As described in the methods section, the full set of twins was used in the survival analysis (4597 individuals). Two SNPs were selected for survival analyses: rs279858 and rs279845, our two strongest signals in the association analyses. For both rs279858 (A/G) and for rs279845 (A/T), A was the risk allele. With respect to rs279858, having 1 or more copies of the A allele was significantly associated with age at first alcohol dependence symptom (p = 0.014) with A/A individuals at increased risk when compared to A/G individuals, and the G/G individuals at least risk (Figure 2a). After adjustment for sex (models 2 and 5 in Table 5), which was significantly associated with age at onset, genotype was still associated with age at onset. However, an interaction with sex was not significant (Models 3 and 6, which included the effects of genotype, sex and their interaction in Table 5) and could be dropped from the model. An additive model fitted well – individuals with two copies of the risk allele were more likely to have an earlier age of onset of alcohol dependence symptoms, when compared with those carrying one copy of the risk allele.
