Acute Myeloid Leukemia

• Class I Mutations (Proliferation/Survival Signals): FLT3-ITD (FLT3 internal tandem duplication), FLT3-TKD (tyrosine kinase domain mutations), and activating mutations in RAS (KRAS, NRAS) lead to constitutive activation of receptor tyrosine kinase signaling, resulting in enhanced phosphoinositide 3-kinase (PI3K) and MAPK pathway activation. These mutations promote blasts to proliferate independently of growth factor signaling and resist apoptosis through enhanced survival signals. FLT3-ITD mutations occur in ~25% of AML cases and are associated with high relapse risk and poor prognosis due to increased blast proliferation rate.

• Class II Mutations (Differentiation Arrest): NPM1 mutations (nucleophosmin, ~30% of AML) cause abnormal cytoplasmic localization of NPM1 protein, disrupting normal nucleolar-nucleoplasmic shuttle function and impairing p53-mediated differentiation pathways. CEBPA mutations disrupt transcription factor binding that normally drives myeloid lineage commitment. TP53 mutations abrogate p53-dependent apoptosis and cell-cycle checkpoints, allowing accumulation of additional genetic hits. PML-RARA fusion (in acute promyelocytic leukemia—APL) interferes with retinoic acid receptor signaling, preventing normal differentiation of promyelocytes. These Class II mutations cause arrest at immature developmental stages, resulting in blast accumulation.

• Epigenetic Dysregulation: DNA methyltransferase inhibitors (azacitidine, decitabine) are therapeutic targets because AML frequently involves hypermethylation of tumor suppressor genes (including p15, p16, VWF) via aberrant DNMT activity. Mutations in DNMT3A, TET2, IDH1/2 disrupt epigenetic regulation of differentiation-associated genes, favoring self-renewal of immature blasts over terminal differentiation. ASXL1 mutations (altered ASXL1 protein) impair polycomb repressive complex 2 function, disrupting chromatin architecture required for normal hematopoietic differentiation.

• Acquired Cytogenetic Abnormalities: Recurrent chromosomal translocations create oncogenic fusion proteins that drive leukemogenesis. t(15;17) generates PML-RARA (APL subtype); t(8;21) produces RUNX1-RUNX1T1 (AML-M2); inv(16)/t(16;16) creates CBFB-MYH11 (AML-M4Eo)—these core binding factor AMLs have better prognosis with appropriate treatment. Complex karyotype (≥3 independent abnormalities) and monosomy 7 or 7q deletion indicate unfavorable prognosis through loss of tumor suppressors and impaired genomic stability.

• Mechanism of Blast Accumulation: Normal myeloid differentiation is blocked at the blast stage through loss of terminal differentiation programs while proliferation signals are amplified. Apoptosis evasion occurs via upregulation of anti-apoptotic proteins (BCL2, MCL1) through FLT3/MAPK signaling, NPM1 cytoplasmic localization effects, or direct TP53 loss. The result is exponential expansion of phenotypically immature CD34+, CD13+, CD33+ blasts that fail to undergo normal myeloid maturation, crowding out normal hematopoiesis through competition for limited bone marrow niche resources and excess myeloid growth factor production.

• Secondary Effects—Cytopenias: Displacement of normal hematopoietic precursors by expanding leukemic blasts leads to anemia (reduced oxygen-carrying capacity), thrombocytopenia (bleeding risk), and neutropenia (infection susceptibility). Increased uric acid production from rapid cell turnover causes hyperuricemia; increased lactate dehydrogenase (LDH) and phosphate from tumor lysis contribute to metabolic complications. Leukostasis can occur with very high white blood cell counts (typically >100,000/μL), particularly in acute monocytic/monoblastic leukemias (AML-M5) and acute promyelocytic leukemia, where the high ratio of blast volume to normal RBC produces microvascular occlusion in lungs and brain.

