Genetic studies of seasonal affective disorder and seasonality.

Genetic studies of seasonal affective disorder (SAD) and seasonality have received considerable attention over the past several years. Studies of the prevalence of SAD and nonseasonal mood disorders among relatives of patients with SAD suggested a familial contribution to the development of SAD. Two twin studies demonstrated a substantial role of genetic variation in seasonality. Two genetic variants related to serotonergic transmission, the 5-HTTLPR and the 5-HT(2A)-1438G/A gene promoter polymorphisms, have been found to be associated with SAD. 5-HTTLPR is also associated with seasonality in SAD patients and in the general population. It is not clear whether SAD is inherited as a distinct entity or whether seasonality and depression are separate heritable traits that happen to coincide in certain individuals. Vulnerability to SAD and disease pathology may be influenced by many genes, perhaps on several chromosomes.