Pediatric Cancer Models in Zebrafish
Authors: Mattie J. Casey, Rodney A. Stewart
Journal: Trends in cancer (2020)

Zebrafish develop tumors that are histologically and genetically similar to human tumors.Zebrafish enable the rapid identification of molecular drivers of tumor development and can be used to model cancers with unknown cells of origin using heat-shock and β-actin promoters.Zebrafish models are amenable to high-throughput drug screening through larval drug submersion approaches, as well as transplantation of primary patient tumors into immunocompromised lines.Tumor-cell dynamics can be visualized in vivo throughout the lifetime of the animal by coupling oncogenes to fluorescent markers. Pediatric cancer is a leading cause of death in children and adolescents. Improvements in pediatric cancer treatment that include the alleviation of long-term adverse effects require a deeper understanding of the genetic, epigenetic, and developmental factors driving these cancers. Here, we review how the unique attributes of the zebrafish model system in embryology, imaging, and scalability have been used to identify new mechanisms of tumor initiation, progression, and relapse and for drug discovery. We focus on zebrafish models of leukemias, neural tumors and sarcomas – the most common and difficult childhood cancers to treat. Pediatric cancer is a leading cause of death in children and adolescents. Improvements in pediatric cancer treatment that include the alleviation of long-term adverse effects require a deeper understanding of the genetic, epigenetic, and developmental factors driving these cancers. Here, we review how the unique attributes of the zebrafish model system in embryology, imaging, and scalability have been used to identify new mechanisms of tumor initiation, progression, and relapse and for drug discovery. We focus on zebrafish models of leukemias, neural tumors and sarcomas – the most common and difficult childhood cancers to treat. Pediatric cancer is one of the leading causes of death in children and adolescents (age 0–19 years) and is mainly comprised of leukemias, cancers of the nervous system, and sarcomas [1.Cunningham R. et al.The major causes of death in children and adolescents in the United States.N. Engl. J. Med. 2018; 379: 2468-2475Crossref PubMed Scopus (313) Google Scholar]. Recent genomic profiling efforts to better understand the etiology of this group of diseases have enabled the stratification of tumor types based on molecular signatures and have led to the identification of potential genetic drivers and cooperating molecular events that underlie the development of different pediatric cancers. Most pediatric malignancies are mutationally quiet and are thought to be driven by a single driver gene, a fusion oncoprotein, or structural/copy number alterations [2.Grobner S.N. et al.The landscape of genomic alterations across childhood cancers.Nature. 2018; 555: 321-327Crossref PubMed Scopus (711) Google Scholar,3.Ma X. et al.Pan-cancer genome and transcriptome analyses of 1,699 paediatric leukaemias and solid tumours.Nature. 2018; 555: 371-376Crossref PubMed Scopus (431) Google Scholar]. In contrast, adult tumors frequently exhibit high mutational burdens, likely due to a longer period of mutational acquisition under selective pressure [2.Grobner S.N. et al.The landscape of genomic alterations across childhood cancers.Nature. 2018; 555: 321-327Crossref PubMed Scopus (711) Google Scholar,3.Ma X. et al.Pan-cancer genome and transcriptome analyses of 1,699 paediatric leukaemias and solid tumours.Nature. 