Abstact

Human telomerase processively adds telomeric repeats (dGGTTAG) to chromosome 3'-ends to maintain telomere length. While mostly absent in somatic cells, telomerase is aberrantly upregulated in most tumor cells to sustain cellular immortality, making it a promising oncology target. However, to date there are no reported structures of human telomerase with inhibitor, impeding structure-based drug design and optimization. We report nine cryo-electron microscopy structures of human telomerase with and without BIBR1532, a highly selective small-molecule telomerase inhibitor. Unexpectedly, BIBR1532 binds a previously unknown pocket between TERT finger and palm. BIBR1532 inhibits each step but disproportionately affects the rate-limiting first step of telomere repeat nucleotide addition. The structures reveal a rigid finger that explains telomerase's slow rate and low fidelity. Our study provides insights into telomerase catalytic mechanism and its inhibition by BIBR1532, explains why prior BIBR derivatives did not improve potency and suggests a rational approach for design of small-molecule telomerase inhibitors.