Louisiana Biomedical Research Network

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Diet induced obesity in flies: a genomic search for modifiers of pathogenesis

Diet-induced obesity in humans and animal models does not guarantee comorbidities, but it does substantially heighten the risk of diabetes, cardiovascular disease, neurological decline, and cancer, which in turn heighten risk of mortality. The ability of obesity to cause disease has been intensely studied in mammals, revealing mechanisms that include chronic inflammation and aberrant cell signaling. An extreme state resembling mammalian obesity results when Drosophila melanogaster are allowed to feed upon solid medium that is high in sugar (HSD) or in fat (HFD). Exposed flies have increased triglyceride levels, increased circulating glucose, reduced insulin response despite increased expression, cardiac abnormalities, a severely reduced lifespan, and are impeded in climbing ability. The latter two qualities are of keen interest in the fly model of diet-induced obesity. With regard to lifespan, this is because the relatively short lifespan of Drosophila melanogaster (on the order of a few months) makes it possible to directly gauge the extent of obesity-induced mortality, and thus discern the genetic basis of the phenotype. With regard to climbing ability, this is because it is an often-used behavioral indicator of neurological health in fruit flies, of interest because mammalian obesity loci are often enriched for neural function. The true strengths of the fly obesity model are its genetic resources/malleability and its provision of an opportunity to study obesity-induced morbidity in the context of mammal-analogous physiological systems. In this project, we will adopt a two-pronged genomic approach. This approach exploits both of the preceding exclusive strengths of the Drosophila model to identify genes and pathways that mediate �tolerance� or �resistance� to obesogenic diets and thus enable understanding of pathogenesis. First, we will take advantage of the Drosophila Genetic Reference Panel (DGRP), which are ~200 genetically diverse wild-caught fly lines that have been whole-genome sequenced and single nucleotide polymorphism (SNP) called, to perform a SNP-based genome-wide association study (GWAS) of lifespan using either HSD or HFD (whichever yields the most trait variance in an initial subset of lines). Second, we will perform RNA-Sequencing of the separated head and body compartments of 3-4 �obesity resistant� lines and 3-4 �obesity susceptible� lines on both a normal diet and an obesogenic diet to identify differentially expressed genes. We will also utilize modulated modularity clustering to group modules of coexpression for the genes with the most expression variance, then use principal component analysis to identify the modules of genes that can separate the fly lines by class. Third, we will screen high-priority candidate loci identified by both of the preceding methods using in vivo RNAi knockdown, followed by assay of lifespan. Finally, we will choose two confirmed candidate genes for energy homeostasis phenotypic profiling as proof of principle.