Differential expression of human COMT alleles in brain and lymphoblasts detected by RT-coupled 5′ nuclease assay

Rationale: A common polymorphism, Val158-Met, alters catechol-O-methyltransferase ( COMT) enzyme activity and has been linked to psychiatric phenotypes. Bray et al. ( 2003) reported that COMT is subject to differential allele expression in brain, finding modest ( 13 - 22%) underexpression of a haplotype containing Val158. However, disparate findings by another group who used the same method, but in lymphoblasts, raise the issues of tissue specificity, magnitude of differential expression, and identity of loci altering expression. Objectives: We measured COMT allele expression ratios in heterozygous human lymphoblast cell lines and brains. Methods: Using transcribed single nucleotide polymorphisms as endogenous reporters, we developed an RT-coupled 5' nuclease assay for allele expression ratios and applied it to 63 COMT rs4818( C> G) heterozygotes and 68 Val158Met [rs4680( G> A)] heterozygotes. Results: For rs4818 ( C> G), the C allele was overexpressed relative to the G allele in 18 of 27 lymphoblast lines and 23 of 36 brains. For Val158Met, Met158 was overexpressed relative to Val158 in all ( 29 of 29) lymphoblast lines and all ( 39 of 39) brains. Each of the 22 rs4818 heterozygotes without differential allele expression was a Val158/Val158 homozygote. The Met158 allele was overexpressed by 65 - 77% when compared with Val158 in lymphoblasts and brain. Haplotype augmented ability to predict expression in brain only. However, the expression of the Val158 allele on the high-expressing haplotype was only 19% higher than Val158 alleles on the other haplotype background. Conclusions: COMT alleles are differentially expressed. The Met158 allele predicts higher mRNA expression in both brain and lymphoblasts. As exemplified here, the RT-coupled 5' nuclease assay is a reliable method for the quantitative evaluation of cis-acting regulatory effects.