Objectives: To evaluate the role of 5-HTTLPR, STin2 VNTR, and rs1042173T>G polymorphisms of the serotonin transporter gene (SLC6A4) as susceptibility factors for medication overuse headache (MOH) and to assess their value as predictors of the number of headache days per month, a potential marker of disease severity.
Methods: Genotyping was performed by PCR and PCR-RFLP on genomic DNA extracted from peripheral blood of 227 MOH patients and 312 control subjects. Logistic regression analysis was used to evaluate the association between the SL6A4 gene polymorphisms and MOH risk. The association between polymorphic variants and monthly headache days was evaluated by linear regression analysis.
Results: Logistic regression analysis, adjusted for age and gender, revealed a nominal association between rs1042173T>G and MOH risk (TT vs. TG + GG, OR: 1.58 95% CI: 1.05–2.37, P = 0.028). In the linear regression analysis adjusted for age, gender, primary headache diagnosis, acute drug overused and monthly drug number, STin2 VNTR was found nominally associated with monthly headache days (12/12 vs. others, difference: 1.55 days, 95% CI: 0.01–3.08, P = 0.050). When STin2 VNTR and rs1042173T>G were analyzed in haplotypic combination, a global haplotype association emerged with monthly headache days which remained significant after Bonferroni correction for multiple comparisons (global haplotype association P = 0.0056).
Conclusion: Although a minor contribution of SLC6A4 variants in the genetic liability of MOH cannot be excluded, haplotype-based analysis of STin2 VNTR and rs1042173T>G polymorphisms allowed to identify a subgroup of MOH patients with a higher number of monthly headache and, possibly, with a more severe disease.
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