The implication of cancer progenitor cells and the role of epigenetics in the development of novel therapeutic strategies for chronic myeloid leukemia

SIGNIFICANCE

Chronic myeloid leukemia (CML) involves the malignant transformation of hematopoietic stem cells, defined largely by the Philadelphia chromosome and expression of the breakpoint cluster region-Abelson (BCR-ABL) oncoprotein. Pharmacological tyrosine kinase inhibitors (TKIs), including imatinib mesylate, have overcome limitations in conventional treatment for the improved clinical management of CML.

RECENT ADVANCES

Accumulated evidence has led to the identification of a subpopulation of quiescent leukemia progenitor cells with stem-like self renewal properties that may initiate leukemogenesis, which are also shown to be present in residual disease due to their insensitivity to tyrosine kinase inhibition.

CRITICAL ISSUES

The characterization of quiescent leukemia progenitor cells as a unique cell population in CML pathogenesis has become critical with the complete elucidation of mechanisms involved in their survival independent of BCR-ABL that is important in the development of novel anticancer strategies. Understanding of these functional pathways in CML progenitor cells will allow for their selective therapeutic targeting. In addition, disease pathogenesis and drug responsiveness is also thought to be modulated by epigenetic regulatory mechanisms such as DNA methylation, histone acetylation, and microRNA expression, with a capacity to control CML-associated gene transcription.

FUTURE DIRECTIONS

A number of compounds in combination with TKIs are under preclinical and clinical investigation to assess their synergistic potential in targeting leukemic progenitor cells and/or the epigenome in CML. Despite the collective promise, further research is required in order to refine understanding, and, ultimately, advance antileukemic therapeutic strategies.

Pathological interactions between hematopoietic stem cells and their niche revealed by mouse models of primary myelofibrosis

Primary myelofibrosis (PMF) belongs to the Philadelphia-negative myeloproliferative neoplasms and is a hematological disorder caused by abnormal function of the hematopoietic stem cells. The disease manifests itself with a plethora of alterations, including anemia, splenomegaly and extramedullary hematopoiesis. Its hallmarks are progressive marrow fibrosis and atypical megakaryocytic hyperplasia, two distinctive features used to clinically monitor disease progression. In an attempt to investigate the role of abnormal megakaryocytopoiesis in the pathogenesis of PMF, several transgenic mouse models have been generated. These models are based either on mutations that interfere with the extrinsic (thrombopoietin and its receptor, MPL) and intrinsic (the GATA1 transcription factor) control of normal megakaryocytopoiesis, or on known genetic lesions associated with the human disease. Here we provide an up-to-date review on the insights into the pathobiology of human PMF achieved by studying these animal models, with particular emphasis on results obtained with Gata1(low) mice.

Epigenetic regulation of cathepsin L expression in chronic myeloid leukaemia

The expression and significance of cathepsin L (CTSL) has been extensively studied in solid tumours. However no such information in chronic myeloid leukaemia (CML) was available. We investigated the activity and expression of this protease in peripheral blood mononuclear cells (PBMCs) of 47 adult CML patients. Thirty adults suffering from systemic diseases and 50 healthy volunteers served as controls. The mRNA levels of CTSL, its specific endogenous inhibitor cystatin C and transcriptional up-regulator vascular endothelial growth factor (VEGF) were quantitated by real-time qPCR. CTSL protease activity and its mRNA expression were significantly higher in CML chronic phase (CP) patients compared to CML accelerated phase/blast crisis (AP/BC) patients and controls (P≤ 0.001). VEGF whose expression was most pronounced in CP and declined (P≤ 0.001) in the advanced phases of the malignancy exhibited a strong positive correlation with CTSL expression (r= 0.97; P≤ 0.001). Cystatin C expression was significantly lower (P≤ 0.001) in CML and displayed inverse correlation with CTSL (r=−0.713; P≤ 0.001) activity. CTSL promoter was significantly hypomethylated in CML CP compared to CML AP/BC patients as well as controls. K562, a BC CML cell line displayed CTSL activity, expression and methylation status of CTSL promoter that was comparable to CML AP/BC patients. Treatment of these cells or PBMCs isolated from CML AP/BC patients with 5′-aza-cytidine resulted in a dramatic increase in CSTL activity and/or expression thereby demonstrating the role of promoter methylation in the stage specific expression of CTSL in CML. Differential expression of CTSL in CML at various stages of malignancy may prove useful in identification of the high-risk patients thereby facilitating better management of disease.

