Targeting of PML/RARa Is Lethal to Retinoic Acid–Resistant Promyelocytic Leukemia Cells

Acute promyelocytic leukemia (APL) cells, containing the t(15;17) rearrangement, express the fusion protein, PML/ RARa. Clinically, patients respond to all-trans retinoic acid (ATRA) through complete remissions associated with myeloid maturation of leukemic cells. This clinical ATRA response of APL is linked to PML/RARa expression. Unfortunately, these remissions are transient and relapsed APL is often ATRA-resistant. The role PML/RARa plays in the growth and maturation of these APL cells with acquired ATRA resistance has not been fully explored. This study uses an ATRA-resistant NB4 cell line (NB4-R1) to investigate the contribution of PML/RARa expression to ATRA resistance. Targeting of PML/RARa in NB4-R1 cells was undertaken using two approaches: homologous recombination and hammerhead ribozyme-mediated cleavage. Reducing PML/RARa protein in NB4-R1 cells rendered these cells more sensitive to ATRA. These cells were growth-inhibited in ATRA, apoptosis was induced, and there was no apparent signaling of differentiation. Sequence analysis identified a mutation in the ligand binding domain (LBD) of the RARa portion of PML/RARa. Results show that these retinoid-resistant NB4 cells require persistent PML/RARa expression for leukemic cell growth. Taken together, these findings can account for why these cells do not respond to ATRA and how reduction of PML/RARa abrogates the antiapoptotic effect it confers to these leukemic cells. r 1998 by The American Society of Hematology.