Betaine-homocysteine methyltransferase: human liver genotype-phenotype correlation

Abstract

Betaine-homocysteine methyltransferase (BHMT) catalyzes the remethylation of homocysteine. BHMT is highly expressed in the human liver. In the liver, BHMT catalyzes up to 50% of homocysteine metabolism. Understanding the relationship between BHMT genetic polymorphisms and function might increase our understanding of the role of this reaction in homocysteine remethylation and in S-adenosylmethionine-dependent methylation. To help achieve those goals, we measured levels of BHMT enzyme activity and immunoreactive protein in 268 human hepatic surgical biopsy samples from adult subjects as well as 73 fetal hepatic tissue samples obtained at different gestational ages. BHMT protein levels were correlated significantly (p<0.001) with levels of enzyme activity in both fetal and adult tissues, but both were decreased in fetal tissue when compared with levels in the adult hepatic biopsies. To determine possible genotype-phenotype correlations, 12 tag SNPs for BHMT and the closely related BHMT2 gene were selected from SNPs observed during our own gene resequencing studies as well as from HapMap. These SNPs data were used to genotype DNA from the adult hepatic surgical biopsy samples, and genotype-phenotype association analysis was performed. Three SNPs (rs41272270, rs16876512, and rs6875201), located 28kb upstream, in the 5'-UTR and in intron 1 of BHMT, respectively, were significantly correlated with both BHMT activity (p=3.41E-8, 2.55E-9 and 2.46E-10, respectively) and protein levels (p=5.78E-5, 1.08E-5 and 6.92E-6, respectively). We also imputed 230 additional SNPs across the BHMT and BHMT2 genes, identifying an additional imputed SNP, rs7700790, that was also highly associated with hepatic BHMT enzyme activity and protein. However, none of the 3 genotyped or one imputed SNPs displayed a "shift" during electrophoretic mobility shift assays. These observations may help us to understand individual variation in the regulation of BHMT in the human liver and its possible relationship to variation in methylation.

Figure 1

Frequency distribution histograms of BHMT phenotypes in adult human liver samples. One sample with undetectable protein was excluded from panel (C).

Figure 2

Correlations between BHMT phenotypes and age for adult liver biopsy samples. * Indicates that samples were excluded from these figures because we lacked patient age information. One additional sample with undetectable protein was excluded from panel (C).

Figure 3

Correlations between BHMT protein and enzyme activity levels.

Figure 4

Correlations between gestational age and BHMT phenotypes in the fetal liver samples.

Figure 5

BHMT genotyped and imputed SNP associations with hepatic BHMT enzyme activity and protein levels. Association of genotyped (red circles) and imputed SNPs (black triangles) with (A) hepatic BHMT enzyme activity and (B) protein levels. (C) The LD structure of BHMT and BHMT2 was plotted using genotyped SNPs and imputed SNPs (p<1E-04). SNPs with the lowest p values are indicated by red rectangles around the rs number.

Figure 6

Genotype-phenotype relationships for rs6875201.