U1 AMO (antisense morpholino oligos) disrupts U1 snRNP structure to promote intronic PCPA (premature cleavage and polyadenylation)

Functional depletion of U1 snRNP with a 25 nt U1 AMO (antisense morpholino oligonucleotides) may lead to intronic premature cleavage and polyadenylation (PCPA) of thousands of genes, a phenomenon known as U1 snRNP telescripting; however, the underlying mechanism remains elusive. In this study, we demonstrated that U1 AMO could disrupt U1 snRNP structure both in vitro and in vivo, thereby affecting U1 snRNP/RNAP polymerase II (RNAPII) interaction. We further showed that U1 AMO treatment might promote RNAPII disassociation with pre-mRNA in an RNA pull-down assay. By performing ChIP-seq for phosphorylation of Ser2 (Ser2P) and Ser5 (Ser5P) of the C-terminal domain (CTD) of RNA polymerase II (RNAPII), we showed that transcription elongation was disturbed upon U1 AMO treatment, with a particular high Ser2P signal at intronic cryptic polyadenylation sites (PASs). In addition, we showed that core 3 processing factors CPSF/CstF are involved in the processing of intronic cryptic PAS. Their recruitment accumulated toward cryptic PASs upon U1 AMO treatment, as indicated by ChIP-seq and iCLIP-seq analysis. Furthermore, we showed that most of these PCPAed transcripts could be exported to cytoplasm and have the potential to be translated. Conclusively, our data provide more insight into U1 snRNP telescripting, and suggest a common theme that modulation of transcription elongation may be an important mode for the regulation of mRNA polyadenylation.