Animal models of BCR/ABL-induced leukemias

'One medicine---one pathology': are veterinary and human pathology prepared?

The American Medical Association and the American Veterinary Medical Association have recently approved resolutions supporting 'One Medicine' or 'One Health' that bridge the two professions. The concept is far from novel. Rudolf Virchow, the Father of Modern Pathology, and Sir William Osler, the Father of Modern Medicine, were outspoken advocates of the concept. The concept in its modern iteration was re-articulated in the 1984 edition of Calvin Schwabe's 'Veterinary Medicine and Human Health.' The veterinary and medical pathology professions are steeped in a rich history of 'One Medicine,' but they have paradoxically parted ways, leaving the discipline of pathology poorly positioned to contribute to contemporary science. The time has come for not only scientists but also all pathologists to recognize the value in comparative pathology, the consequences of ignoring the opportunity and, most importantly, the necessity of preparing future generations to meet the challenge inherent in the renewed momentum for 'One Medicine.' The impending glut of new genetically engineered mice creates an urgent need for prepared investigators and pathologists.

Model mice for BCR/ABL-positive leukemias

p210bcr/abl is detected in almost all chronic myelogenous leukemia (CML) patients and a significant number of acute lymphoblastic leukemia (ALL) cases. It is generated by a reciprocal chromosomal translocation, t(9;22) (q34;q11), and the enhanced kinase activity of the protein is believed to be implicated in the pathogenesis of the diseases. To examine its oncogenicity in vivo and to create an animal model for BCR/ABL-positive leukemias, we generated transgenic mice expressing p210bcr/abl driven by the promoter of the mouse tec gene, a cytoplasmic tyrosine kinase preferentially expressed in early hematopoietic progenitors. While the founder mice showed excessive proliferation of lymphoblasts shortly after birth and were diagnosed as ALL, the transgenic progeny reproducibly exhibited marked granulocyte hyperplasia with thrombocytosis after a long latent period, which closely resembles the clinical course of human CML. In addition, to investigate whether loss of p53 would play a role in the transition from chronic phase to blast crisis of CML, we crossmated p210bcr/abl transgenic (BCR/ABLtg/-) mice with p53 heterozygous (p53+/-) mice and generated p210bcr/abl transgenic, p53 heterozygous (BCR/ABLtg/- p53+/-) mice, in which a somatic alteration in the residual p53 allele directly abrogates p53 function. The BCR/ABLtg/- p53+/- mice exhibited rapid proliferation of blast cells and died in a short period compared with their wild-type (BCR/ABL-/- p53+/+), p53 heterozygous (BCR/ABL-/- p53+/-), and p210bcr/abl transgenic (BCR/ABLtg/- p53+/+) littermates. Interestingly, the normal p53 allele was frequently and preferentially lost in the tumor tissues, providing in vivo evidence that acquired loss of p53 contributes to the blastic transformation of p210bcr/abl-expressing hematopoietic cells. Our transgenic mice will be a useful model for investigating oncogenic properties of p210bcr/abl in vivo and will provide insights into the molecular mechanism(s) underlying the progression from chronic phase to blast crisis of CML.

Expression of bcr-abl mRNA in individual chronic myelogenous leukaemia cells as determined by in situ amplification

We present the results of a novel method developed for evaluation of in situ amplification, a molecular genetic method at the cellular level. Reverse transcription polymerase chain reaction (RT-PCR) was used to study bcr-abl transcript levels in individual cells from patients with chronic myelogenous leukaemia (CML). After hybridizing a fluorochrome-labelled probe to the cell-bound RT-PCR product, bcr-abl mRNA-positive cells were determined using image analysis. A dilution series of bcr-abl-positive BV173 into normal cells showed a good correlation between expected and actual values. In 25 CML samples, the percentage of in situ PCR-positive cells showed an excellent correlation with cytogenetic results (r = 0.94, P < 0.0001), interphase fluorescence in situ hybridization (FISH) (r = 0.95, P = 0.001) and hypermetaphase FISH (r = 0.81, P < 0.001). The fluorescence intensity was higher in residual CML cells after interferon (IFN) treatment than in newly diagnosed patients (P = 0.004), and was highest in late-stage CML resistant to IFN therapy and lowest in CML blast crisis (P = 0.001). Mean fluorescence values correlated with bcr-abl protein levels, as determined by Western blot analysis (r = 0.62). Laser scanning cytometry allowing automated analysis of large numbers of cells confirmed the results. Thus, fluorescence in situ PCR provides a novel and quantitative approach for monitoring tumour load and bcr-abl transcript levels in CML.

