Circulating Homocysteine Levels Research Articles

Circulating homocysteine levels in patients with type 2 diabetes mellitus

Background and aim Previous studies have shown conflicting results regarding circulating homocysteine levels in patients with type 2 diabetes. Methods and results This observational study included 2121 patients with angiographically proven coronary artery disease (507 patients with type 2 diabetes and 1614 patients without diabetes). Circulating homocysteine levels, methylenetetrahydrofolate reductase (MTHFR) C677T gene polymorphism, renal function, presence of coronary artery disease (CAD) diagnosed by coronary angiography, and circulating folate and vitamin B 12 status were assessed. Plasma homocysteine levels [median (25th; 75th percentile)] were significantly higher in patients with diabetes than in those without [12.4 μmol/L (9.9 μmol/L; 15.9 μmol/L) versus 11.7 μmol/L (9.6 μmol/L; 14.5 μmol/L), P = 0.011]. Diabetes affected homocysteine levels only in patients with a glomerular filtration rate <90 mL/min [13.0 μmol/L (10.5 μmol/L; 16.7 μmol/L) in patients with diabetes versus 12.2 μmol/L (10.1 μmol/L; 15.2 μmol/L) in patients without diabetes, P = 0.006] but not in those with a glomerular filtration rate ≥90 mL/min [10.1 μmol/L (8.1 μmol/L; 12.4 μmol/L) versus 10.2 μmol/L (8.8 μmol/L; 12.3 μmol/L), P = 0.267]. Multivariable analysis did not show an independent association between diabetes and homocysteine level ( P = 0.342). Conclusion Circulating homocysteine levels are increased in patients with type 2 diabetes compared with non-diabetic patients due to a more diabetes-associated adverse risk profile rather than to diabetes itself.

Hyperhomocysteinemia in Cancer Patients with Thrombosis is Not Associated With Methylene Tetrahydrofolate Reductase Gene Mutations and Can Be Down Regulated by Low Molecular Weight Heparin Treatment

Methylene tetrahydrofolate reductase (MTHFR) gene mutations, nutritional factors, and anticancer drugs are primarily responsible for the observed hyper-homocysteinemia in cancer patients. In three groups of 210 patients who were diagnosed with cancer and thrombosis and in a group of medical patients (cancer free) with thrombosis enrolled in a treatment study with low molecular weight heparin (LMWH), (Reviparin, n=90), blood levels of homocysteine were measured using an ELISA method (Diazyme Laboratories, San Diego, CA). Molecular methods utilizing PCR were used to measure MTHFR (677 and 1298) variants. On a cumulative basis the cancer patients with thrombosis exhibited a relatively higher level of homocysteine (12.8±4.9; range 2.2–39.4 uM/L) in comparison to normal (n=140, 4.8±3.9; range 2.8=6.9 uM/L) and medical patients with thrombosis (5.3±3.1; range 2.1–9.8 uM/L). Of the 210 cancer patients profiled for the molecular analysis 42/210 (20%) were positive for the 677 variant and 24/210 (11%) were positive for the 1298 variant. One group of cancer patients (n=72) was treated with a LMWH for 4–6 weeks. Blood samples from these patients showed a down-regulation of homocysteine levels (4.8±2.5 uM/L). These results suggest that cancer patients with thrombosis exhibited a relatively higher level of circulating homocysteine levels which are unrelated to a molecular defect in MTHFR.

