Combined MEK and JAK inhibition abrogates murine myeloproliferative neoplasm

Therapeutic efficacy of RAS inhibitor trametinib using a juvenile myelomonocytic leukemia patient-derived xenograft model

Juvenile myelomonocytic leukemia (JMML) is an aggressive pediatric leukemia with few effective treatments and poor outcomes even after stem cell transplantation, the only current curative treatment. We developed a JMML patient-derived xenograft (PDX) mouse model and demonstrated the in vivo therapeutic efficacy and confirmed the target of trametinib, a RAS-RAF-MEK-ERK pathway inhibitor, in this model. A PDX model was created through transplantation of patient JMML cells into mice, up to the second generation, and successful engraftment was confirmed using flow cytometry. JMML PDX mice were treated with trametinib versus vehicle control, with a median survival of 194 days in the treatment group versus 124 days in the control group (p = 0.02). Trametinib's target as a RAS pathway inhibitor was verified by showing inhibition of ERK phosphorylation using immunoblot assays. In conclusion, trametinib monotherapy significantly prolongs survival in our JMML PDX model by inhibiting the RAS pathway. Our model can be effectively used for assessment of novel targeted treatments, including potential combination therapies, to improve JMML outcomes.

A gain-of-function p53 mutant synergizes with oncogenic NRAS to promote acute myeloid leukemia in mice

We previously demonstrated that a subset of acute myeloid leukemia (AML) patients with concurrent RAS pathway and TP53 mutations have an extremely poor prognosis and that most of these TP53 mutations are missense mutations. Here, we report that, in contrast to the mixed AML and T cell malignancy that developed in NrasG12D/+ p53-/- (NP-/-) mice, NrasG12D/+ p53R172H/+ (NPmut) mice rapidly developed inflammation-associated AML. Under the inflammatory conditions, NPmut hematopoietic stem and progenitor cells (HSPCs) displayed imbalanced myelopoiesis and lymphopoiesis and mostly normal cell proliferation despite MEK/ERK hyperactivation. RNA-Seq analysis revealed that oncogenic NRAS signaling and mutant p53 synergized to establish an NPmut-AML transcriptome distinct from that of NP-/- cells. The NPmut-AML transcriptome showed GATA2 downregulation and elevated the expression of inflammatory genes, including those linked to NF-κB signaling. NF-κB was also upregulated in human NRAS TP53 AML. Exogenous expression of GATA2 in human NPmut KY821 AML cells downregulated inflammatory gene expression. Mouse and human NPmut AML cells were sensitive to MEK and NF-κB inhibition in vitro. The proteasome inhibitor bortezomib stabilized the NF-κB-inhibitory protein IκBα, reduced inflammatory gene expression, and potentiated the survival benefit of a MEK inhibitor in NPmut mice. Our study demonstrates that a p53 structural mutant synergized with oncogenic NRAS to promote AML through mechanisms distinct from p53 loss.

MEK inhibitors for neurofibromatosis type 1 manifestations: Clinical evidence and consensus

The wide variety of clinical manifestations of the genetic syndrome neurofibromatosis type 1 (NF1) are driven by overactivation of the RAS pathway. Mitogen-activated protein kinase kinase inhibitors (MEKi) block downstream targets of RAS. The recent regulatory approvals of the MEKi selumetinib for inoperable symptomatic plexiform neurofibromas in children with NF1 have made it the first medical therapy approved for this indication in the United States, the European Union, and elsewhere. Several recently published and ongoing clinical trials have demonstrated that MEKi may have potential benefits for a variety of other NF1 manifestations, and there is broad interest in the field regarding the appropriate clinical use of these agents. In this review, we present the current evidence regarding the use of existing MEKi for a variety of NF1-related manifestations, including tumor (neurofibromas, malignant peripheral nerve sheath tumors, low-grade glioma, and juvenile myelomonocytic leukemia) and non-tumor (bone, pain, and neurocognitive) manifestations. We discuss the potential utility of MEKi in related genetic conditions characterized by overactivation of the RAS pathway (RASopathies). In addition, we review practical treatment considerations for the use of MEKi as well as provide consensus recommendations regarding their clinical use from a panel of experts.

Targeted therapy in juvenile myelomonocytic leukemia: Where are we now?

Juvenile myelomonocytic leukemia (JMML) is a rare and aggressive clonal neoplasm of early childhood, classified as an overlap myeloproliferative/myelodysplastic neoplasm by the World Health Organization. In 90% of the patients with JMML, typical initiating mutations in the canonical Ras pathway genes NF1, PTPN11, NRAS, KRAS, and CBL can be identified. Hematopoietic stem cell transplantation (HSCT) currently is the established standard of care in most patients, although long-term survival is still only 50-60%. Given the limited therapeutic options and the important morbidity and mortality associated with HSCT, new therapeutic approaches are urgently needed. Hyperactivation of the Ras pathway as disease mechanism in JMML lends itself to the use of targeted therapy. Targeted therapy could play an important role in the future treatment of patients with JMML. This review presents a comprehensive overview of targeted therapies already developed and evaluated in vitro and in vivo in patients with JMML.

Epigenetic downregulation of Socs2 contributes to mutant N-Ras-mediated hematopoietic dysregulation

RAS mutations occur in a broad spectrum of human hematopoietic malignancies. Activating Ras mutations in blood cells leads to hematopoietic malignancies in mice. In murine hematopoietic stem cells (HSCs), mutant N-Ras^G12D activates Stat5 to dysregulate stem cell function. However, the underlying mechanism remains elusive. In this study, we demonstrate that Stat5 activation induced by a hyperactive Nras mutant, G12D, is dependent on Jak2 activity. Jak2 is activated in Nras mutant HSCs and progenitors (HSPCs), and inhibiting Jak2 with ruxolitinib significantly decreases Stat5 activation and HSPC hyper-proliferation in vivo in Nras^G12D mice. Activation of Jak2-Stat5 is associated with downregulation of Socs2, an inhibitory effector of Jak2/Stat5. Restoration of Socs2 blocks Nras^G12D HSC reconstitution in bone marrow transplant recipients. SOCS2 downregulation is also observed in human acute myeloid leukemia (AML) cells that carry RAS mutations. RAS mutant AML cells exhibited suppression of the enhancer active marker H3K27ac at the SOCS2 locus. Finally, restoration of SOCS2 in RAS mutant AML cells mitigated leukemic growth. Thus, we discovered a novel signaling feedback loop whereby hyperactive Ras signaling activates Jak2/Stat5 via suppression of Socs2.