2018; 555: 371-376Crossref PubMed Scopus (431) Google Scholar]. Despite these differences, however, pediatric cancer treatments are still largely modeled after treatments designed for the adult version of the disease and can cause debilitating, long-term side effects when administered to children. The development of robust preclinical pediatric cancer models that accurately recapitulate these diseases will ultimately be necessary for the design of more precise, targeted therapies to improve outcomes for pediatric cancer patients. Here, we discuss how the use of zebrafish has advanced the pediatric cancer field as a preclinical model for gene and drug discovery. The zebrafish (Danio rerio) was established as a developmental biology model because it is uniquely amenable to forward genetics, embryology, and imaging, allowing the discovery of new mechanisms controlling embryogenesis, neurogenesis, and organ formation. These features also make zebrafish an attractive model to study cancer because: (i) optically transparent zebrafish embryos and adults enable direct observation of tumor cell behavior in vivo; (ii) rapid production of zebrafish animals provides a highly scalable platform for genetic and drug screening; and (iii) significant conservation of cancer signaling pathways between fish and human enables the identification of new molecular mechanisms of tumorigenesis. Over 50 genetically engineered zebrafish models of human cancer have been established that closely resemble their human counterparts at the histological and/or genomic levels [4.Yen J. et al.Zebrafish models of cancer: progress and future challenges.Curr. Opin. Genet. Dev. 2014; 24: 38-45Crossref PubMed Scopus (39) Google Scholar,5.White R. et al.Zebrafish cancer: the state of the art and the path forward.Nat. Rev. Cancer. 2013; 13: 624-636Crossref PubMed Scopus (262) Google Scholar]. Zebrafish cancer models have accelerated the discovery of new mechanisms driving human cancers and identified new drugs for clinical trials [6.Letrado P. et al.Zebrafish: speeding up the cancer drug discovery process.Cancer Res. 2018; 78: 6048-6058Crossref PubMed Scopus (86) Google Scholar]. A number of recent reviews detailing the experimental approaches for generating different leukemia [7.He S. et al.Zebrafish models of leukemia.in: Detrich H.W. Methods in Cell Biology. Elsevier, 2017: 563-592Google Scholar], sarcoma [8.Hayes M.N. Langenau D.M. Discovering novel oncogenic pathways and new therapies using zebrafish models of sarcoma.in: Detrich H.W. Methods in Cell Biology. Elsevier, 2017: 525-561Google Scholar], neuroblastoma [9.Casey M.J. Stewart R.A. Zebrafish as a model to study neuroblastoma development.Cell Tissue Res. 2017; 372: 223-232Crossref PubMed Scopus (8) Google Scholar], and germ-cell tumor [10.Sanchez A. Amatruda J.F. Zebrafish germ cell tumors.Adv. Exp. Med. Biol. 2016; 916: 479-494Crossref PubMed Scopus (8) Google Scholar] models are published elsewhere. Here, we focus on how genetically engineered zebrafish lines that specifically model pediatric and adolescent cancers, which we define as zebrafish tumors that arise within the first 90 days of life, have informed pediatric cancer research and therapeutics (Figure 1, Key Figure and Table 1).Table 1Models of Pediatric Cancer in ZebrafishCancerDriver oncogeneModelOncogene descriptionRefsT-ALLMycTg(rag2:EGFP-mMyc)Amplification of Myc[15.Langenau D.M. et al.Myc-induced T cell leukemia in transgenic zebrafish.Science. 2003; 299: 887-890Crossref PubMed Scopus (438) Google Scholar]MycTg(rag2-lox-dsRED-lox-EGFP-mMyc)Cre inducible Myc amplification[16.Langenau D.M. et al.Cre/lox-regulated transgenic zebrafish model with conditional myc-induced T cell acute lymphoblastic leukemia.Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 6068-6073Crossref PubMed Scopus (220) Google Scholar]MycTg(rag2:LDL-EGFP-mMyc); Tg(Hsp70:Cre)Heat-shock-inducible Myc amplification[17.Feng H. et al.Heat-shock induction of T-cell lymphoma/leukaemia in conditional Cre/lox-regulated transgenic zebrafish.Br. J. Haematol. 