Abstract NG01: Synthetic lethal interaction between the ESCRT paralog enzymes VPS4A and VPS4B in cancers harboring loss of chromosome 18q or 16q

Background: Discovery of new biomarker-linked cancer therapeutic targets may enable novel drug development and ultimately lead to advances in clinical care. Somatic copy number alterations (CNAs) leading to loss of tumor suppressor gene function constitute important driver events in tumorigenesis. Unfortunately, there are few existing therapeutic options to target the oncogenic processes evoked by tumor suppressor inactivation. However, developing drugs that target tractable synthetic lethal interactions with common somatic CNAs represents a promising approach to attain cancer-selective therapeutics. Synthetic lethality refers to the observation that for certain gene pairs, inactivation of either gene is tolerated but combined loss-of-function of both genes results in decreased cell viability. Synthetic lethal relationships in cancer have been defined in several different contexts, including among paralog genes for which dependency on one paralog is conferred by loss of a second functionally redundant paralog gene. Since targeting synthetic lethal relationships in cancer may yield a wide therapeutic window of efficacy between tumor and normal cells, identification of pharmacologically tractable synthetic lethal targets remains a priority for oncology drug development programs. Results and Discussion: To systematically define synthetic lethal vulnerabilities associated with genomic loss of established tumor suppressor genes, we analyzed genome-scale CRISPR-SpCas9 and RNA interference loss-of-function screening data from over 600 cancer cell lines. We identified and prioritized 193 synthetic lethal interactions with genomic loss of one or more of 51 common tumor suppressor genes. In particular, we discovered that the paralog genes encoding vacuolar protein sorting 4 homolog A and B (VPS4A and VPS4B) are selective genetic vulnerabilities for tumors harboring genomic copy loss of SMAD4 or CDH1 due to co-deletion of VPS4B or VPS4A, respectively. VPS4B is located on the long arm (q) of chromosome 18, 12.3 Mb away from SMAD4, while VPS4A is located 0.476 Mb downstream of CDH1 (encoding E-cadherin) on chromosome 16q. Thus, cancer cells with genomic loss of VPS4B selectively depend on expression of VPS4A for survival, and tumors with loss of VPS4A depend on VPS4B expression. Co-deletion of SMAD4 and VPS4B is commonly observed in approximately 33% of human cancer, with particularly high rates of loss in pancreatic cancers (68%), colorectal (71%) and renal cell carcinomas (17%) and to a lesser extent in cancers of the bile duct, lung, prostate, esophagus, uterus, cervix and ovary. Meanwhile, loss of CDH1 and VPS4A occurs frequently in cancers of the stomach, breast, skin, colon and prostate. VPS4A and B function as AAA ATPases which are critical for the regulation of endosomal sorting complex required for transport (ESCRT), a multimeric protein complex essential for inverse membrane remodeling. The ESCRT machinery is involved in a range of cellular processes, including cytokinesis, membrane repair, autophagy and endosomal processing. VPS4A/B are believed to form asymmetric hexameric complexes that are recruited to ESCRT-III filaments to drive ESCRT-mediated membrane fission and sealing. Here, we demonstrate that suppression of VPS4A in cancer cells with reduced copy number of VPS4B leads to accumulation of CHMP4B-containing ESCRT-III filaments, cytokinesis defects, nuclear membrane abnormalities and micronucleation, ultimately resulting in G2/M cell cycle arrest and apoptosis. We also observed that VPS4 suppression leads to defects in endosomal and endoplasmic reticulum structure. Furthermore, upon VPS4A suppression, we observed potent in vivo tumor regressions, which led to markedly prolonged survival in mouse xenograft models of pancreatic cancer and rhabdomyosarcoma harboring genomic loss of VPS4B. To understand regulators of VPS4A dependency, we performed a CRISPR-SpCas9 genome-scale screen in a pancreatic cancer cell line in the context of VPS4A suppression. We identified multiple genes that promote or suppress VPS4A dependency. Cancer cell sensitivity to VPS4A suppression was potently enhanced by disruption of regulators of the abscission checkpoint, including genes encoding the ULK3 kinase and the ESCRT-III proteins CHMP1A and CHMP1B. The abscission checkpoint is a genome protection mechanism that relies on Aurora B kinase (AURKB) and ESCRT-III subunits to delay abscission in response to chromosome mis-segregation to avoid DNA damage and aneuploidy. These findings suggest that inhibition of the ESCRT pathway and blockade of the abscission checkpoint could provide strategies to further enhance sensitivity of cancer cells to VPS4A suppression. Moreover, through CRISPR-SpCas9 screening and integrative transcriptomic and proteomic analysis, we also identified a strong correlation between baseline interferon response gene expression and VPS4A dependency. Indeed, when we treated VPS4B-deficient cells with interferon-β and interferon-γ to induce interferon signaling, we observed a pronounced sensitization of these cells to VPS4A depletion, thus suggesting that immune signals from the tumor microenvironment may influence VPS4 dependency. These data collectively suggest potential future therapeutic strategies for combination with VPS4A inhibition. Finally, we demonstrate through mutant rescue experiments that the ATPase domain is critical for the function of VPS4A in mediating survival of cells with partial copy loss of VPS4B. Furthermore, we provide data that elucidate the degree to which VPS4A and VPS4B cooperate and form functional complexes in human cancer cells. Although VPS4A and B demonstrate 80.5% homology, the development of small molecules that differentially target VPS4A in cells with VPS4B loss or VPS4B in cells with VPS4A loss remains a tractable possibility due to small structural differences near the ATP-binding pocket. Moreover, combined inhibition of VPS4A and VPS4B may also prove effective and clinically tolerable given a potential therapeutic window arising from gene dosage alterations and differences in total VPS4A/B levels in tumor versus normal cells.

