Alternative polyadenylation alters protein dosage by switching between intronic and 3′UTR sites

Alternative polyadenylation (APA) creates distinct transcripts from the same gene by cleaving the pre-mRNA at poly(A) sites that can lie within the 3′ untranslated region (3′UTR), introns, or exons. Most studies focus on APA within the 3′UTR; however, here, we show that CPSF6 insufficiency alters protein levels and causes a developmental syndrome by deregulating APA throughout the transcript. In neonatal humans and zebrafish larvae, CPSF6 insufficiency shifts poly(A) site usage between the 3′UTR and internal sites in a pathway-specific manner. Genes associated with neuronal function undergo mostly intronic APA, reducing their expression, while genes associated with heart and skeletal function mostly undergo 3′UTR APA and are up-regulated. This suggests that, under healthy conditions, cells toggle between internal and 3′UTR APA to modulate protein expression. CPSF6 loss disrupts polyadenylation site choice and causes a developmental syndrome in humans and zebrafish.