"While a person diagnosed with SAD does not necessarily carry the melanopsin mutation, what we found strongly indicates that people who carry the mutation could very well be diagnosed with SAD," Provencio said. "We think that if an individual has two copies of this gene, he or she has a reasonable chance of having the disorder."
The researchers found that a person with two copies of the gene is five times more likely to have symptoms of SAD than a person without the mutation.
"That is a very high effect for a mental illness, because most mental illnesses have many potential causes," Provencio noted. "A mental illness may arise from many mutations, and we have found one that has a clear link."
The melanopsin gene encodes a light-sensitive protein that is found in a class of photoreceptors in the retina that are not involved with vision, but are linked to many non-visual responses, such as the control of circadian rhythms, the control of hormones, the mediation of alertness and the regulation of sleep.
The mutation in this gene may result in aberrant regulation of these responses to light, leading to the depressive symptoms of SAD. About 29 percent of SAD patients come from families with a history of the disorder, suggesting a genetic or hereditary link.
"The finding suggest that melanopsin mutations may predispose some people to SAD, and that if you have two copies of this mutation, there is a very high probability that you will be afflicted," Provencio said. "An eventual understanding of the mechanisms underlying the pathological response to light in SAD may lead to improved treatments."
Provencio adds that the finding, with further study, could also lead to improved testing for SAD.
|Contact: Ignacio Provencio|
University of Virginia