Citation Format: Jasper E. Neggers, Brenton Paolella, Adhana Asfaw, Michael V. Rothberg, Tom A. Skipper, Radha Kalekar, Michael Burger, Neekesh Dharia, Guillaume Kugener, Jeremie Kalfon, Nancy Dumont, Yvonne Li, Liam Spurr, Annan Yang, Wenbo Wu, AndrewAdam Durbin, Brian M. Wolpin, David E. Root, Jesse Boehm, Andrew D. Cherniack, Aviad Tsherniak, Andrew L. Hong, William C. Hahn, Kimberly Stegmaier, Todd Golub, Francisca Vazquez, Andrew J. Aguirre. Synthetic lethal interaction between the ESCRT paralog enzymes VPS4A and VPS4B in cancers harboring loss of chromosome 18q or 16q [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr NG01.

Abstract PR03: Subtype-specific microenvironmental crosstalk and tumor cell plasticity in metastatic pancreatic cancer

The majority of patients with pancreatic ductal adenocarcinoma (PDAC) present at diagnosis with metastatic disease and have median survival times of less than 12 months. Recent studies have demonstrated that PDAC tumors with distinct transcriptional phenotypes are associated with different clinical outcomes. However, the mechanisms underlying this survival difference, the degree of cellular heterogeneity within a given tumor, and the subtype-specific contributions from the local immune microenvironment are not understood. In addition, there are ongoing efforts to understand if patient-derived organoid models can be used as functional surrogates for an individual patient’s disease. It remains unclear if patient transcriptional phenotypes are preserved in their matched organoid models. Here, we describe a pipeline that permits both direct characterization of the PDAC liver metastatic niche via single-cell RNA-sequencing and functional assessment of PDAC tumor biology in patient-matched organoid models. Starting from core needle biopsies of metastatic PDAC lesions containing 50-100k viable cells, we simultaneously perform: (1) single-cell RNA-sequencing using Seq-Well and (2) three-dimensional organoid culture generation. We have applied this approach to profile 23 patients and their matched early passage organoid models. Our pipeline yields high-quality single-cell measurements across diverse cell types—both malignant and non-malignant—enabling a principled dissection of tumor intrinsic and extrinsic factors. Evaluation of clinically relevant transcriptional signatures (e.g., Basal-like vs Classical) revealed extensive heterogeneity at the single-cell level. Single malignant cells are capable of co-expressing markers of both Basal-like and Classical states suggesting these phenotypes lie on a continuum rather than as discrete types. Basal cells express more stem-like features and inhabit a distinct microenvironment compared to their Classical counterparts. Microenvironmental composition differed on several levels between the two types, most notably their T/NK cell and macrophage populations with specific implications for subtype-specific microenvironmental directed therapy. Finally, we found that the microenvironment in traditional organoid culture selects against the Basal-like subtype and that these tumors are capable of significant phenotypic plasticity in vitro. We are able to recover Basal-like features by altering the organoid growth conditions. These findings suggest the need for distinct environments to support specific transcriptional subtypes in PDAC. Overall, our work provides a framework for the analysis of human cancers and their matched models using single-cell methods, and reveals novel, actionable insights into the heterogeneity and plasticity underlying survival in transcriptionally distinct forms of PDAC.

