Induced Replication Stress Research Articles

Kinetic Features of Degradation of R-Loops by RNase H1 from Escherichia coli

R-loops can act as replication fork barriers, creating transcription–replication collisions and inducing replication stress by arresting DNA synthesis, thereby possibly causing aberrant processing and the formation of DNA strand breaks. RNase H1 (RH1) is one of the enzymes that participates in R-loop degradation by cleaving the RNA strand within a hybrid RNA–DNA duplex. In this study, the kinetic features of the interaction of RH1 from Escherichia coli with R-loops of various structures were investigated. It was found that the values of the dissociation constants Kd were minimal for complexes of RH1 with model R-loops containing a 10–11-nt RNA–DNA hybrid part, indicating effective binding. Analysis of the kinetics of RNA degradation in the R-loops by RH1 revealed that the rate-limiting step of the process was catalytic-complex formation. In the presence of RNA polymerase, the R-loops containing a ≤16-nt RNA–DNA hybrid part were efficiently protected from cleavage by RH1. In contrast, R-loops containing longer RNA–DNA hybrid parts, as a model of an abnormal transcription process, were not protected by RNA polymerase and were effectively digested by RH1.

Abstract B007: Unlocking antiviral immunity against endogenous retroviruses in skin squamous cell carcinomas

Abstract Known as molecular parasites that invaded our genome through ancestors, transposons (mainly retrotransposons) are interspersed genomic repeats that constitute ∼40% of the mammalian genome. Primarily residing in the heterochromatin, retrotransposons exhibit dynamic expressions to orchestrate embryonic development and shape adult functions. Although rare, a cohort of evolutionarily young retrotransposons resisted extinction, whose selfish activities underlie polymorphic variations and genetic diseases including cancer. While the extraordinary molecular and functional heterogeneity of retrotransposons has been long recognized, mechanistic dissection remains challenging. To tackle this challenge, we use murine skin as our model, given its well-characterized, abundant, and highly accessible adult stem cells, not only mediating postnatal remodeling and tissue repair, but also serving as cell of origin for squamous cell carcinomas. By analyzing a genetic model lacking a known retrotransposon suppressor, histone methyltransferase Setdb1, which we found to be essential in the adult skin, we saw hair loss and hair follicle stem cell exhaustion phenotype, accompanied by a robust and selective surge of endogenous retroviruses (ERVs). When combined with squamous cell carcinoma drivers, Setdb1 loss significantly blocked tumor progression in vivo. We detected abundant retroviral peptides originated from its full-length copies through mass spectrometry and viral-like particles through transmission electron microscopy that are immunoreactive to previously reported ERV surface envelope (env) antibody. Similar viral-like particles were broadly evident across several epithelial tissues beyond skin, implicating its conserved function across squamous cancers. Mechanistically, epithelial originated ERVs elicit tissue wide inflammatory response, and can be ameliorated by pharmacological or genetic inhibition of retroviral activity. Moreover, ERV reactivation induced replication stress functionally accounts for stem cell exhaustion, therefore serving as a specific therapeutic vulnerability in squamous cancers. As an evolutionary conundrum, viral-coding ERVs pose a threat to host fitness and yet, they’ve resisted extinction persisting in the mammalian genome. Our findings suggest ERVs wrestle with the host surveillance programs to regulate adult tissue physiology and pathology, thus providing a key target in eliciting innate immunity in squamous cell carcinomas. Citation Format: Ying Lyu, Yejing Ge. Unlocking antiviral immunity against endogenous retroviruses in skin squamous cell carcinomas [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: RNAs as Drivers, Targets, and Therapeutics in Cancer; 2024 Nov 14-17; Bellevue, Washington. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(11_Suppl):Abstract nr B007.

Dissecting the Cell-Killing Mechanisms of Hydroxyurea Using Spot Assays.

Hydroxyurea is an inhibitor of ribonucleotide reductase and is commonly used in laboratories to induce replication stress or arrest cells in the S phase for cell cycle or checkpoint studies. However, hydroxyurea also causes side effects such as oxidative stress, particularly under chronic exposure conditions. This complicates the interpretation of the cell-killing mechanisms, particularly in a previously uncharacterized mutant, and thus hampers the analyses. Here, we describe a few easy and simple spot assays in fission yeast that allow dissecting the cell-killing mechanisms of hydroxyurea.