Summary: Jak2/Stat5 is often considered to be parallel to or upstream of Ras signaling. We have discovered a novel signaling feedback loop whereby hyperactive Ras signaling activates Jak2/Stat5 via suppression of Socs2.

Effects of NRAS Mutations on Leukemogenesis and Targeting of Children With Acute Lymphoblastic Leukemia

Through the advancements in recent decades, childhood acute lymphoblastic leukemia (ALL) is gradually becoming a highly curable disease. However, the truth is there remaining relapse in ∼15% of ALL cases with dismal outcomes. RAS mutations, in particular NRAS mutations, were predominant mutations affecting relapse susceptibility. KRAS mutations targeting has been successfully exploited, while NRAS mutation targeting remains to be explored due to its complicated and compensatory mechanisms. Using targeted sequencing, we profiled RAS mutations in 333 primary and 18 relapsed ALL patients and examined their impact on ALL leukemogenesis, therapeutic potential, and treatment outcome. Cumulative analysis showed that RAS mutations were associated with a higher relapse incidence in children with ALL. In vitro cellular assays revealed that about one-third of the NRAS mutations significantly transformed Ba/F3 cells as measured by IL3-independent growth. Meanwhile, we applied a high-throughput drug screening method to characterize variable mutation-related candidate targeted agents and uncovered that leukemogenic-NRAS mutations might respond to MEK, autophagy, Akt, EGFR signaling, Polo−like Kinase, Src signaling, and TGF−β receptor inhibition depending on the mutation profile.

Molecular Pathogenesis of Chronic Myelomonocytic Leukemia and Potential Molecular Targets for Treatment Approaches

Numerous examples in oncology have shown that better understanding the pathophysiology of a malignancy may be followed by the development of targeted treatment concepts with higher efficacy and lower toxicity as compared to unspecific treatment. The pathophysiology of chronic myelomonocytic leukemia (CMML) is heterogenous and complex but applying different research technologies have yielded a better and more comprehensive understanding of this disease. At the moment treatment for CMML is largely restricted to the unspecific use of cytotoxic drugs and hypomethylating agents (HMA). Numerous potential molecular targets have been recently detected by preclinical research which may ultimately lead to treatment concepts that will provide meaningful benefits for certain subgroups of patients.

RAS/CBL mutations predict resistance to JAK inhibitors in myelofibrosis and are associated with poor prognostic features

The dysregulation of the JAK/STAT pathway drives the pathogenesis of myelofibrosis (MF). Recently, several JAK inhibitors (JAKis) have been developed for treating MF. Select mutations (MTs) have been associated with impaired outcomes and are currently incorporated in molecularly annotated prognostic models. Mutations of RAS/MAPK pathway genes are frequently reported in cancer and at low frequencies in MF. In this study, we investigated the phenotypic, prognostic, and therapeutic implications of NRASMTs, KRASMTs, and CBLMTs (RAS/CBLMTs) in 464 consecutive MF patients. A total of 59 (12.7%) patients had RAS/CBLMTs: NRASMTs, n = 25 (5.4%); KRASMTs, n = 13 (2.8%); and CBLMTs, n = 26 (5.6%). Patients with RAS/CBLMTs were more likely to present with high-risk clinical and molecular features. RAS/CBLMTs were associated with inferior overall survival compared with patients without MTs and retained significance in a multivariate model, including the Mutation-Enhanced International Prognostic Score System (MIPSS70) risk factors and cytogenetics; however, inclusion of RAS/CBLMTs in molecularly annotated prognostic models did not improve the predictive power of the latter. The 5-year cumulative incidence of leukemic transformation was notably higher in the RAS/CBLMT cohort. Among 61 patients treated with JAKis and observed for a median time of 30 months, the rate of symptoms and spleen response at 6 months was significantly lower in the RAS/CBLMT cohort. Logistic regression analysis disclosed a significant inverse correlation between RAS/CBLMTs and the probability of achieving a symptom or spleen response that was retained in multivariate analysis. In summary, our study showed that RAS/CBLMTs are associated with adverse phenotypic features and survival outcomes and, more important, may predict reduced response to JAKis.

Nras Q61R/+ and Kras-/- cooperate to downregulate Rasgrp1 and promote lympho-myeloid leukemia in early T-cell precursors

Kras^−/−; Nras^Q61R/+ mice develop early onset of T-cell malignancy that recapitulates many biological and molecular features of human ETP-ALL.

We identify Rasgrp1 as a negative regulator of Ras/ERK signaling in oncogenic Nras-driven ETP-like leukemia.

Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is an aggressive subtype of T-cell ALL. Although genetic mutations hyperactivating cytokine receptor/Ras signaling are prevalent in ETP-ALL, it remains unknown how activated Ras signaling contributes to ETP-ALL. Here, we find that in addition to the frequent oncogenic RAS mutations, wild-type (WT) KRAS transcript level was significantly downregulated in human ETP-ALL cells. Similarly, loss of WT Kras in Nras^Q61R/+ mice promoted hyperactivation of extracellular signal-regulated kinase (ERK) signaling, thymocyte hyperproliferation, and expansion of the ETP compartment. Kras^−/−; Nras^Q61R/+ mice developed early onset of T-cell malignancy that recapitulates many biological and molecular features of human ETP-ALL. Mechanistically, RNA-sequencing analysis and quantitative proteomics study identified that Rasgrp1, a Ras guanine nucleotide exchange factor, was greatly downregulated in mouse and human ETP-ALL. Unexpectedly, hyperactivated Nras/ERK signaling suppressed Rasgrp1 expression and reduced Rasgrp1 level led to increased ERK signaling, thereby establishing a positive feedback loop to augment Nras/ERK signaling and promote cell proliferation. Corroborating our cell line data, Rasgrp1 haploinsufficiency induced Rasgrp1 downregulation and increased phosphorylated ERK level and ETP expansion in Nras^Q61R/+ mice. Our study identifies Rasgrp1 as a negative regulator of Ras/ERK signaling in oncogenic Nras-driven ETP-like leukemia.

