Investigations of coagulation and fibrinolysis in homocystinuria

Homocysteine activates platelets in vitro

The mechanism of thrombogenicity in hyperhomocysteinemia remains controversial. The authors investigated the association between elevated plasma homocysteine levels, platelet function, and blood coagulation. Blood was collected from healthy subjects and patients with critical limb ischemia. Basal platelet counts and platelet aggregation as well as flow cytometry were performed to assess spontaneous- and agonist-induced platelet aggregation as well as P-selectin and Glycoprotein IIb/IIIa expression at different homocysteine concentrations. Thromboelastography was performed, and platelet shape change was assessed, using a channelyzer, by measuring median platelet volume. Lactate dehydrogenase was measured, to indirectly assess red blood cell membrane integrity, after homocysteine exposure. The study results suggest that platelet activation and hypercoagulability occur after exposure to homocysteine, especially in patients with critical limb ischemia. Homocysteine concentrations of approximately 50 micromol/L appear to be the level at which these changes occur in vitro, and this effect on platelets appears to be indirect.

Plasma homocysteine levels and late outcome in patients with unstable angina

AIM OF THE STUDY

Previous studies have suggested that total plasma homocysteine (HCY) is an important cardiovascular risk factor because of its interaction with vascular smooth muscle cells, endothelium function, plasma lipoprotein, coagulation factors and platelets. The aim of this study was to evaluate a possible relationship between HCY levels and the severity of coronary artery disease (CAD) and its prognostic value in patients with unstable angina (UA).

METHODS AND RESULTS

Ninety-four patients with UA were recruited and underwent coronary angiography and in some cases myocardial revascularization. The primary end point was the severity of CAD. The clinical end points were the recurrence of UA and the compositive end point of the occurrence of cardiac death and re-hospitalization due to acute coronary syndrome. HCY levels were shown to be poorly correlated with the severity of CAD. After 48 months' of follow-up, a graded relationship between HCY levels and recurrence of UA and compositive end point was found (p < 0.001).

CONCLUSIONS

In the light of events occurring during the follow-up period, it was concluded that total plasma HCY is a strong predictor of recurrence of UA.

Platelet activity and in vivo arterial thrombus formation in rats with mild hyperhomocysteinaemia

Elevated plasma levels of total homocysteine (hcy) have been associated with an increased occurrence of arterial thrombosis. In the present study, we investigated the influence of hyperhomocysteinaemia on platelet aggregation and arterial thrombus formation in vivo. Fifty-one rats were included in the study, of which 29 received hcy in the drinking water for 4 weeks. Blood samples were withdrawn for measurement of platelet count and mean platelet volume. Platelet aggregation response in platelet-rich plasma following adenosine diphosphate or collagen stimulation were examined. In vivo thrombus formation was investigated by transillumination and videotape recording of the rat femoral artery after a thrombogenic injury was established. Off-line videotape analysis using computer-assisted planimetry permitted quantification of the thrombus area, and area versus time curves were obtained. In the intervention group receiving hcy, total hcy in plasma increased two-fold to 14.3 micromol/l, as compared with 7.3 micromol/l in the control group (P < 0.001). The platelet count and mean platelet volume did not differ between the two groups. In vivo thrombus formation expressed as the area under the curve or maximum thrombus area was not found to be altered in the presence of an increased homocysteine level, neither was adenosine diphosphate-induced platelet aggregation. However, collagen-induced platelet aggregation significantly decreased in the hcy group (P = 0.02). Pro-thrombotic effects of isolated mild hyperhomocysteinaemia are not supported by the present study in rats.

Homocysteine and atherosclerosis

Elevated plasma total homocysteine is an independent risk factor for atherosclerotic vascular disease. Risk rises continuously across the spectrum of homocysteine concentrations and may become appreciable at levels greater than 10 mumol/l. A compelling case can be made for screening all individuals with atherosclerotic disease or at high risk. A reasonable, but unproven, goal for treatment is a plasma total homocysteine concentration less than 10 mumol/l. Folic acid is the mainstay of treatment, but vitamins B12 and B6 may have added benefit in selected patients. The results of ongoing randomized placebo-controlled trials will not be available for several years, but will help determine whether homocysteine lowering reduces the risk of cardiovascular disease.

Homocysteine and coronary atherosclerosis

Homocysteine is increasingly recognized as a risk factor for coronary artery disease. An understanding of its metabolism and of the importance of vitamins B6 and B12 and folate as well as enzyme levels in its regulation will aid the development of therapeutic strategies that, by lowering circulating concentrations, may also lower risk. Possible mechanisms by which elevated homocysteine levels lead to the development and progression of vascular disease include effects on platelets, clotting factors and endothelium. This review presents the clinical and basic scientific evidence supporting the risk and mechanisms of vascular disease associated with elevated homocysteine concentrations as well as the results of preliminary therapeutic trials.

Homocysteine and vascular disease

There is increasing evidence that a raised blood level of homocysteine (HC) is a risk factor for premature atherosclerosis. With a gene frequency between one in 70 and one in 200 this condition may be more common than previously thought. It should be suspected especially in young patients in whom other risk factors are absent. The diagnosis may be made by demonstrating raised plasma HC levels, either basally or after methionine loading. Studies have shown significantly increased levels of HC in patients with premature coronary artery, peripheral vascular and cerebrovascular disease. The mechanisms by which HC produces vascular damage are, as yet, not completely understood but endothelial injury is probably a central factor. The principle of treatment is to lower HC levels in the blood by administration of vitamin B6, vitamin B12, folate or betaine. How effective this strategy will be in preventing complications is not yet known.

