POSTPRANDIAL LIPOPROTEIN METABOLISM IN TYPE 2 DIABETES MELLITUS AND MOLECULAR MARKERS IN SUBJECTS WITH CORONARY ARTERY DISEASE

Abstract

This thesis is comprised of two different studies to better understand the factors contributing to the development of coronary artery disease (CAD). The first study is a postprandial study, which explores the hypothesis proposed by Zilversmit (1979) on the role of chylomicrons and its remnants, and the relationship of impaired triglyceride tolerance in CAD development. The second study of restricted fragment length polymorphisms (RFLPs), was a pilot project to explore the possibility of identifying molecular markers for identifying individuals who are susceptible to CAD development. The postprandial study is the first study being carried out in Singapore and to my best knowledge, it is also the first being conducted using an Asian population. The study is designed to evaluate the postprandial lipid and lipoprotein profiles of plasma, triglyceride-rich lipoproteins (TRL), and infranatant in Type 2 Diabetes mellitus (DM) after a 50g fat challenge. Thirteen Type 2 DM and eight matched control middle aged men were recruited. They were given a defined test meal (50 g fat, 31 g protein, and 116 g carbohydrate, 1068Kcal), and 340, 0001U (200 mg) of vitamin A at 17 00 hour on the study day. Blood samples were taken pre and postprandially at regular intervals over a 12 hours period. The TRL fraction was isolated by ultracentrifugation (density < l.006glml). Triglyceride (TG), cholesterol, glucose, insulin, apolipoprotein (apo) AI and total apo B, and retinol ester (RE) were measured. Apo B48 and apo B100 were separated by 4-20% gradient gel, detected by enhanced chemiluminesence, and quantitated densitometrically. Two apo B48 peaks were noted in the Type 2 DM group, signifying the possibility of meal induced preformed chylomicrons. Type 2 DM subjects exhibited a significantly different postprandial plasma-, TRL-, and infranatant-TG, TRL-apo B48, and plasma-, and TRL-RE profile (p < 0.05) after the fat challenge. There was no significant in the plasma cholesterol, apo AI, and total apo B, and TRL-, and infranatant-cholesterol in both groups. The study showed that Type 2 DM subjects exhibited an exaggerated postprandial lipaemic response and also suggests that there may be impaired intestinal TRL production. The RFLPs study attempts to explore the possible genetic and biochemical influences of the apo AI-CIII-AIV gene cluster on the serum lipid levels and also to assess the value of the DNA polymorphisms in this gene cluster to CAD development. Six different polymorphic restriction sites — HinfI, HincII, PstI, PvuII, SstI, and MspI on the gene cluster were examined. A total of 140 angiographically proven CAD subjects and 56 angiographically proven non-CAD controls of comparable age were recruited. All six loci were in Hardy-Weinberg equilibrium, with no indication of population heterogeneity. There is no significant difference in the genotype and allele frequency of the six DNA polymorphic sites in both groups. This study failed to analyse the association of lipid parameters with the DNA polymorphism as 39% and 21% of the atherosclerotic and non-atherosclerotic group respectively, were receiving lipid-lowering drugs. Despite this, it was noted from the similar genotype and allele frequency in both groups, that this study suggests DNA loci were unlikely to play a part in influencing the risk of developing angiographic CAD. The study also suggests that it may be counterproductive to develop disease prevention strategies based on these DNA polymorphisms. These two studies had allowed us to better understand the different aspects of factors contributing to CAD development.