• Prior Chemotherapy or Radiation (Therapy-Related AML—tAML): Alkylating agents (cyclophosphamide, melphalan) and topoisomerase II inhibitors (etoposide, doxorubicin) induce secondary AML with latency of 5-10 years and 2-5 years respectively. These therapy-related AMLs demonstrate complex karyotypes, TP53 mutations, and monosomy 7 with uniformly poor prognosis. Radiation therapy for prior malignancies or other conditions increases AML risk proportionally to cumulative dose; this was historically seen in atomic bomb survivors and patients receiving mantle-field radiation for Hodgkin lymphoma.

• Myelodysplastic Syndrome (MDS) Progression: 20-30% of patients with prior MDS evolve to overt AML as disease progresses and additional mutations accumulate. MDS-derived AML typically harbors TP53 and complex karyotypes, conferring worse prognosis than de novo AML with similar morphology.

• Constitutional Genetic Syndromes: Down syndrome (Trisomy 21) confers ~60-fold increased risk of AML, particularly acute megakaryoblastic leukemia (AML-M7). GATA1 mutations in Down syndrome AML are pathognomonic and associated with improved chemotherapy response. Other predisposition syndromes include Fanconi anemia (DEB-positive), Bloom syndrome, and Ataxia-telangiectasia, each with TP53 pathway dysfunction and genomic instability.

• Myeloproliferative Neoplasms (MPN): ~5-10% of patients with chronic myeloid leukemia (CML) progress to blast crisis (AML or ALL). Patients with polycythemia vera or essential thrombocythemia treated with hydroxyurea or anagrelide carry increased risk of transformation to AML. These conditions involve JAK2/CALR/MPL mutations that predispose to additional genetic hits.

• Environmental Exposures: Benzene is a well-established leukemogen causing dose-dependent AML risk through DNA alkylation and epigenetic effects. Smoking doubles AML risk. Occupational exposures to pesticides and formaldehyde carry modest increased risk. Prior HIV infection (though less relevant in modern antiretroviral era) increased AML risk.

• Age and Comorbidity: Advanced age (>70 years) is a major non-genetic risk factor, associated with higher mutation burden (TP53, complex karyotype), worse chemotherapy tolerance, and inferior outcomes. High comorbidity index, reduced performance status, renal/hepatic dysfunction, and cardiac disease limit treatment intensity options.

• Fatigue and Dyspnea (Anemia): Leukemic blast expansion displaces normal erythropoiesis, leading to normocytic anemia. Patients develop fatigue, exertional dyspnea, and presyncope as hemoglobin falls below 7-8 g/dL. The onset is typically acute (over days to weeks) in AML, distinguishing it from chronic cytopenias. Severely anemic patients may present with syncope, stroke, or myocardial infarction if demand exceeds supply, particularly in elderly patients with cardiac comorbidity.

• Bleeding and Bruising (Thrombocytopenia): Profound thrombocytopenia (often <20,000/μL at diagnosis) causes spontaneous bleeding—petechiae on lower extremities and buccal mucosa, ecchymoses from minor trauma, epistaxis, and gingival bleeding. Disseminated intravascular coagulation (DIC) is notably common in acute promyelocytic leukemia (APL) due to tissue factor release from malignant promyelocytes, manifesting as severe bleeding diathesis, microangiopathic hemolytic anemia, and thrombosis. Bleeding into critical organs (intracranial hemorrhage, gastrointestinal hemorrhage) is a leading cause of early death in newly diagnosed AML.

• Fever and Infection (Neutropenia): Absolute neutropenia (<500/μL) with dysfunctional blasts causes severe immunosuppression and rapid progression to overwhelming bacterial and fungal infections. Febrile patients present with pneumonia (cough, hypoxia), bacteremia (sepsis, hypotension), or invasive fungal infections (aspergillus, candida) with high mortality if not promptly treated. Neutropenic enterocolitis (typhlitis) can occur with fever, right lower quadrant pain, and diarrhea. Infection is the most common cause of death in AML patients.

