Метилентетрагидрофолатредуктаза: ключевой фермент метильного цикла и его полиморфизмы.

Rate limiting enzyme in the methyl cycle

Methylenetetrahydrofolate reductase (MTHFR) is the rate limiting enzyme in the methyl cycle, and it is encoded by the MTHFR gene. Methylenetetrahydrofolate reductase catalyzes the conversion of 5,10 methylenetetrahydrofolate to 5 methyltetrahydrofolate, a cosubstrate for homocysteine remethylation to methionine. Natural variation in this gene is common in otherwise healthy people. Although some variants have been reported to influence susceptibility to occlusive vascular disease, neural tube defects, Alzheimer's disease and other forms of dementia, colon cancer, and acute leukemia, findings from small early studies have not been reproduced. Some mutations in this gene are associated with methylenetetrahydrofolate reductase deficiency. Complex I deficiency with recessive spastic paraparesis has also been linked to MTHFR variants. In addition, the aberrant promoter hypermethylation of this gene is associated with male infertility and recurrent spontaneous abortion.

In the rate limiting step of the methyl cycle, MTHFR irreversibly reduces 5,10 methylenetetrahydrofolate (substrate) to 5 methyltetrahydrofolate (product). 5,10 methylene tetrahydrofolate is used to convert dUMP to dTMP for de novo thymidine synthesis. 5 Methyltetrahydrofolate is used to convert homocysteine (a potentially toxic amino acid) to methionine by the enzyme methionine synthase. (Note that homocysteine can also be converted to methionine by the folate independent enzyme betaine homocysteine methyltransferase (BHMT).) MTHFR contains a bound flavin cofactor and uses NAD(P)H as the reducing agent.

Mammalian MTHFR is composed of an N terminal catalytic domain and a C terminal regulatory domain. MTHFR has at least two promoters and two isoforms (70 kDa and 77 kDa).

MTHFR activity may be inhibited by binding of dihydrofolate (DHF) and S adenosylmethionine (SAM, or AdoMet). MTHFR can also be phosphorylated – this decreases its activity by ~20% and allows it to be more easily inhibited by SAM.

The enzyme is coded by the gene with the symbol MTHFR on chromosome 1 location p36.3 in humans. There are DNA sequence variants (genetic polymorphisms) associated with this gene. In 2000 a report brought the number of polymorphisms up to 24. Two of the most investigated are C677T (rs1801133) and A1298C (rs1801131) single nucleotide polymorphisms (SNPs). While multiple published studies have drawn relationships between these SNPs and a wide variety of diseases, the American College of Medical Genetics has issued an official Practice Guideline recommending against testing or reporting on these two variants, citing "Recent meta analyses have disproven an association between hyperhomocysteinemia and risk for coronary heart disease and between MTHFR polymorphism status and risk for venous thromboembolism. There is growing evidence that MTHFR polymorphism testing has minimal clinical utility."

At nucleotide 1298 of the MTHFR, there are two possibilities: A or C. 1298A (leading to a Glu at amino acid 429) is the most common while 1298C (leading to an Ala substitution at amino acid 429) is less common. 1298AA is the "normal" homozygous, 1298AC the heterozygous, and 1298CC the homozygous for the "variant". In studies of human recombinant MTHFR, the protein encoded by 1298C cannot be distinguished from 1298A in terms of activity, thermolability, FAD release, or the protective effect of 5 methyl THF. MTHFR A1298C may play a role as either a driver in the development of major depressive disorder or as a predictive or diagnostic marker, possibly in combination with C677T.

A triplex tetra primer ARMS PCR method was developed for the simultaneous detection of C677T and A1298C polymorphisms with the A66G MTRR polymorphism in a single PCR reaction.

Severe MTHFR deficiency is rare (about 50 cases worldwide) and caused by mutations resulting in 0–20% residual enzyme activity. A study on the Chinese Uyghur population indicated that rs1801131 polymorphism in MTHFR was associated with nsCL/P in Chinese Uyghur population. Given the unique genetic and environmental characters of the Uyghur population, these findings may be helpful for exploring the pathogenesis of this complex disease.

The MTHFR aberrant promoter hypermethylation is associated with male infertility. Furthermore, this improper epigenetic phenomenon was observed in semen samples of infertile males belonging to couples with a history of recurrent spontaneous abortion. The MTHFR improper promoter hypermethylation may affect the two essential roles of DNA methylation in spermatogenetic cells, the global genome methylation process and the genomic imprinting of paternal genes. In addition, MTHFR gene promoter hypermethylation has also been associated with methylation loss at H19 imprinted gene in semen samples from infertile males. The active form of folate, L methylfolate, may be appropriate to target for conditions affected by MTHFR polymorphisms.

The overall reaction catalyzed by MTHFR is illustrated on the right. The reaction uses an NAD(P)H hydride donor and an FAD cofactor. The E. coli enzyme has a strong preference for the NADH donor, whereas the mammalian enzyme is specific to NADPH.

With the growth of direct to consumer genetic testing, the alternative medicine industry has aggressively targeted a range of dubious tests and highly profitable quack treatments for claimed MTHFR polymorphisms, despite the lack of any demonstrated health effects of these mutations. The promotion of supplements and other treatments for MTHFR polymorphisms, especially centered on autistic spectrum disorder, have been characterised as snake oil. Tests for MTHFR, while gaining popularity, are generally unnecessary because the association of MTHFR gene mutations with various diseases have not been established as clear cut cause and effect relationship.