Metastatic dissemination is the leading cause of death in cancer patients, which is particularly evident for high-risk sarcomas such as Ewing sarcoma, osteosarcoma, and rhabdomyosarcoma. Previous research identified a crucial role for YB-1 in the epithelial-to-mesenchymal transition (EMT) and metastasis of epithelial malignancies. Based on clinical data and two distinct animal models, we now report that YB-1 is also a major metastatic driver in high-risk sarcomas. Our data establish YB-1 as a critical regulator of hypoxia-inducible factor 1α (HIF1α) expression in sarcoma cells. YB-1 enhances HIF1α protein expression by directly binding to and activating translation of HIF1A messages. This leads to HIF1α-mediated sarcoma cell invasion and enhanced metastatic capacity in vivo, highlighting a translationally regulated YB-1-HIF1α axis in sarcoma metastasis.

The current preclinical pipeline for drug discovery can be cumbersome and costly, which limits the number of compounds that can effectively be transitioned to use as therapies. Chemical screens in zebrafish have uncovered new uses for existing drugs and identified promising new compounds from large libraries. Xenotransplantation of human cancer cells into zebrafish embryos builds on this work and enables direct evaluation of patient-derived tumor specimens in vivo in a rapid and cost-effective manner. The short time frame needed for xenotransplantation studies means that the zebrafish can serve as an early preclinical drug screening tool and can also help personalize cancer therapy by providing real-time data on the response of the human cells to treatment. In this Review, we summarize the use of zebrafish embryos in drug screening and highlight the potential for xenotransplantation approaches to be adopted as a preclinical tool to identify and prioritize therapies for further clinical evaluation. We also discuss some of the limitations of using zebrafish xenografts and the benefits of using them in concert with murine xenografts in drug optimization.

transplantation (XT) can support the use of patient diagnostic biopsy material, but are better suited to moderate throughput drug-screening than mice. 16,17 Relatively low maintenance costs and short time intervals to complete in vivo studies, as well as their size and transparency that facilitate analyses, make the zebrafish an attractive model for the screening of anti-cancer agents.18 Specifically, zebrafish XT has gained considerable attention as a system to rapidly study and directly observe tumor cell behavior and drug response in a live animal model. 16,17,[19][20][21]

Animal xenografts of human cancers represent a key preclinical tool in the field of cancer research. While mouse xenografts have long been the gold standard, investigators have begun to use zebrafish (Danio rerio) xenotransplantation as a relatively rapid, robust and cost-effective in vivo model of human cancers. There are several important methodological considerations in the design of an informative and efficient zebrafish xenotransplantation experiment. Various transgenic fish strains have been created that facilitate microscopic observation, ranging from the completely transparent casper fish to the Tg(fli1:eGFP) fish that expresses fluorescent GFP protein in its vascular tissue. While human cancer cell lines have been used extensively in zebrafish xenotransplantation studies, several reports have also used primary patient samples as the donor material. The zebrafish is ideally suited for transplanting primary patient material by virtue of the relatively low number of cells required for each embryo (between 50 and 300 cells), the absence of an adaptive immune system in the early zebrafish embryo, and the short experimental timeframe (5-7 days). Following xenotransplantation into the fish, cells can be tracked using in vivo or ex vivo measures of cell proliferation and migration, facilitated by fluorescence or human-specific protein expression. Importantly, assays have been developed that allow for the reliable detection of in vivo human cancer cell growth or inhibition following administration of drugs of interest. The zebrafish xenotransplantation model is a unique and effective tool for the study of cancer cell biology.

