Defining Advances in the WHO Classification for AML

Major updates and revisions for acute monocytic leukemia (AML) based on the World Health Organization (WHO) Classification of Haematolymphoid Tumours, 5th edition, (5th ed)¹ include increased emphasis on genetic and molecular drivers of disease. AML is now divided into 2 broad categories including 1) AML with defining genetic abnormalities and 2) AML defined by differentiation, with the former superseding the latter, whenever applicable. The required blast threshold of 20% has now been omitted from AML with defining genetic abnormalities with the exception of AML with BCR::ABL1 and AML with CEBPA mutation. This article will address AML with defining genetic abnormalities.

The diagnostic criteria for AML with PML::RARA and core binding factor AMLs including AML with RUNX1::RUNX1T1 and AML with CBF::MYH11 remain mostly unchanged with increased emphasis on the role of measurable residual disease (MRD) assessment and concurrent molecular alterations and their therapeutic and management implications. The 20% blast threshold has been omitted from AML with DEK::NUP214 and RBM15::MRTFA (formerly RBM15::MKL1). The A blast cutoff of 20% is retained for AML with BCR::ABL1 to avoid overlap with de novo presentation of myeloid blast phase of chronic myeloid leukemia.

To highlight the promiscuous nature of KMT2A and the numerous partner genes in the setting of AML, the name for “AML with t(9;11)(p22;q23); KMT2A-MLLT3” has been changed to AML with KMT2A (formerly MLL) rearrangement. Initial presentation with marked monoblastic/ monocytic differentiation and hyperleukocytosis should raise the possibility of AML with KMT2A rearrangement (FIGURE 1A) in adults.

AML with KMT2A::MLLT3 and KMT2A::MLLT10 can present with megakaryoblastic differentiation (FIGURE 1B) and/or low blasts, particularly in pediatric populations.

AML with defining genetic abnormalities included 2 novel categories defined by gene rearrangements including AML with MECOM rearrangement (FIGURE 1C) and AML with NUP98 rearrangement (NUP98-r; FIGURE 1D). A blast threshold of 20% is not required for a diagnosis of AML in these entities as patients with less than 20% blasts have similar presentation and outcomes to those with higher blast counts.^2-4 Rearrangements involving NUP98 are often cryptic; MECON and KMT2A rearrangements may also be cryptic but with lesser frequency. Fluorescence in situ hybridization (FISH) or reverse transcription polymerase chain reaction (RT-PCR), or optimal genome mapping (OGM)⁵ should be pursued when these AML subtypes are suspected. Co- occurrence of FLT3 internal tandem duplication and WT1 mutations in cases of AML, particularly in pediatric populations and young adults, should raise the possibility of AML with NUP98-r.⁶

Accurate identification of NUP98-r may have important therapeutic implication, now with the emergence of menin inhibitors in our therapeutic armamentarium.⁷

Myelomonocytic or monocytic differentiation is common and multilineage dysplasia can be seen in 20% to 25% of cases and has no impact on clinical outcome.^8,9 Although blasts with cup-like nuclear morphology can be associated with both NPM1 and FLT3-ITD mutations, identification of cup-like nuclear invaginations in more than 10% of blasts is highly specific for AML with NPM1 mutation (FIGURE 1E).¹⁰ Approximately 80% of AML with NPM1 mutation lack CD34 expression.¹¹ Detection of cytoplasmic NPM1 by immuno-histochemistry can be used as a reliable surrogate marker of NPM1 mutation.^11,12 The 20% blast threshold was eliminated from the diagnostic criteria based on the fact that the NPM1-mutated myeloid neoplasms with less than 20% blasts are associated with shorter survival compared with wild-type cases and can benefit most from upfront intensive (AML-type) treatment regimens.^13,14 Nevertheless, the WHO 5th ed. calls for judicious clinicopathologic correlation for rare cases with low blast count, particularly those with blasts under 10% and low NPM1 mutant allelic burden.

AML with CEBPA mutation has been modified to include single mutations located in the basic leucine zipper (bZIP) region of the gene (smbZIP-CEBPA) in addition to biallelic (biCEBPA) mutations based on recent studies showing a favorable prognosis for smbZIP CEBPA similar to biCEBPA.^15-17 The current data do not support any change in the blast cutoff criterion (>20%) for AML with CEBPA mutation; therefore the 20% minimum blast count has been retained.¹⁸

AML, myelodysplasia-related (AML-MR), which was formerly known as AML with myelodysplasia-related change is defined as a myeloid neoplasm with 20% or greater blasts harboring specific cytogenetic and/ or molecular abnormalities associated with MDS (TABLE), arising de novo or in patients with antecedent myelodysplastic syndrome (MDS) or MDS/myeloproliferative neoplasm (MPN). The main changes to AML-MR criteria include 1) elimination of morphologic criteria as the sole diagnostic criterion; 2) omission of balanced translocations and modifications to cytogenetic criteria; and, 3) addition of MR-defining mutations.^19,20 TP53 mutation is frequently associated with AML-MR and a complex karyotype, so most cases of AML with TP53 mutation fall under this category. However, there are data to suggest that TP53 mutation is an independent adverse prognostic factor in AML-MR and de novo AML.^21,22 and there are data to suggest that TP53- mutated myeloid neoplasms may comprise a distinct group of disease.^23,24

Due to lack of sufficient specificity to define a distinct AML subtype, the provisional entity of AML with mutated RUNX1 has been eliminated from the WHO 5th ed. Notably, the majority of these cases will now be included in the AML-MR category, as RUNX1 mutations frequently co-occur with other MR defining mutations.^{25, 26}

The category of AML defined by differentiation includes cases that lack defining genetic abnormalities. Classification of AML cases based on differentiation for cases without any defining genetic abnormalities still provides valuable practical and prognostic information for clinical purposes. Although classification by differentiation may not be as attractive as molecular methods, phenotype is in part a manifestation of genotype, so why deprive ourselves of potentially relevant and impactful information when we can easily document these data? An example includes the emerging data on therapeutic resistance of certain AML phenotypes (monocytic, erythroid, megakaryocytic) to 1 of the most widely-used AML drugs, venetoclax [Venclexta].²⁶

On this note, let us focus on acute erythroid leukemia (AEL): AEL (FIGURE 1F) is characterized by neoplastic proliferation of erythroid precursors with features of maturation arrest usually 80% or greater of bone marrow (BM) elements, of which 30% or greater are proerythroblasts (immature erythroids) and a high prevalence of biallelic TP53 alterations. Although de novo presentation may occur, most cases of AEL are associated with cytotoxic therapy or progression of a prior myeloid neoplasm, particularly MDS. AEL supersedes the category of AML-MR. In the presence of bi-allelic TP53 inactivation, the erythroid and pronormoblast percentage requirements are not strictly enforced, and prominent erythroid differentiation is sufficient to meet diagnostic criteria for AEL. AEL shows increased venetoclax resistance and BCL-XL dependency²⁶ in cases of acute monocytic leukemia with marked erythroid or megakaryocytic differentiation. In cases with overlap with AML-MR such as those showing a complex karyotype, classification as AML-MR is preferred if 20% or greater blasts with myeloid phenotype are present.

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