The polymeric fluoropyrimidine CF10 overcomes limitations of 5-FU in pancreatic ductal adenocarcinoma cells through increased replication stress

ABSTRACT Pancreatic ductal adenocarcinoma (PDAC) is a lethal disease soon to become the second leading cause of cancer deaths in the US. Beside surgery, current therapies have narrow clinical benefits with systemic toxicities. FOLFIRINOX is the current standard of care, one component of which is 5- Fluorouracil (5-FU), which causes serious gastrointestinal and hematopoietic toxicities and is vulnerable to resistance mechanisms. Recently, we have developed polymeric fluoropyrimidines (F10, CF10) which unlike 5-FU, are, in principle, completely converted to the thymidylate synthase inhibitory metabolite FdUMP, without generating appreciable levels of ribonucleotides that cause systemic toxicities while displaying much stronger anti-cancer activity. Here, we confirm the potency of CF10 and investigate enhancement of its efficacy through combination with inhibitors in vitro targeting replication stress, a hallmark of PDAC cells. CF10 is 308-times more potent as a single agent than 5-FU and was effective in the nM range in primary patient derived models. Further, we find that activity of CF10, but not 5-FU, is enhanced through combination with inhibitors of ATR and Wee1 that regulate the S and G2 DNA damage checkpoints and can be reversed by addition of dNTPs indicative of CF10 acting, at least in part, through inducing replication stress. Our results indicate CF10 has the potential to supersede the established benefit of 5-FU in PDAC treatment and indicate novel combination approaches that should be validated in vivo and may be beneficial in established regimens that include 5-FU.

Human PC4 supports telomere stability and viability in cells utilizing the alternative lengthening of telomeres mechanism.

Cancer cells with an activated Alternative Lengthening of Telomeres (ALT) mechanism elongate telomeres via homology-directed repair. Sustained telomeric replication stress is an essential trigger of ALT activity; however, it can lead to cell death if not properly restricted. By analyzing publicly available data from genome-wide CRISPR KO screenings, we have identified the multifunctional protein PC4 as a novel factor essential for ALT cell viability. Depletion of PC4 results in rapid ALT cell death, while telomerase-positive cells show minimal effects. PC4 depletion induces replication stress and telomere fragility primarily in ALT cells, and increases ALT activity. PC4 binds to telomeric DNA in cells, and its binding can be enhanced by telomeric replication stress. Finally, a mutant PC4 with partly impaired single stranded DNA binding activity is capable to localize to telomeres and suppress ALT activity and telomeric replication stress. We propose that PC4 supports ALT cell viability, at least partly, by averting telomere dysfunction. Further studies of PC4 interactions at ALT telomeres may hold promise for innovative therapies to eradicate ALT cancers.

Comprehensive Analysis of Complex Copy Number Variations in the Plasmodium falciparum Genome via Long-Read Sequencing: Implications for Antimalarial Resistance and Broad Genomic Insights

Abstract Introduction/Objective The genome of Plasmodium falciparum, characterized by a high AT content and a propensity for copy number variations (CNVs), presents a model for understanding how genomes adapt to stress. Our research focused on the prevalence and distribution of CNVs highlights long-read sequencing as a tool for detecting complex structural variants in other models. Methods/Case Report P. falciparum parasites were cultured and treated with DSM1 and aphidicolin that induce replication stress. Long-read sequencing was conducted using the Oxford Nanopore Technology Minion, with an emphasis on an optimizing DNA extraction method for preserving intact chromosomes. Visualization of CNVs, including tandem and inverted duplications, was achieved through a custom R Shiny application that allowed both quantitative and qualitative assessments. Results (if a Case Study enter NA) Long read sequencing identified an enrichment of multiple CNV types in treated samples, with a significant enrichment of inverted duplications as well as other types of structural variants. Aphidicolin and DSM1-treated samples demonstrated significant accumulation of these variations compared to controls (2.0 and 5.6-fold, respectively with a p value of < 0.0001 in both treatment groups). These occurred mainly in core genomic regions that do not contain variable gene copies. An increase in the number of inverted duplications may be related to stress response to the treatment and may be a potential adaptive mechanism of the parasite. Our manual single-read visualization technique made it possible to visualize the structure of various CNV types, which provides unique insight on CNVs as they arise in single genomes. Conclusion We hypothesize that inverted duplications represent stalled replication forks, and we are currently assessing their frequency in replicating and non-replicating cells. This study highlights the role of long-read sequencing in detecting complex genomic structures in P. falciparum, which may influence the parasite’s resistance to drug treatment. These findings not only advance our understanding of malaria pathology but also aid our comprehension of how cancer cells might react to drug-induced stress and the mechanisms behind CNV formation in neoplastic cells.