2007; 138: 169-175Crossref PubMed Scopus (98) Google Scholar]MYCTg(rag2:hMYC-ER)4-hydroxytamoxifen inducible MYC amplification[18.Gutierrez A. et al.Pten mediates Myc oncogene dependence in a conditional zebrafish model of T cell acute lymphoblastic leukemia.J. Exp. Med. 2011; 208: 1595-1603Crossref PubMed Scopus (91) Google Scholar]NOTCHTg(rag2:NICD-EGFP)NOTCH1 intracellular domain amplification[20.Blackburn J.S. et al.Notch signaling expands a pre-malignant pool of T-cell acute lymphoblastic leukemia clones without affecting leukemia-propagating cell frequency.Leukemia. 2012; 26: 2069-2078Crossref PubMed Scopus (59) Google Scholar]ARID5BTg(rag2:ARID5B)ARID5B amplification[23.Leong W.Z. et al.ARID5B as a critical downstream target of the TAL1 complex that activates the oncogenic transcriptional program and promotes T-cell leukemogenesis.Genes Dev. 2017; 31: 2343-2360Crossref PubMed Scopus (39) Google Scholar]B-ALL/T-ALLMYCTg(rag2:hMYC)Mixed-lineage ALL with MYC amplification[30.Borga C. et al.Simultaneous B and T cell acute lymphoblastic leukemias in zebrafish driven by transgenic MYC: implications for oncogenesis and lymphopoiesis.Leukemia. 2019; 33: 333-347Crossref PubMed Scopus (22) Google Scholar]B-ALLTEL(ETV6)-AML1(RUNX1)Tg(XEF:EGFP-TEL-AML1) or Tg(ZBA:EGFP-TEL-AML1)TEL-AML1 fusion[29.Sabaawy H.E. et al.TEL-AML1 transgenic zebrafish model of precursor B cell acute lymphoblastic leukemia.Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 15166-15171Crossref PubMed Scopus (145) Google Scholar]AMLAML1-ETOTg(hsp70:AML1-ETO)AML1-ETO fusion[32.Yeh J.J. et al.AML1-ETO reprograms hematopoietic cell fate by downregulating scl expression.Development. 2008; 135: 401-410Crossref PubMed Scopus (101) Google Scholar]FLT3Tg(CMV:FLT3-ITD) or Tg(CMV:FLT3-TKD)FLT3-ITD (internal tandem duplication) or FLT3-TKD (tyrosine kinase inhibitor resistant mutation D835Y)[33.He B. et al.Functions of flt3 in zebrafish hematopoiesis and its relevance to human acute myeloid leukemia.Blood. 2014; 123: 2518-2529Crossref PubMed Scopus (42) Google Scholar]CNS-PNETnf1a/bnf1a-/-;nf1b-/-Loss of nf1a/b[41.Shin J. et al.Zebrafish neurofibromatosis type 1 genes have redundant functions in tumorigenesis and embryonic development.Dis. Model. Mech. 2012; 5: 881-894Crossref PubMed Scopus (72) Google Scholar]rb1rb1-/-Loss of rb1[42.Solin S.L. et al.Rapid tumor induction in zebrafish by TALEN-mediated somatic inactivation of the retinoblastoma1 tumor suppressor rb1.Sci. Rep. 2015; (Published online September 8, 2015. https://doi.org/10.1038/srep13745)Crossref Scopus (27) Google Scholar,43.Schultz L. et al.Epigenetic regulators Rbbp4 and Hdac1 are overexpressed in a zebrafish model of RB1 embryonal brain tumor, and are required for neural progenitor survival and proliferation.Dis. Model. Mech. 2018; (Published online June 15, 2018. https://doi.org/10.1242/dmm.034124)Crossref PubMed Scopus (28) Google Scholar]NRASTg(sox10:mCherry-NRASWT) or Tg(sox10:mCherry-NRASQ61R)Amplification of wild-type or activated NRAS[39.Modzelewska K. et al.MEK inhibitors reverse growth of embryonal brain tumors derived from oligoneural precursor cells.Cell Rep. 2016; 17: 1255-1264Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar]NBMYCNTg(dβh:EGFP-MYCN)Amplification of EGFP-MYCN[48.Zhu S. et al.Activated ALK collaborates with MYCN in neuroblastoma pathogenesis.Cancer Cell. 2012; 21: 362-373Abstract Full Text Full Text PDF PubMed Scopus (240) Google Scholar]MYCNTg(dβh:MYCN;dβh:EGFP)Amplification of MYCN[49.Tao T. et al.The pre-rRNA processing factor DEF is rate limiting for the pathogenesis of MYCN-driven neuroblastoma.Oncogene. 