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The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has made the management of coronavirus disease-2019 (COVID-19) in patients with hematological disorders a new and important theme for hematologists. Patients with myeloproliferative neoplasms (MPNs) are susceptible to SARS-CoV-2 and are at an increased risk of death after the onset of COVID-19. Thus, infection prevention measures, including vaccination for all patients, are important. Patients with MPNs who have COVID-19 have a poor prognosis, as do patients with other hematological malignancies. The thrombogenic characteristics of MPNs increase the risk of venous thrombosis due to COVID-19. Anticoagulant therapy is adjusted according to the risk of each case after COVID-19 onset. However, thrombosis occurs at a high rate, especially in patients with essential thrombocythemia. Additionally, patients with myelofibrosis have an increased risk of death and bleeding. Ruxolitinib treatment poses a risk of SARAS-CoV-2 infection, and its abrupt discontinuation after infection is associated with an increased risk of death. The emerging evidence of COVID-19 has been quickly reflected in the available treatment recommendations and guidelines.

One of the main molecular features of myeloproliferative neoplasms (MPNs) is the high frequency of JAK2V617F or CALRexon 9 mutations. The mouse models driven by these mutations suggest that they are the direct cause of MPNs and that the expression levels of mutated genes define the disease phenotype. The function of MPN-initiating cells was also elucidated using these mouse models. Furthermore, these mouse models play important roles as disease models to investigate the effects and mechanisms of action of therapeutic drugs such as JAK2 inhibitors and interferon α against MPNs. The mutation landscape of hematological tumors has already been clarified using next-generation sequencing technology, and future research on the importance of the functional analysis of mutant genes in vivo should be emphasized. Thus, it is necessary to promote rapid genetic modification techniques such as genome editing.

The expected survival duration of polycythemia vera (PV) and essential thrombocythemia (ET) patients is not substantially lower than that of the general population. The current goal of therapy is to prevent thrombohemorrhagic events associated with PV and ET. The current first line therapy for PV is phlebotomy, hydroxyurea (HU), and aspirin, while that for ET was HU or anagrelide. The follow-up phase 3 randomized trial wherein the hematological response was evaluated in PV patients treated with ropeginterferon alfa-2b, a next-generation monopegylated IFN-α-2b, or HU, demonstrated a superior hematological effect and a lower incidence of adverse events in patients who were treated with ropeginterferon. The prognosis of primary myelofibrosis (PMF) is poorer than that of PV or ET. The only curative therapeutic option for PMF patients is allogeneic hematopoietic stem cell transplantation (HSCT). Other than HSCT options, ruxolitinib ameliorates splenomegaly and MF-associated symptoms and provides an overall survival benefit in PMF patients with intermediate-2 or high risk. Several different JAK inhibitors have been developed; however, many of them were discontinued because of toxicity concerns. Recently, promising results have been demonstrated for the effect of different JAK inhibitors as well as the drugs that directly target anemia and bone marrow fibrosis.

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.

In myeloid malignancies, mutations have occurred in epigenetic regulator genes, including Ten-Eleven-Translocation 2 (TET2). TET2 is an enzyme that catalyzes the conversion of 5-methylcytosine into 5-hydroxymethylcytosine (5-hmC) which is a key intermediate for oxidative DNA demethylation. We analyzed the in vivo phenotype of TET2 failure using Ayu17-449 (TET2(trap/trap)) mice created by the gene-trap method in which TET2 mRNA levels were decreased to about 20% of the level in wild-type (WT) mice. In TET2(trap/trap) mice the levels of 5-hmC in genomic DNA from bone marrow (BM) cells were decreased in comparison to WT mice. TET2(trap/trap) mice were born at an expected Mendelian frequency but died at a high rate by postnatal day 3, indicating TET2 to be essential for survival. In analysis of the hematopoietic system, transplantation of TET2(trap/trap), but not WT fetal liver cells, led to mild myeloid hyperplasia and splenomegaly in WT recipient mice, but no onsets of lethal hematological malignancies were observed during a follow-up period of 12 months. TET2 knockdown led to an increased serial replating capacity of BM cells in vitro and increased hematopoietic stem cell (HSC) self-renewal in vivo in competitive repopulation and serial transplantation assays. These data indicate that TET2 has a critical role in survival and HSC homeostasis.

Activation of the JAK-STAT pathway by driver mutations of JAK2, CALR or MPL is the pathogenesis of myeloproliferative neoplasms (MPN). Mutations in epigenetic regulators that often coexist with driver mutations reinforce the function of MPN initiating cells and support MPN onset and maintenance. In myelofibrosis patients, JAK2 inhibitors exerted an innovative therapeutic effect on splenomegaly and constitutional symptoms, but had minimal effects on the JAK2V617F allele burden. Epigenetic abnormalities may be a good target for novel therapeutic strategies aiming to induce molecular remission in myelofibrosis patients. New information obtained by next-generation sequencing technologies would greatly contribute to elucidation of the mechanisms underlying MPN onset, progression and leukemic transformation.