Download
1 / 35
E N D
HYPERHOMOCYSTEINEMIA IN CARDIOVASCULAR DISEASE M.Prasad Naidu MSc Medical Biochemistry, Ph.D,.
INTRODUCTION • About 30 years ago,Mc Cully postulated that mildly elevated homocysteine concentrations could increase the risk of cardiovascular disease after observing artery wall lesions in two different metabolic disorders of methionine metabolism which resulted in elevated plasma homocysteine concentrations. • many studies have been conducted to investigate elevated plasma homocysteine concentrations are associated with an increased risk of cardiovascular disease.
normal circulating plasma homocysteine level is less than 15µmol/l. • A modest elevation of plasma homocysteine concentration,commonly referred to as hyperhomocysteinaemia.*(>15µmol/l) • In blood homocysteine is measured widely by high pressure liqid chromatography.(HPLC). • In our lab it is measured by enzyme cycling method.it is cheaper compared to other methods like HPLC and ELISA. • In urine homocysteine level is measured by silver nitroprusside test.
Hyperhomocysteinaemia is an independent and risk factor for both arterial occlusive diseases and venous thrombosis. • In this review,reports are given on both environmental and genetic determinants of hyperhomocysteinaemia.
Determinants of homocysteine concentrations • Disturbance in intracellular homocysteine metabolism lead to elevated plasma homocysteine concentrations. • These defects can have a genetic background ,such as inherited enzyme deficiency,a variation in the genes encoding these enzymes or an environmental aetiology.
Renal function • A strong determinant of homocysteine concentration is kidney function. • Patients with renal failure have markedly elevated plasma homocysteine concentrations and urine homocysteine levels is very low. • It is due to homocysteine metabolism is influenced or even regulated by kidney function or that kidney itself converts a major amount of homocysteine present in blood. • Serum creatinine concentration is an indicator of altered renal function,so it is strongly positively associated with homocysteine concentrations.
Age and sex • Increasing age and male gender is associated with increased homocysteine concentrations. • Sex difference is explained by vitamin status,muscle mass and hormonal factors.
Dietary factors: • Dietary factors like follic acid and other B vitamins are important determinants of plasma homocysteine levels. • Both folic acid supplementation and a high dietary folate intake have been shown to decrease the plasma homocysteine levels and the relationship is dose responsive. • Regular users of multivitamin preparations also have lower levels since they contain vitamin B6 and cobalamin.
Lifestyle factors: • Coffee consumption and smoking associated with increased homocysteine concentrations. • Association between alcohol consumption and homocysteine appears j-shaped. • Alcoholics have high homocysteine concentrations,where as moderate alcohol users have lower homocysteine concentrations compared with non drinkers. • Effect of alcohol consumption dependent on type of alcoholic beverage.
Miscellaneous: • Several drugs like antiepileptic drugs,methotrexate or lipid-lowering drugs,shown to mild increase in plasma homocysteine concentrations. • Lack of exercise ,body mass index and cardiovascular risk factors like cholesterol and blood pressure are determinants of homocysteine concentrations.
Genetics of homocysteine • HOMOCYSTINURIA: • Hyperhomocysteinaemia or homocystinuria first described by Carson and Neil. • Most common cause of homocystinuria is CYSTATHIONE BETA SYNTHASE deficiency. • CLINICAL FEATURES: • Dislocation of optic lens • Osteoporosis • Marfanoid features • Mental retardation
Early onset of vascular disease • Most CBS deficient patients have clearly reduced CBS enzyme activity in extracts of cultured fibroblasts. • In addition to homocysteine,methionine accumulates in the body and is excreted via the kidney. • Two most frequent mutations are • 833T C (1278T) -- Netherlands • 919G A(G307S) -- Irelands
Reduced methionine synthase activity due to inborn errors of methylcobalamin transport or synthesis causes severe homocysteinaemia. • But these causes are very rare. • In methyl tetrahydrofolate reductase deficiency,clinical severity is correlated with degree of enzyme deficiency. • M.c clinical manifestation is developmental delay.
Moderate homocystinuria and homocysteinaemia,with low or relatively normal concentrations of plasma methionine are present. • The excretion of homocysteine is much less than in homocystinuria due to CBS deficiency.
HYPERHOMOCYSTEINAEMIA • Gene coding for the enzymes MTHFR,CBS and MS are possibilities because of their direct catalytic involvement in homocysteine and folate metabolism. • Other enzymes like BHMT,SHMT and MTHFD also cause hyperhomocysteinaemia. • Other cofactors involved in homocysteine metabolism are vitamin B6 ,B12 or folate are also important.
CYSTATATHIONINE BETA SYNTHASE • All genetic studies that have been performed to date have failed to detect any involvement of heterozygosity for CBS deficiency in hyperhomocysteinaemia and premature vascular disease. • 31-BASE PAIR VARIABLE NUMBER OF TANDEM REPEATS POLYMORPHISM • Kraus et al described complete nucleotide sequence of the human CBS gene and reported a variable number of tandem repeats (VNTR) polymorphism of 31 bp in intron 13.
