The FLRT3-UNC5B checkpoint pathway inhibits T cell-based cancer immunotherapies

Cancers exploit coinhibitory receptors on T cells to escape tumor immunity, and targeting such mechanisms has shown remarkable clinical benefit, but in a limited subset of patients. We hypothesized that cancer cells mimic noncanonical mechanisms of early development such as axon guidance pathways to evade T cell immunity. Using gain-of-function genetic screens, we profiled axon guidance proteins on human T cells and their cognate ligands and identified fibronectin leucine-rich transmembrane protein 3 (FLRT3) as a ligand that inhibits T cell activity. We demonstrated that FLRT3 inhibits T cells through UNC5B, an axon guidance receptor that is up-regulated on activated human T cells. FLRT3 expressed in human cancers favored tumor growth and inhibited CAR-T and BiTE + T cell killing and infiltration in humanized cancer models. An FLRT3 monoclonal antibody that blocked FLRT3-UNC5B interactions reversed these effects in an immune-dependent manner. This study supports the concept that axon guidance proteins mimic T cell checkpoints and can be targeted for cancer immunotherapy.

CASZ1 upregulates PI3K-AKT-mTOR signaling and promotes T-cell acute lymphoblastic leukemia

CASZ1 is a conserved transcription factor involved in neural development, blood vessel assembly and heart morphogenesis. CASZ1 has been implicated in cancer, either suppressing or promoting tumor development depending on the tissue. However, the impact of CASZ1 on hematological tumors remains unknown. Here, we show that the T-cell oncogenic transcription factor TAL1 is a direct positive regulator of CASZ1, that T-cell acute lymphoblastic leukemia (T-ALL) samples at diagnosis overexpress CASZ1b isoform, and that CASZ1b expression in patient samples correlates with PI3KAKT- mTOR signaling pathway activation. In agreement, overexpression of CASZ1b in both Ba/F3 and T-ALL cells leads to the activation of PI3K signaling pathway, which is required for CASZ1b-mediated transformation of Ba/F3 cells in vitro and malignant expansion in vivo. We further demonstrate that CASZ1b cooperates with activated NOTCH1 to promote T-ALL development in zebrafish, and that CASZ1b protects human T-ALL cells from serum deprivation and treatment with chemotherapeutic drugs. Taken together, our studies indicate that CASZ1b is a TAL1-regulated gene that promotes T-ALL development and resistance to chemotherapy.

Zebrafish as a high throughput model for organ preservation and transplantation research

Despite decades of effort, the preservation of complex organs for transplantation remains a significant barrier that exacerbates the organ shortage crisis. Progress in organ preservation research is significantly hindered by suboptimal research tools that force investigators to sacrifice translatability over throughput. For instance, simple model systems, such as single cell monolayers or co-cultures, lack native tissue structure and functional assessment, while mammalian whole organs are complex systems with confounding variables not compatible with high-throughput experimentation. In response, diverse fields and industries have bridged this experimental gap through the development of rich and robust resources for the use of zebrafish as a model organism. Through this study, we aim to demonstrate the value zebrafish pose for the fields of solid organ preservation and transplantation, especially with respect to experimental transplantation efforts. A wide array of methods were customized and validated for preservation-specific experimentation utilizing zebrafish, including the development of assays at multiple developmental stages (larvae and adult), methods for loading and unloading preservation agents, and the development of viability scores to quantify functional outcomes. Using this platform, the largest and most comprehensive screen of cryoprotectant agents (CPAs) was performed to determine their toxicity and efficiency at preserving complex organ systems using a high subzero approach called partial freezing (i.e., storage in the frozen state at -10°C). As a result, adult zebrafish cardiac function was successfully preserved after 5 days of partial freezing storage. In combination, the methods and techniques developed have the potential to drive and accelerate research in the fields of solid organ preservation and transplantation.

Therapeutic targeting of LCK tyrosine kinase and mTOR signaling in T-cell acute lymphoblastic leukemia

Relapse and refractory T-cell acute lymphoblastic leukemia (T-ALL) has a poor prognosis, and new combination therapies are sorely needed. Here, we used an ex vivo high-throughput screening platform to identify drug combinations that kill zebrafish T-ALL and then validated top drug combinations for preclinical efficacy in human disease. This work uncovered potent drug synergies between AKT/mTORC1 (mammalian target of rapamycin complex 1) inhibitors and the general tyrosine kinase inhibitor dasatinib. Importantly, these same drug combinations effectively killed a subset of relapse and dexamethasone-resistant zebrafish T-ALL. Clinical trials are currently underway using the combination of mTORC1 inhibitor temsirolimus and dasatinib in other pediatric cancer indications, leading us to prioritize this therapy for preclinical testing. This combination effectively curbed T-ALL growth in human cell lines and primary human T-ALL and was well tolerated and effective in suppressing leukemia growth in patient-derived xenografts (PDX) grown in mice. Mechanistically, dasatinib inhibited phosphorylation and activation of the lymphocyte-specific protein tyrosine kinase (LCK) to blunt the T-cell receptor (TCR) signaling pathway, and when complexed with mTORC1 inhibition, induced potent T-ALL cell killing through reducing MCL-1 protein expression. In total, our work uncovered unexpected roles for the LCK kinase and its regulation of downstream TCR signaling in suppressing apoptosis and driving continued leukemia growth. Analysis of a wide array of primary human T-ALLs and PDXs grown in mice suggest that combination of temsirolimus and dasatinib treatment will be efficacious for a large fraction of human T-ALLs.

Abstract IA014: Stem cell and developmental hierarchies in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma of childhood and is comprised of at least two major molecular subtypes. Despite sharing features with skeletal muscle, the conservation of underlying cellular hierarchy with human muscle development and the identification of molecularly-defined tumor-propagating cells have not been reported. Using single-cell RNA sequencing of patient-derived RMS, DNA-barcode cell fate mapping, antibody enrichment and functional stem cell assays, and mouse xenograft modeling, we have uncovered tumor cell hierarchies in Fusion-negative (FN-) RMS that are shared with normal human muscle development. We also identified common developmental stages at which tumor cells become arrested. FN-RMS resemble early muscle found in embryonic and larval development, while fusion-positive (FP-) RMS express a highly specific developmental gene program found in muscle cells transiting from embryonic to fetal development at 7-7.75 weeks of age. FP-RMS also have neural-pathway enriched cell states, suggesting less-rigid adherence to muscle development hierarchies in this disease. Finally, we identify a new molecularly-defined tumor-propagating cell in FN-RMS that shares remarkable similarity to the newly described bi-potent, muscle mesenchyme stem/progenitor cell that makes both muscle and osteogenic cells.

Citation Format: David M. Langenau, Yun Wei, Qian Qin, Luca Pinello. Stem cell and developmental hierarchies in rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr IA014.

Single-cell analysis and functional characterization uncover the stem cell hierarchies and developmental origins of rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a common childhood cancer that shares features with developing skeletal muscle. Yet, the conservation of cellular hierarchy with human muscle development and the identification of molecularly defined tumor-propagating cells has not been reported. Using single-cell RNA-sequencing, DNA-barcode cell fate mapping and functional stem cell assays, we uncovered shared tumor cell hierarchies in RMS and human muscle development. We also identified common developmental stages at which tumor cells become arrested. Fusion-negative RMS cells resemble early myogenic cells found in embryonic and fetal development, while fusion-positive RMS cells express a highly specific gene program found in muscle cells transiting from embryonic to fetal development at 7-7.75 weeks of age. Fusion-positive RMS cells also have neural pathway-enriched states, suggesting less-rigid adherence to muscle-lineage hierarchies. Finally, we identified a molecularly defined tumor-propagating subpopulation in fusion-negative RMS that shares remarkable similarity to bi-potent, muscle mesenchyme progenitors that can make both muscle and osteogenic cells.

