Acute Promyelocytic Leukemia (APL)

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,...