Mutation that dramatically alters rat titin isoform expression and cardiomyocyte passive tension.

Titin is a very large alternatively spliced protein that performs multiple functions in heart and skeletal muscles. A rat strain is described with an autosomal dominant mutation that alters the isoform expression of titin. While wild type animals go through a developmental program where the 3.0 MDa N2B becomes the major isoform expressed by two to three weeks after birth (approximately 85%), the appearance of the N2B is markedly delayed in heterozygotes and never reaches more than 50% of the titin in the adult. Homozygote mutants express a giant titin of the N2BA isoform type (3.9 MDa) that persists as the primary titin species through ages of more than one and a half years. The mutation does not affect the isoform switching of troponin T, a protein that is also alternatively spliced with developmental changes. The basis for the apparently greater size of the giant titin in homozygous mutants was not determined, but the additional length was not due to inclusion of sequence from larger numbers of PEVK exons or the Novex III exon. Passive tension measurements using isolated cardiomyocytes from homozygous mutants showed that cells could be stretched to sarcomere lengths greater than 4 mum without breakage. This novel rat model should be useful for exploring the potential role of titin in the Frank-Starling relationship and mechano-sensing/signaling mechanisms.

RBM20, a gene for hereditary cardiomyopathy, regulates titin splicing.

Alternative splicing has a major role in cardiac adaptive responses, as exemplified by the isoform switch of the sarcomeric protein titin, which adjusts ventricular filling. By positional cloning using a previously characterized rat strain with altered titin mRNA splicing, we identified a loss-of-function mutation in the gene encoding RNA binding motif protein 20 (Rbm20) as the underlying cause of pathological titin isoform expression. The phenotype of Rbm20-deficient rats resembled the pathology seen in individuals with dilated cardiomyopathy caused by RBM20 mutations. Deep sequencing of the human and rat cardiac transcriptome revealed an RBM20-dependent regulation of alternative splicing. In addition to titin (TTN), we identified a set of 30 genes with conserved splicing regulation between humans and rats. This network is enriched for genes that have previously been linked to cardiomyopathy, ion homeostasis and sarcomere biology. Our studies emphasize the key role of post-transcriptional regulation in cardiac function and provide mechanistic insights into the pathogenesis of human heart failure.