Influence of methionine synthase (A2756G) and methionine synthase reductase (A66G) polymorphisms on plasma homocysteine levels and relation to risk of coronary artery disease

Background — Elevated plasma total homocysteine (tHcy) is increasingly being recognized as a risk factor for coronary artery disease (CAD) and other defects. Recent genetic studies have characterized molecular determinants contributing to altered homocysteine metabolism. Our objectives were therefore to confirm the relationship of tHcy with CAD and to examine the importance of genetic influence on tHcy in the coronary angiograms and conventional cardiovascular risk factors recorded in 230 subjects.We also determined the genotype frequencies distribution of the A2756G transition of the B12-dependent methionine synthase (MTR) gene and the A66G mutation of the methionine synthase reductase (MTRR) gene.Results — Patients with CAD (n = 151) had significantly higher tHcy concentrations than control subjects (15.49 ± 2.75 μmol/l vs. 11.21 ± 3.54 μmol/l, P < 0.001). Hyperhomocysteinaemia (tHcy ≥15 μmol/l) was a risk factor for CAD [RR = 4.07, 95% CI: 2.21 – 7.47, P < 0.001]. The homocysteine concentrations were significantly different between smokers and non-smokers, at 15.63 ± 3.10 vs. 12.45 ± 3.84 μmol/l, P < 0.05. In addition, smokers with hyperhomocysteinaemia demonstrated a markedly increased risk of CAD (OR = 2.50, 95% CI: 1.67 – 3.32, P < 0.05) compared with non-smokers with normal homocysteine.The 2756G and the 66G allele contribute to a moderate increase in homocysteine levels (P = 0.008 and P = 0.007, respectively), but not to CAD (P > 0.05). Combined MTR and MTRR polymorphisms, the 2756AG + 66AG and the 2756AG + 66GG were the combined genotypes that were a significant risk factor for having hyperhomocysteinaemia (14.4 ± 2.8 μmol/l, OR = 2.75, IC 95% = 1.21 – 6.24, P = 0.016 and 17.9 ± 4.1 μmol/l, OR = 6.28, IC 95% = 1.46 – 12.1, P = 0.021, respectively). Statistic analysis using the UniANOVA test shows that these two polymorphisms have an interactive effect circulating homocysteine levels (P < 0.05).Conclusion — Our data suggest that moderately elevated tHcy levels are prevalent in our population and are associated with an increased risk for CAD. This study provides evidence that the MTR A2756G and MTRR A66G polymorphisms significantly influence the circulating homocysteine concentration. In addition, the MTR and MTRR genes may interact to increase the risk for having hyperhomocysteinaemia.

Hyperhomocysteinemia in cancer patients with thrombosis is not associated with methylene tetrahydrofolate reductase (MTHFR) gene mutations is down regulated by low molecular weight heparin (LMWHs) treatment

2056 Background: MTHFR gene mutations, nutritional factors (folic acid/ B12) and anticancer drugs are primarily responsible for the observed hyper-homocysteinemia in cancer patients. In addition, it is well known that cancer patients exhibit increased prevalence of thrombotic events which may be due to the effects of homocysteine on endothelial function. The aim of this study was to demonstrate that an upregulation of homocysteine is multifactorial in cancer associated thrombosis. Methods: In three groups of 210 patients who were diagnosed with cancer and thrombosis and in a group of medical patients (cancer free) with thrombosis who were enrolled in a treatment study with LMWH, (Reviparin, n=90), blood levels of homocysteine were measured using an ELISA method (Diazyme Laboratories, San Diego, CA). In addition, molecular methods utilizing PCR were used to measure the MTHFR (677 and 1298) variants. Results: On a cumulative basis the cancer patients with thrombosis exhibited a relatively higher level of homocysteine (12.8± 4.9; range 2.2–39.4 uM/L) in comparison to normal (n=140, 4.8± 3.9; range 2.8=6.9 uM/L) and medical patients with thrombosis (5.3± 3.1; range 2.1–9.8 uM/L). Of the 210 cancer patients profiled for the molecular analysis 42/210 (20%) were positive for the 677 variant and 24/210 (11%) were positive for the 1298 variant. Similar results were obtained in the medical patients and normal volunteers. In one group of cancer patients (n=72) who were treated with a LMWH for 4–6 weeks, a down-regulation of homocysteine levels (4.8±2.5 uM/L) was noted. Conclusions: These results clearly suggest that, cancer patients with thrombosis exhibited a relatively higher homocysteine levels independent of MTHFR mutations. Medical (cancer free) patients with thrombosis do not exhibit this increase in circulating homocysteine levels. Thus, other relevant factors such as the chemotherapeutic agents contribute to this acquired upregulation of homocysteine. These observations also suggest that LMWHs are capable of reducing the homocysteine levels which may contribute to the observed effects of these agents. Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration sanofi-aventis

