Folates, vitamine B12 et métabolisme monocarboné

Références

1. Indépendant

   Ducker GS, Rabinowitz JD. One-Carbon Metabolism in Health and Disease Cell Metabolism, 2017.

   doi:10.1016/j.cmet.2016.08.009PubMed 27641100

   Financement et résultat

   Financement

   G.S.D. is supported by a postdoctoral fellowship from the American Cancer Society (PF-15-190-01-TBE). J.D.R. is supported by NIH grants R01CA163591 and P30DK019525 and Stand Up to Cancer.

   Résultat

   Review. Animals require dietary folate. Tetrahydrofolate (THF) binds one-carbon units at N5 and N10 at three oxidation states: 5-methyl-THF, the most reduced, 5,10-methylene-THF and 10-formyl-THF, the most oxidised; folic acid is reduced to dihydrofolate (DHF) and then THF. SHMT1 (cytosol) and SHMT2 (mitochondria) interconvert serine and glycine with THF and 5,10-methylene-THF, reversibly; in rapidly proliferating cultured cells the main direct source of cytosolic one-carbon units is formate released from mitochondria, reassimilated by the trifunctional MTHFD1. TYMS methylates dUMP to dTMP with 5,10-methylene-THF and is the only folate enzyme that alters THF itself, taking two electrons from it and producing DHF; DHFR regenerates THF with NADPH and is the target of trimethoprim and methotrexate, which trap folate as DHF. Purine synthesis incorporates two 10-formyl-THF units; in proliferating cells each new DNA or RNA base needs on average one 10-formyl-THF, and 5,10-methylene-THF is needed for one in four DNA bases. 5-Fluorouracil is a suicide inhibitor of TYMS; methotrexate most potently blocks DHFR, whereas pemetrexed more potently inhibits TYMS and AICAR transformylase; 5-formyl-THF (folinic acid, leucovorin) is given as rescue. 5-Methyl-THF, made by cytosolic NADPH-dependent MTHFR, is consumed only by cobalamin-dependent methionine synthase; BHMT is limited to liver and kidney. SAM inhibits MTHFR and activates CBS; 5-methyl-THF inhibits GNMT. Transsulfuration is the only route that degrades homocysteine. Folate and cobalamin supplementation lower homocysteine without reducing cardiovascular events in the general population. Dietary folate deficiency in adults manifests as macrocytic anaemia.

2. Réglementaire

   EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA Panel), Turck D, Bohn T, Castenmiller J, de Henauw S, Hirsch-Ernst KI, et al. Scientific opinion on the tolerable upper intake level for folate EFSA Journal, 2023.

   doi:10.2903/j.efsa.2023.8353PubMed 37965303

   Financement et résultat

   Financement

   European Food Safety Authority, on request from the European Commission (question EFSA-Q-2021-00366). Preparatory systematic reviews were conducted by a contractor, Karolinska Institutet in collaboration with the University of Oslo, under an EFSA procurement procedure.

   Résultat

   Folates consist of a pterin moiety linked to p-aminobenzoic acid and one or more glutamates, substituted at N5 or N10 by one-carbon units; natural folates are reduced polyglutamates usually with five to seven glutamates. A jejunal hydrolase (EC 3.4.17.21) converts polyglutamates to monoglutamates, which PCFT (SLC46A1), the most important intestinal transporter, and RFC (SLC19A1) move into enterocytes; 5-methyl-THF is the main circulating form; RFC, the main transporter from blood to tissues, has about 100 times lower affinity for folic acid than for reduced folates. Folylpolyglutamate synthase retains folates as THF polyglutamates; 5-methyl-THF monoglutamate and folic acid are poor substrates, and 5-methyl-THF is lost from cells unless methionine synthase converts it to THF. Folic acid, a fully oxidised monoglutamate, is reduced by DHFR via dihydrofolate to THF. The MTHFR reaction is essentially irreversible under physiological conditions and SAM is its allosteric inhibitor; in cobalamin deficiency 5-methyl-THF is metabolically trapped, SAM falls, MTHFR becomes fully active, and a functional folate deficiency develops with raised plasma 5-methyl-THF and homocysteine, depleted erythrocyte folate and fewer methyl groups for transmethylation. The megaloblastic anaemia of cobalamin deficiency is identical to that of folate deficiency; neurological manifestations (myeloneuropathy from demyelination) can occur with or without anaemia, are almost never seen in folate deficiency, and have an unknown mechanism, with congenital defects of MTHFR and methionine synthase pointing to methionine synthase rather than methylmalonyl-CoA mutase. Folic acid can initiate DNA synthesis in cobalamin deficiency through DHFR but cannot correct the methylation defect. MTHFR 677TT is associated with lower serum folate and higher homocysteine; in a genome-wide study of 2232 Irish adults aged 18 to 28 years MTHFR 677C>T was the only significant modifier of red cell and serum folate. ULs for folic acid retained: 1000 ug/day for adults including pregnant and lactating women, with progression of neurological symptoms in cobalamin-deficient patients as critical effect.

