Abstract 1452: Mutation of a SWI/SNF-associated Protein Leads to Massive Cardiac Hyperplasia in the Zebrafish Mutant heart of stone
Mature cardiac myocytes are considered postmitotic. The process, how cardiac precursors lose the ability to proliferate is unknown. The identification of factors that control the proliferative potential of cardiac myocytes would be an important step towards unraveling the process and ultimately to overcome it for therapeutic purposes. The zebrafish is an intriguing model organism for this objective, because zebrafish cardiac myocytes retain the ability to proliferate and therefore to regenerate cardiac injuries. In a forward genetics screen in zebrafisch, we have isolated the recessive, embryonically lethal mutant heart of stone (hstHJ163). Hearts of hst-mutant zebrafish are characterized by wall-thickening of the cardiac ventricles up to almost complete obliteration of the ventricular lumen. Histology revealed massive hyperplasia of ventricular cardiac myocytes. Hyperplasia is caused by an increase in cardiomyocyte proliferation. By positional cloning we narrowed down the heart of stone locus to one gene, which encodes an evolutionary highly conserved SWI/SNF-associated protein, known to be involved in transcriptional regulation. Sequencing of cDNA and genomic DNA identified a splice donor-site mutation leading to intron-integration with premature termination of protein translation in hst-mutant zebrafish. Injection of wildtype mRNA into hst-mutant embryos fully rescues the mutant phenotype, while injection of morpholino-modified antisense oligonucleotides directed against hst-transcripts copies the hst phenotype, demonstrating the causality of the identified mutation. In order to define how the mutation of a SWI/SNF-associated protein causes hyperplasia of cardiac myocytes we conducted yeast-two-hybrid screens with a cardiac cDNA-library and the human homologue of hst as a bait. Surprisingly, we identified several sarcomeric proteins such as troponin T and α-actinin as robust interactors. By immunofluorescence staining we indeed found distinct nuclear and sarcomeric pools of the hst homologue in rat heart sections. It appears, that this SWI/SNF-associated protein shuttles between the sarcomere and the nucleus in order to transmit proliferative signals from the contractile apparatus to the transcriptional machinery.