Hyperhomocysteinemia is a known risk factor for vascular disease and commonly occurs in the elderly. Several studies have shown an association between elevated plasma homocysteine levels and ...cognitive impairment, indicating that it may play a role in the pathophysiology of dementia. We studied plasma homocysteine, folate, vitamin B12 levels and the MTHFR C677T genotype in an Italian population of patients with dementia. We confirmed that elevated plasma tHcy (>14 μmol/l) is common in elderly subjects with dementia. Although we found a high prevalence of the MTHFR TT genotype (21.2%) the allele frequency is not over-represented relative to the control population. We also observed a high incidence of folate deficiency (38%) in subjects with dementia. Elevated homocysteine was associated with low plasma folate (<5.7 nmol/l) and the MTHFR TT genotype. Moderate to severe hyperhomocysteinemia (>26.1 nmol/l) was associated with a significantly lower MMSE score. Hyperhomocysteinemia may be neurotoxic by several different mechanisms affecting cognitive function. Further studies are needed to fully explore the potential of B vitamin supplementation to lower plasma homocysteine and improve cognitive function.
Hyperhomocysteinemia has been proposed to inhibit the protein C anticoagulant system through 2 mechanisms: decreased generation of activated protein C (APC) by thrombin, and resistance to APC caused ...by decreased inactivation of factor Va (FVa). We tested the hypotheses that generation of APC by thrombin is impaired in hyperhomocysteinemia in monkeys and that hyperhomocysteinemia produces resistance to APC in monkeys, mice, and humans. In a randomized crossover study, cynomolgus monkeys were fed either a control diet or a hyperhomocysteinemic diet for 4 weeks. Plasma total homocysteine (tHcy) was approximately 2-fold higher when monkeys were on the hyperhomocysteinemic diet than when they were on the control diet (9.8 ± 2.0 μM versus 5.6 ± 1.0 μM; P < .05). After infusion of human thrombin (25 μg/kg of body weight), the peak level of plasma APC was 136 ± 16 U/mL in monkeys fed the control diet and 127 ± 13 U/mL in monkeys fed the hyperhomocysteinemic diet (P > .05). The activated partial thromboplastin time was prolonged to a similar extent by infusion of thrombin in monkeys fed the control diet and in those fed the hyperhomocysteinemic diet. The sensitivity of plasma FV to human APC was identical in monkeys on control diet and those on hyperhomocysteinemic diet. We also did not detect resistance of plasma FV to APC in hyperhomocysteinemic mice deficient in cystathionine β-synthase (plasma tHcy, 93 ± 16 μM) or in human volunteers with acute hyperhomocysteinemia (plasma tHcy, 45 ± 6 μM). Our findings indicate that activation of protein C by thrombin and inactivation of plasma FVa by APC are not impaired during moderate hyperhomocysteinemia in vivo.
Deficiency of methylenetetrahydrofolate reductase (MTHFR) predisposes to hyperhomocysteinemia and vascular disease. We tested the hypothesis that heterozygous disruption of theMthfrgene sensitizes ...mice to diet-induced hyperhomocysteinemia and endothelial dysfunction.Mthfr+/-andMthfr+/+mice were fed 1 of 4 diets: control, high methionine (HM), low folate (LF), or high methionine/low folate (HM/LF). Plasma total homocysteine (tHcy) was higher with the LF and HM/LF diets than the control (P< .01) or HM (P< .05) diets, andMthfr+/-mice had higher tHcy thanMthfr+/+mice (P< .05). With the control diet, the S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) ratio was lower in the liver and brain ofMthfr+/-mice thanMthfr+/+mice (P< .05). SAM/SAH ratios decreased further inMthfr+/+orMthfr+/-mice fed LF or LF/HM diets (P< .05). In cerebral arterioles, endothelium-dependent dilation to 1 or 10 μM acetylcholine was markedly and selectively impaired with the HM/LF diet compared with the control diet for bothMthfr+/+(maximum dilation 5% ± 2% versus 21% ± 4%;P< .01) andMthfr+/-(6% ± 2% versus 21% ± 3%;P< .01) mice. These findings demonstrate that theMthfr+/-genotype sensitizes mice to diet-induced hyperhomocysteinemia and that hyperhomocysteinemia alters tissue methylation capacity and impairs endothelial function in cerebral microvessels.
Hyperhomocysteinemia is associated with increased risk for cardiovascular events, but it is not certain whether it is a mediator of vascular dysfunction or a marker for another risk factor. ...Homocysteine levels are regulated by folate bioavailability and also by the methyl donor S-adenosylmethionine (SAM) and its metabolite S-adenosylhomocysteine (SAH). We tested the hypotheses that endothelial dysfunction occurs in hyperhomocysteinemic mice in the absence of folate deficiency and that levels of SAM and SAH are altered in mice with dysfunction. Heterozygous cystathionine beta-synthase-deficient (CBS(+/-)) and wild-type (CBS(+/+)) mice were fed a folate-replete, methionine-enriched diet. Plasma levels of total homocysteine were elevated in CBS(+/-) mice compared with CBS(+/+) mice after 7 weeks (27.1+/-5.2 versus 8.8+/-1.1 micromol/L; P<0.001) and 15 weeks (23.9+/-3.0 versus 13.0+/-2.3 micromol/L; P<0.01). After 15 weeks, but not 7 weeks, relaxation of aortic rings to acetylcholine was selectively impaired by 35% (P<0.05) and thrombomodulin anticoagulant activity was decreased by 20% (P<0.05) in CBS(+/-) mice. Plasma levels of folate did not differ between groups. Levels of SAH were elevated approximately 2-fold in liver and brain of CBS(+/-) mice, and correlations were observed between plasma total homocysteine and SAH in liver (r=0.54; P<0.001) and brain (r=0.67; P<0.001). These results indicate that endothelial dysfunction occurs in hyperhomocysteinemic mice even in the absence of folate deficiency. Endothelial dysfunction in CBS(+/-) mice was associated with increased tissue levels of SAH, which suggests that altered SAM-dependent methylation may contribute to vascular dysfunction in hyperhomocysteinemia.
