Because 4 of the 10 potentially functional variants associated with CHRNA5 mRNA expression are within the PSMA4 region, we tested whether these variants are also associated with PSMA4 mRNA levels. We observed variability of PSMA4 mRNA levels in both samples of African ancestry and European ancestry. However, the PSMA4 mRNA expression observed in frontal cortices is not correlated with the level of CHRNA5 mRNA expression (correlation = −0.13, p = 0.2 in samples of European ancestry; correlation = 0.03, p = 0.8 in samples of African ancestry). Using linear regression with age as a covariate, none of the 23 variants tested is associated with PSMA4 mRNA levels in frontal cortices of European ancestry (Table S2). In brains of African ancestry, 4 of 22 polymorphic variants tested showed marginal association with PSMA4 mRNA levels (5.96×10−3≤p≤3.91×10−2), but the association is no longer significant after correcting for multiple testing (q value ≥0.13).
