A non-natural nucleotide uses a specific pocket to selectively inhibit telomerase activity

Abstract

Telomerase, a unique reverse transcriptase that specifically extends the ends of linear chromosomes, is up-regulated in the vast majority of cancer cells. Here, we show that an indole nucleotide analog, 5-methylcarboxyl-indolyl-2′-deoxyriboside 5′-triphosphate (5-MeCITP), functions as an inhibitor of telomerase activity. The crystal structure of 5-MeCITP bound to the Tribolium castaneum telomerase reverse transcriptase reveals an atypical interaction, in which the nucleobase is flipped in the active site. In this orientation, the methoxy group of 5-MeCITP extends out of the canonical active site to interact with a telomerase-specific hydrophobic pocket formed by motifs 1 and 2 in the fingers domain and T-motif in the RNA-binding domain of the telomerase reverse transcriptase. In vitro data show that 5-MeCITP inhibits telomerase with a similar potency as the clinically administered nucleoside analog reverse transcriptase inhibitor azidothymidine (AZT). In addition, cell-based studies show that treatment with the cell-permeable nucleoside counterpart of 5-MeCITP leads to telomere shortening in telomerase-positive cancer cells, while resulting in significantly lower cytotoxic effects in telomerase-negative cell lines when compared with AZT treatment.

Biotransformation of Modified Benzylisoquinoline Alkaloids: Boldine and Berberine and In Silico Molecular Docking Studies of Metabolites on Telomerase and Human Protein Tyrosine Phosphatase 1B

“…Telomerase is a multicomponent enzyme which includes a catalytic telomerase reverse transcriptase (TERT) protein and a noncoding RNA molecule which serves as the template for the synthesis of telomere DNA [ 31 ]. Telomerase inhibition results in a sequential telomere shortening that triggers apoptosis or senescence in cancer cells [ 32 ].…”

Mechanisms of telomerase inhibition by oxidized and therapeutic dNTPs

“…Polymerase studies with 8-oxo-dATP are lacking, but mammalian DNA polymerases insert dTTP or dGTP opposite a template 8-oxodA and accommodate the base pairs through base tautomerization and interactions with active site residues 49 . Telomerase may lack contacts required to stabilize an 8-oxodA base pair for catalysis, or 8-oxo-dATP may occupy the active site in a non-productive manner blocking natural dNTP access, similar to telomerase inhibitor 5-MeCITP 50 . In contrast, a 2-OH-dATP:rU base pair is well tolerated in the telomerase active site since 2-OH-dATP insertion opposite rU is only 2-fold less efficient than dATP.…”

Telomerase in Cancer: Function, Regulation, and Clinical Translation

“…Recent efforts aimed at the therapeutic targeting of telomerase have been centered on enzyme inhibition, cytotoxic substrate incorporation, telomere destabilization, and antitelomerase immunotherapy [189]. Enzyme inhibitors include both small molecules and TR template antagonists, both of which have demonstrated antitumor activity in multiple preclinical cancer models [190][191][192]. Moreover, the inhibitors BIBR1532 and GRN163L (also known as Imetelstat) have been assessed in several clinical trials across diverse cancer types and in recurrent and metastatic disease settings [193].…”