ATR, CHK1 and WEE1 inhibitors cause homologous recombination repair deficiency to induce synthetic lethality with PARP inhibitors

PurposePARP inhibitors (PARPi) are effective in homologous recombination repair (HRR) defective (HRD) cancers. To (re)sensitise HRR proficient (HRP) tumours to PARPi combinations with other drugs are being explored. Our aim was to determine the mechanism underpinning the sensitisation to PARPi by inhibitors of cell cycle checkpoint kinases ATR, CHK1 and WEE1.Experimental designA panel of HRD and HRP cells (including matched BRCA1 or 2 mutant and corrected pairs) and ovarian cancer ascites cells were used. Rucaparib (PARPi) induced replication stress (RS) and HRR (immunofluorescence microscopy for γH2AX and RAD51 foci, respectively), cell cycle changes (flow cytometry), activation of ATR, CHK1 and WEE1 (Western Blot for pCHK1S345, pCHK1S296 and pCDK1Y15, respectively) and cytotoxicity (colony formation assay) was determined, followed by investigations of the impact on all of these parameters by inhibitors of ATR (VE-821, 1 µM), CHK1 (PF-477736, 50 nM) and WEE1 (MK-1775, 100 nM).ResultsRucaparib induced RS (3 to10-fold), S-phase accumulation (2-fold) and ATR, CHK1 and WEE1 activation (up to 3-fold), and VE-821, PF-477736 and MK-1775 inhibited their targets and abrogated these rucaparib-induced cell cycle changes in HRP and HRD cells. Rucaparib activated HRR in HRP cells only and was (60-1,000x) more cytotoxic to HRD cells. VE-821, PF-477736 and MK-1775 blocked HRR and sensitised HRP but not HRD cells and primary ovarian ascites to rucaparib.ConclusionsOur data indicate that, rather than acting via abrogation of cell cycle checkpoints, ATR, CHK1 and WEE1 inhibitors cause an HRD phenotype and hence “induced synthetic lethality” with PARPi.

ALTering Cancer by Triggering Telomere Replication Stress through the Stabilization of Promoter G-Quadruplex in SMARCAL1.

Most of the human cancers are dependent on telomerase to extend the telomeres. But ∼10% of all cancers use a telomerase-independent, homologous recombination mediated pathway called alternative lengthening of telomeres (ALT). Due to the poor prognosis, ALT status is not being considered yet in the diagnosis of cancer. No such specific treatment is available to date for ALT positive cancers. ALT positive cancers are dependent on replication stress to deploy DNA repair pathways to the telomeres to execute homologous recombination mediated telomere extension. SMARCAL1 (SWI/SNF related, matrix-associated, actin-dependent regulator of chromatin, subfamily A-like 1) is associated with the ALT telomeres to resolve replication stress thus providing telomere stability. Thus, the dependency on replication stress regulatory factors like SMARCAL1 made it a suitable therapeutic target for the treatment of ALT positive cancers. In this study, we found a significant downregulation of SMARCAL1 expression by stabilizing the G-quadruplex (G4) motif found in the promoter of SMARCAL1 by potent G4 stabilizers, like TMPyP4 and BRACO-19. SMARCAL1 downregulation led toward the increased localization of PML (promyelocytic leukemia) bodies in ALT telomeres and triggered the formation of APBs (ALT-associated promyelocytic leukemia bodies) in ALT positive cell lines, increasing telomere replication stress and DNA damage at a genomic level. Induction of replication stress and hyper-recombinogenic phenotype in ALT positive cells mediated by G4 stabilizing molecules already highlighted their possible application as a new therapeutic window to target ALT positive tumors. In accordance with this, our study will also provide a valuable insight toward the development of G4-based ALT therapeutics targeting SMARCAL1.

