Zebrafish MYC-induced leukemia models: unique in vivo systems to study B and T cell acute lymphoblastic leukemia

Over the past two decades, zebrafish (Danio rerio) has become an increasingly powerful in vivo oncology model system. Technical and practical advantages, such as their low cost, high fecundity and optical translucency, together with the ease of building transgenic zebrafish, all make D. rerio excellent for studying pathways altered in cancer and also facilitate their use in high-throughput anticarcinogenic chemical library/drug screens. In particular, both genetically and functionally, oncogenic and developmental hematopoietic pathways are highly conserved between teleost (bony) fish and mammals. These features have allowed the creation of zebrafish models mimicking several human hematopoietic malignancies, such as acute myeloid leukemia (AML) and T-cell lymphoblastic lymphoma (T-LBL) and acute lymphoblastic leukemia (T-ALL) [1,2]. Surprisingly, zebrafish B cell malignancy models lagged behind, with only a single 2006 report of pre-B-cell ALL (pre-B ALL) in a transgenic line expressing human ETV6-RUNX1 [3]. No subsequent publications using this line exist, likely because it exhibited low incidence and long latency, making it challenging to study. Recently, the scarcity of zebrafish B-cell leukemia models was addressed by the related but distinct discoveries [4,5] that B-ALL occurs in fish expressing either transgenic murine or human c-MYC (mMyc and hMYC, respectively), controlled by a zebrafish rag2 promoter, which is active in immature B and T lymphoblasts. Previously, these lines were thought to exclusively develop T-ALL [6,7], although a logical explanation for this exclusivity was lacking. As noted, in zebrafish and in mammals, rag2 is expressed by both B and T cell progenitors and MYC is known to drive B cell cancers like Burkitt lymphoma and B-ALL [8,9]. These recent papers proved that mMyc/hMYC-induced B-ALL occurs, but was apparently overlooked until now. In the paper by Garcia et al. [4], RNA-seq analysis of a cohort of 12 rag2:mMyc-induced monoclonal tumors [10] revealed the presence of B-ALL. Specifically, the authors found nine T-ALL, two pro-B ALL and one bi-phenotypic ALL that expressed both T and B lineage markers. In contrast, Borga et al. [5] used double-transgenic rag2:hMYC;lck:GFP fish, where differential lck expression drives distinct levels of GFP in Band Tcells, with B cells expressing less GFP than T cells. This study reported data from >50 hMYC-driven ALL, proving the existence of highly penetrant B-ALL that resembled human pre-B-ALL. Of note, the authors also found several instances of ’mixed’ ALL that was not the bi-phenotypic ALL described by Garcia et al., but rather simultaneous cases of Band T-ALL arising in the same animal. Notably, mMyc/hMYC zebrafish are the first animal models described to develop both T-ALL and B-ALL. This highlights MYC’s potency as an oncogene in both lymphocyte lineages, but MYC may use distinct molecular mechanisms and activate unique oncogenic pathways in Tversus B-ALL; these models provide systems to explore these differences. Of note, although c-MYC has pathogenic roles in many types of cancer, MYC-mediated molecular mechanisms of tumorigenesis are not fully understood and likely differ by malignancy type. For example, it is known that overexpressing MYC is not sufficient for tumorigenesis, with varying cooperative genetic lesions needed to