3. Indépendant

   Yamada K, Chen Z, Rozen R, Matthews RG. Effects of common polymorphisms on the properties of recombinant human methylenetetrahydrofolate reductase Proceedings of the National Academy of Sciences of the United States of America, 2001.

   doi:10.1073/pnas.261469998PubMed 11742092

   Financement et résultat

   Financement

   These studies were supported in part by National Institutes of Health Grant GM24908 (to R.G.M.) and by funds from the Medical Research Council of Canada (to R.R.).

   Résultat

   Human MTHFR is a dimer of about 70 kDa subunits, each with non-covalently bound FAD, that reduces 5,10-methylene-THF to 5-methyl-THF with NADPH; SAM binds the C-terminal regulatory domain as an allosteric inhibitor, and SAH competes with SAM there without inhibiting. Purified recombinant Ala222Val enzyme (677C>T) had the same Km for NADPH and 5,10-methylene-THF, the same Vmax and the same SAM inhibition as wild type, but released FAD about 3-fold faster after dilution to 50 nM subunits at 37 degrees C; 5-methyl-THF and SAM slowed FAD loss. Background cited: cell extracts from TT homozygotes have about 30% of CC activity; about 10-15% of Caucasians of European origin and of Japanese are TT, more in Mediterranean countries and fewer among African Americans; 677C>T is the most frequent genetic cause of mild hyperhomocysteinaemia, seen in TT homozygotes with plasma folate below the median.

4. Indépendant

   Gherasim C, Lofgren M, Banerjee R. Navigating the B12 Road: Assimilation, Delivery, and Disorders of Cobalamin Journal of Biological Chemistry, 2013.

   doi:10.1074/jbc.R113.458810PubMed 23539619

   Financement et résultat

   Financement

   This work was supported, in whole or in part, by National Institutes of Health Grant DK45776.

   Résultat

   Review. Mammals have two B12-dependent enzymes: methionine synthase, which uses methylcobalamin, and methylmalonyl-CoA mutase, which uses adenosylcobalamin to convert methylmalonyl-CoA to succinyl-CoA. Methionine synthase works in two half-reactions: methylcobalamin methylates homocysteine to methionine, leaving cob(I)alamin, which removes the methyl group from 5-methyl-THF to give THF and methylcobalamin; occasional oxidation to cob(II)alamin is repaired by reductive methylation needing NADPH, methionine synthase reductase and S-adenosylmethionine. Food cobalamin binds haptocorrin; in the duodenum pancreatic proteases release it, intrinsic factor binds it, and intestinal cells take it up by cubilin/AMN receptor-mediated endocytosis. About 20% of circulating cobalamin is bound to transcobalamin, which delivers it to cells through the transcobalamin receptor. Mutations in the cblG (methionine synthase) and cblE (reductase) loci cause isolated hyperhomocysteinaemia without methylmalonic aciduria.

5. Indépendant

   Blount BC, Mack MM, Wehr CM, MacGregor JT, Hiatt RA, Wang G, et al. Folate deficiency causes uracil misincorporation into human DNA and chromosome breakage: implications for cancer and neuronal damage Proceedings of the National Academy of Sciences of the United States of America, 1997.

   doi:10.1073/pnas.94.7.3290PubMed 9096386

   Financement et résultat

   Financement

   This work was supported by National Cancer Institute Outstanding Investigator Grant CA39910, by National Institute of Environmental Health Sciences Center Grant ESO1896 (B.N.A.), and by the intramural programs of the National Institute of Environmental Health Sciences and National Health and Environmental Effects Research Laboratory (R.B.E.). B.C.B. thanks the Biomedical Mass Spectrometry Unit at the University of New South Wales for financial support during the writing of a portion of the article.

