Abstract A37: Anti-telomerase therapy provokes ALT and mitochondrial adaptive mechanism in cancer

Because telomerase is activated in 85–90% of all human cancers, inhibition of telomerase activity has been proposed as an anti-cancer strategy. However, telomere erosion resulting from telomerase inhibition can generate rampant genomic instability which may fuel adaptive mechanisms. Here, we determined the impact of telomerase activation and its subsequent extinction in T-cell lymphomas arising in Atm −/− mice engineered with an inducible telomerase reverse transcriptase-estrogen receptor ( TERT-ER ) knock-in allele. Endogenous telomerase reactivation in late generation TERT ER/ER Atm −/− mice with telomere dysfunction enabled full malignant progression with accompanying alleviation of telomere checkpoint signaling and acquisition of copy number alterations targeting genes known to be important in human T-cell lymphomas/leukemia. Upon telomerase extinction, tumors eventually showed impaired growth due to reinstatement of telomere dysfunction-induced checkpoint signaling, yet subsequently resumed growth with accompanying alternative lengthening of telomeres (ALT) features. Transcriptomic analysis of ALT+ tumors showed altered transcriptional networks centering on mitochondrial biology and oxidative defense, and genomic analysis showed amplification of PGC-1β, a master regulator of mitochondrial biogenesis and oxidative defense. Relative to telomerase+ tumors, ALT+ tumors showed marked sensitivity to PGC-1β or SOD2 knockdown which compromised mitochondrial function and/or caused increased reactive oxygen species (ROS). Together, these studies provide in vivo genetic evidence that naturally arising tumors adapt to telomerase extinction by acquisition of both ALT and mitochondrial maintenance mechanisms, and show that targeting mitochondrial function and oxidative defense may enhance the effectiveness of anti-telomerase therapy in cancer.