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Ajbr1105005www.AJBlood.us /ISSN: 2160-1992/AJBR1105005 Review Article Mouse models as tools to understand and study BCR-ABL1 diseases Steffen Koschmieder, Mirle Schemionek Medizinische Klinik A, Universitätsklinikum Münster, Münster, Germany. Received May 16, 2011; accepted June 3, 2011; Epub June 7, 2011; published June 15, 2011 Abstract: Mouse models of human malignancy have greatly enhanced our understanding of disease pathophysiology and have led to novel therapeutic approaches, some with extraordinary success, one such example being inhibition of the BCR-ABL1 oncogene in chronic myeloid leukaemia (CML). Here, we review aspects of the biology of CML that have been uncovered at least in part through the generation and analysis of retroviral and transgenic mouse models of BCR-ABL1 disease. It can be expected that these models will also serve as important tools in the future, especially in the rational design of strategies to eradicate leukemic stem cells and potentially cure CML as well as other can-cers. Keywords: BCR-ABL1, mouse models, retroviral, transgenic. leukemic stem cells, hematopoietic stem cells CD34+CD38- cells which are highly enriched in stem cells persist in vitro and in vivo despite Chronic myeloid leukemia (CML) is a malignant kinase inhibitor treatment [13, 14], possibly disorder of hematopoietic stem cells (HSC) [1, explaining the fact that CML frequently relapses 2]. During chronic phase, proliferation and sur- in patients after discontinuation of imatinib vival of HSC and their progeny are enhanced, treatment [15-19]. The reason for the inherent and this is primarily caused by deregulated tyro- resistance of CML stem cells to kinase inhibi- sine kinase signalling. If untreated, the disease tors is not known. In fact, while the effects of progresses to accelerated phase and eventually BCR-ABL1 in more mature progenitor and hema- to a fatal blastic phase which is characterized topoietic precursor cells have been studied ex- by a block in differentiation and accumulation of tensively, the effects of BCR-ABL1 in the HSC immature hematopoietic cells due to inactiva- population are still incompletely understood. tion of important tumor suppressors and mye- Possibly, the cellular context that allows self- loid transcription factors . Preclinical re- renewal in HSC allows these cells to respond search over the past decades has clearly dem- differently to transformation by BCR-ABL1 than onstrated that BCR-ABL1 is the major cause of the cellular composition of the progenitor cell the disease [4-6], and this work has led to the population which lacks self-renewing capacities. development of ABL kinase inhibitors that have Also, the so-called “stem cell niche” in the bone revolutionized CML treatment [7-10] and have marrow may protect stem cells from the effects led to an almost twofold increase in CML preva- of cytostatic agents. This niche is critical for the control of stem cell adherence to stromal cells as well as their migration and egression from Biology of stem cells during chronic phase CML the bone marrow, all of which are critical factors that determine whether stem cells can cause Inhibition of ABL kinase by tyrosine kinase in- overt leukemia or not. Studies of the genes in- hibitors have resulted in impressive rates of volved in CML stem cell migration, transforma- long-term complete cytogenetic remission [8, tion, and homing as well as disease progression 12]. However, BCR-ABL1 positive quiescent are critical in understanding these processes. Mouse models for study of BCR-ABL1 disease expressing the oncogene. In this review, we will focus on retroviral and transgenic mouse mod- While tyrosine kinase inhibitor (TKI) treatment has improved the treatment of patients with chronic-phase CML dramatically, only a fraction Retroviral transduction experiments have identi- of patients with accelerated or blastic phase fied critical requirements for the generation of CML respond to TKIs sufficiently to allow long- leukemia in the recipients. It was shown that term survival following stem cell transplantation the transforming activity of BCR-ABL1 results [9, 20]. A variety of cellular and genetic altera- from deregulated constitutive tyrosine kinase tions has been described in cells from patients activity of the fusion protein and these experi- with