• When reclassifying 2,130 oligomonocytosis cases using the WHO and ICC 2022 classifications, 5-23% of cases are newly classified as CMML.

  • Compared to MD-CMML, newly classified CMML cases show distinct mutational and transcriptional profiles but comparable overall survival.

The World Health Organization Classification of Hematolymphoid Tumors (WHO) and the International Consensus Classification (ICC) of 2022 introduced major changes to the definition of CMML. To assess qualitative and quantitative implications for patient care, we started with 3,311 established CMML cases (according to WHO 2017 criteria) and included also 2,130 oligomonocytosis cases fulfilling the new CMML diagnostic criteria. Applying both classification systems from 2022, 356 and 241 of oligomonocytosis cases were newly classified as myelodysplastic (MD)-CMML (WHO and ICC 2022, respectively), most of which were diagnosed as MDS according to WHO 2017. Importantly, 1.5 times more oligomonocytosis cases were classified as CMML according to WHO 2022 than based on ICC, due to different diagnostic criteria. Genetic analyses of the newly classified CMML cases showed a distinct mutational profile with strong enrichment of MDS-typical alterations, resulting in a transcriptional subgroup separated from established MD- and myeloproliferative (MP)-CMML. Despite a different cytogenetic, molecular, immunophenotypic, and transcriptional landscape, no differences in overall survival were found between newly classified and established MD-CMML cases. To the best of our knowledge, this study represents the most comprehensive analysis of routine CMML cases to date, both in terms of clinical characterization and transcriptomic analysis, placing newly classified CMML cases on a disease continuum between MDS and previously established CMML.

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