Mutant IL7R collaborates with MYC to induce T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive pediatric cancer. Amongst the wide array of driver mutations, 10% of T-ALL patients display gain-of-function mutations in the IL-7 receptor α chain (IL-7Rα, encoded by IL7R), which occur in different molecular subtypes of this disease. However, it is still unclear whether IL-7R mutational activation is sufficient to transform T-cell precursors. Also, which genes cooperate with IL7R to drive leukemogenesis remain poorly defined. Here, we demonstrate that mutant IL7R alone is capable of inducing T-ALL with long-latency in stable transgenic zebrafish and transformation is associated with MYC transcriptional activation. Additionally, we find that mutant IL7R collaborates with Myc to induce early onset T-ALL in transgenic zebrafish, supporting a model where these pathways collaborate to drive leukemogenesis. T-ALLs co-expressing mutant IL7R and Myc activate STAT5 and AKT pathways, harbor reduced numbers of apoptotic cells and remake tumors in transplanted zebrafish faster than T-ALLs expressing Myc alone. Moreover, limiting-dilution cell transplantation experiments reveal that activated IL-7R signaling increases the overall frequency of leukemia propagating cells. Our work highlights a synergy between mutant IL7R and Myc in inducing T-ALL and demonstrates that mutant IL7R enriches for leukemia propagating potential.

Inflammatory response in hematopoietic stem and progenitor cells triggered by activating SHP2 mutations evokes blood defects

Gain-of-function mutations in the protein-tyrosine phosphatase SHP2 are the most frequently occurring mutations in sporadic juvenile myelomonocytic leukemia (JMML) and JMML-like myeloproliferative neoplasm (MPN) associated with Noonan syndrome (NS). Hematopoietic stem and progenitor cells (HSPCs) are the disease propagating cells of JMML. Here, we explored transcriptomes of HSPCs with SHP2 mutations derived from JMML patients and a novel NS zebrafish model. In addition to major NS traits, CRISPR/Cas9 knock-in Shp2D61G mutant zebrafish recapitulated a JMML-like MPN phenotype, including myeloid lineage hyperproliferation, ex vivo growth of myeloid colonies, and in vivo transplantability of HSPCs. Single-cell mRNA sequencing of HSPCs from Shp2D61G zebrafish embryos and bulk sequencing of HSPCs from JMML patients revealed an overlapping inflammatory gene expression pattern. Strikingly, an anti-inflammatory agent rescued JMML-like MPN in Shp2D61G zebrafish embryos. Our results indicate that a common inflammatory response was triggered in the HSPCs from sporadic JMML patients and syndromic NS zebrafish, which potentiated MPN and may represent a future target for JMML therapies.

Antibody-Peptide Epitope Conjugates for Personalized Cancer Therapy

Antibody-peptide epitope conjugates (APEC) are a new class of modified antibody-drug conjugates that redirect T-cell viral immunity against tumor cells. APECs contain a tumor-specific protease cleavage site linked to a patient-specific viral epitope, resulting in presentation of viral epitopes on cancer cells and subsequent recruitment and killing by CD8+ T cells. Here we developed an experimental pipeline to create patient-specific APECs and identified new preclinical therapies for ovarian carcinoma. Using functional assessment of viral peptide antigen responses to common viruses like cytomegalovirus (CMV) in patients with ovarian cancer, a library of 192 APECs with distinct protease cleavage sequences was created using the anti-epithelial cell adhesion molecule (EpCAM) antibody. Each APEC was tested for in vitro cancer cell killing, and top candidates were screened for killing xenograft tumors grown in zebrafish and mice. These preclinical modeling studies identified EpCAM-MMP7-CMV APEC (EpCAM-MC) as a potential new immunotherapy for ovarian carcinoma. Importantly, EpCAM-MC also demonstrated robust T-cell responses in primary ovarian carcinoma patient ascites samples. This work highlights a robust, customizable platform to rapidly develop patient-specific APECs.

SIGNIFICANCE

This study develops a high-throughput preclinical platform to identify patient-specific antibody-peptide epitope conjugates that target cancer cells and demonstrates the potential of this immunotherapy approach for treating ovarian carcinoma.

Single-cell imaging of T cell immunotherapy responses in vivo

T cell immunotherapies have revolutionized treatment for a subset of cancers. Yet, a major hurdle has been the lack of facile and predicative preclinical animal models that permit dynamic visualization of T cell immune responses at single-cell resolution in vivo. Here, optically clear immunocompromised zebrafish were engrafted with fluorescent-labeled human cancers along with chimeric antigen receptor T (CAR T) cells, bispecific T cell engagers (BiTEs), and antibody peptide epitope conjugates (APECs), allowing real-time single-cell visualization of T cell-based immunotherapies in vivo. This work uncovered important differences in the kinetics of T cell infiltration, tumor cell engagement, and killing between these immunotherapies and established early endpoint analysis to predict therapy responses. We also established EGFR-targeted immunotherapies as a powerful approach to kill rhabdomyosarcoma muscle cancers, providing strong preclinical rationale for assessing a wider array of T cell immunotherapies in this disease.

Zebrafish disease models in drug discovery: from preclinical modelling to clinical trials

Numerous drug treatments that have recently entered the clinic or clinical trials have their genesis in zebrafish. Zebrafish are well established for their contribution to developmental biology and have now emerged as a powerful preclinical model for human disease, as their disease characteristics, aetiology and progression, and molecular mechanisms are clinically relevant and highly conserved. Zebrafish respond to small molecules and drug treatments at physiologically relevant dose ranges and, when combined with cell-specific or tissue-specific reporters and gene editing technologies, drug activity can be studied at single-cell resolution within the complexity of a whole animal, across tissues and over an extended timescale. These features enable high-throughput and high-content phenotypic drug screening, repurposing of available drugs for personalized and compassionate use, and even the development of new drug classes. Often, drugs and drug leads explored in zebrafish have an inter-organ mechanism of action and would otherwise not be identified through targeted screening approaches. Here, we discuss how zebrafish is an important model for drug discovery, the process of how these discoveries emerge and future opportunities for maximizing zebrafish potential in medical discoveries.

PHF6 Expression Levels Impact Human Hematopoietic Stem Cell Differentiation

Transcriptional control of hematopoiesis involves complex regulatory networks and functional perturbations in one of these components often results in malignancies. Loss-of-function mutations in PHF6, encoding a presumed epigenetic regulator, have been primarily described in T cell acute lymphoblastic leukemia (T-ALL) and the first insights into its function in normal hematopoiesis only recently emerged from mouse modeling experiments. Here, we investigated the role of PHF6 in human blood cell development by performing knockdown studies in cord blood and thymus-derived hematopoietic precursors to evaluate the impact on lineage differentiation in well-established in vitro models. Our findings reveal that PHF6 levels differentially impact the differentiation of human hematopoietic progenitor cells into various blood cell lineages, with prominent effects on lymphoid and erythroid differentiation. We show that loss of PHF6 results in accelerated human T cell development through reduced expression of NOTCH1 and its downstream target genes. This functional interaction in developing thymocytes was confirmed in vivo using a phf6-deficient zebrafish model that also displayed accelerated developmental kinetics upon reduced phf6 or notch1 activation. In summary, our work reveals that appropriate control of PHF6 expression is important for normal human hematopoiesis and provides clues towards the role of PHF6 in T-ALL development.

Single-cell imaging of human cancer xenografts using adult immunodeficient zebrafish

Zebrafish are an ideal cell transplantation model. They are highly fecund, optically clear and an excellent platform for preclinical drug discovery studies. Traditionally, xenotransplantation has been carried out using larval zebrafish that have not yet developed adaptive immunity. Larval engraftment is a powerful short-term transplant platform amenable to high-throughput drug screening studies, yet animals eventually reject tumors and cannot be raised at 37 °C. To address these limitations, we have recently developed adult casper-strain prkdc^-/-, il2rgα^-/- immunocompromised zebrafish that robustly engraft human cancer cells for in excess of 28 d. Because the adult zebrafish can be administered drugs by oral gavage or i.p. injection, our model is suitable for achieving accurate, preclinical drug dosing. Our platform also allows facile visualization of drug effects in vivo at single-cell resolution over days. Here, we describe the procedures for xenograft cell transplantation into the prkdc^-/-, il2rgα^-/- model, including refined husbandry protocols for optimal growth and rearing of immunosuppressed zebrafish at 37 °C; optimized intraperitoneal and periocular muscle cell transplantation; and epifluorescence and confocal imaging approaches to visualize the effects of administering clinically relevant drug dosing at single-cell resolution in vivo. After identification of adult homozygous animals, this procedure takes 35 d to complete. 7 days are required to acclimate adult fish to 37 °C, and 28 d are required for engraftment studies. Our protocol provides a comprehensive guide for using immunocompromised zebrafish for xenograft cell transplantation and credentials the model as a new preclinical drug discovery platform.

