A paired-sibling analysis of the Xbal polymorphism in the muscle glycogen synthase gene

Aims/hypothesis. We have previously shown an association between a XbaI polymorphism in the muscle glycogen synthase gene (GYS1) and both Type II (non-insulin-dependent) diabetes mellitus and hypertension. Association studies are, however, hampered by the selection of the control group. To circumvent these problems we addressed the same question using a novel genotype discordant paired-sibling approach. Methods. We identified 122 sex-matched sib-pairs discordant for the Xba1 polymorphism among a new set of 743 Finnish subjects from 227 families with Type II diabetes and paired analyses were done by McNemar test of symmetry and by permutation tests. Results. Paired analysis showed that siblings with the A2 variant had more hypertension (p = 0.0067), obesity (p = 0.033) and microalbuminuria (p = 0.031) but not significantly more Type II diabetes (p = 0.27) than siblings with the A1 variant. Siblings with the A2 variant were more often treated by insulin (p = 0.050) or anti-hypertensive medication (p = 0.0060) or both. Diabetic A2 variant carriers had higher triglyceride (p = 0.023) and lower HDL cholesterol (p = 0.0059) concentrations and an earlier age at onset of diabetes (p = 0.022) than diabetic siblings with the A1 variant. In non-diabetic sib-pairs the presence of the A2 variant was associated with higher diastolic (p = 0.0014) blood pressure. Finally, the allele frequency of the XbaI polymorphism differed between 216 randomly chosen unrelated Type II diabetic patients and 115 unrelated healthy control spouses without a family history of Type II diabetes (12.7 vs. 6.5%, p = 0.013). Conclusion/interpretation. The A2 allele of the XbaI polymorphism in the GYS1 confers an increased susceptibility to different features of the metabolic syndrome and Type II diabetes.