Expression of NrasQ61R and MYC transgene in germinal center B cells induces a highly malignant multiple myeloma in mice

NRAS Q61 mutations are prevalent in advanced/relapsed multiple myeloma (MM) and correlate with poor patient outcomes. Thus, we generated a novel MM model by conditionally activating expression of endogenous NrasQ61R and an MYC transgene in germinal center (GC) B cells (VQ mice). VQ mice developed a highly malignant MM characterized by a high proliferation index, hyperactivation of extracellular signal-regulated kinase and AKT signaling, impaired hematopoiesis, widespread extramedullary disease, bone lesions, kidney abnormalities, preserved programmed cell death protein 1 and T-cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibition motif domain immune-checkpoint pathways, and expression of human high-risk MM gene signatures. VQ MM mice recapitulate most of the biological and clinical features of human advanced/high-risk MM. These MM phenotypes are serially transplantable in syngeneic recipients. Two MM cell lines were also derived to facilitate future genetic manipulations. Combination therapies based on MEK inhibition significantly prolonged the survival of VQ mice with advanced-stage MM. Our study provides a strong rationale to develop MEK inhibition-based therapies for treating advanced/relapsed MM.

Dual targeting of JAK2 and ERK interferes with the myeloproliferative neoplasm clone and enhances therapeutic efficacy

Myeloproliferative neoplasms (MPN) show dysregulated JAK2 signaling. JAK2 inhibitors provide clinical benefits, but compensatory activation of MAPK pathway signaling impedes efficacy. We hypothesized that dual targeting of JAK2 and ERK1/2 could enhance clone control and therapeutic efficacy. We employed genetic and pharmacologic targeting of ERK1/2 in Jak2V617F MPN mice, cells and patient clinical isolates. Competitive transplantations of Jak2V617F vs. wild-type bone marrow (BM) showed that ERK1/2 deficiency in hematopoiesis mitigated MPN features and reduced the Jak2V617F clone in blood and hematopoietic progenitor compartments. ERK1/2 ablation combined with JAK2 inhibition suppressed MAPK transcriptional programs, normalized cytoses and promoted clone control suggesting dual JAK2/ERK1/2 targeting as enhanced corrective approach. Combined pharmacologic JAK2/ERK1/2 inhibition with ruxolitinib and ERK inhibitors reduced proliferation of Jak2V617F cells and corrected erythrocytosis and splenomegaly of Jak2V617F MPN mice. Longer-term treatment was able to induce clone reductions. BM fibrosis was significantly decreased in MPLW515L-driven MPN to an extent not seen with JAK2 inhibitor monotherapy. Colony formation from JAK2V617F patients' CD34⁺ blood and BM was dose-dependently inhibited by combined JAK2/ERK1/2 inhibition in PV, ET, and MF subsets. Overall, we observed that dual targeting of JAK2 and ERK1/2 was able to enhance therapeutic efficacy suggesting a novel treatment approach for MPN.

RAS/CBL mutations predict resistance to JAK inhibitors in myelofibrosis and are associated with poor prognostic features

The dysregulation of the JAK/STAT pathway drives the pathogenesis of myelofibrosis (MF). Recently, several JAK inhibitors (JAKis) have been developed for treating MF. Select mutations (MTs) have been associated with impaired outcomes and are currently incorporated in molecularly annotated prognostic models. Mutations of RAS/MAPK pathway genes are frequently reported in cancer and at low frequencies in MF. In this study, we investigated the phenotypic, prognostic, and therapeutic implications of NRASMTs, KRASMTs, and CBLMTs (RAS/CBLMTs) in 464 consecutive MF patients. A total of 59 (12.7%) patients had RAS/CBLMTs: NRASMTs, n = 25 (5.4%); KRASMTs, n = 13 (2.8%); and CBLMTs, n = 26 (5.6%). Patients with RAS/CBLMTs were more likely to present with high-risk clinical and molecular features. RAS/CBLMTs were associated with inferior overall survival compared with patients without MTs and retained significance in a multivariate model, including the Mutation-Enhanced International Prognostic Score System (MIPSS70) risk factors and cytogenetics; however, inclusion of RAS/CBLMTs in molecularly annotated prognostic models did not improve the predictive power of the latter. The 5-year cumulative incidence of leukemic transformation was notably higher in the RAS/CBLMT cohort. Among 61 patients treated with JAKis and observed for a median time of 30 months, the rate of symptoms and spleen response at 6 months was significantly lower in the RAS/CBLMT cohort. Logistic regression analysis disclosed a significant inverse correlation between RAS/CBLMTs and the probability of achieving a symptom or spleen response that was retained in multivariate analysis. In summary, our study showed that RAS/CBLMTs are associated with adverse phenotypic features and survival outcomes and, more important, may predict reduced response to JAKis.

After 95 years, it's time to eRASe JMML

Juvenile myelomonocytic leukaemia (JMML) is a rare clonal disorder of early childhood. Constitutive activation of the RAS pathway is the initial event in JMML. Around 90% of patients diagnosed with JMML carry a mutation in the PTPN11, NRAS, KRAS, NF1 or CBL genes. It has been demonstrated that after this first genetic event, an additional somatic mutation or epigenetic modification is involved in disease progression. The available genetic and clinical data have enabled researchers to establish relationships between JMML and several clinical conditions, including Noonan syndrome, Ras-associated lymphoproliferative disease, and Moyamoya disease. Despite scientific progress and the development of more effective treatments, JMML is still a deadly disease: the 5-year survival rate is ~50%. Here, we report on recent research having led to a better understanding of the genetic and molecular mechanisms involved in JMML.

Blockade of PDGFRβ circumvents resistance to MEK-JAK inhibition via intratumoral CD8+ T-cells infiltration in triple-negative breast cancer

BACKGROUND

Despite the increasing progress in targeted and immune based-directed therapies for other solid organ malignancies, currently there is no targeted therapy available for TNBCs. A number of mechanisms have been reported both in pre-clinical and clinical settings that involve inherent, acquired and adaptive resistance to small molecule inhibitors. Here, we demonstrated a novel resistance mechanism in TNBC cells mediated by PDGFRβ in response to JAK2 inhibition.

METHODS

Multiple in vitro (subG1, western blotting, immunofluorescence, RT-PCR, Immunoprecipitation), in vivo and publically available datasets were used.

RESULTS

We showed that TNBC cells exposed to MEK1/2-JAK2 inhibitors exhibit resistant colonies in anchorage-independent growth assays. Moreover, cells treated with various small molecule inhibitors including JAK2 promote PDGFRβ upregulation. Using publically available databases, we showed that patients expressing high PDGFRβ or its ligand PDGFB exhibit poor relapse-free survival upon chemotherapeutic treatment. Mechanistically we found that JAK2 expression controls steady state levels of PDGFRβ. Thus, co-blockade of PDGFRβ with JAK2 and MEK1/2 inhibitors completely eradicated resistant colonies in vitro. We found that triple-combined treatment had a significant impact on CD44⁺/CD24^- stem-cell-like cells. Likewise, we found a significant tumor growth inhibition in vivo through intratumoral CD8⁺ T cells infiltration in a manner that is reversed by anti-CD8 antibody treatment.

