6.4 NOTCH1 transgenic models
NOTCH1 was first identified in the human T cell ALL-bearing translocation, t(7;9)(q34;q34.3). Activation of the NOTCH1 signaling pathway is the most important tumorigenic event in the pathogenesis of T-ALL(Weng et al., 2004), and this gene is related to the pathogenesis of most T-cell ALL types.
Researchers established a mouse bone marrow transplant (BMT) model in which Cre recombinase induces HSCs Notch activation, thereby inducing T-ALL in mice. The activation of Notch in HSCs leads to the expansion of hematopoietic progenitor cells and T cell precursor cells, and then the production of HSCs and T cells is gradually lost. The over-activated Notch signal in HSCs promoted the leukemia stem cell (LSC) activity of T cell precursor cells and inhibited the self-renewal of HSCs. This conditional transgenic model provides further evidence for T-ALL targeting, and also emphasizes the necessity of strictly regulate Notch signaling to expand the normal HSC population for clinical applications(Chiang, Shestova, Xu, Aster & Pear, 2013). Later, the CD4 promoter was used to express NOTCH1 mutants in Tg mice, including Notch1IC and Notch1EC produced by type I mutations, and Notch1ΔCT produced by type II mutations, double (Notch1IC× myc) or (Notch1ΔCT×myc) Tg mice developed thymoma with a much shorter latency than single Tg mice, providing genetic evidence of a collaboration betweenthese two oncogenes. These Notch1 mutations are very similar to some forms of human T-cell leukemia, and these Tg mice may represent relevant models of these human leukemias(Priceputu et al., 2006). The NOTCH Tg model also plays an important role in the evaluation of drug efficacy. Researchers investigated the drug sensitivity of T-ALL cells in Notch1-induced CD8+CD4+ and CD8+CD4- cell subsets in T-ALL mice and examined the sensitivity of CD8+CD4+ and CD8+CD4- to cyclophosphamide and the mTORC1 inhibitor rapamycin. They found that the combined use of cyclophosphamide and rapamycin reduced the number of leukemia cells and prolonged the lifespan of T-ALL mice compared with the use of drugs alone. This indicates that the combination of cyclophosphamide and rapamycin is expected to improve the clinical treatment of NOTCH-induced T-ALL(Zhang et al., 2013).
7. Non-mouse Models
Insights into the pathophysiology of leukemia have been obtained in non-mouse models. These different animal models help us to understand the underlying mechanisms of leukemia treatment resistance and disease relapse, and are expected to improve treatment strategies.
Zebrafish’s conservative hematopoietic system and unique experimental advantages, combined with the development of transgenic, genome editing and xenotransplantation technologies, reveal the details of leukemogenesis, progression and regression(Harrison, Laroche, Gutierrez & Feng, 2016). Zebrafish has become an attractive model for research on drug screening for hematological malignancies(Idilli, Precazzini, Mione & Anelli, 2017). The first zebrafish T-ALL model was created in 2003 by fusing the mouse c-Myc gene with the Enhanced Green Fluorescence Protein (EGFP) gene, and the lymphocyte-specific promoter rag2 drove the expression of the EGFP-mMyc fusion gene, resulting in T-ALL in zebrafish(Langenau et al., 2003). Later, the researchers modified the model and induced it with 4-hydroxy tamoxifen, and the Myc gene was activated, causing T-ALL in zebrafish(Gutierrez et al., 2011; Langenau, Feng, Berghmans, Kanki, Kutok & Look, 2005). In addition to the T-ALL model induced by Myc gene, researchers also successfully established a zebrafish T-ALL model by overexpressing the human NOTCH1 gene under the rag2 promoter(Sabaawy, Azuma, Embree, Tsai, Starost & Hickstein, 2006). Zebrafish are rapidly expanding our understanding of disease mechanisms and helping to develop treatment strategies to improve the prognosis of patients with leukemia. Based on the progress of zebrafish in high-throughput drug screening, it is expected to accelerate the development of novel leukemia therapeutic medications.
Rats are sometimes used when establishing carcinogens and radiation induced leukemia models(Huggins, Grand & Ueda, 1982; Svejda, Kossey, Hlavayova & Svec, 1958). It is reported that repeated injection of DMBA into Wistar rats, which induced 10% of rat leukemia in 5-9 months(Huggins & Sugiyama, 1966). A subline of inbred Sprague-Dawley rats has a high incidence of T cell malignancies and has been used for preclinical evaluation of treatment(Bua et al., 2018; Otová, Sladká, Panczak & Marinov, 1997). Drosophila has a strong hematopoietic capacity, and its genetic simplicity and ease of manipulation make it an attractive model for studying leukemia caused by complex chromosomal translocations. In order to study the detailed molecular mechanism of leukemia caused by the AML1-ETO chimeric protein, two AML1-ETO drosophila models were established using genetic engineering technology(Lebestky, Chang, Hartenstein & Banerjee, 2000). Later, researchers found in Notch-induced drosophila tumor models that Notch signaling and PI3K-AKT pathway have a synergistic effect in tumorigenesis. The transcriptional control of PTEN and the regulation of the PI3K-AKT pathway are key elements of the leukemia progress activated by NOTCH1. These findings provide the basis for the design of new T-ALL treatment strategies(Palomero et al., 2007).