Citations

Abstract BRCA1 -deficient cancers experience persistent replication stress and structural genome instability yet retain the capacity for sustained proliferation, implying reliance on compensatory genome-maintenance mechanisms. Here, we establish BRCA1–FANCM synthetic lethality in human BRCA1 -deficient breast cancer and exploit temporally controlled FANCM depletion to capture genome evolution over successive cell divisions before declining cellular fitness becomes limiting. We show that FANCM restrains genome-wide structural variation in BRCA1 -deficient breast cancer cells under endogenous replication stress. FANCM loss amplifies the characteristic BRCA1 -associated short tandem duplication (TD) phenotype while permitting larger, including megabase-scale, TDs and diverse rearrangements to emerge. Newly emerged TDs preferentially associate with Pol II-occupied regions, and FANCM depletion increases proximity between the replication machinery and elongating RNAPII in BRCA1 -mutant breast cancer cells, linking FANCM-mediated genome protection to transcription–replication encounters. BRCA1 -altered human tumors with low FANCM expression recapitulate key features of this phenotype, while genome–transcriptome integration links newly emerged SVs to configuration-dependent local transcriptional changes. Together, these findings establish FANCM as a replication-stress safeguard coupling survival to restraint of structural genome evolution.

Abstract Therapeutic resistance in ovarian cancer is frequently driven by persistent replication stress, yet the molecular mechanisms that convert replication stress into a therapeutically exploitable vulnerability remain incompletely understood. Here, we identify drug-induced nuclear cathepsin L (nCTSL) as a previously unrecognized regulator of replication stress and DNA repair. Clofarabine (CLF) combined with the ATR inhibitor AZD6738, or the CHK1 inhibitor prexasertib promoted nuclear accumulation of CTSL, where it remodeled the replication machinery through degradation of CCNE1, MCM3, MCM6, and geminin, accompanied by loss of RAD51 and 53BP1 and increased γH2AX and phospho-RPA2. DNA fiber analysis demonstrated marked inhibition of replication fork progression following CLF-based combinations, whereas CTSL depletion accelerated fork progression and abolished therapy-induced replication stress. Reconstitution with the nuclear M1F CTSL isoform restored replication restraint, confirming a direct role for nuclear CTSL in regulating replication dynamics. GFP-based DNA repair reporter assays further revealed that CLF-based combinations suppress DNA repair competence in a CTSL-dependent manner, indicating that nuclear CTSL couples replication stress amplification with functional inhibition of repair pathways. Functionally, CLF-based combinations selectively targeted transformed fallopian tube secretory epithelial cells while sparing non-transformed counterparts, demonstrated broad activity in patient-derived ovarian cancer ascites spheroids, and significantly inhibited tumor growth and prolonged survival in vivo . Collectively, our findings identify nuclear CTSL as a mechanistic driver of replication stress that remodels the replication machinery, impairs DNA repair, and creates a therapeutically exploitable vulnerability in ovarian cancer. We propose that nuclear CTSL promotes a transition from replication competence to replication catastrophe, thereby establishing a conceptual framework for biomarker-guided therapeutic strategies targeting CTSL-dependent replication stress.

