In the biggest study of its kind, researchers have detected 15 regions of the human genome linked to a higher risk of struggling with serious depression. The study was conducted by drug giant Pfizer and a private genome testing company, 23andMe.

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Generally speaking, depression is a mood disorder that causes a persistent feeling of sadness and loss of interest.  An estimated 19 million American adults are living with major depression, requiring long-term treatment and care, struggling on a day to day basis. However, depression can have varied causes and optimized treatments, and researchers have been struggling for years to come up with a statistically sound approach.

“Everyone is recognizing that this is a numbers problem,” says Ashley Winslow, formerly a neuroscientist at Pfizer and now the director of neurogenetics at the Orphan Disease Center at the University of Pennsylvania. Winslow led the research effort. “It’s hard if not impossible to get to the numbers that we saw in the 23andMe study.”

The results are what the team called a “genome-wide association study.” In this approach, the DNA of many people with a disease is compared to that of healthy controls, using a computerized search. Any genetic difference is a hint that the genes may be involved in depression.

This gene-hunting technique has already been used to study diabetes, schizophrenia, and other common diseases, but depression has remained untouched. That’s where crowdsourcing services, such as 23andMe can step in.

“The big story is that 23andMe got us over the inflection point for depression,” says Douglas Levinson, a psychiatrist and gene researcher at Stanford University involved with the Psychiatric Genomics Consortium, another gene-hunting group. “That is exciting. It makes us optimistic that we are finally there.”

23andMe has sold more than a million gene-test kits. The service, which costs $199 is used to find out general things, like for example one’s ethnic background. However, over half of all clients have agreed to use their genetic data in studies and have answered questions about their health. This allowed researchers to obtain a sample size ten times bigger than the biggest study until now, and the results are promising. Also, with the quickly diminishing prices of genetic tests, access to genetic data might become easier and easier, allowing for a new generation of studies.

Of course, at this time we’re only talking about a correlation, not a causality, but it’s a promising step. According to preliminary results, the genetic risk for depression is actually due to hundreds of genes, each having a very small effect.