Comparison of the 2022 ICC and Who Classification Systems of Myelodysplastic Neoplasms

Introduction: Recently, the World Health Organization (WHO) updated the 5th edition classification of myelodysplastic neoplasms (MDS) and the International Consensus Classification (ICC) of myeloid neoplasms and acute leukemia was published. In this study, we aimed to compare several differences between WHO 2022 and ICC classifications.

Methods: Our study consisted of 989 newly-diagnosed primary MDS subjects classified using the WHO 2016 criteria from 08/2016 to 07/2022. CD34 and CD117 immunostaining were used in 30 of 38 subjects with low blasts and BM fibrosis to evaluate a proportion of immature myeloid precursors. All subjects were negative for CD117+ cells. CD34+ BM cells ranged from 0% to 5%.

Results: According to WHO 2022 classification, 36 MDS subjects with NPM1 mutation were reclassified as AML and 10 MDS-U subjects were reclassified as clonal cytopenia of undetermined significance (CCUS). Applying to the ICC criteria, 20 MDS subjects with NPM1 mutation were reclassified as AML and 8 MDS-U subjects were reclassified as CCUS. We analyzed the remaining patients.

Among the 81 subjects classified as MDS-biTP53 according to WHO 2022, 10 subjects did not meet ICC criteria for MDS or MDS/AML with mutated TP53 because the VAF of TP53 mutation(s) was less than 10%. Subjects not qualifying for MDS or MDS/AML with mutated TP53 had lower Revised International Prognostic Scoring System (IPSS-R) scores, better IPSS-R-defined cytogenetics, a higher frequency of SF3B1 mutations and longer survival compared with subjects with MDS or MDS/AML with mutated TP53 according to ICC (All P-values＜ 0.005, Figure 1 Left). Our data suggested that TP53 mutational burden should also be considered in the diagnostic classification and the cut-off value of 10% seems reasonable.

An additional difference between these two classifications is that ≥15% ring sideroblasts may be a surrogate for SF3B1 mutation in WHO 2022. But only those with SF3B1 mutation (VAF≥10%), without 5q-, -7 or complex karyotype and RUNX1 mutation can be diagnosed as MDS with mutated SF3B1(MDS-SF3B1) according to ICC. 23 of 76 subjects (30.3%) with MDS-SF3B1 according to the WHO 2022 classification did not meet the ICC criteria for MDS-SF3B1. Subjects not meeting the ICC for MDS-SF3B1 had lower WBCs (P = 0.003) ,platelet concentrations (P = 0.001), higher IPSS-M categories (P = 0.024) and shorter survival compared with those with MDS-SF3B1 according to ICC(P = 0.003, Figure 1 middle). Therefore, our study shown that SF3B1-unmutated MDS-RS cases have different clinical features and outcome in comparison with SF3B1-mutated subjects and ICC criteria defined a more homogeneous disease entity of MDS-SF3B1.

Apart from MDS with defining genetic abnormalities, substantial differences in morphologically defined MDS subtypes between WHO 2022 and ICC also need to be addressed. According to the WHO 2022 classification, only MDS-EB patients were furtherly categorized into MDS-EB with or without BM fibrosis. Here, we explore the impact of BM fibrosis in patients with MDS-LB. Among MDS patients without excess blasts and specific genetic abnormalities, 38 patients had fibrosis. Subjects with low blasts and fibrosis (MDS-LBf) had lower platelet concentrations (P = 0.002), worse IPSS-R-defined cytogenetics and IPSS-R categories (P ＜0.05) compared with those without excess blasts and myelofibrosis. MDS-LBf had worse survival compared with those with low blasts and without fibrosis (Figure 1 right, P = 0.007).

Conclusion: Both the WHO 2022 and ICC criteria allow more accurate diagnoses and risk stratification. However, the presence of two separate classifications leads to the dilemma in clinical application. When it comes to MDS with defining genetic abnormalities, our analyses suggest ICC defines more homogeneous disease entities of MDS-SF3B1, MDS or MDS/AML with mutated TP53 subjects compared with WHO 2022. Although WHO 2022 places an emphasis on morphologically defined MDS and listed hypoplastic MDS (MDS-h) and MDS with fibrosis (MDS-f) as new entities. Our data shown that fibrosis is a poor prognostic factor in MDS subjects without excess blasts. These subjects tend to have more aggressive clinical manifestations than other MDS subjects with low blasts. Collectively, we propose to integrate these two classifications into one pipeline for diagnosis of MDS (Figure 2), avoiding disadvantages for each one.

Bing Li and Zhijian Xiao were co-corresponding authors