Rationale: Hypertrophic Cardiomyopathy (HCM) affects as many as 1 in 200 people, equating to about 36 million people worldwide. HCM is characterized by thickening of the heart muscle, with or without left ventricular outflow obstruction, associated with diastolic dysfunction and sudden cardiac death. Previously, we discovered a MYBPC3 polymorphic variant inherited in over 100 million South Asians and associated with pathological left ventricular hypertrophy and heart failure. This variant is a 25-base-pair deletion in intron 32 (MYBPC3Δ25bp variant) that leads to exon 33 splicing (MYBPC3ΔExon33 mRNA) by altering the protein’s C10 domain (cMyBP-CΔC10 protein). To determine the precise molecular mechanism(s), such as the involvement of secondary risk factors, underlying the pathogenicity of MYBPC3Δ25bp, we generated knock-in humanized mice in which intron 32 was replaced with the human intron 32 wildtype with (Mybpc3hInt32-Δ25bp) or without (Mybpc3hInt32-WT) the MYBPC3∆25bp variant. Under baseline conditions, MYBPC3∆25bp displayed no changes in cardiac function or morphology, as measured by echocardiography, heart weight to bodyweight ratio, or histology. However, IonOptix measurements, using freshly isolated cardiomyocytes, revealed a significant reduction in contractility, albeit without affecting calcium transients, at three months of age in Mybpc3hInt32-Δ25bp. In addition, Mybpc3hInt32-Δ25bp mice showed exacerbated cardiac remodeling during pressure-overload cardiac stress. Then, we treated Mybpc3hInt32-Δ25bp mice with nonsense-mediated mRNA decay (NMD) inhibitors and ubiquitin-proteasome system (UPS) inhibitors, demonstrated the accumulation of Mybpc3∆Exon33 mRNA and cMyBP-C∆C10 protein levels. Secondary risk actors like diabetes and pressure overload have impaired mRNA-mediated decay in the Mybpc3hInt32-Δ25bp mice. Finally, treating pressure-overload cardiac stress with anti-sense oligos against Mybpc3∆Exon33 mRNA improved cardiac function. Taken together, our ongoing studies demonstrate that impairment of mRNA-mediated decay, owing to secondary risk factors, leads to the accumulation of Mybpc3∆Exon33 mRNA and cMyBP-C∆C10 protein, resulting in left ventricular hypertrophy and heart failure.