EXTH-77. An AI-engineered bioPROTAC reverses immortality of TERT promoter mutant tumors

Telomerase Reverse Transcriptase promoter (TERTp) mutations enable tumor cell immortality in millions of cancer patients annually. TERTp mutations are the most common non-coding mutations across all cancers, including glioblastoma, oligodendroglioma, medulloblastoma, and high-grade meningioma. A multitude of TERTp mutations, including the two hotspots, create de novo E26 transformation specific transcription factor binding sites. Among the 28 family members, we have shown that only the GA-binding protein (GABP, composed of DNA binding GABPA and transactivating GABPB subunits) activates the mutant TERTp. Prior therapies targeting the telomerase RNA component, TERC, lacked tumor selectivity and were poorly tolerated. The GABP-mediated reactivation of the mutated TERTp presents a unique therapeutic opportunity for tumor specific reversal of cellular immortality. Breakthroughs in the design of biological proteolysis-targeting chimera (bioPROTACs) offer a new approach to selectively degrade previously intractable targets such as transcription factors. We combined in silico protein-protein interaction modeling via AlphaFold and experimental validation to identify a minimal GABPB to bind GABPA and fused it with an E3 ubiquitin ligase. Introduction of this GABPA bioPROTAC into TERTp mutant glioblastoma cells depleted GABPA protein and eliminated GABPA binding to the mutant TERT promoter, reducing transcriptional activating (H3K4me3) and increasing suppressive (H3K27me3) histone marks. Reversion of mutant TERTp to an epigenetically silenced state reduced TERT expression by 73% to 95% and shortened tumor cell telomeres in a promoter mutation-specific manner. The GABP bioPROTAC reduced tumor growth and improved survival of mice bearing an orthotopic xenograft of TERTp mutated glioblastoma cells. The effects of the bioPROTAC were validated in vivo via magnetic resonance imaging of metabolic correlates of TERT expression. These data demonstrate that this GABP bioPROTAC potently degrades GABPA, leading to tumor-specific silencing of telomerase expression, and a reversal of glioblastoma tumor cell immortality. This artificial intelligence-guided approach may be applicable to other intractable cancer specific targets.