The exon junction complex component CASC3 is a major effector of nonsense-mediated mRNA decay in human cells

In metazoans, the exon junction complex (EJC) is a central component of spliced messenger ribonucleoprotein particles (mRNPs). CASC3 has been reported to be a core component of the EJC and to be crucial for assembly, the splicing regulating function of the EJC and nonsense-mediated mRNA decay (NMD). However, recent evidence suggests that CASC3 functions differently from other EJC core components. To elucidate the cellular role of CASC3, we have established human cell lines in which CASC3 was inactivated by means of CRISPR-Cas9 genome editing. We show that in these cells CASC3 is dispensable for the splicing regulatory role of the EJC. However, we find that CASC3 depletion results in the upregulation of many known and novel NMD substrates, suggesting that CASC3 is required for the efficient execution of EJC-dependent NMD. Taken together, our results challenge the model of CASC3 as an assembly factor and core component of the EJC. Our data rather show that CASC3 is involved in the degradation of NMD substrates and therefore uncover the primary molecular function of CASC3 in human cells.