2017; 36: 3852-3867Crossref PubMed Scopus (18) Google Scholar]cMYCTg(dβh:cMYC;dβh:EGFP)Amplification of cMYC[53.Zimmerman M.W. et al.MYC drives a subset of high-risk pediatric neuroblastomas and is activated through mechanisms including enhancer hijacking and focal enhancer amplification.Cancer Discov. 2018; 8: 320-335Crossref PubMed Scopus (108) Google Scholar]ERMSKRASTg(rag2:KRASG12D)Activating KRAS mutation[60.Langenau D.M. et al.Effects of RAS on the genesis of embryonal rhabdomyosarcoma.Genes Dev. 2007; 21: 1382-1395Crossref PubMed Scopus (266) Google Scholar]KRASTg(cdh15:KRASG12D)Activating KRAS mutation[61.Storer N.Y. et al.Zebrafish rhabdomyosarcoma reflects the developmental stage of oncogene expression during myogenesis.Development. 2013; 140: 3040-3050Crossref PubMed Scopus (32) Google Scholar]ARMSPAX3-FOXO1Tg(CMV-GFP-PAX3-FOXO1)PAX3-FOXO1 fusion[69.Kendall G.C. et al.PAX3-FOXO1 transgenic zebrafish models identify HES3 as a mediator of rhabdomyosarcoma tumorigenesis.eLife. 2018; (Published online June 5, 2018 https://doi.org/10.7554/eLife.33800)PubMed Google Scholar] Open table in a new tab Acute lymphoblastic leukemia (ALL) arises in T and B lymphoblasts in the bone marrow and is the most commonly diagnosed cancer in children [11.Ward E. et al.Childhood and adolescent cancer statistics, 2014.CA Cancer J. Clin. 2014; 64: 83-103Crossref PubMed Scopus (1536) Google Scholar]. Current 5-year survival rates for these children are upwards of 90%, with typical treatments involving chemotherapy and allogeneic bone marrow transplantation for high-risk or recurrent cases [11.Ward E. et al.Childhood and adolescent cancer statistics, 2014.CA Cancer J. Clin. 2014; 64: 83-103Crossref PubMed Scopus (1536) Google Scholar]. Although survival for these patients is high, there are many long-term detrimental side effects to the current treatments [11.Ward E. et al.Childhood and adolescent cancer statistics, 2014.CA Cancer J. Clin. 2014; 64: 83-103Crossref PubMed Scopus (1536) Google Scholar]. For these reasons, current efforts are focused on reducing treatment toxicity and developing targeted therapies for high-risk patients and those with recurrent disease. T-ALL accounts for 15% of ALL cases that arise in children [12.Karrman K. Johansson B. Pediatric T-cell acute lymphoblastic leukemia.Genes Chromosom. Cancer. 2017; 56: 89-116Crossref PubMed Scopus (67) Google Scholar]. T-ALL forms in the thymus from immature thymocytes that have acquired genetic or epigenetic changes and migrated to the bone marrow, peripheral blood, and lymph nodes. Genetic alterations in T-ALL patients range from translocations, gene fusions, chromosomal gains and deletions, and epigenetic abnormalities [12.Karrman K. Johansson B. Pediatric T-cell acute lymphoblastic leukemia.Genes Chromosom. Cancer. 2017; 56: 89-116Crossref PubMed Scopus (67) Google Scholar,13.Liu Y. et al.The genomic landscape of pediatric and young adult T-lineage acute lymphoblastic leukemia.Nat. Genet. 2017; 49: 1211-1218Crossref PubMed Scopus (480) Google Scholar]. Unlike most other pediatric cancers that differ significantly from adult cancers in mutational landscape, both pediatric and adult T-ALLs exhibit frequent alterations in the NOTCH, PI3K–AKT, JAK–STAT, and RAS pathways [14.Girardi T. et al.The genetics and molecular biology of T-ALL.Blood. 2017; 129: 1113-1123Crossref PubMed Scopus (195) Google Scholar]. Early zebrafish models of T-ALL recapitulated mouse and human studies and rapidly identified novel mechanisms of tumor progression and survival in this disease. The first genetically engineered zebrafish model of cancer was classified as T-ALL, in which the rag2 promoter was used to drive expression of the mouse proto-oncogene Myc in T and B lymphocytes [15.Langenau D.M. et al.Myc-induced T cell leukemia in transgenic zebrafish.Science. 