Abstract 403: Leveraging the Zebrafish to Overcome Barriers in Heart Transplantation

Cardiac transplantation is the only curative therapy for patients with end-stage heart disease; however, there is a severe shortage of viable donor organs. Heart transplantation faces many interwoven challenges, including both biological factors and research limitations. For example, ischemia-reperfusion injury plays a role in early graft dysfunction and is associated with rejection episodes in heart transplantation. Moreover, experimental transplantation relies heavily on animal studies that are laborious and expensive, prohibiting the discovery of novel, bold solutions. We propose that the zebrafish, Danio rerio , would be a valuable tool for the field since it’s amenable to high-throughput screens, captures the complex structure of organs, and offers a suite of tools to monitor the biology of cardiac injury. Here, we develop a new subzero heart preservation method by strategically leveraging animal models from zebrafish to mammalian hearts. Using zebrafish larvae, we screened for agents which preserve hearts at -10°C. As a result of these screens, we identified promising preservative cocktails which restored heartbeat in 82% of larvae immediately post-recovery. Next, we excised adult zebrafish hearts and developed methods to mimic the ex vivo handling practices of hearts destined for transplant using a heart-on-a-plate assay. Using this assay, we carried forward promising agents identified in our initial zebrafish larvae screen to isolated adult zebrafish hearts that were cooled to -10°C and held for up to 24 hours. After rewarming, heart rate was restored and metabolic rate of zebrafish hearts was like time-matched controls (0.213 ± 0.047 and 0.275 ± 0.060, respectively, p = 0.200). Finally, we report our preliminary scale-up efforts whereby rodent hearts are stored for up to 24 hours at -10°C and viability were assessed by the TUNEL assay. The data shows high viability of cardiomyocytes post-preservation, as compared to controls. In summary, we present data to illustrate our efforts in leveraging the zebrafish to aid new discoveries in subzero heart preservation. Similar efforts to model heart transplantation in zebrafish may provide a different vantage point and enable us to make advances faster.

Abstract B10: Rhabdomyosarcoma requires MYC family genomic events to pathogenically subvert core-regulatory circuitry

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma of childhood. Despite multimodality therapy and trials of molecularly targeted agents, limited improvements in overall survival have been realized for patients with high-risk disease. Thus, we aimed to determine the landscape of tumor-specific gene dependencies that underlie tumorigenesis in RMS and therefore provide a valuable group of targets for the development of novel therapeutics. Using unbiased genome-scale CRISPR-Cas9 approaches, we identified a set of RMS-specific tumor dependencies involved in tumor cell growth and survival. RMS dependencies were enriched for nucleic acid binding proteins, including transcription factors (TFs). We then used genome-wide chromatin-immunoprecipitation coupled to high-throughput sequencing analysis to demonstrate that a small number of essential TFs—MYCN, MYOD1, TCF12, SOX8, ZEB2, ZNF217, and SIX1—are members of the transcriptional core regulatory circuitry (CRC) that maintains the malignant cell state of RMS. Both c-MYC and MYCN were associated with gene and enhancer copy number increases in cell lines and primary tumors and represented strong dependencies in the RMS cell lines screened. c-MYC and MYCN function to similarly invade and regulate the CRC in respectively dependent cells. To disable the CRC, we tested A485, an inhibitor of the histone acetyltransferase enzymes involved in the establishment of super-enhancer elements that are associated with high level expression of the CRC factors. A485 caused a reversible and rapid loss of CRC factor and c-MYC and/or MYCN expression, and prolonged treatment resulted in cell cycle arrest, differentiation, and apoptosis in vitro and in vivo. This phenotype is rescued by exogenous re-expression of either c-MYC or MYCN in a manner insensitive to A485, indicating a mechanism by which these genes subvert a myogenic CRC to produce an oncogenic fate. This study defines a common set of critical dependency genes in RMS and identifies key genomic events surrounding the c-MYC and MYCN loci that lead to elevated expression and tumorigenesis.

Citation Format: Adam D. Durbin, Guillaume Kugener, Mark W. Zimmerman, Chuan Yan, Neekesh V. Dharia, Elizabeth S. Frank, Xiang Chen, Ken N. Ross, Brenton Paolella, Michael Krill-Burger, David E. Root, Jesse S. Boehm, Francisca Vazquez, Andrew L. Hong, Aviad Tsherniak, David M. Langenau, William C. Hahn, Todd R. Golub, Brian J. Abraham, Richard A. Young, A. Thomas Look, Kimberly Stegmaier. Rhabdomyosarcoma requires MYC family genomic events to pathogenically subvert core-regulatory circuitry [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B10.

PRL3 enhances T-cell acute lymphoblastic leukemia growth through suppressing T-cell signaling pathways and apoptosis

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy of thymocytes and is largely driven by the NOTCH/MYC pathway. Yet, additional oncogenic drivers are required for transformation. Here, we identify protein tyrosine phosphatase type 4 A3 (PRL3) as a collaborating oncogenic driver in T-ALL. PRL3 is expressed in a large fraction of primary human T-ALLs and is commonly co-amplified with MYC. PRL3 also synergized with MYC to initiate early-onset ALL in transgenic zebrafish and was required for human T-ALL growth and maintenance. Mass spectrometry phosphoproteomic analysis and mechanistic studies uncovered that PRL3 suppresses downstream T cell phosphorylation signaling pathways, including those modulated by VAV1, and subsequently suppresses apoptosis in leukemia cells. Taken together, our studies have identified new roles for PRL3 as a collaborating oncogenic driver in human T-ALL and suggest that therapeutic targeting of the PRL3 phosphatase will likely be a useful treatment strategy for T-ALL.

Abstract PR12: Dynamic single-cell imaging of human cancer growth and therapy responses following engraftment into immunodeficient zebrafish

Cancer xenograft engraftment studies using immune-deficient mice are indispensable for preclinical drug discovery and are required for IND filings that lead to clinical trials. While immune-deficient mice robustly engraft a wide variety of human cancers, high-resolution intravital imaging of transplanted cells is tedious and its high husbandry costs often limit the scale of experiments. In contrast, zebrafish are an ideal cell transplantation model. They are highly fecund; optically clear, permitting dynamic single-cell imaging of fluorescent cancer cells; and are an excellent platform for high-throughput, large-scale studies. We have recently generated two optically clear prkdc-/-, il2rga-/- and rag2-/-, il2rga-/- immune-compromised zebrafish models that lack T-, B-, and NK-cells. These immune-deficient animals can be grown at 37°C and robustly engraft a variety of human cancers including patient-derived xenografts for >30 days. Importantly, tumors grown in immune-deficient zebrafish and mice are indistinguishable in terms of morphology, proliferation rates, apoptosis, and response to therapy. Engraftment of human cells into the superficial periocular muscle also allowed high-resolution, single-cell imaging using conventional confocal microscopy. Using this approach, we performed photoconversion cell lineage tracing of human rhabdomyosarcoma (RMS), a pediatric cancer of the muscle, and identified mutually exclusive migratory and proliferative cell states. We also demonstrated the preclinical efficacy of combination therapy involving olaparib (PARP-inhibitor) and temozolomide (DNA-damaging agent), including the dynamic visualization of therapeutic responses at single-cell resolution through use of the four-color FUCCI cell cycle fluorescent reporter and serial confocal imaging over days. Importantly, this same drug combination also exhibited remarkable efficacy in studies performed in NSG mice and is now moving forward for clinical evaluation in RMS patients. Finally, recent work has focused on assessing the efficacy of immunotherapies in curbing tumor growth, including assessing CAR-T cell and bispecific T-cell engager antibodies (BITES) in vivo. In both of these platforms, dynamic live cell imaging and automated 3D modeling allowed quantification of migratory potential of T cells into the tumor, dynamic remodeling of T-cell morphology changes following interaction with tumor cells, and real-time visualization of T cell-mediated cancer cell killing using fluorescent caspase reporters. In total, our studies have credentialed the immune-deficient zebrafish as a new platform for preclinical drug studies and provides novel technologies that utilize real-time, single-cell imaging for early endpoint analysis. These models are likely to be particularly useful for discovery of immunomodulatory antibodies and immunomedicines in the future.

