BRCA1-deficient Cancer Research Articles

Positioning loss of PARP1 activity as the central toxic event in BRCA-deficient cancer

The mechanisms by which poly(ADP-ribose) polymerase 1 (PARP1) inhibitors (PARPi)s inflict replication stress and/or DNA damage are potentially numerous. PARPi toxicity could derive from loss of its catalytic activity and/or its physical trapping of PARP1 onto DNA that perturbs not only PARP1 function in DNA repair and DNA replication, but also obstructs compensating pathways. The combined disruption of PARP1 with either of the hereditary breast and ovarian cancer genes, BRCA1 or BRCA2 (BRCA), results in synthetic lethality. This has driven the development of PARP inhibitors as therapies for BRCA-mutant cancers. In this review, we focus on recent findings that highlight loss of PARP1 catalytic activity, rather than PARPi-induced allosteric trapping, as central to PARPi efficacy in BRCA deficient cells. However, we also review findings that PARP-trapping is an effective strategy in other genetic deficiencies. Together, we conclude that the mechanism-of-action of PARP inhibitors is not unilateral; with loss of activity or enhanced trapping differentially killing depending on the genetic context. Therefore, effectively targeting cancer cells requires an intricate understanding of their key underlying vulnerabilities.

Exploring Molecular Drivers of PARPi Resistance in BRCA1-Deficient Ovarian Cancer: The Role of LY6E and Immunomodulation.

Approximately 50% of patients diagnosed with ovarian cancer harbor tumors with mutations in BRCA1, BRCA2, or other genes involved in homologous recombination repair (HR). The presence of homologous recombination deficiency (HRD) is an approved biomarker for poly-ADP-ribose polymerase inhibitors (PARPis) as a maintenance treatment following a positive response to initial platinum-based chemotherapy. Despite this treatment option, the development of resistance to PARPis is common among recurrent disease patients, leading to a poor prognosis. In this study, we conducted a comprehensive analysis using publicly available datasets to elucidate the molecular mechanisms driving PARPi resistance in BRCA1-deficient ovarian cancer. Our findings reveal a central role for the interferon (IFN) pathway in mediating resistance in the context of BRCA1 deficiency. Through integrative bioinformatics approaches, we identified LY6E, an interferon-stimulated gene, as a key mediator of PARPi resistance, with its expression linked to an immunosuppressive tumor microenvironment (TME) encouraging tumor progression and invasion. LY6E amplification correlates with poor prognosis and increased expression of immune-related gene signatures, which is predictive of immunotherapy response. Interestingly, LY6E expression upon PARPi treatment resistance was found to be dependent on BRCA1 status. Gene expression analysis in the Orien/cBioPortal database revealed an association between LY6E and genes involved in DNA repair, such as Rad21 and PUF60, emphasizing the interplay between DNA repair pathways and immune modulation. Moreover, PUF60, Rad21, and LY6E are located on chromosome 8q24, a locus often amplified and associated with the progression of ovarian cancer. Overall, our study provides novel insights into the molecular determinants of PARPi resistance and highlights LY6E as a promising prognostic biomarker in the management of HRD ovarian cancer. Future studies are needed to fully elucidate the molecular mechanisms underlying the role of LY6E in PARPi resistance.

Targeting GPX4-mediated ferroptosis protection sensitizes BRCA1-deficient cancer cells to PARP inhibitors

BRCA1 is one of the most frequently-mutated tumor suppressor genes in ovarian and breast cancers. Loss of BRCA1 triggers homologous recombination (HR) repair deficiency, consequently leading to genomic instability and PARP inhibitors (PARPi)-associated synthetic lethality. Although, the roles of BRCA1 in DNA repair and replication have been extensively investigated, its tumor suppressive functions beyond genome safeguard remain poorly understood. Here, we report that BRCA1 promotes ferroptosis susceptibility through catalyzing K6-linked polyubiquitination of GPX4 and subsequently accelerating GPX4 degradation. Depletion of BRCA1 induces ferroptosis resistance in ovarian cancer cells due to elevated GPX4 protein, and silence of GPX4 significantly suppresses the growth of BRCA1-deficient ovarian cancer xenografts. Importantly, we found that PARPi triggers ferroptosis in ovarian cancer cells, inhibition of GPX4 markedly increase PARPi-induced ferroptosis in BRCA1-deficient ovarian cancer cells. Combined treatment of GPX4 inhibitor and PARPi produces synergistic anti-tumor efficacy in BRCA1-deficient ovarian cancer cells, patient derived organoid (PDO) and xenografts. Thus, our study uncovers a novel mechanism via which BRCA1 exerts tumor suppressive function through regulating ferroptosis, and demonstrates the potential of GPX4 as a therapeutic target for BRCA1-mutant cancers.

BRCAness, DNA gaps, and gain and loss of PARP inhibitor-induced synthetic lethality.