CONCLUSION

These findings reveal a novel regulatory role of JAK2-mediated PDGFRβ proteolysis and provide an example of a PDGFRβ-mediated resistance mechanism upon specific target inhibition in TNBC.

[Advances in the treatment of juvenile myelomonocytic leukemia]

Juvenile myelomonocytic leukemia (JMML) is a rare chronic myeloid leukemia in children and has the features of both myelodysplastic syndrome and myeloproliferative neoplasm. It is highly malignant and has a poor treatment outcome. Children with JMML have a poor response to conventional chemotherapy. At present, hematopoietic stem cell transplantation is the only possible cure for this disease. In recent years, significant progress has been made in targeted therapy for mutant genes in the Ras signaling pathway and demethylation treatment of aberrant methylation of polygenic CpG islands. This article reviews the treatment and efficacy evaluation of JMML.

Unique dependence on Sos1 in Kras G12D -induced leukemogenesis

We and others have previously shown that Kras ^G12D is a much more potent oncogene than oncogenic Nras in hematological malignancies. We attributed the strong leukemogenic activity of Kras^G12D at least partially to its unique capability to hyperactivate wild-type (WT) Nras and Hras. Here, we report that Sos1, a guanine nucleotide exchange factor, is required to mediate this process. Sos1 is overexpressed in Kras ^G12D/+ cells, but not in Nras ^Q61R/+ and Nras ^G12D/+ cells. Kras^G12D proteins form a complex with Sos1 in vivo. Sos1 deficiency attenuates hyperactivation of WT Nras, Hras, and the downstream ERK signaling in Kras ^G12D/+ cells. Thus, Sos1 deletion ameliorates oncogenic Kras-induced myeloproliferative neoplasm (MPN) phenotypes and prolongs the survival of Kras ^G12D/+ mice. In contrast, Sos1 is dispensable for hyperactivated granulocyte-macrophage colony-stimulating factor signaling in Nras ^Q61R/+ cells, and Sos1 ^-/- does not affect MPN phenotypes in Nras ^Q61R/+ mice. Moreover, the survival of Kras ^G12D/+ ; Sos1 ^-/- recipients is comparable to that of Kras ^G12D/+ recipients treated with combined MEK and JAK inhibitors. Our study suggests that targeting Sos1-oncogenic Kras interaction may improve the survival of cancer patients with KRAS mutations.

Downregulating Notch counteracts KrasG12D-induced ERK activation and oxidative phosphorylation in myeloproliferative neoplasm

The Notch signaling pathway contributes to the pathogenesis of a wide spectrum of human cancers, including hematopoietic malignancies. Its functions are highly dependent on the specific cellular context. Gain-of-function NOTCH1 mutations are prevalent in human T cell leukemia, while loss of Notch signaling is reported in myeloid leukemias. Here, we report a novel oncogenic function of Notch signaling in oncogenic Kras-induced myeloproliferative neoplasm (MPN). We find that downregulation of Notch signaling in hematopoietic cells via DNMAML expression or Pofut1 deletion significantly blocks MPN development in Kras^G12D mice in a cell-autonomous manner. Further mechanistic studies indicate that inhibition of Notch signaling significantly upregulates Dusp1, a dual phosphatase that inactivates p-ERK, and downregulates cytokine-evoked ERK activation in Kras^G12D cells. Moreover, mitochondrial metabolism is greatly enhanced in Kras^G12D cells but significantly reprogrammed by DNMAML close to that in control cells. Consequently, cell proliferation and expanded myeloid compartment in Kras^G12D mice are significantly reduced. Consistent with these findings, combined inhibition of the MEK/ERK pathway and mitochondrial oxidative phosphorylation effectively inhibited the growth of human and mouse leukemia cells in vitro. Our study provides a strong rational to target both ERK signaling and aberrant metabolism in oncogenic Ras-driven myeloid leukemia.

Myeloid neoplasms with features intermediate between primary myelofibrosis and chronic myelomonocytic leukemia

Monocytosis can develop during disease course in primary myelofibrosis simulating that seen in chronic myelomonocytic leukemia, and should not lead to disease reclassification. In contrast, at presentation, rare cases have clinical, morphologic, and molecular genetic features truly intermediate between primary myelofibrosis and chronic myelomonocytic leukemia. The taxonomy and natural history of these diseases are unclear. We identified cases which either: (1) fulfilled the 2008 World Health Organization criteria for primary myelofibrosis but had absolute monocytosis and, when available, chronic myelomonocytic leukemia-related mutations (ASXL1, SRSF2, TET2) or (2) fulfilled criteria of chronic myelomonocytic leukemia but had megakaryocytic proliferation and atypia, marrow fibrosis, and myeloproliferative-type driver mutations (JAK2, MPL, CALR). Patients with established primary myelofibrosis who developed monocytosis and those with chronic myelomonocytic leukemia with marrow fibrosis were excluded. By combining the pathology databases of two large institutions, six eligible cases were identified. Patients were predominantly male and elderly with monocytosis at diagnosis (average 17.5%/2.3 × 10³/μl), organomegaly, primary myelofibrosis-like atypical megakaryocytes admixed with a variable number of chronic myelomonocytic leukemia-like hypolobated forms, variable myelodysplasia, marrow fibrosis and osteosclerosis. All had a normal karyotype and no myelodysplasia-associated cytogenetic abnormalities. Five of the patients in whom a more extensive molecular characterization was performed showed co-mutations involving JAK2 or MPL and ASXL1, SRSF2, TET2, NRAS, and/or KRAS. Disease progression has occurred in all and two have died. Rare patients present with features that overlap between primary myelofibrosis and chronic myelomonocytic leukemia and are thus difficult to classify based on current World Health Organization criteria. Biologically, these cases likely represent primary myelofibrosis with monocytosis, dysplasia, and secondary (non-driver) mutations at presentation. Alternatively, they may represent a true gray zone of neoplasms. Their clinical behavior appears aggressive and innovative therapeutic approaches may be beneficial in this particular subset.

Novel Therapies for Myelofibrosis

PURPOSE OF REVIEW

The purpose of the review was to provide a contemporary update of novel agents and targets under investigation in myelofibrosis in the Janus kinase (JAK) inhibitor era.

