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).