Is chronic myelomonocytic leukemia more akin to myelodysplastic or myeloproliferative neoplasms and does it matter?

Chronic myelomonocytic leukemia (CMML) was initially classified as a subcategory of myelodysplastic syndrome (MDS) by the French-American-British (FAB) classification system, in 1982 [1]. Its diagnostic criteria, at the time, included peripheral blood (PB) monocyte count of 41610/L, bone marrow (BM) blast count of 20%, PB blast count of 55% and absence of Auer rods. However, it did not go unnoticed that CMML was somewhat different from the other subcategories of MDS because of its frequent association with granulocytosis and other features of myeloproliferative disorder (MPD) such as splenomegaly. In 1994, the FAB group attended to this issue by defining ‘proliferative’ and ‘dysplastic’ subtypes of CMML, using a leukocyte count cutoff level of 13 000610/L [2]. In 2001, the World Health Organisation (WHO) classification system separated CMML from MDS and placed the former in a new ‘MDS/MPD’ category to underline its shared features with both MDS and MPD [3]. The WHO system also eliminated the FAB MDS category of refractory anemia with excess of blasts in transformation (RAEBT) and established a new blast count cutoff level for acute myeloid leukemia ( 20% BM or PB blasts); the diagnostic criteria for CMML were accordingly modified: PB monocyte count of 41610/L, BM and PB blast and promonocyte count of 520%,+Auer rods. Furthermore, based on available information at the time [4], the WHO group underplayed the biological and prognostic relevance of distinguishing ‘proliferative’ from ‘dysplastic’ CMML and instead identified two prognostic categories of its own: CMML-I (blasts 510% in BM and 55% in PB) and CMML-II (blasts 10–19% in BM or 5–19% in PB or presence of Auer rods) [3,5]. The revised 2008 WHO document has replaced the term ‘MPD’ with ‘myeloproliferative neoplasm (MPN)’ but continues to consider CMML as a subcategory of ‘MDS/MPN’ along with juvenile myelomonocytic leukemia and ‘atypical chronic myeloid leukemia, BCR-ABL1-negative’ (Table I) [6]. Furthermore, CMML is now formally distinguished from PDGFRB-rearranged MPN, which is often associated with monocytosis and eosinophilia [6]. Although CMML and the different clinicopathologic entities currently classified under the WHOdefined MPN category (Table I) share a common stem cell-derived clonal heritage [7,8], there are substantial differences between the two in terms of both molecular pathogenesis and bone marrow histology. For example, ABL1, JAK2, MPL and KIT mutations are infrequently found in CMML [9– 11] whereas they are associated with specific MPN phenotypes including chronic myelogenous leukemia (BCR-ABL), polycythemia vera (JAK2V617F and JAK2 exon 12 mutations), essential thrombocythemia (JAK2V617F and MPLW515 mutations), myelofibrosis (JAK2V617F and MPLW515 mutations), and systemic mastocytosis (KITD816V and other KIT mutations) [12,13]. Instead, CMML is nonspecifically associated with RAS mutations, whose pathogenetic relevance remains dubious [14]. In terms of histology, the ‘cardinal’ morphologic features of MDS (e.g. increased BM erythroblasts, dyserythropoiesis, dysgranulopoiesis) are by definition absent in MPNs but usually present in CMML [2]. Similarly, cytogenetic findings in CMML are more in line with MDS than they are with MPN [15]. The close link between CMML and MDS is further illustrated by the not so infrequent development of