Abstract Introduction Checkpoint blockade revolutionized cancer therapy. However, only a small proportion of cancers respond to checkpoint blockade. B7-H3 is a member of the B7 superfamily, which includes both co-inhibitory and co-stimulatory receptors. B7-H3 is upregulated by many tumors which is generally associated with a worse prognosis. However, the exact mechanism by which B7-H3 suppresses the immune response remains elusive. Methods We transfected human HEK293T cells and assessed the binding to recombinant Siglecs via flow cytometry. We generated monoclonal antibody F7 against human B7-H3 and tested its blocking function using B7-H3 transfected HEK293T cells. We examined the extent of degranulation and NK cell-mediated killing on B7-H3 expressing HEK293T cells in vitro, as well as the CART19 cells mediated killing in relapse model of B7-H3 expressing CD19+ leukemia Nalm6 cells NSG mice. Results We demonstrated that sialylated B7-H3 binds to the inhibitory receptor Siglec-9 on immune cells but not to the related protein Siglec-7. A high affinity monoclonal antibody (mAb) F7 against B7-H3 blocks Siglec-9 binding. Functionally, F7 enhances NK cell killing of B7-H3 expressing target cells in vitro. F7 also dramatically enhances CART19 mediated killing of B7-H3 expressing Nalm6 cells in a relapse model in NSG mice in vivo. Conclusion We have now shown for the first time that B7-H3 binds to the inhibitory receptor Siglec-9 and generated a novel monoclonal antibody F7 that target a unique sequence in B7-H3 as compared to other monoclonal antibodies being examined for clinical efficacy. F7 can promote NK cell killing of B7-H3 expressing target cells in vitro and synergize with CART19 to enhance killing of B7-H3 expressing tumor cells in vivo. As B7-H3 is overexpressed in many human cancers, this work is critical toward understanding how B7-H3 mediates immune evasion through Siglec-9 for enhancing anti-tumor immunity. Funding Source R01CA243545-01A1 Topic Categories Tumor Immunology: Checkpoints, Prevention, and Treatment (TIPT)

10513 Background: Prior reports of high frequency of BRCA1 and BRCA2 ( BRCA1/2 ) PVs among South Asian women from Bangladesh, India, Nepal, and Pakistan with breast or ovarian cancer require validation in large prospective unselected cohorts. Methods: Individual-level anonymized patient data from two prospective cohorts were pooled to evaluate the prevalence of germline PVs among South Asian women: the South Asian Regional Alliance for Hereditary Cancers (SARAH Consortium) in Bangladesh, Nepal, and Pakistan, and the previously reported Nurse-led Genetic Counseling and Awareness (NuGenA) expansion cohort in India. Both studies enrolled consecutive cases of histologically confirmed newly diagnosed breast or epithelial ovarian cancer irrespective of age at diagnosis or family history. In both studies, participants completed questionnaires on personal and family history (first-, second- or third-degree) and provided blood for germline genetic testing after written consent. PVs in cancer predisposition genes including ATM, BARD1, BRCA1, BRCA2, BRIP1, CHEK2, MLH1, MSH2, MSH6, PALB2, RAD51C, RAD51D, and TP53 were identified as part of the studies utilizing a multigene panel and the results were returned to ordering clinicians. The data cutoff date for this analysis was November 30, 2025. Results: The analysis included 1,711 South Asian women with breast (n=1,210) or ovarian (n=501) cancer. A family history of breast or ovarian cancer was noted in 177 (10.3%). Among breast cancers, 94% were invasive, 84% were ductal, 35% were triple-negative and over two-thirds presented with nodal involvement. Germline PVs across 13 predisposition genes were observed in 17.7% of breast and 35.9% of ovarian cancers. Notably, BRCA1/2 PVs accounted for most of these PV (13.6% of breast and 28.3% of ovarian cancer cases) with PVs in all genes except BRCA1/2 under 1%. The frequency of BRCA1/2 PVs was 24.4% in triple-negative breast cancer and 8.0% in ER+/HER2-. BRCA1/2 PV carriers were diagnosed with breast cancer at a younger age than non-carriers (Median age: 42 vs 46 years, P<0.001), with no age difference observed in ovarian cancer (50 vs. 51 years, P=0.79). Notably, among the 306 BRCA1/2 PV carriers identified, 229 (74.2%) did not report a family history of breast or ovarian cancer. Conclusions: We confirm a high prevalence of BRCA1/2 PVs among unselected South Asian women with either breast or ovarian cancer. The observations of younger age at cancer diagnosis, and low rates of family history support the need for population-specific germline genetic testing and risk management strategies in South Asia.