RECENT FINDINGS

Myelofibrosis (MF) is a clonal stem cell disease characterized by marrow fibrosis and a heterogeneous disease phenotype with a variable degree of splenomegaly, cytopenias, and constitutional symptoms that significantly impact quality of life and survival. Overactive JAK/STAT signaling is a hallmark of MF. The only approved therapy for MF, JAK1/2 inhibitor ruxolitinib, can ameliorate splenomegaly, improve symptoms, and prolong survival in some patients. Therapeutic challenges remain, however. Myelosuppression limits the use of ruxolitinib in some patients, eventual drug resistance is common, and the underlying malignant clone persists despite therapy. A deeper understanding of the pathogenesis of MF has informed the development of additional agents. Promising targets under investigation include JAK1 and JAK2 and downstream intermediates in related signaling pathways, epigenetic modifiers, pro-inflammatory cytokines, and immune regulators.

DNA-hypomethylating agents as epigenetic therapy before and after allogeneic hematopoietic stem cell transplantation in myelodysplastic syndromes and juvenile myelomonocytic leukemia

Myelodysplastic syndrome (MDS) is a clonal bone marrow disorder, typically of older adults, which is characterized by ineffective hematopoiesis, peripheral blood cytopenias and risk of progression to acute myeloid leukemia. Juvenile myelomonocytic leukemia (JMML) is an aggressive myeloproliferative neoplasm occurring in young children. The common denominator of these malignant myeloid disorders is the limited benefit of conventional chemotherapy and a particular responsiveness to epigenetic therapy with the DNA-hypomethylating agents 5-azacytidine (azacitidine) or decitabine. However, hypomethylating therapy does not eradicate the malignant clone in MDS or JMML and allogeneic hematopoietic stem cell transplantation (HSCT) remains the only curative treatment option. An emerging concept with intriguing potential is the combination of hypomethylating therapy and HSCT. Possible advantages include disease control with good tolerability during donor search and HSCT preparation, improved antitumoral alloimmunity, and reduced risk of relapse even with non-myeloablative regimens. Herein we review the current role of pre- and post-transplant therapy with hypomethylating agents in MDS and JMML.

JAK1/2 Inhibitors AZD1480 and CYT387 Inhibit Canine B-Cell Lymphoma Growth by Increasing Apoptosis and Disrupting Cell Proliferation

Background

Canine diffuse large B‐cell lymphoma (DLBCL) is a common and aggressive hematologic malignancy. The lack of conventional therapies with sustainable efficacy warrants further investigation of novel therapeutics. The Janus kinase (JAK) and signal transducer and activator of transcription (STAT) pathways play important roles in the pathogenesis of hematologic malignancies in humans including DLBCLs. AZD1480 and CYT387 are novel JAK1/2 inhibitors that have been used in clinical trials for treating various hematologic cancers in humans. No studies have characterized the antitumor effects of JAK inhibitors on DLBCL in dogs.

Hypothesis/Objectives

We hypothesize that JAK1/2 inhibitors AZD1480 and CYT387 can effectively inhibit growth of canine DLBCL in vitro. We aim to assess the antitumor activity of AZD1480 and CYT387 in canine DLBCL and to determine the underlying mechanisms of action.

Methods

In vitro study of canine lymphoma cell growth, proliferation, and apoptosis by viability, proliferation and apoptosis assays.

Results

A significant decrease in viable canine lymphoma cells was observed after AZD1480 and CYT387 treatments. In addition, AZD1480 and CYT387 treatment resulted in decreased lymphoma cell proliferation and increased early apoptosis.

Conclusion and Clinical Importance

AZD1480 and CYT387 inhibit canine lymphoma cell growth in a dose‐dependent manner. Our findings justify further phase I/II clinical investigations of the safety and efficacy of JAK1/2 inhibitors in canine DLBCL and suggest new opportunities for novel anticancer therapies.

Turning the tide in myelodysplastic/myeloproliferative neoplasms

Myelodysplastic syndromes/myeloproliferative neoplasms (MDS/MPN) are aggressive myeloid malignancies recognized as a distinct category owing to their unique combination of dysplastic and proliferative features. Although current classification schemes still emphasize morphology and exclusionary criteria, disease-defining somatic mutations and/or germline predisposition alleles are increasingly incorporated into diagnostic algorithms. The developing picture suggests that phenotypes are driven mostly by epigenetic mechanisms that reflect a complex interplay between genotype, physiological processes such as ageing and interactions between malignant haematopoietic cells and the stromal microenvironment of the bone marrow. Despite the rapid accumulation of genetic knowledge, therapies have remained nonspecific and largely inefficient. In this Review, we discuss the pathogenesis of MDS/MPN, focusing on the relationship between genotype and phenotype and the molecular underpinnings of epigenetic dysregulation. Starting with the limitations of current therapies, we also explore how the available mechanistic data may be harnessed to inform strategies to develop rational and more effective treatments, and which gaps in our knowledge need to be filled to translate biological understanding into clinical progress.

Acquired expression of CblQ367P in mice induces dysplastic myelopoiesis mimicking chronic myelomonocytic leukemia

Chronic myelomonocytic leukemia (CMML) is a hematological malignancy characterized by uncontrolled proliferation of dysplastic myelomonocytes and frequent progression to acute myeloid leukemia (AML). We identified mutations in the Cbl gene, which encodes a negative regulator of cytokine signaling, in a subset of CMML patients. To investigate the contribution of mutant Cbl in CMML pathogenesis, we generated conditional knockin mice for Cbl that express wild-type Cbl in a steady state and inducibly express Cbl^Q367P , a CMML-associated Cbl mutant. Cbl^Q367P mice exhibited sustained proliferation of myelomonocytes, multilineage dysplasia, and splenomegaly, which are the hallmarks of CMML. The phosphatidylinositol 3-kinase (PI3K)-AKT and JAK-STAT pathways were constitutively activated in Cbl^Q367P hematopoietic stem cells, which promoted cell cycle progression and enhanced chemokine-chemokine receptor activity. Gem, a gene encoding a GTPase that is upregulated by Cbl^Q367P , enhanced hematopoietic stem cell activity and induced myeloid cell proliferation. In addition, Evi1, a gene encoding a transcription factor, was found to cooperate with Cbl^Q367P and progress CMML to AML. Furthermore, targeted inhibition for the PI3K-AKT and JAK-STAT pathways efficiently suppressed the proliferative activity of Cbl^Q367P -bearing CMML cells. Our findings provide insights into the molecular mechanisms underlying mutant Cbl-induced CMML and propose a possible molecular targeting therapy for mutant Cbl-carrying CMML patients.

