BCR-ABL1-positive CML and BCR-ABL1-negative chronic myeloproliferative disorders: some common and contrasting features

Myeloproliferative Neoplasms: Diseases Mediated by Chronic Activation of Signal Transducer and Activator of Transcription (STAT) Proteins

Myeloproliferative neoplasms (MPNs) are hematopoietic diseases characterized by the clonal expansion of single or multiple lineages of differentiated myeloid cells that accumulate in the blood and bone marrow. MPNs are grouped into distinct categories based on key clinical presentations and distinctive mutational hallmarks. These include chronic myeloid leukemia (CML), which is strongly associated with the signature BCR::ABL1 gene translocation, polycythemia vera (PV), essential thrombocythemia (ET), and primary (idiopathic) myelofibrosis (PMF), typically accompanied by molecular alterations in the JAK2, MPL, or CALR genes. There are also rarer forms such as chronic neutrophilic leukemia (CNL), which involves mutations in the CSF3R gene. However, rather than focusing on the differences between these alternate disease categories, this review aims to present a unifying molecular etiology in which these overlapping diseases are best understood as disruptions of normal hematopoietic signaling: specifically, the chronic activation of signaling pathways, particularly involving signal transducer and activator of transcription (STAT) transcription factors, most notably STAT5B, leading to the sustained stimulation of myelopoiesis, which underpins the various disease sequalae.

Prognosis in Chronic Myeloid Leukemia: Baseline Factors, Dynamic Risk Assessment and Novel Insights

The introduction of tyrosine kinase inhibitors (TKIs) has changed the treatment paradigm of chronic myeloid leukemia (CML), leading to a dramatic improvement of the outcome of CML patients, who now have a nearly normal life expectancy and, in some selected cases, the possibility of aiming for the more ambitious goal of treatment-free remission (TFR). However, the minority of patients who fail treatment and progress from chronic phase (CP) to accelerated phase (AP) and blast phase (BP) still have a relatively poor prognosis. The identification of predictive elements enabling a prompt recognition of patients at higher risk of progression still remains among the priorities in the field of CML management. Currently, the baseline risk is assessed using simple clinical and hematologic parameters, other than evaluating the presence of additional chromosomal abnormalities (ACAs), especially those at "high-risk". Beyond the onset, a re-evaluation of the risk status is mandatory, monitoring the response to TKI treatment. Moreover, novel critical insights are emerging into the role of genomic factors, present at diagnosis or evolving on therapy. This review presents the current knowledge regarding prognostic factors in CML and their potential role for an improved risk classification and a subsequent enhancement of therapeutic decisions and disease management.

Downregulation of Smad4 expression confers chemoresistance against imatinib mesylate to chronic myeloid leukemia K562 cells

Objective: Imatinib mesylate (IM), a tyrosine kinase inhibitor, exhibits clinically prominent effects against chronic myeloid leukemia (CML); however, a few patients have shown resistance to IM treatment, resulting in disease progression. Smad4 is a tumor inhibitor that transduces TGF-β signaling and modulates genomic stability. Previous studies have indicated that decreased Smad4 expression played a bidirectional role in chemosensitivity in many types of cancers. Therefore, this study aims to evaluate the association between IM sensitivity and decreased Smad4 expression in human CML K562 cells.Methods: Bone marrow (BM) samples were acquired from the patients prior to treatment. qRT-PCR, Western Blotting (WB), colony formation assay (CFA), and apoptosis assay were used to detect relevant indices.Results: Smad4 expression was downregulated in the bone marrow and plasma of patients with multidrug-resistant CML as well as IM-resistant K562 (K562R) cells compared with samples collected from CML patients and K562 cells. Smad4 overexpression inhibited IM-treated K562R cell proliferation and augmented apoptosis, whereas Smad4 silencing promoted viability and inhibited apoptosis in IM-treated K562 cells. In addition, Smad4 expression was inversely correlated with laminin subunit gamma 1 (LAMC1) expression. The upregulation or downregulation of LAMC1 expression partially abolished the effect of Smad4 overexpression or silencing on the IM resistance of CML cells.Conclusion: The downregulation of Smad4 expression might induce drug resistance in CML cells and displayed a possible mechanism through which Smad4 modulates CML cell survival and apoptosis upon IM treatment.

