USMLE Step 1

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Biochemistry

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Leukemias and Lymphomas

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Acute Promyelocytic Leukemia (APL)

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Summary

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Key Points

  • Acute Promyelocytic Leukemia (APL)
    • Etiology
      • Subtype of acute myelogenous leukemia (AML)
        • Formerly known as M3 subtype of AML
      • t(15;17) translocation creates RARA-PML fusion gene
        • Retinoic acid receptor gene (RARA) moved to promyelocytic leukemia (PML) locus
        • Creates an aberrant retinoic acid (Vitamin A) receptor that is less responsive to cell differentiation signals, especially in promyelocytes
    • Presentation
      • Usually seen in adults 20-60 years old
      • Presents with AML symptoms (classic leukemic triad)
        • Fatigue, bleeding, infections
      • DIC is a common presentation
        • Causes systemic bleeding from orifices and purpura, with low platelets
      • Auer rods seen on bone biopsy
        • myeloperoxidase ⊕ cytoplasmic inclusions seen
        • Fused lysosomal granules seen in all subtypes of AML but not ALL (helpful for differentiating APL/AML from ALL)
    • Treatment
      • all-trans retinoic acid/ATRA (Vitamin A)
        • Highly active form of Vitamin A given in high doses can force binding to mutated retinoic acid receptor, inducing differentiation and maturation of promyelocytes