Altered nuclear cofactor switching in retinoic-resistant variants of the PML-RARα oncoprotein of acute promyelocytic leukemia.

Acute promyelocytic leukemia (APL) results from a reciprocal translocation that fuses the gene for the PML tumor suppressor to that encoding the retinoic acid receptor alpha (RARα). The resulting PML-RARα oncogene product interferes with multiple regulatory pathways associated with myeloid differentiation, including normal PML and RARα functions. The standard treatment for APL includes anthracycline-based chemotherapeutic agents plus the RARα agonist all-trans retinoic acid (ATRA). Relapse, which is often accompanied by ATRA resistance, occurs in an appreciable frequency of treated patients. One potential mechanism suggested by model experiments featuring the selection of ATRA-resistant APL cell lines involves ATRA-resistant versions of the PML-RARα oncogene, where the relevant mutations localize to the RARα ligand-binding domain (LBD). Such mutations may act by compromising agonist binding, but other mechanisms are possible. Here, we studied the molecular consequence of ATRA resistance by use of circular dichroism, protease resistance, and fluorescence anisotropy assays employing peptides derived from the NCOR nuclear corepressor and the ACTR nuclear coactivator. The consequences of the mutations on global structure and cofactor interaction functions were assessed quantitatively, providing insights into the basis of agonist resistance. Attenuated cofactor switching and increased protease resistance represent features of the LBDs of ATRA-resistant PML-RARα, and these properties may be recapitulated in the full-length oncoproteins.