Impaired myocardial contractility underlies an Rbfox-mediated zebrafish model of hypoplastic left heart syndrome

Here, we report that zebrafish lacking two orthologs of the RNA binding protein RBFOX2, which was previously linked to HLHS in newborns, display cardiovascular defects overlapping those in HLHS patients. In contrast to current models, we demonstrate that co-existing ventricular, valve, and aortic deficiencies in rbfox mutant zebrafish arise secondary to impaired myocardial contractility. On a molecular and cellular level, we find diminished expression and alternative splicing of sarcomere and mitochondrial components that compromise sarcomere assembly and mitochondrial respiration, respectively. Injection of human RBFOX2 rescues ventricular structure and function in rbfox mutant zebrafish, demonstrating conservation of molecular function between humans and zebrafish Rbfox proteins. However, injection of RBFOX2 variants previously identified in newborns with HLHS do not. Taken together, our data suggest that mutations in RBFOX2 are causal for HLHS and challenge the obligate multigenic etiology of the disease. Instead, we provide a complimentary paradigm where HLHS can arise from a single gene mutation where ventricular, valve, and aortic deficiencies manifest secondary to a primary myocardial defect. Overall design: Heart mRNA profiles of 48hpf wild type (WT) and DKO (rbfox1l-/-; rbfox2-/-) zebrafish