Specific molecular mutation patterns delineate chronic neutrophilic leukemia, atypical chronic myeloid leukemia, and chronic myelomonocytic leukemia.

Chronic neutrophilic leukemia (CNL) and atypical chronic myeloid leukemia (aCML) are rare entities grouped into the World Health Organisation (WHO) categories myeloproliferative and myelodysplastic/myeloproliferative neoplasms (MPN and MDS/MPN overlap), respectively. According to the WHO 2008 classification, both entities are characterized by leukocytosis and a hypercellular bone marrow, predominantly consisting of granulocytic cells. Chronic neutrophilic leukemia is diagnosed by the expansion of neutrophils in the peripheral blood, the exclusion of an elevated blast count, and hepatosplenomegaly. In contrast to other MPNs, before 2013, no molecular marker was known to prove clonality or could shed light on the molecular nature of the disease. Therefore, CNL had been diagnosed by a number of exclusion criteria eliminating evidence for other neoplasms or myelodysplastic syndromes. Atypical CML is diagnosed by a similar approach, since diagnosis according to WHO has featured an increased number of neutrophil precursors, a defined threshold of blasts and monocytes, and dysplasia in the granulocytic lineage. A distinctive feature for differential diagnosis of CNL and aCML is the proportion of immature neutrophils (≥10% in aCML and <10% in CNL). As in CNL, other neoplasms and myelodysplastic syndromes should be excluded. In addition, also chronic myelomonocytic leukemia (CMML) shares several of these characteristics and, therefore, needs to be discriminated from the other two entities, especially by the absolute number of monocytes for clinical decision making. In the last three years, important markers have been identified for the diagnosis and differential diagnosis in these entities. ASXL1, SRSF2, and TET2 were found to be frequently mutated in CMML. SETBP1 was identified to be frequently mutated in aCML, which was shown to cooccur frequently with mutations in ASXL1 and CBL. CSF3R mutations were found to associate with CNL and aCML. In Philadelphia negative MPNs, cytogenetic abnormalities occur, but the frequency differs and no specific abnormality has been defined in the different entities so far. Therefore, the aim of our study was to determine the frequencies of the new armamentarium of genes, i.e. ASXL1, CBL, CSF3R, SETBP1, SRSF2, and TET2mutations in CNL, aCML, and CMML, to help guide the diagnosis and clinical decisions of these three, in part overlapping, entities. A total of 218 patients were diagnosed according to the WHO 2008 criteria, including 14 cases with CNL, 58 with aCML, and 146 with CMML (for more clinical details see Online Supplementary Table S1). Cytogenetics was available in 211 (97%) cases. In all cases, BCR-ABL1 was excluded by RTPCR and/or FISH, and JAK2V617F mutation was analyzed by melting curve analyses, as were JAK2 exon 12 and MPL mutations in JAK2wild-type (wt) patients. CALR mutations were analyzed in JAK2wt CNL and aCML patients by Sanger sequencing. Presence of PDGFR-rearrangements was excluded in CNL by expression analyses of PDGFRA and PDGFRB. In all patients the mutational hot spot regions of ASXL1, CBL, CSF3R, SETBP1, and SRSF2 were analyzed by Sanger sequencing. The complete coding region of TET2 was analyzed by next generation sequencing in 217 of 218 cases. For more details see Online Supplementary Appendix. Cytogenetic aberrations were detected in 54 of 211 cases (26%); the most frequent were trisomie 8 (n=14), deletion of the Y chromosome (n=7), del(20q) (n=3), and i(17)(q10) (n=3). However, there was no association to one of these entities. Mutational analyses showed that ASXL1 was frequently mutated in all three diseases, resulting in mutation frequencies of 57% in CNL (8 of 14), 66% in aCML (38 of 58), and 45% in CMML (66 of 146), respectively (Figures 1 and 2). A similar frequency of ASXL1 mutations has previously been published in CMML. However, the frequency in aCML was higher than the 23% reported by Piazza et al. This finding of frequent appearance in CNL was surprising since little is known about clonality markers in addition to CSF3R in CNL. Furthermore, in CMML, MDS, and also PMF there is evidence that mutations in ASXL1 provide prognostic information, with faster leukemic transformation. CBLmutations clustered mostly in CMML patients (21%, 31 of 146), were less frequent in aCML (10%, 6 of 58), and not found in CNL (0 of 14), matching reported data. In contrast, CSF3R was often mutated in CNL patients (43%, 6 of 14), but rarely in aCML and CMML cases, with only 2 patients each harboring a CSF3R mutation (3%, 2 of 58; 1%, 2 of 146). However, these 4 cases with CSF3R mutation in aCML and CMML showed neutrophil counts below 80% and increased monocyte numbers above 1000/mL in CMML cases and more than 10% neutrophilic precursors in aCML. Although a higher mutation frequency was reported for CNL patients, the rare occurrence of CSF3R mutations in aCML and CMML is in accordance with other reports. Two mutation types have