p53-/- synergizes with enhanced NrasG12D signaling to transform megakaryocyte-erythroid progenitors in acute myeloid leukemia

Somatic mutations in TP53 and NRAS are associated with transformation of human chronic myeloid diseases to acute myeloid leukemia (AML). Here, we report that concurrent RAS pathway and TP53 mutations are identified in a subset of AML patients and confer an inferior overall survival. To further investigate the genetic interaction between p53 loss and endogenous Nras^G12D/+ in AML, we generated conditional Nras^G12D/+p53^-/- mice. Consistent with the clinical data, recipient mice transplanted with Nras^G12D/+p53^-/- bone marrow cells rapidly develop a highly penetrant AML. We find that p53^-/- cooperates with Nras^G12D/+ to promote increased quiescence in megakaryocyte-erythroid progenitors (MEPs). Nras^G12D/+p53^-/- MEPs are transformed to self-renewing AML-initiating cells and are capable of inducing AML in serially transplanted recipients. RNA sequencing analysis revealed that transformed MEPs gain a partial hematopoietic stem cell signature and largely retain an MEP signature. Their distinct transcriptomes suggests a potential regulation by p53 loss. In addition, we show that during AML development, transformed MEPs acquire overexpression of oncogenic Nras, leading to hyperactivation of ERK1/2 signaling. Our results demonstrate that p53^-/- synergizes with enhanced oncogenic Nras signaling to transform MEPs and drive AML development. This model may serve as a platform to test candidate therapeutics in this aggressive subset of AML.

Diagnosis and treatment of juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is a rare myelodysplastic/myeloproliferative disorder that occurs during infancy and early childhood; this disorder is characterized by hypersensitivity of the myeloid progenitor cells to granulocyte-macrophage colony-stimulating factor in vitro. JMML usually involves somatic and/or germline mutations in the genes of the RAS pathway, including PTPN11, NRAS, KRAS, NF1, and CBL, in the leukemic cells. Almost all patients with JMML experience an aggressive clinical course, and hematopoietic stem cell transplantation (HSCT) is the only curative treatment. A certain proportion of patients with somatic NRAS and germline mutations in CBL, however, have spontaneous resolution. A suitable treatment after diagnosis and conditioning regimen prior to HSCT are yet to be determined, but several clinical trials have been initiated throughout the world to develop suitable pre- or post-allogeneic HSCT treatments and new targeted therapies that are less toxic, to improve patient outcome.

Stat5 is critical for the development and maintenance of myeloproliferative neoplasm initiated by Nf1 deficiency

Juvenile myelomonocytic leukemia is a rare myeloproliferative neoplasm characterized by hyperactive RAS signaling. Neurofibromin1 (encoded by the NF1 gene) is a negative regulator of RAS activation. Patients with neurofibromatosis type 1 harbor loss-of-function mutations in NF1 and have a 200- to 500-fold increased risk of juvenile myelomonocytic leukemia. Leukemia cells from patients with juvenile myelomonocytic leukemia display hypersensitivity to certain cytokines, such as granulocyte-macrophage colony-stimulating factor. The granulocyte-macrophage colony-stimulating factor receptor utilizes pre-associated JAK2 to initiate signals after ligand binding. JAK2 subsequently activates STAT5, among other downstream effectors. Although STAT5 is gaining recognition as an important mediator of growth factor signaling in myeloid leukemias, the contribution of STAT5 to the development of hyperactive RAS-initiated myeloproliferative disease has not been well described. In this study, we investigated the consequence of STAT5 attenuation via genetic and pharmacological approaches in Nf1-deficient murine models of juvenile myelomonocytic leukemia. We found that homozygous Stat5 deficiency extended the lifespan of Nf1-deficient mice and eliminated the development of myeloproliferative neoplasm associated with Nf1 gene loss. Likewise, we found that JAK inhibition with ruxolitinib attenuated myeloproliferative neoplasm in Nf1-deficient mice. Finally, we found that primary cells from a patient with KRAS-mutant juvenile myelomonocytic leukemia displayed reduced colony formation in response to JAK2 inhibition. Our findings establish a central role for STAT5 activation in the pathogenesis of juvenile myelomonocytic leukemia and suggest that targeting this pathway may be of clinical utility in these patients.

Kras is Required for Adult Hematopoiesis

Previous studies indicate that Kras is dispensable for fetal liver hematopoiesis, but its role in adult hematopoiesis remains unclear. Here, we generated a Kras conditional knockout allele to address this question. Deletion of Kras in adult bone marrow (BM) is mediated by Vav-Cre or inducible Mx1-Cre. We find that loss of Kras leads to greatly reduced thrombopoietin (TPO) signaling in hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs), while stem cell factor-evoked ERK1/2 activation is not affected. The compromised TPO signaling is associated with reduced long term- and intermediate-term HSC compartments and a bias toward myeloid differentiation in MPPs. Although granulocyte macrophage colony-stimulating factor (GM-CSF)-evoked ERK1/2 activation is only moderately decreased in Kras(-/-) myeloid progenitors, it is blunted in neutrophils and neutrophil survival is significantly reduced in vitro. At 9-12 months old, Kras conditional knockout mice develop profound hematopoietic defects, including splenomegaly, an expanded neutrophil compartment, and reduced B cell number. In a serial transplantation assay, the reconstitution potential of Kras(-/-) BM cells is greatly compromised, which is attributable to defects in the self-renewal of Kras(-/-) HSCs and defects in differentiated hematopoietic cells. Our results demonstrate that Kras is a major regulator of TPO and GM-CSF signaling in specific populations of hematopoietic cells and its function is required for adult hematopoiesis. Stem Cells 2016;34:1859-1871.

An International MDS/MPN Working Group's perspective and recommendations on molecular pathogenesis, diagnosis and clinical characterization of myelodysplastic/myeloproliferative neoplasms

In the 2008 WHO classification, chronic myeloid malignancies that share both myelodysplastic and myeloproliferative features define the myelodysplastic/myeloproliferative group, which includes chronic myelomonocytic leukemia, juvenile myelomonocytic leukemia, atypical chronic myeloid leukemia, refractory anemia with ring sideroblasts and thrombocytosis, and myelodysplastic/myeloproliferative unclassified. With the notable exception of refractory anemia with ring sideroblasts and thrombocytosis, there is much overlap among the various subtypes at the molecular and clinical levels, and a better definition of these entities, an understanding of their biology and an identification of subtype-specific molecular or cellular markers are needed. To address some of these challenges, a panel comprised of laboratory and clinical experts in myelodysplastic/myeloproliferative was established, and four independent academic MDS/MPN workshops were held on: 9(th) March 2013, in Miami, Florida, USA; 6(th) December 2013, in New Orleans, Louisiana, USA; 13(th) June 2014 in Milan, Italy; and 5(th) December 2014 in San Francisco, USA. During these meetings, the current understanding of these malignancies and matters of biology, diagnosis and management were discussed. This perspective and the recommendations on molecular pathogenesis, diagnosis and clinical characterization for adult onset myelodysplastic/myeloproliferative is the result of a collaborative project endorsed and supported by the MDS Foundation.