DIPG-26. H3.3K27M ENHANCES SENSITIVITY TO ATR INHIBITION IN PEDIATRIC GLIOMA

Abstract BACKGROUND Diffuse midline glioma (DMG) is an aggressive, uniformly fatal childhood brain tumor. Radiation remains the standard of care but provides limited survival benefit. The most prevalent driver mutation in DMG is lysine to methionine mutation at position 27 (K27M) of histone H3.3, resulting in global loss of H3K27 trimethylation and remodeled chromatin, thus promoting DMG tumorigenesis. We have previously shown that inhibition of de novo pyrimidine synthesis induces replication stress that enhances DMG dependency on ATR. In this study, we sought to explore the contribution of H3K27M to basal replication stress in DMGs and vulnerability to ATR inhibition. METHODS To investigate ATR sensitivity of DMG, we employed ART0380, a potent and selective ATR inhibitor currently in Phase 2 clinical trials in adults. Using isogenic models and a panel of patient tumor-derived DMG cell lines we perform viability assays, immunoblotting, microscopy and flow cytometry-based assays to determine various endpoints/measures of ATR inhibition in DMGs. RESULTS Our data show that H3K27M histone oncoproteins enhance dependency on ATR and ATR inhibition synergizes with radiation to induce DMG cell death. After ART0380 treatment, H3K27M oncoproteins result in greater induction in apoptosis and accumulation of DNA damage (as measured by g-H2AX) compared with their histone H3 wild-type isogenic counterparts. Inhibition of ATR by ART0380 is confirmed by effects on phosphorylation of bona fide ATR targets-CHK1 and RPA. To gain mechanistic insights into how H3K27M modulates dependency on ATR, we have evaluated accumulation of chromatin-bound RPA, R-loops, and frequency of transcription and replication collisions (TRC) in isogenic cells. CONCLUSIONS Our data indicate a heightened sensitivity of H3K27M mutated DMG to ATR inhibition thus identifying a therapeutic window for DMG treatment with ATR inhibitor as monotherapy or in combination with radiotherapy.

Replication Stress in Activated Human NK Cells Induces Sensitivity to Apoptosis.

NK cells are innate immune effectors that kill virally infected or malignant cells. NK cell deficiency (NKD) occurs when NK cell development or function is impaired and variants in MCM4, GINS1, MCM10, and GINS4 result in NKD. Although NK cells are strongly impacted by mutational deficiencies in helicase proteins, the mechanisms underlying this specific susceptibility are poorly understood. In this study, we induced replication stress in activated NK cells or T cells by chemical and genetic methods. We found that the CD56bright subset of NK cells accumulates more DNA damage and replication stress during activation than do CD56dim NK cells or T cells. Aphidicolin treatment increases apoptosis of CD56bright NK cells through increased pan-caspase expression and decreases perforin expression in surviving cells. These findings show that sensitivity to replication stress affects NK cell survival and function and contributes to NKD.

CK2-dependent degradation of CBX3 dictates replication fork stalling and PARP inhibitor sensitivity.

DNA replication is a vulnerable cellular process, and its deregulation leads to genomic instability. Here, we demonstrate that chromobox protein homolog 3 (CBX3) binds replication protein A 32-kDa subunit (RPA2) and regulates RPA2 retention at stalled replication forks. CBX3 is recruited to stalled replication forks by RPA2 and inhibits ring finger and WD repeat domain 3 (RFWD3)-facilitated replication restart. Phosphorylation of CBX3 at serine-95 by casein kinase 2 (CK2) kinase augments cadherin 1 (CDH1)-mediated CBX3 degradation and RPA2 dynamics at stalled replication forks, which permits replication fork restart. Increased expression of CBX3 due to gene amplification or CK2 inhibitor treatment sensitizes prostate cancer cells to poly(ADP-ribose) polymerase (PARP) inhibitors while inducing replication stress and DNA damage. Our work reveals CBX3 as a key regulator of RPA2 function and DNA replication, suggesting that CBX3 could serve as an indicator for targeted therapy of cancer using PARP inhibitors.

Targeting AURKA to induce synthetic lethality in CREBBP-deficient B-cell malignancies via attenuation of MYC expression.

Loss-of-function mutations in CREBBP, which encodes for a histone acetyltransferase, occur frequently in B-cell malignancies, highlighting CREBBP deficiency as an attractive therapeutic target. Using established isogenic cell models, we demonstrated that CREBBP-deficient cells are selectively vulnerable to AURKA inhibition. Mechanistically, we found that co-targeting CREBBP and AURKA suppressed MYC transcriptionally and post-translationally to induce replication stress and apoptosis. Inhibition of AURKA dramatically decreased MYC protein level in CREBBP-deficient cells, implying a dependency on AURKA to sustain MYC stability. Furthermore, in vivo studies showed that pharmacological inhibition of AURKA was efficacious in delaying tumor progression in CREBBP-deficient cells and was synergistic with CREBBP inhibitors in CREBBP-proficient cells. Our study sheds light on a novel synthetic lethal interaction between CREBBP and AURKA, indicating that targeting AURKA represents a potential therapeutic strategy for high-risk B-cell malignancies harboring CREBBP inactivating mutations.

