Rb Pathway Research Articles

CTNI-58. A PHASE 0/2 TRIAL OF ABEMACICLIB PLUS LY3214996 IN RECURRENT GLIOBLASTOMA

Abstract BACKGROUND This Phase 0/2 study (NCT04391595) evaluates tumor pharmacokinetics (PK), tumor pharmacodynamics (PD), and clinical efficacy of abemaciclib, a CDK4 and 6 inhibitor, plus LY3214996, an ERK 1/2 inhibitor, in recurrent glioblastoma (rGBM). METHODS Adult rGBM patients (n=42) with intact RB, >30% pERK expression, and CDKN2A/B deletion or CDK4/6 amplification received six days of abemaciclib plus LY3214996 prior to a planned resection. Time-escalation cohorts (3-5 and 7-9 hour intervals from last dose to tumor resection) identified the optimal time interval (OTI). Subsequent Phase 0 patients had tumor tissue (gadolinium [Gd]-enhancing and nonenhancing regions), cerebrospinal fluid (CSF), and plasma collected at the OTI. Total and unbound drug concentrations were measured using validated LC-MS/MS methods. Tumor PD was compared to matched archival or pre-treatment biopsied tissue. A PK ‘trigger’ (i.e., unbound concentration > 5 x IC50) was set for each drug. Gd-nonenhancing tumor demonstrating PK response to both drugs qualified patients for the Phase 2 component of the study. RESULTS Forty two patients were accrued to Phase 0 and no dose-limiting toxicities were observed. Median unbound concentrations in Gd-nonenhancing tumor regions were 31 nM and 10 nM for abemaciclib and LY3214996, respectively. Tumor RB, RSK phosphorylation, and tumor proliferation (MIB-1) decreased in (47.5%), (25%) and (72.5%) patients, respectively. Nine of the 40 (22.5%) patients exceeded PK thresholds for both abemaciclib (12 nM) and LY3214996 (25 nM), qualifying them for Phase 2. Here, median PFS and median OS were 2.4 and 7.2 months, respectively. CONCLUSION Abemaciclib and LY3214996 achieved pharmacologically-relevant concentrations in Gd-non-enhancing GBM tissue in 80% and 30% of the patients, respectively. Suppression of the RB pathway and tumor proliferation, but not ERK pathway was observed. Inadequate CNS penetration and tumor PD modulation by LY3214996 contributed to the unremarkable clinical efficacy profile of this drug combination. Targeted ERK inhibition remains an unmet need in experimental GBM therapy.

Merkel cell polyomavirus pan-T antigen knockdown reduces cancer cell stemness and promotes neural differentiation independent of RB1.

Merkel cell carcinoma (MCC) is a highly aggressive skin cancer associated with integration of Merkel cell polyomavirus (MCPyV). MCPyV-encoded T-antigens (TAs) are pivotal for sustaining MCC's oncogenic phenotype, i.e., repression of TAs results in reactivation of the RB pathway and subsequent cell cycle arrest. However, the MCC cell line LoKe, characterized by a homozygous loss of the RB1 gene, exhibits uninterrupted cell cycle progression after shRNA-mediated TA repression. This unique feature allows an in-depth analysis of the effects of TAs beyond inhibition of the RB pathway, revealing the decrease in expression of stem cell-related genes upon panTA-knockdown. Analysis of gene regulatory networks identified members of the E2F family (E2F1, E2F8, TFDP1) as key transcriptional regulators that maintain stem cell properties in TA-expressing MCC cells. Furthermore, minichromosome maintenance (MCM) genes, which encodes DNA-binding licensing proteins essential for stem cell maintenance, were suppressed upon panTA-knockdown. The decline in stemness occurred simultaneously with neural differentiation, marked by the increased expression of neurogenesis-related genes such as neurexins, BTG2, and MYT1L. This upregulation can be attributed to heightened activity of PBX1 and BPTF, crucial regulators of neurogenesis pathways. The observations in LoKe were confirmed in an additional MCPyV-positive MCC cell line in which RB1 was silenced before panTA-knockdown. Moreover, spatially resolved transcriptomics demonstrated reduced TA expression in situ in a part of a MCC tumor characterized by neural differentiation. In summary, TAs are critical for maintaining stemness of MCC cells and suppressing neural differentiation, irrespective of their impact on the RB-signaling pathway.

