The histone deacetylase inhibitor ITF2357 selectively targets cells bearing mutated JAK2(V617F)

The oral histone deacetylase inhibitor ITF2357 reduces cytokines and protects islet β cells in vivo and in vitro

In type 1 diabetes, inflammatory and immunocompetent cells enter the islet and produce proinflammatory cytokines such as interleukin-1β (IL-1β), IL-12, tumor necrosis factor-α (TNFα) and interferon-γ (IFNγ); each contribute to β-cell destruction, mediated in part by nitric oxide. Inhibitors of histone deacetylases (HDAC) are used commonly in humans but also possess antiinflammatory and cytokine-suppressing properties. Here we show that oral administration of the HDAC inhibitor ITF2357 to mice normalized streptozotocin (STZ)-induced hyperglycemia at the clinically relevant doses of 1.25-2.5 mg/kg. Serum nitrite levels returned to nondiabetic values, islet function improved and glucose clearance increased from 14% (STZ) to 50% (STZ + ITF2357). In vitro, at 25 and 250 nmol/L, ITF2357 increased islet cell viability, enhanced insulin secretion, inhibited MIP-1α and MIP-2 release, reduced nitric oxide production and decreased apoptosis rates from 14.3% (vehicle) to 2.6% (ITF2357). Inducible nitric oxide synthase (iNOS) levels decreased in association with reduced islet-derived nitrite levels. In peritoneal macrophages and splenocytes, ITF2357 inhibited the production of nitrite, as well as that of TNFα and IFNγ at an IC(50) of 25-50 nmol/L. In the insulin-producing INS cells challenged with the combination of IL-1β plus IFNγ, apoptosis was reduced by 50% (P < 0.01). Thus at clinically relevant doses, the orally active HDAC inhibitor ITF2357 favors β-cell survival during inflammatory conditions.

Deactylase inhibition in myeloproliferative neoplasms

Myeloproliferative neoplasms (MPN) are clonal haemopoietic progenitor cell disorders characterized by the proliferation of one or more of the haemopoietic lineages (myeloid, erythroid and/or megakaryocytic). The MPNs include eight haematological disorders: chronic myelogenous leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF), systemic mastocytosis (SM), chronic eosinophilic leukemia, not otherwise specified (CEL, NOS), chronic neutrophilic leukemia (CNL), and unclassifiable MPN (MPN, U). Therapeutic interventions for MPNs include the use of tyrosine kinase inhibitors (TKIs) for BCR-ABL1(+) CML and JAK2 inhibitors for PV, ET and PMF. Histone deacetylase inhibitors (HDACi) are a novel class of drugs capable of altering the acetylation status of both histone and non-histone proteins, thereby affecting a repertoire of cellular functions in neoplastic cells including proliferation, differentiation, immune responses, angiogenesis and survival. Preliminary studies indicate that HDACi when used in combination with tyrosine kinase or JAK2 inhibitors may overcome resistance to the latter agents and enhance the pro-apoptotic effects on MPN cells. This review provides a review of pre-clinical and clinical studies that have explored the use of HDACi as potential therapeutics for MPNs.

Phase II study of obatoclax mesylate (GX15-070), a small-molecule BCL-2 family antagonist, for patients with myelofibrosis

BACKGROUND

Myelofibrosis (MF) is a disease characterized by the overexpression of the antiapoptotic BCL-2 family of proteins (eg, BCL-XL and MCL-1).

PATIENTS AND METHODS

We conducted a multicenter, open-label, noncomparative phase II study of obatoclax mesylate, a small-molecule pan-BCL-2 antagonist, in patients with MF. Obatoclax was administered as a 24-hour infusion (on an outpatient basis) every 2 weeks at a fixed dose of 60 mg.

