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Abstract
Dogs offer a unique population structure that can be exploited to identify genetic risk factors of complex disease. Herein, I used the canine model to investigate the genetic underpinnings of primary hypoadrenocorticism, a rare endocrine disorder of the adrenal gland. Primary hypoadrenocorticism occurs naturally in dogs and with increased incidence in certain breeds. Affected dogs and people experience similar clinical signs, disease progression, and treatment responses, thereby making the dog an excellent comparative genetics model. To characterize the genetic risk factors associated with primary hypoadrenocorticism, I employed next-generation and Sanger sequencing technologies in two high-risk breeds: the Portuguese Water Dog and the Standard Poodle. I first generated low-coverage whole genome sequencing data from unrelated cases and controls. Based on a multi-breed reference panel, I imputed low-coverage genomes for increased coverage and genotype accuracy. With these data, I performed a genome-wide association study and cross-population extended haplotype homozygosity testing in Standard Poodles. I identified a novel locus associated with primary hypoadrenocorticism on chromosome 17, and two additional risk loci on chromosome 27 containing signatures of selection unique to cases. Notably, all three loci contain genes canonically related to the central nervous system. I also uncovered shared and breed-defining genetic factors that may contribute to the high frequency of primary hypoadrenocorticism in these breeds. Runs of homozygosity and FST analyses revealed shared signatures of selection over six loci containing genes that may have pleiotropic effects related to adrenocortical dysfunction. Because it has been clearly linked to human primary hypoadrenocorticism, I next evaluated the major histocompatibility complex (MHC) class II genes and found that the two breeds have different haplotype compositions, where only Portuguese Water Dogs possess a clear risk haplotype. Finally, I filtered for unique variants based on minor allele frequencies in high-coverage data and identified four missense mutations associated with primary hypoadrenocorticism, within ARID5B and SMYD1 for Standard Poodles and within MYPN and ASCC3 for Portuguese Water Dogs. Collectively, these data highlight the genetic complexity underlying disease risk while also implicating the immune system, and for the first time, the central nervous system in primary hypoadrenocorticism etiology.