Polymorphic forms of the protein L-isoaspartate (D-aspartate) O-methyltrans-ferase involved in the repair of age-damaged proteins

Abstract

The protein L-isoaspartate (D-aspartate) Omethyltransferase (PCMT1) can initiate the repair of agedamaged aspartyl and asparaginyl residues of intracellular proteins. The human gene PCMT1 encoding this enzyme has at least four polymorphic sites, one of which results in two major isoforms with either an Ile residue or a Val residue at amino acid position 119. The frequencies of the alleles encoding the Ile 119 and Val 119 variants are similar in Caucasian populations, but a predominance of the Ile 119 allele exists in Asian and African populations. Analyses of the enzymatic activities of the Ile 119 and Val 119 variants in red blood cell lysates show that the higher specific activity and thermostability of the Ile 119 isoform is balanced by the potentially compensating higher substrate affinity of the Val 119 isoform. In a preliminary attempt to find an association between genotype frequency at the PCMT1 locus and healthy aging, we compared the distribution of genotypes in a healthy older population of Ashkenazi Jewish individuals with that in a younger ethnically matched control group. We found that 65% of the healthy older population had the heterozygous genotype, greater than the 50% expected by Hardy-Weinberg equilibrium, suggesting a possible selection for having both alleles of the repair methyltransferase in successful aging. Three additional polymorphisms in noncoding regions of the methyltransferase gene were found to be biallelic and demonstrated nonrandom association in a specific haplotype with the codon 119 polymorphism. Finally, we also detected a heterozygous mutation in the splicing branch site of intron 2 that did not appear to affect activity. This study will help define the normal physiological range of activity for this repair methyltransferase and give us a better understanding of its role in the processes of aging and disease.

A known functional polymorphism (Ile120Val) of the human PCMT1 gene and risk of spina bifida

“…These observed lowered risks suggest that the affinity for AdoMet by PIMT may play a more important role during neural tube closure in embryonic development than what is inferred simply by its specific activity. In vitro studies showed that the Ile/Ile genotype had 20% higher specific activity but 30% lower substrate affinity than Val/Val [12]. Our findings were consistent with DeVry's hypothesis that a 30% increase in substrate affinity in Val/Val genotype could account for a substantial decrease in damaged protein accumulation.…”

“…A complete linkage disequilibrium (LD) was observed between the two SNPs (rs4816 and rs4552) in both subpopulations (nonHispanic White and Hispanic white) in our study. This finding is in line with previously published work [12]. We further interrogated available SNP genotyping data in PCMT1 gene from www.hapmap.org [19], using Haploview 3.2 (http://www.broad.mit.edu/mpg/haploview) [20].…”

“…Population variation of PCMT1 polymorphism was first reported by DeVry and colleagues. The frequencies of the alleles encoding the Ile 120 and Val 120 variants are similar in Caucasian populations, but there is a noted predominance of the Ile 120 allele present in Asian and African populations [12]. In our study, population variation did not appear to be a significant confounding factor.…”

“…The higher specific activity and thermo-stability of the Ile 120 isoform is thought to be balanced by the potentially compensating higher substrate affinity of the Val 120 isoform. This polymorphism was found to be involved in the repair process of agedamaged proteins [12,13].…”

Mass spectrometry in aging research

“…Some protein deamidation is subject to repair by the enzyme protein L-isoaspartate (Daspartate) O-methyltransferase (PCMT1). Polymorphism results in different human PCMT1 isoforms with either isoleucine or valine at position 119; these differ in specific activity and substrate affinity (De Vry & Clarke, 1999;Clarke, 2003). It appears that healthy aging may correlate with an increased frequency of a heterozygous genotype, simultaneously expressing both isoforms (Clarke, 2003).…”