A 38-year-old Indian woman presents to the emergency department with severe epistaxis, gum bleeding, and petechiae. Peripheral blood smear shows hypergranular promyelocytes with bundles of Auer rods. Coagulation studies reveal prolonged PT/aPTT, low fibrinogen (80 mg/dL), and elevated D-dimer. FISH analysis of bone marrow is performed. The fusion signal marked **A** in the diagram is detected in >98% of blast cells. Which of the following best describes the molecular mechanism by which this fusion signal drives the pathogenesis of this leukemia?

Explanation

## Why "Blocks myeloid differentiation at the promyelocyte stage through aberrant recruitment of HDAC/DNMT corepressor complexes to RARA target genes" is right The PML-RARA fusion signal marked **A** is the pathognomonic t(15;17)(q24;q21) translocation found in >98% of acute promyelocytic leukemia (APL, AML-M3). This chimeric transcription factor acts as a dominant negative by recruiting histone deacetylase (HDAC) and DNA methyltransferase (DNMT) corepressor complexes to retinoic acid receptor-α (RARA) target genes, blocking the normal differentiation signal and trapping cells at the promyelocyte stage. This is the defining molecular mechanism of APL pathogenesis (Robbins 10e Ch 13; Harrison 21e Ch 103). ## Why each distractor is wrong - **Constitutive activation of FLT3-ITD signaling pathway leading to uncontrolled myeloid proliferation**: FLT3-ITD is a common mutation in other AML subtypes (particularly normal karyotype AML) but is NOT the defining mechanism of APL. The PML-RARA fusion blocks differentiation, not simply increases proliferation. - **Loss of tumor suppressor function through TP53 mutation causing failure of apoptosis**: TP53 mutations occur in some AML cases but are not the primary driver of APL pathogenesis. APL is defined by the PML-RARA translocation, not TP53 loss. - **Overexpression of BCR-ABL fusion protein resulting in enhanced tyrosine kinase activity**: BCR-ABL is the hallmark fusion of chronic myeloid leukemia (CML), not APL. The mechanism is also fundamentally different—BCR-ABL drives proliferation via kinase activity, whereas PML-RARA blocks differentiation via corepressor recruitment. **High-Yield:** APL = PML-RARA blocks differentiation via HDAC/DNMT corepressors; ATRA works by degrading PML-RARA and releasing the block—start ATRA immediately on clinical suspicion before genetic confirmation because DIC-driven hemorrhage kills within hours. [cite: Robbins 10e Ch 13; Harrison 21e Ch 103]