Molecular Genetic Analysis Of Blood HDL Cholesterol And Triglyceride Levels

Today, molecular genetic technology can identify disease-causing genes for essentially all types of human diseases, which immediately provides the molecular mechanisms underlying the pathogenesis of the disease, and lead to development of new methods for diagnosis and treatment. Positional cloning based on genome-wide linkage analysis is the most efficient method for identifying genes for single gene disorders or monogenic diseases, and has led to identification of many human disease genes, for example, long QT syndrome, atrial fibrillation, retinitis pigmentosa, and many others.However, the most common diseases that threaten human health and burden the society are the complex diseases caused by multiple genes (not a single gene) and environment factors. The examples are coronary artery disease and myocardial infarction, diabetes, and hypertension. Complex diseases are different from monogenic disorders in that the pattern of the disease can not be simply determined to be either dominant or recessive. As a result, genome-wide linkage analysis of common complex diseases or traits usually requires hundreds of small families. After linkage is found, candidate genes within the linked chromosomal region can be analyzed for association with the trait. Alternatively, a candidate association study can be used to identify risk factors or genes for a complex disease or trait. In this case, a candidate variant or gene is selected and its frequency is compared between a group of patients (or people with a trait under study) and a group of controls without the disease or trait. A significant difference suggests that a specific variant is associated with the disease or trait.In our study, using differents populations, we applied linkage anaylsis and association astudy on the two important traits of CAD: HDL and triglyceride, and finally found the most significant QTL of HDL: 15q25. The result may help researcher to find the real variant or gene affecting HDL levels in this locus.In the first part, we have completed a genome-wide linkage scan for HDL-C in a U.S. cohort consisting 388 multiplex families with premature CAD (GeneQuest population). The heritability of HDL-C in GeneQuest was 0.37 with gender and age as covariates (P = 5.1 x 10-4). Two major quantitative trait loci (QTLs) for log-transformed HDL-C adjusted for age and gender were identified onto chromosomes 7p22 and 15q25 with maximum multi-point LOD scores of 3.76 and 6.69 respectively. Fine mapping decreased the 7p22 LOD score to a non-significant level of 3.09 and split the 15q25 QTL into two loci, one minor QTL on 15q22 (LOD=2.73) and spanning the LIPC gene, and the other QTL remained at 15q25 (LOD=5.63). A family-based QTDT revealed significant association between variant rs1800588 in LIPC and HDL-C in the GeneQuest population (P=0.0067), which may account for the minor QTL on 15q22. The 15q25 QTL is the most significant locus identified for HDL-C to date, and these results provide a framework for the ultimate identification of the underlying HDL-C variant and gene on chromosomes 15q25, which will provide insights into novel regulatory mechanisms of HDL-C metabolism.In the second part, recent genome wide association studies (GWAS) identified significant associations between single nucleotide polymorphisms (SNPs) rs1323432, rs2338104, and rs4846914 and HDL-C, and between SNPs rs4846914, rs16996148, rs17321515, rs17145738, rs1748195, and rs12130333 and triglyceride, But those results need to be replicated in other populations. In our study, the newly discovered HDL-C and triglyceride SNPs by GWAS were genotyped for 1,231 MI cases and 560 controls from the Cleveland GeneBank by TaqMan SNP genotyping assays. In the result, All eight SNPs, including rs1323432, rs2338104, rs4846914, rs16996148, rs17321515, rs17145738, rs1748195, and rs12130333, showed a P value of >0.05 for plasma HDL-C and triglyceride concentrations in 1,231 cases, 560 controls and a combined 1,791 study subjects. A P value of 0.007 was obtained for association between SNP rs12130333 and MI (odds ratio of 0.773). As a conclution, no significant association could be found between the eight newly-identified HDL-C and triglyceride SNPs by GWAS and HDL-C and triglyceride, at lease in a Cleveland Caucasian population.