Paradoxical absence of a prothrombotic phenotype in a mouse model of severe hyperhomocysteinemia

Abstract

Hyperhomocysteinemia confers a high risk for thrombotic vascular events, but homocysteine-lowering therapies have been ineffective in reducing the incidence of secondary vascular outcomes, raising questions regarding the role of homocysteine as a mediator of cardiovascular disease. Therefore, to determine the contribution of elevated homocysteine to thrombosis susceptibility, we studied Cbs ؊/؊ mice conditionally expressing a zinc-inducible mutated human CBS (I278T) transgene. Tg-I278T Cbs ؊/؊ mice exhibited severe hyperhomocysteinemia and endothelial dysfunction in cerebral arterioles. Surprisingly, however, these mice did not display increased susceptibility to arterial or venous thrombosis as measured by photochemical injury in the carotid artery, chemical injury in the carotid artery or mesenteric arterioles, or ligation of the inferior vena cava. A survey of hemostatic and hemodynamic parameters revealed no detectible differences between control and Tg-I278T Cbs ؊/؊ mice. Our data demonstrate that severe elevation in homocysteine leads to the development of vascular endothelial dysfunction but is not sufficient to promote thrombosis. These findings may provide insights into the failure of homocysteinelowering trials in secondary prevention from thrombotic vascular events. (Blood. 2012;119(13):3176-3183) IntroductionHyperhomocysteinemia is defined as an elevation of plasma total homocysteine (tHcy) 1 and is an independent risk factor for arterial and venous vascular diseases. [2][3][4] The classic form of severe hyperhomocysteinemia arises from a rare homozygous deficiency in cystathionine ␤-synthase (CBS) which metabolizes homocysteine to cystathionine. Patients with severe hyperhomocysteinemia have plasma tHcy levels Ͼ 200M and, if untreated, ϳ 50% suffer from life-threatening thromboembolic events before the age of 30. 5 Treatment with B vitamins and betaine in conjunction with restriction of methionine improves vascular outcomes, 6 though in most patients this intervention only partially lowers tHcy levels. Therefore, the causal relationship between elevated tHcy and vascular events remains poorly defined.Mild to moderate hyperhomocysteinemia, with plasma tHcy levels of 15-50M, occurs more commonly than severe hyperhomocysteinemia and is also associated with elevated risk for myocardial infarction, stroke, and thrombosis. [2][3][4] Despite this established clinical association, treatments to lower tHcy with B vitamins paradoxically fail to decrease secondary vascular events in these patients. [7][8][9][10][11][12] This lack of benefit of homocysteine-lowering therapy is in contrast to patients with severe hyperhomocysteinemia and raises further questions regarding whether homocysteine is a mediator or biomarker of vascular disease.Both genetic and dietary animal models of hyperhomocysteinemia have been used to examine the vascular effects of elevated tHcy levels. Mouse models with genetic deficiency in CBS have been demonstrated to have endothelial dysfunction. [13][14][15] With dietary induction of hype...

S100A10: A Key Regulator of Fibrinolysis

“…However, CBS deficiency did not result in a prothrombotic phenotype. In fact the CBS-null animals displayed normal rates of fibrinolysis [59]. Since these authors were unable to reproduce the findings of the Hajjar study [57,60], they speculated that the prothrombotic phenotype observed in diet-induced hyperhomocysteinemia might not be due to elevated homocysteine but possibly due to other dietary metabolites.…”

Betaine supplementation is less effective than methionine restriction in correcting phenotypes of CBS deficient mice

“…However, it has been reported to have decreased clotting times in a tail bleeding assay and that this effect is reversible by short-term betaine treatment. No differences in clotting behavior have been observed in Tg-I278T Cbs −/− mice (Gupta et al 2009; Dayal et al 2012). It is unclear why the two models behave so differently, although strain background differences are a distinct possibility.…”

Protein methionine oxidation augments reperfusion injury in acute ischemic stroke

“…Susceptibility to thrombosis in the venous system was measured as described previously (72). Briefly, mice were anesthetized with ketamine/xylazine (87.5 mg/kg ketamine and 12.5 mg/kg xylazine i.p.).…”