Homocysteine as a Risk Factor in Atherosclerotic Cardiovascular Disease

Nomenclature

Lack of standardization and wide variation in the literature as to what constitutes abnormally elevated levels of homocysteine make direct comparison among studies difficult and might account for some of the variation in evidence for the clinical implications of hyperhomocysteinemia. It is present as protein (albumin)-bound, free circulating disulfide, and sulfhydryl forms. Current laboratory methods detect all three forms and report this as total homocysteine concentration.

In general, reference intervals published for clinical practice are not corrected for factors known to influence circulating homocysteine levels (e.g., age, ethnicity, gender) or protein-rich diets. In particular, methionine is found in high concentrations in red meat, and as the substrate for the homocysteine reactions, it directly influences homocysteine levels. In fact, a methionine-load test may be used to measure homocysteine levels in high-risk patients with normal basal levels of homocysteine to identify patients with post-load hyperhomocysteinemia. This test has been shown to uncover up to 39% of persons with homocysteine-related cardiovascular disease risk but with normal basal homocysteine levels.

Ideally, homocysteine should be measured when the patient is fasting. Many studies have simply used any value above the 95th percentile for their control group, leading to the suggested cut-off point varying from 9 to 15 μmol/L. According to the American Heart Association (AHA) advisory statement, normal fasting homocysteine concentrations range from 5 to 15 μmol/L. Intermediately elevated homocysteine levels are between 31 and 100 μmol/L, and severely elevated levels are greater than 100 μmol/L. Severely elevated levels are essentially pathognomonic for the presence of an inborn error of homocysteine metabolism, causing homocystinuria.

Pathophysiology

Homocysteine is a sulfhydryl-containing amino acid produced from the metabolism of the essential amino acid methionine. Homocysteine can undergo auto-oxidation, resulting in the formation of key biologically reactive products that participate in signaling pathways associated with increased cell toxicity. Homocysteine has been identified as a contributor to four fundamental mechanisms of disease: thrombosis, oxidant stress, apoptosis, and cellular proliferation.

There are several proposed mechanisms by which homocysteine could inflict vascular injury. Homocysteine administration has been shown to cause endothelial cell injury, in both in vitro and in vivo experimental models. Homocysteine can cyclize under acidic conditions to form homocysteine thiolactone. Hydroxyl radicals (OH ⁻ , OH•), and superoxide anions (O ₂ ⁻ ) are byproducts of these reactions. The superoxide anion can be converted to H ₂ O ₂ in the presence of superoxide dismutase or spontaneously undergoes dismutation to H ₂ O ₂ . Homocysteine can also promote oxidant stress by directly impairing glutathione peroxidase expression (Gpx-1), an antioxidant enzyme that reduces H ₂ O ₂ to water. Homocysteine-induced formation of reactive oxygen species decreases levels of bioavailable nitric oxide (NO) either by reducing the availability of key NOS cofactors, such as tetrahydrobiopterin (BH ₄ ) or by inducing conversion of NO to peroxynitrite (ONOO ⁻ )

By inducing oxidative stress to the endothelium, homocysteine reduces bioavailability of NO. It can also generate free radicals and inhibit the production of other antioxidants. Endothelial injury in turn results in platelet aggregation and thrombus formation. Furthermore, it impairs endothelial-mediation vasodilation and control of vascular tone. Toxic endothelial damage is also related to the stimulation of smooth muscle cell proliferation and susceptibility to oxidation of low-density lipoproteins.

Another mechanism by which homocysteine can induce vascular injury is the increased thrombogenicity mediated by increased platelet adherence and the release of platelet-derived growth factors; activated factor V, X, and XII; inhibition of protein C activation; inhibition of cell surface expression of thrombomodulin; and decreased tissue plasminogen activator activity. Homocysteine has also been thought to increase arterial stiffness by damaging elastin fibers, increasing collagen production, and stimulating smooth muscle activity

Homocysteine metabolism occurs via three pathways: remethylation of homocysteine to form methionine by methionine synthase in a vitamin B ₁₂ - and folate-dependent reaction; the trans-sulfuration pathway, in which, after the addition of a serine group, homocysteine is converted to cystathionine by cystathionine β-synthase (CBS), requiring vitamin B ₆ as a cofactor; and in certain tissues such as liver and kidney by remethylation of homocysteine to methionine via betaine–homocysteine methyltransferase (BHMT).

Mutations in the 5,10-methyl-tetrahydrofolate reductase (MTHFR) and cystathionine β-synthase genes impair homocysteine conversion to methionine and cystathionine, respectively. In particular, a highly prevalent C677T point mutation has been associated with a thermolabile MTHFR variant. It is estimated that between 5% and 12% of the white population may be homozygous for this genotype, which results in a reduction of MTHFR activity and increased levels of plasma homocysteine. However, several studies on the clinical implications of genotype status have been inconclusive and the exact independent effects of genotype status are not yet known.

More commonly, elevated levels of homocysteine can result from nutritional deficiencies of folic acid, vitamin B ₆ , and vitamin B ₁₂ , which are key enzyme cofactors required for normal homocysteine metabolism. Mild hyperhomocysteinemia levels are seen in about 5% to 12% of the general population, and in specific populations such as alcoholic patients (as a result of poor vitamin intake), those who smoke cigarettes, or patients with chronic kidney disease this may be more common. Specifically, homocysteine levels increase markedly with age, and hyperhomocysteine rates are estimated to be as high as 30% to 40% in the elderly population.