AML (Part 1): Biology, Diagnosis, Classification and Risk

Epidemiology & Etiology

• Incidence: ~20,000/yr US; median age 68; M>F (1.3:1).
• Risk factors: prior chemo (alkylators 5 to 7 y latency w/ -5/-7; topo II inhibitors 1 to 3 y latency w/ KMT2A/MLL rearrangements); radiation; benzene; smoking; antecedent MDS/MPN/aplastic anemia; germline (RUNX1, CEBPA, DDX41, GATA2, ANKRD26, ETV6, TP53/Li-Fraumeni); Down syndrome (transient abnormal myelopoiesis to ML-DS w/ GATA1 mutation, excellent prognosis).
• Alkylating agents: melphalan, cyclophosphamide, nitrogen mustard, chlorambucil, busulfan, carboplatin, cisplatin, dacarbazine, bendamustine. Topoisomerase II inhibitors: etoposide, doxorubicin, mitoxantrone. Treatment-related myeloid neoplasia shows 11q23 (e.g. t(9;11)) or 21q22 (t(8;21), t(3;21)) abnormalities.
• Therapy-related AML (t-AML): now a qualifier (MN-pCT) in WHO 2022, not a subtype of AML-MR; complex/monosomal karyotype, TP53 common; poor prognosis.

Pathogenesis (clonal evolution)

• Leukemia and MDS develop through sequential acquisition of mutations (generally 3 to 4 drivers); mutations do not cause cancer immediately but drive clonal expansion. Roughly 1 protein-coding mutation is acquired per decade, so the question is not whether a person acquires mutations but whether a clone grows out to become detectable.
• Most common clonal-hematopoiesis/preleukemic founder mutations: DNMT3A, TET2, ASXL1, TP53. In overt AML the most frequently mutated genes are FLT3, NPM1, and DNMT3A (TCGA).
• Functional classes: epigenetic regulators (ASXL1, TET2, EZH2, IDH1/2, BCOR, DNMT3A); RNA splicing (SF3B1, SRSF2, U2AF1); cohesin (STAG2, RAD21, SMC3); DNA-damage response (TP53); transcription factors (RUNX1, ETV6); tyrosine-kinase signaling (JAK2, NRAS, KRAS, BRAF). TPO acts via MPL mainly on hematopoietic stem/progenitor cells and megakaryocytes; preserved counts in AML do not imply elevated TPO.

Presentation & Diagnostic Workup

• Symptoms: cytopenias (fatigue, infection, bleeding), leukostasis (WBC >50 to 100k w/ pulmonary/CNS sx; myeloblasts cause leukostasis more than lymphoblasts), DIC (especially APL, monocytic), gum hypertrophy/skin (monocytic, M4/M5), chloroma (myeloid sarcoma, extramedullary mass, may precede marrow involvement).
• Diagnosis: ≥20% blasts in marrow OR peripheral blood (WHO/ICC). Exception: defining genetic abnormalities establish diagnosis at ANY blast %: t(15;17) PML::RARA, t(8;21) RUNX1::RUNX1T1, inv(16)/t(16;16) CBFB::MYH11, KMT2A-r, NUP98-r, NPM1-mut (ICC requires ≥10% for these, including PML::RARA; WHO 2022 sets no blast cutoff for them). AML w/ BCR::ABL1 requires ≥20% blasts in both WHO and ICC.
• Required workup: CBC w/ diff, peripheral blast review, BM aspirate + biopsy, flow cytometry (CD34, CD117, CD13, CD33, MPO, lineage markers), conventional cytogenetics (G-banding 20 metaphases), FISH (core-binding factor, MLL/KMT2A, -5/-7, -17), comprehensive molecular panel (NPM1, FLT3-ITD/TKD, CEBPA, IDH1/2, TP53, ASXL1, RUNX1, KIT, DDX41), HLA typing if transplant candidate, baseline echo, LP for suspected CNS involvement (CNS symptoms); defer during circulating blasts or coagulopathy; screening LP in selected high-risk patients may be done at first remission.
• Cytochemistry classics: MPO+ (myeloid), nonspecific esterase + (monocytic), PAS+ (erythroid/lymphoid). Flow cytometry characterizes lineage and can quantify abnormal cells/MRD, but the morphologic marrow and blood differential remains the reference for diagnostic blast enumeration.
• MPAL (mixed-phenotype acute leukemia): myeloid = MPO (flow/IHC/cytochemistry) or monocytic (≥2 of NSE, CD11c, CD14, CD64, lysozyme); T = cytoplasmic or surface CD3; B = strong CD19 plus ≥1 of CD79a/cCD22/CD10, or weak CD19 plus ≥2 of these. Aggressive; treat as ALL.