Loss of wild-type Kras promotes activation of all Ras isoforms in oncogenic Kras-induced leukemogenesis

Despite the well-established role of oncogenic RAS in promoting tumor formation, whether and how wild-type (WT) Ras inhibits tumorigenesis under physiological conditions remains controversial. Here, we show that in a fraction of endogenous oncogenic Kras-induced hematopoietic malignancies, including acute T-cell lymphoblastic leukemia/lymphoma (T-ALL) and myeloproliferative neoplasm (MPN), WT Kras expression is lost through epigenetic or genetic mechanisms. Using conditional Kras(G12D/-) mice, we find that WT Kras deficiency promotes oncogenic Kras-induced MPN, but not T-ALL, in a cell-autonomous manner. Loss of WT Kras rescues oncogenic Kras-mediated hematopoietic stem cell depletion and further enhances granulocyte-macrophage colony-stimulating factor signaling in myeloid cells expressing oncogenic Kras. Quantitative signaling studies reveal that oncogenic Kras but not oncogenic Nras leads to cross-activation of WT Ras, whereas loss of WT Kras further promotes the activation of all Ras isoforms. Our results demonstrate the tumor suppressor function of WT Kras in oncogenic Kras-induced leukemogenesis and elucidate its underlying cellular and signaling mechanisms.

Clinical management of myelodysplastic syndrome/myeloproliferative neoplasm overlap syndromes

The myelodysplastic/myeloproliferative neoplasms (MDS/MPNs) are a unique group of hematologic malignancies characterized by concomitant myelodysplastic and myeloproliferative features. According to the 2008 WHO classification, the category includes atypical chronic myeloid leukemia (aCML), chronic myelomonocytic leukemia (CMML), juvenile myelomonocytic leukemia (JMML), MDS/MPN-unclassifiable (MDS/MPN-U), and the provisional entity refractory anemia with ring sideroblasts and thrombocytosis (RARS-T). Although diagnosis currently remains based on clinicopathologic features, the incorporation of next-generation platforms has allowed for the recent molecular characterization of these diseases which has revealed unique and complex mutational profiles that support their distinct biology and is anticipated to soon play an integral role in diagnosis, prognostication, and treatment. Future goals of research should include the development of disease-modifying therapies, and further genetic understanding of the category will likely form the foundation of these efforts.

Evolving Therapeutic Strategies for the Classic Philadelphia-Negative Myeloproliferative Neoplasms

Despite the emergence of JAK inhibitors, there is a need for disease-modifying treatments for Philadelphia-negative myeloproliferative neoplasms (MPNs). JAK inhibitors ameliorate symptoms and address splenomegaly, but because of the heterogeneous contributors to the disease process, JAK inhibitor monotherapy incompletely addresses the burden of disease. The ever-growing understanding of MPN pathogenesis has provided the rationale for testing novel and targeted therapeutic agents, as monotherapies or in combination, in preclinical and clinical settings. A number of intriguing options have emerged, and it is hoped that further progress will lead to significant changes in the natural history of MPNs.

Single agent BMS-911543 Jak2 inhibitor has distinct inhibitory effects on STAT5 signaling in genetically engineered mice with pancreatic cancer

The Jak/STAT pathway is activated in human pancreatic ductal adenocarcinoma (PDAC) and cooperates with mutant Kras to drive initiation and progression of PDAC in murine models. We hypothesized that the small-molecule Jak2 inhibitor (BMS-911543) would elicit anti-tumor activity against PDAC and decrease immune suppressive features of the disease. We used an aggressive genetically engineered PDAC model with mutant KrasG12D, tp53R270H, and Brca1 alleles (KPC-Brca1 mice). Mice with confirmed tumor burden were treated orally with vehicle or 30 mg/kg BMS-911543 daily for 14 days. Histologic analysis of pancreata from treated mice revealed fewer foci of adenocarcinoma and significantly decreased Ki67+ cells versus controls. In vivo administration of BMS-911543 significantly reduced pSTAT5 and FoxP3 positive cells within the pancreas, but did not alter STAT3 phosphorylation. Continuous dosing of KPC-Brca1 mice with BMS-911543 resulted in a median survival of 108 days, as compared to a median survival of 87 days in vehicle treated animals, a 23% increase (p = 0.055). In vitro experiments demonstrated that PDAC cell lines were poorly sensitive to BMS-911543, requiring high micromolar concentrations to achieve targeted inhibition of Jak/STAT signaling. Similarly, BMS-911543 had little in vitro effect on the viability of both murine and human PDAC-derived stellate cell lines. However, BMS-911543 potently inhibited phosphorylation of pSTAT3 and pSTAT5 at low micromolar doses in human PBMC and reduced in vitro differentiation of Foxp3+ T regulatory cells. These results indicate that single agent Jak2i deserves further study in preclinical models of PDAC and has distinct inhibitory effects on STAT5 mediated signaling.

Chronic myelomonocytic leukemia: Forefront of the field in 2015

Chronic myelomonocytic leukemia (CMML) includes components of both myelodysplastic syndrome and myeloproliferative neoplasms and is associated with a characteristic peripheral monocytosis. CMML is caused by the proliferation of an abnormal hematopoietic stem cell clone and may be influenced by microenvironmental changes. The disease is rare and has undergone revisions in its classification. We review the recent classification strategies as well as diagnostic criteria, focusing on CMML's genetic alterations and unique pathophysiology. We also discuss the latest molecular characterization of the disease, including how molecular factors affect current prognostic models. Finally, we focus on available treatment strategies, with a special emphasis on experimental and forthcoming therapies.