Mutations in the tumor-suppressor genes BRCA1 and BRCA2 resulting in BRCA1/2 deficiency are frequently identified in breast, ovarian, prostate, pancreatic, and other cancers. Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPis) selectively kill BRCA1/2-deficient cancer cells by inducing synthetic lethality, providing an effective biomarker-guided strategy for targeted cancer therapy. However, a substantial fraction of cancer patients carrying BRCA1/2 mutations do not respond to PARPis, and most patients develop resistance to PARPis over time, highlighting a major obstacle to PARPi therapy in the clinic. Recent studies have revealed that changes of specific functional defects of BRCA1/2-deficient cells, particularly their defects in suppressing and protecting single-stranded DNA gaps, contribute to the gain or loss of PARPi-induced synthetic lethality. These findings not only shed light on the mechanism of action of PARPis, but also lead to revised models that explain how PARPis selectively kill BRCA-deficient cancer cells. Furthermore, new mechanistic principles of PARPi sensitivity and resistance have emerged from these studies, generating potentially useful guidelines for predicting the PARPi response and design therapies for overcoming PARPi resistance. In this Review, we will discuss these recent studies and put them in context with the classic views of PARPi-induced synthetic lethality, aiming to stimulate the development of new therapeutic strategies to overcome PARPi resistance and improve PARPi therapy.

How to sensitize glioblastomas to temozolomide chemotherapy: a gap-centered view.

Temozolomide (TMZ) is a methylating agent used as the first-line drug in the chemotherapy of glioblastomas. However, cancer cells eventually acquire resistance, necessitating the development of TMZ-potentiating therapy agents. TMZ induces several DNA base adducts, including O 6 -meG, 3-meA, and 7-meG. TMZ cytotoxicity stems from the ability of these adducts to directly (3-meA) or indirectly (O 6 -meG) impair DNA replication. Although TMZ toxicity is generally attributed to O 6 -meG, other alkylated bases can be similarly important depending on the status of various DNA repair pathways of the treated cells. In this mini-review we emphasize the necessity to distinguish TMZ-sensitive glioblastomas, which do not express methylguanine-DNA methyltransferase (MGMT) and are killed by the futile cycle of mismatch repair (MMR) of the O 6 -meG/T pairs, vs. TMZ-resistant MGMT-positive or MMR-negative glioblastomas, which are selected in the course of the treatment and are killed only at higher TMZ doses by the replication-blocking 3-meA. These two types of cells can be TMZ-sensitized by inhibiting different DNA repair pathways. However, in both cases, the toxic intermediates appear to be ssDNA gaps, a vulnerability also seen in BRCA-deficient cancers. PARP inhibitors (PARPi), which were initially developed to treat BRCA1/2-deficient cancers by synthetic lethality, were re-purposed in clinical trials to potentiate the effects of TMZ. We discuss how the recent advances in our understanding of the genetic determinants of TMZ toxicity might lead to new approaches for the treatment of glioblastomas by inhibiting PARP1 and other enzymes involved in the repair of alkylation damage (e.g., APE1).

A RAD18-UBC13-PALB2-RNF168 axis mediates replication fork recovery in BRCA1-deficient cancer cells.

BRCA1/2 proteins function in genome stability by promoting repair of double-stranded DNA breaks through homologous recombination and by protecting stalled replication forks from nucleolytic degradation. In BRCA1/2-deficient cancer cells, extensively degraded replication forks can be rescued through distinct fork recovery mechanisms that also promote cell survival. Here, we identified a novel pathway mediated by the E3 ubiquitin ligase RAD18, the E2-conjugating enzyme UBC13, the recombination factor PALB2, the E3 ubiquitin ligase RNF168 and PCNA ubiquitination that promotes fork recovery in BRCA1- but not BRCA2-deficient cells. We show that this pathway does not promote fork recovery by preventing replication fork reversal and degradation in BRCA1-deficient cells. We propose a mechanism whereby the RAD18-UBC13-PALB2-RNF168 axis facilitates resumption of DNA synthesis by promoting re-annealing of the complementary single-stranded template strands of the extensively degraded forks, thereby allowing re-establishment of a functional replication fork. We also provide preliminary evidence for the potential clinical relevance of this novel fork recovery pathway in BRCA1-mutated cancers, as RAD18 is over-expressed in BRCA1-deficient cancers, and RAD18 loss compromises cell viability in BRCA1-deficient cancer cells.

Abstract PR009: Cytidine diphosphate diacylglycerol synthase 2 is a synthetic lethal target in mesenchymal cancers