RESULTS

A total of 22 patients were enrolled, with a median age of 63 years (range, 43-89 years). Twelve were men, and all 22 patients were previously treated (median of 2 previous therapies). Ten patients (45%) had a Lille score of 1, and 9 patients (41%) had a Lille score of 2. Thirteen (59%) were red blood cell transfusion dependent. A median of 7 cycles of obatoclax were administered. No patient achieved complete or partial response according to International Working Group criteria. One patient (4%) demonstrated a clinical improvement (in terms of hemoglobin and platelet count) after 7 cycles of therapy. The improvement was sustained for 4 cycles of therapy, after which he underwent allogeneic stem cell transplantation. The most common adverse events included low-grade ataxia and fatigue in 50% of the patients. Dose reduction because of toxicity was required in 1 patient, whereas 2 patients were taken off the study because of grade 3 ataxia and grade 3 heart failure. Grade 3/4 anemia and thrombocytopenia were evident in 6 (27%) and 4 (18%) patients, respectively.

CONCLUSION

Obatoclax exhibits no significant clinical activity in patients with MF at the dose and schedule evaluated.

Chronic myeloproliferative neoplasms: a collaborative approach

The classic chronic myeloproliferative neoplasms (MPN) include different entities that pose significant challenges for their optimal diagnosis, treatment and overall management. Polycythemia Vera and Essential Thrombocythemia are the most common among chronic myeloproliferative neoplasms (MPNs); major causes of morbidity and mortality are represented by arterial and venous thrombosis, as well as evolution to myelofibrosis or transformation to acute leukemia. However, survival is only minimally affected. Therapy aims at reducing the rate of thrombosis without increasing the risk of hematologic transformation which could be caused by exposure to cytotoxic drugs. On the other hand, survival is significantly reduced in primary myelofibrosis, and the clinical manifestations may be disabling. In the absence of therapies with the potential of curing the disease, a careful risk-oriented approach is employed for stratifying patients to the most appropriate, currently available, therapeutic options. In this brief review, we will discuss some of the key issues that can arise along the clinical course of MPNs and require an integrated, strictly patient-oriented, approach.

A pilot study of the Histone-Deacetylase inhibitor Givinostat in patients with JAK2V617F positive chronic myeloproliferative neoplasms

A phase II A study was conducted to evaluate the safety and efficacy of Givinostat, a novel Histone-Deacetylases inhibitor, in patients with Polycythaemia Vera (PV, n = 12), Essential Thrombocythaemia (ET, n = 1) and Myelofibrosis (n = 16), bearing the JAK2V617F mutation. The study was approved by the local ethics committees and all human participants gave written informed consent. Givinostat was given orally for 24 weeks at a starting dose of 50 mg twice daily. The median treatment duration was 20 weeks. Reasons for treatment discontinuation were disease progression (n = 6), grade 2 thrombocytopenia (n = 1), psychiatric symptoms (n = 1) and withdrawn consent (n = 2). A dose reduction was applied in 10 patients while a temporary interruption occurred in 15. Among 13 PV/ET patients, 1 complete, 6 partial and 4 no responses were documented at study end while 2 patients went off-study, prematurely. Three major responses were registered among 16 MF patients. Pruritus disappeared in most patients and reduction of splenomegaly was observed in 75% of PV/ET and 38% of MF patients. Reverse transcription polymerase chain reaction identified a trend to reduction of the JAK2V617F allele burden. Givinostat was well tolerated and could induce haematological response in most PV and some MF patients.

Trichostatin A blocks type I interferon production by activated plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDC) represent the main type I interferon (IFN-I) producing cells. Emerging evidence supports a role for IFN-I in autoimmune diseases. Given the central role of PDC in the pathogenesis of systemic lupus erythematosus (SLE), we investigated the effect of Trichostatin A (TSA), a prototypic histone deacetylase inhibitor, on PDC activation. TSA inhibited the production of IFN-I, TRAIL and of the pro-inflammatory cytokines TNFalpha and IL-6 by CpG-activated PDC. These effects were associated with the inhibition of IFN Regulatory Factor (IRF)-7 nuclear translocation. Furthermore, TSA was also effective in inhibiting the production of IFNalpha by PDC cultured in vitro in the presence of serum obtained from SLE patients. This study describes a new level of regulation of immune responses by histone deacetylase inhibitors and defines the molecular basis for new strategies to be exploited in the treatment of autoimmune diseases.

