Ah Siew Sim

Genetic Contribution of the Endothelial Constitutive Nitric Oxide Synthase Gene to Plasma Nitric Oxide Levels

Nitric oxide (NO) has an important physiological role in regulating vascular tone and is also relevant to many pathological processes including hypertension and atherosclerosis. Endothelial constitutive nitric oxide synthase (ecNOS) is the key enzyme in determining basal vascular wall NO production. We used a combination of maximum-likelihood-based statistical genetic methods to explore the contributions of the ecNOS gene and other unmeasured genes to basal NO production measured by its metabolites (NOx: nitrite and nitrate) in 428 members of 108 nuclear families. Our initial quantitative genetic analysis estimated that approximately 30% of the variance in fasting NOx levels is due to genes (chi 2(1) = 16.04, P = .000062). Complex segregation analysis detected the effects of both a single locus and residual polygenes on NOx levels, and measured genotype analysis showed that plasma NOx levels in those homozygous for the rare allele (64.9 +/- 7.8 mumol/L) were significantly higher (P = .000242) than those homozygous for the common allele (30.2 +/- 3.1 mumol/L). The results of the variance component linkage analysis were consistent with linkage of a quantitative trait locus in or near the ecNOS gene to variation in plasma NOx levels (P = .0066). While many environmental factors have been shown to alter transiently plasma NOx levels, our study is the first to identify a substantial effect of the ecNOS locus on the variance of plasma NOx, i.e. basal NO production. This finding may be relevant to atherogenesis and NO-related disorders.

Genotype dependent and cigarette specific effects on endothelial nitric oxide synthase gene expression and enzyme activity

We explored the interactive effects of endothelial nitric oxide synthase (eNOS) genotypes and cigarette smoking on protein levels and enzyme activity in 33 postpartum placentas. Whilst the eNOS protein levels were lower in the rare allele (0.48±0.11, n=9 vs. 1.05±0.10, n=24, P<0.01), the eNOS enzyme activity was about 7‐fold higher in the rare allele (4556.2±255.4 vs. 621.8±180.5 cpm/mg/min, P<0.01). Smokers had lower eNOS protein levels (1.07±0.09 vs. 0.50±0.19, P<0.05) in both alleles. It reduced the eNOS activities only in the rare allele (non‐smokers: 6143.8±251.2, n=5, smokers: 2968.5±259.4, n=4, 52% reduction, P<0.01). We conclude that associations between eNOS polymorphism and protein levels and enzyme activities are modifiable by smoking, the effects of smoking are dependent on the eNOS genotypes.

Distribution in Healthy and Coronary Populations of the Methylenetetrahydrofolate Reductase (MTHFR) C677T Mutation

Modest elevations of circulating homocyst(e)ine are common in patients with vascular disease. We explored in normal and coronary artery disease (CAD) populations the distribution of a mutation in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene that results in enzyme thermolability and reduced activity and in homocyst(e)ine elevation to assess its relevance to risk. We identified the C to T substitution at the MTHFR locus and compared the distributions of genotypes in 565 patients aged < or = 65 years without and with angiographically documented CAD and in 225 healthy subjects. In the patients, we also assessed interrelations between genotypes and CAD occurrence and severity, as well as standard risk factors. The frequency of homozygotes for the mutation was the same in patients with and without CAD and in healthy subjects (11.6%, 11.0%, and 10.7%, respectively: P > .5 for each). There was also no excess among the 419 patients with severe disease (ie, one or more vessels with > 50% luminal obstruction) compared with those with no or mild CAD (odds ratio: 1.004; 95% confidence interval: 0.59 to 1.70). Homozygosity for the mutation was also not associated with a history of myocardial infarction or the presence or severity of angina. However, body mass index increased linearly with the presence of the mutant allele (P = .005), and the mutation and hypertension were weakly associated (P = .036). We conclude that the MTHFR genotype is not a risk factor for coronary disease in this Australian population but that the strong association found with body mass index should be explored further.

Improved method for plasma malondialdehyde measurement by high-performance liquid chromatography using methyl malondialdehyde as an internal standard

Measurement of malondialdehyde (MDA) is an important contribution to the assessment of oxidative stress. We report a sensitive and reproducible high-performance liquid chromatography (HPLC) method for measurement of plasma MDA in the assessment of lipid peroxidation. Using methyl malondialdehyde (Me-MDA) as an internal standard with reversed-phase HPLC and UV detection and derivatisation with 2,4 dinitrophenylhydrazine (DNPH), we obtained maximum MDA values with 60-min incubation of 10% plasma with 1 M NaOH at 60 degrees C. The dilution of the plasma and a longer incubation time in the alkaline hydrolysis step greatly improved recovery of MDA from its bound form. Ratios of peak height of MDA/Me-MDA were linear over a range of 0-100 microM with correlation coefficients >0.99. The recovery was 88.5%. Within and between run variations were <4 and <7%, respectively. The mean MDA value measured in 20 healthy volunteers was 13.8 microM (+/-1.32).

