TARGETING THE PML RAR-ALPHA TRANSLOCATION PRODUCT TRIGGERS APOPTOSIS IN PROMYELOCYTIC LEUKEMIA-CELLS/

The t(15;17) rearrangement found in acute promyelocytic leukemia (APL) yields a fusion transcript, PML/RAR alpha. PML/RAR alpha. expression is linked to leukemogenesis and to clinical sensitivity to all-trans r etinoic acid (RA), Paradoxically, RA treatment causes transient comple te remissions in most t(15;17) APL cases. The precise roles of PML/RAR alpha in triggering leukemia or in causing a maturation block are not yet known. This study explores directly these PML/RAR alpha. function s in the growth and differentiation of APL cells using a hammerhead ri bozyme to target PML/RAR alpha mRNA in the NB4 APL cell line. When the PML/RAR alpha cleaving but not the non-catalytic control ribozyme is introduced into the NB4 APL cell line, PML/RAR alpha protein expressio n is reduced. This catalysis signals growth suppression, cytotoxicity, and apoptosis without overcoming the maturation block found in these leukemic cells. These biologic effects depend on the selective pressur e used to express the ribozyme from an episomal vector. Introduction o f a non-catalytic, control ribozyme into NB4 cells caused no observed phenotype due to anti-sense activities, Expression of the catalytic or non-catalytic ribozymes in control cells lacking PML/RAR alpha mRNA y ielded no apparent growth or differentiation effects. Thus, use of a h ammerhead ribozyme that targets PML/RAR alpha. expression in APL cells reveals the anti-apoptotic function of this translocation product and demonstrates that PML/RAR alpha cleavage is insufficient to overcome the differentiation block observed in these leukemic cells. Taken toge ther, these findings indicate that persistent PML/RAR alpha expression is required to maintain basal leukemic cell growth and point to the t herapeutic potential of targeting PML/RAR alpha in APL.