Clonal acquisition of multiple chromosomal abnormalities in hematopoietic stem and progenitor cells (HSPCs) is a hallmark of high-risk acute myeloid leukemias with complex karyotype (AML-CK). AML-CK is associated with TP53 mutations and chromosome 5q deletions (del5q); however, the drivers and clonal trajectories of aneuploid evolution in HSPCs remain unknown. We have developed a patient-derived induced pluripotent stem cell (iPSC) model in which preleukemic HSPCs clonally evolve to distinct, highly aneuploid states following transient mitotic inhibition. By tracking chromosome evolution at single cell resolution, we show that TP53-mutant HSPCs with del5q, but not TP53­ mutation alone, evolved complex chromosomal changes. Clonal evolution was marked by stepwise acquisition of numerical and structural chromosome changes seen in AML-CK patients, with individual abnormalities conferring fitness advantage. iPSC-derived aneuploid HSPCs and primary AML-CK patient samples exhibited a conserved gene expression signature marked by upregulation of PTEN, cohesins, and anti-apoptotic factor BCL2, indicative of a shared aneuploid cell state in HSPCs. Clinical BCL2 inhibitor venetoclax eradicated BCL2-dependent aneuploid clones, with resistant clones undergoing a lineage switch to upregulate alternative BCL2 factors. In summary, we demonstrate that mutant TP53 and del5q drive chromosome evolution marked by stepwise acquisition of individual abnormalities. Moreover, aneuploid HSPCs exhibit a shared gene expression state which confers unique targetable therapeutic vulnerabilities in AML-CK.

Abstract M.D.-Ph.D. programs in the United States have traditionally followed a “2- n -2” curricular model, with the graduate phase occurring between the pre-clerkship and clerkship portions of medical training. While well established, this format can limit trainee autonomy in shaping their physician-scientist training trajectories. In response, some programs have introduced a “3- n -1” model, allowing students to complete clerkships before beginning Ph.D. training. Our institution implemented multiple flexible curricula in 2017. Understanding why trainees choose one pathway is important as programs consider implementing more adaptable training structures. To investigate these factors, we surveyed M.D.-Ph.D. students at our institution, which offers multiple flexible curricular alternatives, about contributions to their curricular decisions. Responses supported that trainees weigh considerations across medical education, scientific development, and integrated physician-scientist training domains. Although the traditional pathway was a popular option, most students pursued one of the flexible pathways. Our findings suggest that introducing flexibility in the timing of undergraduate medical education and graduate training can support diverse educational, logistical, and personal priorities while maintaining the rigor of physician-scientist training. Offering multiple pathways empowers trainees to design trajectories that best fit their needs. Continued longitudinal studies are needed to assess the long-term impacts of flexible curricula on physician-scientist career outcomes.

Abstract Despite long-standing interest in combining topoisomerase I (TOP1) poisons with PARP inhibitors (PARPis), the mechanistic basis for the variable and often unpredictable response to this strategy remains unclear. Early preclinical studies suggested that PARP inhibition amplifies TOP1 poison-induced DNA damage, implying broad therapeutic potential; however, emerging translational and clinical data—particularly in ovarian cancer—indicate that synergy is context-dependent and governed by tumor-intrinsic features that have not been fully defined.Here we assessed determinants of response in five HR proficient ovarian cancer (OC) patient-derived xenografts (PDXs) previously treated with the SN-38 prodrug etirinotecan pegol (NKTR-102) in combination with the PARPi rucaparib (C. Nitschmann, Clin. Cancer Res. 22(2) Suppl. B50, 2016). Although all models were refractory to single-agent rucaparib, 50% exhibited enhancement of NKTR-102-induced tumor regression with the addition of rucaparib. Neither PARP1 abundance nor extent of rucaparib-mediated poly(ADP-ribose) suppression correlated with therapeutic benefit. Instead, sensitization was restricted to PDXs intrinsically responsive to NKTR-102 and was strongly associated with high levels of NKTR-102-induced TOP1-DNA covalent complexes. In further analysis, 2 of 3 resistant PDXs exhibited high levels of the ABC transporter ABCG2 despite lack of previous therapy, revealing a previously underappreciated mechanism of de novo resistance that limits effective TOP1 inhibitor activity independent of homologous recombination status. Notably, NKTR-102 response did not track with platinum/taxane sensitivity, highlighting the distinct molecular circuitry governing TOP1 poison susceptibility.Collectively, these findings demonstrate that rucaparib can potentiate NKTR-102-induced regression in tumors capable of forming abundant TOP1-DNA covalent complexes. This work identifies TOP1-DNA complex trapping and lack of ABCG2 expression as mechanistic predictors of benefit, providing an updated framework for selecting patients most likely to respond to TOP1 poison/PARPi combinations and challenging the long-standing assumption that PARP inhibition can universally resensitize tumors to TOP1 poisons. Citation Format: Annapoorna Venkatachalam, Caroline C. Nitschmann, Karen S. Flatten, Rachel Hurley, Xiaonan Hou, Cristina Correia, Paula A. Schenider, Krista Goergen, Matthew Maurer, Ethan P. Heinzen, Ann L. Oberg, S John Weroha, Paul Haluska, Scott H. Kaufmann, Elizabeth M. Swisher. TOP1-DNA protein complex as a predictive biomarker for NKTR-102/Rucaparib combination therapy in PARP-inhibitor resistant ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr LB364.