Abstract PO1-08-09: Drynaria fortunie, a nutritional herb for prevention in triple negative breast cancer

Abstract Study Rationale: The triple negative breast cancer (TNBC) lacks the expression of hormone and growth factor receptors and responds only to anthracycline/taxol-based conventional chemotherapy. Major therapeutic limitations include systemic toxicity and acquired resistance to chemo-therapeutics. Relatively non-toxic nutritional herbs from traditional Chinese medicine (TCM) may represent testable alternatives against TNBC. These herbs effectively target multiple signaling pathways (Yang et al: J. Ethno-pharmacology 264: 113249, 2021). Growth inhibitory efficacy of nutritional herbs in a cellular model for TNBC is associated with downregulation of RB, RAS, PI3K and AKT signaling pathways (Telang et al: Pharmaceuticals 14: 1318, 2021). Drynaria fortunie (DF) is a nutritional herb that is used in many TCM herbal formulations. The present study is designed to examine the effects of DF in a cellular model for TNBC and to identify mechanistic leads for its efficacy. Experimental Model, herbal extract and biomarkers: Estrogen receptor negative, progesterone receptor negative and human epidermal growth factor receptor-2 negative MDA-MB-231 human breast cancer cell line represented a cellular model for TNBC. Non-fractionated aqueous extract from the bark of DF represented the test agent. Cell cycle progression, RB signaling and caspase 3/7 activity represented quantitative end point biomarkers. Study Outcome: Treatment with DF at cytostatic concentrations induced S phase arrest and inhibited RB signaling via downregulated expression of cyclin E, CDK2, pRB and E2F1. DF treatment also induced caspase 3/7 activity, a marker for cellular apoptosis. Collectively, these data provide potential mechanistic leads for anti-proliferative and pro-apoptotic effects of DF in the present model. Conclusion: Present data validate a mechanism-driven experimental approach to prioritize efficacious nutritional herbs as testable alternatives for prevention/therapy of TNBC. Citation Format: Nitin TELANG, Hareesh Nair, George Wong. Drynaria fortunie, a nutritional herb for prevention in triple negative breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-08-09.

Therapeutic Strategies for RB1-Deficient Cancers: Intersecting Gene Regulation and Targeted Therapy.

The retinoblastoma (RB) transcriptional corepressor 1 (RB1) is a critical tumor suppressor gene, governing diverse cellular processes implicated in cancer biology. Dysregulation or deletion in RB1 contributes to the development and progression of various cancers, making it a prime target for therapeutic intervention. RB1's canonical function in cell cycle control and DNA repair mechanisms underscores its significance in restraining aberrant cell growth and maintaining genomic stability. Understanding the complex interplay between RB1 and cellular pathways is beneficial to fully elucidate its tumor-suppressive role across different cancer types and for therapeutic development. As a result, investigating vulnerabilities arising from RB1 deletion-associated mechanisms offers promising avenues for targeted therapy. Recently, several findings highlighted multiple methods as a promising strategy for combating tumor growth driven by RB1 loss, offering potential clinical benefits in various cancer types. This review summarizes the multifaceted role of RB1 in cancer biology and its implications for targeted therapy.

Abstract 6390: Exploratory biomarker analysis of phase 3 ASTRUM-005 study: Serplulimab versus placebo plus chemotherapy for extensive-stage small cell lung cancer

