Regulation of alternative polyadenylation by the C2H2-zinc finger protein Sp1 [CLIP-seq]

About 70% of human genes carry multiple polyadenylation signals, and mRNA 3’end formation is dynamically regulated under different physiological conditions. Global 3’end shortening through alternative polyadenylation (APA) correlates with enhanced cellular proliferation, and 3’ untranslated region (UTR) shortening is a widespread phenomenon in tumour cells, where it appears to enhance tumorigenic properties. However, the mechanisms responsible for this dynamic APA regulation remain incompletely understood. Here we show that transcription factor Sp1 binds directly to RNA in vivo and is a common repressor of distal poly (A) site usage. RNA-sequencing (RNA-seq) analysis identified 2344 genes (36% of total mapped mRNA transcripts) with lengthened 3’UTRs upon Sp1 depletion. Sp1 preferentially binds in vivo within the 3’UTRs of many of these lengthened transcripts and inhibits cleavage at distal sites by interacting physically with subunits of the core cleavage and polyadenylation (CPA) machinery. The 3’UTR lengths of Sp1 target genes in breast cancer patient RNA-seq data correlate with Sp1 expression levels, implicating Sp1-mediated APA regulation in modulating tumorigenic properties. Taken together, our findings reveal an important mechanism for dynamic APA regulation by unraveling a novel function of Sp1. iCLIP-seq of GFP-tagged and endogenous proteins in HEK293 cells