Abstract Body: Introduction: Dilated cardiomyopathy (DCM) is a common inherited cardiac disease characterized by systolic dysfunction, ventricular dilation, myocyte hypertrophy, and fibrosis. Myofilament activators have emerged as a new class of targeted therapies, yet initial clinical use has shown limited benefit. Because patients often present with DCM after substantial remodeling has occurred, this limited efficacy may reflect incomplete understanding of cardiac plasticity and the reversibility in genetic DCM.

Goal: Determine the extent to which the heart can recover from established genetic DCM across cellular and extracellular compartments.

Approach: Genetic DCM was modeled in mice using a doxycycline (DOX)-repressible I61Q mutation in cardiac troponin C (cTnC) that reduces Ca²⁺ sensitivity, inducing hypocontractility and eccentric remodeling. DOX was used in this study as a treatment mimetic to silence the I61Q variant and replace it with native cTnC.

Results: DOX normalized Ca²⁺ sensitivity within 7 days, confirming rapid transgene silencing. Mice aged to advanced DCM preceding heart failure (6 months) exhibited systolic dysfunction and dilation. One month of DOX induced complete structural, functional, and genomic recovery in I61Q myocytes. Whole heart function and dilation, however, was only partially rescued, suggesting a tissue-level barrier to recovery. I61Q hearts exhibited increased fibroblast number and interstitial fibrosis that was not reversible with one or three months of DOX. Single-nucleus RNAseq revealed these fibroblasts were largely quiescent, suggesting that increased cell number, not sustained activation, underlies matrix persistence. ECM stiffness also remained elevated despite myocyte correction. Accordingly, inhibition of collagen crosslinking with beta-aminopropionitrile combined with DOX achieved complete functional recovery. Even at end-stage disease (12 months), DOX alone markedly improved systolic function, though hearts remained partially impaired and fibrosis persisted.

Conclusions: Cardiomyocytes exhibit remarkable plasticity, even in advanced disease. In contrast, secondary adaptations in the fibroblast and extracellular matrix persist despite myocyte genetic correction and limit organ-level recovery. These findings demonstrate compartment-specific reversibility in genetic DCM and support combinatorial strategies targeting both myocytes and matrix to achieve complete disease reversal.