accelerated phase and blast crisis, includ- ments have identified regions within the fusion ing large genomic changes (i.e. +8, +Ph, +19, protein which are essential for transformation and i(17)q, del 2, del 5, and del 7 [21, 22]) and ([4, 31, 32] and others, as reviewed in ). gene mutations leading to disrupted differentia- tion and tumor suppressor pathways (i.e. ABL1 is able to transform bone marrow-derived CEBPA, PP2A, p53, p16, and Rb [23-28]). At hematopoietic cells which, upon transplanta- what stage during development of CML these tion, induce hematopoietic tumors in recipient alterations take place and in which cell popula- mice. Thus, together with the initial description tion they occur, is still unknown. Since CML of the “minute chromosome” in chronic granulo- blast crisis cells are generally but not always cytic leukemia by Nowell and Hungerford in BCR-ABL1 positive, clonal evolution of both BCR 1960 , the description of the “Philadelphia -ABL1 positive and BCR-ABL1 negative cells has chromosome” by Rowley in 1973 , and the been discussed. According to these hypotheses, cloning of the BCR-ABL1 fusion gene by Shtivel- genetic changes occur in a subset of cells dur- man et al in 1985 , these experiments pro- ing chronic phase CML, conferring a growth ad- vided the basis for our current understanding of vantage to these cells which can then outgrow BCR-ABL1 oncogenic activity, and they have the rest of the clones and contribute to the pro- been critical for the rational design and develop- gression from chronic phase to blast crisis . ment of tyrosine kinase inhibitors which repre- Genetic changes may be promoted through BCR sent the current standard of care in patients -ABL1 effects on genetic stability and survival . Despite the therapeutic success of imatinib treatment, formation of resistance to Three major BCR-ABL1 fusion proteins have tyrosine kinase inhibitors and the inherent in- been described in patients (p185, p210, and sensitivity of CML and Ph+ ALL stem cells are p230), and these are associated with three dif- still problematic and make the goal of prevent- ferent clinical phenotypes of BCR-ABL1 disease ing the progression to blast crisis more difficult. (acute lymphoblastic leukemia, CML, and CML Studies of genes involved in the progression to characterized by slower progression kinetics, acute phase CML are therefore critical in under- respectively). In vivo experiments in mice standing the course of CML disease and improv- showed that all three BCR-ABL1 translocation products (p185, p210, and p230) were able to transform 5-FU enriched bone marrow cells and Retroviral mouse models of BCR-ABL1 disease cause a similar phenotype (CML-like disease) in recipient mice [37-39]. Interestingly, these ex- Murine models of CML have not only greatly periments demonstrated that the leukemic phe- enhanced our understanding of leukemogene- notype was influenced by the type of condition- sis  but also of physiologic human hemato- ing regimen of the donor mice (pure CML with 5- poiesis and have been indispensable for pre- FU treated vs. mixed phenotypes with untreated clinical drug testing of BCR-ABL1 inhibitors. Sev- donor cells), suggesting that the type of target eral technical approaches were used to gener- cell in which the oncogenic fusion protein was ate mouse models of CML-like disease: injection expressed is more relevant in determining the of cell lines or primary cells from CML patients disease phenotype than the type of BCR-ABL1 into recipient mice, transduction of bone mar- fusion protein (p185, p210, or p230) . row-derived cells with retroviral vectors that Major progress in identifying prospectively iso- transplantation into lethally irradiated congenic lated hematopoietic stem cell (HSC) and pro- recipient mice or generation of transgenic mice genitor populations has been made by the use Mouse models for study of BCR-ABL1 disease of a set of defined surface markers in combina- els are the variability of BCR-ABL1 expression tion with high-speed cell sorting techniques and disease phenotype between recipients and which allowed for the efficient sorting of rare the relatively rapid onset and fatal outcome of cell populations [40-42]. Using these tech- the disease soon after transplantation, which niques, the characteristics of murine long-term may hamper the analysis of the disease during and short-term HSC (LT- and ST-HSC) and their chronic phase. Moreover, since a transplanta- progeny (CMP, CLP, GMP, and MEP) have now tion step is required for this method, this prohib- been analyzed both under physiologic condi- its the study of BCR-ABL1 disease under steady- tions as well as in selected models of leukemia. These experiments have led to the discovery of stem-cell specific and progenitor-specific gene Therefore, researchers have generated trans- expression profiles . Moreover, these experi- genic mouse models, and the data gathered ments have corroborated the unique ability of from these mice complement and extend the HSC to both self-renew and undergo differentia- results obtained with the retroviral models. tion into more mature cell populations and dem- While transgenic approaches are inherently onstrated that CMP, CLP, MEP, and GMP have time-consuming due to founder selection as limited self-renewing capacities [40, 44, 45]. well as breeding and genotyping procedures, Prospective isolation of HSC and progenitors they offer highly-reproducible expression among has greatly facilitated the targeting of specific transgenic offspring, versatile matings with dif- HSC and progenitor populations. Using these ferent other transgenic mouse strains including techniques, it was shown that BCR-ABL1 causes gene knockout strains, and analysis of leukemic transplantable disease when using whole bone phenotypes under steady-state conditions. marrow as a source but not when using bone marrow-derived CMP or GMP cells . Another Transgenic animals carry the exogenous gene in group reported that transduction of purified Lin- every cell, but expression is restricted by the Sca-1+c-kit+ (LSK) cells which contain the HSC use of cell type specific promoter/enhancer population with the BCR-ABL1 oncogene is suffi- constructs. In the past, non-conditional and con- cient to induce CML-like disease in mice . These data suggest that BCR-ABL1 exerts its conditional models have utilized the metal- effects in the HSC compartment, in agreement lothionein (MT) promoter , the Tec promoter with early studies of human CML that demon- , and the MRP8 promoter  among oth- strated the Philadelphia chromosome in several ers. The hematopoietic neoplasms detected in hematopoietic lineages including granulocytes MT p210BCR-ABL1 mice showed an exclusively and eryhroid cells  and confirming the notion T-lymphoid phenotype in contrast to patients, that chronic phase-CML is a stem cell disease. where p210 is almost exclusively associated This is in contrast to acute myeloid leukemia with chronic myeloid disease . While tec- (AML) where transduction of both unfraction- p210-BCR-ABL1 mice did develop CML-like dis- ated bone marrow containing HSC and FACS- ease in the second generation, this model does purified progenitor cell populations are able to not focus on targeting the HSC compartment induce acute leukemia in mice [44, 47, 48]. lower disease penetrance (4 to 31%) and, in Finally, retroviral mouse models have allowed addition, a highly variable onset of disease (3 to the functional analysis of individual genes in 10 months) . One of the major problems of vivo by transduction of transgenic cells with a non-conditional transgenic mouse models is targeted disruption of genes such as STAT5 [49, that the BCR-ABL1 oncogene is expressed con- 50] and p53  and others. These experi- tinuously throughout life, including embryogene- ments showed that STAT5 is indispensable for sis. Early studies had demonstrated that expres- BCR-ABL1 mediated leukemogenesis [49, 50] sion of the fusion gene is detrimental causing and that the proapoptotic function of p53 is intra-uterine lethality or selection for animals required for BCR-ABL1 positive cells to undergo with low levels of expression . Embryonic lethality has also complicated several knockout mouse models where non-conditional targeted Transgenic mouse models of BCR-ABL1 disease gene disruption has resulted in embryonic le- thality. A possible solution to this problem is the The major drawbacks of retroviral mouse mod- use of conditional promoter/enhancer con- Mouse models for study of BCR-ABL1 disease structs which allow induction of gene expression that expression decreases as the cells maturate , SCLtTA/BCR-ABL1 mice were generated in order to express BCR-ABL1 in these cell popula- In order to model p230-induced BCR-ABL1 