Abstract Synthetic lethal interactions (SLIs) can provide a therapeutic index, as illustrated by PARP inhibition of BRCA-deficient cancers. This clinical success, and other examples, have spurred efforts to identify additional cancer-associated SLIs. Whereas additional SLIs based on genomic alterations in cancer have been identified, we set out to explore the SLI space as a function of differential RNA expression profiles in cancer and normal tissue, covering all ∼3.4e8 gene pairs. In our bioinformatic pipeline, synthetic lethality is scored by correlating DepMap dependency and expression data (n = 913 cancer cell lines), while cancer specificity is scored by comparing TCGA expression data for 9264 tumors and 741 healthy samples. Cancer specificity is confirmed by comparing also calibrated GTEx data for 17382 healthy tissue samples with calibrated DepMap expression data. With this pipeline we uncovered a frequent cancer-specific SLI between the paralogs cytidine diphosphate diacylglycerol synthase 1 (CDS1) and CDS2 (p<0.001 in all three analyses). Essentiality of CDS2 is observed in one third of cultured cancers (DepMap). Using CRISPR-Cas9 we confirm the CDS1-dependency for CDS2 essentiality in a panel of 12 cultured cancer cell lines, with lethality by CDS2 ablation reaching up to 98% in CDS1-negative cell lines. We also confirm the SLI using admixing experiments in tumor-bearing mice for two cell lines (6 mice per group, up to 95% synthetic lethality, p<0.001). The essentiality of CDS2 for cell survival is observed for mesenchymal-like cancers, which commonly express low levels of CDS1. To biochemically define the effects of CDS2 perturbation in CDS1-negative cancer cells, we performed multi-omic analyses in a panel of CDS1-negative cancer cell lines. The results show that mechanistically, the CDS1-2 SLI is accompanied by disruption of lipid homeostasis, including extensive accumulation of cholesterol esters and triglycerides. Additionally, quantitative western blotting for cleaved caspase-3 showed induction of apoptotic cell death (p<0.001 in 2 cell lines). To challenge the robustness of the SLI, we performed genome-wide CRISPR-Cas9 knockout screens in a panel of four CDS1-negative cancer cell lines. This failed to identify a common escape mechanism of death caused by CDS2 ablation. These findings suggest that no common escape to the combined loss of CDS1 and CDS2 is possible through loss-of-function. In conclusion, by computational, genetic, biochemical and functional analyses we demonstrate that CDS2 may serve as a target in mesenchymal cancers, meriting therapeutic exploration. Citation Format: Tim Arnoldus, Alex van Vliet, Adriaan F.H. de Groot, Niek Blomberg, Onno B. Bleijerveld, Susan E. van Hal-van Veen, Anita E. Grootemaat, Rolf Harkes, Nicole N. van der Wel, Maarten Altelaar, Martin Giera, Daniel S. Peeper. Cytidine diphosphate diacylglycerol synthase 2 is a synthetic lethal target in mesenchymal cancers [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr PR009.

BRCA1-Mediated Dual Regulation of Ferroptosis Exposes a Vulnerability to GPX4 and PARP Co-Inhibition in BRCA1-Deficient Cancers.

Resistance to poly (ADP-ribose) polymerase inhibitors (PARPi) limits the therapeutic efficacy of PARP inhibition in treating breast cancer susceptibility gene 1 (BRCA1)-deficient cancers. Here we reveal that BRCA1 has a dual role in regulating ferroptosis. BRCA1 promotes the transcription of voltage-dependent anion channel 3 (VDAC3) and glutathione peroxidase 4 (GPX4); consequently, BRCA1 deficiency promotes cellular resistance to erastin-induced ferroptosis but sensitizes cancer cells to ferroptosis induced by GPX4 inhibitors (GPX4i). In addition, nuclear receptor coactivator 4 (NCOA4)-mediated ferritinophagy and defective GPX4 induction unleash potent ferroptosis in BRCA1-deficient cancer cells upon PARPi and GPX4i co-treatment. Finally, we show that xenograft tumors derived from patients with BRCA1-mutant breast cancer with PARPi resistance exhibit decreased GPX4 expression and high sensitivity to PARP and GPX4 co-inhibition. Our results show that BRCA1 deficiency induces a ferroptosis vulnerability to PARP and GPX4 co-inhibition and inform a therapeutic strategy for overcoming PARPi resistance in BRCA1-deficient cancers. Significance: BRCA1 deficiency promotes resistance to erastin-induced ferroptosis via blocking VDAC3 yet renders cancer cells vulnerable to GPX4i-induced ferroptosis via inhibiting GPX4. NCOA4 induction and defective GPX4 further synergizes GPX4i with PARPi to induce ferroptosis in BRCA1-deficient cancers and targeting GPX4 mitigates PARPi resistance in those cancers. See related commentary by Alborzinia and Friedmann Angeli, p. 1372.

Abstract 3234: A new vulnerability to BET inhibition due to enhanced autophagy in BRCA2-deficient pancreatic cancer

