Hyperhomocysteinemia

INTRODUCTION

Homocysteine is a naturally occurring, sulfur containing amino acid formed during the metabolism of methionine, an essential amino acid derived from the diet.  The interconversion of methionine and homocysteine depends on the availability of the methyl donor 5-methyltetrahydrofolate, cofactors vitamin B₁₂ and folate, and the enzyme activity of methionine synthase.  Elevated intracellular homocysteine concentrations with corresponding increases in blood levels can result from augmented production or reduced metabolism.  Although severe hyperhomocysteinemia is rare, mild hyperhomocysteinemia occurs in approximately 5 to 7 percent of the general population.^1,2  Patients with mild hyperhomocysteinemia are asymptomatic until the third or fourth decade of life when premature coronary artery disease may develop, as well as recurrent arterial and venous thrombosis.

MEASUREMENT 

SPECIMEN REQUIREMENTS

Plasma homocysteine is measured on a morning specimen collected in an EDTA (lavender top) tube after an overnight fast.  Because homocysteine is continuously released by blood cells, the specimen must be centrifuged and the plasma separated immediately to avoid falsely elevated values.  Alternatively, the specimen can be placed on wet ice until it can be centrifuged.  Specimens that are not sent to the lab the same day must be spun down and the plasma frozen until testing is performed. 

METHODS

Chromatography (HPLC and gas) and enzyme immunoassay are the two main analytical methods used to measure homocysteine.  The latter method is simple, rapid, and has good to excellent performance data, making it suitable for routine lab analysis. Among the commercially available assays, the fluorescence polarization immunoassay is used extensively.^3,4

A methionine load challenge (100 mg/kg body weight oral dose of methionine) can be given to individuals with suspected hyperhomocysteinemia who have normal homocysteine concentrations on fasting specimens.  This procedure requires measurement of plasma homocysteine concentration before the methionine challenge and between four and eight hours afterward.⁵  The methionine challenge test cannot adequately assess thermolabile variants of the methyltetrahydrofolate reductase (MTHFR) protein and should be utilized when assessing enzymes of the transulfuration pathway (Cystathionine b-Synthase).

RESULTS

Using any of the standard analytical methods, values between 5 and 15 mmol/L are generally considered normal in the fasting state, albeit not optimal (<10 mmol/L).^6,7  Kang and coworkers have classified hyperhomocysteinemia as moderate (15 to 30 mmol/L), intermediate (>30 to 100 mmol/L) and severe (>100 mmol/L) on the basis of concentrations measured during fasting.⁸   Levels tend to increase with age.

CAUSES 

Elevations in plasma homocysteine are typically caused either by genetic defects in the enzymes involved in homocysteine metabolism or by nutritional deficiencies in vitamin cofactors.  Homocystinuria and severe hyperhomocysteinemia are caused by rare inborn errors of metabolism  (most commonly Cystathionine beta-synthase deficiency) resulting in marked elevations of plasma and urine homocysteine concentrations.  Deficiencies of the B complex vitamins and folate in particular can also cause large increases in homocysteine levels (exceeding 100 mmol/L).

More recently, two common polymorphisms of MTHFR (C677T and A1298C) have been shown to contribute to moderate hyperhomocysteinemia.^9,10  These mutations are associated with reduced MTHFR activity and thermolability, requiring increased levels of folate intake.  Homozygosity for the C677T mutation (9-17% population) and C677T/A1298C combined heterozygosity both have been associated with increased homocysteine levels and a mild prothrombotic tendency.

Nutritional deficiencies in the vitamin cofactors (folate, vitamin B₁₂ and vitamin B₆) required for homocysteine metabolism may also promote hyperhomocysteinemia.  It has been speculated that these types of nutritional deficiencies contribute to approximately two-thirds of all cases of hyperhomocysteinemia.¹¹  In addition to vitamin deficiencies, several therapeutic drugs (methotrexate, theophylline, cyclosporine and most anticonvulsants) and chronic disease states (liver and renal disease, hypothyroidism and malignancies) can lead to moderate hyperhomocysteinemia.

ASSOC. WITH VASCULAR DISEASE 

High homocysteine levels can damage blood vessels in several ways, including injury to arterial endothelial cells and promotion of smooth muscle growth, both of which result in lesions (plaques) that narrow the lumens of the affected vessels.  Increased homocysteine concentrations can also disrupt normal blood clotting mechanisms, increasing the risk of thrombi formation that can lead to heart attack or stroke.

A growing body of literature indicates that elevated homocysteine is a risk factor for coronary, cerebrovascular, and peripheral atherosclerotic disease, as well as arterial and venous thrombosis.  A homocysteine level above 15 mmol/L is associated with a significantly higher risk compared to lower levels.  Furthermore, plasma homocysteine is independent of, but interacts with, conventional coronary vascular disease (CVD) risk factors, enhancing their effect.

TREATMENT 

Vitamin therapy and dietary modification can work together to help lower plasma homocysteine levels.  Blood levels of homocysteine become elevated if the dietary folic acid intake is <250 mg per day, so dietary folic acid supplementation and maintaining an adequate intake of vitamins B₆ and B₁₂ is also recommended. Studies are currently in place to determine whether or not normalizing homocysteine levels will improve cardiovascular morbidity and mortality.

SUMMARY 

Homocysteine has been shown to be an independent risk factor for the development of vascular disease.  Homocysteine measurements should be included in the evaluation of individuals in high-risk groups.

These groups include patients with:

1)      Evidence of increased urinary homocysteine

2)      Premature arteriovascular disease

3)      Strong family history of:

a.      Myocardial infarction

b.      Peripheral vascular disease

c.       Stroke

d.      Recurrent pulmonary embolism

e.      Venous thrombosis

f.        Renal Failure

g.      Cardiac or renal transplant

For the regular use of homocysteine testing as a marker of CVD and the use of folic acid supplementation to prevent CVD, the medical community will have to wait for the results of the numerous ongoing outcomes studies.

REFERENCES

1. Ueland PM, Refsum H.  J. Lab Clin Med 1989; 114:473-501
2. McCully KS.  Nat Med 1996; 2: 386-389.
3. Blanco-Vaca F, Arcelus R, et al.  Clin Chem Lab Med 2000; 38: 327-329.
4. Fritsche-Polanz R, Huber A, et al. Laboratory Medicine 2003; 34: 538-542.
5. Dudman NP, Wilcken DE, et al. Arterioscler Thromb 1993; 13:1253-1260.
6. Ueland PM, Refsum H. et al.  Clin Chem 1993; 39: 1764-1779.
7. Jacobsen DW, Gatautis VJ, et al. Clin Chem 1994; 40: 873-881.
8. Kang SS, Wong PW, Malinow MR. Ann Rev Nutr 1992; 12: 279-298.
9. Frosst P, Blom HJ, et al. Nat Genet 1995; 10: 111-113.
10. Weisberg I, Tran P, et al. Molec. Genet 1998; 64: 169-172.
11. Selhub J, Jacques PF, et al. JAMA 1993; 270: 2693-2698.

Copyright ©2003, Institute For Transfusion Medicine 

Editor: Donald L. Kelley, M.D., MBA: dkelley@itxm.org