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The pitfalls of consumer breast cancer gene testing

Consumer genetics company 23andMe is offering breast cancer testing without going to your doctor. @NightshiftMD tells you if it's worth it.
Dr. Brian Goldman · CBC ·
An artist's illustration of a DNA double helix. For people who test positive for variants of the BRCA1 and BRCA2 genes, the implications are life-altering. (U.S. National Human Genome Research Institute/Reuters)

Earlier this month, the U.S. Food and Drug Administration gave personal genetics company 23andMe approval to offer the first consumer testing for cancer. No appointment and no doctor's prescription are needed. Is it convenient?  You bet. Should you do it? Let's take a closer look.

23andMe will report on three variants in the BRCA1 and BRCA2 genes. These genes are associated with a significantly higher risk of breast and ovarian cancer in women, and breast cancer in men. Compared to other ethnic groups, these variants are most found most often in people of Ashkenazi Jewish descent. Roughly one in 40 individuals of that particular genetic heritage has one of these three variants.

Studies have shown that women with one of these variants are at least five times more likely to develop breast cancer. They have a 45 to 85 per cent chance of developing the disease by the age of 70. They are also up to 20 times more likely to develop ovarian cancer. The genetic variants tested by 23andMe are also associated with an increased risk for prostate and pancreatic cancer, though the precise numeric risk is unclear.

To get tested by 23andMe, you set up an account and order a kit for collecting a saliva sample. The company says that typically, it arrives within three to five days, or faster if you order express shipping. When the kit arrives, follow the instructions to spit in the tube provided. You can do that at home. You register your saliva collection tube using the barcode label provided and mail it back to the lab in the pre-paid package provided. In six to eight weeks, the company sends you an email to let you know your reports are ready in your online account. Log in and get your test results.

For people who test positive, the implications are life-altering. A woman who has a BRCA1 or BRCA2 mutation may decide to have her breast and ovaries removed. These do not need to be done immediately. Armed with the test results, you can discuss the timing of surgery with your doctor. Knowing that you possess one or more of the genes might also affect your desire to have children.

The implications don't end with prevention. Some women receive genetic testing after they've been diagnosed with breast cancer because the results may affect treatment. Last year, the FDA approved a drug called Lynparza for women with metastatic breast cancer who have a particular BRCA gene mutation. To be approved for the drug, patients need to have an FDA-approved genetic test, called the BRCAnalysis CDx. Last year, the CADTH pan-Canadian Oncology Drug Review gave its stamp of approval for the use of Lynparza in women with BRCA-mutated ovarian cancer.

So, what's not to like about the genetic testing being offered by 23andMe? For starters, whatever information you get will not tell your entire genetic story as it pertains to breast cancer. There are thousands of mutations for BRCA1 and BRCA2. Your doctor can test up to 30 mutations. 23andMe tests just three of them, each of which is found mainly in people of Ashkenazi Jewish descent.

If you don't belong to that narrow genetic group, you're likely to test negative for the three mutations. If that sounds like good news, keep reading. You could still carry the other BRCA1 and BRCA2 mutations not tested by 23andMe.

The other issue cancer doctors have is that the testing does not come with counselling. Doctors who do genetic testing on a regular basis generally spend an hour or more counselling patients on how to interpret the tests and what to do about them. 

I'm not criticizing 23andMe for offering the tests. I'm a huge believer in giving diagnostic power to patients so that they are more engaged in their health and wellness. My sense is that the company is presenting information about the testing clearly and objectively. But people hear what they want to hear, and I believe some people will buy the testing and misinterpret the results.

Talk to your health professional if you're worried about your risk of breast and ovarian cancer. If several members of your family have the disease, you may be eligible to be followed by a genetic cancer clinic, which means you get detailed genetic testing free of charge.

The bottom line is that a negative test result could still mean you have an increased genetic risk of cancer due to gene mutations. 

And here's one more thing to think about. Most cases of cancer are not caused by genetic mutations. It's lifestyle factors such as obesity, hormone use and smoking that increase your risk. For the vast majority of people at risk, addressing these is a lot more bang for your buck than 23andMe.