dis- tion and mimic human CML . Expression of ease, non-conditional transgenic mice were gen- tTA mRNA was confirmed in FACS-sorted hema- erated . These mice showed a late-onset topoietic stem cells (HSC), common myeloid mild neutrophilia and progressive thrombocyto- progenitors (CMP), and common lymphoid pro- sis as well as signs of a myeloproliferative neo- genitors (CLP) but was very low or negative in plasm (MPN). However, only a fraction of these granulocyte-macrophage progenitors (GMP) and mice succumbed to the disease. Thus, the phe- megakaryocyte-erythrocyte progenitors (MEP). notype of these mice does mimic the clinical After induction of BCR-ABL1 expression, all dou- characteristics of patients with p230 BCR-ABL1- ble-transgenic mice developed neutrophilia and leucocytosis reminiscent of chronic-phase CML, the clinical condition of the mice deteriorated, The development of binary expression systems and the mice died within 29 to 122 days. Upon using two separate strains of mice (a transacti- autopsy, splenomegaly was found in all mice, vator and a transresponder strain) has greatly and histological analysis demonstrated granulo- improved the generation of inducible transgenic cytic hyperplasia of the bone marrow and ex- mouse models and provides the means to pre- tramedullary organs. CML-like disease was re- vent oncogene expression during embryogene- peatedly reversible upon re-administration of sis . Several “driver” transgenic mouse lines tetracycline, suggesting that the disease re- have been generated using the tTA gene under the control of the MMTV-LTR , human CD34 expression. Further experiments demonstrated genomic locus , and the murine stem cell that CP-CML was transplantable using bone leukemia (SCL) gene 3´ enhancer . These marrow cell fractions highly enriched in HSC mice were crossbred with mice expressing p210 and that this population was necessary and BCR-ABL1 under the control of the tetracycline sufficient to induce CML-like disease in synge- responsive element (TRE) . Similar to previ- neic transplant recipient mice . In addition, ous retroviral and transgenic mouse models the experiments revealed that the phenotype using p190 BCR-ABL1 [5, 37, 39, 51, 61, 62], was re-inducible after complete abrogation of BCR-ABL1 expression, suggesting that the leu- developed acute pre-B cell leukemia (ALL) kemic stem cell population was not oncogene- within three weeks after induction of BCR-ABL1 addicted and persisted despite the absence of expression by removal of tetracycline from the BCR-ABL1. Moreover, imatinib was unable to drinking water . This binary system was eradicate the disease in the mice. These results highly reliable with 100% of animals developing are in keeping with data from retroviral mouse the phenotype. Re-administration of tetracycline models which have also shown that imatinib is led to abrogation of BCR-ABL1 expression and unable to eradicate BCR-ABL1 positive leukemic complete reversion of the leukemia, suggesting stem cells . Insensitivity of very immature that continued BCR-ABL1 expression is required hematopoietic cells to imatinib and other TKIs for maintenance of the disease. When the en- has been shown in patients [14, 65-67]. More- tire human CD34 locus was used to direct ex- over, clinical data confirmed that most CML pression of BCR-ABL1 to more immature pro- patients that have discontinued imatinib ther- genitors and HSC, induction of these mice led to apy relapse within a few months after stopping an MPN with predominant involvement of the imatinib , again suggesting that imatinib megakaryocytic lineage . The disease la- does not lead to eradication of the leukemic tency in this model was longer than that of the stem cell population in the majority of patients. pre-B ALL, and this may be due to different ex- pression levels in the targeted cells . These Another tetracycline-responsive transgenic mice provided further evidence for a critical role mouse model was generated using a vector of the cell type expressing BCR-ABL1 in deter- expressing both tTA driven by the CMV promoter mining the disease phenotype. Since a fragment and p190 BCR-ABL1 under the control of the of the 3' enhancer of the murine SCL gene is tetracycline-responsive element . Two trans- sufficient to direct expression of exogenous genic founder lines were established which transgenes to HSC and myeloid