Abstract Background First-line serplulimab plus chemotherapy (chemo) significantly improved OS compared with chemo alone for patients with extensive-stage small cell lung cancer (ES-SCLC) in the phase 3 ASTRUM-005 study. In this exploratory biomarker analysis, we retrospectively evaluated the association of proteome signature, genetic mutations, and hematological indices with efficacy. Methods: Serum proteomics data were generated via Olink® Explore 3072 platform for 168 patients (training: 80; validation: 88). Genetic mutations were assessed by Med1CDxTM panel for 305 patients. Baseline neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), and lactate dehydrogenase (LDH) levels were analyzed in all 585 patients. Clinical data cutoff date was June 13, 2022. Results: From Olink data, we generated a 15-protein signature. Results in the validation cohort are shown in the table below. Patients with upper signature score derived higher PFS (7.89 vs 4.17 mo, HR 0.37) and OS (17.18 vs 9.74 mo, HR 0.27) benefit from adding serplulimab, than those with lower signature score (PFS: 4.40 vs 4.34 mo, HR 0.68; OS: 9.05 vs 9.22 mo, HR 0.76). Patients with mutations in RB1 or NOTCH pathway had better tumor responses to the treatment of serplulimab-chemo (ORR: 80% and 86%, respectively) when compared to patients without mutations in RB1 or NOTCH pathway (ORR: 58% and 68%). Patients with NLR ≤4.6, PLR ≤338.3, and LDH ≤ upper limit of normal tended to have longer PFS and OS in both treatment groups; NLR and LDH were further proven to be independent prognostic biomarkers in multivariate regression (P = 0.002 and P <0.001, respectively). Conclusions: We found a 15-protein signature showing predictive power for serplulimab-chemo treatment; RB1 or NOTCH pathway mutations were enriched in serplulimab-chemo responders; baseline NLR and LDH were independent prognostic biomarkers for ES-SCLC. The above findings warrant further investigations. Serplulimab-chemo (n=48) Placebo-chemo (n=40) HR (95% CI) P Upper signature score, n 31 34 Median PFS (95% CI), months 7.89 (1.27, 10.74) 4.17 (2.96, 4.34) 0.37 (0.21, 0.66) <0.001 Median OS (95% CI), months 17.18 (11.01, NE) 9.74 (6.90, 12.65) 0.27 (0.15, 0.50) <0.001 Lower signature score, n 17 6 Median PFS (95% CI), months 4.40 (3.61, 5.75) 4.34 (2.66, NE) 0.68 (0.23, 2.02) 0.488 Median OS (95% CI), months 9.05 (6.67, 11.73) 9.22 (4.53, NE) 0.76 (0.29, 1.98) 0.570 Citation Format: Ying Cheng, Liang Han, Lin Wu, Jun Chen, Hongmei Sun, Guilan Wen, Yinghua Ji, Anastasia Zimina, Jianhua Shi, Zhijie Pan, Jinsheng Shi, Xicheng Wang, Fang Yang, Xinyi Yang, Mengkai Chen, Shiqi Zhong, Qingyu Wang, Jing Li, Jiancheng Cheng, Chen Ling, Yongqiang Shan, Jun Zhu, The ASTRUM-005 Study Group. Exploratory biomarker analysis of phase 3 ASTRUM-005 study: Serplulimab versus placebo plus chemotherapy for extensive-stage small cell lung 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 6390.

Abstract 5728: YB1 is a novel therapeutic for the treatment of triple negative breast cancer tumors

Abstract Metastatic breast cancer (BC) is the 2nd leading cause of death in women in the US, annually accounting for more than 43,000 deaths and 281,000 new cases of invasive BC. Amongst genetically distinct BC subtypes, those classified as being “triple-negative” (TNBC) are especially devastating due to their highly metastatic behavior, their propensity to recur rapidly, and their low response to standard-of-care therapies. In fact, the acquisition of chemoresistant phenotypes represents the main cause of disease recurrence, metastasis, and death in TNBC patients. Currently, the molecular mechanisms that regulate TNBC progression and metastasis remain unknown, as does the way these metastatic tumors acquire resistance to standard-of-care therapies. We recently established YB1 as a novel driver of these deadly TNBC activities, doing so by stimulating the cancer stem cell phenotype, and by disrupting normal cell cycle progression, therefore promoting therapy resistance and metastasis of TNBC tumors. YB1 is a multifunctional protein that acts as a transcription factor of cancer stem cell genes Nanog, Oct and Sox. Moreover, aberrant activation of YB1 contributes to the metastatic progression of several cancers, including TNBC. Our investigations revealed a major role of YB1 in the regulation of several hallmarks of cancer that drive TNBC tumors progression and metastasis, both in vitro and in preclinical mouse models of TNBC tumors. In extending these discoveries, we now show that aberrant YB1 expression activates oncogenic signaling leading to the dysregulation of cell cycle progression through the regulation of the Cyclin D/CDK4/6 complex signaling and the RB pathway. We also report the identification of a novel small molecule inhibitor, SU056, that specifically targets YB1 and inhibits its oncogenic activity in TNBC cell lines. We further show that the SU056-mediated inhibition of YB1, either as monotherapy or in combination with the CDK4/6 inhibitors, has a significant impact on inhibiting TNBC tumor progression and metastasis. In fact, the SU056-CDK4/6 inhibitors combination showed a synergistic effect when compared to monotherapy, at the same time reducing the toxicity associated with CDK4/6 inhibitors. Mechanistically, our studies revealed that genetic or pharmacologic targeting of YB1 inhibits TNBC tumor progression and metastasis through the regulation cyclin D-CDK4/6-RB pathway and blockade of cell cycle progression. Together, our data support the notion that targeting YB1 represents a potential therapeutic option for the treatment of TNBC, which could be enhanced when combined with standard of care treatment modalities. Citation Format: Wei Wang, Lamyae El Khalki, Neelum Yousaf Zai, Justin Szpendyk, Akram Alkrekshi, Bin Su, Khalid Sossey-Alaoui. YB1 is a novel therapeutic for the treatment of triple negative breast cancer tumors [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 5728.

