Is the common 844ins68 polymorphism in the cystathionine beta-synthase gene associated with atherosclerosis?

Targeting Homocysteine and Hydrogen Sulfide Balance as Future Therapeutics in Cancer Treatment

A high level of homocysteine (Hcy) is associated with oxidative/ER stress, apoptosis, and impairment of angiogenesis, whereas hydrogen sulfide (H2S) has been found to reverse this condition. Recent studies have shown that cancer cells need to produce a high level of endogenous H2S to maintain cell proliferation, growth, viability, and migration. However, any novel mechanism that targets this balance of Hcy and H2S production has yet to be discovered or exploited. Cells require homocysteine metabolism via the methionine cycle for nucleotide synthesis, methylation, and reductive metabolism, and this pathway supports the high proliferative rate of cancer cells. Although the methionine cycle favors cancer cells for their survival and growth, this metabolism produces a massive amount of toxic Hcy that somehow cancer cells handle very well. Recently, research showed specific pathways important for balancing the antioxidative defense through H2S production in cancer cells. This review discusses the relationship between Hcy metabolism and the antiapoptotic, antioxidative, anti-inflammatory, and angiogenic effects of H2S in different cancer types. It also summarizes the historical understanding of targeting antioxidative defense systems, angiogenesis, and other protective mechanisms of cancer cells and the role of H2S production in the genesis, progression, and metastasis of cancer. This review defines a nexus of diet and precision medicine in targeting the delicate antioxidative system of cancer and explores possible future therapeutics that could exploit the Hcy and H2S balance.

Genes and genetics in hyperhomocysteinemia and the "1-carbon metabolism": implications for retinal structure and eye functions

Homocysteine (Hcy), a sulfur-containing nonproteinogenic amino acid, is generated as a metabolic intermediate. Hcy constitutes an important part of the "1-carbon metabolism" during methionine turnover. Elevated levels of Hcy known as hyperhomocysteinemia (HHcy) results from vitamin B deficiency, lack of exercise, smoking, excessive alcohol intake, high-fat and methionine-rich diet, and the underlying genetic defects. These factors directly affect the "1-carbon metabolism (methionine-Hcy-folate)" of a given cell. In fact, the Hcy levels are determined primarily by dietary intake, vitamin status, and the genetic blueprint of the susceptible individual. Although Hcy performs an important role in cellular functions, genetic alterations in any of the key enzymes responsible for the "1-carbon metabolism" could potentially upset the metabolic cycle, thus causing HHcy environment in susceptible people. As such, HHcy relates to several clinical conditions like atherosclerosis, myocardial infarction, stroke, cognitive impairment, dementia, Parkinson's disease, multiple sclerosis, epilepsy, and ocular disorders, among others. This article summarizes the findings from our laboratory and public database regarding genetics of HHcy and its effects on ocular disorders, their respective management during dysregulation of the 1-carbon metabolism.

Association between Hcy levels and the CBS844ins68 and MTHFR C677T polymorphisms with essential hypertension

The aim of the present study was to investigate the association between the homocysteine (Hcy) levels and polymorphisms of the CBS844ins68 and MTHFR C677T genes in essential hypertension (EH). The effects of the MTHFR C677T and CBS844ins68 haploid genotypes and the combined genotypes on EH and levels of Hcy were further explored. The polymorphisms of CBS844ins68 and MTHFR C677T genes in 200 EH and 200 normal tensive (NT) patients were detected using polymerase chain reaction-restriction fragment length polymorphism and analysis of the distribution of genotypes. An automated biochemical analyzer was used to measure the plasma Hcy levels and the clinical biochemistry data. The plasma Hcy levels in EH were significantly higher than those of the NT group (P<0.05). There were no significant differences (P>0.05) between males and females. Two genotypes, deletion/deletion (DD) and deletion/insertion (DI), of the CBS844ins68 polymorphism were found in two groups with no clear differences in two genotypes and allele frequency distribution (P>0.05). There were significant differences in the three genotype frequencies (χ²=6.658, χ²=4.410, P<0.05) for MTHFR C677T locus genotypes CC, CT and TT. The Hcy levels in genotypes DD and DI had no significant differences (P>0.05) and the CT and TT types were significantly higher compared to the CC genotype (P<0.05). The CC/DD combined genotype in the two groups was significantly different (P<0.05), and the odds ratio (OR), 0.569 showed that the CC/DD genotype may be a protective factor of hypertension. In the two groups, the Hcy levels for combined genotypes CC/DD, CT/DD, TT/DD and TT/DI were significantly different (P<0.05). The SHEsis software analysis linkage disequilibrium coefficient=0.216, indicates that there is probably a weak linkage for MTHFR C677T and CBS844ins68. Haplotype analysis suggested that the C-D haplotype was negatively correlated with EH (OR, 0.727) and that there was a positive correlation between T-D haplotype and EH (OR, 1.376). MTHFR C677T and CBS844ins68 polymorphisms were present in the populations studied and the CBS844ins68 homozygous mutation was not present. Therefore, there is a correlation between the polymorphisms of the MTHFR C677T gene and EH, and allele T may be one of the predisposing factors. MTHFR C677T and CBS844ins68 may exist with a certain linkage and the T-D haplotype may be a risk factor for EH.