progenitors and showed tetracycline-regulated expression of Mouse models for study of BCR-ABL1 disease p190 BCR-ABL1 transcripts in the peripheral scribed in Spa1-/- mice with CML-like disease blood (PB), bone marrow (BM), and spleen. After , and increased numbers of HSC were found a latency of 5-11 months, these animals devel- in an AML1-ETO retroviral transplant model oped hepatosplenomegaly, and the authors . However, LSK cell expansion may not be reported a B-lineage ALL phenotype, with cells required for the development of more acute from the PB, BM, and spleen co-expressing early leukemias since LSK cells were not expanded in B-cell and myeloid markers. Treatment of the MRP8/BCR-ABL1/bcl2-transgenic mice  or mice with imatinib did not alter the course of even decreased in the bone marrow [44, 73] the disease, and the mice died within 15 weeks while GMPs were increased and exerted abnor- of tetracycline withdrawal. When tetracycline mal self-renewal [44, 73, 74]. It would be of was re-administered to diseased animals, BCR- interest to obtain more information about LSK ABL1 expression was no longer detected. How- and progenitor populations in other existing ever, the animals did not get better, and the MPN mouse models such as retroviral trans- phenotype was enhanced with decreasing BCR- plant models expressing BCR-ABL1 [46, 75], ABL1 expression. Together with the late onset FLT3-ITD  or transgenic mice expressing K- and 15-week progression of the disease which Ras  to understand the role of GMP self- are unexpected for an ALL, this suggests that renewal and LSK cell expansion in acute and secondary events in addition to BCR-ABL1 ex- chronic leukemias. It is yet not clear why com- mon myeloid progenitors (CMP) in mouse mod- els of myeloproliferative disease are not ex- In order to achieve BCR-ABL1 expression exclu- panded to the same extent as LSK and GMP sively in the HSC compartment, a transgenic populations [60, 70]. One explanation may be a mouse model was generated, expressing p210 rapid transition through the CMP to the GMP BCR-ABL1 under the control of the Sca-1 pro- stage in these mice and subsequent slower dif- moter . After a latency of 4-12 months, ferentiation of GMP into their more mature prog- these mice developed leucocytosis, neutro- eny. Another explanation would be that the leu- philia, and evidence of extramedullary disease, kemic stem cells in these mice are programmed and 70% of the mice progressed to an acute leukemia characterized by the appearance of stage. To date, the transition kinetics from HSC myeloid or lymphoid blasts in the PB, BM, via CMP to GMP under physiologic conditions spleen and liver. In addition, a significant por- are not known, and experiments to test either of tion of the mice developed solid tumors (10% these possibilities need to be carried out. Inter- lung cancer, 4% sarcoma, 3% liver cancer, 2% estingly, two recent transgenic mouse models Sertoli cell tumor) which was attributed to the expressing the JAK2 V617F mutation have re- expression of BCR-ABL1 in Sca-1 positive non- hematopoietic cells. The leukemia was trans- erythrocyte progenitor (MEP) compartment in plantable into secondary recipients and was the bone marrow [78, 79], and one of the mod- unresponsive to imatinib treatment. However, els also showed an increase of the LSK cell the disease was at least in part dependent on compartment , suggesting that, like CML, BCR-ABL1 expression, as demonstrated by an these MPN may be stem cell-derived malignan- cies. However, in a third transgenic model that ABL1p210 transgenic mice that were treated with ganciclovir to eradicate BCR-ABL1 positive sizes, neither LSK nor MEP cell pools were in- LSK and/or GMP cell expansion in murine MPN HSC and progenitor populations in human CML Jamieson et al. analyzed specific subpopula- macrophage progenitor (GMP) cell pool may be tions of hematopoietic stem and progenitor cells a common pathogenetic event of murine MPN. of patients with CML at different stages of the This phenomenon has been described in vari- disease . They found that the percentage of ous MPN mouse models, including junB-/-ubi junB CD34+CD38-CD90+Lin- cells in the bone mar- mice and SCLtTA/BCR-ABL1 mice [60, 64, 70] row, which are highly enriched in stem cells, which develop CML-like disease. In addition, was not significantly different in healthy donors expansion of LSK