Pulmonary valve stenosis (PS) is the most common congenital heart defect in dogs. The condition is due to abnormal valve anatomy present at birth that leads to stenosis of the right ventricular outflow tract. The valve stenosis results in pressure overload of the right ventricle. If severe enough, the stenosis leads to right ventricular hypertrophy, arrhythmias, exercise intolerance, and right-sided heart failure. Severity of disease is typically determined based on velocity of blood flow across the lesion via echocardiography. The median survival time for dogs with severe PS is 4-5 years with medical management, while moderately affected cases have a variable prognosis. The prognosis for mild PS is generally good.
The Bulldog, which is the fourth most popular breed according to AKC dog rankings, is a breed highly overrepresented in PS cases. Due to this breed predisposition PS is suspected to be inherited. Mildly affected dogs are frequently not diagnosed as traditional auscultation-based tests are insensitive in dogs with profound airway sounds and barrel-shaped chests such as the Bulldog. These subclinical cases make it incredibly difficult to successfully screen and remove affected individuals from the breeding program. Thus, hindering breeding efforts to reduce disease prevalence. Therefore, the development of a genetic screening test represents the most practical approach to identification and reduction of disease frequency in the breed. Additionally, severe and moderately affected PS cases are treated with traditional balloon valvuloplasty, which has good but variable success. The procedure has a significant cost, does not restore normal function, and there is considerable anesthetic risk for the brachycephalic breeds that are predisposed to PS. Additionally, some PS-affected dogs have concurrent coronary anomalies, which are expensive to identify and make balloon valvuloplasty contraindicated, further highlighting the need for genetic screening tests for this condition.
There is limited information about the genetic cause of PS in humans with the majority of mutations involving the RAS-MAPK pathway. Although PS is commonly seen in puppies, no mutation to date has been identified, although literature suggests a possible recessive pattern of inheritance. Through the combined efforts of the clinical cardiology service at the University of California Davis, clinical cardiology service at Colorado State University, and the Translational Cardiac Genetics and Pharmacogenomics Laboratory in the University of California Davis School of Veterinary Medicine we aim to elucidate the genetic mechanisms of PS in Bulldogs.
The goal of this dissertation is to compile and contribute to what is currently known about the genetics of PS in humans and animals. A retrospective study in a large referral hospital population allowed determination of the prevalence of this disease in dogs and confirmed common breed predisposition. A pedigree analysis was performed in the most affected breed, the Bulldog, to elucidate a common mode of inheritance. A genome-wide association study was performed to refine a region of interest in the canine genome that is associated with PS. Whole genome sequencing followed by variant analysis allowed identification of mutations associated with the disease. The most plausible variants underwent validation in a large population of dogs using MassArray and those that continued to segregate were followed up with immunohistochemistry in cardiac tissue. Finally, the results of this project will contribute to the understanding of heart development and congenital heart disease in dogs overall, thus guiding novel prevention and therapeutic strategies for PS.