Fractions of Total Plasma Homocysteine in Patients with Ischemic Stroke Before the Age of 55 Years

The mechanism responsible for the association between elevated circulating homocysteine levels and ischemic stroke remains unclear. Therefore, the authors assessed total plasma homocysteine (tHcy) and its fractions (free [fHcy] and protein-bound [bHcy] homocysteine) in patients with ischemic stroke before the age of 55 years. Fifty patients (23 men, mean age 46.8+/-7.6 years) with ischemic stroke or transient ischemic attacks, with symptoms lasting < 72 hours were enrolled. In this group: 32 (64%) patients had hypertension; 12 (24%), ischemic heart disease (IHD); and 20 (40%), type 2 diabetes mellitus (DM). The control group consisted of 30 matched healthy individuals (17 men, mean age 44.6+/-6.2 years). The tHcy, fHcy, and bHcy levels were determined by high-performance liquid chromatography. tHcy and its fractions did not differ significantly between patients and controls. However, stroke patients with hypertension had significantly higher concentrations of tHcy and bHcy compared to stroke patients without hypertension (tHcy 13.0+/-3.3 vs 10.7 +/-3.2 micromol/L, p < 0.05; bHcy 9.7+/-2.6 vs 7.8+/-2.3 micromol/L, p < 0.01, respectively); fHcy was borderline significant: 3.1 (1.5-6.5) vs 2.5 (1.8-5.3) micromol/L, p = 0.05. The presence of IHD, DM, hyperlipoproteinemia, clinical subtypes of stroke, smoking, and family history of stroke did not influence these parameters. In the group of 50 patients, tHcy correlated with mean systolic blood pressure (BP) (r = 0.3, p < 0.05) and bHcy correlated with mean systolic and mean diastolic BP (r = 0.3, p < 0.05). These findings suggest an association between hypertension and redox status of Hcy in patients with ischemic stroke before the age of 55 years. This observation supports the hypothesis that elevated BP may contribute to Hcy-related vascular injury.

Hyperhomocysteinemia in Cancer Patients with Thrombosis Is Independent of Methylene Tetrahydrofolate Reductase Gene Mutation.

Plasma homocysteine levels are considered to be an important surrogate marker of vasculopathy in thrombotic disorders. Methylene tetrahydrofolate reductase (MTHFR) gene mutations, nutritional factors (folic acid/ B12) and anticancer drugs are primarily responsible for the observed increase in this surrogate marker. In addition, it is well known that cancer patients exhibit increased prevalence of thrombotic events. The aim of these studies was to demonstrate that an upregulation of homocysteine plays a role in cancer associated thrombosis. However, initial screening of cancer patients showed that the malignancy related thrombotic state was independent of molecular defects in Factor V Leiden, Prothrombin 20210 and MTHFR ( 677 and 1298) variants (Hoppensteadt et al. Proceedings ASCO 2003. 22:346;861). In a group of 102 patients who were diagnosed with cancer and thrombosis and in a group of medical patients (cancer free) with thrombosis enrolled in a treatment study with low molecular weight heparin (CORTES, n=50), blood levels of homocysteine were measured using a highly specific ELISA method (Diazyme Laboratories, San Diego, CA). In addition, molecular methods utilizing PCR were used to measure the MTHFR (677 and 1298) variants. On a cumulative basis the cancer patients with thrombosis exhibited a relatively higher level of homocysteine (11.8± 4.7 uM/L) in comparison to normal (n=140, 4.6± 2.9 uM/L) and medical patients with thrombosis (5.3± 3.1 uM/L). Of the 102 cancer patients profiled for the molecular analysis 18% were positive for the 677 variant and 12% were positive for the 1298 variant. Similar results were obtained in the medical patients and normal volunteers. These results clearly suggest that, cancer patients with thrombosis exhibited a relatively higher level of circulating homocysteine levels which are unrelated to a molecular defect in MTHFR. Medical (cancer free) patients with thrombosis do not exhibit this increase in circulating homocysteine levels. Thus, other relevant factors such as the use of chemotherapeutic agents like methotrexate and purine analogs may contribute to this apparently acquired upregulation of homocysteine. This observation warrants periodic monitoring of homocysteine in patients with thrombosis and cancer. Nutritional supplementation of folic acid and B12 may be helpful in the overall management of these patients including those who are treated with methotrexate and purine analogues.

