Many of the murine models described above have been utilized to study the vascular pathophysiology of hyperhomocysteinemia (). Because of the poor growth and survival of mice with homozygous defects in the Cbs
, or Mtr
genes, most investigators have chosen to study mice with heterozygous Cbs
gene defects, often in combination with dietary approaches to produce hyperhomocysteinemia. Despite the considerable variability in the diets and genotypes used to produce hyperhomocysteinemia, several consistent vascular phenotypic effects have been observed in these studies.32, 81
Vascular phenotypes of hyperhomocysteinemic mice.
The most commonly observed vascular abnormality in murine models of hyperhomocysteinemia is endothelial vasomotor dysfunction due to impaired bioavailability of endothelium-derived nitric oxide.81
Endothelial vasomotor dysfunction has been observed in large arteries, such as the aorta and carotid artery, as well as in smaller vessels, such as mesenteric, cremasteric, and cerebral arterioles. Susceptibility to endothelial impairment may vary with the degree of hyperhomocysteinemia. Smaller arterioles appear to be susceptible to endothelial dysfunction in the presence of mild hyperhomocysteinemia, whereas large arteries tend to exhibit functional abnormalities only in the presence of higher concentrations of plasma tHcy.39-42, 48, 58, 62, 63
Investigators interested in studying endothelial function in large conduit arteries such as the aorta or carotid artery generally should utilize a dietary or genetic model that produces plasma tHcy levels of 20 μmol/L or higher (). On the other hand, studies of endothelial function in small resistance vessels, such as mesenteric or cerebral arterioles, can be readily performed using murine models that produce plasma tHcy concentrations of 10 to 20 μmol/L.
In addition to abnormal vascular function, hyperhomocysteinemic mice also develop structural alterations of the vessel wall. Several studies have demonstrated vascular hypertrophy, remodeling, altered vascular mechanics, or increased stiffness of arteries or arterioles in mice with hyperhomocysteinemia.39, 49, 82, 83
Increased neointima formation in response to arterial injury also has been observed.38
Accelerated thrombosis of the carotid artery has been detected using the rose bengal photochemical thrombosis method33
in Cbs+/+, Cbs+/−,
mice fed a high methionine/low folate diet.69, 77
Several groups have utilized murine models to investigate the effects of hyperhomocysteinemia on the development and progression of atherosclerosis. None of the murine models of hyperhomocysteinemia described above has been observed to spontaneously produce advanced atherosclerotic lesions in the absence of hyperlipidemia. However, hyperhomocysteinemia has been observed to potentiate the development of atherosclerosis in susceptible strains of mice, such as hyperlipidemic Apoe
or C57BL/6 mice fed an atherogenic diet containing cholate.84
Hofmann et al. reported that Apoe
−/− mice fed a hyperhomocysteinemic diet developed atherosclerotic lesions in the aortic sinus that were of greater size and complexity than those seen in Apoe
−/− mice fed normal chow.37
Several other investigators, using a variety of dietary and genetic approaches to induce hyperhomocysteinemia in Apoe
−/− mice, have confirmed and extended the observations of Hofmann.46, 56, 57, 85, 86
An interesting study by Troen et al suggested that a high dietary intake of methionine may be atherogenic in Apoe
−/− mice, even in the absence of significant hyperhomocysteinemia.36
Taken together, the vascular phenotypes observed in multiple murine models of hyperhomocysteinemia strongly suggest that hyperhomocysteinemia, whether induced by dietary or genetic approaches, is a causative factor in the development of atherothrombotic vascular disease. It still remains an open question, however, whether it is homocysteine itself or a related metabolic factor that is the key etiological agent.