FAB Classification of AML: M1

• The French-American-British (FAB) system is an outdated method for classifying acute leukemia, replaced by newer systems.
• The FAB system, established in 1976, classified acute leukemias based on Romanowsky-stained blood and bone marrow smears, requiring at least 30% blasts in the peripheral blood (PB) or bone marrow (BM).
• Subclassification criteria included cell morphology and cytochemical staining results.
• The FAB system had seven subclassifications.

M1: AML Without Maturation (Acute Myeloblastic Leukemia without Maturation)

• Characterized by low red blood cell, granulocyte, and platelet counts.
• Nucleus to cytoplasm maturation asynchrony is common.
• Immature nucleus is more prominent than cytoplasm.
• Leukemic blasts exhibit phagocytosis.
• May contain a few azurophilic granules and Auer rods; may show a chloroma.
• ≥30% blasts in bone marrow; mature granulocytic cells is 10%
• Auer rods are common.
• Associated with the chromosomal abnormality translocation t(8;21) (10%).

M3: Acute Promyelocytic Leukemia (APL)

• Most aggressive type of acute leukemia, characterized by cells with dumbbell-shaped or bilobed nuclei.
• Similar to M1 and M2, but frequently associated with disseminated intravascular coagulation (DIC) due to thromboplastin-like activity of primary granules.
• Bundles of Auer rods are present, forming "faggot cells."
• Associated with the chromosomal abnormality translocation t(15;17), t(11;17), t(5;17), which affects prognosis.
• M3m is a microgranular promyelocytic leukemia variant.
• Treatable with ATRA (All-Trans Retinoic Acid), which targets the PML/RARa (Retinoic Acid Receptor Alpha) gene.

M4: Acute Myelomonocytic Leukemia

• Also known as Naegeli monocytic leukemia.
• Characterized by proliferation of granulocytes and monocytic cells.
• Associated with soft-tissue infiltrates, meningeal symptoms, and leukostasis.
• Bone marrow contains 30%-80% monocytic cells.
• Elevated serum or urine lysozyme (muramidase) is important for recognition.
• Associated with deletion or conversion of the long arm of chromosome 16.

M5: Acute Monocytic Leukemia

• Also known as Schilling's leukemia.
• Blasts have muddy or smudgy gray-blue cytoplasm and pseudopods.
• Two categories:
  • M5a: Poorly differentiated; large blasts dominate the bone marrow (≥80% monoblasts in BM), with 1-5 large nucleoli.
  • M5b: Well-differentiated; all stages of monocyte development are present (≥80% monocytic cells in BM).
• Cytogenetic abnormality: Chromosome translocation t(9;11) is most common.

Rarely seen subtypes

• t(9;22) or BCR/ABL (Philadelphia chromosome) is more typically associated with CML (Chronic Myeloid Leukemia).

M6: Acute Erythroleukemia/Erythremic Myelosis

• Also known as Di Guglielmo syndrome.
• ≥50% erythrocytic precursors in bone marrow with abnormal morphology.
• ≥30% blasts in bone marrow of myeloid origin; may have Auer rods.
• May progress to M1, M2, or M4.

M7: Acute Megakaryocytic Leukemia

• Characterized by immature megakaryocytic cells in bone marrow.
• Uncommon type; megakaryoblasts may express CD41 or CD61.

M0: Acute Undifferentiated Leukemia/Acute Myeloid Leukemia, Minimally Differentiated