Adverse vascular effects of homocysteine are modulated by endothelium-derived relaxing factor and related oxides of nitrogen

Elevated levels of homocysteine are associated with an increased risk of atherosclerosis and thrombosis. The reactivity of the sulfhydryl group of homocysteine has been implicated in molecular mechanisms underlying this increased risk. There is also increasingly compelling evidence that thiols react in the presence of nitric oxide (NO) and endothelium-derived relaxing factor (EDRF) to form S-nitrosothiols, compounds with potent vasodilatory and antiplatelet effects. We, therefore, hypothesized that S-nitrosation of homocysteine would confer these beneficial bioactivities to the thiol, and at the same time attenuate its pathogenicity. We found that prolonged (> 3 h) exposure of endothelial cells to homocysteine results in impaired EDRF responses. By contrast, brief (15 min) exposure of endothelial cells, stimulated to secrete EDRF, to homocysteine results in the formation of S-NO-homocysteine, a potent antiplatelet agent and vasodilator. In contrast to homocysteine, S-NO-homocysteine does not support H2O2 generation and does not undergo conversion to homocysteine thiolactone, reaction products believed to contribute to endothelial toxicity. These results suggest that the normal endothelium modulates the potential, adverse effects of homocysteine by releasing EDRF and forming the adduct S-NO-homocysteine. The adverse vascular properties of homocysteine may result from an inability to sustain S-NO formation owing to a progressive imbalance between the production of NO by progressively dysfunctional endothelial cells and the levels of homocysteine.

Cerebral infarction after caesarean section due to heterozygosity for homocystinuria; a case report

Eleven days after a caesarean section cerebral infarction was diagnosed. The underlying cause appeared to be a heterozygous form of homocystinuria. This inborn error in methionine metabolism increases the risk of premature arteriosclerosis, even in heterozygotes. The biochemical abnormalities can be prevented in most of the patients by treatment with vitamin B6 (pyridoxine).

Blood coagulation changes in homocystinuria: effects of pyridoxine and other specific therapy

The aim of this study was to investigate the blood coagulation changes in three patients with homocystinuria, in baseline condition and during therapy. At baseline, antithrombin III activity and factor VII levels were reduced in all three patients; antithrombin III protein and protein C antigen were also slightly lowered in one patient, and factor X in another. beta-Thromboglobulin, a measure of platelet activation, was increased in one case. During pyridoxine treatment, antithrombin III activity was rapidly restored to normal; factor VII increased and beta-thromboglobulin decreased. These data suggest that, in addition to platelet activation, abnormalities of blood clotting, and particularly reduction of antithrombin III, may play a role in the thrombotic tendency associated with homocystinuria. The nature of these clotting alterations is still uncertain, but their improvement during active metabolic treatment suggests that the defect in amino acid transsulfuration of homocystinuria may directly affect synthesis or activity of some liver-dependent clotting factors.

Ectopia lentis

Ectopia lentis was first described more than 200 years ago, but its value as a significant diagnostic clue to the presence of other ocular and systemic disorders has only recently been appreciated. Ectopia lentis may cause a marked reduction in visual acuity, which varies with the type and degree of dislocation and the presence of other ocular abnormalities. An approach to the diagnostic evaluation of the patient with ectopia lentis is outlined. The differential diagnosis is reviewed in detail both clinically and histologically. The complications of ectopia lentis and the appropriate management of affected patients are discussed.

Kinetics and distribution of 111Indium-labeled platelets in patients with homocystinuria

Homocystinuria is an inborn error of metabolism involving a high incidence of thromboembolism. It sometimes improves with large doses of pyridoxine. We investigated the kinetics and distribution of 111Indoxine-labeled platelets in 11 normal volunteers and 12 patients with homocystinuria, none of whom had clinical evidence of acute thrombosis at the time of the study. Six of the patients were resistant to pyridoxine and had homocystinemia. There were no statistical differences in mean platelet-survival times between pyridoxine responders and nonresponders or between normal subjects and pyridoxine responders or nonresponders, regardless of whether a linear, exponential, or multiple-hit model was used to analyze the kinetic data. Plasma homocystine levels had no apparent effect on mean platelet-survival time. There was no abnormal accumulation of platelets in any of the patients, and the distribution of platelets in liver and spleen was similar to that in normal subjects. Our results suggest that the kinetics and distribution of platelets in patients with homocystinuria who have no clinical evidence of thromboembolism are normal. Thus, the data do not provide evidence for disordered platelet function or for an ongoing interaction of platelets with vessel walls in this condition.

Platelet survival and morphology in homocystinuria due to cystathionine synthase deficiency

In homocystinuria due to cystathionine synthase deficiency thromboembolism is a major cause of mortality and morbidity. Recent studies by others identified an abnormally shortened platelet survival and increased platelet vacuolization in patients with homocystinuria. When we studied six additional patients, however, we found the platelet survival to be within normal limits for each. The mean survival (+/-1 S.D.) was 9.75+/-0.94 days (normal, 9.27+/-1.06). In addition, platelets from five patients with homocystinuria and three obligate heterozygotes could not be distinguished from those of seven normal control subjects by electron microscopy. Specifically, no increased vacuolization was observed. Genetic heterogeneity, technical differences of differences in plasma homocystine concentrations could account for these descrepant results. The mechanism of thrombosis in homocystinuria remains an open question.