According to a February 22, 2016 report appearing in PNAS, scientists have discovered a gene which may provide a link between certain patterns of sleep and seasonal affective disorder.
Seasonal affective disorder is a type of depression which is characterized by its tendency to recur during a particular season each year. Although it can occur during summer, it more commonly affects people during the winter months when the hours of daylight become shorter. It is believed that those who tend to suffer from seasonal affective disorder have brains which are more sensitive to the lack of exposure to light, causing their circadian rhythm to be more easily disturbed.
The authors of the report, Ying-Hui-Fu, Ph.D., and Louis Ptacek, MD, had previously studied many different types of sleep patterns, including one where people seem to have a faster than normal biological clock. When a person has this sleep pattern, known as Familial Advanced Sleep Phase (FASP), they might go to sleep quite early and wake up early as well.
The authors were able to study three people belonging to the same family who happened to have both FASP and seasonal affective disorder and identify a genetic mutation which occurred in all three members of the family. This discovery suggests that perhaps a gene called PER3 has something to do with both conditions.
According to Ptacek, this discovery is significant because prior to this, no one had ever linked a gene mutation to the phenomenon of seasonal depression. Past research has only been able to make a more general connection between sleep and depression. This study demonstrates how the two might have a genetic link.
In order to further study the mutated gene, Ptacek and Fu introduced it into mice and then exposed the mice to conditions simulating the shorter daylight hours of winter, which would normally trigger the appearance of seasonal depression in humans. When the mice were treated in this manner, the researchers found that they began to behave in ways similar to their human study subjects who possessed the mutant PER3 gene, developing altered circadian rhythms and symptoms similar to depression. And, when the scientists deleted the gene altogether, the mice developed even more severe depression symptoms.
Ptacek and Fu do admit, however, that studying mice is not an entirely adequate way to evaluate the effects of the mutant gene on humans. It is simply not known whether mice experience depression in any way that is comparable to humans. They do, however, remain a useful tool for learning more about the gene’s effects.
The researchers were able to further study how the gene might affect circadian rhythms by placing the mutation into isolated cells contained in petri dishes. What they found was that the cells produced less of the PER3 protein than normal. This would have the likely effect of destabilizing another protein, the PER2 protein, which is responsible for regulating the biological clock. Most likely this lowered protein production is what causes people with FASP to experience altered sleep schedules.
Although Ptacek and Fu are not sure how this genetic mutation might also affect mood, they do have plans to continue their studies in order to further clarify the mechansim.