HOMOCYSTEINE: THE SUBTLE CULPRIT

Hussien H. Rizk, MD

Homocysteine � An intermediary amino acid in conversion of

methionine to cysteine. � Homocystinuria (severe hyperhomocysteinemia): rare

autosomal recessive disorder. � Severe elevations in plasma & urine homocysteine. � Clinical manifestations: developmental delay,

osteoporosis, ocular abnormalities, thromboembolic disease, and severe premature atherosclerosis.

� Less marked elevations in homocysteine are much more common, 5-7% of the population.

� Unassociated with the clinical stigmata of homocystinuria

� Moderate hyperhomocysteinemia is an independent risk factor for atherosclerotic vascular disease and recurrent venous thromboembolism

[Ueland & Refsom. J Lab Clin Med 1989; 114:473; McCully. Nat Med 1996; 2:386]..

� Homocysteine is metabolized by� Transsulfuration to cysteine: catalyzed by

cystathionine-ß-synthase, needs vit. B6. � Remethylation to methionine: catalyzed by:

� methionine synthase (needs vit. B12)� betaine-homocysteine methyltransferase.

Causes of hyperhomocysteinemia

� Genetic enzyme defects [Andersson et al. Eur J Clin Invest 1992; 22:79] � Nutritional vit. Deficiencies [D'Angelo et al. Thromb Haemost 2000; 83:563]

� Other factors: � Chronic medical conditions (CRF)

[Mezzano et al. Thromb Haemost 1999; 81:913]

� Drugs� Methotrexate [Refsum et al. Cancer Res 1986;

46:5385]

� Trimethoprim [Smulders et al. Lancet 1998; 352:1827]

� Other: Niacin, Bile acid sequestrants

� Smoking [Bazzano et al. Ann Intern Med 2003; 138:891]

Thermolabile variant of MTHFR� Most common genetic hyperhomocysteinemia. � Thermolabile variant of methylene tetrahydrofolate

reductase (MTHFR), reduced activity (T mutation)

[Kang et al. Am J Hum Genet 1991; 48:536] � TT mutation in 5-14% of the population.

[Gallagher et al. Circulation 1996; 94:2154]

� Of 625 men: Prevalence of TTAll subjects 11.5%

Top 10% plasma homocysteine 36.0%

Top 5% plasma homocysteine 48.0%[Harmon et al. QJM 1996; 89:571]

Vitamin deficiencies� Folate & B12 are particularly strong determinants. � Homocysteine level drops with rising folate

consumption, to a plateau at intake of 400 µg/day.Robinson et al. CIrculation 1998; 97:437

Rimm et al. JAMA 1998; 279:359Voutilainen et al. Circulation 2001; 103:2674

Vermeulen et al. Lancet 2000; 355:517

� In a cohort of 1000 elderly: [Selhub et al. JAMA 1993; 270:2693]

� 2/3 of hyperhomocysteinemia: low folate, B12, or B6. � Low B12 was more common than in younger cohorts. � B12 deficiency is more important in older age. � Folate deficiency is prevalent in the general population,

particularly with alcohol consumption.

FDA regulation of folic acid supplementation of grain products (1997) led to significantly higher blood folate and lower homocysteine concentrations.

Marked hyperhomocysteinemia (>13 µmol/L) was reduced by 50%

[Jacques et al. NEJM 1999; 340:1449]

HOMOCYSTEINE HAS ATHERO-THROMBOTIC

PROPERTIES

� Intimal thickening� Elastic lamina disruption� Smooth muscle hypertrophy� Marked platelet accumulation� Platelet-enriched occlusive thrombi

Harker et al NEJM 1974; 291:537

Stuhlinger et al. Circulation 2001; 104:2569

Mechanisms of vascular injury� Promotes leukocyte recruitment & activation. � Metabolite of homocysteine aggregates with LDL and

is taken by macrophages.� Increases smooth muscle proliferation & collagen.� Free radicals (during the oxidation of homocysteine)

directly injure endothelium.� Platelet accumulation: proaggregatory effects of

homocysteine + endothelial impairment.� Prolonged exposure of endothelial cells to

homocysteine impairs the production of NO.