G runs in cystathionine beta-synthase c.833C/c.844_845ins68 mRNA are splicing silencers of pathogenic 3' splice sites

The c.844_845ins68 is an evolutionary conserved polymorphism of the cystathionine beta-synthase gene that segregates with the pathogenic c.833C mutation and consists of a 68nt insertion duplicating the 3' splice site between intron 7 and exon 8. The gene rearrangement brought two GGGG runs close to each other and generated a splicing control element that allows the constitutive selection of the more distal 3' splice site in the c.844_854ins68 carriers. In this study, we have characterized functionally the two G4 runs within the duplication and have found that they work as silencers of the upstream potentially pathogenic 3' splice sites has been functionally characterized. This selection allows skipping of both the 68nt-insertion and the c.833C mutation, and is essential to preserve the wild-type ORF. Knocking down hnRNP H and F expression modulated the rescue of the proximal 3' splice site more than hnRNP H alone. These observations suggest that hnRNP H/F contribute jointly to prevention of CBS deficiency in c.844_854ins68 carriers by silencing the potentially pathogenic upstream acceptor site.

Diversity of cystathionine beta-synthase haplotypes bearing the most common homocystinuria mutation c.833T>C: a possible role for gene conversion

Homozygosity or compound heterozygosity for the c.833T>C transition (p.I278T) in the cystathionine beta-synthase (CBS) gene represents the most common cause of pyridoxine-responsive homocystinuria in Western Eurasians. However, the frequency of the pathogenic c.833C allele, as observed in healthy newborns from several European countries (q_c.833C ≊ 3.3 × 10^–3), is ∼20-fold higher than expected on the basis of the observed number of symptomatic homocystinuria patients carrying this mutation (q_c.833C ≊ 0.18 × 10^–3), implying clinical underascertainment. Intriguingly, the c.833C mutation is also present in combination with a 68-bp insertion, c.[833C; 844_845ins68], in a substantial proportion of chromosomes from nonhomocystinuric individuals worldwide. We have sought to study the relationship between the pathogenic and nonpathogenic c.833C-bearing chromosomes and to determine whether the pathogenic c.[833C; −] chromosomes are identical-by-descent or instead arose by recurrent mutation. Initial haplotype analysis of 780 randomly selected Czech and sub-Saharan African wild-type chromosomes, employing 12 intragenic markers, revealed 29 distinct CBS haplotypes, of which 10 carried the c.[833C; 844_845ins68] combination; none carried an isolated c.833C or c.844_845ins68 mutation. Subsequent examination of 69 pathogenic c.[833C; −] chromosomes, derived from homocystinuria patients of predominantly European origin, disclosed three unrelated haplotypes that differed from their wild-type counterparts by virtue of the presence of c.833C, thereby indicating that c.833T>C transition has occurred repeatedly and independently in the past. Since c.833T does not reside within an obvious mutational hotspot, we surmise that the three pathogenic and comparatively prevalent c.[833C; −] chromosomes may have originated by recurrent gene conversion employing the common nonpathogenic c.[833C; 844_845ins68] chromosomes as templates. Hum Mutat 28(3), 255–264, 2007. Published 2006 Wiley-Liss, Inc.†

