TARGETING OF PML RAR-ALPHA IS LETHAL TO RETINOIC ACID-RESISTANT PROMYELOCYTIC LEUKEMIA-CELLS/

Acute promyelocytic leukemia (APL) cells, containing the t(15;17) rear rangement, express the fusion protein, PML/ RAR alpha. Clinically, pat ients respond to all-trans retinoic acid (ATRA) through complete remis sions associated with myeloid maturation of leukemic cells. This clini cal ATRA response of APL is linked to PML/RAR alpha expression. Unfort unately, these remissions are transient and relapsed APL is often ATRA -resistant. The role PML/RAR alpha plays in the growth and maturation of these APL cells with acquired ATRA resistance has not been fully ex plored. This study uses an ATRA-resistant NB4 cell line (NB4-R1) to in vestigate the contribution of PML/RAR alpha expression to ATRA resista nce. Targeting of PML/RAR alpha in NB4-R1 cells was undertaken using t wo approaches: homologous recombination and hammerhead ribozyme-mediat ed cleavage. Reducing PML/RAR alpha protein in NB4-R1 cells rendered t hese cells more sensitive to ATRA. These cells were growth-inhibited i n 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 RAR alpha portion of PML/RAR alpha. Resul ts show that these retinoid-resistant NB4 cells require persistent PML /RAR alpha expression for leukemic cell growth, Taken together, these findings can account for why these cells do not respond to ATRA and ho w reduction of PML/RAR alpha abrogates the antiapoptotic effect it con fers to these leukemic cells. (C) 1998 by The American Society of Hema tology.