Novel mutations and their functional and clinical relevance in myeloproliferative neoplasms: JAK2, MPL, TET2, ASXL1, CBL, IDH and IKZF1

Myeloproliferative neoplasms (MPNs) originate from genetically transformed hematopoietic stem cells that retain the capacity for multilineage differentiation and effective myelopoiesis. Beginning in early 2005, a number of novel mutations involving Janus kinase 2 (JAK2), Myeloproliferative Leukemia Virus (MPL), TET oncogene family member 2 (TET2), Additional Sex Combs-Like 1 (ASXL1), Casitas B-lineage lymphoma proto-oncogene (CBL), Isocitrate dehydrogenase (IDH) and IKAROS family zinc finger 1 (IKZF1) have been described in BCR-ABL1-negative MPNs. However, none of these mutations were MPN specific, displayed mutual exclusivity or could be traced back to a common ancestral clone. JAK2 and MPL mutations appear to exert a phenotype-modifying effect and are distinctly associated with polycythemia vera, essential thrombocythemia and primary myelofibrosis; the corresponding mutational frequencies are approximately 99, 55 and 65% for JAK2 and 0, 3 and 10% for MPL mutations. The incidence of TET2, ASXL1, CBL, IDH or IKZF1 mutations in these disorders ranges from 0 to 17%; these latter mutations are more common in chronic (TET2, ASXL1, CBL) or juvenile (CBL) myelomonocytic leukemias, mastocytosis (TET2), myelodysplastic syndromes (TET2, ASXL1) and secondary acute myeloid leukemia, including blast-phase MPN (IDH, ASXL1, IKZF1). The functional consequences of MPN-associated mutations include unregulated JAK-STAT (Janus kinase/signal transducer and activator of transcription) signaling, epigenetic modulation of transcription and abnormal accumulation of oncoproteins. However, it is not clear as to whether and how these abnormalities contribute to disease initiation, clonal evolution or blastic transformation.

JAK2 kinase inhibitors and myeloproliferative disorders

PURPOSE OF REVIEW

The pathophysiology of Philadelphia-chromosome negative myeloproliferative disorders has significantly advanced with the discovery of JAK2V617F. The prevalence of JAK2V617F mutation has made it a much anticipated target for inhibition; this review will update and assess progress.

RECENT FINDINGS

Many agents have been studied in preclinical trials, of which few have entered clinical trials. Data from the clinical trials are limited and mostly in the form of abstracts and reviews.

SUMMARY

The prevalence of the JAK2V617F mutation in the classic Philadelphia-chromosome negative myeloproliferative disorders has made it a much anticipated target for inhibition. Present in greater than 90% of patients with polycythemia vera and approximately 50% of patients with essential thrombocythemia and primary myelofibrosis, it has been hoped that targeted inhibition of JAK2V617F would achieve similar disease control as imatinib mesylate has produced in chronic myeloid leukemia. However, JAK2V617F in the Philadelphia-chromosome negative myeloproliferative disorders, unlike bcr/abl tyrosine kinase in chronic myeloid leukemia, is not a causative but rather a secondary somatic mutation. As the JAK2 inhibitors move into phase III clinical trials, their efficacy and role in therapy is becoming clearer; however, there are still many questions needing answers.

Anti-inflammatory Agents: Present and Future

Inflammation involving the innate and adaptive immune systems is a normal response to infection. However, when allowed to continue unchecked, inflammation may result in autoimmune or autoinflammatory disorders, neurodegenerative disease, or cancer. A variety of safe and effective anti-inflammatory agents are available, including aspirin and other nonsteroidal anti-inflammatories, with many more drugs under development. In particular, the new era of anti-inflammatory agents includes "biologicals" such as anticytokine therapies and small molecules that block the activity of kinases. Other anti-inflammatories currently in use or under development include statins, histone deacetylase inhibitors, PPAR agonists, and small RNAs. This Review discusses the current status of anti-inflammatory drug research and the development of new anti-inflammatory therapeutics.

Therapy of myelofibrosis (excluding JAK2 inhibitors)

Myelofibrosis shows a progressive clinical course and usually a poor, lethal prognosis. Allogeneic transplantation is an effective, potentially curable treatment approach although only a minority of patients may currently benefit from it. New effective treatment strategies are becoming available for this disease, including not only JAK2 inhibitors, but also other innovative drugs, targeting more general oncogenic mechanisms and the epigenetic control of cell proliferation and differentiation.