2003; 299: 887-890Crossref PubMed Scopus (438) Google Scholar]. In the Tg(rag2:mMyc) transgenic model, mosaic expression of Myc induces T-ALL in 6% of animals, with a mean latency of 44 days [15.Langenau D.M. et al.Myc-induced T cell leukemia in transgenic zebrafish.Science. 2003; 299: 887-890Crossref PubMed Scopus (438) Google Scholar]. Conditional germline transgenic models of Myc-driven T-ALL have now been established that use one of three inducible approaches: (i) injecting Cre mRNA into Tg(rag2-lox-dsRED-lox-EGFP-mMyc) embryos [16.Langenau D.M. et al.Cre/lox-regulated transgenic zebrafish model with conditional myc-induced T cell acute lymphoblastic leukemia.Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 6068-6073Crossref PubMed Scopus (220) Google Scholar]; (ii) crossing Tg(rag2:LDL-EGFP-mMyc) and Tg(hsp70:Cre) fish [17.Feng H. et al.Heat-shock induction of T-cell lymphoma/leukaemia in conditional Cre/lox-regulated transgenic zebrafish.Br. J. Haematol. 2007; 138: 169-175Crossref PubMed Scopus (98) Google Scholar]; and (iii) treating Tg(rag2:hMYC-ER) fish with 4-hydroxytamoxifen [18.Gutierrez A. et al.Pten mediates Myc oncogene dependence in a conditional zebrafish model of T cell acute lymphoblastic leukemia.J. Exp. Med. 2011; 208: 1595-1603Crossref PubMed Scopus (91) Google Scholar]. As detailed later, the increased reproducibility and tumor penetrance in these refined models (80–100% tumor penetrance by 35 days of life) have allowed the identification of new genes and/or genetic pathways that promote or repress Myc-driven T-ALL initiation, progression and survival as well as the discovery of drugs that inhibit tumorigenesis in vivo. The NOTCH signaling pathway is aberrantly activated in over 60% of T-ALL cases [13.Liu Y. et al.The genomic landscape of pediatric and young adult T-lineage acute lymphoblastic leukemia.Nat. Genet. 2017; 49: 1211-1218Crossref PubMed Scopus (480) Google Scholar] and can promote T-ALL tumorigenesis through both MYC-dependent and -independent pathways [19.Sanchez-Martin M. Ferrando A. The NOTCH1-MYC highway toward T-cell acute lymphoblastic leukemia.Blood. 2017; 129: 1124-1133Crossref PubMed Scopus (140) Google Scholar,20.Blackburn J.S. et al.Notch signaling expands a pre-malignant pool of T-cell acute lymphoblastic leukemia clones without affecting leukemia-propagating cell frequency.Leukemia. 2012; 26: 2069-2078Crossref PubMed Scopus (59) Google Scholar]. Moreover, NOTCH1 and Myc collaborate during T-ALL pathogenesis in zebrafish since rag2-mediated expression of both Myc and the constitutively active intracellular domain of NOTCH1 (NICD) enhances T-ALL progression compared with expression of either Myc or NICD alone [20.Blackburn J.S. et al.Notch signaling expands a pre-malignant pool of T-cell acute lymphoblastic leukemia clones without affecting leukemia-propagating cell frequency.Leukemia. 2012; 26: 2069-2078Crossref PubMed Scopus (59) Google Scholar]. Enhanced Notch signaling does not increase the number of leukemia-propagating cells, indicating that NOTCH controls premalignant cell expansion, while Myc primarily drives clonal cell growth and survival [20.Blackburn J.S. et al.Notch signaling expands a pre-malignant pool of T-cell acute lymphoblastic leukemia clones without affecting leukemia-propagating cell frequency.Leukemia. 2012; 26: 2069-2078Crossref PubMed Scopus (59) Google Scholar]. Notch pathway mutations are also associated with activation of the Hedgehog pathway in ~16% of relapsed pediatric T-ALL cases [21.Burns M.A. et al.Hedgehog pathway mutations drive oncogenic transformation in high-risk T-cell acute lymphoblastic leukemia.Leukemia. 