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After the discovery of the JAK2V617F mutation in 2005, followed by the discovery of the MPL and JAK2 exon 12 mutations, and the CALR mutation in 2013, the majority of patients with Philadelphia negative myeloproliferative neoplasms (MPN) have been shown to display one of these somatic mutations. In regards to the pathogenesis of MPN, the CALR mutation has been shown to activate the JAK2 downstream cascade. However, the mechanism has yet to be elucidated. Nevertheless, these investigations have changed our understanding of MPN, especially in terms of diagnosis and treatment. Reports on the clinical effects of the different somatic mutations have revealed that MPN patients with CALR-mutated ET are less likely to have thrombosis, and that PMF patients with the CALR mutation have better overall survival. Hereafter, evaluations of the status of these gene mutations are necessary for the diagnosis of MPN, and it is important to consider how to put these genetic data to the best possible practical use.

To investigate the current situation and issues regarding the diagnosis of Philadelphia-negative myeloproliferative neoplasms (MPN) in Japan, we retrospectively analyzed an accumulated cohort consisting of 1,081 patients with suspected MPN. Based on WHO2008 diagnostic criteria, we diagnosed 101 of these patients with polycythemia vera, 179 with essential thrombocythemia, 36 with primary myelofibrosis, 45 with unclassifiable MPN, and 4 with myelodysplastic syndromes. Out of 716 patients, 235 were not diagnosed with MPN despite the detection of a JAK2, CALR, or MPL mutation. Among 156 patients with undefined MPN receiving further follow-up, none underwent bone marrow examination and screening for BCR-ABL1 was not performed in 88 cases. Thus, diagnosis was not possible in these cases due to a lack of essential examinations. Since the prognosis and treatment strategy associated with MPN differ among disease types, in addition to mutation analysis, the importance of bone marrow examination and screening for BCR-ABL1 must be re-recognized.