Abstract Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related deaths in the United States, with the lowest five-year survival rate of all cancers. Around 10% of pancreatic cancer diagnoses are thought to be familial pancreatic cancer (FPC). BRCA2 pathogenic variants have been identified to be closely associated with FPC. Personalized medicine approaches that are tailored to FPC patients based on their mutation profiles could improve patient outcomes and minimize adverse effects. This study aims to identify novel potential drug candidates for BRCA2-deficient PDAC and dissect its underlying molecular mechanisms. To discover new vulnerabilities for BRCA2-deficient pancreatic cancer, we performed high-throughput drug screening (HTS) using isogenic murine Brca2 knockout (KO) and control PDAC cells. The HTS assay revealed that JQ1, a small molecule inhibitor of the bromodomain and extra-terminal domain protein (BET) family, selectively induced cytotoxicity in Brca2 KO cells. JQ1 significantly decreased the cell viability of Brca2 KO cells in vitro and suppressed the growth of Brca2 KO tumors in vivo compared to the controls. The transcriptomic analyses showed that JQ1 treatment resulted in upregulation of the gene sets associated with macroautophagy. Multi-orthogonal autophagy assays, including live cell imaging of fluorescence-based autophagy reporter, also supported that Brca2 KO cells had a constitutively higher basal level of autophagic activities compared to the controls, and JQ1 further induced autophagic flux in Brca2 KO cells. Moreover, blocking the autophagy process by pharmacological inhibition or knocking down essential autophagy genes rescued JQ1-induced cell death in Brca2 KO cells, indicating that the increased autophagy is responsible for JQ1-mediated cell death in Brca2 KO cells. In conclusion, we found that BRCA2 deficiency elevated autophagic flux, and extensive autophagy was further activated by BET inhibition, culminating in autophagy-dependent cell death in Brca2-deficient pancreatic cancer cells. Our findings suggest that BET inhibition could be a promising therapeutic strategy for treating BRCA2-deficient pancreatic cancer. Citation Format: Suyakarn Archasappawat, EunJung Lee, Keely Ji, Jocelyn Pena, Virneliz Fernandez Vega, Ritika Gangaraju, Nitin Sai Beesabathuni, Martin Jean Kim, Qi Tian, Priya Shah, Louis Scampavia, Timothy Spicer, Chang-Il Hwang. A new vulnerability to BET inhibition due to enhanced autophagy in BRCA2-deficient pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3234.

Abstract 4644: Single strand DNA GAP accumulation as a functional biomarker for USP1 inhibitor sensitivity in ovarian cancer patient-derived organoid models

Abstract Recent studies suggest that PARP inhibitors and POLQ inhibitors confer synthetic lethality in BRCA1-deficient tumors by accumulation of single-stranded DNA gaps (ssDNA gaps) at replication forks. Loss of USP1, a deubiquitinating enzyme, is also synthetic lethal with BRCA1 deficiency, and USP1 inhibitors are now undergoing clinical development for these cancers. Here, we show that USP1 inhibitors also promote the accumulation of ssDNA gaps during replication in BRCA1-deficient cells. USP1 inhibition increased monoubiquitinated PCNA at replication forks and resulted in accumulation of ssDNA gaps in BRCA1-deficient cancer cells. Knockdown of RAD18, the E3 ubiquitin ligase known to ubiquitinate PCNA, caused USP1 inhibitor resistance and suppression of ssDNA gaps. ssDNA gap accumulation induced by USP1 inhibition correlates with drug sensitivity of BRCA1-mutated cancer cells and overcomes PARP inhibitor resistance in cell line and xenograft models. Furthermore, USP1 inhibition was synergistic with PARP and POLQ inhibition in BRCA1-mutant cells, with enhanced ssDNA gap accumulation. A set of patient-derived ovarian cancer organoids (PDOs) were used to confirm the sensitivity of BRCA1-deficient cells to USP1 inhibition. Five PDOs were derived from patients with high grade-serous ovarian cancer (HGSOC). Among the five PDOs, two of them were from tumors harboring germline BRCA1 pathogenic mutations. BRCA1-mutant PDOs, but not BRCA1-WT PDOs, were sensitive to a USP1 inhibitor and to PARP inhibitor monotherapies. The accumulation of ssDNA gaps after treatment with a USP1 inhibitor or a PARP inhibitor correlated with the sensitivity to these drugs in all the models tested. Moreover, the combination of a PARP inhibitor and a USP1 inhibitor showed synergy in a BRCA1-WT model, which was resistant to both monotherapies. Interestingly, ssDNA gaps accumulated with the combination treatment and not with monotherapy in this model. Ovarian cancer PDOs therefore provide a powerful tool for rapid in vitro sensitivity testing. The detection of ssDNA gap accumulation may be a useful predictive biomarker for response to USP1 inhibition as monotherapy or in combination in ongoing clinical trials. Citation Format: Alexandre Andre B. A. Da Costa, Ozge Somuncu, Ramya Ravindranathan, Sirisha Mukkavalli, David Martigneti, Huy Nguyen, Yuqing Jiao, Benjamin Lamarre, Golbahar Sadatrezai, Lisa Moreau, Joyce Liu, Divya Iyer, Jean-Bernard Lazaro, Geoffrey Shapiro, Kalindi Parmar, Alan D. D'Andrea. Single strand DNA GAP accumulation as a functional biomarker for USP1 inhibitor sensitivity in ovarian cancer patient-derived organoid models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4644.