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We have optimized point mutation knock-ins into zebrafish genomic sites using clustered regularly interspaced palindromic repeats (CRISPR)/Cas9 reagents and single-stranded oligodeoxynucleotides. The efficiency of knock-ins was assessed by a novel application of allele-specific polymerase chain reaction and confirmed by high-throughput sequencing. Anti-sense asymmetric oligo design was found to be the most successful optimization strategy. However, cut site proximity to the mutation and phosphorothioate oligo modifications also greatly improved knock-in efficiency. A previously unrecognized risk of off-target trans knock-ins was identified that we obviated through the development of a workflow for correct knock-in detection. Together these strategies greatly facilitate the study of human genetic diseases in zebrafish, with additional applicability to enhance CRISPR-based approaches in other animal model systems.

Purpose: Ewing sarcoma is an aggressive solid tumor malignancy of childhood. Although current treatment regimens cure approximately 70% of patients with localized disease, they are ineffective for most patients with metastases or relapse. New treatment combinations are necessary for these patients. Experimental Design: Ewing sarcoma cells are dependent on focal adhesion kinase (FAK) for growth. To identify candidate treatment combinations for Ewing sarcoma, we performed a small-molecule library screen to identify compounds synergistic with FAK inhibitors in impairing Ewing cell growth. The activity of a top-scoring class of compounds was then validated across multiple Ewing cell lines in vitro and in multiple xenograft models of Ewing sarcoma. Results: Numerous Aurora kinase inhibitors scored as synergistic with FAK inhibition in this screen. We found that Aurora kinase B inhibitors were synergistic across a larger range of concentrations than Aurora kinase A inhibitors when combined with FAK inhibitors in multiple Ewing cell lines. The combination of AZD-1152, an Aurora kinase B-selective inhibitor, and PF-562271 or VS-4718, FAK-selective inhibitors, induced apoptosis in Ewing sarcoma cells at concentrations that had minimal effects on survival when cells were treated with either drug alone. We also found that the combination significantly impaired tumor progression in multiple xenograft models of Ewing sarcoma. Conclusions: FAK and Aurora kinase B inhibitors synergistically impair Ewing sarcoma cell viability and significantly inhibit tumor progression. This study provides preclinical support for the consideration of a clinical trial testing the safety and efficacy of this combination for patients with Ewing sarcoma.

T-cell acute lymphoblastic leukemia (T-ALL) is a high-risk subset of ALL, for which there is a need for new therapeutic strategies and efficient preclinical screening methods. We have pioneered an innovative zebrafish human cancer xenotransplantation (XT) model to examine drug-tumor interactions in vivo. T-ALL cell lines and primary patient T-ALL samples were microinjected into 48-hour zebrafish embryos, a stage at which the adaptive immune system has not yet developed. Fluorescent labelling of tumor cells prior to injection and use of casper pigment mutant fish facilitates evaluation of drug response both by direct observation in transparent fish and enumeration of human cells following embryo dissociation. Proliferation rates are rapidly determined by directly counting fluorescent cells using in silico-based programs and/or utilizing immunohistochemical approaches to distinguish human cancer cells from host cell populations. T-ALL cell lines harboring defined mutations in the NOTCH1, phosphoinositide 3-kinase (PI3K)/AKT and mTOR pathways differentially responded to targeted inhibition using the γ-secretase inhibitor Compound E, triciribine, and rapamycin, when xenografted into embryos, consistent with responses in vitro. Primary patient-derived T-ALL bone marrow samples similarly engrafted and proliferated in zebrafish embryos. Using this in vivo chemical genomic approach, a targetable mutation sensitive to γ-secretase inhibition was identified from the diagnostic bone marrow sample of a child with T-ALL, which was confirmed by exome Sanger sequencing, and validated as a gain-of-function mutation in the NOTCH1 gene by luciferase assay and Western blot. Focused chemical genomics using the zebrafish T-ALL XT model provides a means of tailoring therapy using a real time in vivo assay that more accurately recapitulates the tumor microenvironment than in vitro methods and more rapidly than mouse xenografts. Moreover, the efficiency and cost-effectiveness of this innovative platform provides a novel intermediary for the prioritization of much-needed drug candidates in the preclinical pipeline. Disclosures: No relevant conflicts of interest to declare.