Relationship between total plasma homocysteine, polymorphisms of homocysteine metabolism related enzymes, risk factors and coronary artery disease in the Australian hospital-based population

Modest elevations of circulating homocysteine are common in patients with vascular disease. We explored interrelations between total plasma homocysteine levels and mutations in genes for three key enzymes in methionine-homocysteine metabolism. Methyltetrahydrofolate reductase (MTHFR) 677C-->T, cystathionine beta synthase (CBS) 68-bp insertion at exon 8, and methionine synthase (MS) 2756A-->G were typed in 685 Australian caucasian patients aged < or =65 years with and without angiographically documented coronary artery disease (CAD). We also assessed associations between homocysteine levels and extracellular superoxide dismutase (EC-SOD) and other CAD risk factors. There were significant correlations between plasma total homocysteine, and EC-SOD (r = 0.170, p = 0.001 for men; r = 0.241, p = 0.003 for women) and LDL (r = 0.153, p = 0.001 for men; r = 0.132, p = 0.081 for women). Levels were also significantly higher among patients with unstable angina (15.30+/-0.44 micromol/l for men, 14.44+/-0.74 micromol/l for women) than those without angina (13.98+/-0.38 micromol/l for men, 13.41+/-0.98 micromol/l for women) or with stable angina (14.00+/-0.37 micromol/l for men, 12.88+/-0.71 micromol/l for women). There were no significant associations between the levels and the presence or severity of CAD. The mutant MTHFR homozygotes tended to have higher levels and those with the MS and CBS mutations tended to have lower levels. We conclude that there is a significant correlation between plasma homocysteine levels and EC-SOD suggesting that elevated homocysteine may exert oxidative stress and that levels are associated with unstable angina, but not the occurrence or extent of coronary stenosis. The contributions to total plasma homocysteine levels of the common mutations of genes coding for the enzymes controlling homocysteine metabolism are modest.

Glutathione S-Transferase Mu1 Deficiency, Cigarette Smoking and Coronary Artery Disease:

Background While genetic variation accounts for a large proportion of interindividual differences in coronary artery disease (CAD) development, environmental factors such as cigarette smoking may genotype-dependently initiate or accelerate the risk. Glutathione S-transferase mu1 (GSTM1) is one of the GST isoenzymes and contributes to the detoxification process of organic compounds produced by cigarette smoking. In the present study we explored the hypothesis that GSTM1 deficiency, caused by GSTM1 null allele, may predispose subjects to cigarette smoking related CAD risk. Design Cross-sectional. Methods We genotyped the GSTM1 null allele in 868 angiographically characterized CAD patients who were consecutively recruited in the present study. Results The frequency of the null genotype in this high-risk patient population was 57.1% (55.4% for males and 61.0% for females). While 75.7% male and 50.7% female null GSTM1 patients had significant CAD as defined by one or more significantly stenosed coronary arteries, 79.3% male and 48.3% female patients with positive GSTM1 also had the significant CAD (P > 0.05). However, although 54.3% male and 55.2% female GSTM1 null patients had triple vessel disease, only 45.7% male and 44.5% female GSTM1 positive patients had the severe disease. Controlling for cigarette smoking did not change the relationship. The occurrences of MI were 37.9% in male and 31.4% in female with the null genotype whereas they were 42.8% in male 37.6% in female with positive GSTM1 (P > 0.05). Using logistic regression analyses, we found no interactions between GSTM1 genotype and cigarette smoking in relation to CAD or MI. Conclusions While our data may be consistent with that the GSTM1 null genotype predisposes subjects to cigarette smoking related severe CAD, interactive effect on CAD risk is minor and insignificant. GSTM1 deficiency alone is not sufficient to cause CAD.

Asymmetric dimethylarginine in homocystinuria due to cystathionine β-synthase deficiency: Relevance of renal function

SummaryObjective Vascular disease is associated with increased plasma asymmetric dimethylarginine (ADMA) and homocysteine, and both are increased in renal failure. In cystathionine β-synthase deficiency (CBS) there is severe hyperhomocysteinaemia, precocious vascular disease, and endothelial dysfunction. We investigated whether ADMA levels are elevated in CBS patients with and without renal impairment, and whether lowering plasma homocysteine also lowers ADMA.Methods We measured plasma homocysteine, arginine, asymmetric and symmetric dimethylarginines, nitrate + nitrite, creatinine and cystatin C in 23 CBS-deficient patients and 24 age-matched controls.Results In the patients, nitrate + nitrite and the ratio L-arginine/ADMA were markedly reduced (21.6 ± 6.1 vs 57.7 ± 7.5 μmol/L and 132.9 ± 24.7 vs 181.9 ± 56.1, respectively, p < 0.001 for both), reflecting endothelial dysfunction. Plasma ADMA for the group was moderately increased (0.55 ± 0.08 vs 0.49 ± 0.07 μmol/L, p = 0.018), but this was due to significantly higher levels than controls in only those 7 of the 23 patients who had elevated cystatin C levels (0.59 ± 0.08 vs 0.49 ± 0.07 mg/L, p = 0.007). Posttreatment total homocysteine in patients varied widely (15–285, median 92 μmol/L), but was not correlated with ADMA or other measured variables. In three newly-diagnosed patients, marked reduction of total homocysteine during treatment produced minimal changes in ADMA.Conclusions ADMA levels were significantly increased only in the CBS-deficient patients with elevated cystatin C levels, and not in those with normal renal function. The reported relationship between hyperhomocysteinaemia and ADMA may not be direct, but could be secondary to reduced renal function.