Citation Format: Chuan Yan, Qiqi Yang, Daniel Do, Dalton Brunson, John Iafrate, John Rawls, David M. Langenau. Dynamic single-cell imaging of human cancer growth and therapy responses following engraftment into immunodeficient zebrafish [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 PR12.

Zebrafish patient avatars in cancer biology and precision cancer therapy

In precision oncology, two major strategies are being pursued for predicting clinically relevant tumour behaviours, such as treatment response and emergence of drug resistance: inference based on genomic, transcriptomic, epigenomic and/or proteomic analysis of patient samples, and phenotypic assays in personalized cancer avatars. The latter approach has historically relied on in vivo mouse xenografts and in vitro organoids or 2D cell cultures. Recent progress in rapid combinatorial genetic modelling, the development of a genetically immunocompromised strain for xenotransplantation of human patient samples in adult zebrafish and the first clinical trial using xenotransplantation in zebrafish larvae for phenotypic testing of drug response bring this tiny vertebrate to the forefront of the precision medicine arena. In this Review, we discuss advances in transgenic and transplantation-based zebrafish cancer avatars, and how these models compare with and complement mouse xenografts and human organoids. We also outline the unique opportunities that these different models present for prediction studies and current challenges they face for future clinical deployment.

The macrophage-expressed gene (mpeg) 1 identifies a subpopulation of B cells in the adult zebrafish

The mononuclear phagocytic system consists of many cells, in particular macrophages, scattered throughout the body. However, there is increasing evidence for the heterogeneity of tissue-resident macrophages, leading to a pressing need for new tools to discriminate mononuclear phagocytic system subsets from other hematopoietic lineages. Macrophage-expressed gene (Mpeg)1.1 is an evolutionary conserved gene encoding perforin-2, a pore-forming protein associated with host defense against pathogens. Zebrafish mpeg1.1:GFP and mpeg1.1:mCherry reporters were originally established to specifically label macrophages. Since then more than 100 peer-reviewed publications have made use of mpeg1.1-driven transgenics for in vivo studies, providing new insights into key aspects of macrophage ontogeny, activation, and function. Whereas the macrophage-specific expression pattern of the mpeg1.1 promoter has been firmly established in the zebrafish embryo, it is currently not known whether this specificity is maintained through adulthood. Here we report direct evidence that beside macrophages, a subpopulation of B-lymphocytes is marked by mpeg1.1 reporters in most adult zebrafish organs. These mpeg1.1⁺ lymphoid cells endogenously express mpeg1.1 and can be separated from mpeg1.1⁺ macrophages by virtue of their light-scatter characteristics using FACS. Remarkably, our analyses also revealed that B-lymphocytes, rather than mononuclear phagocytes, constitute the main mpeg1.1-positive population in irf8^null myeloid-defective mutants, which were previously reported to recover tissue-resident macrophages in adulthood. One notable exception is skin macrophages, whose development and maintenance appear to be independent from irf8, similar to mammals. Collectively, our findings demonstrate that irf8 functions in myelopoiesis are evolutionary conserved and highlight the need for alternative macrophage-specific markers to study the mononuclear phagocytic system in adult zebrafish.

Insights into pediatric rhabdomyosarcoma research: Challenges and goals

Overall survival rates for pediatric patients with high-risk or relapsed rhabdomyosarcoma (RMS) have not improved significantly since the 1980s. Recent studies have identified a number of targetable vulnerabilities in RMS, but these discoveries have infrequently translated into clinical trials. We propose streamlining the process by which agents are selected for clinical evaluation in RMS. We believe that strong consideration should be given to the development of combination therapies that add biologically targeted agents to conventional cytotoxic drugs. One example of this type of combination is the addition of the WEE1 inhibitor AZD1775 to the conventional cytotoxic chemotherapeutics, vincristine and irinotecan.

Visualizing Engrafted Human Cancer and Therapy Responses in Immunodeficient Zebrafish

Xenograft cell transplantation into immunodeficient mice has become the gold standard for assessing pre-clinical efficacy of cancer drugs, yet direct visualization of single-cell phenotypes is difficult. Here, we report an optically-clear prkdc-/-, il2rga-/- zebrafish that lacks adaptive and natural killer immune cells, can engraft a wide array of human cancers at 37°C, and permits the dynamic visualization of single engrafted cells. For example, photoconversion cell-lineage tracing identified migratory and proliferative cell states in human rhabdomyosarcoma, a pediatric cancer of muscle. Additional experiments identified the preclinical efficacy of combination olaparib PARP inhibitor and temozolomide DNA-damaging agent as an effective therapy for rhabdomyosarcoma and visualized therapeutic responses using a four-color FUCCI cell-cycle fluorescent reporter. These experiments identified that combination treatment arrested rhabdomyosarcoma cells in the G2 cell cycle prior to induction of apoptosis. Finally, patient-derived xenografts could be engrafted into our model, opening new avenues for developing personalized therapeutic approaches in the future.

Vangl2/RhoA Signaling Pathway Regulates Stem Cell Self-Renewal Programs and Growth in Rhabdomyosarcoma

Tumor growth and relapse are driven by tumor propagating cells (TPCs). However, mechanisms regulating TPC fate choices, maintenance, and self-renewal are not fully understood. Here, we show that Van Gogh-like 2 (Vangl2), a core regulator of the non-canonical Wnt/planar cell polarity (Wnt/PCP) pathway, affects TPC self-renewal in rhabdomyosarcoma (RMS)-a pediatric cancer of muscle. VANGL2 is expressed in a majority of human RMS and within early mononuclear progenitor cells. VANGL2 depletion inhibited cell proliferation, reduced TPC numbers, and induced differentiation of human RMS in vitro and in mouse xenografts. Using a zebrafish model of embryonal rhabdomyosarcoma (ERMS), we determined that Vangl2 expression enriches for TPCs and promotes their self-renewal. Expression of constitutively active and dominant-negative isoforms of RHOA revealed that it acts downstream of VANGL2 to regulate proliferation and maintenance of TPCs in human RMS. Our studies offer insights into pathways that control TPCs and identify new potential therapeutic targets.

Single-cell trajectories reconstruction, exploration and mapping of omics data with STREAM

Single-cell transcriptomic assays have enabled the de novo reconstruction of lineage differentiation trajectories, along with the characterization of cellular heterogeneity and state transitions. Several methods have been developed for reconstructing developmental trajectories from single-cell transcriptomic data, but efforts on analyzing single-cell epigenomic data and on trajectory visualization remain limited. Here we present STREAM, an interactive pipeline capable of disentangling and visualizing complex branching trajectories from both single-cell transcriptomic and epigenomic data. We have tested STREAM on several synthetic and real datasets generated with different single-cell technologies. We further demonstrate its utility for understanding myoblast differentiation and disentangling known heterogeneity in hematopoiesis for different organisms. STREAM is an open-source software package.

tp53 deficiency causes a wide tumor spectrum and increases embryonal rhabdomyosarcoma metastasis in zebrafish

The TP53 tumor-suppressor gene is mutated in >50% of human tumors and Li-Fraumeni patients with germ line inactivation are predisposed to developing cancer. Here, we generated tp53 deleted zebrafish that spontaneously develop malignant peripheral nerve-sheath tumors, angiosarcomas, germ cell tumors, and an aggressive Natural Killer cell-like leukemia for which no animal model has been developed. Because the tp53 deletion was generated in syngeneic zebrafish, engraftment of fluorescent-labeled tumors could be dynamically visualized over time. Importantly, engrafted tumors shared gene expression signatures with predicted cells of origin in human tissue. Finally, we showed that tp53^del/del enhanced invasion and metastasis in kRAS^G12D-induced embryonal rhabdomyosarcoma (ERMS), but did not alter the overall frequency of cancer stem cells, suggesting novel pro-metastatic roles for TP53 loss-of-function in human muscle tumors. In summary, we have developed a Li-Fraumeni zebrafish model that is amenable to large-scale transplantation and direct visualization of tumor growth in live animals.