The role of FAS-mediated apoptosis in chronic myelogenous leukemia

Clinical observation and laboratory evidence suggest that immune mechanisms play an important role in the natural control of evolution of the Ph+ clone in chronic phase as well as during progression of chronic myelogenous leukemia (CML). The understanding of these mechanisms could facilitate development of innovative therapeutic approaches. Due to bcr-abl translocation, CML cells carry an intrinsic resistance to apoptotic signals. However, resistance to apoptosis is not absolute and can be overcome through enhancement of immune-mediated pathways, e.g., during graft vs. leukemia reaction after allogeneic bone marrow transplantation, or during interferon-alpha (IFN-alpha) therapy. Among the effector mechanisms, T-lymphocyte-mediated killing of target cells via Fas-receptor (Fas-R) triggering plays an important role in the elimination of malignant cells, including CML cells. Although CML Ph+ progenitor cells express Fas-R, the expression levels are variable and do not correlate with clinical parameters. In addition, CML progenitor cells also express functional Fas-ligand (Fas-L), which may be an important immune surveillance escape factor. IFN-alpha can greatly upmodulate Fas-R expression, an effect that seems to be more pronounced in CML compared to normal cells, while Fas-L expression levels are not affected by IFN-alpha, thereby improving their susceptibility to elimination by the immune system. Responsiveness to Fas-induced apoptosis following stimulation with IFN-alpha correlates with the clinical effects of IFN-alpha therapy. This effect seems to be associated with decreased bcr-abl protein levels, which are influenced by Fas via posttranscriptional modulation. In comparison to the chronic phase, CML cells derived from patients in blast crisis are refractory to Fas-mediated apoptosis, regardless of the expression levels of Fas, suggesting that an immune-mediated selection pressure could result in acquisition of Fas-resistance. In the future, enhancement of immunological recognition and elimination of CML cells may prove to be an effective therapeutic approach directed towards the cure of CML.

Acquired loss of p53 induces blastic transformation in p210bcr/abl-expressing hematopoietic cells: a transgenic study for blast crisis of human CML

Chronic myelogenous leukemia (CML) begins with an indolent chronic phase but inevitably progresses to a fatal blast crisis. Although the Philadelphia chromosome, which generates p210bcr/abl, is a unique chromosomal abnormality in the chronic phase, additional chromosomal abnormalities are frequently detected in the blast crisis, suggesting that superimposed genetic events are responsible for disease progression. To investigate whether loss of p53 plays a role in the evolution of CML, we crossmated p210bcr/abl-transgenic (BCR/ABLtg/−) mice with p53-heterozygous (p53+/−) mice and generated p210bcr/abl-transgenic, p53-heterozygous (BCR/ABLtg/−p53+/−) mice, in which a somatic alteration in the residual normal p53 allele directly abrogates p53 function. TheBCR/ABLtg/−p53+/− mice died in a short period compared with their wild-type (BCR/ABL−/−p53+/+), p53 heterozygous (BCR/ABL−/−p53+/−), and p210bcr/abl transgenic (BCR/ABLtg/−p53+/+) litter mates. They had rapid proliferation of blast cells, which was preceded by subclinical or clinical signs of a myeloproliferative disorder resembling human CML. The blast cells were clonal in origin and expressed p210bcr/abl with an increased kinase activity. Interestingly, the residual normal p53 allele was frequently and preferentially lost in the tumor tissues, implying that a certain mechanism facilitating the loss of p53 allele exists in p210bcr/abl-expressing hematopoietic cells. Our study presents in vivo evidence that acquired loss of p53 contributes to the blastic transformation of p210bcr/abl-expressing hematopoietic cells and provides insights into the molecular mechanism for blast crisis of human CML.

