Abstract 5988: A non-canonical CDK9 complex mediates endocycling in polyaneuploid cancer cell (PACC) state

Abstract Once prostate cancer has spread from its primary site, treatment is limited to systemically administered therapy. While tumors initially respond to these therapies, eventually, all therapies fail, and the tumor recurs. Ours and others' recent studies have shown the presence of a universal mechanism of resistance: accession of the polyaneuploid cancer cell (PACC) state. The PolyAneuploid Transition (PAT) is initiated when a cancer cell is subjected to external stress, including therapy (we have tested three classes of chemotherapy and radiation), and accesses an alternative non-mitotic cell cycle known as endocycling, resulting in a greater-than-G2 polyploidization of the aneuploid genome (polyaneuploidy). While in this pro-survival PACC state, we have shown that cells are resistant to all forms of cytotoxic therapy. Initial RNA sequencing of cells that have accessed the PACC state in response to multiple classes of chemotherapy, including docetaxel and cisplatin, demonstrate a remodeled transcriptional profile that is distinct from control cultures. We hypothesize that accession of the resistance-mediating PACC state requires transcriptional remodeling. Our data show that cells in the PACC state had increased overall transcriptional activity. The multi protein complex pTEFB (positive transcription elongation factor) is a critical driver of RNA Polymerase II function and is composed of multiple subunits, including CDK9, a cyclin regulatory subunit, and a transcription factor. CDK9 phosphorylation was significantly elevated during accession and persistence in the PACC state. Genetic inhibitionof CDK9 by siRNA led to decreased expression of cyclin A which plays a role in S phase, both in the mitoticcycle and in the PACC endocycle. Interestingly, despite increased CDK9 levels, known CDK9 pTEFb complex members cyclin T1 and BRD4 were both decreased in cells in the PACC state. We treated cancer cells induced to enter the PACC state upon exposure to cisplatin with pharmaceutical inhibitors of CDK9, Flavopiridol and Fadraciclib. Combination of CDK9 inhibition with chemotherapy showed a suppressive effect on the PACC population. Our findings therefore suggest that a non-canonical CDK9 complex could be critical role for the endocycling to access and maintain the PACC state and represents a therapeutic candidate to eliminate the resistance-mediating PACC state. Citation Format: Cheng-fan Lee, Michael Loycano, Luke LOFTUS, Laurie Kostecka, Melvin Li, Sarah Amend, Kenneth Pienta. A non-canonical CDK9 complex mediates endocycling in polyaneuploid cancer cell (PACC) state. [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 5988.