Leukemic cellular retinoic acid resistance and missense mutations in the PML-RARα fusion gene after relapse of acute promyelocytic leukemia from treatment with all-trans retinoic acid and intensive chemotherapy

This study evaluated whether relapse of acute promyelocytic leukemia (APL) patients from clinical remissions achieved and/or maintained with all- trans retinoic acid (RA) in combination with intensive chemotherapy is associated with leukemic cellular resistance to RA and with alterations in the PML-RARα fusion gene. We studied matched pretreatment and relapse specimens from 12 patients who received variable amounts of RA, primarily in nonconcurrent combination with daunorubicin and cytarabine (DA) on Eastern Cooperative Oncology Group (ECOG) protocol E2491, and from 8 patients who received DA only on protocol E2491. Of 10 RA-treated patients evaluable for a change in APL cell sensitivity to RA-induced differentiation in vitro, 8 showed diminished sensitivity at relapse, whereas, of 6 evaluable patients treated with DA alone, only 1 had marginally reduced sensitivity. From analysis of sequences encoding the principal functional domains of the PML and RARα portions of PML-RARα, we found missense mutations in relapse specimens from 3 of 12 RA-treated patients and 0 of 8 DA-treated patients. All 3 mutations were located in the ligand binding domain (LBD) of the RARα region of PML-RARα. Relative to normal RARα1, the mutations were Leu290Val, Arg394Trp, and Met413Thr. All pretreatment analyses were normal except for a C to T base change in the 3'-untranslated (UT) region of 1 patient that was also present after relapse from DA therapy. No mutations were detected in the corresponding sequences of the normal RARα or PML (partial) alleles. Minor additional PML-RARα isoforms encoding truncated PML proteins were detected in 2 cases. We conclude that APL cellular resistance occurs with high incidence after relapse from RA + DA therapy administered in a nonconcurrent manner and that mutations in the RARα region of the PML-RARα gene are present in and likely mechanistically involved in RA resistance in a subset of these cases.