Triiodothyronine Treatment Attenuates the Induction of Hepatic Glycine N-Methyltransferase by Retinoic Acid and Elevates Plasma Homocysteine Concentrations in Rats

Recent studies indicated that hormonal imbalances have a role in modulating the metabolism of methyl groups and homocysteine, interrelated pathways that when disrupted, are associated with a number of pathologies. Retinoic acid (RA) was shown to induce hepatic glycine N-methyltransferase (GNMT), a key regulatory protein in methyl group metabolism, and to reduce circulating homocysteine levels. Because thyroid status influences the hepatic folate-dependent one-carbon pool and retinoids can alter thyroid hormone levels, the aim of this study was to examine the interaction between retinoids and thyroid function. For hypothyroid studies, rats were administered 0.5 g/L propylthiouracil in the drinking water for 15 d, and RA [30 micromol/(kg . d)] for the final 5 d. For hyperthyroid studies, rats were treated with RA [30 micromol/(kg . d)] for 8 d and triiodothyronine [T(3); 50 microg/(100 g . d)] the last 4 d. T(3) treatment prevented the RA-mediated increase in GNMT activity. However, GNMT abundance remained elevated, indicating that GNMT regulation by T(3) in RA-treated rats may be, at least in part, at the post-translational level. In addition, T(3) treatment elevated plasma levels of homocysteine 177%, an elevation that was prevented by RA. T(3)-mediated hyperhomocysteinemia may be due to a 70% decrease in hepatic betaine-homocysteine S-methyltransferase, the enzyme that catalyzes folate-independent remethylation of homocysteine, whereas the RA-mediated stimulation of hepatic homocysteine remethylation by folate-dependent methionine synthase may contribute to lowering plasma homocysteine levels. These findings indicate that thyroid hormones, alone and in conjunction with RA, play an important role in the regulation of methyl group and homocysteine metabolism.

Hyperhomocysteinemia in cancer patients with thrombosis is independent of methylene tetrahydrofolate reductase gene mutation

6152 Background: Methylene Tetrahydrofolate reductase (MTHFR) gene mutations and nutritional factors (folic acid/ B12) are primarily responsible for the observed increase in this surrogate marker. Since, cancer patients exhibit increased prevalence of thrombotic events, we hypothesized that upregulation of homocysteine may play a role in cancer associated thrombosis. However, initial screening of cancer patients in the ONCENOX study showed that the malignancy related thrombotic state was independent of molecular defects in Factor V Leiden, Prothrombin 20210 and MTHFR variants. (Hoppensteadt et al. Pathophysiology of Hemostasis and Thrombosis 2003; 33. S1. 03 A. 56). Methods: In the same group of 102 patients who were enrolled in a randomized open label, multidose, active comparator, parallel design study (ONCENOX) and in another group of cancer free patients with thrombosis enrolled in a treatment study with low molecular weight heparin (CORTES, n=50), blood levels of homocysteine were measured using a highly specific ELISA method (Diazyme Laboratories, San Diego, CA). Results: On a cumulative basis ONCENOX patients exhibited a relatively higher level of homocysteine (11.8± 4.7 μM/L) in comparison to normal (n=140, 4.6± 2.9 μM/L) and in contrast to cancer patients exhibiting homocysteine levels (7.8± 3.1 μM/L). No difference in the MTHFR gene mutation profile was noted in the three groups. These results clearly suggest that, cancer patients with thrombosis exhibited a relatively higher level of circulating homocysteine levels which are unrelated to a molecular defect in MTHFR. Conclusions: Cancer free patients with thrombosis do not exhibit this increase in circulating homocysteine levels. Thus, other relevant factors such as the use of chemotherapeutic agents like methotrexate and purine analogs may contribute to this apparently acquired upregulation of homocysteine. This observation warrants periodic monitoring of homocysteine in patients with thrombosis and cancer. Nutritional supplementation of folic acid and B12 may be helpful in the overall management of these patients. Author Disclosure Employment or Leadership Consultant or Advisory Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Aventis; Mitsubishi; Pfizer