Abstract Breast and ovarian cancers are major causes of cancer mortality in women with frequent relapse after curative intent treatment, underscoring the need for novel therapeutics. To this end, we identified CTPS1 as an essential gene in multiple breast and ovarian cancer models, including those resistant to chemotherapy and PARP inhibitors. CTPS1 and CTPS2 are responsible for de novo synthesis of cytidine triphosphate (CTP) which is required for DNA/RNA polymerization, phospholipid synthesis and protein glycosylation. We found CTPS1 to be the predominant isoform expressed in cancer cells, with significantly higher levels in advanced and resistant disease models compared to normal tissue, benign lesions and treatment naïve cancers. Using CTPS1 siRNAs and CTPS1-dTAG (degradation tag) cell lines, we discovered that depletion of CTPS1 induces S-phase cell cycle arrest followed by apoptosis. Leveraging a first-in-class, highly selective, and orally bioavailable CTPS1 inhibitor (STP938) developed by Step Pharma, we identified nanomolar IC50 values across a panel of cell lines, both in 2D and 3D culture systems, and potent anti-neoplastic activity ex vivo and in vivo using patient-derived xenograft (PDX) models. STP938 also synergized with standard-of-care chemotherapy agents and PARP inhibitors, including in therapy-resistant models. Given the essentiality of CTP for biosynthetic processes, we performed bulk and single cell RNA-seq, total and phospho-proteomics, metabolomics, and lipidomic strategies to examine changes induced by acute and chronic STP938 exposure. This revealed that cancer cells are able to overcome pharmacologic CTPS1 inhibition by rewiring of pyrimidine and purine synthesis pathways, upregulation of the pentose phosphate pathway, and extensive remodeling of lipid synthesis pathways including shifts in PC/PE content, increased lysophospholipid production, and altered mitochondrial lipid composition (decreased cardiolipin and increased PG). To identify genetic drivers of STP938 resistance and synthetic lethal vulnerabilities, we performed a genome-wide CRISPR knockout screen together with a PRISM drug sensitivity screen. Genes regulating dNTP biosynthesis, S-phase progression, and RNA processing were identified as primary escape mechanisms, while DHODH inhibitors, purine and nucleoside analogs, and mitosis modifiers were implicated as synthetic lethal opportunities. Notably, we discovered that 25% of ovarian tumors lack CTPS2 protein expression, suggesting near complete reliance on CTPS1 for survival. Thus, a Phase 1a/b clinical trial of STP938 (NCT06297525) has been initiated, including an expansion cohort for patients with CTPS2-null ovarian tumors. These laboratory discoveries and ongoing clinical efforts aim to enable broader uptake of this novel therapeutic approach to improve long-term outcomes in patients with CTPS1-dependent disease. Citation Format: Xiyin Wang, Michael Emch, Lauren Voll, Rebecca Epp, Esther Rodman, Noa Odell, Hannah Smith, Nicole Pearson, Xiaonan Hou, Matthew Goetz, Scott Kaufmann, S. John Weroha, Phillip Beer, John Hawse. CTPS1 as a metabolic vulnerability and novel therapeutic target in breast and ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4560.