Gene-gene and gene-environment interactions in mild hyperhomocysteinemia

Mild/moderate hyperhomocysteinemia (HHcy), a highly prevalent condition, is independently associated with an increased risk of arterial and venous thromboembolic diseases. Early reports of the association of mild/moderate HHcy with juvenile venous thromboembolism have shown familiarity for HHcy in relatives of index cases with thrombosis. Similar to inherited thrombophilia defects, inheritance of the HHcy phenotype was accordingly retained important for the definition of HHcy as an independent risk factor for thrombosis. A number of common polymorphisms in genes coding for methylenetetrahydrofolate reductase(MTHFR), methionine-synthase, methionine-synthase reductase and cysthationine beta-synthase (CBS) have been explored for their association with homocysteine levels, fasting and post-methionine load, and with thrombotic diseases. MTHFR thermolability accounts for a 10-fold increase in the risk of mild/moderate HHcy. With the possible exception of the CBS844ins68 insertion, there is no evidence for an increased risk of HHcy for any of these polymorphisms, isolated or in association with MTHFR thermolability. Environmental factors and MTHFR thermolability are main determinants of the HHcy phenotype.If mild/moderate HHcy is a pathogenetic risk factor for thrombosis, intervention aimed to improve the vitamin status appears of major importance, irrespective of common gene polymorphisms of the homocysteine metabolism.

Cystathionine beta-synthase c.844ins68 gene variant and non-syndromic cleft lip and palate

Non-syndromic cleft lip with or without cleft palate (CL/P) is a common birth defect with substantial clinical and social impact and whose causes include both genetic and environmental factors. Folate and homocysteine (Hcy) metabolism have been indicated to play a role in the etiology of CL/P, and polymorphisms in folate and Hcy genes may act as susceptibility factors. We investigated a common polymorphism in the cystathionine beta-synthase (CBS) gene (c.844ins68) in 134 Italian CL/P cases and their parents using the transmission disequilibrium test (TDT). Although no overall linkage disequilibrium was observed, considering the parent-of-origin transmission of the CBS 68 bp insertion a significant (P = 0.002) transmission distortion was detected. When children receive the c.844ins68 allele from the mother compared to the father, they show a 18.7-fold increase in risk for CL/P. This evidence suggests CBS as a candidate gene for CL/P and supports a role of maternal-embryo interactions in the etiology of CL/P.

Genetic variants of homocysteine metabolizing enzymes and the risk of coronary artery disease

It is unresolved whether elevated homocysteine in coronary artery disease (CAD) is the cause of arteriosclerosis or its consequence. In contrast, genetic variants of enzymes that metabolize homocysteine cannot be altered by arteriosclerosis. Consequently, their association with CAD would permit to imply causality. We modeled by regression analysis the effect of 11 variants in the methionine cycle upon CAD manifestation in 591 controls and 278 CAD patients. Among the examined variants only the carriership for the c.844ins68 in the cystathionine beta-synthase (CBS) gene was associated with a significantly lowered risk of CAD (OR=0.56; 95% CI=0.35-0.90 in the univariable, and OR=0.41, 95% CI=0.19-0.89 for obese people in the multivariable analysis, respectively). Healthy carriers of the c.844ins68 variant exhibited, compared to the wild type controls, significantly higher postload ratios of blood S-adenosylmethionine to S-adenosylhomocysteine (61.4 vs. 54.9, p=0.001) and of plasma total cysteine to homocysteine (8.6 vs. 7.3, p=0.004). The changes in these metabolites are compatible with an improved methylation status and with enhanced activity of homocysteine transsulfuration. In conclusion, the coincidence of clinical and biochemical effects of a common c.844ins68 CBS variant supports the hypothesis that compounds relating to homocysteine metabolism may play role in the development and/or progression of CAD.

Regulation of 3' splice site selection in the 844ins68 polymorphism of the cystathionine Beta -synthase gene

