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The acquired JAK2 V617F mutation is observed in the majority of patients with BCR-ABL1 negative chronic myeloproliferative neoplasms (MPN). BCR-ABL1 negative MPN displays myeloproliferation with an elevated leucocyte alkaline phosphatase (LAP) activity, a neutrophil activation marker. We tried to separate the downstream signalling of JAK2 V617F to stimulate myeloproliferation and LAP activity. NB4, a myeloid lineage cell line, was transduced with Jak2 V617F mutation or wild-type Jak2. We found that Jak2 V617F mutation, but not wild-type Jak2 enhanced LAP expression in NB4-derived neutrophils and proliferation of NB4 cells. JAK2 V617F induces constitutive phosphorylation of STAT3 and STAT5, and uses signalling targets such as Ras/MEK/ERK and PI3K/Akt pathways. By using MEK1/2 inhibitor U0126, PI3K inhibitor LY294002, and STAT3 or STAT5 siRNAs, JAK2 V617F was found to specifically use the STAT3 pathway to enhance LAP expression, while STAT5, Ras/MEK/ERK and PI3K/Akt, but not STAT3 pathways, were able to stimulate cell proliferation. These data strongly suggest that JAK2 V617F uses distinct signalling pathways to induce typical pathological features of MPN, such as high LAP activity and enhanced cell proliferation. [ABSTRACT FROM AUTHOR]

Cytogenetic abnormalities were often observed in primary myelofibrosis patients. The presence of specific cytogenetic abnormalities, such as sole abnormalities of chromosome 13q−, 20q−, or −7/7q−, is reported to have the influence on the prognosis of primary myelofibrosis. We analyzed the data from the prospective survey of Japanese primary myelofibrosis patients which was conducted from 1999 to clarify the impact of cytogenetic abnormalities on the prognosis of primary myelofibrosis. A total of 202 primary myelofibrosis patients had the cytogenetic and the prognostic data. Eighty (40%) out of 202 cases had cytogenetic abnormalities, and an association was evident for platelet counts. Although the presence of an abnormal karyotype did not affect the prognosis, primary myelofibrosis patients with cytogenetic abnormalities other than 13q− and 20q− showed an inferior prognosis compared to patients with a normal karyotype or sole 13q− or 20q− abnormalities. Patients with an unfavorable cytogenetic profile (abnormal cytogenetics other than 13q− or 20q−) also had a greater tendency to transform to leukemia than patients with a favorable cytogenetic profile (normal cytogenetics, sole abnormalities of either chromosome 13q−, or 20q−). Abnormal cytogenetics other than 13q− or 20q− in primary myelofibrosis patients has the poor prognostic effect for both survival and the risk of leukemic transformation. [ABSTRACT FROM AUTHOR]

Acquired mutations of JAK2 and TET2 are frequent in myeloproliferative neoplasms (MPNs). We examined the individual and cooperative effects of these mutations on MPN development. Recipients of JAK2V617F cells developed primary myelofibrosis-like features; the addition of loss-of-TET2 worsened this JAK2V617F-induced disease, causing prolonged leukocytosis, splenomegaly, extramedullary hematopoiesis, and modestly shorter survival. Double mutant (JAK2V617F and loss-of-TET2) myeloid cells were more likely to be in a proliferative state than JAK2V617F single mutant myeloid cells. In a serial competitive transplantation assay, JAK2V617F cells resulted in decreased chimerism in the 2nd recipients, which did not develop MPNs. In marked contrast, cooperation between JAK2V617F and loss-of-TET2 developed and maintained MPNs in the 2nd recipients by compensating for impaired hematopoietic stem cell (HSC) functioning. In vitro sequential colony formation assays also supported the observation that JAK2V617F did not maintain HSC functioning over the long term, but concurrent loss-of-TET2 mutation restored it. Transcriptional profiling revealed that loss-of-TET2 affected the expression of many HSC signature genes. We conclude that loss-of-TET2 has two different roles in MPNs; one is as a disease accelerator, while the other is as a disease initiator and sustainer in combination with JAK2V617F. [ABSTRACT FROM AUTHOR]

The activating mutations in JAK2 (including JAK2V617F) that have been described in patients with myeloproliferative neoplasms (MPNs) are linked directly to MPN pathogenesis. We developed R723, an orally bioavailable small molecule that inhibits JAK2 activity in vitro by 50% at a concentration of 2nM, while having minimal effects on JAK3, TYK2, and JAK1 activity. R723 inhibited cytokine-independent CFU-E growth and constitutive activation of STAT5 in primary hematopoietic cells expressing JAK2V617F. In an anemia mouse model induced by phenylhydrazine, R723 inhibited erythropoiesis. In a leukemia mouse model using Ba/F3 cells expressing JAK2V617F, R723 treatment prolonged survival and decreased tumor burden. In V617F-transgenic mice that closely mimic human primary myelofibrosis, R723 treatment improved survival, hepatosplenomegaly, leukocytosis, and thrombocytosis. R723 preferentially targeted the JAK2-dependent pathway rather than the JAK1- and JAK3-dependent pathways in vivo, and its effects on T and B lymphocytes were mild compared with its effects on myeloid cells. Our preclinical data indicate that R723 has a favorable safety profile and the potential to become an efficacious treatment for patients with JAK2V617F-positive MPNs. [ABSTRACT FROM AUTHOR]