Clinical meaning
Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia (AML-M3 in the FAB classification) characterized by the accumulation of abnormal promyelocytes in the bone marrow and a uniquely dangerous coagulopathy. APL accounts for approximately 5 to 8% of all AML cases and is defined by the balanced reciprocal translocation t(15;17)(q24.1;q21.2), which fuses the promyelocytic leukemia gene (PML) on chromosome 15 with the retinoic acid receptor alpha gene (RARA) on chromosome 17, creating the PML-RARA fusion oncoprotein.
Under normal hematopoiesis, retinoic acid receptor alpha (RARA) binds retinoic acid and functions as a transcription factor that drives myeloid differentiation. The PML-RARA fusion protein acts as a dominant-negative transcriptional repressor: it recruits nuclear co-repressor complexes (N-CoR and SMRT) and histone deacetylases (HDACs) to retinoic acid response elements (RAREs) in the promoter regions of genes essential for myeloid differentiation. This epigenetic silencing blocks the differentiation of promyelocytes into mature granulocytes, causing accumulation of malignant promyelocytes arrested at the promyelocytic stage. The PML-RARA fusion protein also disrupts PML nuclear bodies (PML-NBs), which normally function as tumor suppressors involved in apoptosis, DNA repair, senescence, and antiviral defense, further contributing to leukemogenesis.
The hallmark clinical feature of APL is a severe, life-threatening coagulopathy that distinguishes it from all other AML subtypes. This coagulopathy is driven by three simultaneous mechanisms. First, disseminated intravascular coagulation (DIC): the abnormal promyelocytes express and release tissue factor (TF) and cancer procoagulant (CP, a cysteine protease that directly activates factor X), triggering systemic activation of the coagulation cascade with consumption of clotting factors and platelets. Second, hyperfibrinolysis: the malignant promyelocytes overexpress annexin II on their cell surface, which serves as a receptor for tissue plasminogen activator (tPA), markedly increasing plasmin generation and fibrinolysis. They also produce urokinase-type plasminogen activator (uPA) and release elastase, which degrades fibrinogen and other coagulation proteins. Third, proteolysis: granules within the abnormal promyelocytes release elastase and other proteases upon cell lysis, directly degrading fibrinogen, von Willebrand factor, and other hemostatic proteins. The combination of DIC with superimposed hyperfibrinolysis creates a catastrophic bleeding diathesis that is the leading cause of early death in APL, accounting for the majority of fatalities within the first 30 days of diagnosis.