Small-Molecule PAPD5 Inhibitors Restore Telomerase Activity in Patient Stem Cells

Genetic lesions that reduce telomerase cause a range of incurable diseases including dyskeratosis congenita (DC) and pulmonary fibrosis (PF), and restoring telomere length in these patients would be curative. Ectopic expression of telomerase reverse transcriptase (TERT) risks cellular immortalization, and how to target telomerase in stem cells throughout the body remains unclear. Here we describe a successful screen for small molecules that augment TERC, the non-coding telomerase RNA component, and thereby specifically elongate telomeres in stem cells. PAPD5 is a non-canonical polymerase that oligo-adenylates and destabilizes TERC. Using a high-throughput screen we identify BCH001, a specific PAPD5 inhibitor that decreases TERC 3′- oligo(A) tailing and increases telomerase activity and telomere length by thousands of nucleotides in DC patient induced pluripotent stem cells (iPSCs). BCH001 does not result in immortalization or telomere elongation in somatic cells which lack TERT, establishing a favorable safety profile. When human hematopoietic stem and progenitor cells (HSPCs) engineered by CRISPR-Cas9 to carry PARN mutations that cause DC and PF are xenotransplanted into immunodeficient mice, oral treatment with PAPD5 inhibitors rescues TERC 3′-end maturation and telomere length. Our data demonstrate telomere restoration in human stem cells in vivo using small molecules.