Retinoic acid, but not arsenic trioxide, degrades the PLZF/RARα fusion protein, without inducing terminal differentiation or apoptosis, in a RA-therapy resistant t(11;17)(q23;q21) APL patient

Primary blasts of a t(11; 17)(q23; q21) acute promyelocytic leukaemia (APL) patient were analysed with respect to retinoic acid (RA) and arsenic trioxide (As sub(2)O sub(3)) sensitivity as well as PLZF/RAR alpha status. Although RA induced partial monocytic differentiation ex vivo, but not in vivo, As203 failed to induce apoptosis in culture, contrasting with t(15; 17) APL and arguing against the clinical use of As203 in t(11; 17)(q23; q21) APL. Prior to cell culture, PLZF/RAR alpha was found to exactly co-localize with PML onto PML nuclear bodies. However upon cell culture, it quickly shifted towards microspeckles, its localization found in transfection experiments. Arsenic trioxide, known to induce aggregation of PML nuclear bodies, left the microspeckled PLZF/RAR alpha localization completely unaffected. RA treatment led to PLZF/RAR alpha degradation. However, this complete PLZF/RAR alpha , degradation was not accompanied by differentiation or apoptosis, which could suggest a contribution of the reciprocal RAR alpha /PLZF fusion product in leukaemogenesis or the existence of irreversible changes induced by the chimera.