Prothrombotic mechanisms

� Attenuation of endothelial cell tPA binding� Activation of factor VIIa & V� Inhibition of protein C & heparin sulfate� Increased fibrinopeptide A & prothrombin fragments 1

and 2� Increased blood viscosity� Decreased endothelial antithrombotic activity due to

changes in thrombomodulin function [Nappo et al. JAMA 1999; 281:2113].

[Al-Obaidi et al. Circulation 2000; 101:372][Thambyrajah et al. JACC 2001; 37:1858]

Hyperhomocysteinemia impairs DNA methylation leading to altered gene expression (Cytosine bases, Coiling)

[van Guldener. Lancet 2003; 361:1668]

An alternate view

Laboratory diagnosis

� Normal concentration: 5-15 µmol/L. � Hyperhomocysteinemia classified to:

� Moderate (15-30 µmol/L)� Intermediate (30-100 µmol/L)� Severe (>100 µmol/L)

[Kang et al. Annu Rev Nutr 1992; 12:279]

� Oral methionine challenge (100 mg/kg) in ? Cases.� Homocysteine measured fasting, 4 & 8 H later. � Significance of oral methionine challenge is doubted.

Role in vascular disease

� Meta-analysis of 30 studies � (5073 CAD & 1113 stroke events) � After adjustment for known CVD risk factors,

a 25% lower homocysteine was associated with a lower risk of:

� CAD (OR 0.89, 95% CI 0.83-0.96) � Stroke (OR 0.81, 95% CI 0.69-

0.95)[JAMA 2002; 288:2015]

TT-MTHFR & vascular events

� Meta-analysis of 40 studies � (11,162 TT-MTHFR + 12,758 matched

controls� TT genotype associated with: � 16% higher risk of CAD (95% CI 5-28%). � Increased risk of silent brain infarcts

[Klerk et al. JAMA 2002; 288:2023][Kohara et al. Stroke 2003; 34:1130]

Hyperhomocysteinemia linked to:� MI and other ACS [Stampfer et al. JAMA 1992; 268:877]

� Premature CAD [Schwartz et al. Circulation 1997; 96:412]

� CVD mortality [Nygard et al. NEJM 1997; 337:230] � Total mortality [Hoogeveen et al. Circulation 2000; 101:1506] � Adverse outcomes Post-PCI [Schnyder et al. JACC 2002; 40:1769]

� Carotid artery stenosis [Selhub et al. NEJM 1995; 332:286]

� Stroke [Tanne et al. Stroke 2003; 34:632] � Recurrent stroke [Boysen et al. Stroke 2003; 34:1258]

� Heart failure [Vasan et al. JAMA 2003; 289:1251]

Nygård, O. et. al. NEJM 1997;337:230-237

Dose-responserelationship

between plasmahomocysteine &

mortality

Nygård, O. et. al. NEJM 1997;337:230-237

Survival of CAD patients by plasmahomocysteine level

Selhub, J. et. al. NEJM 1995;332:286-291

Age-adjusted Prevalence of Maximal Extracranial Carotid Stenosis of >25%, by Quartile of plasma Homocysteine

MenWomen

Homocysteine level, vitamin status and CVD

Graded inverse relation of dietary folate & B6 vs CAD. � The highest quintile of intake had the greatest RRR (40%) � Intake in the lowest risk group was > 545 mcgm folate/D and > 5.9

mg B6/D (above the current RDA: 400 mcgm/D & 1.6 mg/D)

� Benefits of folate supplementation greatest in women consuming the most alcohol.

