Identifying host genetic determinants of Coxiella burnetii pathogenesis

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Project Summary/Abstract: Coxiella burnetii is a Gram-negative, obligate intracellular, macrophage-tropic bacterium and the causative agent of the zoonotic disease Q fever. Acute phase of the disease in humans is characterized primarily by influenza-like symptoms, and approximately 3-5% of the infected individuals develop chronic infection, which is characterized by endocarditis. Association between host genetic background and the development of Coxiella infection has been demonstrated both in humans and animals; however, specific genes associated with susceptibility to infection remain largely unknown. A powerful tool to study the impact of host genetics on pathogenic infection is the model organism Drosophila melanogaster. The Drosophila Genetics Reference Panel (DGRP) is a fully sequenced panel of fly lines derived from a natural population that is used to as a living library to test how polymorphisms affect complex host responses. Thus, to address the knowledge gap regarding how host genetics affects Coxiella pathogenesis, we propose to identify host genetic variants implicated in susceptibility to Coxiella infection using Drosophila melanogaster. Our preliminary data demonstrate that Drosophila exhibit mortality and host immune responses following infection with the BSL2 phase II clone 4 strain of C. burnetii that has a deletion in lipopolysaccharide synthesis genes but retains other virulence factors. The phase II strain has similar intra-macrophage replication kinetics as BSL3 phase I C. burnetii and causes lethality in wax moth larvae. Similarly, Drosophila succumb to Coxiella phase II clone 4 and thus can be used as a novel host model for Coxiella infection. Moreover, our pathway-unbiased screening approach will significantly improve upon ongoing studies of Coxiella in mammalian systems. We hypothesize that the DGRP lines will reveal genetic variants in immune response- related genes associated with susceptibility to Coxiella infection. In Specific Aim 1 we will use DGRP lines to perform a genome-wide association study (GWAS) and screen for genetic variants associated with susceptibility to Coxiella infection. In Specific Aim 2 we will use genetically recombinant fly lines and in vitro assays to knockdown or over-express genes with identified variants for validation and further functional evaluation of the gene's impact on Coxiella pathogenesis. Additionally, since a majority of known human disease genes have recognizable homologues in Drosophila, we will take advantage of these genetic similarities to ask how homologous genes identified in our screen function during Coxiella infection in human cells. Through the proposed work we will identify host genetic determinants of susceptibility to Coxiella infection in Drosophila and mammalian hosts. Our findings will have broad-ranging impacts by identifying host factors that when dysfunctional confer susceptibility to Coxiella infection. These results will be important for better prediction of pathogen spread in domestic ruminant reservoirs and disease progression in humans.