Abstract 7569: Characterizing pathways for metastasis of retinoblastoma in forebrain organoids

Abstract Retinoblastoma (Rb) is a childhood cancer of the eye, which usually presents in children under the age of five. When detected early, Rb has favorable prognosis in developed countries, but metastasis into the central nervous system (CNS) through the optic nerve correlate with a high mortality rate. Even if detected early, the available harsh and unspecific treatments can cause serious side effects, and enucleations are still performed frequently. There is still uncertainty regarding the mechanism for metastasis into the central nervous system. We performed bioinformatic analysis of single cell sequencing data for retinoblastoma, retinal tissues, optic nerve, and brain, to identify potential pathways and cell-cell communications fostering Rb invasion into the optic nerve and brain. Our analysis revealed several potential metastatic drivers, including immune modulation involving MIF, secreted by the Rb cells, as well as cell-adhesion pathways promoting a change in cellular phenotype. Given that animal models do not faithfully recapitulate human Rb initiation and progression, we developed a novel human iPSC-derived forebrain organoid model to investigate mechanisms of Rb CNS metastasis. Microinjections of GFP-labelled Rb cells allow to track growth and spread. In this fully human model including microglial cells, we can, for the first time, study the Rb forebrain interactions and pathways involved. RNA-seq analysis of Rb cells growing in forebrain organoids shows increased activity in neuron projection development and neuron migration, overlapping with the observed phenotype switch of invading Rb. Together, we present a novel iPSC-derived human forebrain model to monitor growth and spread of metastatic Rb, and describe new pathways involved in invading and metastasizing Rb. Citation Format: Yulia Shtanko, Sara Rodriguez, Benjamin Schachner, Emily Adis, Samantha McLaughlin, Sebastian Borges, James Dollar, Ashley Zuniga, Gabriela Quintana, Sofia Lopez, Dawn Owens, Chloe Moulin, Steffanie Braun, Gulum Yenisehirli, Daniel Pelaez, J. William Harbour, Stefan Kurtenbach. Characterizing pathways for metastasis of retinoblastoma in forebrain organoids [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 7569.

MAP4K4 and WT1 mediate SOX6-induced cellular senescence by synergistically activating the ATF2-TGFβ2-Smad2/3 signaling pathway in cervical cancer.

SRY-box transcription factor 6 (SOX6) is a member of the SOX gene family and inhibits the proliferation of cervical cancer cells by inducing cell cycle arrest. However, the final cell fate and significance of these cell-cycle-arrested cervical cancer cells induced by SOX6 remains unclear. Here, we report that SOX6 inhibits the proliferation of cervical cancer cells by inducing cellular senescence, which is mainly mediated by promoting transforming growth factor beta 2 (TGFB2) gene expression and subsequently activating the TGFβ2-Smad2/3-p53-p21WAF1/CIP1-Rb pathway. SOX6 promotes TGFB2 gene expression through the MAP4K4-MAPK (JNK/ERK/p38)-ATF2 and WT1-ATF2 pathways, which is dependent on its high-mobility group (HMG) domain. In addition, the SOX6-induced senescent cervical cancer cells are resistant to cisplatin treatment. ABT-263 (navitoclax) and ABT-199 (venetoclax), two classic senolytics, can specifically eliminate the SOX6-induced senescent cervical cancer cells, and thus significantly improve the chemosensitivity of cisplatin-resistant cervical cancer cells. This study uncovers that the MAP4K4/WT1-ATF2-TGFβ2 axis mediates SOX6-induced cellular senescence, which is a promising therapeutic target in improving the chemosensitivity of cervical cancer.

