The emerging significance of splicing in vertebrate development.

Splicing is a crucial regulatory node of gene expression that has been leveraged to expand the proteome from a limited number of genes. Indeed, the vast increase in intron number that accompanied vertebrate emergence might have aided the evolution of developmental and organismal complexity. Here, we review how animal models for core spliceosome components have provided insights into the role of splicing in vertebrate development, with a specific focus on neuronal, neural crest and skeletal development. To this end, we also discuss relevant spliceosomopathies, which are developmental disorders linked to mutations in spliceosome subunits. Finally, we discuss potential mechanisms that could underlie the tissue-specific phenotypes often observed upon spliceosome inhibition and identify gaps in our knowledge that, we hope, will inspire further research.

Trp53 ablation fails to prevent microcephaly in mouse pallium with impaired minor intron splicing.

Minor spliceosome inhibition due to mutations in RNU4ATAC are linked to primary microcephaly. Ablation of Rnu11, which encodes a minor spliceosome snRNA, inhibits the minor spliceosome in the developing mouse pallium, causing microcephaly. There, cell cycle defects and p53-mediated apoptosis in response to DNA damage resulted in loss of radial glial cells (RGCs), underpinning microcephaly. Here, we ablated Trp53 to block cell death in Rnu11 cKO mice. We report that Trp53 ablation failed to prevent microcephaly in these double knockout (dKO) mice. We show that the transcriptome of the dKO pallium was more similar to the control compared with the Rnu11 cKO. We find aberrant minor intron splicing in minor intron-containing genes involved in cell cycle regulation, resulting in more severely impaired mitotic progression and cell cycle lengthening of RGCs in the dKO that was detected earlier than in the Rnu11 cKO. Furthermore, we discover a potential role of p53 in causing DNA damage in the developing pallium, as detection of γH2aX+ was delayed in the dKO. Thus, we postulate that microcephaly in minor spliceosome-related diseases is primarily caused by cell cycle defects.

Disruption of exon-bridging interactions between the minor and major spliceosomes results in alternative splicing around minor introns.

Vertebrate genomes contain major (>99.5%) and minor (<0.5%) introns that are spliced by the major and minor spliceosomes, respectively. Major intron splicing follows the exon-definition model, whereby major spliceosome components first assemble across exons. However, since most genes with minor introns predominately consist of major introns, formation of exon-definition complexes in these genes would require interaction between the major and minor spliceosomes. Here, we report that minor spliceosome protein U11-59K binds to the major spliceosome U2AF complex, thereby supporting a model in which the minor spliceosome interacts with the major spliceosome across an exon to regulate the splicing of minor introns. Inhibition of minor spliceosome snRNAs and U11-59K disrupted exon-bridging interactions, leading to exon skipping by the major spliceosome. The resulting aberrant isoforms contained a premature stop codon, yet were not subjected to nonsense-mediated decay, but rather bound to polysomes. Importantly, we detected elevated levels of these alternatively spliced transcripts in individuals with minor spliceosome-related diseases such as Roifman syndrome, Lowry-Wood syndrome and early-onset cerebellar ataxia. In all, we report that the minor spliceosome informs splicing by the major spliceosome through exon-definition interactions and show that minor spliceosome inhibition results in aberrant alternative splicing in disease.

Minor intron-containing genes as an ancient backbone for viral infection?

Minor intron-containing genes (MIGs) account for <2% of all human protein-coding genes and are uniquely dependent on the minor spliceosome for proper excision. Despite their low numbers, we surprisingly found a significant enrichment of MIG-encoded proteins (MIG-Ps) in protein-protein interactomes and host factors of positive-sense RNA viruses, including SARS-CoV-1, SARS-CoV-2, MERS coronavirus, and Zika virus. Similarly, we observed a significant enrichment of MIG-Ps in the interactomes and sets of host factors of negative-sense RNA viruses such as Ebola virus, influenza A virus, and the retrovirus HIV-1. We also found an enrichment of MIG-Ps in double-stranded DNA viruses such as Epstein-Barr virus, human papillomavirus, and herpes simplex viruses. In general, MIG-Ps were highly connected and placed in central positions in a network of human-host protein interactions. Moreover, MIG-Ps that interact with viral proteins were enriched with essential genes. We also provide evidence that viral proteins interact with ancestral MIGs that date back to unicellular organisms and are mainly involved in basic cellular functions such as cell cycle, cell division, and signal transduction. Our results suggest that MIG-Ps form a stable, evolutionarily conserved backbone that viruses putatively tap to invade and propagate in human host cells.