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Mutations in JAK2, myeloproliferative leukemia virus (MPL), or calreticulin (CALR) occur in hematopoietic stem cells (HSCs) and are detected in more than 80% of patients with myeloproliferative neoplasms (MPNs). They are thought to play a driver role in MPN pathogenesis via autosomal activation of the JAK-STAT signaling cascade. Mutant CALR binds to MPL, activates downstream MPL signaling cascades, and induces essential thrombocythemia in mice. However, embryonic lethality of Calr-deficient mice precludes determination of a role for CALR in hematopoiesis. To clarify the role of CALR in normal hematopoiesis and MPN pathogenesis, we generated hematopoietic cell-specific Calr-deficient mice. CALR deficiency had little effect on the leukocyte count, hemoglobin levels, or platelet count in peripheral blood. However, Calr-deficient mice showed some hematopoietic properties of MPN, including decreased erythropoiesis and increased myeloid progenitor cells in the bone marrow and extramedullary hematopoiesis in the spleen. Transplantation experiments revealed that Calr haploinsufficiency promoted the self-renewal capacity of HSCs. We generated CALRdel52 mutant transgenic mice with Calr haploinsufficiency as a model that mimics human MPN patients and found that Calr haploinsufficiency restored the self-renewal capacity of HSCs damaged by CALR mutations. Only recipient mice transplanted with Lineage-Sca1+c-kit+ cells harboring both CALR mutation and Calr haploinsufficiency developed MPN in competitive conditions, showing that CALR haploinsufficiency was necessary for the onset of CALR-mutated MPNs., (© 2020 by The American Society of Hematology.)

Background/aim: In myeloproliferative neoplasms (MPN), Janus kinase 2 (JAK2) is activated by mutations including JAK2V617F (JAK2VF). It is unclear whether JAK kinases [i.e. JAK1, JAK2, JAK3, or tyrosine kinase 2 (TYK2)] other than JAK2 have cooperative actions such as enhancement or suppression of JAK2. If other kinases enhance activation, therapies that co-target them could have a therapeutic efficacy. We examined the role of TYK2 in Jak2VF-induced murine MPN., Materials and Methods: We crossed Jak2VF transgenic mice and Tyk2-knockout (Tyk2KO) mice to generate Jak2VF/Tyk2KO mice. The disease severity and treatment effect with a JAK2 inhibitor was compared between Jak2VF and Jak2VF/Tyk2KO mice., Results: Both types of mice developed MPN, and there were no differences in peripheral blood counts, spleen weight, or survival period. Upon JAK2 inhibitor therapy, both types of mice had equally improved leukocytosis and splenomegaly., Conclusion: TYK2 does not have cooperative effects with JAK2VF upon MPN onset nor in the presence of a JAK2 inhibitor., (Copyright© 2017, International Institute of Anticancer Research (Dr. George J. Delinasios), All rights reserved.)

Myeloproliferative neoplasms are frequently complicated by thrombosis and bleeding. Therefore, not only their primary prevention, but also the management of special clinical issues is important. These issues include venous thromboembolic disorders such as splanchnic venous thromboembolism, as well as major bleeding, surgery, and pregnancy. As for the primary prevention of thrombosis, it has been proposed that low-risk essential thrombocythemia with newly reported risk factors (cardiovascular risks or JAK2V617F) might be included among the factors meriting prevention. As for management of the aforementioned special clinical issues, an expert consensus has been established, wherein the recommended treatment strategies are described. In Japan as well, clinical practice based on this consensus would be preferable.

Mutations in JAK2, MPL and CALR are regarded as driver mutations, and are mutually exclusively detected in more than 90% of myeloproliferative neoplasms (MPNs). In addition, mutations in epigenetic regulator genes such as TET2 or DNMT3A are detected in MPNs. Although the roles of mutations in epigenetic regulator genes were clarified in normal hematopoiesis, their roles have remained unclear in malignant hematopoiesis of MPNs. We analyzed three lines of mutant mice: mice with JAK2V617F, a representative of driver gene mutations; mice with loss of TET2, a representative of epigenetic abnormalities; and mice with both. We thereby clarified two roles of loss of TET2 in malignant hematopoiesis of JAK2-mutated MPNs: one is "disease initiator and sustainer" via reinforcing the function of JAK2-mutated hematopoietic stem cells, and the other is "disease accelerator". New strategies in risk assessment or treatment are required, considering not only single but also multiple mutations.

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 plus 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 second recipients, which did not develop MPNs. In marked contrast, cooperation between JAK2V617F and loss of TET2 developed and maintained MPNs in the second 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: disease accelerator and disease initiator and sustainer in combination with JAK2V617F., (© 2015 by The American Society of Hematology.)

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.