2018; 32: 2126-2137Crossref PubMed Scopus (41) Google Scholar]. Loss of ptch1, a negative regulator of the Hedgehog pathway, accelerates the onset of notch1-induced T-ALL in zebrafish, suggesting that activation of the Hedgehog signaling pathway cooperates with NOTCH1 to drive T‐cell transformation [21.Burns M.A. et al.Hedgehog pathway mutations drive oncogenic transformation in high-risk T-cell acute lymphoblastic leukemia.Leukemia. 2018; 32: 2126-2137Crossref PubMed Scopus (41) Google Scholar]. These zebrafish models highlight the therapeutic potential for targeting multiple developmental pathways during T-ALL disease progression. Zebrafish have been used to identify new oncogenic cofactors that drive T-ALL pathogenesis by different mechanisms. For example, the thymocyte selection-associated high mobility group box factor (TOX) was found to synergize with Myc, as well as NICD, to promote T-ALL in zebrafish by inducing genomic instability through impaired nonhomologous end-joining DNA repair [22.Lobbardi R. et al.TOX regulates growth, DNA repair, and genomic instability in T-cell acute lymphoblastic leukemia.Cancer Discov. 2017; 7: 1336-1353Crossref PubMed Scopus (40) Google Scholar]. Elevated expression of the ARID family of genes results in T-ALL tumors that have elevated myc levels, suggesting that ARID5B contributes to the overexpression of Myc during T-ALL tumorigenesis [23.Leong W.Z. et al.ARID5B as a critical downstream target of the TAL1 complex that activates the oncogenic transcriptional program and promotes T-cell leukemogenesis.Genes Dev. 2017; 31: 2343-2360Crossref PubMed Scopus (39) Google Scholar]. Aberrant lymphocyte-specific expression of JDP2, a gene that encodes a bZIP protein whose expression correlates with poor overall survival in pediatric T-ALL cases [24.Mansour M.R. et al.JDP2: An oncogenic bZIP transcription factor in T cell acute lymphoblastic leukemia.J. Exp. Med. 2018; 215: 1929-1945Crossref PubMed Scopus (18) Google Scholar], was sufficient to drive T-ALL in over 50% of zebrafish at 40 weeks post-fertilization (wpf), and caused a higher tumor penetrance when coexpressed with Myc [24.Mansour M.R. et al.JDP2: An oncogenic bZIP transcription factor in T cell acute lymphoblastic leukemia.J. Exp. Med. 2018; 215: 1929-1945Crossref PubMed Scopus (18) Google Scholar]. These studies therefore revealed new mechanisms that could potentially be targeted to disable MYC-driven T-ALL tumors. Pediatric leukemia patients have a >90% chance of survival. Yet, current treatment regimens cause systemic toxicity, and a subset of patients eventually relapse. To identify novel T-ALL therapies to potentially address this problem, a small molecule screen of 4880 FDA-approved compounds was performed using the zebrafish Tg(rag2:MYC-ER) model of T-ALL [25.Gutierrez A. et al.Phenothiazines induces PP2A-mediated apoptosis in T cell acute lymphoblastic leukemia.J. Clin. Invest. 2014; 124: 644-655Crossref PubMed Scopus (158) Google Scholar]. Perphenazine, a phenothiazine antipsychotic, was found to reduce thymic fluorescence in embryo assays, overall tumor burden in zebrafish with established MYC-induced T-ALL, and growth of human T-ALL cells in a murine xenograft model [25.Gutierrez A. et al.Phenothiazines induces PP2A-mediated apoptosis in T cell acute lymphoblastic leukemia.J. Clin. Invest. 