Histone demethylase RBP2 mediates the blast crisis of chronic myeloid leukemia through an RBP2/PTEN/BCR-ABL cascade

Epigenetic disorders play a key role in tumorigenesis and development, among which histone methylation abnormalities are common. While patients living with chronic myeloid leukemia in the chronic phase (CML-CP) have a good response to TKI, blastic phase (CML-BP) patients demonstrate poor efficacy and high fatality rates. However, while the mechanism of blast crisis of chronic myeloid leukemia remains unclear, high expression and activation of BCR-ABL are usually related to CML blast crisis transition. We found that histone H3 lysine 4 (H3K4) demethylase RBP2 expression is negatively correlated with BCR-ABL expression, which suggests a regulatory link between these two genes. We also discovered that RBP2 mediates the dephosphorylation of BCR-ABL by directly downregulating PTEN expression, depending on histone demethylase activity, while PTEN targets protein phosphatase activity of BCR-ABL, a phosphatase which directly dephosphorylates BCR-ABL. In clinical specimens, the mRNA expression of RBP2 was found to be positively correlated with that of PTEN. These data suggest that the under-expression of RBP2 promotes blast crisis transition by activating an RBP2/PTEN/BCR-ABL cascade.

Occurrence of chromosomal abnormalities in Philadelphia chromosome-negative metaphases in patients with chronic-phase chronic myeloid leukemia undergoing TKI treatments

Forty-three chromosomal abnormalities in Philadelphia-negative metaphases (Ph-CAs) appeared in 35 of 432 patients in chronic phase chronic myeloid leukemia (CP-CML) undergoing tyrosine kinase inhibitor (TKI) treatments. These CAs were mostly common in trisomy-8 (16 cases), trisomy-Y (five cases), and monosomy-7 (five cases). Furthermore, Ph- CAs were significantly associated with higher platelet count (494 × 10⁹/L vs. 326 × 10⁹/L, p = .006), and higher incidence of true clonal evolution in Ph-positive metaphase (22.9% vs. 9.1%, p = .017). Additionally, patients with Ph- CAs had worse rates of complete cytogenetic remission (76% vs. 86%, p = .0091), major molecular remission (55% vs. 76%, p = .001), progression-free survival (47% vs. 86%, p < .001), but a similar overall survival rates compared to those in patients without Ph- CAs. In conclusion, Ph- CAs may predict worse response to TKI therapies and survival in patients with CP-CML, thus requiring close cytogenetic monitoring.

DNA Damage Response in Quiescent Hematopoietic Stem Cells and Leukemia Stem Cells

In humans, hematopoietic stem cells (HSCs) adopt unique responsive pathways counteracting with the DNA-damaging assaults to weigh the balance between the maintenance of normal stem cell poor for whole-life blood regeneration and the transformation to leukemia stem cells (LSCs) for leukemia initiation. LSCs also take actions of combating with the attack launched by externally therapeutic drugs that can kill most leukemic cells, to avoid extermination and promote disease relapse. Therefore, the collection of knowledge about all these underlined mechanisms would present a preponderance for later studies. In this chapter, the universal DNA damage response (DDR) mechanisms were firstly introduced, and then DDR of HSCs were presented focusing on the DNA double-strand breaks in the quiescent state of HSCs, which poses a big advantage in promoting its transformation into preleukemic HSCs. Lastly, the DDR of LSCs were summarized based on the major outcomes triggered by different pathways in specific leukemia, upon which some aspects for future investigations were envisioned under our currently limited scope of knowledge.

Molecular-defined clonal evolution in patients with chronic myeloid leukemia independent of the BCR-ABL status

To study clonal evolution in chronic myeloid leukemia (CML), we searched for BCR-ABL-independent gene mutations in both Philadelphia chromosome (Ph)-negative and Ph-positive clones in 29 chronic-phase CML patients by targeted deep sequencing of 25 genes frequently mutated in myeloid disorders. Ph-negative clones were analyzed in 14 patients who developed clonal cytogenetic abnormalities in Ph-negative cells during treatment with tyrosine kinase inhibitors (TKI). Mutations were detected in 6/14 patients (43%) affecting the genes DNMT3A, EZH2, RUNX1, TET2, TP53, U2AF1 and ZRSR2. In two patients, the mutations were also found in corresponding Ph-positive diagnostic samples. To further investigate Ph-positive clones, 15 randomly selected CML patients at diagnosis were analyzed. Somatic mutations additional to BCR-ABL were found in 5/15 patients (33%) affecting ASXL1, DNMT3A, RUNX1 and TET2. Analysis of individual hematopoietic colonies at diagnosis revealed that most mutations were part of the Ph-positive clone. In contrast, deep sequencing of subsequent samples during TKI treatment revealed one DNMT3A mutation in Ph-negative cells that was also present in Ph-positive cells at diagnosis, implying that the mutation preceded the BCR-ABL rearrangement. In summary, BCR-ABL-independent gene mutations were frequently found in Ph-negative and Ph-positive clones of CML patients and may be considered as important cofactors in the clonal evolution of CML.

