Polymorphisms in MTHFR, MS and CBS Genes and Homocysteine Levels in a Pakistani Population

Abstract

BackgroundHyperhomocysteinemia (>15 µmol/L) is highly prevalent in South Asian populations including Pakistan. In order to investigate the genetic determinants of this condition, we studied 6 polymorphisms in genes of 3 enzymes - methylenetetrahydrofolate reductase (MTHFR; C677T; A1298C), methionine synthase (MS; A2756G), cystathionine-β-synthase (CBS; T833C/844ins68, G919A) involved in homocysteine metabolism and investigated their interactions with nutritional and environmental factors in a Pakistani population.Methodology/Principal FindingsIn a cross-sectional survey, 872 healthy adults (355 males and 517 females; age 18–60 years) were recruited from a low-income urban population in Karachi. Fasting venous blood was obtained and assessed for plasma/serum homocysteine; folate, vitamin B12, pyridoxal phosphate and blood lead. DNA was isolated and genotyping was performed by PCR-RFLP (restriction-fragment-length- polymorphism) based assays. The average changes in homocysteine levels for MTHFR 677CT and TT genotypes were positive [β(SE β), 2.01(0.63) and 16.19(1.8) µmol/L, respectively]. Contrary to MTHFR C677T polymorphism, the average changes in plasma homocysteine levels for MS 2756AG and GG variants were negative [β(SE β), −0.56(0.58) and −0.83(0.99) µmol/L, respectively]. The average change occurring for CBS 844ins68 heterozygous genotype (ancestral/insertion) was −1.88(0.81) µmol/L. The combined effect of MTHFR C677T, MS A2756G and CBS 844ins68 genotypes for plasma homocysteine levels was additive (p value <0.001). Odds of having hyperhomocysteinemia with MTHFR 677TT genotype was 10-fold compared to MTHFR 677CC genotype [OR (95%CI); 10.17(3.6–28.67)]. Protective effect towards hyperhomocysteinemia was observed with heterozygous (ancestral/insertion) genotype of CBS 844ins68 compared to homozygous ancestral type [OR (95% CI); 0.58 (0.34–0.99)]. Individuals with MTHFR 677CT or TT genotypes were at a greater risk of hyperhomocysteinemia in folate and vitamin B12 deficiencies and high blood lead (p value <0.05) level.ConclusionsGene polymorphism (especially MTHFR C677T transition), folate and vitamin B12 deficiencies, male gender and high blood lead level appear to be contributing towards the development of hyperhomocysteinemia in a Pakistani population.

Interactions among methylenetetrahydrofolate reductase (MTHFR) and cystathionine β-synthase (CBS) polymorphisms - a cross-sectional study: multiple heterozygosis as a risk factor for higher homocysteine levels and vaso-occlusive episodes

“…Since the P values in this study were >0.05 for the MTHFR C677T, MTHFR A1298C and CBS 844ins68 polymorphisms, we have no reason to reject the hypothesis that the genotype frequencies of the CBS T833C polymorphism are also in HardyWeinberg proportions, because this polymorphism segregates in cis with 844ins68 in carriers of the insertion (Franco et al, 1998;Dutta et al, 2005;Yakub et al, 2012), corroborating our results. CBS G919A showed a significant deviation from HWE, but that was expected, since in the Brazilian population it seems to be rare, and only the wild-type GG genotype has been reported (Aléssio et al, 2008).…”

“…On the other hand, despite the rarity of carriers of the insertion (I) allele reported for Asian patients, both heterozygous (34.48%) and homozygous (13.79%) for the mutant C allele of the CBS T833C polymorphism have been reported in Chinese patients with cerebral thrombosis (Shao et al, 2005). Although apparently contradictory, because of segregation in cis with 844ins68 (Franco et al, 1998;Dutta et al, 2005;Yakub et al, 2012), this may indicate interethnic differences among Asian ethnic groups in this CBS polymorphism, like those observed for African, Afro-American, European, and North American populations (Tsai et al, 1996;Franco et al, 1998;Pepe et al, 1999;Dutta et al, 2005). In our study, the frequency of 844ins68/T833C heterozygotes was 28.2% in the total sample of patients, and was found in 25% of white patients, 27.5% of mulatto patients and 38.5% of black patients, considering each group separately.…”

Concept mapping One-Carbon Metabolism to model future ontologies for nutrient–gene–phenotype interactions

“…Heterozygosity may confer a risk for post-methionine load hyperhomocysteinemia with increased risk of vascular disease (Clarke et al 1991). However, in the case of one particular insertional mutation in CBS, a protective effect toward hyperhomocysteinemia was observed in heterozygous individuals (Yakub et al 2012). Low levels of homocysteine are also observed in individuals with Down syndrome who have trisomy of chromosome 21.…”

Vitamin B12 Gene Polymorphisms and Chronic Diseases

“…While various studies in the West have shown that 677T MTHFR allele is associated with increased levels of plasma homocysteine in ethnic Europeans [7], it has been suggested that genetic determinants of plasma homocysteine in Asians vary to a great extent. T-allele frequency for C677T mutation of MTHFR gene in the South Indian population (0.10) is lower compared to UK (0.186) and USA (0.322) and much higher when compared to Sri Lanka (0.049) [8] [10].…”