� No association between dietary B12 & CADThe Nurses' Health Study [Rimm et al. JAMA 1998; 279:359]

Role in venous thrombo-embolism� Meta-analyses of case-control studies: OR of 2.5-2.95

for DVT-PE with homocysteine >2 SD of the mean control value

Ray. Arch Intern Med 1998; 158:2101den Heijer et al. Thromb Haemost 1998; 80:874

den Heijer et al. NEJM1996; 334:759

Recurrent DVT more likely with hyperhomocysteinemia (18.2% vs 8.1%)

[Eichinger et al. Thromb Haemost 1998; 80:566]

Some [Ridker et al. Circulation 1997; 95:1777] but not all [De Stefano et al. Semin

Thromb Hemost 2000; 26:305] studies suggested the risk of DVT is 10-50 X with hyperhomocysteinemia + inherited thrombophilia (eg, factor V Leiden)

Obstetric complications

� Thermolabile MTHFR was linked to severe pre-eclampsia, abruptio placentae, fetal growth restriction, and stillbirth, which are associated with inadequate placental perfusion

[Kupferminc et al. NEJM 1999; 340:9]

� This adds to the risk of neural tube defects with folate deficiency (diet, alcohol, or antibodies to folate receptors)

Arguments against population screening� Dx of TT genotype of MTHFR unlikely to be cost-

effective. From prevalence (11%) & RR (1.16), the population attributable risk is 1-2% [Wilson JAMA 2002; 288:2042]

� Adequate folic acid intake reduces the impact of the TT� Benefit of lowering homocysteine on CVD & DVT

remains unproven [Stampfer & Malinow. NEJM 1995; 332:328] � Even if treatment is beneficial, it may be more cost

effective to recommend a daily multivitamin, there may be other benefits as well.

Lowering Homocysteine in Patients With Ischemic Stroke to Prevent Recurrent Stroke, MI, & Death

The Vitamin Intervention for Stroke Prevention (VISP) Randomized Controlled Trial

Toole et al. JAMA. 2004;291:565.

� 3680 pts, cerebral infarction, 56 centers (US, Canada, Scotland. � Randomly assigned to QD:

� high-dose (n = 1827) [25 mg B6, 0.4 mg B12, & 2.5 mg FA]� low-dose (n = 1853), [200 µg B12, 6 µg B12 & 20 µg FA].

� FU: 2Y� Results:

� Mean reduction of homocysteine: 2 µmol/L greater with high-dose.� No treatment effect on any end point.

Who to screen?

� Patients with premature atherosclerotic disease and absent conventional risk factors.

� Otherwise unexplained venous thrombosis. � AHA [Circulation 1999; 99:178] recommend screening in

malnutrition, malabsorption, hypothyroidism, CRF, SLE, medications ( niacin, theophylline, bile acid resins, methotrexate, L-dopa).

Treatment

� Hyperhomocysteinemia: � Folic acid, vitamin B12, and vitamin B6. � Diet rich in fruits, vegetables, low-fat dairy, and

low in saturated and total fat can lower homocysteine [Appel et al. Circulation 2000; 102:852]

� Cardiovascular disease� Randomized trial: daily FA 1 mg + B12 400 µg + B6 10 mg. � 553 pts undergoing PCI � 29% had baseline homocysteine >12 µmol/L, � mean FU 11 m� Primary EP: death + nonfatal MI + need for repeat PCI.

Comarator Vit suppl.(%) Placebo (%)Composite EP 15.4 22.8Target lesion revascularization 9.9 16.0Death 1.5 2.8 NSNon-fatal MI 2.6 4.3 NS

[Schnyder et al. JAMA 2002; 288:973]

Current recommendations� Treat with premature CAD: FA (1 mg/D) + B6 (10 mg/D) + B12

(0.4 mg/D). � Always add B12 (to avoid masking pernicious anemia &

precipitating PN). � Increase dose of FA up to 5 mg/D as needed. � With homocysteine >30 µmol/L or with CRF the initial dose of

folic acid is 5 mg/D.� Normalization of the homocysteine occurs in 2 w, further

lowering occurs by 6 w [Brattstrom et al. Atherosclerosis 1990; 81:51].

� Refractory cases: trimethylglycine (750 mg bid).

Summary � Screen:

� Premature CVD without conventional risk factors � Unexplained venous thrombosis.

� Treat: � Premature CAD:

• Give a B complex vitamin. • Normalization of homocysteine expected in 2-6 w. • Increase folic acid up to 5 mg/day as needed. • With homocysteine >30 µmol/L or CRF, start folic acid at

5 mg/D.

� Adults at average risk: • Daily B complex vitamin.