Homocysteine levels and the risk of osteoporotic fracture.

Very high plasma homocysteine levels are characteristic of homocystinuria, a rare autosomal recessive disease accompanied by the early onset of generalized osteoporosis. We therefore hypothesized that mildly elevated homocysteine levels might be related to age-related osteoporotic fractures. We studied the association between circulating homocysteine levels and the risk of incident osteoporotic fracture in 2406 subjects, 55 years of age or older, who participated in two separate prospective, population-based studies. In the Rotterdam Study, there were two independent cohorts: 562 subjects in cohort 1, with a mean follow-up period of 8.1 years; and 553 subjects in cohort 2, with a mean follow-up period of 5.7 years. In the Longitudinal Aging Study Amsterdam, there was a single cohort of 1291 subjects, with a mean follow-up period of 2.7 years. Multivariate Cox proportional-hazards regression models were used for analysis of the risk of fracture, with adjustment for age, sex, body-mass index, and other characteristics that may be associated with the risk of fracture or with increased homocysteine levels. During 11,253 person-years of follow-up, osteoporotic fractures occurred in 191 subjects. The overall multivariable-adjusted relative risk of fracture was 1.4 (95 percent confidence interval, 1.2 to 1.6) for each increase of 1 SD in the natural-log-transformed homocysteine level. The risk was similar in all three cohorts studied, and it was also similar in men and women. A homocysteine level in the highest age-specific quartile was associated with an increase by a factor of 1.9 in the risk of fracture (95 percent confidence interval, 1.4 to 2.6). The associations between homocysteine levels and the risk of fracture appeared to be independent of bone mineral density and other potential risk factors for fracture. An increased homocysteine level appears to be a strong and independent risk factor for osteoporotic fractures in older men and women.

Betaine analogues alter homocysteine metabolism in rats

Glycine betaine supplementation lowers homocysteine levels in homocystinuria and in chronic renal failure patients through methylation catalysed by betaine-homocysteine methyltransferase (BHMT). The aim of this study was to determine the effect of glycine betaine analogues on homocysteine metabolism in Lewis rats. Glycine betaine, proline betaine, trigonelline, dimethylsulfoniopropionate (DMSP) or dimethylthetin (1.5 mmoles) was subcutaneously administered to rats fed a low betaine diet. The effect of each betaine on total plasma homocysteine and urinary and plasma betaine concentrations was monitored for 24 h following administration. Baseline plasma homocysteine was 8.5±0.2 μmol/l (S.E.M., n=44) and compared to controls concentrations decreased following glycine betaine (0.8±0.4 μmol/l, P=0.064), DMSP (1.0±0.5 μmol/l, P=0.041) and dimethylthetin (1.5±0.7 μmol/l, P=0.033) treatment, while concentrations increased following proline betaine (2.2±0.7 μmol/l, P=0.002) and trigonelline (1.6±0.3 μmol/l, P<0.001) treatment. The effect of glycine betaine, DMSP and dimethylthetin on circulating homocysteine concentrations was thought to be mediated by BHMT in vivo. This hypothesis was supported by the finding that circulating glycine betaine concentrations increased following DMSP and dimethylthetin treatment. Proline betaine and trigonelline appeared to be poor BHMT substrates, being largely excreted in the urine unchanged, yet increased circulating homocysteine levels. This suggests they are inhibitors of BHMT. Urinary excretion of glycine betaine increased following treatment with all betaines, suggesting that the resorption of glycine betaine in the kidney was inhibited. The study shows that glycine betaine analogues have multiple effects on homocysteine metabolism (250).