JDP2: An oncogenic bZIP transcription factor in T cell acute lymphoblastic leukemia

A substantial subset of patients with T cell acute lymphoblastic leukemia (T-ALL) develops resistance to steroids and succumbs to their disease. JDP2 encodes a bZIP protein that has been implicated as a T-ALL oncogene from insertional mutagenesis studies in mice, but its role in human T-ALL pathogenesis has remained obscure. Here we show that JDP2 is aberrantly expressed in a subset of T-ALL patients and is associated with poor survival. JDP2 is required for T-ALL cell survival, as its depletion by short hairpin RNA knockdown leads to apoptosis. Mechanistically, JDP2 regulates prosurvival signaling through direct transcriptional regulation of MCL1. Furthermore, JDP2 is one of few oncogenes capable of initiating T-ALL in transgenic zebrafish. Notably, thymocytes from rag2:jdp2 transgenic zebrafish express high levels of mcl1 and demonstrate resistance to steroids in vivo. These studies establish JDP2 as a novel oncogene in high-risk T-ALL and implicate overexpression of MCL1 as a mechanism of steroid resistance in JDP2-overexpressing cells.

Abstract 3171: Vangl2 regulates cancer stem cell self-renewal and growth in rhabdomyosarcoma

Growth and relapse are driven by cancer stem cells (CSCs) in a subset of tumors, yet mechanisms driving cancer cell fate choices, maintenance and self-renewal are not fully understood. Here, we show that Van Gogh-like 2 (Vangl2), a core regulator of the non-canonical Wnt/planar cell polarity pathway (Wnt/PCP), regulates CSCs self-renewal in human rhabdomyosarcoma (RMS) – a common pediatric cancer of muscle. Wnt/PCP signaling is essential during development and recent work has linked this pathway to cancer growth, invasion and metastasis. However, roles for Vangl2 in regulating tumor self-renewal have not been previously described. Here, we show that VANGL2 is expressed in a majority of human RMS, specifically within early mononuclear progenitor-like cells. VANGL2 depletion inhibited proliferation, reduced self-renewal, and induced differentiation of human RMS. VANGL2 was also required for continued tumor growth and maintenance following engraftment of human RMS using mouse xenografts. Using a zebrafish model of embryonal rhabdomyosarcoma (ERMS) and limiting dilution cell transplantation approaches, we identified that Vangl2 expression enriches for CSCs in vivo and when transgenically expressed, at high levels elevates cancer stem cell number by 9-fold. Mechanistic studies revealed a role for RhoA downstream of Vangl2 in regulating maintenance of stem cell programs in human RMS. Our studies offer novel opportunities to isolate and characterize RMS cancer stem cells in vivo, and identify potential therapeutic targets for patient treatment.

Citation Format: Madeline Hayes, Karin McCarthy, Alexander Jin, Sowmya Iyer, Sara Garcia, Mariana L. Oliveira, Sivasish Sindiri, Berkley Gryder, Zainab Motala, G Petur Nielsen, Jean-Paul Borg, Matt van de Rijn, David Malkin, Javed Khan, Myron Ignatius, David M. Langenau. Vangl2 regulates cancer stem cell self-renewal and growth in rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3171.

Abstract 4127: A tumor evolution screen in zebrafish identifies PlexinA1 as a driver of metastasis and growth in human rhabdomyosarcoma

Relapse and metastasis are major clinical problems facing patients with rhabdomyosarcoma (RMS), a devastating pediatric malignancy of the muscle. Alveolar Rhabdomyosarcoma (ARMS) is characterized by genetic translocations while Embryonal Rhabdomyosarcoma (ERMS) is characterized by Ras pathway activation. Treatment for either RMS subtype requires surgical resection, chemotherapy, and radiation with overall poor prognosis for patients with high-risk features including metastasis and relapse. Thus, there is great interest in elucidating key molecular pathways and genetic factors that are involved in continued RMS growth and tumor maintenance. In the Langenau lab, we utilize a powerful transgenic zebrafish model of kRASG12D- induced ERMS and have recently developed a p53-/- (null) zebrafish model. p53 is one of the most common tumor suppressor genes, and upwards of 1/3 of RMS patients have mutations or Loss-of-Heterozygosity (LOH) of this locus. This model accurately mimics the molecular underpinnings and histopathology of human ERMS and has been used to discover molecular pathways that drive relapse. Using our tp53-/- (null) zebrafish along with large-scale cell transplantation assays, we have assessed metastatic potential of our ERMS tumors following serial passaging and acquisition of new driver mutations. Specifically, we performed a tumor evolution screen using serial engraftment of ERMS into recipient fish followed by RNA sequencing to discover novel pathways that are associated with elevated tumor growth and metastasis. Plexin A1 was one of our top hits in this screen, being expressed at >8 fold in metastatic lesions of the zebrafish and expressed in >80% of human RMS. Plexins are a large family of receptors for transmembrane, secreted and GPI-anchored semaphorins. Knockdown of PLXNA1 decreased human ERMS cell proliferation, increased differentiation and impaired anchorage-independent growth and migration. Collectively, our data supports the hypothesis that PLXNA1 is an important moderator of tumor growth in a large fraction of human RMS and likely regulates local invasion and metastasis. This work will provide much needed data for discovery of novel therapeutically-actionable pathways for relapsed ERMS and likely inform future treatment for high-risk patients.

Citation Format: Alessandra M. Welker, Madeline N. Hayes, Qin Tang, David M. Langenau. A tumor evolution screen in zebrafish identifies PlexinA1 as a driver of metastasis and growth in human rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4127.

The NOTCH1/SNAIL1/MEF2C Pathway Regulates Growth and Self-Renewal in Embryonal Rhabdomyosarcoma

Tumor-propagating cells (TPCs) share self-renewal properties with normal stem cells and drive continued tumor growth. However, mechanisms regulating TPC self-renewal are largely unknown, especially in embryonal rhabdomyosarcoma (ERMS)-a common pediatric cancer of muscle. Here, we used a zebrafish transgenic model of ERMS to identify a role for intracellular NOTCH1 (ICN1) in increasing TPCs by 23-fold. ICN1 expanded TPCs by enabling the de-differentiation of zebrafish ERMS cells into self-renewing myf5+ TPCs, breaking the rigid differentiation hierarchies reported in normal muscle. ICN1 also had conserved roles in regulating human ERMS self-renewal and growth. Mechanistically, ICN1 upregulated expression of SNAIL1, a transcriptional repressor, to increase TPC number in human ERMS and to block muscle differentiation through suppressing MEF2C, a myogenic differentiation transcription factor. Our data implicate the NOTCH1/SNAI1/MEF2C signaling axis as a major determinant of TPC self-renewal and differentiation in ERMS, raising hope of therapeutically targeting this pathway in the future.

Identification and characterization of T reg-like cells in zebrafish

Kasheta et al. report the identification of T reg–like cells in zebrafish, a means to track and live-image these cells, and foxp3a-deficient mutants that display lymphoproliferation, severe inflammation, and other hallmarks of T reg deficiency syndromes.

Regulatory T (T reg) cells are a specialized sublineage of T lymphocytes that suppress autoreactive T cells. Functional studies of T reg cells in vitro have defined multiple suppression mechanisms, and studies of T reg–deficient humans and mice have made clear the important role that these cells play in preventing autoimmunity. However, many questions remain about how T reg cells act in vivo. Specifically, it is not clear which suppression mechanisms are most important, where T reg cells act, and how they get there. To begin to address these issues, we sought to identify T reg cells in zebrafish, a model system that provides unparalleled advantages in live-cell imaging and high-throughput genetic analyses. Using a FOXP3 orthologue as a marker, we identified CD4-enriched, mature T lymphocytes with properties of T reg cells. Zebrafish mutant for foxp3a displayed excess T lymphocytes, splenomegaly, and a profound inflammatory phenotype that was suppressed by genetic ablation of lymphocytes. This study identifies T reg–like cells in zebrafish, providing both a model to study the normal functions of these cells in vivo and mutants to explore the consequences of their loss.