Citation Format: Peter S. Winter, Srivatsan Raghavan, Andrew Navia, Hannah Williams, Alan DenAdel, Radha Kalekar, Jennyfer Galvez-Reyes, Kristen Lowder, Nolawit Mulugeta, Manisha Raghavan, Ashir Borah, Raymond Ng, Junning Wang, Emma Reilly, Dorisanne Ragon, Lauren Brais, Kimmie Ng, James Cleary, Lorin Crawford, Scott Manalis, Jonathan Nowak, Brian Wolpin, William Hahn, Andrew Aguirre, Alex Shalek. Subtype-specific microenvironmental crosstalk and tumor cell plasticity in metastatic pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PR03.

Abstract PR02: Matched metastatic pancreatic ductal adenocarcinoma biopsies and organoid models reveal tumor cell transcriptional plasticity and subtype-specific microenvironmental crosstalk

The majority of patients with pancreatic ductal adenocarcinoma (PDAC) present with metastatic disease at diagnosis and have median survival times of less than 12 months. Recent studies have demonstrated that PDAC tumors with distinct transcriptional signatures are associated with different clinical outcomes, and that the tumor microenvironment may contribute to PDAC pathogenesis. In parallel, there are ongoing efforts to understand if patient-derived organoid models can be used as functional surrogates for an individual patient’s disease. However, it remains unclear if patient transcriptional phenotypes are preserved in their matched organoid models. Here, we describe a pipeline that permits both direct characterization of the PDAC liver metastatic niche via single-cell RNA-sequencing and functional assessment of PDAC tumor biology in patient-matched organoid models. Starting from core needle biopsies of metastatic PDAC lesions containing 50-100k viable cells, we simultaneously perform (1) low-input single-cell RNA-sequencing using Seq-Well and (2) three-dimensional organoid culture generation. We have applied this approach to profile 21 patients and their matched early passage organoid models. Our pipeline yields high-quality single-cell measurements across diverse cell types—both tumor and nontumor stromal—enabling a principled dissection of tumor intrinsic and extrinsic factors. Evaluation of clinically relevant transcriptional signatures (e.g., basal-like vs. classical) revealed extensive heterogeneity at the single-cell level and identified new, hybrid expression states. We also observed evidence of significant subtype-specific crosstalk between immune populations and tumor cells—specifically between T cells and tumor cells originating from basal-like tumors. Serial sampling at different stages of treatment revealed transcriptional shifts in tumor cells suggestive of significant plasticity. We similarly found that organoids derived from basal-like tumors exhibited considerable plasticity in vitro and had decreased fitness in standard organoid culture conditions, suggesting the need for distinct environments to support specific transcriptional subtypes. Overall, our approach provides actionable insights into the heterogeneity and plasticity of human PDAC, as well as a pipeline and framework for the analysis of PDAC and other cancers.

This abstract is also being presented as Poster A50.

Citation Format: Peter S. Winter, Srivatsan Raghavan, Andrew W. Navia, Hannah Williams, Jennyfer Galvez-Reyes, Radha Kalekar, Ashir Borah, Alan DenAdel, Manisha Raghavan, Kristen Lowder, Nolawit Mulugeta, Junning Wang, Emma Reilly, Lauren Brais, Lorin Crawford, James McFarland, James M. Cleary, Jonathan Nowak, Brian M. Wolpin, Andrew J. Aguirre, William C. Hahn, Alex K. Shalek. Matched metastatic pancreatic ductal adenocarcinoma biopsies and organoid models reveal tumor cell transcriptional plasticity and subtype-specific microenvironmental crosstalk [abstract]. In: Proceedings of the AACR Special Conference on the Evolving Landscape of Cancer Modeling; 2020 Mar 2-5; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2020;80(11 Suppl):Abstract nr PR02.

