Abstract 18015: Loss of Swi/snf-complex Subunit Heart of Stone Leads to Cardiac Hyperplasia in Zebrafish

Fish and also neonatal mice possess the capacity to regenerate their hearts after myocardial damage. The adult mammalian heart only has a very limited capacity to regenerate after cardiac injury. Myocardial regeneration in zebrafish and newborn mice involves the dedifferentiation and proliferation of resident cardiomyocytes to replace the injured tissue. Unfortunately, only little is known about the factors and signaling cascades that regulate this regenerative capacity. In a forward genetics ENU-screen in zebrafish, we recently isolated the mutant heart of stone (hos). Hearts of hos mutant embryos show a severe thickening of the ventricle with almost complete obliteration of the ventricle. Based on cell count assays using histological sections we find significantly increased cell numbers in hos mutant ventricles. Based on BrdU incorporation assays as well as H3P and PCNA stainings, we find that ventricular hyperplasia in hos is caused by significantly enhanced cardiomyocyte proliferation. By positional cloning we show a splice donor site mutation within the zebrafish hos gene. hos is a subunit of the SWI/SNF chromatin remodeling complex, which is known to be involved in transcriptional regulation. The identified hos mutation specifically leads to an integration of Intron7 resulting in a reading frame shift and thereby to premature termination of hos protein translation. Injection of wild-type hos mRNA in hos mutant embryos restores the wild-type, while injection of morpholinos directed against the splice donor site of Exon7 phenocopy the hos mutant phenotype. In addition, by generating hos-null zebrafish using morpholinos directed against the translational start site of hos, we are also able to induce ventricular hyperplasia, suggesting that the hos mutation results in the complete loss of hos function. Interestingly, in contrast to hos-deficient embryos, targeted knockdown of the zebrafish SWI/SNF-complex core component BRG1 leads to severe hypoplastic hearts, indicating that hos in zebrafish might play a repressive role on SWI/SNF complex function and the control of cardiomyocyte proliferation in vivo.