• Bone Pain and Leukostasis Symptoms: Rapid leukemic proliferation within marrow cavities causes diffuse bone pain, particularly in long bones and ribs. Leukostasis occurs when white blood cell count exceeds 100,000-200,000/μL (particularly in monocytic subtypes), causing viscous blood flow through small vessels and producing acute dyspnea, hypoxia, altered mental status, retinal hemorrhages, or stroke from pulmonary or cerebral microvascular occlusion. This is a medical emergency requiring urgent cytoreduction via hydroxyurea or chemotherapy.

• Gingival and Skin Involvement: Gingival infiltration manifests as swollen, friable, bleeding gums, most prominent in monocytic differentiation (AML-M5). Leukemia cutis (infiltration of dermis with leukemic blasts) appears as violaceous papules or nodules and indicates systemic disease burden. Sweet syndrome (acute febrile neutrophilic dermatosis) is an immune-mediated phenomenon sometimes associated with AML.

• Hepatosplenomegaly and Lymphadenopathy: Variable expansion of extramedullary hematopoiesis and leukemic infiltration causes hepatomegaly and/or splenomegaly in 40-50% of patients. Lymphadenopathy is less common in AML than ALL but can occur with monocytic differentiation. Anterior mediastinal mass may be seen with acute monocytic leukemia (AML-M5) or when a large leukemic mass occurs.

• Metabolic and Biochemical Abnormalities: Hyperuricemia from rapid cell turnover predisposes to acute urate nephropathy and gout-like arthralgia. Hyperkalemia can occur from rapid cell lysis. Hypophosphatemia and hypocalcemia develop in some patients. Tumor lysis syndrome is less common at diagnosis than in ALL but can occur rapidly upon initiation of chemotherapy, particularly in high-burden disease, requiring aggressive hydration and allopurinol/febuxostat prophylaxis. Lactic acidosis may develop from high tumor burden and anaerobic metabolism.

• Important Variants—APL (t(15;17)): Acute promyelocytic leukemia presents with bleeding/DIC dominance and is distinguished by characteristic abnormal promyelocytes with abundant Auer rods and abnormal morphology (often hypergranular, sometimes microgranular). This subtype has transformed from lethal disease to >80% cure rate with ATRA (all-trans retinoic acid) plus arsenic trioxide.

• Important Variants—Acute Monocytic Leukemia (AML-M5): Monocytic differentiation causes particularly aggressive disease with high risk of leukostasis, CNS involvement (meningeal disease), and gingival infiltration. These patients require careful monitoring for CNS prophylaxis.

• Complete Blood Count and Differential: Peripheral blood typically shows leukocytosis (WBC >50,000/μL in ~50%), though "subleukemic AML" can present with normal or low WBC. Anemia is nearly universal (hemoglobin <10 g/dL in most patients). Thrombocytopenia is profound, typically <50,000/μL and often <20,000/μL. Circulating blasts are usually visible on differential (defined as >20% of nucleated cells = peripheral blood AML; if <20% blasts in blood, diagnosis requires bone marrow). Dysplastic changes in other lineages (pseudo-Pelger-Huet nuclei, hypogranular neutrophils, giant platelets) suggest underlying dysplasia and may indicate secondary AML.

• Bone Marrow Aspiration and Biopsy: Gold standard for AML diagnosis and subtyping. Hypercellular marrow (cellularity >90%) is typical. Myeloid blasts comprise ≥20% of nucleated cells (WHO diagnostic criterion). Morphologic assessment identifies blast type (myeloblasts, monoblasts, promyelocytes) and degree of maturation. Auer rods (abnormal cytoplasmic inclusions) are pathognomonic for AML when present and support diagnosis. Biopsy provides cellularity assessment and allows identification of fibrosis (particularly in therapy-related AML or monocytic differentiation).

• Cytochemistry: Myeloperoxidase (MPO) staining positive (in >3% of blasts) confirms myeloid lineage differentiation and distinguishes AML from ALL or other malignancies. Sudan Black B is similarly positive. Esterase staining patterns help classify monocytic differentiation (specific vs non-specific). These methods are increasingly supplemented/replaced by flow cytometry but remain useful adjunctive confirmation.

