In acute promyelocytic leukemia (APL), the PML-RARA fusion protein causes leukemic block at the promyelocyte stage. The molecular mechanism by which ATRA (all-trans retinoic acid) restores differentiation is:

• A ATRA at pharmacologic doses displaces co-repressor complexes (NCoR/SMRT/HDAC) from the PML-RARA fusion, allowing differentiation gene transcription to resume ✓
• B ATRA directly cleaves the PML-RARA fusion protein via caspase-dependent proteolysis
• C ATRA activates PU.1 transcription factor independently of RARA, bypassing the block
• D ATRA upregulates wild-type RARA synthesis to competitively dilute the PML-RARA fusion

Explanation

The PML-RARA fusion protein recruits nuclear co-repressor complexes (NCoR, SMRT) and histone deacetylases (HDACs) at physiologic RA concentrations, silencing differentiation genes. At pharmacologic doses (micromolar range), ATRA binds the RARA ligand-binding domain of the fusion protein with sufficient affinity to conformationally displace the co-repressor complex, switching PML-RARA from a transcriptional repressor to an activator. This restores expression of myeloid differentiation genes including C/EBPα and PU.1. ATRA does not directly cleave the fusion; arsenic trioxide (ATO) causes PML-RARA proteasomal degradation via PML oxidation and SUMOylation. Wild-type RARA upregulation is not the pharmacologic mechanism.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.