Novel histone deacetylase inhibitors in clinical trials as anti-cancer agents

Histone deacetylases (HDACs) can regulate expression of tumor suppressor genes and activities of transcriptional factors involved in both cancer initiation and progression through alteration of either DNA or the structural components of chromatin. Recently, the role of gene repression through modulation such as acetylation in cancer patients has been clinically validated with several inhibitors of HDACs. One of the HDAC inhibitors, vorinostat, has been approved by FDA for treating cutaneous T-cell lymphoma (CTCL) for patients with progressive, persistent, or recurrent disease on or following two systemic therapies. Other inhibitors, for example, FK228, PXD101, PCI-24781, ITF2357, MGCD0103, MS-275, valproic acid and LBH589 have also demonstrated therapeutic potential as monotherapy or combination with other anti-tumor drugs in CTCL and other malignancies. At least 80 clinical trials are underway, testing more than eleven different HDAC inhibitory agents including both hematological and solid malignancies. This review focuses on recent development in clinical trials testing HDAC inhibitors as anti-tumor agents.

Perspectives for the use of structural information and chemical genetics to develop inhibitors of Janus kinases

Gain-of-function mutations in the genes encoding Janus kinases have been discovered in various haematologic diseases. Jaks are composed of a FERM domain, an SH2 domain, a pseudokinase domain and a kinase domain, and a complex interplay of the Jak domains is involved in regulation of catalytic activity and association to cytokine receptors. Most activating mutations are found in the pseudokinase domain. Here we present recently discovered mutations in the context of our structural models of the respective domains. We describe two structural hotspots in the pseudokinase domain of Jak2 that seem to be associated either to myeloproliferation or to lymphoblastic leukaemia, pointing at the involvement of distinct signalling complexes in these disease settings. The different domains of Jaks are discussed as potential drug targets. We present currently available inhibitors targeting Jaks and indicate structural differences in the kinase domains of the different Jaks that may be exploited in the development of specific inhibitors. Moreover, we discuss recent chemical genetic approaches which can be applied to Jaks to better understand the role of these kinases in their biological settings and as drug targets.

Cotreatment with panobinostat and JAK2 inhibitor TG101209 attenuates JAK2V617F levels and signaling and exerts synergistic cytotoxic effects against human myeloproliferative neoplastic cells

The mutant JAK2V617F tyrosine kinase (TK) is present in the majority of patients with BCR-ABL-negative myeloproliferative neoplasms (MPNs). JAK2V617F activates downstream signaling through the signal transducers and activators of transcription (STAT), RAS/mitogen-activated protein kinase (MAPK), and phosphatidylinositol 3 (PI3)/AKT pathways, conferring proliferative and survival advantages in the MPN hematopoietic progenitor cells (HPCs). Treatment with the pan-histone deacetylase (HDAC) inhibitor panobinostat (PS) is known to inhibit the chaperone function of heat shock protein 90, as well as induce growth arrest and apoptosis of transformed HPCs. Here, we demonstrate that PS treatment depletes the autophosphorylation, expression, and downstream signaling of JAK2V617F. Treatment with PS also disrupted the chaperone association of JAK2V617F with hsp90, promoting proteasomal degradation of JAK2V617F. PS also induced apoptosis of the cultured JAK2V617F-expressing human erythroleukemia HEL92.1.7 and Ba/F3-JAK2V617F cells. Treatment with the JAK2 TK inhibitor TG101209 attenuated JAK2V617F autophosphorylation and induced apoptosis of HEL92.1.7 and Ba/F3-JAK2V617F cells. Cotreatment with PS and TG101209 further depleted JAK/STAT signaling and synergistically induced apoptosis of HEL92.1.7 and Ba/F3-JAK2V617F cells. Cotreatment with TG101209 and PS exerted greater cytotoxicity against primary CD34(+) MPN cells than normal CD34(+) HPCs. These in vitro findings suggest combination therapy with HDAC and JAK2V617F inhibitors is of potential value for the treatment of JAK2V617F-positive MPN.

