With the development of society and the aging of population, the incidence of coronary artery disease (CAD) is gradually increasing, which seriously threatens human health. Diabetes, hypertension, hyperlipidemia, smoking and old age are the traditional risk factors for CAD. However, many patients do not have these risk factors, but still have a higher incidence of CAD. Later studies found that these patients had elevated plasma homocysteine (Hcy) levels. The role of plasma Hcy in the pathogenesis of CAD has been paid more and more attention.1,2

Hcy is a sulfur-containing amino acid, which is an important intermediate product in methionine metabolism. There are two ways of Hcy metabolism: sulfur transfer and methylation. It can be remethylated to methionine in the body. Methionine is formed by methionine synthase catalyzed by methionine synthase (MS) through the folate cycle, with methyltetrahydrofolate as a methyl donor and vitamin B12 as a cofactor.

MS, a key enzyme in the remethylation metabolic pathway, is a vitamin B12 dependent enzyme that catalyzes the remethylation of Hcy to methionine. The human MS gene is located on chromosome 1q43 and consists of one 4 kb exon and one intron. At present, eight mutations of MS gene have been found, among which D919G mutation is most common. That is, the Adenine at the 2756th is replaced by Guanine(G). The A2756G (rs185087) mutation leads to the substitution of Aspartic acid (D) at the 919th for Glycine(G) residues. In 1996, the polymorphism of MS gene A2756G was first discovered by Leclerc et al., and its polymorphism is significantly correlated with plasma Hcy concentration, and the mutation resulted in the defect of MS activity and thus hyperhomocysteinemia (HHcy) .3 Since the amino acid encoded by the codon of the A2756G mutation is located in the enzymatic active region of MS, it is speculated that the mutation at this site may weaken the activity of MS by changing the secondary structure of the protein, resulting in HHcy.

There are many studies on the relationship between MS gene A2756G polymorphism and CAD, but there is no consensus yet. In 2003, Klerk et al. found that the GG genotype of the MS gene D919G polymorphism presented a four-fold increased risk of CAD when compared to the AA genotype in a Dutch population.4 In 2007, Li et al. found that the MS gene D919G mutation could promote the CAD occurrence by elevating the serum Hcy level in a Chinese population.5 Similarly, in 2020, Raina et al. found a significant association of MS gene A2756G polymorphism and CAD susceptibility in an Indian population.6 Inversely, in 1999, Morita et al. found that the MS gene D919G polymorphism unlikely has a major effect on the CAD onset in a Japanese population.7 In 2016, Iqbal et al. found that no association was observed between MS A2756G polymorphism and premature AMI in a Pakistani population.8

Cystathionine-β-synthase (CBS) is a key enzyme in the sulfur transfer pathway. Under the catalysis of CBS, Hcy combines with serine to produce cystathionine, which is decomposed into cysteine and α ketobutyric acid under the action of cysteine sulfide γ-enzyme. The decrease of CBS activity could lead to the obstruction of the sulfur transfer pathway, which lead to HHcy. CBS is a multidomain tetramer enzyme that regulates its activity by catalyzing pyridoxal 5′ phosphate and S-adenosine methionine. Missense mutation of CBS is the main cause of homocystinuria.9 The human CBS gene is located on chromosome 21q22.3 and consists of 23 exons. At least 164 polymorphism loci of CBS gene have been found, most of which are distributed in the 3rd exon and 8th exon.10 Different mutation sites have different effects on the activity of CBS, among which 844ins68 polymorphism mutation is relatively common. That is, a 68 bp insertion mutation appears in the 8th exon. The 844ins68 polymorphism is divided into three genotypes as DD (wild type, deletion/deletion), DI (heterozygous type, deletion/insertion), and II (mutation type, insertion /insertion). Several studies on the relationship between CBS gene 844ins68 polymorphism and CAD have been conducted and no significant association was found between them.8,11, 12, 13, 14, 15 However, the result from each study is still different yet.

To determine whether MS gene A2756G or CBS gene 844ins68 polymorphism was associated with increased CAD risk, this meta-analysis was conducted (Supplement S1).