Transient Zn2+ deficiency induces replication stress and compromises daughter cell proliferation

Cells must replicate their genome quickly and accurately, and they require metabolites and cofactors to do so. Ionic zinc (Zn2+) is an essential micronutrient that is required for hundreds of cellular processes, including DNA synthesis and adequate proliferation. Deficiency in this micronutrient impairs DNA synthesis and inhibits proliferation, but the mechanism is unknown. Using fluorescent reporters to track single cells via long-term live-cell imaging, we find that Zn2+ is required at the G1/S transition and during S phase for timely completion of S phase. A short pulse of Zn2+ deficiency impairs DNA synthesis and increases markers of replication stress. These markers of replication stress are reversed upon resupply of Zn2+. Finally, we find that if Zn2+ is chelated during the mother cell's S phase, daughter cells enter a transient quiescent state, maintained by sustained expression of p21, which disappears upon reentry into the cell cycle. In summary, short pulses of mild Zn2+ deficiency in S phase specifically induce replication stress, which causes downstream proliferation impairments in daughter cells.

Genomic Frequencies of Dynamic DNA Sequences and Mammalian Lifespan.

Dynamic DNA sequences (i.e. sequences capable of forming hairpins, G-quadruplexes, i-motifs, and triple helices) can cause replication stress and associated mutations. One example of such a sequence occurs in the RACK7 gene in human DNA. Since this sequence forms i-motif structures at neutral pH that cause replication stress and result in spontaneous deletions in prostate cancer cells, our initial aim was to determine its potential utility as a biomarker of prostate cancer. We cloned and sequenced the region in RACK7 where i-motif deletions often occur in DNA obtained from eight individuals. Expressed prostatic secretions were obtained from three individuals with a positive biopsy for prostate cancer and two with individuals with a negative biopsy for prostate cancer. Peripheral blood specimens were obtained from two control healthy bone marrow donors and a marrow specimen was obtained from a third healthy marrow donor. Follow-up computer searches of the genomes of 74 mammalian species available at the NCBI ftp site or frequencies of 6 dynamic sequences known to produce mutations or replication stress using a program written in Mathematica were subsequently performed. Deletions were found in RACK7 in specimens from both older normal adults, as well as specimens from older patients with cancer, but not in the youngest normal adult. The deletions appeared to show a weak trend to increasing frequency with patient age. This suggested that endogenous mutations associated with dynamic sequences might accumulate during aging and might serve as biomarkers of biological age rather than direct biomarkers of cancer. To test that hypothesis, we asked whether or not the genomic frequencies of several dynamic sequences known to produce replication stress or mutations in human DNA were inversely correlated with maximum lifespan in mammals. Our results confirm this correlation for six dynamic sequences in 74 mammalian genomes studied, thereby suggesting that spontaneously induced replication stress and mutations linked to dynamic sequence frequency may limit lifespan by limiting genome stability.

Abstract LB405: CTP synthase 1 is a synthetic vulnerability in prostate cancer treated with supraphysiological androgen

Abstract Bipolar androgen therapy (BAT) is the cyclical administration of supraphysiological androgen (SPA), which can be an effective treatment for 30-40% of patients with castration-resistant prostate cancer. We sought to improve the efficacy of BAT by better understanding its molecular consequences in prostate cancer. Given that the androgen receptor (AR) regulates cellular metabolism, we hypothesized that SPA induces dependencies on specific metabolic pathways that might be exploited therapeutically. A negative selection metabolism-focused CRISPR-KO screen in LNCaP cells treated with SPA suggested that the deletion of several enzymes involved in de novo nucleotide synthesis enhances growth suppression by SPA. A top hit was cytidine triphosphate synthase 1 (CTPS1). CTPS1 converts UTP to CTP in the final step of de novo pyrimidine synthesis. Exposure of SPA-treated prostate cancer cell lines to STP-B, a potent and highly selective inhibitor of CTPS1 (PMID 37008165), increased cell death in vitro and in vivo, indicating that CTPS1 constitutes a metabolic vulnerability in this context. Inhibition of CTPS1 with STP-B induced S phase cell cycle arrest and replication stress, as indicated by phosphorylation of RPA and CHEK proteins, which was augmented by treatment with SPA. CTPS1 likely becomes a vulnerability in SPA-treated prostate cancer due to the downregulation of MYC by SPA, which we found leads to a reduced abundance of enzymes required for nucleotide synthesis and nucleotides, particularly CTP. This rewires nucleotide synthesis and salvage pathway such that de novo synthesis of CTP is required to avoid replication stress and cell death. Altogether, this work suggests that inhibition of CTPS1 may enhance the efficacy of BAT through the induction of replication stress. Selective inhibitors of CTPS1 have entered clinical development (NCT05463263), increasing the feasibility of a combination therapy clinical trial design for patients with prostate cancer. Citation Format: Sheila Jonnatan, Rajendra Kumar, Varsha Vakkala, Karthik Vasan, Zachary R. Chalmers, Daniel R. Schmidt, Philip A. Beer, Matthew G. Vander Heiden, Samuel R. Denmeade, Navdeep S. Chandel, Laura A. Sena. CTP synthase 1 is a synthetic vulnerability in prostate cancer treated with supraphysiological androgen [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB405.

