EVALUATION OF 6-THIO-2’-DEOXYGUANOSINE (6-Thio-dG), A POTENTIAL TELOMERASE-TARGETED ANTI-CANCER AGENT, IN TELOMERASE-POSITIVE AND TELOMERASE-NEGATIVE CELLS
Telomeres are repetitive DNA sequences (TTAGGG) at the end of linear chromosomes. Telomeres are capped by protein complexes that protect them from DNA damage and from being recognized as double-stranded breaks. As cells continuously divide, telomere length shortens due to the end replication problem. Very short or uncapped telomeres are recognized as DNA damage causing the cells to undergo replicative senescence or apoptosis. Cancer cells evade replicative senescence and acquire genetic mutations that maintain telomeres’ length either by activating telomerase or via a homologous recombination-based mechanism termed, Alternative Lengthening of Telomeres (ALT). Telomerase is reactivated in ~90% of cancers and thus represents an attractive target for anti-cancer therapy. Recent studies have shown that 6-thio-2’-deoxyguanosine (6-thio-dG) is preferentially incorporated into telomeres by telomerase, leading to telomere uncapping and selectively triggering cell death in telomerase-positive cancer cells. However, more insight is needed into the mechanism of action of 6-thio-dG and its relative efficacy in a broader spectrum of cancers, including the ~10% of cancers that maintain telomeres via ALT. In the present study, we utilized cancerous and non-cancerous isogenic cell lines predominately of human prostate origin, that differ in their telomere maintenance mechanism (TMM); being either telomerase-positive or telomerase-negative,. Based on the proposed mechanism of 6-thio-dG, we hypothesized that telomerase-negative cells would be more resistant to 6-thio-dG treatment compared to telomerase-positive cells, especially within the same isogenic background. Cytotoxicity, cell growth, clonogenicity, and telomere-length analyses in our panel of telomerase-positive and telomerase-negative cells, however, did not show a significant dependence of 6-thio-dG on telomerase expression. Both telomerase-positive and -negative cells were almost equally sensitive (cell death) to treatment with 6-thio-dG. We also demonstrate that neither apoptosis nor the cGAS/STING pathway were activated upon treatment with 6-thio-dG regardless of telomerase expression status. Furthermore, DNA damage was upregulated only in mouse embryonic fibroblasts (MEFs) in response to 6-thio-dG, but this was independent of telomerase expression. In conclusion, based on our present study, 6-thio-dG treatment results in non-selective cell death independent of telomerase status. As this is the first study to investigate the effects of 6-thio-dG in benign and malignant prostate cells, such telomerase-independent cytotoxicity by 6-thio-dG could be specific to prostate cells; thus, warranting further study in other tumor types
