Coronary artery disease (CAD) is the most common cause of morbidity and mortality worldwide. A cluster of metabolic phenotypes known as metabolic syndrome has been established as a major risk factor for CAD. The mechanisms that link these phenotypes to one another and to CAD are not well understood. We have identified the disease causing gene in a large family with autosomal dominant early onset coronary artery disease (CAD), diabetes, hyperlipidemia, and hypertension. The mutation, substitutes cysteine for arginine (R611C) at a highly conserved residue of the second epidermal growth factor precursor domain in the LDL receptor like protein (LRP6), which encodes a co-receptor in the Wnt signaling pathway. A second mutation (R473Q) was identified in a large kindred with early onset CAD and phenotypes that are consistent with the overall picture of the metabolic syndrome. In vitro studies have indicated that these mutations impair Wnt signaling. Genotype phenotype correlation showed that the mutations impact number of the metabolic phenotypes that are present in these kindreds. These findings have established a causal link between Wnt signaling impairment caused by LRP6 mutation and coronary artery/metabolic syndrome and raise the possibility of complex downstream effects of the mutation which warrants further investigation. Oral glucose tolerance tests in nondiabetic R611C mutation carriers have shown that they are hyperinsulinemic, indicating that insulin resistance is likely the major cause of impaired glucose tolerance in the mutation carriers. The mechanisms of insulin resistance caused by LRP6 mutation are not known. Defect in glycogen synthesis of the skeletal muscle is thought to be one of the major causes of insulin resistance. The rate limiting enzyme for glycogen synthesis, glycogen synthase (GS), is phosphorylated and inactivated by glycogen synthase kinase 3ss (GSK3ss). GSK3ss is negatively regulated by Wnt signaling activation. The expression and activity of GSK3ss is increased in lymphoblastoid cells of the mutation carriers. We hypothesize that LRP6 mutation directly affect glycogen synthesis by impairment of glycogen synthase activity. To examine this hypothesis and to identify the major site of insulin resistance, insulin-stimulatedglucose disposal rate and sensitivity to insulin suppression of hepatic glucose output will be measured with infusion of [6, 6- 2H]-glucose isotope in both nondiabetic mutation carriers and non-carrier family members in the two kindreds. Glycogen synthesis in liver and muscle wil be assessed using 13C MRS and insulin stimulated changes in intramuscular glucose-6-phosphate concentration will be measured using 31P MRS. Glycogen synthesis and GS activity in the skeletal muscle biopsies of the mutation carriers and healthy controls will be compared. In addition, to explore the prevalence and spectrum of LRP6 mutations in patients with metabolic syndrome and CAD, a cohort of 200 cases with early onset CAD, diabetes and metabolic syndrome will be screened for non conservative mutation(s) in the coding region of LRP6. These studies hold great promise in providing important insight into the pathophysiology of metabolic syndrome and CAD and identifying novel biomarkers and therapeutic targets for these disorders.
Coronary artery disease (CAD) and its consequences are the single largest cause of death in the United States. CAD risk factors include high blood pressure, high cholesterol and diabetes. These risk factors are often associated with one another and referred to as metabolic syndrome. The causes of metabolic syndrome remain vastly unknown. We have identified a novel gene mutation in families with early coronary artery disease and metabolic syndrome. The identified gene (LRP6) is a receptor that in normal condition activates a certain pathway known as Wnt signaling pathway, which is impaired in the affected family members due to the gene mutation. Our finding is the first evidence for relationship between impairment of this pathway and development of CAD and metabolic syndrome in humans. The current study goals are investigating the effect of the mutation on metabolism of glucose and insulin secretion and function in patients with this mutation.
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