Abstract 7149: Discovery of a novel USP1 inhibitor: Targeted therapy for BRCA-mutated and PARPi-resistant cancers

Abstract Harnessing synthetic lethality offers an innovative approach to targeted cancer treatment. USP1, a specific deubiquitinating enzyme, has gained attention as a potential synthetic lethality target for BRCA-mutated cancers. At AIGEN Sciences, Inc., we've integrated the advanced capabilities of our human-in-the-loop (HITL) AI drug discovery platform, AIGEN ChemTailor. This cutting-edge platform thrives on continuous learning enhanced by expert feedback, refining our approach to drug discovery. Using AIGEN ChemTailor, we synthesized a specific USP1 inhibitor. Its inhibitory capabilities were validated through the Ubiquitin-rhodamine 110 assay. Notably, our inhibitor presented remarkable anti-proliferative effects on BRCA1 mutant cells (MDA-MB-436) while showing no activity against BRCA wild-type cells (HCC1954), highlighting its specificity. The efficacy of our inhibitor was enhanced when combined with PARP inhibitors, such as olaparib or AZD5305, especially in UWB1.289 cells. Impressively, the USP1 inhibitor also effectively targeted PARPi-resistant cell lines, including A2780-ola, UWB1.289-nir, HCC38-ola, MDA-MB-468-ola, U251MG-ola, and OVCAR8-ola. Clonogenic assays with MDA-MB-436 and MCF-10A emphasized our inhibitor's selectivity towards BRCA mutant phenotypes. Subsequent studies demonstrated the compound's mechanism: inducing replication stress by impairing of DNA synthesis and activating DNA damage pathways via ATR signaling. Preclinical mouse studies highlighted favorable pharmacokinetics and involved the assessment of anti-tumor effects in animal models. In summation, our USP1 inhibitor, developed using AIGEN ChemTailor and advanced molecular modeling techniques, stands out as a potent therapeutic against BRCA-mutated cancers and PARP inhibitor-resistant strains. Citation Format: Jihye Kim, Sunkyu Kim, Sejeong Park, Sanghoon Lee, Kiwoong Yoo, Ho-Jeong Lee, Jaewoo Kang, Kwang-Ok Lee. Discovery of a novel USP1 inhibitor: Targeted therapy for BRCA-mutated and PARPi-resistant cancers [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 7149.

Abstract 4527: TNG348 is synergistic with PARP inhibitors in tumor models with elevated replication stress

Abstract Defective maintenance of genomic integrity is a hallmark of cancer cells that can result from oncogene-induced replication stress and by loss of DNA repair mechanisms. DNA repair deficiencies and elevated replication stress present targetable vulnerabilities for cancer treatment. Notably, BRCA1/2 mutant and homologous recombination deficient (HRD) tumors cannot repair double-strand breaks by homologous recombination and rely on alternative pathways of DNA repair. PARP inhibitors (PARPi), which are a standard of care in many BRCA1/2 mutant tumors, cause synthetic lethality with BRCA1/2 mutation by inhibiting the DNA base excision repair pathway. Despite the clinical benefit of PARPi, they are not effective in every HRD tumor and the acquisition of PARPi resistance limits long-term response. TNG348, a selective allosteric inhibitor of the deubiquitinating enzyme USP1, was specifically designed to target HRD vulnerabilities through an alternative mechanism. We previously showed that the anti-tumor activity of USP1 inhibition results from disruption of the translesion synthesis DNA damage tolerance pathway, a mechanism of action that is functionally distinct from base excision repair targeted by PARPi. Our preclinical studies show that TNG348 is active in HRD models and strongly synergizes with PARP inhibitors to drive strong anti-tumor responses. We have identified replication stress as a predictive biomarker of TNG348 response using cell line profiling and genome-wide CRISPR screens. For example, overexpression of oncogenes known to induce replication stress sensitized to the TNG348 and PARPi combination both in vitro and in vivo. These data indicate that cancer-specific elevated DNA replication stress could contribute to tumor sensitivity to TNG348 and provide additional patient stratification strategies and opportunities for indication expansion. Citation Format: Antoine Simoneau, Hsin-Jung Wu, Charlotte Pratt, Grace Comer, Shangtao Liu, Samuel Meier, Tenzing Khendu, Ashley Choi, Hongxiang Zhang, Binzhang Shen, Douglas Whittington, Sirimas Sudsakorn, Wenhai Zhang, Yi Yu, Yingnan Chen, Brian Haines, Adam Crystal, Jannik N. Andersen, John Maxwell, Scott Throner. TNG348 is synergistic with PARP inhibitors in tumor models with elevated replication stress [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 4527.

