Genetic and environmental predictors of seasonality and seasonal affective disorder in individuals with depression

Background The etiology and nosological status of seasonal affective disorder (SAD) as a specifier of depressive episodes versus a transdiagnostic disorder are the subject of debate. In this study, we investigated the underlying etiology of SAD and dimensional seasonality by examining their association with latitude and genetic risk for a range of traits, and investigated gene-environment interactions. Methods This study included 12,460 adults (76% female, aged 18–90) with a history of depression from the Australian Genetics of Depression Study. Regression models included predictors for latitude (distance from equator) and polygenic scores for eight traits; major depressive disorder, bipolar disorder, anxiety disorders, chronotype, sleep duration, body mass index, vitamin D levels, and educational attainment. Outcomes were SAD status and general seasonality score. Results SAD was positively associated with latitude (OR[95%CI] = 1.05[1.03–1.06], p adjusted < 0.001), and there was nominal evidence of additive and multiplicative interactions between chronotype genetic risk and latitude (OR = 0.99[0.99–0.99], p adjusted = 0.381; OR = 0.98[0.97–0.99], p adjusted = 0.489). General seasonality score was associated with latitude (IRR = 1.01[1.01–1.01], p adjusted 0.001) and genetic risk for major depressive disorder (IRR =1.02[1.01–1.03], p adjusted < 0.001), bipolar disorder (IRR = 1.02[1.01–1.03], p adjusted = 0.001), anxiety disorders (IRR = 1.03[1.01–1.04], p adjusted < 0.001), vitamin D levels (OR = 0.89[0.80–0.95], p adjusted = 0.048), and educational attainment (IRR = 0.97[0.96–0.99], p adjusted < 0.001). Conclusions These findings enhance understanding of SAD etiology, highlighting contributions of psychiatric genetic risk and geographic measures on seasonal behavior, and support examining seasonality as a continuous dimension.