Wild-Type N-Ras, Overexpressed in Basal-like Breast Cancer, Promotes Tumor Formation by Inducing IL-8 Secretion via JAK2 Activation

Basal-like breast cancers (BLBCs) are aggressive, and their drivers are unclear. We have found that wild-type N-RAS is overexpressed in BLBCs but not in other breast cancer subtypes. Repressing N-RAS inhibits transformation and tumor growth, whereas overexpression enhances these processes even in preinvasive BLBC cells. We identified N-Ras-responsive genes, most of which encode chemokines; e.g., IL8. Expression levels of these chemokines and N-RAS in tumors correlate with outcome. N-Ras, but not K-Ras, induces IL-8 by binding and activating the cytoplasmic pool of JAK2; IL-8 then acts on both the cancer cells and stromal fibroblasts. Thus, BLBC progression is promoted by increasing activities of wild-type N-Ras, which mediates autocrine/paracrine signaling that can influence both cancer and stroma cells.

Deficiency of β Common Receptor Moderately Attenuates the Progression of Myeloproliferative Neoplasm in NrasG12D/+ Mice

Activating Ras signaling is a major driver in juvenile and the myeloproliferative variant of chronic myelomonocytic leukemia (JMML/MP-CMML). Numerous studies suggest that GM-CSF signaling plays a central role in establishing and maintaining JMML/MP-CMML phenotypes in human and mouse. However, it remains elusive how GM-CSF signaling impacts on JMML/MP-CMML initiation and progression. Here, we investigate this issue in a well characterized MP-CMML model induced by endogenous Nras(G12D/+) mutation. In this model, Nras(G12D/+) hematopoietic stem cells (HSCs) are required to initiate and maintain CMML phenotypes and serve as CMML-initiating cells. We show that the common β chain of the GM-CSF receptor (βc) is dispensable for Nras(G12D/+) HSC function; loss of βc does not affect the expansion, increased self-renewal, or myeloid differentiation bias in Nras(G12D/+) HSCs. Therefore, βc(-/-) does not abrogate CMML in Nras(G12D/+) mice. However, βc deficiency indeed significantly reduces Nras(G12D/+)-induced splenomegaly and spontaneous colony formation and prolongs the survival of CMML-bearing mice, suggesting that GM-CSF signaling plays an important role in promoting CMML progression. Together, our results suggest that inhibiting GM-CSF signaling in JMML/MP-CMML patients might alleviate disease symptoms but would not eradicate the disease.

Loss of Dnmt3a and endogenous Kras(G12D/+) cooperate to regulate hematopoietic stem and progenitor cell functions in leukemogenesis

Oncogenic NRAS and KRAS mutations are prevalent in human juvenile and chronic myelomonocytic leukemia (JMML/CMML). However, additional genetic mutations cooperating with oncogenic RAS in JMML/ CMML progression and/or their transformation to acute myeloid leukemia (AML) remain largely unknown. Here we tested the potential genetic interaction of DNMT3A mutations and oncogenic RAS mutations in leukemogenesis. We found that Dnmt3a(-/-) induces multiple hematopoietic phenotypes after a prolonged latency, including T-cell expansion in the peripheral blood, stress erythropoiesis in the spleen and myeloid malignancies in the liver. Dnmt3a(-/-) significantly promoted JMML/CMML progression and shortened the survival of Kras(G12D/+) mice in a cell-autonomous manner. Similarly, downregulating Dnmt3a also promoted myeloid malignancies in Nras(G12D/+) mice. Further studies show that Dnmt3a deficiency rescues Kras(G12D/+)-mediated depletion of hematopoietic stem cells and increases self-renewal of Kras(G12D/+) myeloid progenitors (MPs). Moreover, ~33% of animals developed an AML-like disease, which is driven by Kras(G12D/+); Dnmt3a(-/-) MPs. Consistent with our result, COSMIC database mining demonstrates that the combination of oncogenic RAS and DNMT3A mutations exclusively occurred in patients with JMML, CMML or AML. Our results suggest that DNMT3A mutations and oncogenic RAS cooperate to regulate hematopoietic stem and progenitor cells and promote myeloid malignancies.

Bedside to bench in juvenile myelomonocytic leukemia: insights into leukemogenesis from a rare pediatric leukemia

Juvenile myelomonocytic leukemia (JMML) is a typically aggressive myeloid neoplasm of childhood that is clinically characterized by overproduction of monocytic cells that can infiltrate organs, including the spleen, liver, gastrointestinal tract, and lung. JMML is categorized as an overlap myelodysplastic syndrome/myeloproliferative neoplasm (MDS/MPN) by the World Health Organization and also shares some clinical and molecular features with chronic myelomonocytic leukemia, a similar disease in adults. Although the current standard of care for patients with JMML relies on allogeneic hematopoietic stem cell transplant, relapse is the most frequent cause of treatment failure. Tremendous progress has been made in defining the genomic landscape of JMML. Insights from cancer predisposition syndromes have led to the discovery of nearly 90% of driver mutations in JMML, all of which thus far converge on the Ras signaling pathway. This has improved our ability to accurately diagnose patients, develop molecular markers to measure disease burden, and choose therapeutic agents to test in clinical trials. This review emphasizes recent advances in the field, including mapping of the genomic and epigenome landscape, insights from new and existing disease models, targeted therapeutics, and future directions.

STAT3 supports experimental K-RasG12D-induced murine myeloproliferative neoplasms dependent on serine phosphorylation

Juvenile myelomonocytic leukemia, acute myeloid leukemia (AML), and other myeloproliferative neoplasms (MPNs) are genetically heterogeneous but frequently display activating mutations in Ras GTPases and activation of signal transducer and activator of transcription 3 (STAT3). Altered STAT3 activity is observed in up to 50% of AML correlating with poor prognosis. Activated STAT proteins, classically associated with tyrosine phosphorylation, support tumor development as transcription factors, but alternative STAT functions independent of tyrosine phosphorylation have been documented, including roles for serine-phosphorylated STAT3 in mitochondria supporting transformation by oncogenic Ras. We examined requirements for STAT3 in experimental murine K-Ras-dependent hematopoietic neoplasia. We show that STAT3 is phosphorylated on S727 but not Y705 in diseased animals. Moreover, a mouse with a point mutation abrogating STAT3 S727 phosphorylation displayed delayed onset and decreased disease severity with significantly extended survival. Activated K-Ras required STAT3 for cytokine-independent growth of myeloid progenitors in vitro, and mitochondrially restricted STAT3 and STAT3-Y705F, both transcriptionally inert mutants, supported factor-independent growth. STAT3 was dispensable for growth of normal or K-Ras-mutant myeloid progenitors in response to cytokines. However, abrogation of STAT3-S727 phosphorylation impaired factor-independent malignant growth. These data document that serine-phosphorylated mitochondrial STAT3 supports neoplastic hematopoietic cell growth induced by K-Ras.