Triptolide inhibits Jak2 transcription and induces apoptosis in human myeloproliferative disorder cells bearing Jak2V617F through caspase-3-mediated cleavage of Mcl-1

The discovery of oncogene addiction in myeloproliferative disorders (MPDs) driven by the gain-of-function mutant Jak2V617F has attracted intense interest in targeted therapy for MPDs. In this report, we demonstrate that triptolide potently downregulated the transcription of Jak2 by inhibiting the activity of RNA polymerase. Triptolide inhibited the in vitro and in vivo growth of tumor cells harboring Jak2V617F. Triptolide induced abundant apoptosis with a prominent decline of Bcl-2, Bcl-X(L), survivin and Mcl-1. As well, triptolide induced caspase-3-dependent Mcl-1 cleavage, which may potentiate apoptosis. These findings suggest that triptolide is a promising agent to kill Jak2V617F-harboring cells.

Pleiotropic anti-myeloma activity of ITF2357: inhibition of interleukin-6 receptor signaling and repression of miR-19a and miR-19b

BACKGROUND

The histone deacetylase inhibitor ITF2357 has potent cytotoxic activity in multiple myeloma in vitro and has entered clinical trials for this disease.

DESIGN AND METHODS

In order to gain an overall view of the activity of ITF2357 and identify specific pathways that may be modulated by the drug, we performed gene expression profiling of the KMS18 multiple myeloma cell line treated with the drug. The modulation of several genes and their biological consequence were verified in a panel of multiple myeloma cell lines and cells freshly isolated from patients by using polymerase chain reaction analysis and western blotting.

RESULTS

Out of 38,500 human genes, we identified 140 and 574 up-regulated genes and 102 and 556 down-modulated genes at 2 and 6 h, respectively, with a significant presence of genes related to transcription regulation at 2 h and to cell cycling and apoptosis at 6 h. Several of the identified genes are particularly relevant to the biology of multiple myeloma and it was confirmed that ITF2357 also modulated their encoded proteins in different multiple myeloma cell lines. In particular, ITF2357 down-modulated the interleukin-6 receptor alpha (CD126) transcript and protein in both cell lines and freshly isolated patients' cells, whereas it did not significantly modify interleukin-6 receptor beta (CD130) expression. The decrease in CD126 expression was accompanied by decreased signaling by interleukin-6 receptor, as measured by STAT3 phosphorylation in the presence and absence of inter-leukin-6. Finally, the drug significantly down-modulated the MIRHG1 transcript and its associated microRNA, miR-19a and miR-19b, known to have oncogenic activity in multiple myeloma.

CONCLUSIONS

ITF2357 inhibits several signaling pathways involved in myeloma cell growth and survival.

Epigenetic therapy in myeloproliferative neoplasms: evidence and perspectives

The classic Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), which include polycythaemia vera, essential thrombocythaemia and primary myelofibrosis, originate from a stem cell-derived clonal myeloproliferation that manifests itself with variable haematopoietic cell lineage involvement; they are characterized by a high degree of similarities and the chance to transform each to the other and to evolve into acute leukaemia. Their molecular pathogenesis has been associated with recurrent acquired mutations in janus kinase 2 (JAK2) and myeloproliferative leukemia virus oncogene (MPL). These discoveries have simplified the diagnostic approach and provided a number of clues to understanding the phenotypic expression of MPNs; furthermore, they represented a framework for developing and/or testing in clinical trials small molecules acting as tyrosine kinase inhibitors. On the other hand, evidence of abnormal epigenetic gene regulation as a mechanism potentially contributing to the pathogenesis and the phenotypic diversity of MPNs is still scanty; however, study of epigenetics in MPNs represents an active field of research. The first clinical trials with epigenetic drugs have been completed recently, whereas others are still ongoing; results have been variable and at present do not allow any firm conclusion. Novel basic and translational information concerning epigenetic gene regulation in MPNs and the perspectives for therapy will be critically addressed in this review.