Concomitant BCR-ABL1 translocation and JAK2(V617F) mutation in three patients with myeloproliferative neoplasms

Chronic myeloproliferative neoplasms (MPN) are clonal disorders of hematopoietic stem cells, which fall into distinct categories based on a number of characteristics including the presence of the BCR-ABL1 gene fusion (chronic myelogenous leukemia) or the JAK2(V617F) mutation (polycythemia vera, primary myelofibrosis, and essential thrombocythemia). One of the criteria in the 2008 World Health Organization Classification divides MPN into different categories based on the presence of an underlying genetic abnormality, however the WHO does not currently address the classification of myeloproliferative neoplasms that have more than one genetic abnormality. The coexistence of a JAK2(V617F) mutation and BCR-ABL1 is rare, and to our knowledge, less than 25 cases have been reported in the literature. Our case series examines the clinical, histopathologic, and genetic features of 3 patients with myeloproliferative neoplasms characterized by concomitant BCR-ABL1 and JAK2(V617F). The implications for diagnosis and treatment of patients with concomitant BCR-ABL1 and JAK2(V617F) are discussed as well as how the BCR-ABL1 and JAK2(V617F)-positive clones may be related to one another.

The sixth annual translational stem cell research conference of the New York Stem Cell Foundation

The New York Stem Cell Foundation's "Sixth Annual Translational Stem Cell Research Conference" convened on October 11-12, 2011 at the Rockefeller University in New York City. Over 450 scientists, patient advocates, and stem cell research supporters from 14 countries registered for the conference. In addition to poster and platform presentations, the conference featured panels entitled "Road to the Clinic" and "The Future of Regenerative Medicine."

A potential role of ruxolitinib in leukemia

INTRODUCTION

An increased understanding of cellular signaling pathways, like the JAK?STAT pathway, and the identification of the JAK2 V617F mutation in the classic Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), has generated great interest in the development of targeted JAK2 inhibitors. In a recently completed Phase I?II study, ruxolitinib, a selective orally available JAK1 and JAK2 inhibitor, has shown efficacy in patients with advanced myelofibrosis. Constitutive activation of the JAK?STAT pathway has also been implicated in other hematological malignancies suggesting a potential role of JAK kinase inhibitors in these malignancies.

AREAS COVERED

This article reviews the chemistry, pharmacodynamics, pharmacokinetics, clinical efficacy, safety and tolerability of ruxolitinib. The literature for this article was retrieved from PubMed database searches using the keywords ?ruxolitinib?, ?INCB 018424?, ?JAK2 inhibitors? and ?leukemia?.

EXPERT OPINION

The JAK?STAT signaling pathway plays a vital role in leukemogenesis. Ruxolitinib, a potent JAK1 and JAK2 inhibitor, known to decrease spleen size and alleviate constitutional symptoms in myelofibrosis, represents a potentially promising agent for the treatment of leukemias by inhibiting the JAK?STAT signaling. Further studies of ruxolitinib, in patients with acute and chronic leukemias, are now needed to establish the clinical usefulness of this promising drug.

Chronic myeloid leukemia stem cells and developing therapies

Chronic myeloid leukemia therapy has remarkably improved with the use of frontline BCR-ABL kinase inhibitors such that newly diagnosed patients have minimal disease manifestations or progression. Effective control of disease may also set the stage for eventual 'cure' of this leukemia. However, the existence of Philadelphia chromosome-positive leukemic cells that are unaffected by BCR-ABL inhibition represents a major barrier that may delay or prevent curative therapy with the current approaches. The most commonly reported mechanism of resistance to tyrosine kinase inhibitor-based therapies involves BCR-ABL gene mutations and amplification, but these changes may not be solely responsible for disease relapse when inhibitor-based therapies are curtailed. Therefore new targets may need to be defined before significant advancement in curative therapies is possible. Emerging evidence suggests that persistence of chronic myeloid leukemia stem cells or acquisition of stem cell-like characteristics prevents complete elimination of chronic myeloid leukemia by tyrosine kinase inhibition alone. This review focuses on several recently emerging concepts regarding the existence and characteristics of chronic myeloid leukemia stem cells. Definitions based on human primary cells and animal model studies are highlighted as are the potential signaling pathways associated with disease repopulating cells. Finally, several recently defined therapeutic targets and active compounds that have emerged from stem cell studies are described. Our goal is to provide an unbiased report on the current state of discovery within the chronic myeloid leukemia stem cell field and to orient the reader to emerging therapeutic targets and strategies that may lead to elimination of this leukemia.