and progenitor cells was de- or patients irrespective of the disease stage. Mouse models for study of BCR-ABL1 disease Table 1. Potential future applications of mouse models of BCR-ABL1 disease Application Study disease pathogenesis and characterize cancer Inducible stem-cell specific oncogene and targeted gene stem cells to better understand leukemias and disruption in the same cell to identify critical target genes solid tumors Investigate development of resistance during TKI therapy in mouse models Test therapies targeting leukemic stem cells Current strategies include combination of TKIs and inter-feron alpha, sonic hedgehog signalling inhibitors, PP2A activators, or immunotherapies Develop novel transplantation approaches Inducible Assessment of the functional consequences of onco- expression of oncogenes in different hematopoi- genes at the HSC level and exploitation in the posttrans- Investigate mechanism of transition from chronic Loss-of-function and gain-of-function modification of exist- ing mouse models (i.e. crossbreeding with tumor suppres- However, the percentage of the MEP population cell is critical in determining the disease pheno- was increased in chronic phase-CML but de- type, and although the promoter and enhancer creased in blast crisis, while the CMP popula- constructs used may be similar to the ones driv- tion was increased in accelerated phase-CML ing expression of BCR-ABL1 and other onco- but largely unchanged in chronic-phase and genes in humans, differences of expression blast crisis. The GMP population was decreased between mouse and man are still very likely. in chronic and accelerated phase-CML but in- Last but not least, the situation of transgenic creased in blast crisis. This population also mice where multiple clones start to express BCR showed an increase of self-renewal during blast -ABL1 at the same time is probably different crisis, possibly caused by an increased expres- from the setting of human CML where the dis- ease is thought to arise from a few clones ex- These results show some discrepancies be- tween the human disease and murine models of CML. However, several points need to be con- sidered. Firstly, the markers used for stem and In spite of obvious differences between mouse progenitor cell isolation are not identical in hu- models and human disease, mouse models of leukemia have been essential for the under- have different expression patterns in mouse standing of leukemogenesis, the development and man , making direct comparisons diffi- of specific molecular treatment approaches, cult. Secondly, the percentage of MEP, CMP, and preclinical testing of these drugs in vivo. and GMP under healthy conditions is different in More information on HSC and progenitor com- humans and mice . Specifically, the ratio of partments in humans is rapidly evolving [83, 84].It can thus be expected that the stem-cell found to be 2.0 but only 0.75 in humans, while specific mouse models which are currently be- the ratio of MEP/CMP was essentially the same ing developed will be integral parts of stem-cell (0.5 and 0.45, respectively) . These ratios directed treatment strategies to improve long- also show that the percentage of cells that are term survival of patients with acute and chronic neither MEP, CMP, nor GMP differs between human and murine bone marrow, although the nature of these cells has not been defined. Thirdly, as has been shown for mice by the use of inducible disease models, the type of target Mouse models for study of BCR-ABL1 disease Innovative Medizinische Forschung an der  Kantarjian H, Giles F, Wunderle L, Bhalla K, Mediznischen Fakultät Münster KO 1 2 08 19. O'Brien S, Wassmann B, Tanaka C, Manley P, Deutsche José Carreras-Stiftung DJCLS R Rae P, Mietlowski W, Bochinski K, Hochhaus A, Griffin JD, Hoelzer D, Albitar M, Dugan M, Cor-tes J, Alland L and Ottmann OG. Nilotinib in imatinib-resistant CML and Philadelphia chro- mosome-positive ALL. 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STATE OF THE EVIDENCE What is the Connection Between Chemicals and Breast Cancer? Breast cancer now strikes more women than any other type of cancer, the result of adramatic increase that has taken place in the industrialized world during the past halfcentury. Whereas one in 22 women were diagnosed with breast cancer in the 1940s, nowthe incidence is one in eight. Of all women who die of