Possible Genetic Cause for SAD Found

A genetic mutation affecting the eye may account for some cases of seasonal affective disorder (SAD), according to researchers at the University of Virginia.

SAD is a type of depression that occurs during fall and winter, but subsides during spring and summer. It is believed to be caused by a disturbance in the normal circadian rhythm of the body, which is influenced by light entering through the eyes.

Because the treatment for SAD is exposure to bright light, the researchers speculated that people with SAD might have eyes which are less sensitive to light. When they looked at the genes of 220 people who had SAD, they found that seven of them had two copies of the mutated gene believed to be involved in SAD. None of the study participants without SAD had the double copy of the mutated gene.

The gene, which is called the melanopsin gene, produces a light-sensitive protein found in the light receptors of the eye's retina. This protein is involved in non-visual functions, such as circadian rhythms, hormones, altertness and sleep.

"Not all people with SAD have this mutation," said study author Ignacio Provencio, "But, at least in our study, all the people who had two mutated copies of this gene were in the SAD group. We think we have found a cause of SAD among a subset of patients."

The study appears in the November online edition of the Journal of Affective Disorder.