Abstract PR15: Assessment of tumor heterogeneity, clonal evolution, and the stromal microenvironment in metastatic pancreatic ductal adenocarcinoma and matched patient-derived organoids

Patients with metastatic pancreatic ductal adenocarcinoma (PDAC) have few treatment options and continue to have dismal prognoses due to the rapid development of resistance to both standard-of-care and experimental therapies. Several recent studies have demonstrated that patients with distinct PDAC transcriptional subtypes have differing clinical courses, and that the tumor microenvironment can also contribute to patient outcome. However, deep cellular characterization of metastatic PDAC tumors and their stromal microenvironments has been challenging due to limited tissue availability from metastatic liver biopsies. Here, we present a focused assessment of the PDAC liver metastatic niche—encompassing tumor, immune, and stromal cells—via low-input single-cell transcriptional profiling of patient specimens with the goal of developing a deeper understanding of tumor heterogeneity and the tumor microenvironment. Our pipeline accesses core needle biopsies from liver metastases, splitting each core for 1) single-cell RNA sequencing using Seq-Well and 2) organoid generation. Using this pipeline, we have successfully profiled liver metastases from 15 patients along with matched early-passage organoid models. Assessment of clinically relevant transcriptional signatures reveals extensive heterogeneity at the single-cell level and identifies new, hybrid transcriptional states occupied by these metastases. In addition, we observe evidence of significant crosstalk between stromal and immune populations and tumor cells. Serial samples at different stages of therapy show transcriptional shifts in tumor cells suggestive of significant plasticity that likely contributes to therapeutic resistance. Initial analysis of matched organoids at successive passages demonstrates a skew in their clonal composition, as well as evolution of their transcriptional state as compared to their in vivo phenotypes. Overall, our work provides an important window into the biology of metastatic PDAC, as well as some of the first direct comparisons of clonality and transcriptional phenotypes across in vivo specimens and their in vitro organoid counterparts.

This abstract is also being presented as Poster C43.

Citation Format: Srivatsan Raghavan, Peter S. Winter, Andrew Navia, Radha Kalekar, Jennyfer Galvez-Reyes, Sanjay Prakadan, Junning Wang, Emma Reilly, Lauren Brais, James M. Cleary, Jonathan Nowak, Brian M. Wolpin, Alex K. Shalek, Andrew J. Aguirre, William C. Hahn. Assessment of tumor heterogeneity, clonal evolution, and the stromal microenvironment in metastatic pancreatic ductal adenocarcinoma and matched patient-derived organoids [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr PR15.

Insights into the initiation of JC virus DNA replication derived from the crystal structure of the T-antigen origin binding domain

JC virus is a member of the Polyomavirus family of DNA tumor viruses and the causative agent of progressive multifocal leukoencephalopathy (PML). PML is a disease that occurs primarily in people who are immunocompromised and is usually fatal. As with other Polyomavirus family members, the replication of JC virus (JCV) DNA is dependent upon the virally encoded protein T-antigen. To further our understanding of JCV replication, we have determined the crystal structure of the origin-binding domain (OBD) of JCV T-antigen. This structure provides the first molecular understanding of JCV T-ag replication functions; for example, it suggests how the JCV T-ag OBD site-specifically binds to the major groove of GAGGC sequences in the origin. Furthermore, these studies suggest how the JCV OBDs interact during subsequent oligomerization events. We also report that the OBD contains a novel "pocket"; which sequesters the A1 & B2 loops of neighboring molecules. Mutagenesis of a residue in the pocket associated with the JCV T-ag OBD interfered with viral replication. Finally, we report that relative to the SV40 OBD, the surface of the JCV OBD contains one hemisphere that is highly conserved and one that is highly variable.