Therapeutic Targeting Of Telomere Diseases Via Novel Small Molecule Modulators Of Non-Coding Rna Biogenesis

Germline mutations that impair telomere maintenance cause dyskeratosis congenita (DC), and predispose to hematologic diseases including bone marrow failure, myelodysplastic syndrome, and acute myeloid leukemia. Mutations in several DC-associated genes (DKC1, NOP10, NHP2, NAF1, PARN, TERC) act by disrupting steady-state levels of the essential non-coding RNA component of telomerase (TERC), which is limiting for telomerase activity and consequently self-renewal capacity in human stem cells. Ectopic expression of TERC RNA restores telomere length in cellular models of DC, but this approach poses major challenges for clinical translation. Based on insights from genetic discoveries in DC, we hypothesized that manipulating non-coding RNA pathways could be therapeutically useful to augment TERC in patients with telomere diseases. We and others recently demonstrated that PARN mutations cause DC via destabilization of nascent TERC RNA. Specifically, PARN is an RNA exonuclease that is required to remove post-transcriptionally added, non-genomically encoded adenosine residues - “oligo(A) tails” - that target TERC for destruction by the nuclear exosome (Fig. A). We subsequently identified the non-canonical poly(A) polymerase PAPD5 as the enzyme responsible for TERC oligo-adenylation and destabilization (Fig. A), and demonstrated that RNA interference of PAPD5 restored telomere length in DC patient cells. To further address this, we disrupted PAPD5 by CRISPR/Cas9-mediated genome editing in DC patient induced pluripotent stem cells (iPSCs) and heterologous human cell lines. We found that graded deficiencies in PAPD5 resulted in dose-dependent increases in TERC RNA and telomere length, and that PAPD5-null clones could be isolated and propagated indefinitely. These results provide genetic evidence of an unanticipated therapeutic window for PAPD5 inhibition, as a potential strategy to modulate TERC RNA levels.