Human telomerase RNA processing and quality control

Primary telomerase RNA transcripts are processed into shorter mature forms that assemble into a complex with the catalytic subunit and provide the template for telomerase activity. In diverse fungi telomerase RNA 3' end processing involves a single cleavage reaction by the spliceosome akin to the first step of splicing. Longer forms of human telomerase RNA (hTR) have been reported, but how the mature form of precisely 451 nucleotides is generated is still unknown. We now show that the splicing inhibitor isoginkgetin causes accumulation of long hTR transcripts, but find no evidence for a direct role for splicing in hTR processing. Instead, isoginkgetin mimics the effects of inhibiting the RNA exosome. Depletion of exosome components and accessory factors reveals functions for the cap binding complex (CBC) and the nuclear exosome targeting (NEXT) complex in hTR turnover. Whereas longer transcripts are predominantly degraded, shorter precursor RNAs are oligo-adenylated by TRF4-2 and either processed by poly (A) specific ribonuclease (PARN) or degraded by the exosome. Our results reveal that hTR biogenesis involves a kinetic competition between RNA processing and quality control pathways and suggest new treatment options for dyskeratosis congenita caused by mutations in RNA processing factors. Overall design: We cloned and sequenced 3' ends by RLM-RACE coupled with high-throughput sequencing to gain further insights into hTR processing.