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The genetic testing company 23andMe received approval this week from regulators to sell individuals reports on their genetic 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.

23andMe is still fine-tuning the reports, but its tests will also tell you how the presence (or absence) of variants affects the risk of getting a disease during your lifetime. 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 lifetime 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 pulmonary disorder alpha-1 antitrypsin deficiency, and Gaucher disease. The company might soon also offer testing 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.

Do I need a doctor to do this?

Not at all; this is a direct-to-consumer product. Patient empowerment, you own your DNA, and all that.

Is 23andMe my only option?

Not if you’re willing to go through a physician or genetic counselor, who can order genetic tests from any of the companies, from large ones like Quest Diagnostics and Lab Corp. to small ones that sell them. That will usually cost more than 23andMe’s $199, however.

If I’m told I do not have any of the genetic variants associated with these diseases, I can breathe easy, right?

No. Genes account for most of the risk of developing rare disorders like Factor XI Deficiency (three variants in one gene, F11, are linked to excessive bleeding after accidents or surgery). But especially for Alzheimer’s and Parkinson’s, most of the risk is either non-genetic or from genes in addition to the ones 23andMe tests for. “Even if you test negative for all known [disease] genes, your risk for that disease may still be increased based on your family history,”said Mary Freivogel, president of the National Society of Genetic Counselors. “A negative 23andMe test might provide false assurance.” The company will therefore warn customers that the Parkinson’s test, for instance, “does not describe a person’s overall risk of developing” the disease.

Does the meaning of the test depend on whether anyone in my family has the disease?

Yes, and this is what experts most fault 23andMe for. “My concern is that they’re giving the same report to patients with and without a family history of a particular disease,” said Freivogel. For some diseases, she said, the chance that a disease-linked gene will lead to the disease is greater if the disease runs in the family. That sort of analysis is what genetic counselors provide.

This sounds like a good way to freak people out, no?

23andMe is aware of that possibility. Before customers opt to know whether they carry either of two genetic variants that raise a person’s risk of Parkinson’s, or of one variant that increases the risk of Alzheimer’s, 23andMe therefore requires them to read additional information. It warns that tests of these genes “are about serious diseases that may not currently have an effective treatment or cure,” and that people who “have ever been diagnosed with anxiety or depression” may experience “emotional difficulty” as a result of the report.

Might genetic variants not picked up by the tests increase or decrease my risk of acquiring these diseases?

Definitely. That’s why many expert groups recommend against testing for ApoE4, hemochromatosis, and others. The American College of Medical Genetics warns that because Alzheimer’s develops in the absence of ApoE4 and because many people with ApoE4 “seem to escape disease,” it does not recommend testing for the variant.

Did I pass a bad gene to my kids?

As with any gene, it depends on whether you have two copies or one. If two copies, your children inherited it. If one, they have a 50-50 chance of doing so.

Will 23andMe or other companies begin testing for the risk of other conditions?

This is an expanding market. Illumina has a pilot program for this, as does Invitae.

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