial cases of colorectal cancer are not associated with mutations in the MUTYH gene. Depending on the specifics of your family history of colorectal cancer, it may make sense to consider clinical genetic testing through a diagnostic medical laboratory. Hereditary cancer genetic testing panels often include the MUTYH gene, but Lynch syndrome is more common culprit.

If you have no family history of colorectal cancer, will this test be helpful for you?

Possibly!

1-2% of individuals of European ancestry are estimated to be carriers of a mutation in the MUTYH gene. So it’s not uncommon to have a mutation in One copy of the MUTYH gene.

And most individuals (of European ancestry at least) who carry a MUTYH mutation do have one of the two mutations that 23andMe is testing for.

Carrying mutations in both copies of the MUTYH gene confers a significantly increased risk for developing colorectal cancer. Individuals with two mutations are said to have the autosomal recessive condition MAP (MUTYH-associated polyposis).

Since MAP is an autosomal recessive condition, individuals with MAP typically do not have a strong family history of colorectal cancer. In the absence of family history and without positive genetic results pointing to a diagnosis of MAP, there is often no reason to start colonoscopies at a younger age or to have them repeated with the same frequency that may be recommended for an individual with MAP. Learning you have a diagnosis of MAP could be life-saving. With breast screening, the best case scenario is to diagnosis and treat a breast cancer at an early age. With colorectal cancer, however, removing polyps actually reduces the risk of developing colorectal cancer.

What are the odds that you have MAP? Let’s do the math!

Assuming the higher carrier estimate of 2% (or 1/50), for most individuals of European ancestry, the odds would be 1/50 x ½ x 1/50 x ½  : That is–the chance that your mother was a carrier (1/50) and that she passed her mutation down to you (½); the chance that your father was a carrier (1/50) and that he passed his mutation down to you (½) = 1/50 x 1/50 x ½ x ½ = 1/10,000. That number got a lot smaller pretty fast.

 

Wish you could just talk to a genetics expert?

Grey Genetics has a growing Network of Genetic Counselors available for telehealth appointments. You can book an appointment online for a time that’s convenient for you.

A genetic counselor can help you make sense of genetic testing options and results within the context of your own personal and family health history. A genetic counselor can also help you understand the implications of test results not only for your own health, but also for that of family members.

 

Are you concerned about your risk for colorectal cancer? Are you a DIY-er but just want a little more guidance?

Grey Genetics is also test driving a few services for individuals who are looking for some more guidance but prefer a lower cost option or just prefer not to talk with someone one-on-one at this point. Much of genetic counseling consists of gathering information about your personal and family health history. If you take the time to provide us with this information, we can provide you with a report, personalized to you, commenting on your risk for colorectal cancer taking your family history into account. 

Further Reading & Quality News Coverage:

23andMe Receives FDA Clearance for Genetic Health Risk report that looks at a Hereditary Colorectal Cancer Syndrome.” 23andMe Blog. January 22, 2019.

23andMe Receives FDA Clearance for Direct-to-Consumer Genetic Test on a Hereditary Colorectal Cancer Syndrome.” The ASCO Post. January 29, 2019.

MUTYH-Associated Polyposis. Gene Reviews.

“23andMe Has a New Genetic Cancer Test. But the Results Don’t Tell You The Whole Story” Ducharme, Jamie, Time Magazine. January 24, 2019.  

 

Do you meet genetic testing criteria for hereditary breast cancer risk?

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