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ALC1 and PARP activities coordinate chromatin accessibility and viability in homologous recombination deficient cells (ATAC-seq)

ABSTRACT: The response to Poly (ADP-ribose) polymerase inhibitors (PARPi) is dictated by homologous recombination (HR) DNA repair mechanisms and the abundance of lesions that trap PARP enzymes on chromatin. It remains unclear, however, if the established role of PARP in promoting chromatin accessibility impacts viability in these settings. Using a CRISPR based screen, we identify the PAR-binding Snf2-like ATPase, ALC1 as a key determinant of PARPi toxicity in HR-deficient cells. ALC1 loss reduced viability of BRCA mutant cells and enhanced their sensitivity to PARPi by up to 250-fold, while overcoming several known resistance mechanisms. ALC1 loss was not epistatic to other repair pathways that execute the PARPi response. Instead, ALC1 deficiency reduced chromatin accessibility concomitant with a decrease in the association of repair factors. This resulted in an accumulation of replication associated DNA damage and a reliance on HR. These findings establish PAR-dependent chromatin remodeling as a mechanistically distinct aspect of PARPi responses, implicating ALC1 inhibition as a new approach to overcome therapeutic resistance in HR-deficient cancers.

ORGANISM(S): Homo sapiens

PROVIDER: GSE150955 | GEO | 2020/12/02

REPOSITORIES: GEO

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Project description:Extending the therapeutic spectrum of PARP-inhibition (PARPi) beyond HR-deficiency or reverting PARPi resistance is of high clinical interest. This is particularly true for the identification of innovative therapeutic strategies for ovarian cancer, given the recent advances in the use of PARPi in clinical practice. In this regard, the combination of PARPi with chemotherapy is a promising strategy for defining new therapeutic standards. In this study, we analysed the therapeutic effect of novel triazene derivatives, including the drug CT913 and its metabolite CT913-M1 on ovarian cancer cells and describe their interaction with PARPi. This is the first study analysing the potential therapeutic effect of these components. In vitro assays for drug characterization including RNA-seq were applied in a selected panel of ovarian cancer cell lines. CT913 treatment conferred a dose-dependent reduction of cell viability in a set of platinum-sensitive and platinum-resistant ovarian cancer cell lines with an IC50 in the micro- to almost millimolar range (107-940µM), whereas its metabolite CT913-M1 was about 10-fold more potent (IC50 of 17-93µM). Neither of the drugs increased the cytotoxic effect of cisplatin, CT913 might even antagonize it. Furthermore, CT913 conferred synthetic lethality in BRCA1-deficient ovarian cancer cells, indicating that homologues recombination (HR) repair may contribute to its mechanism of action. Importantly, CT913 sensitized for olaparib treatment, independently of BRCA-1 mutational status, supporting the finding that CT913 may act partially independent of an impaired HR. CT913 treatment led to changes in gene transcription. CT913 strongly induced CDKN1A transcription, suggesting cell cycle arrest as an early response to this drug. It also downregulated a variety of transcripts involved in prominent DNA-repair pathways, such as HR, mismatch repair (MMR) or nucleotide excision repair (NER). This is the first study, suggesting the triazene drug class CT913 as auxiliary drug for extending the therapeutic spectrum of PARPi.

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ALC1 and PARP activities coordinate chromatin accessibility and viability in homologous recombination deficient cells (ATAC-seq)

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