Abstract A050: CD8+ T cell exhaustion and their DNA damage in BRCA1 mutated TNBC

Abstract PARP Poly (ADP-ribose) polymerase inhibitors (PARPi) are treatment options in TNBC patients with inherited BRCA1 mutations. Recent studies have shown that immune checkpoint therapies (ICT) in combination with PARPi are effective in other solid cancer types such as melanoma, pancreatic and BRCA deficient ovarian cancer. In early and advanced BRCA1 TNBC, ICT studies are limited and not fully elucidated especially for new ICT that target LAG-3, CTLA-4, ICOS. There is a need for identifying potential prognostic markers suggesting which patients could benefit most from ICT. Therefore, we conducted a detailed single cell analysis of the tumor microenvironment (TME) mainly focused on immune checkpoint expression, CD8 T cell exhaustion and DNA damage associated with BRCA1 TNBC compared to WT TNBC. Combining single cell spatial tissue multiplexing (PhenoCycler) (BRCA1 TNBC n=77, WT TNBC n=55), scRNA seq (BRCA2 n=1, WT n=5), FLOW (PBMCs: BRCA1 n=13, BRCA2 n=17, WT n=62) as well as bulk RNA seq (BRCA1 TNBC n=35, WT TNBC n=11), we performed a detailed analysis of the TME associated with BRCA1 and WT breast cancer. We developed a 32-plex antibody panel that evaluated T-cells and other immune cells with markers of exhaustion, DNA damage and immune checkpoint expression. CD8+ T cell frequency was significantly increased in BRCA1 TNBC compared to WT TNBC (p=0.0015). A detailed CD8 T cell characterization revealed increased dysfunctional/exhausted cells (e.g., TOX+/LAG-3+/PD-1+), T cell DNA damage (CD3e+/CD8+/RAD51+, p=0.0047) and LAG-3 transcript levels in BRCA1 TNBC compared to WT TNBC (p=0.0625). Furthermore, immune checkpoint protein expression in tumor- (PD-L1) and immune cells (PD-1, LAG-3, ICOS) was increased in the TME associated with BRCA1 TNBC compared to WT TNBC. Of note, we also found that PBMCs of healthy tumor-free BRCA mutation carriers contained significantly increased CD8+/PD+1 T cells compared to WT (p=0.012). Our data indicates that BRCA1 haploinsufficiency leads to DNA damaged and dysfunctional CD8+ T cells within the TME compared to WT. Thus, the TME of BRCA1 TNBC patients is more immune suppressive due to increased exhausted CD8+ T cell frequency, their DNA damage as well as immune checkpoint expression of both tumor-/and immune cells compared to WT TNBC. The effect of BRCA1 haploinsufficiency on immune cells is important to consider when trials of ICT options are designed. Citation Format: Dana Pueschl, Anupma Nayak, Derek A. Oldridge, William Chandler, Bria Fulmer, Sokratis Apostolidis, Danielle Bragen, Bradley Wubbenhorst, John Pluta, kConFab Investigator, Antonis Antoniou, Georgia Chenevix-Trench, E. John Wherry, Susan M. Domchek, Katherine L. Nathanson. CD8+ T cell exhaustion and their DNA damage in BRCA1 mutated TNBC [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr A050.

Abstract A079: A new vulnerability to BET inhibition due to enhanced autophagy in BRCA2-deficient pancreatic cancer

Abstract Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related deaths in the United States, with the lowest five-year survival rate of all cancers. Around 10% of pancreatic cancer diagnoses are thought to be familial pancreatic cancer (FPC). BRCA2 pathogenic variants have been identified to be closely associated with FPC. Personalized medicine approaches that are tailored to FPC patients based on their mutation profiles could improve patient outcomes and minimize adverse effects. This study aims to identify novel potential drug candidates for BRCA2-deficient PDAC and dissect its underlying molecular mechanisms. To discover new vulnerabilities for BRCA2-deficient pancreatic cancer, we performed high-throughput drug screening (HTS) using isogenic murine Brca2 knockout (KO) and control PDAC cells. The HTS assay revealed that JQ1, a small molecule inhibitor of the bromodomain and extra-terminal domain protein (BET) family, selectively induced cytotoxicity in Brca2 KO cells. JQ1 significantly decreased the cell viability of Brca2 KO cells in vitro and suppressed the growth of Brca2 KO tumors in vivo compared to the controls. The transcriptomic analyses showed that JQ1 treatment resulted in upregulation of the gene sets associated with macroautophagy. Multi-orthogonal autophagy assays, including live cell imaging of fluorescence-based autophagy reporter, also supported that Brca2 KO cells had a constitutively higher basal level of autophagic activities compared to the controls, and JQ1 further induced autophagic flux in Brca2 KO cells. Moreover, blocking the autophagy process by pharmacological inhibition or knocking down essential autophagy genes rescued JQ1-induced cell death in Brca2 KO cells, indicating that the increased autophagy is responsible for JQ1-mediated cell death in Brca2 KO cells. In conclusion, we found that BRCA2 deficiency elevated autophagic flux, and extensive autophagy was further activated by BET inhibition, culminating in autophagy-dependent cell death in Brca2-deficient pancreatic cancer cells. Our findings suggest that BET inhibition could be a promising therapeutic strategy for treating BRCA2-deficient pancreatic cancer. Citation Format: Suyakarn Archasappawat, EunJung Lee, Keely Ji, Jocelyn Pena, Virneliz Fernandez-Vega, Ritika Gangaraju, Nitin Sai Beesabathuni, Martin Jean Kim, Qi Tian, Priya Shah, Louis Scampavia, Timothy Spicer, Chang-Il Hwang. A new vulnerability to BET inhibition due to enhanced autophagy in BRCA2-deficient pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Pancreatic Cancer; 2023 Sep 27-30; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(2 Suppl):Abstract nr A079.

