The L348P point mutation in cardiac myosin binding protein-C alters transient responses to stretch, slows cardiac relaxation, and is embryonic lethal in homozygous CRISPR gene-edited mice.

Mutations in cardiac myosin binding protein-C (cMyBP-C) are a common cause of hypertrophic cardiomyopathy (HCM), an inherited autosomal dominant disease affecting 1 in 250-500 people. We previously identified a single amino acid substitution (L348P) in the regulatory motif (M-domain) of cMyBP-C that slowed relaxation and caused diastolic dysfunction in transgenic mice. Here we attempted to increase expression of the mutant protein by creating a CRISPR gene-edited knock-in mouse model (L348P-CR) and breeding mice to homozygosity for the mutant allele. Results showed that L348P-CR homozygous mice died in utero, but that heterozygous knock-in mice developed contractile deficits and diastolic dysfunction comparable to transgenic mice. To overcome the lethal homozygous expression of the L348P mutation, we used our "cut-and-paste" approach to fully replace endogenous wild-type cMyBP-C with recombinant L348P cMyBP-C in permeabilized cardiomyocytes from SpyC mice. Results showed that replacement of wild-type cMyBP-C with recombinant L348P recapitulated mechanical effects seen in transgenic and L348P-CR mice, validating the utility of our cut-and-paste method for evaluating functional effects of cMyBP-C. We conclude that L348P-CR knock-in mice are a robust model of diastolic dysfunction due to a single point mutation in cMyBP-C and that the cut-and-paste approach offers a rapid and cost-effective approach for evaluating mutations in cMyBP-C, especially those that are lethal in traditional animal models.