Abstract 1435: A GABP dominant negative approach to targeting tumor cell immortality

Abstract

Abstract Immortality is a hallmark of human cancer cells and therapeutic reversal is of great interest. Telomerase Reverse Transcriptase promoter (TERTp) mutations reactivate TERT expression, the rate limiting step in telomerase activity and cellular immortality. While prior telomerase blocking therapies lack tumor selectivity and were poorly tolerated, TERTp mutations and their regulation offer a unique opportunity for tumor specific reversal of cellular immortality. TERTp mutations are the most common non-coding mutation across all cancers, including glioblastoma (GBM), melanoma, urothelial carcinoma, and many others. The mutations generate a de novo E26 Transformation Specific (ETS) binding motif that recruits the GA-Binding Protein (GABP) multimeric complex to reactivate TERT expression. While GABP can form both a dimeric and tetrameric complex, we found that the tetrameric complex is responsible for reactivation of the mutant TERTp, suggesting selectively targeting the tetramer may induce telomere shortening and tumor cell death. However, through knocking out the tetramer, we found that the GABP dimer is sufficiently upregulated to maintain TERT expression. In the absence of the dimer and the tetramer isoform, many tumor cells senesce or undergo cell death. However, some escape this fate via upregulation of a second tetramer forming GABP paralogue that maintains TERT expression. Therefore, we devised a transactivation domain null GABP subunit that we hypothesized would act in a dominant negative manner to block the GABP dimer and both GABP tetramers. Introduction of the dominant negative construct into TERTp mutant GBM, meningioma, medulloblastoma, oligodendroglioma, and urothelial carcinoma cells significantly reduced TERT expression while not altering expression in TERTp wildtype GBM cells. Furthermore, the dominant negative is able to shorten telomere length in TERTp mutant GBM cells. While therapeutic targeting of the three GABP complexes seems particularly challenging, our initial dominant negative appears to do so. Ultimately the dominant negative could be delivered directly into tumors via clinically approved viral delivery systems such as retroviral replicating vectors. Citation Format: Nicholas O. Stevers, Sara A. Collins, Samuel H. Wu, Noriyuki Kasahara, Joseph F. Costello. A GABP dominant negative approach to targeting tumor cell immortality [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1435.