Relation of Helicobacter pylori infection to plasma vitamin B 12, folic acid, and homocysteine levels in patients who underwent diagnostic coronary arteriography

OBJECTIVE: The aim of this study was to test the hypothesis that chronic atrophic gastritis induced by Helicobacter pylori ( H. pylori) causes malabsorption of vitamin B 12 and folate in food, leading ultimately to an increase in circulating homocysteine levels. METHODS: We performed endoscopy with stomach biopsy and measured fasting plasma homocysteine, vitamin B 12, and folate levels in 93 patients who underwent diagnostic coronary arteriography. The patients were divided into two groups according to the presence (n = 57) or absence (n = 36) of H. pylori infection. Positive H. pylori infection was defined as positive H. pylori histology of biopsy specimens from the stomach. The extent of atrophic gastritis was endoscopically graded from 0 to 6. RESULTS: There were no differences in age, sex, or traditional coronary risk factors between the two groups. Atrophy scores of the stomach were greater in patients with H. pylori infection than in patients without (3.9 ± 1.4 vs 2.2 ± 1.8, p < 0.0001). Patients with H. pylori infection had lower levels of vitamin B 12 (630 ± 222 vs 747 ± 259 pg/ml, p = 0.02) and folate (6.2 ± 2.1 vs 7.4 ± 2.8, p = 0.046), as well as higher levels of homocysteine (11 ± 4.9 vs 8.3 ± 2.1 nmol/ml, p = 0.01), than did patients without H. pylori infection. Plasma homocysteine levels correlated inversely with plasma vitamin B 12 and folate levels and positively with atrophic scores. CONCLUSIONS: This study suggests that H. pylori-induced chronic atrophic gastritis decreases plasma vitamin B 12 and folic acid levels, thereby increasing homocysteine levels. However, this effect does not seem to be strong.

Elevated plasma homocysteine concentrations six months after gastroplasty in morbidly obese subjects.

To investigate whether the increased homocysteine levels occur in the first 6 months postoperatively, when nutritional intake is the most inadequate and weight reduction is the most drastic. Fasting glucose, insulin, lipoprotein, homocysteine, folic acid and vitamin B12 levels and oral glucose, tolerance test (OGTT) were determined in 12 morbidly obese subjects (3 men and 9 women with a mean age of 31+/-3 years, mean+/-SEM) before, 6 and 12 months after banded gastroplasty. Gastroplasty resulted in significant weight loss, from 120+/-6 to 92+/-6 and 88+/-7 kgs, 6 and 12 months postoperatively (all p<0.001). Fasting plasma insulin and triglyceride concentrations, the ratio of total cholesterol to HDL cholesterol, glucose and insulin responses to OGTT, and the degree of insulin resistance as expressed by the Homeostasis model index decreased significantly (p<0.05-0.001) following gastroplasty. Fasting plasma homocysteine concentrations increased from 10.2+/-0.8 to 12.1+/-0.6 at 6 months (p=0.036) and 12.0+/-1.2 micromol/l at 12 months (p=0.040), respectively. Pooled plasma homocysteine levels were negatively correlated with serum folate concentrations (r=-0.42, p=0.013). However, serum folate and vitamin B12 levels did not change after gastroplasty, nor did the relation between the loss of body weight and increase in homocysteine levels. We observed that elevated circulating homocysteine levels occurred as early as 6 months after gastroplasty despite improvement in carbohydrate and lipoprotein metabolism in morbidly obese Chinese subjects.

Serum homocysteine concentration as an indicator of survival in patients with acute coronary syndromes.