Abstract 1987: PARP inhibitor (PARPi) rechallenge shows differential sensitivity in PARPi olaparib-resistant BRCA1-mutated (BRCA1m) high-grade serous ovarian cancer (HGSC)

Abstract Background: PARPis have changed the treatment paradigm in HGSC, particularly for women with BRCA mutations. PARPi, olaparib (O), is a standard of care maintenance therapy following platinum-based chemotherapy in newly diagnosed BRCA-mutated (BRCAm) HGSC. But patients ultimately develop resistance to O. We investigated whether rechallenge with different PARPi monotherapy could cause cytotoxicity in BRCAm HGSC after becoming O-resistant. We also aimed to identify possible mechanisms of actions of PARPi’s differential sensitivity. Methods: We developed a PARPi-resistant line (UWB-OlaJR) from BRCA1m PARPi-sensitive cells (UWB1.289) by gradient exposure of O with initial lower doses to a final concentration of 20 μM over 10 months. Four PARPis including O, niraparib (N), talazoparib (T), and veliparib (V) were tested. Cell growth was assessed using 5-day XTT and 10-day clonogenic assays. DNA damage and homologous recombination (HR) repair activity were evaluated using immunofluorescence (IF) staining of γH2AX and RAD51 foci, respectively. Replication fork (RF) dynamics was measured by DNA fiber assay. All data were repeated in triplicate and analyzed using t-test or one-way ANOVA. Data were shown as mean ± SD and P < 0.05 was considered statistically significant. Results: 5-day XTT assay shows a 53.7-fold increase in resistance to O in UWB-OlaJR relative to UWB1.289 (IC50 2.2 μM vs. 118.1 μM). Cross-resistance was observed with other 3 PARPis, showing increased IC50 values compared to its parental line (V: 23.4-fold, N: 160.5-fold, T: 229.6-fold). However, 10-day clonogenic assay showed no resistance against N (1.09-fold) and T (0.51-fold) in UWB-OlaJR, indicating differential sensitivity to different PARPis by a long-term cytotoxicity measure. Surprisingly, γH2AX foci did not increase in response to PARPis in UWB-OlaJR compared to UWB1.289 (O: 2.58-fold, V: 2.33-fold; N: 2.89-fold, T: 2.58-fold; all P < 0.05), suggesting possible restored DNA repair function. All four PARPis induced replication stress in both O-sensitive and -resistant cell lines as evidenced by decreased RF speed compared to untreated group (O: 0.57- and 0.7-fold, V: 0.68- and 0.74-fold, N: 0.47- and 0.47-fold, T: 0.47- and 0.52-fold; all P < 0.001). Notably, N and T treatments reduced fork speed to a greater extent than O and V in UWB-OlaJR. However, RF protection was not affected when UWB-OlaJR treated with different PARPis compared to UWB1.289 (O: 0.89-fold, V: 0.87-fold, N: 0.71-fold, T: 0.83-fold; all P < 0.05), suggesting that differential sensitivity to different PARPis was unlikely due to modulation of RF stabilization. Conclusions: Our findings demonstrate different PARPis may induce varied levels of cytotoxicity and replication stress in O-resistant BRCA1m HGSC, possibly via modulating alternative RF dynamics besides RF protection. Citation Format: Chi-Ting Shih, Tzu-Ting Huang, Jayakumar R. Nair, Jung-Min Lee. PARP inhibitor (PARPi) rechallenge shows differential sensitivity in PARPi olaparib-resistant BRCA1-mutated (BRCA1m) high-grade serous ovarian cancer (HGSC) [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 1987.

