Summer and SERT: Effect of daily sunshine hours on SLC6A4 promoter methylation in seasonal affective disorder

Meteorological factors affect the serotonergic system, potentially influencing SLC6A4 promoter methylation in seasonal affective disorder (SAD). However, studies on how sunlight impacts methylation and modulates SERT activity in this context remain limited. This study aims to investigate the effect of average daily sunshine duration on SLC6A4 promoter methylation in a cohort consisting of both patients with SAD as well as healthy controls (HC). Methylation data were collected from 28 patients with SAD and 40 healthy controls (HC). Average methylation from four SLC6A4 promoter CpG sites was assessed. Daily sunlight data for Vienna, Austria (mean of 28 days before blood sampling), were obtained from (C)GeoSphere Austria. A general linear model (GLM) analyzed SLC6A4 promoter methylation as the dependent variable, with sunlight hours as the independent variable, and group (SAD, HC), age, sex, and 5-HTTLPR/rs25531 as covariates. Exploratory analyses examined sunlight hours and methylation effects on Beck Depression Inventory (BDI) scores. Sunlight had a significant effect on SLC6A4 promoter methylation (p = 0.03), with more sunlight hours resulting in lower methylation (r = -0.25). However, the interaction between sunlight and group was non-significant, suggesting a rather general effect across both groups. Sunlight also influenced BDI scores (p < 0.01), with fewer sunlight hours leading to higher BDI scores (r = -0.25), which aligns with previous research. SLC6A4 promoter methylation had no significant effect on BDI scores. Our findings suggest that sunlight impacts SLC6A4 promoter methylation, but this effect appears general, not specific to SAD pathophysiology.