FAB classification & phenotype

• M0-M7: M3=APL; M4eo=inv(16); M5=monocytic; M6=erythroid; M7=megakaryocytic (associated w/ Down syndrome, transformed MF, GATA1 mutations).

WHO 2022 / ICC 2022 Classification: Key Categories

• AML w/ defining genetic abnormalities: APL w/ PML::RARA, AML w/ RUNX1::RUNX1T1 [t(8;21)], AML w/ CBFB::MYH11 [inv(16)/t(16;16)], AML w/ NPM1 mutation, AML w/ CEBPA mutation (WHO: biallelic or single bZIP; ICC: in-frame bZIP only, = ELN favorable), AML w/ MECOM rearrangement [inv(3)/t(3;3)], AML w/ KMT2A rearrangement, AML w/ NUP98 rearrangement, AML w/ BCR::ABL1, AML w/ DEK::NUP214 [t(6;9)], AML w/ RBM15::MRTFA [t(1;22)] (pediatric).
• AML, myelodysplasia-related (AML-MR): WHO defined by prior MDS or MDS/MPN, OR MDS-related cytogenetics, OR one of 8 MDS-related mutations (SRSF2, SF3B1, U2AF1, ZRSR2, ASXL1, EZH2, BCOR, STAG2). Replaces older "AML-MRC."
• AML, NOS (ICC) / AML defined by differentiation (WHO 2022): lacks defining genetics; subclassified by maturation.
• Myeloid neoplasm post cytotoxic therapy (MN-pCT): qualifier; does not exclude defining-genetic AML categories.
• Myeloid sarcoma: extramedullary AML; diagnostic of AML regardless of marrow blast %.
• Key WHO vs ICC difference: WHO 2022 dropped the 20% blast threshold for most defining-genetic AML (any %), EXCEPT AML w/ CEBPA mutation and AML w/ BCR::ABL1, which still require ≥20%; ICC 2022 created an "MDS/AML" category for 10 to 19% blasts and requires ≥10% blasts for AML w/ recurrent genetic abnormalities including PML::RARA (abnormal promyelocytes count as blast equivalents) (BCR::ABL1 requires ≥20% in both systems).

Response criteria in AML (ELN)

Risk stratification

• Major changes from ELN 2017 to 2022: FLT3-ITD allelic ratio dropped (any FLT3-ITD = intermediate when with NPM1; adverse role of high ratio removed); CEBPA limited to bZIP in-frame mutations (mono- or biallelic); TP53 explicitly adverse (VAF ≥10%, irrespective of allelic status); new MDS-related mutations added to adverse; t(9;11) is intermediate.
• Caveats: NPM1 with adverse-risk cytogenetics is categorized as adverse. MDS-related mutations (RUNX1, ASXL1, and the others) should NOT be used as adverse markers if they co-occur with a favorable-risk subtype. Initial risk assignment may change during treatment based on MRD.

Common mutations (cytogenetics)

• NPM1: encodes a nucleolar shuttle protein; found in ~30% of AML and up to 60% of cytogenetically normal AML. Isolated NPM1 mutation gives better EFS/OS; with normal cytogenetics it triages to favorable risk if no FLT3-ITD (under ELN 2017, also with FLT3-ITD-low ratio <0.5). In ELN 2022, good prognosis if no ITD; otherwise intermediate; adverse cytogenetics override to poor.
• CEBPA: transcription factor for granulocyte differentiation; seen in 7 to 11%. Favorable outcome historically only with double (biallelic) mutations; ELN 2022 recognizes bZIP in-frame mutations (mono- or biallelic) as favorable.
• KIT: seen in ~20% of core-binding-factor AML; associated with lower survival in t(8;21).
• FLT3-ITD: ~30% of AML; inferior outcome (shorter remission, poorer OS); under ELN 2017 prognosis depended on ITD allelic ratio (ELN 2022: any FLT3-ITD without a favorable or adverse defining subtype is intermediate (CBF AML stays favorable despite concurrent FLT3)). Under ELN 2017, NPM1-mut with low ratio (<0.5) behaved like NPM1-mut without ITD (favorable), NPM1-wt with low ratio was intermediate, and NPM1-wt with high ratio (≥0.5) was adverse. FLT3-TKD (~10%) has more controversial significance.
• TP53: poor prognosis (very bad).
• t(6;9): associated with FLT3 mutation and basophilia.
• inv(3): rearrangement causing MECOM(EVI1) overexpression.
• inv(16)(p13q22): usually FAB M4 with abnormal eosinophil component.
• Pearl: RUNX1, TP53, or ASXL1 triage to poor risk (FLT3-ITD is intermediate in ELN 2022); but RUNX1 or ASXL1 should not be used as adverse markers when co-occurring with a favorable-risk subtype.

Continued in: AML (Part 2): Treatment, APL, Relapse and Special Populations