The Role of Alternative mRNA Splicing in Heart Development

Research in the last 10 years has led to improved understanding of the genetic regulation of vertebrate heart development, but despite this effort, approximately 70% of all congenital heart defects (CHDs) still have an unknown etiology. Alternative splicing of mRNA has been documented to play roles in normal and abnormal development. Dysregulated splicing of mRNA has been shown to cause heart defects in mice, however a link between mRNA splicing and CHDs has not yet been shown in humans. We reported that more than 50% of genes associated with heart development were alternatively spliced in the right ventricle (RV) of infants with tetralogy of Fallot (TOF) relative to the RV of normally developing infants. Moreover, there was a significant decrease in the level of 12 scaRNAs (small cajal body associated RNAs) in the RV from infants with TOF. These small noncoding RNAs guide the biochemical modification of specific nucleotides in spliceosomal RNAs that are critical for spliceosomal function. We used primary cells derived from the RV of infants with TOF to show a direct link between scaRNA levels and alteration in mRNA splicing of several genes that regulate heart development. We modified the expression of sets of scaRNAs and consequentially documented distinctive mRNA splicing, accompanied by corresponding protein isoform changes suggesting a unique contribution by each scaRNA. Furthermore, we knocked down two homologous scaRNAs in zebrafish and saw a disruption of heart development with an accompanying alteration in splice isoforms of cardiac regulatory genes. These combined results provide compelling evidence that scaRNAs contribute to the regulation of cardiac development by fine-tuning the fidelity of the spliceosome that adjusts exon retention as cell differentiation occurs. Importantly, our findings are consistent with the concept that disruption of mRNA splicing patterns during early embryonic development disturbs normal signaling pathways, resulting in conotruncal misalignment and TOF.