Methionine synthase (MS) is a key enzyme involved in remethylation of homocysteine to methionine. We tested the hypothesis that deficiency of MS influences endothelial function. Mice heterozygous for ...disruption of the Ms gene (Ms+/−), and wild type littermates (Ms+/+), were fed either a control diet or a low folate (LF) diet. Plasma total homocysteine was similar in Ms+/+ and Ms+/− mice fed the control diet (4.5±0.3 and 5.3±0.4 μmol/L, respectively), and was mildly elevated in Ms+/+ or Ms+/− mice fed the LF diet (7.5±0.7 and 9.6±1.2 μmol/L, respectively; p<0.001 vs. control diet). Dilatation of cerebral arterioles (~30 μm diameter) to the endothelium-dependent dilator, acetylcholine (10 μM), was blunted in Ms+/− mice compared with Ms+/+ mice fed the control diet (21±4 vs. 32±4%; p<0.05). Both Ms+/+ and Ms+/− mice exhibited impaired dilatation of cerebral arterioles to acetylcholine when they were fed the LF diet (12±2 and 17±3%, respectively; p<0.01 vs. Ms+/+ mice fed the control diet). Dilatation of cerebral arterioles in response to the endothelium-independent dilator, nitroprusside, was similar in all groups of mice. Relaxation of aortic rings to acetylcholine was not influenced by Ms genotype or diet. Elevated levels of superoxide and hydrogen peroxide were detected by dihydroethidium (DHE) and dichlorodihydrofluorescein (DCF), respectively, in cerebral arterioles of Ms+/− mice fed the control diet and in both Ms+/+ and Ms+/− mice fed the LF diet. These findings demonstrate that defective homocysteine remethylation caused by deficiency of either MS or folate produces endothelial dysfunction and increased oxidative stress in the cerebral microcirculation of mice. Interestingly, impairment of cerebral vascular function was independent of plasma tHcy concentration.
Homocysteine (Hcy), excitatory sulfur amino acids (ESAA), and methylation were investigated in humans and transgenic animals with hyperhomocysteinemia. Patients treated with methotrexate, had ...significantly increased Hcy and ESAA levels in plasma and CSF. Specifically, elevated levels of the ESAA compounds, homocysteic acid (HCA) and cysteine sulfinic acid (CSA) were present. Although patients with methylenetetrahydrofolate reductase (MTHFR), methionine synthase, and cystathionine-β-synthase (CBS) deficiency, and transgenic animals with targeted disruption of the MTHFR or CBS gene had severe hyperhomocysteinemia, no significant increase in HCA or CSA was detected. Accumulation of ESAA appears to be specific to MTX administration. Hcy and ESAA are neurotoxic through activation of N-methyl-D-aspartate receptors. This mechanism may be involved in neurological complications associated with hyperhomocysteinemia. Increased plasma Hcy levels were found in patients with dementia although this appears to be age related. Presence of the MTHFR C677T and methionine synthase A2756G mutations is associated with higher plasma Hcy levels especially in combination with folate deficiency. A genetic-nutrient interaction is a major determinant of mild hyperhomocysteineimia. CSF Hcy has also been studied and was not found to be significantly higher in the demented group overall. However, patients with moderate hyperhomocysteinemia (plasma Hcy>26μM) had significantly higher CSF Hcy values. Higher plasma Hcy values were associated with poorer measures of cognitive function as assessed by the Mini Mental State Examination and Clinical Dementia Rating. Transgenic animals deficient in MTHFR and CBS were studied in terms of Hcy and methylation. Both models had elevated levels of S-adenosylhomocysteine (SAH). S-adenosylmethionine (SAM) was decreased in MTHFR-deficient mice and increased in CBS-deficient mice. In both models the SAM/SAH ratio was decreased to varying degrees in different tissues. The MTHFR-deficient mouse had a decreased SAM/SAH ratio in liver and brain tissue, associated with hypomethylation of DNA. An altered SAM/SAH ratio may affect other methyltransferase reactions, and remains to be studied in the CBS-deficient mouse. Neuropathological changes were observed in both animal models although they differed histologically. Hyperhomocysteinemia in heterozygous CBS-deficient mice is associated with endothelial vascular dysfunction. Transgenic models may be useful to define the biochemical mechanisms involved in neurological and vascular damage associated with hyperhomocysteinemia.