TOX Regulates Growth, DNA Repair, and Genomic Instability in T-cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy of thymocytes. Using a transgenic screen in zebrafish, thymocyte selection-associated high mobility group box protein (TOX) was uncovered as a collaborating oncogenic driver that accelerated T-ALL onset by expanding the initiating pool of transformed clones and elevating genomic instability. TOX is highly expressed in a majority of human T-ALL and is required for proliferation and continued xenograft growth in mice. Using a wide array of functional analyses, we uncovered that TOX binds directly to KU70/80 and suppresses recruitment of this complex to DNA breaks to inhibit nonhomologous end joining (NHEJ) repair. Impaired NHEJ is well known to cause genomic instability, including development of T-cell malignancies in KU70- and KU80-deficient mice. Collectively, our work has uncovered important roles for TOX in regulating NHEJ by elevating genomic instability during leukemia initiation and sustaining leukemic cell proliferation following transformation.Significance: TOX is an HMG box-containing protein that has important roles in T-ALL initiation and maintenance. TOX inhibits the recruitment of KU70/KU80 to DNA breaks, thereby inhibiting NHEJ repair. Thus, TOX is likely a dominant oncogenic driver in a large fraction of human T-ALL and enhances genomic instability. Cancer Discov; 7(11); 1336-53. ©2017 AACR.This article is highlighted in the In This Issue feature, p. 1201.

Dissecting hematopoietic and renal cell heterogeneity in adult zebrafish at single-cell resolution using RNA sequencing

The work by Tang et al. provides a comprehensive, single-cell, transcriptomic analysis of kidney and blood cells from the adult zebrafish, identifying novel cell types, including two classes of NK immune cells, classically defined and erythroid-primed hematopoietic stem and progenitor cells, mucin-secreting kidney cells, and kidney stem/progenitor cells.

Recent advances in single-cell, transcriptomic profiling have provided unprecedented access to investigate cell heterogeneity during tissue and organ development. In this study, we used massively parallel, single-cell RNA sequencing to define cell heterogeneity within the zebrafish kidney marrow, constructing a comprehensive molecular atlas of definitive hematopoiesis and functionally distinct renal cells found in adult zebrafish. Because our method analyzed blood and kidney cells in an unbiased manner, our approach was useful in characterizing immune-cell deficiencies within DNA–protein kinase catalytic subunit (prkdc), interleukin-2 receptor γ a (il2rga), and double-homozygous–mutant fish, identifying blood cell losses in T, B, and natural killer cells within specific genetic mutants. Our analysis also uncovered novel cell types, including two classes of natural killer immune cells, classically defined and erythroid-primed hematopoietic stem and progenitor cells, mucin-secreting kidney cells, and kidney stem/progenitor cells. In total, our work provides the first, comprehensive, single-cell, transcriptomic analysis of kidney and marrow cells in the adult zebrafish.

Efficient Transduction of Zebrafish Melanoma Cell Lines and Embryos Using Lentiviral Vectors

The establishment of in vitro cultures of zebrafish cancer cells has expanded the potential of zebrafish as a disease model. However, the lack of effective methods for gene delivery and genetic manipulation has limited the experimental applications of these cultures. To overcome this barrier, we tested and optimized vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped lentiviral and retroviral vector transduction protocols. We show that lentivirus successfully and efficiently transduced zebrafish melanoma cell lines in vitro, allowing antibiotic selection, fluorescence-based sorting, and in vivo allotransplantation. In addition, injection of concentrated lentiviral particles into embryos and tumors established the feasibility of in vivo gene delivery.

Satellite-like cells contribute to pax7-dependent skeletal muscle repair in adult zebrafish

Satellite cells, also known as muscle stem cells, are responsible for skeletal muscle growth and repair in mammals. Pax7 and Pax3 transcription factors are established satellite cell markers required for muscle development and regeneration, and there is great interest in identifying additional factors that regulate satellite cell proliferation, differentiation, and/or skeletal muscle regeneration. Due to the powerful regenerative capacity of many zebrafish tissues, even in adults, we are exploring the regenerative potential of adult zebrafish skeletal muscle. Here, we show that adult zebrafish skeletal muscle contains cells similar to mammalian satellite cells. Adult zebrafish satellite-like cells have dense heterochromatin, express Pax7 and Pax3, proliferate in response to injury, and show peak myogenic responses 4-5 days post-injury (dpi). Furthermore, using a pax7a-driven GFP reporter, we present evidence implicating satellite-like cells as a possible source of new muscle. In lieu of central nucleation, which distinguishes regenerating myofibers in mammals, we describe several characteristics that robustly identify newly-forming myofibers from surrounding fibers in injured adult zebrafish muscle. These characteristics include partially overlapping expression in satellite-like cells and regenerating myofibers of two RNA-binding proteins Rbfox2 and Rbfoxl1, known to regulate embryonic muscle development and function. Finally, by analyzing pax7a; pax7b double mutant zebrafish, we show that Pax7 is required for adult skeletal muscle repair, as it is in the mouse.

Myogenic regulatory transcription factors regulate growth in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric malignacy of muscle with myogenic regulatory transcription factors MYOD and MYF5 being expressed in this disease. Consensus in the field has been that expression of these factors likely reflects the target cell of transformation rather than being required for continued tumor growth. Here, we used a transgenic zebrafish model to show that Myf5 is sufficient to confer tumor-propagating potential to RMS cells and caused tumors to initiate earlier and have higher penetrance. Analysis of human RMS revealed that MYF5 and MYOD are mutually-exclusively expressed and each is required for sustained tumor growth. ChIP-seq and mechanistic studies in human RMS uncovered that MYF5 and MYOD bind common DNA regulatory elements to alter transcription of genes that regulate muscle development and cell cycle progression. Our data support unappreciated and dominant oncogenic roles for MYF5 and MYOD convergence on common transcriptional targets to regulate human RMS growth.

DOI:http://dx.doi.org/10.7554/eLife.19214.001

Discovering novel oncogenic pathways and new therapies using zebrafish models of sarcoma

Sarcoma is a type of cancer affecting connective, supportive, or soft tissue of mesenchymal origin. Despite rare incidence in adults (<1%), over 15% of pediatric cancers are sarcoma. Sadly, both adults and children with relapsed or metastatic disease have devastatingly high rates of mortality. Current treatment options for sarcoma include surgery, radiation, and/or chemotherapy; however, significant limitations exist with respect to the efficacy of these strategies. Strong impetus has been placed on the development of novel therapies and preclinical models for uncovering mechanisms involved in the development, progression, and therapy resistance of sarcoma. Over the past 15 years, the zebrafish has emerged as a powerful genetic model of human cancer. High genetic conservation when combined with a unique susceptibility to develop sarcoma has made the zebrafish an effective tool for studying these diseases. Transgenic and gene-activation strategies have been employed to develop zebrafish models of rhabdomyosarcoma, malignant peripheral nerve sheath tumors, Ewing's sarcoma, chordoma, hemangiosarcoma, and liposarcoma. These models all display remarkable molecular and histopathological conservation with their human cancer counterparts and have offered excellent platforms for understanding disease progression in vivo. Short tumor latency and the amenability of zebrafish for ex vivo manipulation, live imaging studies, and tumor cell transplantation have allowed for efficient study of sarcoma initiation, growth, self-renewal, and maintenance. When coupled with facile chemical genetic approaches, zebrafish models of sarcoma have provided a strong translational tool to uncover novel drug pathways and new therapeutic strategies.

