What a 23andMe Disease Risk Report Can Tell You and What It Can't

DNA double helix (National Human Genome Research Institute, National Institutes of Health)

The genetic testing company 23andMe received approval this week from regulators to sell genetic reports on an individual’s risk for 10 diseases, most prominently Alzheimer’s and Parkinson’s. Before you send in your saliva sample and $199, here’s what you should know:

What will a genetic test actually tell me?

At most, that you carry a DNA variant that, according to research, is associated with a higher risk of a disease. For the rare clotting disorder hereditary thrombophilia, for instance, the report will say that you do or do not carry a variant called Factor V Leiden in the F5 gene and a variant called Prothrombin G20210A in the F2 gene.

23and me is still fine-tuning the reports, but its tests will also tell you how the presence (or absence) of variants affects risk. If there’s enough science to quantify that, the report will specify a percentage, like “your risk is 3 percent.” If not, it will just say there’s an (unspecified) increased risk. Of course, you can also look it up. For Alzheimer’s, carrying two copies of the Apoe4 variant (one from each parent), as 1 to 2 percent of the population does, raises the risk of the disease to as much as 87 percent, for instance, compared to about 9 percent in the general population.

What diseases can 23andMe tell me about?

This month, late-onset Alzheimer’s disease, Parkinson’s disease, the clotting disorder alpha-1 antitrypsin deficiency, and Gaucher disease. Soon — the company hasn’t said exactly when — it will also test for genetic variants linked to factor XI deficiency (excessive bleeding), celiac disease, anemia-causing G6PD deficiency, the movement disorder early-onset primary dystonia, and the blood illness hereditary hemochromatosis.

Will the test tell me if I’m doomed to get one of these terrible disorders?

No. None of the genetic variants that 23andMe tests for is what’s called “fully penetrant,” meaning that 100 percent of those who carry the variant develop the disease. By not “fully,” we mean really not fully, as in the risk might be measured in the single-digit percentages. “It’s important for people to know that even if they have a mutation in the genes [associated with Parkinson’s that 23andMe will test for], by and large they won’t get Parkinson’s disease,” said James Beck, chief scientific officer of the Parkinson’s Foundation. The N370S variant in the GBA gene, for instance, triples the risk of Parkinson’s, Beck said, but with a baseline risk of 0.3 percent that means about a 1 percent risk.

Do these tests work better for some ethnic groups than others?

Geneticists have studied more people of European descent than other groups, so they have more data on white people. 23andMe knows this, so its reports will include warnings such as that the test results are “most relevant for people of European descent” (for Alzheimer’s), “. . . for people of European, Ashkenazi Jewish, and North African Berber descent” (for Parkinson’s), and “. . . for people of Ashkenazi Jewish descent” (Factor XI Deficiency). With Alzheimer’s, the effect of the ApoE4 variant is weaker in African-Americans, for instance.

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This story was originally published by STAT, an online publication of Boston Globe Media that covers health, medicine, and scientific discovery.

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