Analysis of E2F1 single-nucleotide polymorphisms reveals deleterious non-synonymous substitutions that disrupt E2F1-RB protein interaction in cancer

Cancer is a medical condition that is caused by the abnormal growth and division of cells, leading to the formation of tumors. The E2F1 and RB pathways are critical in regulating cell cycle, and their dysregulation can contribute to the development of cancer. In this study, we analyzed experimentally reported SNPs in E2F1 and assessed their effects on the binding affinity with RB. Out of 46, nine mutations were predicted as deleterious, and further analysis revealed four highly destabilizing mutations (L206W, R232C, I254T, A267T) that significantly altered the protein structure. Molecular docking of wild-type and mutant E2F1 with RB revealed a docking score of −242 kcal/mol for wild-type, while the mutant complexes had scores ranging from −217 to −220 kcal/mol. Molecular simulation analysis revealed variations in the dynamics features of both mutant and wild-type complexes due to the acquired mutations. Furthermore, the total binding free energy for the wild-type E2F1-RB complex was −64.89 kcal/mol, while those of the L206W, R232C, I254T, and A267T E2F1-RB mutants were −45.90 kcal/mol, −53.52 kcal/mol, −55.67 kcal/mol, and −61.22 kcal/mol, respectively. Our study is the first to extensively analyze E2F1 gene mutations and identifies candidate mutations for further validation and potential targeting for cancer therapeutics.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression.

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

A small-molecule inhibitor of TopBP1 exerts anti-MYC activity and synergy with PARP inhibitors.

We have previously identified TopBP1 (topoisomerase IIβ-binding protein 1) as a promising target for cancer therapy, given its role in the convergence of Rb, PI(3)K/Akt, and p53 pathways. Based on this, we conducted a large-scale molecular docking screening to identify a small-molecule inhibitor that specifically targets the BRCT7/8 domains of TopBP1, which we have named 5D4. Our studies show that 5D4 inhibits TopBP1 interactions with E2F1, mutant p53, and Cancerous Inhibitor of Protein Phosphatase 2A. This leads to the activation of E2F1-mediated apoptosis and the inhibition of mutant p53 gain of function. In addition, 5D4 disrupts the interaction of TopBP1 with MIZ1, which in turn allows MIZ1 to bind to its target gene promoters and repress MYC activity. Moreover, 5D4 inhibits the association of the TopBP1-PLK1 complex and prevents the formation of Rad51 foci. When combined with inhibitors of PARP1/2 or PARP14, 5D4 synergizes to effectively block cancer cell proliferation. Our animal studies have demonstrated the antitumor activity of 5D4 in breast and ovarian cancer xenograft models. Moreover, the effectiveness of 5D4 is further enhanced when combined with a PARP1/2 inhibitor talazoparib. Taken together, our findings strongly support the potential use of TopBP1-BRCT7/8 inhibitors as a targeted cancer therapy.

Metastatic Competency and Tumor Spheroid Formation Are Independent Cell States Governed by RB in Lung Adenocarcinoma.

Members of the RB pathway are frequently mutated in lung adenocarcinoma. We show that RB regulates cell state plasticity, tumor spheroid formation, and metastatic competency. Our data indicate that these are independent states where spheroid formation is distinct from metastatic competency. Thus, we caution against conflating spheroid formation and other signs of cell state plasticity with advanced metastatic cell states. Nevertheless, our work supports clinical strategies to reactivate RB pathways.

Randomized phase 2 study of gemcitabine with or without ATR inhibitor berzosertib in platinum-resistant ovarian cancer: Final overall survival (OS) and biomarker analyses.

