Background: Although, in general, cancer is considered a multifactorial disease, clustering of particular cancers in pedigrees suggests a genetic predisposition and could explain why some dog breeds appear to have an increased risk of certain cancers. To our knowledge, there have been no published reports of whole genome sequencing to investigate inherited canine mammary tumor (CMT) risk, and with little known about CMT genetic susceptibility, we carried out whole genome sequencing on 14 purebred dogs diagnosed with mammary tumors from four breedspecific pedigrees. Following sequencing, each dog's data was processed through a bioinformatics pipeline. This initial report highlights variants in orthologs of human breast cancer susceptibility genes. Results: The overall whole genome and exome coverage averages were 26.0X and 25.6X, respectively, with 96.1% of the genome and 96.7% of the exome covered at least 10X. Of the average 7.9 million variants per dog, initial analyses involved surveying variants in orthologs of human breast cancer susceptibility genes, BRCA1, BRCA2, CDH1, PTEN, STK11, and TP53, and identified 19 unique coding variants that were validated through PCR and Sanger sequencing. Statistical analyses identified variants in BRCA2 and STK11 that appear to be associated with CMT, and breed-specific analyses revealed the breeds at the highest risk. Several additional BRCA2 variants showed trends toward significance, but have conflicting interpretations of pathogenicity, and correspond to variants of unknown significance in humans, which require further investigation. Variants in other genes were noted but did not appear to be associated with disease.

Genetic homology with humans and breeding practices that limit intrabreed heterogeneity make canines a useful genetic model for identifying heritable disease markers that have parallels in humans. Ultimately, the homogeneity of individual canine breeds is advantageous in identifying the modes of inheritance for human disorders that have resisted investigative efforts for decades. Particularly few genetic research studies have focused on identifying inherited risk factors for canine mammary tumor (CMT), and as such there has been little exploration of the potential overlaps between the ~65% of human hereditary breast cancer cases that are currently genetically unsolved. DNA samples from 85 CMT-affected canines were extracted from blood/buccal swabs for whole-genome sequencing. These included 32 different canine breeds from six of the seven American Kennel Club (AKC) recognized breed groups; the most represented breeds were Golden Retrievers (n=20), Siberian Huskies (n=8), and Standard Schnauzers (n=7). The cohort also included 3 male CMT cases, which is a hallmark of hereditary CMT and comparable to hereditary breast cancer incidence in human males. Furthermore, pedigree analyses identified dogs within the CMT cohort who stem from a breed-specific common ancestor, and this indicates increased genetic homogeneity within a breed. Currently, these dogs exist in twelve different canine pedigrees corresponding to twelve different breeds and representing 72% of the CMT cohort. The most impressively linked pedigrees contain 20/20 Golden Retrievers, 7/8 Siberian Huskies, 7/7 Standard Schnauzers, 5/5 Welsh Springer Spaniels, 4/4 Newfoundlands, 4/4 Golden Setters, 3/3 Dalmatians, and 3/3 Doberman Pinschers. These CMT pedigrees highlight the characteristics of hereditary disease and demonstrate potential to identify mutations that are specific to a breed or kennel. Through pedigree analysis, fourteen samples were selected for genome sequencing, and they represented Golden Retrievers, Siberian Huskies, Standard Schnauzers, and Dalmatians. Following genome sequencing, orthologs of human genes are being investigated for comparison to human hereditary breast cancer sequencing data that were generated in our laboratory. This comparative genomics aspect may lead to the discovery of novel genes that influence breast cancer risk. Citation Format: Anna L.W. Huskey, Carlos Lloveras-Fuentes, Katie Goebel, Nancy D. Merner. Canine pedigree analysis as a model of hereditary breast cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr A62.