844ins68 is a frequent polymorphism of the cystathionine beta-synthase gene (CBS) that consists of a 68-bp insertion duplicating the 3' splice site of intron 7 and the 5'-end of exon 8. The presence of two identical 3' splice sites spaced by 68 bp should lead to either a selection of the proximal site or to at least two alternatively spliced CBS mRNA variants. Instead, an accurate selection of the distal 3' splice site is observed in the 844ins68 carriers. The duplication has generated a gene re-arrangement at the 3' splice site where two GGGG runs have been brought close to each other. Using a minigene system, we have investigated the effect this peculiar configuration might have on the selection of the 3' splice site of intron 7 in the CBS gene. Minimal disruption of the G runs resulted in a dramatic shift toward the proximal 3' splice site selection with inclusion of the 68-bp insertion and a consequent change of the reading frame. The insertional event created this peculiar configuration of two G repeats close to each other that subsequently acquired the ability to strongly bind heterogeneous nuclear ribonucleoprotein (hnRNP) H1, a specific trans-acting factor. The interaction of hnRNP H1 with G runs within the 844ins68 context might interfere with the recruitment of splicing factors to the proximal 3' splice site thus favoring the selection of the distal 3' splice site. Our results therefore suggest the possibility that the insertion was an evolutionary event that allowed the rescue of the wild-type sequence, so preserving protein function.

Genetic risk factors in young adults with 'cryptogenic' ischemic cerebrovascular disease

Mutations such as factor V Leiden G1691A (FVL), prothrombin G20210A (FIIM), methylenetetrahydrofolate reductase (MTHFR) C677T, cystathionine beta-synthase (CBS) 844ins68 and endothelial cell protein C receptor (EPCR) 4031ins23 are risk factors for thromboembolism. To assess the role of these mutations in young adults with cerebral ischemia of otherwise undetermined etiology, 93 patients younger than 50 years old with thromboembolic strokes or transient ischemic attacks were studied. One hundred and eighty-six healthy age-matched and sex-matched blood donors served as controls. The FVL mutation was detected in 15/93 patients and 13/186 controls. After adjustment for smoking, arterial hypertension, and hyperlipidemia, the association of the FVL mutation with cerebral ischemia [odds ratio (OR), 3.19; 95% confidence interval (CI), 1.38-7.39] remained significant. One of 93 patients and 6/186 controls were carriers of FIIM (OR, 0.33; 95% CI, 0.04-2.75). We detected the MTHFR TT677 genotype in 9/93 patients and 26/186 controls (OR, 0.66; 95% CI, 0.30-1.47), a CBS 844ins68 mutation in 12/93 patients and 19/186 controls (OR, 1.30; 95% CI, 0.60-2.81), and an EPCR 4031ins23 mutation in 1/93 patients and in no control individual (P = 0.33). In conclusion, in younger adults the FVL mutation is a risk factor for cerebrovascular disease. FIIM, the MTHFR TT677 genotype and the CBS 844ins68 mutation did not contribute to the risk in this group of patients. The EPCR 4031ins23 mutation is very rare, its possible role needs further investigation.

Mutations in the genes regulating methylene tetrahydrofolate reductase (MTHFR C-->T677) and cystathione beta-synthase (CBS G-->A919, CBS T-->c833) are not associated with myocardial infarction in African Americans

Moderate hyperhomocysteinemia is a putative risk factor for cardiovascular disease. Molecular studies have demonstrated increased plasma homocysteine levels in the presence of DNA mutations in either the methylenetetrahydrofolate reductase (MTHFR) enzyme found in the remethylation pathway or the enzyme cystathione beta-synthase (CBS) of the transsulfuration pathway. To determine whether the mutation C-->T677 in the MTHFR gene or the T-->C833/844ins68 and G-->A919 mutations in the CBS gene are associated with myocardial infarction (MI) in African Americans, DNA was analyzed from samples obtained from a case-control study conducted at a large, inner-city hospital. One-hundred ten African American subjects with a diagnosis of MI and 185 race- and age-matched controls were recruited. Our results demonstrated that 15% of the MI cases were heterozygous for the C-->T677 (MTHFR) mutation, while 1.8% were homozygous. When compared to the controls in which 15% were heterozygous and 2.1% were homozygous, no significant association with MI was observed. In addition, 34% of the cases were heterozygous for the T-->C833 (CBS) mutation while 6% were homozygous. This is compared to 32% and 5% of the controls having the heterozygous and homozygous genotype, respectively. No significant association was observed for the T-->C833 (CBS) mutation among the cases and controls. Although this mutation has no significant association with MI, the prevalence of the heterozygous state was higher than what has been reported for whites (12%). No mutations for G-->A919 (CBS) were detected in the cases or controls. The racial differences of the CBS T-->C833 polymorphism suggest that further investigation into the other areas of the CBS gene is needed.