• Flow Cytometry (Immunophenotyping): Essential for blast enumeration and lineage assignment. Blasts typically express CD34+ (immature), CD13+ and/or CD33+ (myeloid markers), HLA-DR+, and CD45+ (dim), with variable expression of CD11b, CD14 (monocytic), CD15, CD56. Aberrant antigen expression (e.g., CD7+ on myeloid blasts, loss of normal antigen expression) supports clonality. Flow cytometry is more sensitive than morphology for blast detection, allowing identification of minimal disease and MRD monitoring by detecting abnormal blast immunophenotype after treatment. Normal B-cell and T-cell populations are typically decreased (lymphopenia) due to marrow space occupancy by blasts.

• Cytogenetics (Conventional Karyotype): Essential for risk stratification. Modern AML risk stratification incorporates favorable cytogenetics (t(8;21), inv(16),

Immediate stabilisation (before subtype is known)

• Rule out APL first: send STAT PML-RARA FISH/PCR and review the smear. Per NCCN AML guidelines, if APL is suspected on morphology or coagulopathy, start all-trans retinoic acid (ATRA) immediately — do not wait for cytogenetic confirmation, because early hemorrhagic death is the main killer.
• Coagulopathy support: aggressive platelet and cryoprecipitate/fresh frozen plasma transfusion to keep platelets and fibrinogen well above usual thresholds in APL-associated DIC.
• Tumor lysis prophylaxis: IV hydration plus a xanthine oxidase inhibitor (allopurinol); rasburicase (recombinant urate oxidase) for high burden or established hyperuricemia — contraindicated in G6PD deficiency (hemolysis/methemoglobinemia). This risk-adapted approach follows the widely used 2008 expert consensus TLS guideline (Coiffier et al.) and NCCN supportive-care recommendations.
• Leukostasis: cytoreduction with hydroxyurea ± leukapheresis; avoid red cell transfusion until the count falls, since it raises whole-blood viscosity. Leukapheresis is avoided in APL (worsens coagulopathy).
• Febrile neutropenia: empiric antipseudomonal beta-lactam (cefepime or piperacillin-tazobactam) within an hour, per IDSA guidance.

Definitive therapy — fitness- and genotype-driven (NCCN; ELN 2022 risk stratification)

• Fit patients: intensive induction with continuous-infusion cytarabine plus an anthracycline ("7+3", daunorubicin or idarubicin). Add the FLT3 inhibitor midostaurin for FLT3-mutated disease; gemtuzumab ozogamicin (anti-CD33 antibody-drug conjugate) for CD33+ core-binding-factor AML; CPX-351 (liposomal daunorubicin/cytarabine) for therapy-related AML or AML with myelodysplasia-related changes.
• Unfit/older patients: hypomethylating agent (azacitidine or decitabine) plus the BCL2 inhibitor venetoclax. Mutation-directed alternatives: the IDH1 inhibitor ivosidenib (± azacitidine) is approved for newly diagnosed IDH1-mutated AML in patients unfit for intensive chemotherapy, whereas the IDH2 inhibitor enasidenib is used mainly in relapsed/refractory IDH2-mutated disease.
• APL: ATRA plus arsenic trioxide, a largely chemotherapy-free regimen curing the majority; anthracycline or gemtuzumab is added for high-risk disease.
• Post-remission: high-dose cytarabine consolidation for favorable-risk disease; allogeneic hematopoietic stem cell transplant in first remission for intermediate/adverse-risk AML — the only reliably curative option there. Relapsed FLT3-mutant disease: gilteritinib.