Efficient and Rapid Induction of a Chronic Myelogenous Leukemia-Like Myeloproliferative Disease in Mice Receiving P210 bcr/abl-Transduced Bone Marrow

Expression of the 210-kD bcr/abl fusion oncoprotein can cause a chronic myelogenous leukemia (CML)-like disease in mice receiving bone marrow cells transduced by bcr/abl-encoding retroviruses. However, previous methods failed to yield this disease at a frequency sufficient enough to allow for its use in the study of CML pathogenesis. To overcome this limitation, we have developed an efficient and reproducible method for inducing a CML-like disease in mice receiving P210 bcr/abl-transduced bone marrow cells. All mice receiving P210 bcr/abl-transduced bone marrow cells succumb to a myeloproliferative disease between 3 and 5 weeks after bone marrow transplantation. The myeloproliferative disease recapitulates many of the hallmarks of human CML and is characterized by high white blood cell counts and extensive extramedullary hematopoiesis in the spleen, liver, bone marrow, and lungs. Use of a retroviral vector coexpressing P210 bcr/abl and green fluorescent protein shows that the vast majority of bcr/abl-expressing cells are myeloid. Analysis of the proviral integration pattern shows that, in some mice, the myeloproliferative disease is clonal. In multiple mice, the CML-like disease has been transplantable, inducing a similar myeloproliferative syndrome within 1 month of transfer to sublethally irradiated syngeneic recipients. The disease in many of these mice has progressed to the development of acute lymphoma/leukemia resembling blast crisis. These results demonstrate that murine CML recapitulates important features of human CML. As such, it should be an excellent model for addressing specific issues relating to the pathogenesis and treatment of this disease.

Efficient and Rapid Induction of a Chronic Myelogenous Leukemia-Like Myeloproliferative Disease in Mice Receiving P210 bcr/abl-Transduced Bone Marrow

Expression of the 210-kD bcr/abl fusion oncoprotein can cause a chronic myelogenous leukemia (CML)-like disease in mice receiving bone marrow cells transduced by bcr/abl-encoding retroviruses. However, previous methods failed to yield this disease at a frequency sufficient enough to allow for its use in the study of CML pathogenesis. To overcome this limitation, we have developed an efficient and reproducible method for inducing a CML-like disease in mice receiving P210 bcr/abl-transduced bone marrow cells. All mice receiving P210 bcr/abl-transduced bone marrow cells succumb to a myeloproliferative disease between 3 and 5 weeks after bone marrow transplantation. The myeloproliferative disease recapitulates many of the hallmarks of human CML and is characterized by high white blood cell counts and extensive extramedullary hematopoiesis in the spleen, liver, bone marrow, and lungs. Use of a retroviral vector coexpressing P210 bcr/abl and green fluorescent protein shows that the vast majority of bcr/abl-expressing cells are myeloid. Analysis of the proviral integration pattern shows that, in some mice, the myeloproliferative disease is clonal. In multiple mice, the CML-like disease has been transplantable, inducing a similar myeloproliferative syndrome within 1 month of transfer to sublethally irradiated syngeneic recipients. The disease in many of these mice has progressed to the development of acute lymphoma/leukemia resembling blast crisis. These results demonstrate that murine CML recapitulates important features of human CML. As such, it should be an excellent model for addressing specific issues relating to the pathogenesis and treatment of this disease.

