Metabolic architecture of insulin action in Southwest American Indians

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Project Summary Southwest American Indians (SWAI) suffer from the highest lifetime risk of type 2 diabetes (T2D) and its adverse health consequences of any ethnic group. Over the last two decades, our collaborators (led by Dr. C. Bogardus; NIDDK-Phoenix) have pioneered longitudinal studies of SWAls in the Gila River Indian Community in Phoenix, Arizona, to characterize clinical and genetic predictors of T2D in this at-risk population. Through careful integrative metabolic studies (e.g. measurement of insulin action using hyperinsulinemic-euglycemic clamp), they have defined insulin resistance (IR) and reduced acute insulin secretion (IS) as significant predictors of T2D. Of note, while these physiologies are linked to T2D in other ethnicities, SWAls have a greater degree of IR and IS at a similar level of obesity relative to other Americans, the mechanisms of which are elusive. While studies have found small molecule metabolites proximal and specific to metabolic dysfunction may presage T2D, these studies (1) do not identify precise biologic mechanisms of IR/IS due to lack of mechanistic measures of IR/IS (e.g., via clamp); (2) are focused on Caucasians, whose clinical risk and severity of T2D is lower relative to SWAls. Furthermore, pilot studies in Indians suggest that metabolites linked to T2D may not be the same as those found in Caucasians. Here, we identify metabolic pathways linked to T2D via their effect on IR/IS by measuring circulating metabolites in high-risk SWAls alongside exquisite characterization of in vivo insulin physiology and molecular genetics. We collaborate with the NIDDK-Phoenix Epidemiology/Clinical Research Branch to measure metabolites in SWAI adults to define the molecular architecture of metabolism, focused on insulin physiology. Our central hypothesis is that circulating metabolites will identify mechanisms of IR and IS in SWAls. We will study two different populations: (1) the Gila River Indian Cohort Study, a prospective cohort study of >650 SWAls with baseline measures of body composition and insulin physiology and longitudinal follow-up for T2D; (2) the Phoenix Cohort Study, a longitudinal study of >650 SWAls with oral glucose tolerance testing (OGTT) and ongoing clinical surveillance. In Aim 1, we will use measures of IR and IS based on the euglycemic-hyperinsulinemic- 3H-glucose clamp (HEC) and oral or IV glucose tolerance testing in the Gila River Indian Study to identify metabolic pathways linked to IR/IS in SWAls without T2D. In Aim 2, we will identify the genetic architecture of metabolism in SWAls via (1) association of metabolite patterns with >500,000 directly genotyped SNP variants and >4.5m imputed variants to identify quantitative trait loci for metabolism (mQTLs), with verification of derived genetic risk scores with T2D in a separate, large population of SWAls (N=6936) to demonstrate a causal role ininstrumental variables analysis; (2) studying relationship between IR and expression of genes implicated in metabolic pathways from skeletal muscle. If successful, this application defines underlying architecture of metabolism in SWAls, with resources accessible by the general scientific community for future discovery and comparison in broaderpopulations.