Informational Suppression to Probe RNA:RNA Interactions in the Context of Ribonucleoproteins: U1 and 5' Splice-Site Base-Pairing

Alternative polyadenylation expands the mRNA isoform repertoire of human CD46

Alternative polyadenylation is a prevalent mechanism regulating mammalian gene expression. While tandem 3'-Untranslated-Region (3'UTR) polyadenylation changes expression levels, Intronic PolyAdenylation generates shorter transcripts encoding truncated proteins. Intronic PolyAdenylation regulates 20% of genes and is especially common in receptor tyrosine-kinase transcripts, generating soluble repressors. Here we report that human CD46, encoding a TransMembrane repressor of complement and T-cell co-stimulator, expresses multiple isoforms by alternative polyadenylation. We provide evidence for polyadenylation at several introns by RT-PCR of 5' intronic fragments, and by increase in such isoforms via functional U1 knockdown. We mapped various Intronic PolyAdenylation Sites by 3' Rapid Amplification of cDNA Ends (3'RACE), which could generate soluble or membrane-bound but tail-less CD46. Intronic PolyAdenylation could add to the source of soluble CD46 isoforms in fluids and tissues, which increase in cancers and autoimmune syndromes. Furthermore, 3'RACE identified three PolyAdenylation Sites within the last intron and exon, whose transcripts with shortened 3'UTRs could support higher CD46 expression. Finally, 3'RACE revealed that the CD46 Pseudogene only expresses short transcripts by early polyadenylation in intron 2. Overall, we report a wide variety of CD46 mRNA isoforms which could generate new protein isoforms, adding to the diverse physiological and pathological roles of CD46.

Characterization of the Regulation of CD46 RNA Alternative Splicing

Here we present a detailed analysis of the alternative splicing regulation of human CD46, which generates different isoforms with distinct functions. CD46 is a ubiquitous membrane protein that protects host cells from complement and plays other roles in immunity, autophagy, and cell adhesion. CD46 deficiency causes an autoimmune disorder, and this protein is also involved in pathogen infection and cancer. Before this study, the mechanisms of CD46 alternative splicing remained unexplored even though dysregulation of this process has been associated with autoimmune diseases. We proved that the 5' splice sites of CD46 cassette exons 7 and 8 encoding extracellular domains are defined by noncanonical mechanisms of base pairing to U1 small nuclear RNA. Next we characterized the regulation of CD46 cassette exon 13, whose inclusion or skipping generates different cytoplasmic tails with distinct functions. Using splicing minigenes, we identified multiple exonic and intronic splicing enhancers and silencers that regulate exon 13 inclusion via trans-acting splicing factors like PTBP1 and TIAL1. Interestingly, a common splicing activator such as SRSF1 appears to repress CD46 exon 13 inclusion. We also report that expression of CD46 mRNA isoforms is further regulated by non-sense-mediated mRNA decay and transcription speed. Finally, we successfully manipulated CD46 exon 13 inclusion using antisense oligonucleotides, opening up opportunities for functional studies of the isoforms as well as for therapeutics for autoimmune diseases. This study provides insight into CD46 alternative splicing regulation with implications for its function in the immune system and for genetic disease.