Chronic myeloid leukemia: mechanisms of blastic transformation

The BCR-ABL1 oncoprotein transforms pluripotent HSCs and initiates chronic myeloid leukemia (CML). Patients with early phase (also known as chronic phase [CP]) disease usually respond to treatment with ABL tyrosine kinase inhibitors (TKIs), although some patients who respond initially later become resistant. In most patients, TKIs reduce the leukemia cell load substantially, but the cells from which the leukemia cells are derived during CP (so-called leukemia stem cells [LSCs]) are intrinsically insensitive to TKIs and survive long term. LSCs or their progeny can acquire additional genetic and/or epigenetic changes that cause the leukemia to transform from CP to a more advanced phase, which has been subclassified as either accelerated phase or blastic phase disease. The latter responds poorly to treatment and is usually fatal. Here, we discuss what is known about the molecular mechanisms leading to blastic transformation of CML and propose some novel therapeutic approaches.

The blast phase of chronic myeloid leukaemia

The blast phase of chronic myelogeneous leukaemia (BP-CML) still remains a difficult entity to understand pathogenetically and to treat. In recent years, advances have been made in our understanding of the molecular biology and the pathogenesis of this disorder. Although three new agents have been introduced with some success for treating this disease, namely imatinib, dasatinib and nilotinib, response durations are limited. For those patients who achieve a remission following the use of one of the aforementioned drugs, bone marrow transplantation is recommended.

A gene expression signature of primary resistance to imatinib in chronic myeloid leukemia

Using gene expression profiling we show that the expression of 105-probe sets in mononuclear cells collected from chronic myeloid leukemia (CML) chronic phase (CP) patients with raised leukocyte counts who subsequently achieved complete cytogenetic response after 12 months on imatinib, differed substantially from that of patients who failed to achieve any degree of cytogenetic response. In the non-responder cohort, 9 of the 50 overexpressed genes were involved in DNA repair by homologous recombination, whereas 36 genes, including PTEN, were downregulated. This pattern of altered gene expression in responders and non-responders was validated in another independent dataset. These findings may prove useful for identifying at the time of diagnosis a subset of CP-CML patients who are likely to be resistant to imatinib and require an alternative treatment.

Therapeutic potential of JAK2 inhibitors

The discovery of an activating tyrosine kinase mutation JAK2V617F in myeloproliferative neoplasms (MPNs), polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF) has resulted in the development of JAK2 inhibitors, of which several are being evaluated in phase I/II clinical studies. It is important to recognize that because the V617F mutation is localized in a region outside the adenosine triphosphate (ATP)-binding pocket of JAK2 enzyme, ATP-competitive inhibitors of JAK2 kinase (like the current JAK2 inhibitors in the clinic) are not likely to discriminate between wild-type and mutant JAK2 enzymes. Therefore, JAK2 inhibitors, by virtue of their near equipotent activity against wild-type JAK2 that is important for normal hematopoiesis, may have adverse myelosuppression as an expected side effect, if administered at doses that aim to completely inhibit the mutant JAK2 enzyme. While they may prove to be effective in controlling hyperproliferation of hematopoietic cells in PV and ET, they may not be able to eliminate mutant clones. On the other hand, JAK inhibitors may have great therapeutic benefit by controlling the disease for patients with MPNs who suffer from debilitating signs (eg, splenomegaly) or constitutional symptoms (which presumably result from high levels of circulating cytokines that signal through JAK enzymes). Indeed, the primary clinical benefits observed so far in MF patients have been significant reduction is splenomegaly, elimination of debilitating disease-related symptoms, and weight gain. Most importantly, patients with and without the JAK2V617F mutation appear to benefit to the same extent. In this review we summarize current clinical experience with JAK2 inhibitors in MPNs.