Circulating homocysteine levels are predictive of survival in patients with stable coronary artery disease. The prognostic value of serum homocysteine levels, obtained in the acute phase in patients with myocardial infarction or unstable angina, is unknown. To test the hypothesis that circulating homocysteine levels, obtained during the first 24 hours following hospital admission in patients with acute coronary syndromes, are predictive of long-term mortality. To test this hypothesis we performed a prospective inception cohort study at a teaching hospital in Gothenburg, Sweden. A total of 579 patients (179 women and 400 men; median age, 67 years) were included (Q-wave myocardial infarction in 163 patients, non-Q-wave myocardial infarction in 210 patients, unstable angina pectoris in 206 patients). All-cause mortality. During a median follow-up of 628 days, 65 patients died. The serum homocysteine level (mean [SD]) was significantly lower in long-term survivors (n = 514) than in nonsurvivors (n=65) (12.3 [7.0] vs 14.3 [5.9] pmol/L; P=.003). The relative risk (all-cause mortality) for patients with homocysteine levels in the upper quartile was 2.4 (95% confidence interval, 1.5-4.0) compared with that of patients in the 3 lower quartiles. After adjustment for relevant confounders, the relative risk estimate remained significant (relative risk= 1.69; 95% confidence interval, 1.02-2.80). In a stepwise model the homocysteine level provided prognostic information additional to that of patient age, diabetes mellitus, and diuretic usage prior to hospital admission (P=.03). The serum homocysteine level on hospital admission is an independent predictor of long-term survival in patients with acute coronary syndromes.

Folate deficiencies and cardiovascular pathologies.

Although folates are widely distributed in foods, folate deficiencies may be more frequent than expected because their true availability may be impaired due to their lability under various food cooking and processing conditions. Folate deficiency is frequently observed in elderly people, smokers, alcoholics and oral contraceptive users. It is also associated with the mutation leading to the thermolabile variant of N5,10-methylenetetrahydrofolate reductase which is observed in about 10% of the population. In addition to the essential role of the intracellular pool of polyglutamates in de novo biosynthesis of deoxyribonucleotides which allow cell growth and division, the reduced and methylated form of folate, N5-methyltetrahydrofolate, is required for the remethylation of homocysteine to methionine. By inhibiting this remethylation pathway, folate deficiency induces homocysteine efflux into the circulation. Many studies have shown a negative correlation between plasma folate, particularly N5-methyltetrahydrofolate, and circulating homocysteine levels. In addition, folate deficiency is a major cause of hyperhomocysteinemia which is fully recognised as an independent risk factor for atherothrombosis. Epidemiological and recent experimental studies have demonstrated that folate deficiency might increase the risk of cardiovascular disease by increasing circulating homocysteine levels. Thus, the clinical efficiency of folate supplementation, especially N5-methyltetrahydrofolate, in reducing homocysteine-dependent cardiovascular risk should be evaluated.

Hyperhomocysteinaemia and vascular disease

The amino-acid homocysteine plays a crucial role in cell metabolism. It participates in the remethylation pathway enabling maintenance of adequate cellular levels of methionine or is catabolized by transsulphuration. A number of hereditary defects in the enzymes involved in homocysteine metabolism and acquired deficiencies in the vitamin cofactors of these enzymes are associated with the development of hyperhomocysteinaemia. The association between high circulating homocysteine levels and premature vascular thrombosis is well established in individuals with hereditary homocystinuria. There is now good epidemiological evidence that mild hyperhomocysteinaemia is an independent risk factor in the development of arterial disease and venous thrombosis although the causes of the elevated plasma homocysteine are unclear. A good candidate is homozygosity for the common thermolabile variant of methylenetetrahydrofolate reductase but the evidence for a causative association is conflicting. A number of in vitro effects of homocysteine on vascular endothelium, platelets and coagulation have been described which may predispose to vascular disease but the exact in vivo mechanisms remain to be elucidated. Dietary folate supplementation may normalize homocysteine in hyperhomocysteinaemic individuals and modify the risk of vascular disease.