Tissue factor pathway inhibitor levels in patients with homocystinuria

Hyperhomocysteinemia in Cardiovascular Diseases: Revisiting Observational Studies and Clinical Trials

Thromboembolic manifestations are relatively frequent in patients with intermediate/severe hyperhomocysteinemia (>30 µmol/L) related to inherited disorders and deficiencies in vitamin B12 and folate. In contrast, moderate hyperhomocysteinemia (15-30 µmol/L) is a modest predictor of cardiovascular risk. The recognition of homocysteine as a cardiovascular risk factor has been challenged by some but not all randomized clinical trials. We reviewed the main data of this controversy and formulated conclusions to be translated in clinical practice.Homocysteine-lowering trials have been performed in cardiovascular subjects with moderate but not intermediate/severe hyperhomocysteinemia despite the dose-effect risk association. The first meta-analyses found no benefit and led cardiology societies not recommending homocysteine in the assessment of cardiovascular risk. This guideline challenged the need to diagnose and treat the nutritional and genetic causes of intermediate/major hyperhomocysteinemia and was not revised when larger meta-analyses concluded to a reduced risk of stroke. In a recent observational study, 84% of consecutive cardiovascular patients assessed for homocysteine had intermediate or major hyperhomocysteinemia, which was properly assessed in only half of the cases and related to B12 and/or folate deficiency and Addison/Biermer disease in 55% of these cases.In conclusion, revisiting observational studies and clinical trials suggests that cardiovascular patients should be screened for hyperhomocysteinemia, when no other risk factor is found. Patients with intermediate/major hyperhomocysteinemia should be properly assessed and treated for B vitamin deficiencies and inherited disorders according to current guidelines. Further trials are needed to assess the effect of lowering homocysteine according to hyperhomocysteinemia categories at baseline.

Hyperhomocysteinemia and thrombosis: an overview

CONTEXT

Homocysteine, a sulfur-containing amino acid, absent in natural diets, is a metabolic intermediary in transmethylation and transsulfuration reactions. Such reactions are essential to normal cellular growth, differentiation, and function. Excess homocysteine is associated with vascular disease and related disorders.

OBJECTIVE

To review homocysteine metabolism, the pathogenesis and classification of hyperhomocysteinemia, and the published literature investigating the association of homocysteine and methylenetetrahydrofolate reductase defects with arterial and venous thromboembolism and related disorders. The role of vitamin supplementation in patients with hyperhomocysteinemia is addressed.

DATA SOURCES

Published medical and scientific literature. Articles addressing the objectives were selected and reviewed. Pertinent studies and conclusions were summarized, grouped, and contrasted.

CONCLUSIONS

The association of hyperhomocysteinemia and arterial and venous thrombosis is controversial. Severe hyperhomocysteinemia is associated with atherosclerosis. The effect of mild hyperhomocysteinemia is less certain. Coinheritance of methylenetetrahydrofolate reductase defects and factor V Leiden is likely to increase the risk of venous thromboembolism. The association of methylenetetrahydrofolate reductase defects combined with no additional thrombophilic risk factors with venous thrombosis is less clear. High doses of folic acid to lower homocysteine levels might not be necessary.

Association between factor V Leiden, prothrombin G20210A, and methylenetetrahydrofolate reductase C677T mutations and events of the arterial circulatory system: a meta-analysis of published studies

BACKGROUND

The association between the inherited gene mutations of factor V, prothrombin, and homocysteine metabolism and venous thromboembolic events is accepted widely; however, their influence on the arterial circulatory system remains controversial.

METHODS

We performed a MEDLINE search to identify published case-control and cohort studies correlating the factor V Leiden, prothrombin (PT) G20210A, and methylenetetrahydrofolate reductase (MTHFR) C677T (TT genotype) mutations with myocardial infarction, ischemic stroke, or peripheral vascular disease. Studies were included only when they adhered to specific diagnostic criteria for ischemic events and met the published methodological criteria. Odds ratios (ORs) with accompanying 95% CIs were calculated for each mutation and clinical end points with a random-effects model (DerSimonian and Laird method).

RESULTS

The association between inherited gene mutations and arterial ischemic events was modest: factor V Leiden mutation (OR, 1.21; 95% CI, 0.99-1.49), PT G20210A mutation (OR, 1.32; 95% CI, 1.03-1.69), and MTHFR TT mutation (OR, 1.20; 95% CI, 1.02-1.41). Subgroup analyses of younger patients (<55 years old) and of women revealed slightly stronger associations overall.

CONCLUSIONS

Genetic abnormalities specific to factor V, prothrombin,and homocysteine metabolism increase the risk for myocardial infarction and ischemic stroke, particularly among younger patients and women. Because the overall association is only modest, screening studies should be limited to carefully selected patient populations. The individual propensity for arterial and venous thrombosis is likely influenced by differing local mechanisms, systemic mechanisms, or both.

Hyperhomocysteinemia due to folate deficiency is thrombogenic in rats

Although hyperhomocysteinemia (HH) appears to be an independent risk factor for thrombosis, the pathophysiologic mechanisms seen in thrombus formation are largely unresolved. The aim of the present study was to investigate whether HH is accompanied by thrombogenic alterations as assessed by whole blood thromboelastographic profiles. Severe (111 micro mol/L) and intermediate (42 micro mol/L) HH were induced in two series of rats by feeding them a folate-depleted diet. Each group was compared with a separate control group. In rats suffering severe HH (n = 30), initiation of the coagulation was protracted, with a clotting time of 79.7 s (95% CI, 76.2-83.4) compared with 70.4 s (95% CI, 66.5-74.6) in controls (P = 0.001). The velocity of the propagation of coagulation was increased, reaching 0.119 mm/s (95% CI, 0.111-0.127) in HH rats compared with 0.104 mm/s (95% CI, 0.099-0.108) in controls (P < 0.001). The maximum clot firmness also was increased, i.e., 41.9 mm (95% CI, 40.5-43.4) in HH rats compared with 37.6 mm (95% CI, 36.5-38.7) in controls (P < 0.001). The resting thrombin-antithrombin complex concentration in plasma tended to be lower in HH rats [5.60 micro g/L (95% CI, 3.76-8.34) than in controls [8.56 micro g/L (95% CI, 6.44-11.37)] (P = 0.074). In the series of rats with intermediate HH (n = 16), the changes in the whole blood coagulation profile (WBCP) were similar to those in rats with severe HH although only the prolongation of the initiation phase was significant. In conclusion, our study revealed that the WBCP was influenced by high plasma homocysteine by 1) prolonging the initiation phase, 2) increasing the velocity of the coagulation propagation and 3) increasing the maximum clot firmness. These changes in WBCP may contribute to the increased risk of thrombosis in hyperhomocysteinemic individuals.

Hyperhomocysteinemia is common in patients with antiphospholipid syndrome and may contribute to expression of major thrombotic events

Homocysteine plasma levels were measured by high-performance liquid chromatography in 52 patients with primary antiphospholipid syndrome (APS). Elevated homocysteine concentrations (>or= 15 micromol/l) were found in 16/52 (30.8%) APS patients. Elevated homocysteine levels were found to be associated with an increased risk for a major thromboembolic event (50 versus 16.7%, P = 0.014). Hyperhomocysteinemia is a common finding in APS patients, and may contribute to severity of thrombotic tendency observed in this syndrome.