5512 Background: The multicenter, open-label, randomized phase 2 NCI-9944 study (NCT02595892) demonstrated that addition of ATR inhibitor (ATRi) berzosertib to gemcitabine increased progression-free survival (PFS) compared to gemcitabine alone (HR = 0.57, one-sided log-rank p = 0.044, which met the one-sided significance level of 0.1 used for sample size calculation). Final overall survival (OS) and corresponding biomarker analyses are reported here. Methods: Patients (pts) with platinum-resistant high-grade serous ovarian cancer and unlimited previous lines of cytotoxic therapy in the platinum-sensitive setting but no more than one line of cytotoxic therapy in the platinum-resistant setting, were randomized 1:1 to gemcitabine/berzosertib versus gemcitabine alone. Randomization was stratified based on platinum free interval (PFI), PFI≤3 months versus > 3 months. Crossover from gemcitabine to gemcitabine/berzosertib was allowed upon disease progression by RECIST 1.1. OS was a secondary endpoint while preplanned exploratory correlative studies included assessment of DNA repair pathways and replication stress (RS) alterations by targeted gene sequencing and/or immunohistochemistry (IHC). Results: Seventy pts were randomly assigned to treatment with gemcitabine/berzosertib (34 pts) or gemcitabine alone (36 pts); 15 pts crossed over from gemcitabine to gemcitabine/berzosertib. At the final OS analysis (92.9% maturity), median follow-up was 53.2 weeks in the gemcitabine/berzosertib and 43 weeks in the gemcitabine alone groups. Median OS in the intent-to-treat (ITT) population was 59.4 weeks in the gemcitabine/berzosertib group versus 43.0 weeks in the gemcitabine alone group (HR 0.79, 90% CI 0.52-1.2, one-sided p = 0.18). However, when patients who crossed over to gemcitabine/berzosertib were excluded from analysis, a significant OS benefit was observed with gemcitabine/berzosertib (HR 0.60, 90%CI 0.37−0.97); HR was 0.26 (90% CI 0.1−0.7) in pts with PFI≤3 months and 0.89 (90%CI 0.50−1.59) in pts with PFI > 3 months. Furthermore, significant OS benefit was observed in pts with RS-low tumors (HR 0.39, 90%CI 0.17−0.91, defined as tumors harboring no genomic RS alterations related to loss of RB pathway regulation and/or oncogene-induced RS) but not in pts with RS-high tumors (HR 0.74, 90%CI 0.35−1.56). Additional targeted gene sequencing and IHC analyses as well as analyses adjusting for patient crossover will be reported at the meeting. Conclusions: In the ITT population, gemcitabine/berzosertib did not significantly improve OS versus gemcitabine alone. Pts with PFI≤3 months and pts with RS-low tumors may derive a survival advantage from addition of berzosertib to gemcitabine in the platinum-resistant setting. Clinical trial information: NCT02595892 .

Abstract 6521: Subtype-specific molecular signatures of field cancerization in patients with sporadic breast cancer

Abstract Background: We have previously shown the presence of molecular field cancerization in patients with sporadic breast cancer. Given the heterogeneity of breast cancer, we hypothesized that such field cancerization is likely to be subtype-specific. Methods: RNA exome sequencing was performed on 360 samples obtained from 75 breast cancer patients undergoing mastectomy; 25 cases were selected from each subtype: hormone receptor (HR) positive, Her2 positive, triple negative (TN) breast cancer. Four sites were sampled from each patient: primary tumor (A), adjacent normal parenchyma within 2cm of the tumor (B), 2 separate, histologically normal sites at least 2cm away from the tumor (C &D) or, if available, tissue from the unaffected contralateral breast (E). Normal breast tissue (N) from cancer-free controls was obtained from reduction mammoplasty. Tumor-associated genes (TAGs) were identified by comparison of primary tumor to tissue from cancer free controls. We estimated tumor content that was shared across samples B-E by applying deconvolutional analysis to the tumor associated genes, to calculate indices that ranged from 0, indicating normal, to 1, indicating tumor. Molecular field-associated genes and pathways were identified by Spearman’s correlation coefficient analysis between the differential genes/pathways and ISTC. Results: Across all subtypes, the proportion of tumor content present in the histologically normal breast tissue samples (samples B, C, D, E) obtained from cancer patients ranged from 80% in tissue adjacent to the tumor to approximately 50% in tissue obtained from the contralateral breast. We found over 600 deregulated genes to constitute the molecular field across all of the 3 major subtypes tested (Her2+, HR, and TNBC), of which approximately 20% were shared in the molecular field of all 3 subtypes of breast cancer. Among the 664 genes noted to be part of the molecular field in TNBC, 23.8% were unique to this subtype. In contrast, only 13% of the genes that constituted the field cancerization in the Her2 and HR+ subtypes were subtype-specific. PIP3-AKT, TCR signaling, and DNA synthesis were among the top pathways specifically activated in the molecular field of the Her2+ subtype of breast cancer. The molecular field of HR+ breast cancer was uniquely characterized by upregulation in the mitotic cell cycle, G1- S transition, and RB pathway. The field of cancerization in the TNBC subtype showed an upregulation in MEK-MAPK, mTOR, and JNK-JUN-TAK1 pathways. Conclusions: Our study suggests the presence of a breast cancer molecular field effect that extends beyond the adjacent normal breast tissue and includes the entire mammary gland. A substantial proportion of this field cancerization is subtype specific with uniquely deregulated pathways within each subtype. These findings provide new opportunities for developing subtype-specific chemoprevention strategies. Citation Format: Anjana Bhardwaj, Zhenlin Ju, Constance Albarracin, Celestine Trinidad, Preethi Gunaratne, Jing Wang, Randa El-Zein, Isabelle Bedrosian. Subtype-specific molecular signatures of field cancerization in patients with sporadic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6521.

