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Telomerase reverse transcriptase expression protects transformed human cells against DNA-damaging agents, and increases tolerance to chromosomal instability

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Reactivation of telomerase reverse transcriptase (TERT) expression is found in more than 85% of human cancers. The remaining cancers rely on the alternative lengthening of telomeres (ALT), a recombination-based mechanism for telomere-length maintenance. Prevalence of TERT reactivation over the ALT mechanism was linked to secondary TERT function unrelated to telomere length maintenance. To characterize this non-canonical function, we created a panel of ALT cells with recombinant expression of TERT and TERT variants: TERT-positive ALT cells showed higher tolerance to genotoxic insults compared with their TERT-negative counterparts. We identified telomere synthesis-defective TERT variants that bestowed similar genotoxic stress tolerance, indicating that telomere synthesis activity is dispensable for this survival phenotype. TERT expression improved the kinetics of double-strand chromosome break repair and reduced DNA damage-related nuclear division abnormalities, a phenotype associated with ALT tumors. Despite this reduction in cytological abnormalities, surviving TERT-positive ALT cells were found to have gross chromosomal instabilities. We sorted TERT-positive cells with cytogenetic changes and followed their growth. We found that the chromosome-number changes persisted, and TERT-positive ALT cells surviving genotoxic events propagated through subsequent generations with new chromosome numbers. Our data confirm that telomerase expression protects against double-strand DNA (dsDNA)-damaging events, and show that this protective function is uncoupled from its role in telomere synthesis. TERT expression promotes oncogene-transformed cell growth by reducing the inhibitory effects of cell-intrinsic (telomere attrition) and cell-extrinsic (chemical- or metabolism-induced genotoxic stress) challenges. These data provide the impetus to develop new therapeutic interventions for telomerase-positive cancers through simultaneous targeting of multiple telomerase activities.

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We thank members of Judy Wong’s laboratory for providing helpful insights. Kathleen Collins and Bill Hahn kindly provided reagents. Shirley Yeung, Irene Iu and Arthur Chan assisted with method development. Nigel O’Neil assisted with optimization of the chromosomal healing assay. Andy Johnson from the UBC Flow-Cytometry Facility provided expertise with FACS procedures. We thank them for their assistance. Financial support for this work was provided by the Canadian Cancer Society Research Institute (operating grant #019250), and Research Reinvestment Funds from the Faculty of Pharmaceutical Sciences at UBC. JMYW is supported by the Michael Smith Foundation for Health Research Career Development program.

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Correspondence to J M Y Wong.

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Fleisig, H., Hukezalie, K., Thompson, C. et al. Telomerase reverse transcriptase expression protects transformed human cells against DNA-damaging agents, and increases tolerance to chromosomal instability. Oncogene 35, 218–227 (2016).

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