ABSTRACT Somatic cell genome editing (SCGE) has remarkable promise to transform our therapeutic toolbox for heart failure due to Titin truncation variants (TTNtvs). However, despite having the knowledge of the pathogenic TTNtvs, their molecular consequences, and SCGE technologies including CRISPR genome editing and transcriptional activation to target TTNtvs, the lack of in vivo studies using humanized TTNtv animal models has hindered clinical development. There remain key knowledge gaps limiting SCGE development for TTNtvs including the lack of a systematic examination of the proportion of TTNtv alleles and cardiomyocytes that need to be corrected for clinical benefit, and the toxicology profile of SCGE enzymes and delivery systems including genotoxicity and immunogenicity. Our long-term goal is to develop SCGE for heart failure patients. While in vitro studies to evaluate the efficacy and safety of SCGE editors and delivery systems such as by utilizing human engineered heart tissues (EHTs) are important, they do not replace the need for animal model studies that recapitulate the complex pathophysiology of heart failure. Guided by our comprehensive preliminary data and unique reagents including SCGE components functionally validated in TTNtv EHTs, humanized Ttntv mice and cardiotropic adeno-associated viruses; our 1-year study will include two Specific Aims to determine efficacy and safety of 1) TTNtv reading frame repair and 2) TTN transcriptional activation in a humanized Ttntv mouse. Execution of these Aims will provide editing thresholds, safety, and toxicity for two SCGE approaches to treat TTNtvs that we expect will serve as a resource for the broader scientific community.