Chronic myeloproliferative diseases with and without the Ph chromosome: some unresolved issues

Ph-positive chronic myeloid leukemia (CML) and Ph-negative chronic myeloproliferative diseases (MPDs), characterized in many cases by the presence of the JAK2(V617F) mutation, have many features in common and yet also show fundamental differences. In this review, we pose five discrete and related questions relevant to both categories of hematological malignancy, namely: What are the mechanisms that underlie disease progression from a relatively benign or chronic phase? By what therapeutic methods might one target residual leukemia stem cells in CML? Is JAK2(V617F) the original molecular event in MPD? What epigenetic events must have a role in dictating disease phenotype in MPDs? And finally, Will the benefits conferred by current or future JAK2(V617F) inhibitors equal or even surpass the clinical success that has resulted from the use of tyrosine kinase inhibitors in CML? These and others questions must be addressed and in some cases should be answered in the foreseeable future.

How do JAK2-inhibitors work in myelofibrosis: an alternative hypothesis

The clinical efficacy of JAK2-inhibitors in patients with myelofibrosis, that involves a rapid and massive reduction of spleen enlargement and improvement of clinical symptoms, is not accompanied by significant modifications of hematologic parameters nor of the burden of JAK2V617F allele. Furthermore, clinical improvement has been reported to occur irrespective of patient's JAK2-mutated status. On the other hand, dramatic changes in plasma cytokine levels have been observed. Based on available information about the role of cytokines in the pathogenesis of myelofibrosis, the hypothesis that the clinical efficacy of JAK2-inhibitors could be mainly ascribed to a general down-regulation of cytokine production and cytokine signaling is discussed.

A new era for small molecule screening: from new targets, such as JAK2 V617F, to complex cellular screens

Traditionally reserved to research and development in pharmaceutical companies, screening of small molecule libraries is rapidly becoming an approach undertaken by academic laboratories. Novel cellular assays, sensitive systems to probe function, emerging new molecular targets are just some of the reasons explaining this shift. Targets of small molecules identified in cellular screens begin to be amenable to identification by elegant genetic approaches, such as probing toxicity of candidate small molecules on libraries of genetically modified yeast strains. Several new targets, such as JAK2 V617F, an activated JAK2 (Janus Kinase 2) mutant genetically associated with the majority of human myeloproliferative neoplasms, are being actively pursued. In this Review Series, we will learn how libraries of small molecules are harnessed to identify novel molecules, that alone or in combination, have the ability to alter cell fate, cell signalling, gene expression or response to extracellular cues.

Myeloproliferative disorders

In 1951 William Dameshek classified polycythemia vera (PV), essential thombocytosis (ET), and primary myelofibrosis (PMF) as pathogenetically related myeloproliferative disorders (MPD). Subsequent studies demonstrated that PV, ET, and PMF are clonal disorders of multipotent hematopoietic progenitors. In 2005, a somatic activating mutation in the JAK2 nonreceptor tyrosine kinase (JAK2V617F) was identified in most patients with PV and in a significant proportion of patients with ET and PMF. Subsequent studies identified additional mutations in the JAK-STAT pathway in some patients with JAK2V617F(-) MPD, suggesting that constitutive activation of this signaling pathway is a unifying feature of these disorders. Although the discovery of mutations in the JAK-STAT pathway is important from a pathogenetic and diagnostic perspective, important questions remain regarding the role of this single disease allele in 3 related but clinically distinct disorders, and the role of additional genetic events in MPD disease pathogenesis. In addition, these observations provide a foundation for development of small molecule inhibitors of JAK2 that are currently being tested in clinical trials. This review will discuss our understanding of the pathogenesis of PV, ET, and PMF, the potential role of JAK2-targeted therapy, and the important unanswered questions that need to be addressed to improve clinical outcome.

