One in 10 people have a major depressive disorder (MDD) during their lives, which makes depression the most common mental illness. Women are twice as likely as men to suffer from depression. Like many ailments, it is known that depression has both a genetic component and environmental factors.
The genetic component has been difficult to determine. Researchers from Erasmus University Medical Centre in The Netherlands published a new study, in the journal Biological Psychiatry, which may shed some light on this, They identified one gene implicated in MDD called NKPD1.
How did they find this gene? The researchers took advantage of a genetically homogeneous group of people called the Erasmus Rucphen Family (ERF) - a cohort of 3000 people who are all descendants of 22 couples from the 18th century. The group lives in an isolated village in the Netherlands and has had very few people join from the outside, resulting in a restricted genetic population. When given a self-reported questionnaire, 2,353 of them—a very high percentage—showed symptoms of depression.
Using the genetic sequences of the ERF, certain NKPD1 sequences were found to coincide with with people who also reported symptoms of depression. Further, the study also found an association between NKPD1 and people outside of the ERF, when they looked in the general population.
NKPD1 is thought to be involved in the biosynthesis of sphingolipids, a class of lipids that have been implicated in mental illness even though there is no evidence for this. The hypothesis is based on the fact that NKPD1 encodes a protein that is similar to others that make sphingolipids.
Although this discovery is exciting, it is a small step forward in the complex field of the genetics of depression. The authors wrote that they have "detected significant association of depressive symptoms with a gene NKPD1" and "we are the first to show a possible genetic connection in this respect.”
This is not to say that this is "the depression gene" as some media outlets are implying. The Independent's headline that "Depression Could Be Linked To Single Gene, Say Dutch Scientists" is misleading.
There are very few diseases, that we know of that are caused by mutations (or errors) in one single gene. Our genetics are usually not that simple. This does happen, but, it is rare.
Some of the most well understood single gene diseases are cystic fibrosis (caused by a mutation in the gene for the cystic fibrosis transmembrane receptor) and Duchenne Muscular Dystrophy (caused by a mutation in the gene that makes dystrophin.) In these cases, it is usually a yes or no situation. You have cystic fibrosis or your don't.
The rest of our diseases are called 'polygenic' —caused by multiple genes. And this type of genetic situation is more complicated and can be harder to target when designing drugs or therapies. The polygenic concept is not limited to just diseases. Most of our traits are polygenic as well. For example, we are not either 5'1" or 6'1" in height - short or tall. Our height is a spectrum like most of our other traits.
So, although this study highlights a genetic component that may have an important role in depression, it does not mean that depression is caused by mutations in a single gene. It is not that simple and the complexities that underlie the genetics of mental illness should not be oversimplified.