2014; 124: 644-655Crossref PubMed Scopus (158) Google Scholar]. In Myc;Akt-driven T-ALLs, the combination of the glucocorticoid dexamethasone and an AKT inhibitor (MK2206) is highly effective against leukemia-propagating cells [26.Blackburn J.S. et al.Clonal evolution enhances leukemia-propagating cell frequency in T cell acute lymphoblastic leukemia through Akt/mTORC1 pathway activation.Cancer Cell. 2014; 25: 366-378Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar]. Using serial transplantation, MK2206 also overcomes dexamethasone resistance, a finding recapitulated in human T-ALL cells [26.Blackburn J.S. et al.Clonal evolution enhances leukemia-propagating cell frequency in T cell acute lymphoblastic leukemia through Akt/mTORC1 pathway activation.Cancer Cell. 2014; 25: 366-378Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar]. MK2206 entered a phase I clinical trial for refractory childhood malignancies and was well tolerated at tested doses, but only a limited benefit has been observed so far [27.Fouladi M. et al.A phase I trial of MK-2206 in children with refractory malignancies: a children's oncology group study.Pediatr. Blood Cancer. 2014; 61: 1246-1251Crossref PubMed Scopus (34) Google Scholar]. Expression of jdp2 has been found to confer dexamethasone resistance in zebrafish Myc-driven T-ALL models [24.Mansour M.R. et al.JDP2: An oncogenic bZIP transcription factor in T cell acute lymphoblastic leukemia.J. Exp. Med. 2018; 215: 1929-1945Crossref PubMed Scopus (18) Google Scholar]. The zebrafish data suggests that stratification of patients with activation of the AKT pathway and/or elevated JDP2 could be useful for predicting responses to therapy involving glucocorticoids. B-ALL comprises 85% of all childhood ALL cases. While B-ALL and T-ALL are morphologically indistinguishable [12.Karrman K. Johansson B. Pediatric T-cell acute lymphoblastic leukemia.Genes Chromosom. Cancer. 2017; 56: 89-116Crossref PubMed Scopus (67) Google Scholar], each disease is characterized by distinct molecular subtypes. Similar genetic and chromosomal abnormalities exist in both adult and pediatric B-ALL, including fusion genes such as BCR-ABL1 and ETV6-RUNX, MLL-rearrangements, hyperploidy, and hypoploidy, albeit at different frequencies [28.Lilljebjorn H. Fioretos T. New oncogenic subtypes in pediatric B-cell precursor acute lymphoblastic leukemia.Blood. 2017; 130: 1395-1401Crossref PubMed Scopus (54) Google Scholar]. The first model of B-ALL in zebrafish ubiquitously expressed TEL-AML1 (ETV6-RUNX) using the xenopus elongation factor 1α and the zebrafish β-actin promoters to generate B-ALL tumors with 3% penetrance [29.Sabaawy H.E. et al.TEL-AML1 transgenic zebrafish model of precursor B cell acute lymphoblastic leukemia.Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 15166-15171Crossref PubMed Scopus (145) Google Scholar]. In addition, it was recently found that a subset of animals from the Tg(rag2:hMYC) line also develop B-ALL tumors with distinct gene-expression profiles from the T-ALL tumors [30.Borga C. et al.Simultaneous B and T cell acute lymphoblastic leukemias in zebrafish driven by transgenic MYC: implications for oncogenesis and lymphopoiesis.Leukemia. 2019; 33: 333-347Crossref PubMed Scopus (22) Google Scholar], indicating that this transgenic line actually models mixed-lineage ALL. These studies emphasize the need to incorporate modern comparative oncogenomics methods to verify the exact type and/or subtype of human tumor that the zebrafish lines are modeling. AML is defined by an accumulation of immature myeloid cells that comprises at least 20% of the patient's bone marrow. The overall 5-year survival rate for AML is 64%, but rates differ significantly depending on the molecular subtype [11.Ward E. et al.Childhood and adolescent cancer statistics, 2014.CA Cancer J. Clin. 2014; 64: 83-103Crossref PubMed Scopus (1536) Google Scholar]. Unlike other liquid malignancies, some pediatric AML patients have strikingly different somatic variations than those observed in adult AML. Thus, childhood AML is uniquely typified by gene fusion events such as AML1-ETO (also called RUNX1-RUNX1T1), NUP98-NSD1, and KMT2A-MLLT3 [31.Bolouri H. et al.The molecular landscape of pediatric acute myeloid leukemia reveals recurrent structural alterations and age-specific mutational interactions.Nat. Med. 2018; 24: 103-112Crossref PubMed Scopus (355) Google Scholar]. Expression of AML1-ETO driven by the hsp70 promoter [32.Yeh J.J. et al.AML1-ETO reprograms hematopoietic cell fate by downregulating scl expression.Development. 