To obtain a normal distribution, we log transformed relative total mRNA expression. Association between SNP genotype and relative total mRNA expression of CHRNA5 and PSMA4 was performed in PLINK [27] using linear regression under an additive genetic model. To minimize possible effects of sample heterogeneity, we performed our association analysis in subjects of each ancestral group separately. Similar to our observation in European Americans, postmortem interval (PMI = 13.9±6.0 hour in samples of African ancestry; 22.5±14.7 hour in samples of European ancestry) had a weak effect on CHRNA5 mRNA expression levels in brains of African ancestry and this variable was included as a covariate for association analysis. Age and gender did not affect CHRNA5 expression and were not included as covariates. For PSMA4 mRNA expression analysis, age had a weak effect in brains of European ancestry but not in samples of African ancestry; therefore this variable was included as a covariate for association analysis in the European ancestry datasets. PMI and gender did not influence PSMA4 mRNA expression.