Targeting the Molecular and Immunologic Features of Leiomyosarcoma.

Leiomyosarcoma (LMS) is a rare, aggressive mesenchymal tumor with smooth muscle differentiation. LMS is one of the most common histologic subtypes of soft tissue sarcoma; it most frequently occurs in the extremities, retroperitoneum, or uterus. LMS often demonstrates aggressive tumor biology, with a higher risk of developing distant metastatic disease than most sarcoma histologic types. The prognosis is poor, particularly in patients with uterine disease, and there is a need for the development of more effective therapies. Genetically, LMS is karyotypically complex and characterized by a low tumor mutational burden, with frequent alterations in TP53, RB1, PTEN, and DNA damage response pathways that may contribute to resistance against immune-checkpoint blockade monotherapy. The LMS immune microenvironment is highly infiltrated with tumor-associated macrophages and tumor-infiltrating lymphocytes, which may represent promising biomarkers. This review provides an overview of the clinical and pathologic behavior of both soft tissue and uterine LMS and summarizes the genomic and immune characteristics of these tumors and how they may provide opportunities for the development of biomarker-based immune therapies.

H3 G34-mutant high-grade gliomas: integrated clinical, imaging and pathological characterisation of a single-centre case series.

Diffuse hemispheric glioma, H3 G34-mutant, is a novel paediatric tumour type in the fifth edition of the WHO classification of CNS tumours associated with an invariably poor outcome. We present a comprehensive clinical, imaging and pathological review of this entity. Patients with confirmed H3 G34R-mutant high-grade glioma were included in a single-centre retrospective cohort study and examined for clinical, radiological and histo-molecular data. Twelve patients were enrolled in the study - 7 males/5 females; the mean age was 17.5years (10-57years). Most patients presented with signs of raised intracranial pressure (8/12). The frontal lobe (60%) was the prevalent location, with a mixed cystic-nodular appearance (10/12) and presence of vascular flow voids coursing through/being encased by the mass (8/12), and all tumours showed cortical invasion. Nine patients had subtotal resection limited by functional margins, two patients underwent supra-total resection, and one patient had biopsy only. 5-ALA was administered to 6 patients, all of whom showed positive fluorescence. Histologically, the tumours showed a marked heterogeneity and aggressive spread along pre-existing brain structures and leptomeninges. In addition to the diagnostic H3 G34R/V mutation, pathogenic variants in TP53 and ATRX genes were found in most cases. Potential targetable mutations in PDGFRA and PIK3CA genes were detected in five cases. The MGMT promoter was highly methylated in half of the samples. Methylation profiling was a useful diagnostic tool and highlighted recurrent structural chromosome abnormalities, such as PDGFRA amplification, CDKN2A/B deletion, PTEN loss and various copy number changes in the cyclin D-CDK4/Rb pathway. Radiochemotherapy was the most common adjuvant treatment (9/12), and the average survival was 19.3months. H3 G34R-mutant hemispheric glioma is a distinct entity with characteristic imaging and pathological features. Genomic landscaping of individual tumours can offer an opportunity to adapt individual therapies and improve patient management.

A non-genetic switch triggers alternative telomere lengthening and cellular immortalization in ATRX deficient cells

Alternative Lengthening of Telomeres (ALT) is an aberrant DNA recombination pathway which grants replicative immortality to approximately 10% of all cancers. Despite this high prevalence of ALT in cancer, the mechanism and genetics by which cells activate this pathway remain incompletely understood. A major challenge in dissecting the events that initiate ALT is the extremely low frequency of ALT induction in human cell systems. Guided by the genetic lesions that have been associated with ALT from cancer sequencing studies, we genetically engineered primary human pluripotent stem cells to deterministically induce ALT upon differentiation. Using this genetically defined system, we demonstrate that disruption of the p53 and Rb pathways in combination with ATRX loss-of-function is sufficient to induce all hallmarks of ALT and results in functional immortalization in a cell type-specific manner. We further demonstrate that ALT can be induced in the presence of telomerase, is neither dependent on telomere shortening nor crisis, but is rather driven by continuous telomere instability triggered by the induction of differentiation in ATRX-deficient stem cells.

Integrative analysis of multi-omics data reveals inhibition of RB1 signaling promotes apatinib resistance of hepatocellular carcinoma.