Development of Acute Lymphoblastic Leukemia and Myeloproliferative Disorder in Transgenic Mice Expressing p210bcr/abl: A Novel Transgenic Model for Human Ph1-Positive Leukemias

The Philadelphia (Ph1) chromosome can be detected in chronic myelogenous leukemia (CML) and a significant number of acute lymphoblastic leukemia (ALL) cases. Generation of p210bcr/abl, a chimeric protein with enhanced kinase activity, is thought to be involved in the pathogenesis of these diseases. To elucidate the biological properties of p210bcr/abl and to create an animal model for human Ph1-positive leukemias, we generated transgenic mice expressing p210bcr/abl driven by the promoter of the tec gene, a cytoplasmic tyrosine-kinase preferentially expressed in the hematopoietic lineage. The founder mice showed excessive proliferation of lymphoblasts shortly after birth and were diagnosed as suffering from ALL based on surface marker and Southern blot analyses. Expression and enhanced kinase activity of the p210bcr/abl transgene product were detected in the leukemic tissues. In contrast, transgenic progeny exhibited marked granulocyte hyperplasia with thrombocytosis after a long latent period and developed myeloproliferative disorders (MPDs) closely resembling human CML. Expression of p210bcr/abl mRNA in the proliferating granulocytes was detected by RT-PCR. In particular, one MPD mouse showed remarkable proliferation of blast cells in the lung, which might represent an extramedullar blast crisis. The results demonstrate that the expression of p210bcr/abl in hematopoietic progenitor cells in transgenic mice can contribute to two clinically distinct hematopoietic malignancies, CML and ALL, indicating that this transgenic system provides a novel transgenic model for human Ph1-positive leukemias.

Development of Acute Lymphoblastic Leukemia and Myeloproliferative Disorder in Transgenic Mice Expressing p210bcr/abl: A Novel Transgenic Model for Human Ph1-Positive Leukemias

The Philadelphia (Ph1) chromosome can be detected in chronic myelogenous leukemia (CML) and a significant number of acute lymphoblastic leukemia (ALL) cases. Generation of p210bcr/abl, a chimeric protein with enhanced kinase activity, is thought to be involved in the pathogenesis of these diseases. To elucidate the biological properties of p210bcr/abl and to create an animal model for human Ph1-positive leukemias, we generated transgenic mice expressing p210bcr/abl driven by the promoter of the tec gene, a cytoplasmic tyrosine-kinase preferentially expressed in the hematopoietic lineage. The founder mice showed excessive proliferation of lymphoblasts shortly after birth and were diagnosed as suffering from ALL based on surface marker and Southern blot analyses. Expression and enhanced kinase activity of the p210bcr/abl transgene product were detected in the leukemic tissues. In contrast, transgenic progeny exhibited marked granulocyte hyperplasia with thrombocytosis after a long latent period and developed myeloproliferative disorders (MPDs) closely resembling human CML. Expression of p210bcr/abl mRNA in the proliferating granulocytes was detected by RT-PCR. In particular, one MPD mouse showed remarkable proliferation of blast cells in the lung, which might represent an extramedullar blast crisis. The results demonstrate that the expression of p210bcr/abl in hematopoietic progenitor cells in transgenic mice can contribute to two clinically distinct hematopoietic malignancies, CML and ALL, indicating that this transgenic system provides a novel transgenic model for human Ph1-positive leukemias.

The chimeric BCR-ABL gene

The 1982 discovery that in chronic myeloid leukaemia (CML) the ABL proto-oncogene is translocated to the BCR gene located on chromosome 22 initiated many studies on the structural organization and function of these genes. The nucleotide sequence of the entire BCR and major parts of the ABL gene has now been determined. However, the actual cause of the fusion of BCR with ABL remains essentially unknown. Mouse models have been helpful to unravel the normal cellular function of BCR and ABL, as well the activity of BCR-ABL, although a single mechanism explaining the transforming activity of the latter has not been discovered. The cause of progression of the disease remains unknown, and no single genetic abnormality has been linked to the blast phase of CML. Much has been learned concerning the molecular biology of CML, but answers to the fundamental questions above may be expected in the coming years in parallel to increasing knowledge of genome structure, signal transduction and cell cycle control.