Abstract High-grade serous ovarian cancer (HGSOC), the most prevalent and lethal ovarian cancer subtype, is typically diagnosed at an advanced stage due to the absence of effective early detection strategies. Mounting evidence indicates that HGSOC originates from the fallopian tube (FT), particularly from secretory epithelial cells in the fimbria. However, incomplete understanding of the cellular and molecular origins of HGSOC continues to hinder early interception efforts. To address this, we established a clinically and genetically annotated living FT organoid biobank at Mayo Clinic, representing over 200 patient donors. Using optimized enrichment protocols, we isolated FT epithelial stem/progenitor cells from tissue and Tao brushings, generating organoids independent of anatomical site or laterality that recapitulate native FT epithelial architecture and lineage differentiation. We applied integrated multi-omic analyses, including single-cell RNA sequencing, single-nucleus RNA/ATAC sequencing, and bulk RNA-seq, across fresh cells and organoids. Integration with public datasets produced the largest single-cell atlas of normal and high-risk FTs to date. This analysis defined high-specificity gene signatures for secretory and multiciliated lineages that outperform canonical lineage markers (e.g. PAX8, FOXJ1), identified lineage-defining transcription factors and regulatory networks, and showed concordant protein expression in the human FT epithelium. Notably, subcluster-level analysis revealed a rare hybrid epithelial-mesenchymal population transcriptionally and proteomically aligned with the mesenchymal-like subtype of HGSOC. Spatial immunoprofiling demonstrated that these cells express multiple defining markers and reside above the basement membrane, intercalated between epithelial cells and lack luminal contact, reminiscent of the histologically described peg cell population. Comparative analyses with fetal mesonephric tissues uncovered developmental parallels, and this population was stably maintained in 3D organoid cultures, providing a robust ex-vivo model to study early transformation. Together, our findings identify a rare, multipotent epithelial population with mesenchymal features as a candidate origin for mesenchymal-like HGSOC and establish the FT organoid biobank as a powerful platform for mechanistic and translational studies in ovarian carcinogenesis. Citation Format: Megan L. Ritting, Wenmei Yang, Syed Mohammed Musheer Aalam, Hui Zhao, Liang Feng, Jianning Song, Mihai G. Dumbrava, Wazim M. Ismail, Kenneth Schaufelberger, S. John Weroha, Scott H. Kaufmann, Alexandre Gaspar-Maia, Mark E. Sherman, Jamie N. Bakkum-Gamez, Nagarajan Kannan. A rare multipotent peg-like epithelial cell is a candidate cell-of-origin for high-grade serous ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7636.

PURPOSE To compare mortality risk among long-term survivors of high-grade serous ovarian cancer (HGSOC) with that of the general US population and to examine how mortality and recurrence trends have evolved under changing treatment approaches. MATERIALS AND METHODS This is a retrospective cohort study including patients with histologically confirmed HGSOC treated at a single referral cancer center from 1991 to 2022. Mortality risk, expressed as standardized mortality ratios (SMRs, ratio of observed deaths in the study population to expected deaths in an age-matched US population), was assessed at diagnosis and yearly on patients without events (disease persistence, recurrence, or death from any cause). RESULTS A total of 2,074 consecutive patients were included. The median follow-up was 12.5 years (IQR, 11.7-13.8). Most patients were stage III (1,396, 69.2%), 1,299 (62.8%) underwent primary cytoreductive surgery, and 1,688 (87.4%) received platinum-taxane as first-line therapy. At diagnosis, the mortality rate was 7.40 times higher than that in the general population (95% CI, 7.04 to 7.77). By 7 event-free years, the 95% CI for the SMR included 1 for the first time, and at 10 years, the SMR was 1.05 (95% CI, 0.72 to 1.49), indicating no statistically significant excess mortality compared with the general population. In those event-free at 10 years, the 5-year cumulative incidence of ovarian cancer–related deaths (8.2% [95% CI, 4.2 to 16.0]) was comparable with that of non–ovarian cancer–related deaths (6.3% [95% CI, 2.9 to 13.8]). CONCLUSION In this cohort, mortality risk steadily declined for patients who remained event-free, approaching that of the general population by 7 years after diagnosis. These results emphasize the importance of individualized, long-term follow-up to address both recurrence risk and survivorship needs.