Genetics of metabolic syndrome and related traits

In this thesis several aspects of metabolic syndrome are addressed. The focus involves questions concerning the genetics of obesity, TG and cholesterol and hyperglycemia. Since we hypothesized that obesity is the most important trigger of metabolic impairment, the MetS definition in this thesis was chosen to include the obesity measure waist circumference as an essential component. In the study described in chapter 2, the heritability of the metabolic syndrome was addressed and compared to the heritability of its individual components. Since the individual components of MetS were shown to be more heritable than MetS itself, the studies described in chapter 3 and 4 focused on the genetics of the individual MetS component plasma TG. For this purpose, a candidate gene approach was employed using HTG patients and healthy controls. The involvement of a series of candidate genes was confirmed. The study described in chapter 5 followed a similar approach to that used in the studies described in chapter 3 and 4. Several candidate genes were studied in patients suffering from hyperlipoproteinemia (HLP) type III, which is characterized by elevated levels of total plasma cholesterol and plasma TG. HLP type III is characterized by APOE2 homozygosity. Contributing genetic factors in the (metabolically stressed) APOE2/2 environment were confirmed. Plasma adiponectin, an adipose tissue secreted hormone (adipokine), has been suggested to be a biomarker for MetS. In chapter 6 we describe a study which particularly aimed to determine the effect of menopause on the discriminating accuracy of adiponectin to predict MetS. Especially low levels of plasma adiponectin in postmenopausal women were found to be a risk for MetS. However, the discriminating accuracy of adiponectin for the presence of MetS was exceeded by BMI in men and pre –and post menopausal women. Since plasma adiponectin levels are very well correlated with MetS components or related traits, the study described in chapter 7 addressed the question whether these correlations are caused by a genetic overlap (genetic correlation). The genetic correlation was mono-laterally validated with regard to the adiponectin gene (ADIPOQ). Chapter 8 describes a study towards finding novel loci associated with adiponectin or loci that are possibly involved in the genetic overlap between adiponectin and MetS components or related traits. This study followed a genome-wide association (GWA) approach. The results of this GWA were used in a joined analysis with two other cohorts in a meta-analysis. In addition, a selected proportion of SNPs was submitted for replication in several cohorts. Chapter 9 provides a general discussion by reviewing all previous chapters in the thesis. Furthermore, chapter 9 includes suggestions and proposals for future analyses towards unraveling genetic and environmental factors involved in the expression and manifestation of metabolic risk factors.