Oligomonocytic Chronic Myelomonocytic Leukemia (O-CMML) and Chronic Myelomonocytic Leukemia (CMML) Show Similar Clinical, Morphological, Immunophenotypic and Molecular Features

INTRODUCTION The 2017 WHO classification requires the presence of ≥1x109/L and ≥10% of monocytes in peripheral blood (PB) for the diagnosis of CMML. Recently, Geyer et al. defines oligomonocytic CMML (O-CMML) as those MDS cases with relative monocytosis (≥10% monocytes) and monocyte count 0.594% is a highly sensitive and specific diagnostic marker for CMML. In the extent of our knowledge, there are no data about PB monocyte subset distribution by FC in O-CMML. Moreover, CD2 and CD56 expression is common in CMML and rarely observed in MDS, the group where O-CMML are currently included. Furthermore, we compared: the molecular profile; cytogenetic abnormalities; cytopenias; BM dysplasia; BM blast and monocyte percentage; PB monocyte percentage, and monocyte and leukocyte counts. METHODS 50 CMML and 33 O-CMML from a single institution were prospectively studied from 02/2016 to date. Table 1 summarizes morphologic, cytogenetic, molecular and clinical findings. We studied PB monocyte subsets by FC: Mo1 (CD14bright/CD16-), Mo2 (CD14bright/CD16+) and Mo3 (CD14dim or -/CD16bright). In addition, we assessed the expression of CD56 and CD2 in monocytes (positivity ≥ 20%). Finally, targeted NGS of the entire exonic sequence of 25 genes recurrently mutated in myeloid malignancies was performed (VAF sensitivity: 2%). Chi-Square, Fisher exact or Man-Whitney U tests were used as appropriate. RESULTS AND DISCUSSIONThe Mo1 percentage (%) was significantly inferior in O-CMML (P=0.007), but it is noteworthy that median and mean of Mo1% in O-CMML were upper the cutoff of 94% (median: 96.1 vs 98.1; mean: 94.7 vs 96.9). Moreover, the % of patients with >94% Mo1 was no significantly different when comparing O-CMML and CMML although a clear trend was observed (72% vs 90%; P=0.082). This result is impressive since, as previously reported, the specificity of the Mo1 >94% test is around 90-95% and only 5-10% of false positive rate (FP) should be expected. However, in O-CMML a 72% of FP was observed since following 2017 WHO recommendation these patients should be considered as MDS.No differences were observed neither in the % of patients showing CD56+ monocytes (65.6% vs 66.7%; P=0.923) nor in the % of th