LAUSR

BACKGROUND Glioblastoma is among the deadliest of all cancers, with 5-year survival rates of only 6%. Glioblastoma targeted therapeutics have been challenging to develop due to significant inter- and intra-tumoral heterogeneity. TERT promoter mutations are the most common known clonal oncogenic mutations in glioblastoma. Telomerase is therefore considered to be a promising therapeutic target against this tumor. However, an important limitation of this strategy is that cell death does not occur immediately after telomerase ablation, but rather after several cell divisions required to reach critically short telomeres. We therefore hypothesize that telomerase inhibition would only be effective in low tumor burden glioblastomas. METHODS We used CRISPR interference to knock down TERT expression in TERT promoter-mutant glioblastoma cell lines and patient derived models. We then measured viability using serial proliferation assays. We also assessed for features of telomere crisis by measuring telomere length and chromatin bridge formation. Lastly, we used a doxycycline-inducible CRISPR interference system to knock down TERT expression in vivo early and late in the tumor formation process. RESULTS Upon TERT inactivation, glioblastoma cells lose their proliferative ability over time and exhibit evidence of telomere crisis with telomere shortening and chromatin bridge formation. In vivo, tumor formation is only inhibited when TERT knockdown is induced shortly after tumor implantation, but not when tumor burden is high. CONCLUSIONS Our results support the idea that telomerase inhibition would be most effective at treating glioblastomas with low tumor burden, for example in the adjuvant setting after surgical debulking and chemoradiation.