Disease-related — emergencies

• APL-associated DIC/hemorrhage (emergency): malignant promyelocytes release tissue factor and annexin II–driven fibrinolytic activity, producing a consumptive plus hyperfibrinolytic coagulopathy; the key signals are falling fibrinogen, rising D-dimer, prolonged PT/aPTT, and profound thrombocytopenia. Schistocytes may appear but are variable and not a reliable feature. Intracranial hemorrhage is the leading cause of early death.
• Leukostasis (emergency): bulky, poorly deformable blasts (especially monocytic subtypes) plug pulmonary and cerebral microvasculature — new hypoxia with a relatively clear chest film, confusion, or retinal hemorrhages. Arterial blood gas PaO2 may be spuriously low from in-vitro oxygen consumption by blasts (leukocyte larceny); pulse oximetry is the more reliable measure, and the specimen should be iced and processed immediately.
• Tumor lysis syndrome (emergency): massive purine and intracellular ion release after cytoreduction — hyperkalemia, hyperphosphatemia, hyperuricemia, and hypocalcemia with acute kidney injury.
• Febrile neutropenia and sepsis (emergency): fever in a neutropenic host is infection until proven otherwise; neutropenic enterocolitis (typhlitis) and invasive aspergillosis carry high mortality.

Treatment-related

• Differentiation syndrome (emergency): with ATRA, arsenic, or IDH inhibitors, differentiating blasts release cytokines and become adhesive — fever, weight gain, hypotension, pulmonary infiltrates, pleural/pericardial effusions. Treat promptly with dexamethasone; hold the differentiating agent if severe.
• Anthracycline cardiotoxicity: topoisomerase IIβ inhibition and free-radical injury cause cumulative dose-dependent, often irreversible LVEF decline — signalled by falling ejection fraction on surveillance echocardiography.
• High-dose cytarabine toxicities: cerebellar toxicity (nystagmus, dysmetria, ataxia — check a cerebellar exam before each dose, higher risk in the elderly and in renal impairment), chemical conjunctivitis (prevented with steroid eye drops), and palmar-plantar erythrodysesthesia.
• Arsenic trioxide: QT prolongation with torsades risk — serial ECGs and repletion of potassium and magnesium.
• Late/other: extramedullary disease (myeloid sarcoma/chloroma), CNS relapse in monocytic subtypes, graft-versus-host disease after allogeneic transplant, transfusional iron overload, and relapse — the dominant long-term threat.

• Auer rods: azurophilic, needle-shaped cytoplasmic crystals of fused primary granules — seen only in myeloid blasts, and heaviest in APL (faggot cells). Their presence excludes ALL.
• The single best next step in suspected APL: start ATRA immediately on clinical/morphologic suspicion, before PML-RARA confirmation returns. A stem with a young patient, low platelets, prolonged PT, low fibrinogen, and promyelocytes is testing this reflex — the wrong answers are "await cytogenetics" or "start 7+3."
• AML vs ALL: myeloperoxidase-positive, Sudan Black-positive, CD13/CD33/CD34+ blasts = AML; TdT-positive, CD10/CD19 (B) or CD3 (T) blasts = ALL. TdT is the classic distractor.
• Leukostasis management trap: give hydroxyurea ± leukapheresis and avoid packed red cell transfusion until the count falls — transfusing raises viscosity and can precipitate stroke or respiratory failure. Leukapheresis is contraindicated in APL.
• Differentiation syndrome: fever, hypotension, weight gain, and pulmonary infiltrates days into ATRA or arsenic → dexamethasone, not antibiotics alone or diuresis alone.
• Cytogenetics drive prognosis: t(15;17), t(8;21), and inv(16) are favorable; complex karyotype, monosomy 7, and TP53 mutation are adverse. FLT3-ITD portends relapse and prompts adding midostaurin; NPM1-mutated/FLT3-wild-type disease is relatively favorable.
• Down syndrome association: markedly increased risk of AML, classically acute megakaryoblastic leukemia (M7) with GATA1 mutation, and often chemosensitive; transient abnormal myelopoiesis in the neonate is the precursor lesion.
• Rasburicase is contraindicated in G6PD deficiency — a favorite distractor in a tumor lysis question.
• Blast threshold nuance: the ≥20% rule is the general teaching point, but AML-defining recurrent genetic abnormalities such as PML-RARA, RUNX1-RUNX1T1, and CBFB-MYH11 are handled differently by the two current systems — the WHO 5th edition removes any blast requirement for these defining abnormalities, whereas the ICC 2022 classification still requires ≥10% blasts. Boards most often reward recognizing that these fusions define AML even when blasts fall short of 20%.