Aberrant signal transduction pathways in myeloproliferative neoplasms

The BCR-ABL-negative myeloproliferative neoplasms (MPNs), polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF), entered the spotlight in 2005 when the unique somatic acquired JAK2 V617F mutation was described in >95% of PV and in 50% of ET and PMF patients. For the very rare PV patients who do not harbor the JAK2 V617F mutation, exon 12 JAK2 mutants were discovered also to result in activated forms of JAK2. A minority of ET and PMF patients harbor mutations that constitutively activate the thrombopoietin receptor (TpoR). In bone marrow reconstitution models based on retroviral transduction, the phenotype induced by JAK2 V617F is less severe and different from the rapid fatal myelofibrosis induced by TpoR W515L. The reasons for these differences are unknown. Exactly by which mechanism(s) one acquired somatic mutation, JAK2 V617F, can promote three different diseases remains a mystery, although gene dosage and host genetic variation might have important functions. We review the recent progress made in deciphering signaling anomalies in PV, ET and PMF, with an emphasis on the relationship between JAK2 V617F and cytokine receptor signaling and on cross-talk with several other signaling pathways.

Comparison of mutated ABL1 and JAK2 as oncogenes and drug targets in myeloproliferative disorders

Constitutively activated mutants of the non-receptor tyrosine kinases (TK) ABL1 (Abelson murine leukemia viral (v-abl) homolog (1) protein) and JAK2 (JAnus Kinase 2 or Just Another Kinase 2) play a central role in the pathogenesis of clinically and morphologically distinct chronic myeloproliferative disorders but are also found in some cases of de novo acute leukemia and lymphoma. Ligand-independent activation occurs as a consequence of point mutations or insertions/deletions within functionally relevant regulatory domains (JAK2) or the creation of TK fusion proteins by balanced reciprocal translocations, insertions or episomal amplification (ABL1 and JAK2). Specific abnormalities are correlated with clinical phenotype, although some are broad and encompass several World Health Organization-defined entities. TKs are excellent drug targets as exemplified by the activity of imatinib in BCR-ABL1-positive disease, particularly chronic myeloid leukemia. Resistance to imatinib is seen in a minority of cases and is often associated with the appearance of secondary point mutations within the TK domain of BCR-ABL1. These mutations are highly variable in their sensitivity to increased doses of imatinib or alternative TK inhibitors such as nilotinib or dasatinib. Selective and non-selective inhibitors of JAK2 are currently being developed, and encouraging data from pre-clinical experiments and initial phase-I studies regarding efficacy and potential toxicity of these compounds have already been reported.

New Advances in the Pathogenesis and Therapy of Essential Thrombocythemia

Essential thrombocythemia (ET) is a hematopoietic disorder that manifests clinically as thrombocytosis, and patients with ET are at increased risk for developing thrombosis, myelofibrosis, and transformation to acute myeloid leukemia. Although ET was recognized as a distinct clinical syndrome more than 6 decades ago and was classified as a myeloproliferative neoplasm (MPN) by William Dameshek in 1951, the molecular pathogenesis of ET remained unknown until 2005, when activating mutations in the JAK2 tyrosine kinase (JAK2V617F) were identified in a significant proportion of patients with ET, polycythemia vera (PV) and primary myelofibrosis (PMF). In addition, subsequent studies have identified gain-of-function mutations in the thrombopoietin receptor (MPL) in a subset of patients with JAK2V617F-negative ET, suggesting that JAK2 activation by distinct mechanisms contributes to the pathogenesis of ET. Despite these important observations, important questions remain regarding the role of JAK2/MPL mutations in ET pathogenesis, the etiology of JAK2/MPL negative ET, the factors that distinguish ET from other MPNs with the JAK2V617F mutation, and the role of JAK2-targeted therapies for the treatment of these MPNs.

From palliation to epigenetic therapy in myelofibrosis

Myelofibrosis shows a progressive clinical course and usually a poor, lethal prognosis. The molecular pathogenesis of this disease largely remains to be fully understood but the identification of the JAK2V617F mutation in more than half of patients was a major improvement in our understanding of the disease biology and may represent the first biologic marker useful for risk stratification, independently from conventional clinical predictors. After many elusive efforts, new effective treatment strategies are becoming available for this disease. Allogeneic transplantation following reduced-intensity conditioning programs, at least in some patients, may induce not only a hematologic response but also a molecular remission, thus supporting the hope of a possible, definitive eradication of the disease. Moreover, new innovative drugs, targeting either the JAK2V617F mutation or more general oncogenic mechanisms, may provide widely applicable, effective treatments to many patients for whom allogeneic transplantation is not feasible.