2008; 135: 401-410Crossref PubMed Scopus (101) Google Scholar], and FLT3-ITD or FLT3-TKD driven by the CMV promoter [33.He B. et al.Functions of flt3 in zebrafish hematopoiesis and its relevance to human acute myeloid leukemia.Blood. 2014; 123: 2518-2529Crossref PubMed Scopus (42) Google Scholar], lead to myeloid cell expansion in the early zebrafish embryo. Hematopoietic defects in AML1-ETO embryos could be reversed using the COX2 inhibitor nimesulide to block a β-catenin-mediated increase in myelopoiesis [34.Yeh J.J. et al.Discovering chemical modifiers of oncogene-regulated hematopoietic differentiation.Nat. Chem. Biol. 2009; 5: 236-243Crossref PubMed Scopus (132) Google Scholar]. Additionally, myeloid expansion in FLT3-ITD, but not FLT3-TKD, animals could be abrogated by treatment with a tyrosine kinase inhibitor (AC220), suggesting the existence of different therapeutic opportunities for seemingly similar genetic aberrations [33.He B. et al.Functions of flt3 in zebrafish hematopoiesis and its relevance to human acute myeloid leukemia.Blood. 2014; 123: 2518-2529Crossref PubMed Scopus (42) Google Scholar]. These studies illustrate the utility of the zebrafish embryo for understanding basic mechanisms of oncogenic fusion genes in hematopoiesis and for the discovery of new therapies for pediatric leukemia. Brain and CNS tumors are the leading cause of pediatric-cancer-related deaths [35.Curtin S.C. et al.Declines in cancer death rates among children and adolescents in the United States, 1999-2014.NCHS Data Brief. 2016; 257: 1-8Google Scholar]. Approximately 75% of pediatric brain tumors are malignant and have a collective 5-year survival rate of 78% [36.Siegel R.L. et al.Cancer statistics, 2018.CA Cancer J. Clin. 2018; 68: 7-30Crossref PubMed Scopus (6500) Google Scholar]. Moreover, surgical debulking of the tumor, radiation, and general chemotherapy cause long-term complications on a developing brain [11.Ward E. et al.Childhood and adolescent cancer statistics, 2014.CA Cancer J. Clin. 2014; 64: 83-103Crossref PubMed Scopus (1536) Google Scholar]. Fortunately, significant progress in the genomic characterization of pediatric brain tumors has identified specific brain tumor entities with distinct molecular profiles, allowing for more accurate classification of tumors with recurrent genomic features. The challenge now is to generate cell- and animal-based models of the new brain tumor entities for gene and drug discovery. CNS-PNETs typically arise in the cerebrum and account for 3–5% of pediatric brain tumors [37.Picard D. et al.Integrative genomic analyses identify LIN28 and OLIG2 as markers of survival and metastatic potential in childhood central nervous system primitive neuro-ectodermal brain tumours.Lancet Oncol. 2012; 13: 838-848Abstract Full Text Full Text PDF PubMed Scopus (129) Google Scholar,38.Sturm D. et al.New brain tumor entities emerge from molecular classification of CNS-PNETs.Cell. 2016; 164: 1060-1072Abstract Full Text Full Text PDF PubMed Scopus (527) Google Scholar]. One subgroup of CNS-PNET, called CNS NB-FOXR2, expresses transcription factors critical for oligodendrocyte precursor cells (OPCs) and activation of the RAS/MAPK pathway [39.Modzelewska K. et al.MEK inhibitors reverse growth of embr