They observed negative correlation between CBS enzyme activity and an increasing number of repeat units,which corroborates the positive association between the VNTR and post-load homocysteine concentrations. • 699C T(Y233Y) and 1080C T(A360A) • These are the two possible functional variants in the CBS gene,which determine homocysteine concentrations.
METHIONINE SYNTHASE • A single nucleotide change at 2756 position i.e.,from A to G causes significant change in homocysteine concentration. • 2756GG genotype significantly associated with increased risk of coronary artery disease when compared with 2756AG.
METHIONINE SYNTHASE REDUCTASE • The enzyme methionine synthase reductase(MTRR) is needed to maintain MS in its active state.so it influence the homocysteine concentrations. • An A to G substitution at bp 66 is a risk factor for the devolopment of premature CAD,with a relative risk of 1.49 in 66GG subjects compared with 66AA individuals.
BETAINE-HOMOCYSTEINE METHYL TRANSFERASE • This enzyme remethylates homocysteine to methionine,but is primarly expressed in liver and kidney. • There are three varients which have the relation with homocysteine metabolism. • These are • 1218G T(Q406H) • 595G A(G199S) • 716G A(Q239R)
5,10 – methylenetetrahydrofolate • 677C T(A226V) • Kang et al.detected a variant of the MTHFR enzyme which was associated with decreased enzyme activity,reduced stability after heating at 46*C and increased homocysteine concentrations. • In their study thermolabile MTHFR enzyme was identified in patients with different forms of premature vascular disease and was associated with fasting as well as post-methionine- load homocysteine concentrations.
This variant is associated with altered distribution of RBC folates was tested by a chromatographic method in vitro. • Chromatographic method involves the analysis of RBC folates by affinity/high performance liquid chromatography with electrochemical (coulometric)detection. • 1298A C(E433A) • This is the Second common polymorphism in the MTHFR gene which mandates an aminoacid substitution of glutamate by alanine.
This variant was observed only in trans with the 677CC T variant and was associated with decreased MTHFR enzyme activity. • In their study they observed the incrased concentration of homocyteine concentration and risk of CVD.
SERINE HYDROXYMETHYL TRANSFERASE • In humans,this enzyme is present in twoisoenzymes,one located in mitochondria and the other in the cytoplasm. • One polymorphism in cytosolic SHMT i.e.1420C T(L474F),was associated with increased homocysteine concentration and decreased plasma and RBC folate levels in 1420 CC homozygotes.
METHYLENETETRAHYDROFOLATE DEHYDROGENASE • In the gene coding for this enzyme , a 1958G A(R653Q)substitution was detected in NTD patients and controls,but no association between this variant and homocysteine concentration was found. • TRANSCOBALAMIN(TC) • Since TC is the transporter of vitamin B12 to the cells and vitamin B12 is the cofactor for MS,variation in TC gene could affect homocysteine concentrations.
GLUTAMATE CARBOXYPEPTIDASE GENE 2 • This gene codes for folylpoly-gamma-glutamate carboxypeptidase(FGCP),which hydrolyses polyglutamylated folates to monoglutamylated folates. • A variant 1561C T resulted in significant reduction in FGCP activity.there by reduced blood folate levels and increased homocysteine concentration.
REDUCED-FOLATE CARRIER • A polymorphism,80G A(R27H), in the reduced folate carrier gene,the candidate gene for 5-methyl THF internalization with in cells,was reported by Chango etal. • But this variant is not associated with a change in homocysteine,plama and RBC folate concentrations.
Gene-gene interactions • A combination of multiple variants was studied by Tsai et al. • They studied the combined effect of the CBS 844ins68 variant,the 677C T MTHFR variant and the 2756A G polymorphism in the MS gene,on either fasting or post-methionine-load homocysteine concentrations. • They found that the effect of these three variants were additive and therefore demonstrated a polygenic regulation of plasma homocysteine concentrations.
Dekou et al conducted genitic interaction study and found the above three varients are independent and additive risk factors for homocysteine concentration.
GENE-ENVIRONMENT INTERACTIONS • An interaction between micronutrient deficiencies and variants in genes regulating homocysteine metabolism has in particular been demonstrated for the 677C T polymorphism in the MTHFR gene.
conclusion • 677C T polymorphism in the MTHFR gene has pronounced effect on plasma homocysteine concentrations. • All the genetic variations explained so far constitutes only 9% of over variance in the plasma homocysteine concentration,among mostly explained by MTHFR 677C T. • Variants of genes involved in transport or absorption of folate,such as folate-binding protein,or cofactors such as vitamin B12 and vitamin B6 also influence the homocysteine concentration.
In addition to genes of the key enzymes in homocysteine metabolism,other genes coding for enzymes that might be relevant for homocysteine homeostasis also important. • Therefore,a genome-wide approach using linkage and association studies in large family cohorts is a powerful tool in the discovery of unknown polymorphism that influence homocysteine concentrations in humans.