   Résultat

   Folate-deficient adults (erythrocyte folate below 140 ng/ml, n = 8) had a geometric mean of 3,960,000 uracils per diploid cell in blood DNA, 8-fold more than controls (498,000, n = 14; P = 0.003); after 5 mg folic acid daily, measured at 6 to 8 weeks, the value fell to 186,000 (n = 5; P = 0.0003). Mechanism proposed: low 5,10-methylene-THF decreases dTMP synthesis, raises the dUMP/dTMP ratio and leads to DNA polymerase-mediated dUTP misincorporation; excision by uracil-DNA glycosylase and apyrimidinic endonuclease creates transient nicks, and two opposing nicks can form a double-strand break. Vitamin B12 deficiency traps folate as 5-methyl-THF, causing functional folate deficiency, homocysteine accumulation and uracil misincorporation.

6. Indépendant

   Lu SC, Mato JM. S-adenosylmethionine in liver health, injury, and cancer Physiological Reviews, 2012.

   doi:10.1152/physrev.00047.2011PubMed 23073625

   Financement et résultat

   Financement

   This work was supported by National Institutes of Health Grants R01DK51719, R01DK092407 and P30DK48522 (to S. C. Lu), R01AT1576 and R01AT004896 (to S. C. Lu and J. M. Mato) as well as Plan Nacional of I+D SAF 2011-29851 and Departamento de Educación Gobierno Vasco 2011 (to J. M. Mato).

   Résultat

   Review. Methionine adenosyltransferase synthesises S-adenosylmethionine (SAM) from methionine and ATP; tripolyphosphate formed in the reaction is hydrolysed to orthophosphate and pyrophosphate before product release. MAT1A is largely expressed in normal liver and MAT2A in all extrahepatic tissues. Over 200 human proteins are known or putative SAM-dependent methyltransferases; GNMT is the most abundant in liver (1% of cytosolic protein); the liver carries out up to 85% of all methylation reactions. S-adenosylhomocysteine (SAH) inhibits methyltransferases with a Ki in the micromolar range of their Km for SAM. AHCY cleaves SAH reversibly into adenosine and homocysteine; the thermodynamics favour SAH synthesis, and hydrolysis proceeds only if adenosine and homocysteine are rapidly removed. Homocysteine is remethylated by methionine synthase, expressed in all tissues, or by BHMT, limited to liver and kidney, with betaine; transsulfuration converts it to cysteine through CBS and cystathionase, both requiring vitamin B6. SAM inhibits MTHFR and activates CBS.

7. Indépendant

   Martí-Carvajal AJ, Solà I, Lathyris D, Dayer M. Homocysteine-lowering interventions for preventing cardiovascular events Cochrane Database of Systematic Reviews, 2017.

   doi:10.1002/14651858.CD006612.pub5PubMed 28816346

   Financement et résultat

   Participants

   71 422

   Suivi

   1 to 7.3 years

   Financement

   Internal sources: no sources of support provided. External sources: Iberoamerican Cochrane Centre, Spain (academic); Cochrane Heart Group, UK (academic).

   Résultat

   15 RCTs, 71,422 participants, follow-up 1 to 7.3 years. Against placebo, homocysteine-lowering interventions showed no difference in myocardial infarction (7.1% against 6.0%; RR 1.02, 95% CI 0.95-1.10; 12 trials, 46,699 participants), death from any cause (11.7% against 12.3%; RR 1.01, 0.96-1.06; 11 trials, 44,817) or serious adverse events (8.3% against 8.5%; RR 1.07, 1.00-1.14), and were associated with reduced stroke (4.3% against 5.1%; RR 0.90, 0.82-0.99; 10 trials, 44,224). Background: normal total homocysteine is close to 10 umol/L; fasting 12-30 umol/L is termed mild or moderate, 31-100 intermediate and above 100 severe hyperhomocysteinaemia.

8. Financé par l'industrie

   Aroda VR, Edelstein SL, Goldberg RB, Knowler WC, Marcovina SM, Orchard TJ, et al. Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes Study Journal of Clinical Endocrinology and Metabolism, 2016.

   doi:10.1210/jc.2015-3754PubMed 26900641

   Financement et résultat

   Participants

   1 800

   Suivi

   mean 5 and 13 years

   Financement

   The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the National Institutes of Health is the primary funding agency, with further funding from other NIH institutes, the Indian Health Service, the Department of Veterans Affairs, the Centers for Disease Control and Prevention and the American Diabetes Association. Bristol-Myers Squibb and Parke-Davis provided additional funding and material support during the DPP, Lipha (Merck-Santé) provided medication, and LifeScan Inc donated materials during the DPP and DPPOS.

   Soutien en nature

   Medication from Lipha (Merck-Santé); materials from LifeScan Inc.

