Because polymorphisms in the methyl group metabolism genes methylene-tetrahydrofolate reductase (MTHFR), methionine synthase (MS), and cystathione beta-synthetase (CBS) affect plasma homocysteine levels and intracellular concentrations of S-adenosylmethionine (SAM), they modify the susceptibility to cardiovascular diseases and cancer.
We discuss the possibility that a mild deficiency of methionine synthase activity could be associated with mild hyperhomocysteinemia, a risk factor for cardiovascular disease and possibly neural tube defects.
In addition, they suggest directly that mutations in methionine synthase can lead to elevated homocysteine, implicated both in neural tube defects and in cardiovascular diseases.