Correct Answer: B. PML-RARA
All-trans retinoic acid (ATRA) is the gold-standard targeted therapy for acute promyelocytic leukemia (APL), which is characterized by the PML-RARA fusion gene. This fusion results from a t(15;17) translocation, where the promyelocytic leukemia gene (PML) on chromosome 15 fuses with the retinoic acid receptor alpha gene (RARA) on chromosome 17. The PML-RARA fusion protein acts as an aberrant transcriptional repressor that blocks myeloid differentiation. ATRA binds to the RARA moiety of the fusion protein, causing a conformational change that releases the transcriptional block and allows differentiation of leukemic promyelocytes into mature neutrophils. This mechanism of action—targeting the specific molecular lesion—represents the paradigm of molecular-targeted therapy. APL is the most curable acute leukemia in India and worldwide, with cure rates >90% when treated with ATRA plus arsenic trioxide (ATO). The PML-RARA fusion is pathognomonic for APL and is used for diagnosis and risk stratification (long vs. short isoform). ATRA's efficacy is directly dependent on the presence of this specific fusion protein, making it a textbook example of precision oncology.
Why the other options are wrong
A. BCR ABL — BCR-ABL is the hallmark fusion in chronic myeloid leukemia (CML), not APL. While BCR-ABL is also a target for molecular therapy, it is treated with tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib), not ATRA. This is a common trap pairing two fusion oncogenes with different leukemias and different targeted drugs. C. cMYC — cMYC translocations (t(8;14) in Burkitt lymphoma) result in oncogene activation and uncontrolled proliferation, but do not respond to ATRA. cMYC-driven lymphomas are treated with chemotherapy (CHOP, R-CHOP) or targeted agents like BTK inhibitors, not differentiation therapy. ATRA does not reverse the cMYC-driven phenotype. D. RUNX1 ETO — RUNX1-ETO (AML1-ETO) is the fusion in acute myeloid leukemia with t(8;21), a core binding factor AML. While this is a favorable-risk AML, it is not treated with ATRA as monotherapy. RUNX1-ETO does not respond to retinoic acid differentiation therapy; these patients receive standard chemotherapy (cytarabine + daunorubicin).
High-Yield Facts
- PML-RARA fusion (t(15;17)) is pathognomonic for acute promyelocytic leukemia (APL) and is the only leukemia where ATRA is a primary therapeutic agent.
- ATRA mechanism: binds RARA moiety of PML-RARA fusion protein, reverses transcriptional repression, and forces differentiation of leukemic promyelocytes into mature neutrophils.
- APL cure rate in India is >90% with ATRA + arsenic trioxide (ATO); APL is the most curable acute leukemia worldwide.
- PML-RARA isoforms (long vs. short) determine risk stratification and influence ATRA sensitivity; long isoform has higher relapse risk.
- Differentiation syndrome (formerly ATRA syndrome) is a life-threatening complication of ATRA therapy in APL, characterized by fever, respiratory distress, and pulmonary infiltrates; managed with dexamethasone and early chemotherapy.
Mnemonics
APL = ATRA (Acute Promyelocytic Leukemia = All-Trans Retinoic Acid) APL is the only acute leukemia where a single targeted agent (ATRA) can induce complete remission by forcing differentiation. Remember: Acute Promyelocytic Leukemia → All-Trans Retinoic Acid. PML-RARA = t(15;17) = APL The fusion is between chromosome 15 (PML) and chromosome 17 (RARA). The numbers 15 and 17 are close together—think of them as 'stuck together' in the translocation. This is the diagnostic hallmark.
NBE Trap
NBE pairs multiple fusion oncogenes (BCR-ABL, cMYC, RUNX1-ETO) with ATRA to test whether students confuse different leukemia subtypes and their targeted therapies. The trap is assuming any fusion gene in leukemia responds to ATRA—only PML-RARA does.
Clinical Pearl
In Indian tertiary centers, APL is now recognized as a medical emergency requiring immediate ATRA + ATO initiation upon morphologic suspicion, even before cytogenetic confirmation, because early treatment dramatically improves survival. Coagulopathy and differentiation syndrome are the two life-threatening complications to watch for in the first 2 weeks of therapy.
_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 13 (Hematopoietic and Lymphoid Systems); Harrison's Principles of Internal Medicine, Ch. 110 (Acute Leukemias)_