It is important to remember that there are often a large number of SNPs associated with particular diseases and 23andMe only tests a small fraction of these SNPs. It is therefore quite possible to test negative for a particular disease but still to be a carrier. To its credit, 23andMe does a commendable job of educating its customers. There is an excellent post on their blog, Absence of evidence is not evidence of absence: understanding what genetic testing can (and can’t) tell you, which explains the problem of interpretation very succinctly.
One of the most controversial aspects of the 23andMe test is the inclusion of the SNPs for three BRCA breast cancer mutations. The company make it very clear that "the absence of these mutations does not rule out the possibility that a person may carry another genetic variation that increases the risk of these diseases". (See screenshot below - you will need to click on the image to enlarge it.)
Interestingly again, the SNPs selected account for the majority of breast cancer cases in Ashkenazi Jewish women. Only a tiny proportion (0.2%) of 23andMe customers test positive for the BRCA mutations. Anne Wojcicki's presentation to the Food and Drug Administration last week provides statistics on the prevalence of some of the diseases and conditions in the 23andMe database.
My only quibble with the presentation of the carrier status data is that you are urged to consult a genetics counsellor if you have any questions about your results. The link provided is to a commercial genetics counselling organisation in America. This might be appropriate for an American customer who presumably has to pay to see a doctor and for whom a genetics counsellor might be a cheaper option. It is however not much help if you don't live in the US. For customers in the UK it would be more appropriate to make an appointment with a general practitioner for which of course there is no charge.
The drug responses are probably the most useful part of my 23andMe data. I have learnt that my genetics indicate that I might possibly have a reduced response to hepatitis C treatment and an increased sensitivity to warfarin. It is of course quite possible that I will never need either of these treatments but if I should do so then I am sure that the information will be beneficial to my general practitioner who might then be able to target the dosage accordingly and perhaps save the National Health Service some money in the process.
Of no practical use, but interesting nonetheless, one preliminary research report suggests that I have a genotype which puts me at substantially higher odds of becoming addicted to heroin! The study was carried out on just 139 heroin addicts primarily of Swedish origin and it remains to be seen if these results can be replicated.
Part 1 Disease risks
Part 3 Traits
Part 4 Ancestry
© Debbie Kennett 2010