Abstract A015: 53BP1 loss mediated PARP inhibitor resistance in BRCA1-deficient pancreatic cancer is overcome with immune checkpoint blockade

Abstract Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal cancer characterized by the presence of mutations in homologous recombination (HR) repair genes, including BRCA1 and BRCA2, in approximately 20% of diagnosed cases. 53BP1 (tumor protein p53 binding protein 1) nucleates the anti-end resection machinery at DNA double strand break (DSB)s thereby countering BRCA1 activity. Importantly, BRCA1-deficient PDACs that are typically sensitive to poly-ADP ribose polymerase inhibitors (PARPi) become resistant in the absence of 53BP1. Here, we demonstrate that loss of 53BP1 results in enhanced DNA end-resection, increased micronuclei, and cytoplasmic dsDNA, leading to activation of the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway, activating pro-inflammatory signaling. This not only enhances CD8+ T cell infiltration but also activates macrophages and natural killer (NK) cells culminating in tumor growth inhibition. Loss of 53BP1 correlates with response to immune checkpoint blockade (ICB) and improved overall survival. Finally, BRCA1-deficient tumors that develop resistance to PARPi due to the loss of 53BP1 are susceptible to ICB. Taken together, our findings provide a rationale for combining PARPi with ICB to overcome PARPi-resistance in BRCA1-deficient pancreatic cancers. Citation Format: Jeffrey Patterson-Fortin, Yajie Sun, Sen Han, Zhe Li, Zuzanna Nowicka, Yuna Hirohashi, Susan Kilgas, Jae Kyo Yi, Alexander Spektor, Wojciech Fendler, Panagiotis Konstantinopoulos, Alan D. D'Andrea, Dipanjan Chowdhury. 53BP1 loss mediated PARP inhibitor resistance in BRCA1-deficient pancreatic cancer is overcome with immune checkpoint blockade [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr A015.

Abstract IA009: Scars of faulty DNA repair in cancer whole genomes

Abstract Cancer genomes provide a record of the genetic alterations acquired from DNA damage and DNA repair defects during normal cell development and carcinogenesis. Genome-wide somatic alteration patterns in BRCA1-deficient (BRCA1d) and BRCA2-deficient (BRCA2d) cancers are attributed to a deficiency in homologous recombination (HR), a major pathway for the repair of double-strand breaks (DSBs) in human cells. Some of these mutational patterns may reflect specific error-prone repair mechanisms of DSB repair that cells use in the absence of HR. Such mutational patterns can provide biomarkers of HR-deficiency and help identify clinically relevant therapeutic vulnerabilities. HRD is has been associated with specific DNA rearrangements and cytogenetic aberrations. Paradoxically, the types of DNA rearrangements specifically associated with HR-deficient (HRD) cancers only minimally impact chromosomal structure. Addressing this, we combined a genome graph analysis of short-read whole genome sequencing (WGS) profiles across thousands of tumors with deep linked-read (LR) WGS of 46 BRCA1- or BRCA2-mutant breast cancers to discover a distinct class of HR deficiency-enriched rearrangements called reciprocal pairs. LR WGS showed that reciprocal pairs with identical rearrangement orientations gave rise to one of two distinct chromosomal outcomes, distinguishable only with long molecule data. While one (cis) outcome corresponded to the copy and pasting of a small segment to a distant site, a second (trans) outcome was a quasi-balanced translocation or multi-megabase inversion with substantial (10kb) duplications at each junction. We propose an HR-independent replication restart repair mechanism to explain the full spectrum of reciprocal pair outcomes. LR WGS additionally identified single-strand annealing (SSA) as a BRCA2-deficiency specific repair pathway in human cancers. Integrating these features in a classifier improved discrimination between BRCA1- vs. BRCA2-deficient genomes. In conclusion, our data reveal classes of BRCA1- and BRCA2-deficiency specific rearrangements as a source of cytogenetic aberrations in HRD cells. Citation Format: Jeremy Setton, Simon Powell, Marcin Imielinski. Scars of faulty DNA repair in cancer whole genomes [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr IA009.

Abstract B032: USP1 inhibition induces single strand DNA gap accumulation and overcomes PARP inhibitor resistance in BRCA1 deficient cancer cells