   Résultat

   Serum B12 at or below 203 pg/mL at a mean 5 years: 37 of 859 (4.3%) on metformin against 20 of 856 (2.3%) on placebo, P = 0.02; at a mean 13 years: 56 of 753 (7.4%) against 40 of 736 (5.4%), P = 0.12. B12 at or below 298 pg/mL: 19.1% against 9.5% at 5 years and 20.3% against 15.6% at 13 years. Odds ratio for B12 deficiency per year of metformin use 1.13 (95% CI 1.06 to 1.20).

9. Réglementaire

   EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA). Scientific Opinion on Dietary Reference Values for cobalamin (vitamin B12) EFSA Journal, 2015.

   doi:10.2903/j.efsa.2015.4150PubMed 42109795

   Financement et résultat

   Financement

   European Food Safety Authority, on request from the European Commission (question EFSA-Q-2011-01227); preparatory work by the EFSA Working Group on Dietary Reference Values for vitamins and EFSA staff.

   Résultat

   A combination of biomarkers of cobalamin status, namely serum cobalamin, holotranscobalamin, methylmalonic acid and plasma total homocysteine, was judged the most suitable basis for Dietary Reference Values; an Average Requirement could not be determined. Adequate Intake for adults 4 ug/day, based on biomarker data and observed mean intakes of 4.2 to 8.6 ug/day in adults in several EU countries; 4.5 ug/day in pregnancy and 5 ug/day in lactation; 1.5 ug/day at 7 to 11 months rising to 4 ug/day at 15 to 17 years.

10. Indépendant

    Clarke R, Bennett DA, Parish S, Verhoef P, Dötsch-Klerk M, Lathrop M, et al. Homocysteine and coronary heart disease: meta-analysis of MTHFR case-control studies, avoiding publication bias PLoS Medicine, 2012.

    doi:10.1371/journal.pmed.1001177PubMed 22363213

    Financement et résultat

    Participants

    116 136

    Financement

    Supported by grants from the British Heart Foundation and UK Medical Research Council to the University of Oxford Clinical Trial Service Unit and Epidemiological Studies Unit (CTSU), and the Oxford BHF Centre for Research Excellence (JCH). No funding bodies played any role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

    Soutien en nature

    Unpublished genotype datasets contributed by collaborating groups, among them deCODE (Reykjavik), whose dataset was contributed by an employee co-author.

    Résultat

    In 19 unpublished datasets (48,175 CHD cases, 67,961 controls), the MTHFR 677TT against CC genotype gave a CHD odds ratio of 1.02 (95% CI 0.98-1.07), and 1.01 (0.95-1.07) in unsupplemented low-folate populations; homocysteine was expected to be about 20% higher with TT than CC. In 86 published studies (28,617 cases, 41,857 controls), the odds ratio was 1.15 (1.09-1.21), significantly discrepant (p = 0.001); the authors conclude that the discrepancy reflects publication bias or methodological problems.

11. Financé par l'industrie

    Holmes MV, Newcombe P, Hubacek JA, Sofat R, Ricketts SL, Cooper J, et al. Effect modification by population dietary folate on the association between MTHFR genotype, homocysteine, and stroke risk: a meta-analysis of genetic studies and randomised trials Lancet, 2011.

    doi:10.1016/S0140-6736(11)60872-6PubMed 21803414

    Financement et résultat

    Financement

    Public and charitable funders listed in the Acknowledgments, among them the UK Medical Research Council, British Heart Foundation, Wellcome Trust, UK Department of Health, US National Institute of Neurological Disorders and Stroke, US National Institute on Aging, National Health and Medical Research Council of Australia, National Center for Geriatrics and Gerontology of Japan, Erasmus MC and Dutch ministries, the Netherlands Heart Foundation, the Danish Medical Research Council and Danish Heart Foundation. The funding sources had no role in study design, data collection, analysis, interpretation, writing or the decision to submit.

    Résultat

    The TT against CC difference in homocysteine was 3.12 umol/L (95% CI 2.23 to 4.01) in Asia and 0.13 umol/L (-0.85 to 1.11) in high-folate America, Australia and New Zealand; the stroke odds ratios were 1.68 (1.44 to 1.97) and 1.03 (0.84 to 1.25). In trials of homocysteine-lowering interventions the summary stroke RR was 0.94 (0.85 to 1.04); no trial had evaluated homocysteine lowering exclusively in a low-folate region. Classed industry because employees of GlaxoSmithKline took part in the analyses.