Advances in Acute Myeloid Leukemia Classification: A Comparative Analysis of Who-HAEM4R, Who-HAEM5, and International Consensus Classification
Authors: Chen Xue , Yuan Lili , Zhang Yang , Wang Fang , Ma Xiaoli , Fang Jiancheng , Cao Panxiang , Liu Yijun , Liu Zhixiu , Liu Ming , Chen Jiaqi , Zhou Xiaosu , Liu Mingyue , Jin David , Lu Peihua , Liu Hongxing 
Published: December 2023

Introduction
Two guidelines were recently published to refine the diagnostic criteria of acute myeloid leukemia (AML): the 5th edition of the World Health Organization Classification of Haematolymphoid Tumors (WHO-HAEM5, PMID: 35732831) and the International Consensus Classification (ICC; PMID: 35767897), and both expanded molecularly defined AML subtypes. In this study, we utilized a cohort of 1135 AML cases to examine their performance.
Methods
From Sep 1, 2018 to Dec 31, 2022, a consecutive cohort of 1135 cases diagnosed with de novo AML according to WHO-HAEM4R (PMID: 27069254) were enrolled, with a median follow-up of 20 months. All underwent high-throughput sequencing (PMID: 34135310, 29932212).
Results
When classified according to WHO-HAEM4R, 55% of the 1135 cases were diagnosed as AML with recurrent genetic abnormalities, 16% were AML with myelodysplasia-related changes (AML-MRC), 2% were therapy-related AML, 4% were AML with germline predisposition, and 24% fell into AML not otherwise specified (AML, NOS).
Major changes in WHO-HAEM5 include the inclusion of AML subtypes of KMT2A rearrangement (KMT2A-r), MECOM-r, and NUP98-r; adding AML with other defined genetic alterations (AML-ODGA) as a basket entity; and annulling the entity of AML with mutated RUNX1 (AML-RUNX1). The entity of AML with CEBPA mutation (AML-CEBPA) has changed to include not only biallelic (biCEBPA) but also single bZIP (smbZIP-CEBPA) mutations. AML-MRC in WHO-HAEM4R is redefined as AML, myelodysplasia-related (AML-MR), with a mutation-based definition based on an 8-gene panel (without RUNX1).
When comparing WHO-HAEM4R and WHO-HAEM5 (Fig. 1), AML with KMT2A-r increased from 2% to 10% because WHO-HAEM5 no longer restricts the partner genes of KMT2A. AML with NUP98-r and AML-ODGA in WHO-HAEM5 account for 6% and 4%, respectively. AML-MR increased from 12% AML-MRC to 21% benefited from the 8-gene panel-based definition. AML-RUNX1 has been annulled in WHO-HAEM5 and 63% cases of this subtype were reclassified as AML-MR, and 30 cases with RUNX1 mutations as the only basis for classification were reclassified as AML defined by differentiation. In total, AML without defining genetic abnormalities dropped from 24% to 11%.
When comparing WHO-HAEM4R and ICC (Fig. 1), AML with mutated TP53 (AML-TP53) is a new category in ICC and accounted for 4% of AML, 79% of them were previously classified as AML-MRC in WHO-HAEM4R. AML-MR in WHO-HAEM5 is divided into two entities in ICC: AML with MR gene mutations (including RUNX1) and AML with MR cytogenetic abnormalities, comprising 13% and 5% of cases, respectively. ICC only includes in-frame bZIP CEBPA mutations. Thus 6 cases classified as AML-biCEBPA based on WHO-HAEM4R fell into AML, NOS under ICC since their CEBPA mutations were not in bZIP. Conversely, 21 cases with smbZIP-CEBPA and diagnosed as AML, NOS according to WHO-HAEM4R were classified as AML-CEBPA under ICC.
Cases classified as AML-CEBPA based on WHO-HAEM5 tended to have a better overall survival (OS) than cases classified as AML-biCEBPA under WHO-HAEM4R because AML-smbZIP-CEBPA was added while cases with MR gene mutations which had a worse prognosis fell into AML-MR under WHO-HAEM5. Despite ICC only included in-frame bZIP-CEBPA mutations, the prognosis of AML-CEBPA diagnosed under ICC was not superior to that under WHO-HAEM5. This might be because cases with both MR cytogenetic abnormalities and CEBPA mutations were reclassified as AML-CEBPA under ICC's hierarchical principle. ICC distinguished AML with KMT2A::MLLT3 from AML with KMT2A-r, while WHO-HAEM5 only included AML with KMT2A-r entity. Survival analysis showed significantly better OS in AML with KMT2A::MLLT3 than AML with KMT2A-r.
ICC separated AML-TP53 from AML-MR due to the notorious prognosis of TP53 defects, and cases classified as AML-TP53 showed the worst OS in this cohort. But prognostic analysis showed no difference between AML-TP53 and cases with concurrent TP53 mutation and other AML-defining genetic abnormalities. The rationality of considering AML-TP53 as a distinct entity remains debatable.
Conclusions
Both new classification systems consider genomic features more extensively and enhance the precise classification of AML. Discrepancies between WHO-HAEM5 and ICC, like classifying AML with RUNX1 mutations and the rationality of defining AML-TP53 as a unique entity, remain open questions that require further research for clearer answers.