Single-cell imaging of normal and malignant cell engraftment into optically clear prkdc-null SCID zebrafish

Cell transplantation into immunodeficient mice has revolutionized our understanding of regeneration, stem cell self-renewal, and cancer; yet models for direct imaging of engrafted cells has been limited. Here, we characterize zebrafish with mutations in recombination activating gene 2 (rag2), DNA-dependent protein kinase (prkdc), and janus kinase 3 (jak3). Histology, RNA sequencing, and single-cell transcriptional profiling of blood showed that rag2 hypomorphic mutant zebrafish lack T cells, whereas prkdc deficiency results in loss of mature T and B cells and jak3 in T and putative Natural Killer cells. Although all mutant lines engraft fluorescently labeled normal and malignant cells, only the prkdc mutant fish reproduced as homozygotes and also survived injury after cell transplantation. Engraftment into optically clear casper, prkdc-mutant zebrafish facilitated dynamic live cell imaging of muscle regeneration, repopulation of muscle stem cells within their endogenous niche, and muscle fiber fusion at single-cell resolution. Serial imaging approaches also uncovered stochasticity in fluorescently labeled leukemia regrowth after competitive cell transplantation into prkdc mutant fish, providing refined models to assess clonal dominance and progression in the zebrafish. Our experiments provide an optimized and facile transplantation model, the casper, prkdc mutant zebrafish, for efficient engraftment and direct visualization of fluorescently labeled normal and malignant cells at single-cell resolution.

Abstract 4177: Dynamic visualization of cancer cell engraftment into immune compromised zebrafish

Cell transplantation into immune compromised mice has transformed our understanding of cancer and is now the gold standard for assessing therapeutic responses in vivo. However, mouse models are expensive and engraftment is often difficult to visualize directly. To overcome these challenges, we have developed immune compromised zebrafish (ICZ) in the transparent casper background using genome editing techniques. We have successfully targeted genes required for immune cell function and are well known to cause immune deficiency in human and mice. To date, we have developed homozygous viable mutants for recombination-activating gene 2 (rag2), DNA-dependent protein kinase (prkdc), janus kinase 3 (jak3), interleukin 2 receptor gamma (Il2rg), zeta-chain (TCR) associated protein kinase 70 (zap70), and forkhead box N1 (foxn1/nude). Gene expression analysis of marrow cells using RNAseq has identified novel transcript changes correlated with loss of specific cell types, and in conjunction with large-scale single cell transcriptional profiling, has identified specific cellular defects associated with T, B, and NK cell loss. For example, homozygous prkdc (SCID) mutant fish lack mature T and B cells, but have intact NK cell signaling. By contrast, il2rg-deficient zebrafish lack T and NK cells. Importantly, these ICZ models accurately recapitulate known human severe combined immune deficiencies and established mouse models that are commonly used for cell transplantation. Thus, it is not unexpected that a subset of zebrafish mutants have reduced immune cell function, permitting engraftment of normal hematopoietic and muscle satellite cells from allogeneic donors. Additionally, we have demonstrated robust and persistent engraftment of fluorescently labeled leukemia, rhabdomyosarcoma, neuroblastoma, and melanoma from a wide range of zebrafish strains. Because mutations have been created in optically-clear, casper-strain zebrafish and cancers are fluorescently labeled, we now have unprecedented access to directly visualize tumor cells at single cell resolution in live animals. To date, we have optimized our models to visualize neovascularization, intratumoral cell heterogeneity, clonal evolution and metastisis. The ability to transplant non-immune matched cell types will likely revolutionize the types and scale of cell transplantation experiments performed in the zebrafish and will likely permit engraftment of mouse and human cells into compound mutant ICZ models in the near future.

Citation Format: John C. Moore, Qin Tang, Nora Torres Yordan, Timothy Mulligan, Finola E. Moore, Riadh Lobbardi, Ashwin Ramakrishnan, Anthony Anselmo, Ruslan Sadreyev, Jason Berman, Robert Liwski, Brant Weinstein, John Rawls, David M. Langenau. Dynamic visualization of cancer cell engraftment into immune compromised zebrafish. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4177.

Abstract 3583: Thymocyte selection-associated HMG box protein (TOX) induces genomic instability in T-cell acute lymphoblastic leukemia

MYC and NOTCH are major oncogenic drivers in T-cell Acute Lymphoblastic Leukemia (T-ALL), yet additional collaborating genetic lesions collaborate to induce frank malignancy. To identify these factors, a large-scale transgenic screen was completed where 28 amplified and over-expressed genes found in human T-ALL were assessed for accelerating leukemia onset in the zebrafish transgenic model. From this analysis, Thymocyte selection-associated HMG protein (TOX) synergized with both MYC and NOTCH to induce T-ALL. Here, we show that TOX is highly expressed in 95% of human primary and relapse T-ALL when compared with both normal T cells and B-ALL. TOX is highly and specifically expressed in human T-ALL due to both genomic amplification and transcriptional regulation by the master transcription factors MYB/LMO2. Characterization of zebrafish T-ALLs revealed that TOX promoted genomic instability as assessed by changes in DNA content and Whole Genome Sequencing. Effects on genomic instability were confirmed by metaphase spread following TOX expression in MEF cells, confirming roles for TOX in regulating genomic instability and elevated DNA translocation potential in a wider range of cell types. To identify TOX binding partners, Tandem Mass Spectrometry was performed in human T-ALL cells. TOX was found to interact with KU70/KU80 but not other DNA repair enzymes, a result verified by co-immunoprecipitation studies. Given that TOX elevated genomic instability in the zebrafish model, that Ku70 or Ku80 loss lead to genomic instability and T cell lymphoma in mice, and that TOX bound specifically to KU70/KU80 - the initiating factors required for Non-Homologous End Joining (NHEJ) repair - we hypothesized that TOX is a negative regulator of double-strand break repair. Fluorescent repair assays were completed in 3T3 fibroblasts and confirmed that TOX inhibits NHEJ. Dynamic real-time imaging studies showed that TOX suppresses recruitment of fluorescent-tagged KU80 to DNA breaks. Importantly, TOX loss of function increased NHEJ in human T-ALL cells and reduced time to DNA repair as assessed by fluorescent Traffic Light Reporter assays and quantitative assessment of 53BP1 and γH2A.X foci resolution following irradiation. Our results show that TOX is aberrantly re-activated in 95% of human T-ALL, thereby suppressing KU70/KU80 function to promote genomic instability and ultimately elevating rates at which acquired mutations and rearrangements are amassed in developing pre-malignant T cells. Our work shows that TOX is the major oncogenic driver of genomic instability human T-ALL and locks cells in a constant state of dampened repair.

Citation Format: Riadh Lobbardi, Jordan Pinder, Barbara Martinez-Pastor, Jessica Blackburn, Brian J. Abraham, Marc Mansour, Nouran S. Abdelfattah, Aleksey Molodtsov, Gabriela Alexe, Debra Toiber, Manon de Waard, Esha Jain, Deepak Bhere, Khalid Shah, Alejandro Gutierrez, Kimberly Stegmaier, Lewis B. Silverman, Ruslan Sadreyev, John Asara, A Thomas Look, Richard A. Young, Raul Mostoslavsky, Graham Dellaire, David M. Langenau. Thymocyte selection-associated HMG box protein (TOX) induces genomic instability in T-cell acute lymphoblastic leukemia. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3583.

Single-cell transcriptional analysis of normal, aberrant, and malignant hematopoiesis in zebrafish

Moore et al. reports the first single-cell gene expression analysis in zebrafish blood to distinguish major blood lineages, identify new cell types, and delineate heterogeneity in T cell leukemia.

Hematopoiesis culminates in the production of functionally heterogeneous blood cell types. In zebrafish, the lack of cell surface antibodies has compelled researchers to use fluorescent transgenic reporter lines to label specific blood cell fractions. However, these approaches are limited by the availability of transgenic lines and fluorescent protein combinations that can be distinguished. Here, we have transcriptionally profiled single hematopoietic cells from zebrafish to define erythroid, myeloid, B, and T cell lineages. We also used our approach to identify hematopoietic stem and progenitor cells and a novel NK-lysin 4⁺ cell type, representing a putative cytotoxic T/NK cell. Our platform also quantified hematopoietic defects in rag2^E450fs mutant fish and showed that these fish have reduced T cells with a subsequent expansion of NK-lysin 4⁺ cells and myeloid cells. These data suggest compensatory regulation of the innate immune system in rag2^E450fs mutant zebrafish. Finally, analysis of Myc-induced T cell acute lymphoblastic leukemia showed that cells are arrested at the CD4⁺/CD8⁺ cortical thymocyte stage and that a subset of leukemia cells inappropriately reexpress stem cell genes, including bmi1 and cmyb. In total, our experiments provide new tools and biological insights into single-cell heterogeneity found in zebrafish blood and leukemia.

