Deletion of the Chromatin Remodeling Factor Brg1 Leads to Genomic Instability During Development, and Confers Sensitivity to Adult-onset Doxorubicin Cardiotoxicity in Mice

Cardiovascular disease is the number one killer of adults in the United States. Paradoxically, a certain subtype of the most prevalent morbidity and mortality factor (heart disease) is due to treatment for the second most prevalent morbidity and mortality factor (cancer): this is the case for doxorubicin cardiotoxicity. Doxorubicin continues to be an efficacious treatment for a variety of solid and hematopoietic liquid tumors, and as such receives continued clinical use. Cardiac toxicity is an under-recognized side effect of several biopharmaceutical and environmental chemical agents. The degree of toxicity may be mediated by epigenetic events, such as chromatin remodeling. It is known the chromatin remodeling factor Brg1, a catalytic subunit of the SWI/SNF ATPase chromatin remodeling complex, is required in cardiovascular development and pathologic cardiac hypertrophy. However, BRG1 necessity during development beyond certain embryonic stages has not been fully investigated, nor has its potential role in doxorubicin cardiotoxicity. Due to the known chromatin remodeling activities of BRG1, the requirement of Brg1 during early embryonic development and its role in the response to cardiotoxicity caused by doxorubicin was investigated. BRG1 ablation beginning at E6.5 results in arrested growth and embryonic death by E9.5 due to developmental defects. Microarray analysis revealed BRG1's role in maintaining genomic integrity, without which there is aberrant expression of cell cycle, proliferation, and apoptosis pathways leading to the observed pathologic phenotype. This demonstrates that BRG1's role in genomic surveillance is essential for survival after the pre-implantation period for normal cellular proliferation and differentiation. Adult Brg1 WT and Brg1 KO mice were subjected to a doxorubicin regimen with repeated dosing over several weeks, analogous to repeat-dose paradigms used in the human clinical setting. Analyses of tissues including protein and RNA biochemistry, microarray analysis, serum biochemistry, and light and electronic microscopic analyses confirmed the presence of doxorubicinassociated lesions in treated mice; however, morphologically-similar lesions were also observed at a lesser incidence and severity in untreated Brg1 KO mice. Microarray analysis revealed significant differentially expressed genes with common clustering between both the treated groups and the Brg1 KO groups, suggesting both common and dissimilar molecular mechanisms between Brg1 KO-induced cardiomyocyte lesions and those induced by doxorubicin treatment. This analysis additionally identified differentially expressed genes previously unreported to be associated with doxorubicin cardiotoxicity. The work in this dissertation has revealed that not only is Brg1 required for the maintenance of normal cardiomyocyte homeostasis during development, but found that despite previous reports it is required in adulthood as well. Furthermore, its absence has common and dissimilar mechanisms to that suffered from treatment with doxorubicin, suggesting mechanisms unique to Brg1 knockout irrespective of treatment may reveal new pathways for therapeutic targeting in patients at risk for doxorubicin cardiotoxicity.