Although apatinib is a promising drug for the treatment of liver cancer, the underlying drug resistance mechanism is still unclear. Here, we constructed apatinib-resistant HepG2 cells. We then characterized the epigenomic, transcriptomic, and proteomic landscapes both in apatinib-resistant and non-resistant HepG2 cells. Differential expression, ATAC-seq, and proteomic data analyses were performed. We found that the cell cycle related protein RB1 may play an essential role in the process of apatinib resistant to hepatocarcinoma. Moreover, there were extensive variations at the transcriptome, epigenetic, and proteomic level. Finally, quantitative PCR (qPCR) and western blot analysis showed that expression level of RB1 in apatinib-resistant cell as well as the samples of patients in progressive disease were significantly lower than that in controls. Those results also showed that the RB1 pathway inhibitors CDK2-IN-73 and Palbociclib could relieve the resistance of apatinib resistant cells. Our results further enhance our understanding of the anti-tumorigenic and anti-angiogenic efficacy of apatinib in liver cancer and provide a novel perspective regarding apatinib resistance. Furthermore, we proved that CDKN2B inhibition of RB1 signaling promoted apatinib resistance in hepatocellular carcinoma. Those findings have greatly important biological significance for the resistance of apatinib and the treatment of liver cancer.

Disruption of Multiple Overlapping Functions Following Stepwise Inactivation of the Extended Myc Network.

Myc, a member of the "Myc Network" of bHLH-ZIP transcription factors, supervises proliferation, metabolism, and translation. It also engages in crosstalk with the related "Mlx Network" to co-regulate overlapping genes and functions. We investigated the consequences of stepwise conditional inactivation of Myc and Mlx in primary and SV40 T-antigen-immortalized murine embryonic fibroblasts (MEFs). Myc-knockout (MycKO) and Myc × Mlx "double KO" (DKO)-but not MlxKO-primary MEFs showed rapid growth arrest and displayed features of accelerated aging and senescence. However, DKO MEFs soon resumed proliferating, indicating that durable growth arrest requires an intact Mlx network. All three KO MEF groups deregulated multiple genes and functions pertaining to aging, senescence, and DNA damage recognition/repair. Immortalized KO MEFs proliferated in Myc's absence while demonstrating variable degrees of widespread genomic instability and sensitivity to genotoxic agents. Finally, compared to primary MycKO MEFs, DKO MEFs selectively downregulated numerous gene sets associated with the p53 and retinoblastoma (Rb) pathways and G2/M arrest. Thus, the reversal of primary MycKO MEF growth arrest by either Mlx loss or SV40 T-antigen immortalization appears to involve inactivation of the p53 and/or Rb pathways.

Loss of Brca1 and Trp53 in adult mouse mammary ductal epithelium results in development of hormone receptor-positive or hormone receptor-negative tumors, depending on inactivation of Rb family proteins

BackgroundBreast cancer is a heterogenous disease with several histological and molecular subtypes. Models that represent these subtypes are essential for translational research aimed at improving clinical strategy for targeted therapeutics.MethodsDifferent combinations of genetic aberrations (Brca1 and Trp53 loss, and inhibition of proteins of the Rb family) were induced in the mammary gland by injection of adenovirus expressing Cre recombinase into the mammary ducts of adult genetically engineered mice. Mammary tumors with different genetic aberrations were classified into molecular subtypes based on expression of molecular markers and RNAseq analysis. In vitro potency assays and Western blots were used to examine their drug sensitivities.ResultsInduction of Brca1 and Trp53 loss in mammary ductal epithelium resulted in development of basal-like hormone receptor (HR)-negative mammary tumors. Inhibition of Rb and Trp53 loss or the combination of Rb, Trp53 and Brca1 aberrations resulted in development of luminal ductal carcinoma positive for ER, PR, and Her2 expression. HR positivity in tumors with Rb, Trp53 and Brca1 aberrations indicated that functionality of the Rb pathway rather than Brca1 status affected HR status in these models. Mammary tumor gene expression profiles recapitulated human basal-like or luminal B breast cancer signatures, but HR-positive luminal cancer models were endocrine resistant and exhibited upregulation of PI3K signaling and sensitivity to this pathway inhibition. Furthermore, both tumor subtypes were resistant to CDK4/6 inhibition.ConclusionsExamination of molecular expression profiles and drug sensitivities of tumors indicate that these breast cancer models can be utilized as a translational platform for evaluation of targeted combinations to improve chemotherapeutic response in patients that no longer respond to hormone therapy or that are resistant to CDK4/6 inhibition.