Abstract 5784: Structure-based drug design against caPCNA for selective cancer chemotherapy development

Abstract Proliferating cell nuclear antigen (PCNA) is critical to DNA replication and repair processes, and it is also a proliferation biomarker in a variety of human tumors. A unique cancer-associated isoform of the protein, caPCNA, has been previously identified, allowing for rational-based drug discovery studies to develop leads with the potential for selective therapeutic targeting of cancer cells. Several strategies have been employed to develop agents targeting caPCNA, including peptide and small molecule-based inhibitors, but the success in developing therapeutically tractable compounds has been limited so far. Here, we present the crystal structures and corresponding biochemical characterizations of our novel small molecule-based caPCNA inhibitors, which bind into the PIP-box binding pocket of PCNA. One of our lead compounds is now an investigational new drug that we observed selectively kills cancer cells, and it appears to induce replication stress, apoptosis and increase cancer cell sensitivity to genotoxic agents, while these effects are not observed in non-malignant cell controls. This caPCNA inhibitor is orally administrable, metabolic stable and it suppresses tumor growth as a monotherapy or as a combination treatment, and it has entered clinical trials in the United States. PCNA plays a critical role in temporarily dislodging RNA polymerase II from transcription replication conflict sites, to enable the replication fork to proceed. Thus, structure-based drug discovery approaches to target transcription replication conflict resolution machinery, such as PCNA protein, may open a novel therapeutic avenue for exploiting this cancer-selective vulnerability. Citation Format: Jennifer Jossart, Pouya Haratipour, Long Gu, Carone M. li, Robert Lingeman, Yilun Liu, Robert J. Hickey, Linda H. Malkas, Jeff J. Perry. Structure-based drug design against caPCNA for selective cancer chemotherapy development [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 5784.

Abstract 138: UCHL5 and PAX3-FOXO1 resolve replication stress in rhabdomyosarcoma

Abstract PAX3-FOXO1 positive rhabdomyosarcoma (RMS) is an aggressive pediatric sarcoma with a terrible prognosis. The resultant fusion acts as a chimeric transcription factor, in part remodeling chromatin to inhibit terminal differentiation and maintain cancer cell proliferation. As this oncoprotein is not directly druggable, we have used genetic screens to identify dependencies downstream of PAX3-FOXO1 to improve our understanding of pathogenesis and develop new therapeutic targets. We performed pooled CRISPRi screening in an isogenic system of a patient-derived RMS cell line with or without knockdown of PAX3-FOXO1. Loss of the ubiquitin hydroxylase UCHL5 led to selective depletion of PAX3-FOXO1 positive Rh30 cells compared to PAX3-FOXO1 knockdown cells, suggesting that the fusion creates UCHL5 dependence. We validated this finding in competition growth assays in additional fusion positive (FP) and fusion negative (FN) RMS cell lines, and observed UCHL5 dependency only in the former. By contrast, UCHL5 was dispensable for cell growth and survival in FN RMS cell lines. These data were recapitulated in the large DepMap dataset, which also identified the top genetic co-dependencies of UCHL5 as INO80-complex (INO80c) members. We therefore investigated whether the fusion selective effects of UCHL5 were mediated by the INO80c, of which UCHL5 is an N-terminal member. We observed that knockdown of another N-terminal component of INO80c, NFRKB, phenocopies the fusion-selective effects of UCHL5 loss. Conversely, the catalytic ATPase INO80 was pan-essential, suggesting that only N-terminal subunits of INO80c regulate PAX3-FOXO1 specific functions. INO80c plays an essential role in mitigating DNA replication stress. Indeed, loss of UCHL5 led to DNA damage as measured by γH2AX and slowing of replication forks as measured by fiber assays in FP, but not FN RMS. Unexpectedly, acute degradation of PAX3-FOXO1 also slowed DNA replication fork speed, suggesting a genetic model in which UCHL5 and PAX3-FOXO1 cooperate to resolve replication stress in FP RMS. Finally, in vivo experiments using cell line xenografts confirms that loss of UCHL5 slows the growth of FP RMS, but not FN RMS. These findings suggest that loss of UCHL5 induces replication stress in a fusion-specific manner, and offers both an opportunity to study the mechanisms by which the PAX3-FOXO1 oncoprotein mitigates replication stress, and to develop UCHL5 inhibitors as precision therapies for FP RMS patients. Citation Format: Amit J. Sabnis, Pushpendra K. Sahu, Yue Pan. UCHL5 and PAX3-FOXO1 resolve replication stress in rhabdomyosarcoma [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 138.