LAUSR

Rationale: Hypertrophic cardiomyopathy occurs with a frequency of about 1 in 500 people. Approximately 30% of those affected carry mutations within the gene encoding cMyBP-C (cardiac myosin binding protein C). Cardiac stress, as well as cMyBP-C mutations, can trigger production of a 40kDa truncated fragment derived from the amino terminus of cMyBP-C (Mybpc340kDa). Expression of the 40kDa fragment in mouse cardiomyocytes leads to hypertrophy, fibrosis, and heart failure. Here we use genetic approaches to establish a causal role for excessive myofibroblast activation in a slow, progressive genetic cardiomyopathy—one that is driven by a cardiomyocyte-intrinsic genetic perturbation that models an important human disease. Objective: TGF&bgr; (transforming growth factor-&bgr;) signaling is implicated in a variety of fibrotic processes, and the goal of this study was to define the role of myofibroblast TGF&bgr; signaling during chronic Mybpc340kDa expression. Methods and Results: To specifically block TGF&bgr; signaling only in the activated myofibroblasts in Mybpc340kDa transgenic mice and quadruple compound mutant mice were generated, in which the TGF&bgr; receptor II (T&bgr;RII) alleles (Tgfbr2) were ablated using the periostin (Postn) allele, myofibroblast-specific, tamoxifen-inducible Cre (Postnmcm) gene-targeted line. Tgfbr2 was ablated either early or late during pathological fibrosis. Early myofibroblast-specific Tgfbr2 ablation during the fibrotic response reduced cardiac fibrosis, alleviated cardiac hypertrophy, preserved cardiac function, and increased lifespan of the Mybpc340kDa transgenic mice. Tgfbr2 ablation late in the pathological process reduced cardiac fibrosis, preserved cardiac function, and prolonged Mybpc340kDa mouse survival but failed to reverse cardiac hypertrophy. Conclusions: Fibrosis and cardiac dysfunction induced by cardiomyocyte-specific expression of Mybpc340kDa were significantly decreased by Tgfbr2 ablation in the myofibroblast. Surprisingly, preexisting fibrosis was partially reversed if the gene was ablated subsequent to fibrotic deposition, suggesting that continued TGF&bgr; signaling through the myofibroblasts was needed to maintain the heart fibrotic response to a chronic, disease-causing cardiomyocyte-only stimulus.