Abstract Targeting BRCA-deficient tumors with PARP inhibitors has improved the progression-free survival of ovarian and breast cancer patients in the clinic. However, the majority of patients develop acquired resistance. Recent studies suggest that PARP and POLQ inhibitors confer synthetic lethality in BRCA1-deficient tumors by accumulating single-stranded DNA (ssDNA) gaps at replication forks. We have previously shown that USP1 deficiency is synthetic lethal with BRCA1 deficiency, suggesting USP1 is a potential therapeutic target for overcoming PARPi resistance (Lim et al., Mol Cell, 2018). Accordingly, USP1 inhibitors (USPi) are now undergoing clinical development. Here, we show that USP1i also promote ssDNA gap accumulation during replication in BRCA1-deficient cells. USP1 inhibition results in the accumulation of monoubiquitinated PCNA at the replication fork and increased levels of non-cleaved USP1 and RAD18 at the replication fork. Accumulation of ssDNA gaps is dependent on RAD18 activity and monoubiquitinated PCNA levels. Knockdown of RAD18, the E3 ubiquitin ligase known to ubiquitinate PCNA, resulted in reduced PCNA-Ub levels, resistance to the USP1i, and suppression of ssDNA gaps. Moreover, the accumulation of ssDNA gaps is associated with increased S-phase PAR levels and is independent of PRIMPOL, suggesting defects on the lagging strand of the replication fork. Interestingly, the induction of ssDNA gaps was detected in USP1i-sensitive but not USP1i-resistant BRCA1-deficient cells, suggesting this is the mechanism of cytotoxicity by USP1 inhibition. Indeed, USP1i also sensitized BRCA1-deficient cancer cells to other ssDNA gap-inducing agents such as POLQ and PARP inhibitors. Notably, USP1i exhibited monotherapy activity in ovarian and breast cancer cell lines with acquired PARPi resistance and re-sensitized them to PARP inhibition. PARPi-resistant BRCA1-mutated patient-derived ovarian cancer organoids were also sensitized by USP1 inhibition. Similar to the cell lines, the ssDNA gap accumulation induced by USP1i in organoids strongly correlated with the drug sensitivity of both PARPi-sensitive and PARPi-resistant models. Finally, USPi overcame PARPi resistance in a BRCA1-mutant PARPi-resistant PDX model. In conclusion, USPi exhibits monotherapy activity in both PARPi-sensitive. Citation Format: Alexandre A. B. A. da Costa, Ozge Somuncu, Ramya Ravindranathan, Sirisha Mukkavalli, David B. Martignetti, Huy Nguyen, Yuqing Jiao, Benjamin Lamarre, Lisa Moreau, Joyce Liu, Divya Iyer, Jean-Bernard Lazaro, Geoffrey I. Shapiro, Kalindi Parmar, Alan D. D’Andrea. USP1 inhibition induces single strand DNA gap accumulation and overcomes PARP inhibitor resistance in BRCA1 deficient cancer cells [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr B032.

Abstract IA001: Single strand DNA GAP accumulation as a functional biomarker for USP1 inhibitor sensitivity

Abstract Recent studies suggest that PARP inhibitors and POLQ inhibitors confer synthetic lethality in BRCA1-deficient tumors by accumulation of single-stranded DNA gaps (ssDNA gaps) at replication forks. Loss of USP1, a deubiquitinating enzyme, is also synthetic lethal with BRCA1 deficiency, and USP1 inhibitors are now undergoing clinical development for these cancers. Here, we show that USP1 inhibitors also promote the accumulation of ssDNA gaps during replication in BRCA1-deficient cells. USP1 inhibition increased monoubiquitinated PCNA at replication forks and resulted in accumulation of ssDNA gaps in BRCA1-deficient cancer cells. Knockdown of RAD18, the E3 ubiquitin ligase known to ubiquitinate PCNA, caused USP1 inhibitor resistance and suppression of ssDNA gaps. ssDNA gap accumulation induced by USP1 inhibition correlates with drug sensitivity of BRCA1-mutated cancer cells and overcomes PARP inhibitor resistance in cell line and xenograft models. Furthermore, USP1 inhibition was synergistic with PARP and POLQ inhibition in BRCA1-mutant cells, with enhanced ssDNA gap accumulation. A set of patient-derived ovarian cancer organoids (PDOs) were used to confirm the sensitivity of BRCA1-deficient cells to USP1 inhibition. Five PDOs were derived from patients with high grade-serous ovarian cancer (HGSOC). Among the five PDOs, two of them were from tumors harboring germline BRCA1 pathogenic mutations. BRCA1-mutant PDOs, but not BRCA1-WT PDOs, were sensitive to a USP1 inhibitor and to PARP inhibitor monotherapies. The accumulation of ssDNA gaps after treatment with a USP1 inhibitor or PARP inhibitor correlated with the sensitivity to these drugs in all the models tested. Moreover, the combination of a PARP inhibitor and USP1 inhibitor showed synergy in a BRCA1-WT model, which was resistant to both monotherapies. Interestingly, ssDNA gaps accumulated with the combination treatment and not with monotherapy in this model. Ovarian cancer PDOs therefore provide a powerful tool for rapid in vitro sensitivity testing. The detection of ssDNA gap accumulation may be a useful predictive biomarker for response to USP1 inhibition as monotherapy or in combination in ongoing clinical trials. Citation Format: Alan D. D'Andrea. Single strand DNA GAP accumulation as a functional biomarker for USP1 inhibitor sensitivity [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr IA001.

PP2A inhibition causes synthetic lethality in BRCA2-mutated prostate cancer models via spindle assembly checkpoint reactivation.

