Supplementary Material for: Population Stratification in a Case-Control Study of Brain Arteriovenous Malformation in Latinos

Background: Genetic association studies conducted in admixed populations may be confounded by population stratification resulting in spurious associations. The purpose of this pilot study was to determine the presence and effect of population stratification in a case-control study of brain arteriovenous malformation (BAVM). Methods: We tested 83 ancestry informative markers in BAVM cases and healthy controls of self-reported Latino race/ethnicity (n = 294). Individual ancestry estimates (IAE) were obtained using the Structure program, assuming 3 underlying subpopulations. Summary χ² tests comparing genotype frequency of ancestry informative markers were used to detect stratification and IAE were included as covariates in logistic regression analysis to account for differences in genetic background. Results: Admixture estimates for Latinos (overall 47% native American, 45% European and 8% African ancestry) revealed heterogeneity between individuals within ancestral groups. The summary χ² test was significant (p = 0.005), suggesting ancestral differences between cases and controls. Furthermore, genetic ancestry was associated with frequency differences in a promoter variant in the IL-6 gene (IL-6 –174G>C). On average, subjects with the IL6 –174 GG genotype had 6% greater Native American ancestry (p = 0.023). Age- and sex-adjusted risk of BAVM associated with the IL-6 –174 GG genotype was 1.85 (95% CI 0.99–3.48, p = 0.055), and further adjustments for IAE yielded an OR of 1.96 (95% CI 1.03–3.72, p = 0.039). Conclusion: The IL-6 –174G>C polymorphism was associated with increased risk of BAVM among Latinos after accounting for differences in ancestral background. These results suggest subtle, negative confounding and illustrate the importance of addressing population stratification in case-control studies conducted in admixed populations.