High STAT5 levels mediate imatinib resistance and indicate disease progression in chronic myeloid leukemia

In BCR-ABL1(+) leukemia, drug resistance is often associated with up-regulation of BCR-ABL1 or multidrug transporters as well as BCR-ABL1 mutations. Here we show that the expression level of the transcription factor STAT5 is another parameter that determines the sensitivity of BCR-ABL1(+) cells against tyrosine kinase inhibitors (TKIs), such as imatinib, nilotinib, or dasatinib. Abelson-transformed cells, expressing high levels of STAT5, were found to be significantly less sensitive to TKI-induced apoptosis in vitro and in vivo but not to other cytotoxic drugs, such as hydroxyurea, interferon-β, or Aca-dC. The STAT5-mediated protection requires tyrosine phosphorylation of STAT5 independent of JAK2 and transcriptional activity. In support of this concept, under imatinib treatment and with disease progression, STAT5 mRNA and protein levels increased in patients with Ph(+) chronic myeloid leukemia. Based on our data, we propose a model in which disease progression in BCR-ABL1(+) leukemia leads to up-regulated STAT5 expression. This may be in part the result of clonal selection of cells with high STAT5 levels. STAT5 then accounts for the resistance against TKIs, thereby explaining the dose escalation frequently required in patients reaching accelerated phase. It also suggests that STAT5 may serve as an attractive target to overcome imatinib resistance in BCR-ABL1(+) leukemia.

Programmed cell death proteins and chronic leukemia

Apoptosis or programmed cell death is a genetically regulated process of cellular suicide. Apoptosis has been implicated in a wide range of pathological conditions, and mutations in apoptotic genes play important roles in the process of malignant transformation. Chronic leukemia represents a neoplastic disorder caused primarily by defective programmed cell death, as opposed to increased cell proliferation. This paper presents the main results of our ten-year research on the apoptosis of leukemia cells. The research included the morphological aspects of the process, the effect of antineoplastic agents on the induction of apoptosis in leukemia cells and expression analysis of the proteins involved in programmed cell death. Special attention was paid to the expression and interaction of the Bcl-2 family of proteins in leukemia cells. The ultimate aim of the study of apoptosis of leukemic cells is the discovery of new biological agents that might be used in the treatment of chronic leukemia.

Prognostic significance of cellular vascular endothelial growth factor (VEGF) expression in the course of chronic myeloid leukaemia

Introduction. Increased angiogenesis in bone marrow is one of the characteristics of chronic myeloid leukaemia (CML), a clonal myeloproliferative disorder that expresses a chimeric bcr/abl protein. Vascular endothelial growth factor (VEGF) is one of the most potent and a specific regulator of angiogenesis which principally targets endothelial cells and regulates several of their functions, including mitogenesis, permeability and migration. The impact of elevated VEGF expression on the course of chronic myeloid leukaemia is unknown. Objective. The aim of this study was the follow-up of VEGF expression during the course of CML. Methods. We studied VEGF expression of 85 CML patients (median age 50 years, range 16-75 years). At the commencement of the study, 29 patients were in chronic phase (CP), 25 in an accelerated phase (AP), and 31 in the blast crisis (BC). The temporal expression (percentage positivity per 1000 analysed cells) VEGF proteins over the course of CML were studied using the immunohistochemical technique utilizing relevant monoclonal antibodies. It was correlated with the laboratory (Hb, WBC and platelet counts, and the percentage of blasts) and clinical parameters (organomegaly, duration of CP, AP, and BC) of disease progression. Results. The expression of VEGF protein was most pronounced in AP (ANOVA, p=0.033). The level of VEGF expression correlated inversely with the degree of splenomegaly (Pearson, r=-0.400, p=0.011). High expression of VEGF correlated with a shorter overall survival (log rank, p=0.042). Conclusion. Immunohistochemically confirmed significance of the expression of VEGF in dependence of the CML stage could be of clinical importance in deciding on the timing therapy. These data suggest that VEGF plays a role in the biology of CML and that VEGF inhibitors should be investigated in CML.