Mutations in the BRCA2 tumor suppressor gene have been associated with an increased risk of developing prostate cancer. One of the paradoxes concerning BRCA2 is the fact that its inactivation affects genetic stability and is deleterious for cellular and organismal survival, while BRCA2-mutated cancer cells adapt to this detriment and malignantly proliferate. Therapeutic strategies for tumors arising from BRCA2 mutations may be discovered by understanding these adaptive mechanisms. In this study, we conducted forward genetic synthetic viability screenings in Caenorhabditis elegans brc-2 (Cebrc-2) mutants and found that Ceubxn-2 inactivation rescued the viability of Cebrc-2 mutants. Moreover, loss of NSFL1C, the mammalian ortholog of CeUBXN-2, suppressed the spindle assembly checkpoint (SAC) activation and promoted the survival of BRCA2-deficient cells. Mechanistically, NSFL1C recruited USP9X to inhibit the polyubiquitination of AURKB and reduce the removal of AURKB from the centromeres by VCP, which is essential for SAC activation. SAC inactivation is common in BRCA2-deficient prostate cancer patients, but PP2A inhibitors could reactivate the SAC and achieve BRCA2-deficient prostate tumor synthetic lethality. Our research reveals the survival adaptation mechanism of BRCA2-deficient prostate tumor cells and provides different angles for exploring synthetic lethal inhibitors in addition to targeting DNA damage repair pathways.

Identification of aberrant luminal progenitors and mTORC1 as a potential breast cancer prevention target in BRCA2 mutation carriers.

Inheritance of a BRCA2 pathogenic variant conveys a substantial life-time risk of breast cancer. Identification of the cell(s)-of-origin of BRCA2-mutant breast cancer and targetable perturbations that contribute to transformation remains an unmet need for these individuals who frequently undergo prophylactic mastectomy. Using preneoplastic specimens from age-matched, premenopausal females, here we show broad dysregulation across the luminal compartment in BRCA2mut/+ tissue, including expansion of aberrant ERBB3lo luminal progenitor and mature cells, and the presence of atypical oestrogen receptor (ER)-positive lesions. Transcriptional profiling and functional assays revealed perturbed proteostasis and translation in ERBB3lo progenitors in BRCA2mut/+ breast tissue, independent of ageing. Similar molecular perturbations marked tumours bearing BRCA2-truncating mutations. ERBB3lo progenitors could generate both ER+ and ER- cells, potentially serving as cells-of-origin for ER-positive or triple-negative cancers. Short-term treatment with an mTORC1 inhibitor substantially curtailed tumorigenesis in a preclinical model of BRCA2-deficient breast cancer, thus uncovering a potential prevention strategy for BRCA2 mutation carriers.

Retraction Note: Polymerase θ inhibition activates the cGAS-STING pathway and cooperates with immune checkpoint blockade in models of BRCA-deficient cancer

Retraction Note: Polymerase θ inhibition activates the cGAS-STING pathway and cooperates with immune checkpoint blockade in models of BRCA-deficient cancer

Abstract B055: Characterization of highly selective and CNS-penetrant PARP1 inhibitors

Abstract First-generation PARP1/2 inhibitors are FDA-approved for cancers such as breast, ovarian prostate and pancreas and provide significant therapeutic benefit to patients whose tumors harbor BRCA1 or BRCA2 mutations or homologous recombination deficiency (HRD). However, their use is associated with hematological toxicities that narrow their therapeutic index and restrict their application, particularly in combination with standard-of-care chemotherapy. All four FDA-approved PARP1 inhibitors are largely non-selective for the closely related enzyme PARP2, inhibition of which has been shown to drive hematotoxicity. Hence, the development of second-generation molecules highly selective for PARP1 over PARP2 offers a significant opportunity to 1) dramatically enhance therapeutic index, 2) enable additional precision medicine/combination approaches with chemotherapy, radiotherapy, immunotherapy and targeted agents and 3) expand the addressable patient population to those whose tumors harbour additional DDR defects. Utilising X-ray crystallography and structure-based drug design, we describe the characterization of exquisitely selective PARP1 inhibitors. These molecules demonstrate high potency in multiple biochemical and cell-based assays, including viability and colony forming unit read-outs in BRCA-deficient cancer cells, whilst sparing HR-proficient cancer cell lines and primary cells. The molecules are potent PARP1-DNA trappers and exhibit high selectivity across the PARP family and wider secondary pharmacology targets. The molecules potentiate the activity of ATR inhibitors when tested in ATM-deficient cancer cell lines. This chemical series generally exhibits highly desirable physico-chemical and in vitro ADME properties, which translate to high oral bioavailability and low clearance in rodent PK studies. Exemplar molecules also demonstrate high rat CNS penetrance using in vivo microdialysis. Lead molecules yield deep and durable dose-dependent anti-tumor efficacy in a variety of BRCAm xenograft models with responses continuing after cessation of treatment. The compounds demonstrate strong PK/PD relationships which correlate with anti-tumor efficacy using inhibition of tumor PARylation as a PD read-out. Strikingly, the molecules demonstrate compelling anti-tumor efficacy in BRCA-deficient xenograft models that are known to be largely resistant to FDA-approved non-selective PARP1/2 inhibitors. Taken together, these data predict low once daily therapeutic oral dosing based on a highly favorable pre-clinical PK profile and potent in vivo efficacy, with the potential to demonstrate improved efficacy and tolerability compared to marketed PARP inhibitors. In summary, we describe the characterization of next generation potent and selective CNS-penetrant PARP1 inhibitors. These molecules have the potential for an improved safety profile and greater combination potential with standard-of-care and targeted anti-cancer therapies. Citation Format: Phillip M Cowley, Barry E McGuinness, Gillian M Campbell, Alan Wise. Characterization of highly selective and CNS-penetrant PARP1 inhibitors [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B055.