Nov. 8 -- FRIDAY, Nov. 7 (HealthDay News) -- A mutated gene in the eye may account for some cases of seasonal affective disorder, that annual bout of "winter blues" experienced by an estimated 6 percent of the U.S. population as the days get shorter.
"SAD [seasonal affective disorder] is a kind of major depression that recurs every year right around the fall," said lead researcher Ignacio Provencio, an associate professor of biology at the University of Virginia. "By the spring and early summer, it goes away."
Provencio noted that the treatment for SAD is light therapy, which usually takes about two hours a day. "Exposing patients to bright light can actually get rid of some of these symptoms and allow patients to function normally during the winter," he said.
Since light is the treatment for SAD, Provencio's group speculated that people with the condition may be less sensitive to light. "They can overcome that insensitivity by increasing the amount of light they're exposed to," he noted.
But only 50 percent of SAD patients respond to light therapy, Provencio said. "So it could be that this mutation could allow a way of predicting patients that may be responsive to light therapy," he added.
For the study, published in the November online edition of the Journal of Affective Disorders, Provencio's team looked at genes in 220 people. One hundred thirty of the people had been diagnosed with SAD, and 90 had no history of SAD or any other depressive disorder.
The researchers found that seven people had two copies of the mutated gene thought to be involved in SAD. All seven were in the group that had been diagnosed with the depressive condition. The researchers concluded that someone with two copies of the mutation was five times more likely to develop SAD, compared with someone without the mutation, Provencio said.
The gene is called the melanopsin gene, and it produces a light-sensitive protein found in photoreceptors in the eye's retina. The protein is not involved with vision, but it is linked to non-visual responses, such as circadian rhythms, hormones, alertness and sleep, Provencio said.
A mutation of the melanopsin gene may cause a change in responses to light, which can lead to symptoms of depression. Among SAD patients, about 29 percent have a family history of the condition, suggesting there may be a genetic component, according to background information in the study.
"Not all people with SAD have this mutation," Provencio said. "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 may have found a cause of SAD among a subset of patients."
Dr. Daniel F. Kripke, a professor of psychiatry emeritus at the University of California, San Diego, called the finding interesting, but said it only accounts for a small number of SAD cases.
"This is a responsible piece of work," Kripke said. "The finding is promising, but it needs to be repeated and may not be correct. If it is correct, it appears to explain only about 5 percent of the cause of SAD."
To learn more about SAD, visit the U.S. National Library of Medicine.
SOURCES: Ignacio Provencio, Ph.D., associate professor, biology, University of Virginia, Charlottesville; Daniel F. Kripke, M.D., professor emeritus, psychiatry, University of California, San Diego; November 2008 Journal of Affective Disorders, online