This Mentored Patient-Oriented Research Career Development proposal will provide for a structured environment with expert mentorship and an adequate subject population facilitating Dr. Jonathan Soslow's development as an independent clinical investigator. Dr. Soslow is an Assistant Professor of Pediatric Cardiology at Vanderbilt University Medical Center. He trained in pediatric cardiology at Vanderbilt University, including a fourth year in advanced cardiac imaging (complex transthoracic echocardiography, transesophageal echocardiography, and cardiac MRI). Dr. Soslow's long-term goal is to use non-invasive methods to improve diagnosis and monitoring in DMD cardiomyopathy and other forms of pediatric cardiomyopathy. This K23 proposal will strengthen his knowledge of physics and physiology, allowing him to better interface with bioengineers and physicists in the design and implementation of new MRI sequences. He will gain a unique skillset in advanced pediatric cardiac imaging, biomarker evaluation, and statistical analysis that will facilitate his transitionto an independent investigator. Dr. Soslow's mentorship team is uniquely qualified to support him in this endeavor. Strengths include expertise in non-invasive imaging and physiology (Damon), specialist in DMD cardiac care and research and general clinical research design and study conduct (Markham), expertise in heart failure and biomarkers (Sawyer), and expertise in cardiac MRI (Raman and Arai). His mentors have a documented history of successful mentorship of junior faculty. The skills that Dr. Soslow will develop during this award, combined with his current didactic coursework in the Master of Science in Clinical Investigation (MSCI) program, his excellent mentorship, and the research environment at Vanderbilt, will set him on the path towards a career as an independent investigator with R01 funding. Duchenne muscular dystrophy (DMD) is a devastating disease that affects 1 in 3500 boys. Despite significant research advances, patients with DMD continue to die, wheelchair bound, in their twenties. Cardiovascular disease is the leading cause of death in DMD, but the timing and pace of progression to cardiomyopathy is variable. While echocardiographic screening can detect cardiac dysfunction, echocardiography is a poor surrogate marker of disease. Echocardiographic dysfunction likely represents an end stage fibrotic myocardium that is unresponsive to remodeling therapies. Studies demonstrate that early initiation of cardiac specific therapy, prior to manifest systolic dysfunction, can decrease morbidity and delay mortality. Therefore, a better surrogate marker would allow for earlier, focused therapy, potentially altering disease progression and reducing cardiovascular mortality. Fibrosis plays an integral role in the development of DMD cardiomyopathy. Serum and imaging biomarkers of fibrosis are promising methods to identify patients at risk for an accelerated pace of progression to heart failure. Research has demonstrated that proteins secreted by skeletal muscle tissue, or myokines, can modulate the function of distant organs. This concept led us to hypothesize that maintenance of skeletal muscle mass and function, through the secretion of said myokines, is cardioprotective. Our preliminary data fit this model. We have demonstrated a direct relationship between preserved skeletal and cardiac function after loss of ambulation and have established that DMD patients with heart failure have lower levels of neuregulin- 1B (NRG), a candidate myokine. The central hypothesis of this proposal is that serum and imaging biomarkers detect subclinical myocardial fibrosis and can be used as surrogate markers of disease in DMD. We further postulate that maintaining residual skeletal muscle function is cardioprotective, and that this cardioprotection is modulated by NRG and insulin-like growth factor I (IGF-I). Aim 1 will evaluate serum and imaging biomarkers of fibrosis and assess whether abnormalities in these biomarkers precede left ventricular dysfunction. This aim will have a direct, meaningful application to clinical care in DMD, allowing for early cardiovascular therapy, assessment and modification of this therapy, and evaluation of novel therapeutic agents. Aim 2 will evaluate the interaction between skeletal and cardiac muscle function. The results of this aim will inform future studies to prevent DMD cardiomyopathy, including a study evaluating continued corticosteroid therapy after loss of ambulation and a study on the effects of isometric exercise on cardiomyopathy. Aim 3 will assess the postulated cell-signaling mechanism of skeletal and cardiac interaction by evaluating two candidate myokines, NRG and IGF-I. This aim will potentially lead to future therapeutic strategies in DMD. This project will advance our understanding of DMD cardiomyopathy and the interaction between skeletal and cardiac muscle, allowing for meaningful changes in DMD care and spring-boarding the candidate to a successful career as an independent investigator.