Preclinical Models Provide Scientific Justification and Translational Relevance for Moving Novel Therapeutics into Clinical Trials for Pediatric Cancer

Despite improvements in survival rates for children with cancer since the 1960s, progress for many pediatric malignancies has slowed over the past two decades. With the recent advances in our understanding of the genomic landscape of pediatric cancer, there is now enthusiasm for individualized cancer therapy based on genomic profiling of patients' tumors. However, several obstacles to effective personalized cancer therapy remain. For example, relatively little data from prospective clinical trials demonstrate the selective efficacy of molecular-targeted therapeutics based on somatic mutations in the patient's tumor. In this commentary, we discuss recent advances in preclinical testing for pediatric cancer and provide recommendations for providing scientific justification and translational relevance for novel therapeutic combinations for childhood cancer. Establishing rigorous criteria for defining and validating druggable mutations will be essential for the success of ongoing and future clinical genomic trials for pediatric malignancies.

Abstract A33: Role for the tumor suppressor phf6 in hematopoiesis

Plant Home domain Finger 6 (PHF6) is a tumor suppressor of unknown function for blood malignancies such as T-cell Acute Lymphoblastic Leukemia (T-ALL), Acute Myeloid leukemia (AML), and Chronic Myeloid Leukemia (CML). PHF6 contains two zinc finger-like PH domains and interacts with the Nucleosome Remodeling and Deacetylation (NuRD) complex, suggesting a role in chromatin remodeling. Although PHF6 loss-of-function mutations are found in nearly 40% of T-ALL patients, little is known about how PHF6 mutations contribute to blood development and leukemogenesis. To understand the function of PHF6, beginning with its role in hematopoiesis, we have undertaken developmental studies in zebrafish to discover how loss of phf6 affects blood development and to determine which pathways are regulated by phf6. Zebrafish will be used to study hematopoiesis due to the remarkable conservation of molecular pathways that regulate blood development, genetic tractability, and ability to observe embryonic development over a short window of time. RNA in situ hybridization studies of zebrafish embryos showed that phf6 is expressed broadly during zebrafish development, and especially in the dorsal aorta, a site analogous to the aorta-gonad-mesonephros (AGM) in mammals, from which hematopoietic stem cells (HSCs) arise. Further, phf6 is highly expressed in lymphocytes of adult zebrafish, reminiscent of the expression patterns found in human and mouse. To determine the effect of phf6 loss on hematopoiesis, phf6 expression was knocked down by morpholino injection. We find that phf6 morphants have increased numbers of HSCs by RNA in situ hybridization of runx1/cmyb in the AGM and caudal hematopoietic tissue ((CHT) analogous to mammalian fetal liver), sites of HSC emergence and migration. Later in development, phf6 morphants demonstrate increased lymphocytes by RNA in situ hybridization of rag1 at the thymus. Similar phenotypes were observed in homozygous phf6-null mutants generated by TALEN-mediated knockout. Phf6-null mutant zebrafish survive to Mendelian ratios and are fertile as adults. In total, we found a new role for phf6 in regulation of HSC formation.

Citation Format: Finola E. Moore, Virginie Esain, Riadh Lobbardi, Jessica S. Blackburn, Trista E. North, David M. Langenau. Role for the tumor suppressor phf6 in hematopoiesis. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr A33.

Abstract 5153: Myogenic regulatory factors and their role in embryonal rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric sarcoma of muscle. RAS pathway activation is the dominant oncogenic driver event in fusion negative RMS, which includes the Embryonal RMS (ERMS). We have previously shown in a transgenic zebrafish model of kRASG12D induced ERMS that tumor-propagating potential is confined to molecularly defined cells that express Myf5, m-Cadherin, c-Met and additional satellite cell markers. MYF5 and MYOD1 are bHLH myogenic regulatory factors (MRFs) that orchestrate skeletal muscle differentiation during development and regeneration. Both are sufficient to reprogram human mesenchymal cells into a myogenic fate. Importantly, these same factors are highly expressed a subset of mouse and human ERMS. Given that Myf5 is highly and specifically expressed in the TPC compartment in our ERMS model, we hypothesized that Myf5 and its transcriptional targets may regulate self-renewal and growth of TPCs.

To address this question, we utilized transgenic zebrafish and expressed Myf5 in differentiated ERMS cells that lack proliferative capacity and can not make tumors when transplanted into recipient fish. Induced expression of Myf5 was sufficient to confer tumor-propagating ability to differentiated populations of ERMS cells. These “Induced TPCs” proliferate and generate aggressive tumors that re-express satellite cell markers and yet retain expression of mature muscle markers. Next, we assessed if Myf5 is required for ERMS initiation in the zebrafish model. Remarkably, kRASSG12D-induced ERMS could be generated in Myf5 loss-of-function zebrafish. Moreover, Myf5-deficient ERMS could regrow following transplantation into rag2E450fs immune-compromised zebrafish; however, these tumors are histologically and molecularly distinct when compared with ERMS arising in wild-type fish. Given the redundancy of function between Myf5 and MyoD in muscle and their differential expression in satellite cells and muscle progenitors, our current hypothesis is that Myf5-deficient zebrafish ERMS likely recapitulate a molecularly-distinct class of human ERMS. This idea is consistent with the fact that MYF5 is expressed in only 50% of primary human ERMS and a small fraction of human cell lines. Importantly, knock down experiments in human ERMS cell lines confirm independent roles of either MYF5 or MYOD1 in maintenance of ERMS cell viability in vitro.

Collectively, our results support a previously unappreciated role for bHLH MRFs in ERMS cell survival and intra-tumor functional heterogeneity and suggest that myogenic factor expression may define unique subtypes of ERMS.

Citation Format: Ines M. Tenente, Myron Ignatius, Eleanor Chen, Madeline Hayes, David M. Langenau. Myogenic regulatory factors and their role in embryonal rhabdomyosarcoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5153. doi:10.1158/1538-7445.AM2015-5153

Zebrafish as a model to assess cancer heterogeneity, progression and relapse

Clonal evolution is the process by which genetic and epigenetic diversity is created within malignant tumor cells. This process culminates in a heterogeneous tumor, consisting of multiple subpopulations of cancer cells that often do not contain the same underlying mutations. Continuous selective pressure permits outgrowth of clones that harbor lesions that are capable of enhancing disease progression, including those that contribute to therapy resistance, metastasis and relapse. Clonal evolution and the resulting intratumoral heterogeneity pose a substantial challenge to biomarker identification, personalized cancer therapies and the discovery of underlying driver mutations in cancer. The purpose of this Review is to highlight the unique strengths of zebrafish cancer models in assessing the roles that intratumoral heterogeneity and clonal evolution play in cancer, including transgenesis, imaging technologies, high-throughput cell transplantation approaches and in vivo single-cell functional assays.

Normal and malignant muscle cell transplantation into immune compromised adult zebrafish

Zebrafish have become a powerful tool for assessing development, regeneration, and cancer. More recently, allograft cell transplantation protocols have been developed that permit engraftment of normal and malignant cells into irradiated, syngeneic, and immune compromised adult zebrafish. These models when coupled with optimized cell transplantation protocols allow for the rapid assessment of stem cell function, regeneration following injury, and cancer. Here, we present a method for cell transplantation of zebrafish adult skeletal muscle and embryonal rhabdomyosarcoma (ERMS), a pediatric sarcoma that shares features with embryonic muscle, into immune compromised adult rag2^E450fs homozygous mutant zebrafish. Importantly, these animals lack T cells and have reduced B cell function, facilitating engraftment of a wide range of tissues from unrelated donor animals. Our optimized protocols show that fluorescently labeled muscle cell preparations from α-actin-RFP transgenic zebrafish engraft robustly when implanted into the dorsal musculature of rag2 homozygous mutant fish. We also demonstrate engraftment of fluorescent-transgenic ERMS where fluorescence is confined to cells based on differentiation status. Specifically, ERMS were created in AB-strain myf5-GFP; mylpfa-mCherry double transgenic animals and tumors injected into the peritoneum of adult immune compromised fish. The utility of these protocols extends to engraftment of a wide range of normal and malignant donor cells that can be implanted into dorsal musculature or peritoneum of adult zebrafish.