CBIO-26. A NEW MODEL OF THE GABP-TERT REGULATORY AXIS IN MAINTAINING IMMORTALITY OF TERT PROMOTER MUTANT TUMOR CELLS

Tumor cell immortality is a fundamental hallmark of human cancers. Normally silenced during somatic cell differentiation, 90% of human tumors reactivate Telomerase Reverse Transcriptase (TERT) expression to achieve cellular immortality. TERT, the catalytic subunit of telomerase, complexes with the RNA template molecule TERC to maintain telomeres. Mutations in the TERT promoter (TERTp) are the most common non-coding mutation across all cancer types and the most frequent mutation within many cancers, such as IDH wildtype glioblastoma (GBM), Melanoma, and Bladder Cancer. TERTp mutations generate de novo E26 Transformation Specific (ETS) binding motifs that are spaced full helical turns from TERTp native ETS sites. Together the de novo and native ETS motifs specifically recruit the GABP tetrameric complex but not the GABP dimer. CRISPR-cas9 mediated insertion/deletion mutagenesis of the unique exon of GABP tetramer forming subunit, GABPB1L (B1L), reduces TERT transcriptional activity in a TERT promoter-mutation dependent manner. Here we show that GABPB1S (B1S), the GABP dimer restricted alternative isoform of GABPB1, is consistently and significantly increased following B1L reduction, a process we have determined to be driven by a conserved homeostatic mechanism whereby the GABP tetramer suppresses expression of one of its own components, GABPB1. In contrast to the native setting, in the absence of B1L the elevated B1S expression leads to dimer binding to the mutant TERTp and maintenance of TERT expression. Indeed, co-targeting B1L and B1S together, but not B1L alone, via CRISPR-cas9 knockout resulted in a near complete elimination of GABP recruitment to the TERTp and TERT expression, and lead to tumor cell death and eventual senescence in a telomere length dependent manner. Together, this data suggests a new model of the TERT-GABP axis involving the tetramer and dimer and highlights a new and potentially more potent therapeutic strategy to eliminate TERT expression and reverse tumor cell immortality.