Response to histone deacetylase inhibition of novel PML/RARα mutants detected in retinoic acid-resistant APL cells

Resistance to all-trans retinoic acid (ATRA) remains a clinical problem in the treatment of acute promyelocytic leukemia (APL) and provides a model for the development of novel therapies. Molecular alterations In the ligand-binding domain (LBD) of the PML/RARalpha fusion gene that characterizes APL constitute one mechanism of acquired resistance to ATRA. We identified missense mutations In PML/RARalpha from an additional ATRA-resistant patient at relapse and In a novel ATRA-resistant cell line, NB4-MRA1. These cause altered binding to ligand and transcriptional coregulators, leading to a dominant-negative block of transcription. These mutations are In regions of the LBD that appear to be mutational hot spots occurring repeatedly In ATRA-resistant APL patient cells. We evaluated whether histone deacetylase (HDAC) Inhibition could overcome the effects of these mutations on ATRA-Induced gene expression. Cotreatment with ATRA and TSA restored RARbeta gene expression In NB4-MRA1 cells, whose PML/RARalpha mutation Is In helix 12 of the LBD, but not In an APL cell line harboring the patient-derived PML/RARalpha mutation, which was between helix 5 and 6. Furthermore, ATRA combined with TSA increases histone 4 acetylation on the RARbeta promoter only In NB4-MRA1 cells, Consistent with these results, the combined treatment Induces differentiation of NB4-MRA1 only. Thus, the ability of an HDAC Inhibitor to restore ATRA sensitivity in resistant cells may depend on their specific molecular defects. The variety of PML/RARalpha mutations arising In ATRA-resistant patients begins to explain how APL patients In relapse may differ In response to transcription therapy with HDAC Inhibitors. (C) 2002 by The American Society of Hematology.