The serotonin transporter promoter repeat length polymorphism, seasonal affective disorder and seasonality

Background. Conflicting results have been reported in previous association studies of the serotonin transporter promoter repeat length polymorphism (5-HTTLPR), seasonal affective disorder (SAD) and seasonality (seasonal variations in mood and behaviour). The aim of this study was to test for association in new case–control and population-based materials, and to perform a combined analysis of all published studies of 5-HTTLPR and SAD. Method. One hundred and forty-seven new SAD cases and 115 controls were genotyped for 5-HTTLPR and in total 464 patients and 414 controls were included in the pooled analysis. In addition, 226 individuals selected for unusually high or low seasonality scores from a population based material and 46 patients with non-seasonal depression were analysed. Different genetic models were tested and seasonality was analysed both as a qualitative (high v. low) and as a quantitative trait in the different sample sets. Results. No association between 5-HTTLPR and SAD was found in the new case–control material, in the combined analysis of all samples, or when only including 316 patients with controls (N=298) selected for low seasonality. A difference was detected between the population based high and low seasonality groups, when assuming a recessive effect of the short allele (20 % and 10 % short allele homozygotes, respectively, OR (95 % CI) : 2 . 24 (1 . 03–4 . 91)). Quantitative analysis of seasonality revealed no association with 5-HTTLPR in any sample set. Conclusions. These results do not suggest a major role of the short variant of 5-HTTLPR in susceptibility to SAD, but provide modest evidence for an effect on seasonality.