Formation of PML/RAR alpha high molecular weight nuclear complexes throughthe PML coiled-coil region is essential for the PML/RAR alpha-mediated retinoic acid response

Retinoic Acid (RA) treatment induces disease remission of Acute Promyelocyt ic Leukaemia (APL) patients by triggering terminal differentiation of neopl astic cells. RA-sensitivity in APL is mediated by its oncogenic protein, wh ich results from the recombination of the PML and the RA receptor alpha (RA R alpha) genes (PML/RAR alpha fusion protein). Ectopic expression of PML/RA R alpha into haemopoietic cell lines results in increased response to RA-in duced differentiation. By structure-function analysis of PML/RAR alpha-medi ated RA-differentiation, we demonstrated that fusion of PML and RAR alpha s equences and integrity of the PML dimerization domain and of the RAR alpha DNA binding region are required for the effect of PML/RAR alpha on RA-diffe rentiation. Indeed, direct fusion of the PML dimerization domain to the Nor C-terminal extremities' of RAR alpha retained full biological activity. Al l the biologically active PML/RAR alpha mutants formed high molecular weigh t complexes in vivo. Functional analysis of mutations within the PML dimeri zation domain revealed that the capacity to form PML/RAR alpha homodimers, but not PML/RAR alpha-PML heterodimers, correlated with the RA response. Th ese results suggest that targeting of RAR alpha sequences by the PML dimeri zation domain and formation of nuclear PML/RAR alpha homodimeric complexes are crucial for the ability of PML/RAR alpha to mediate RA-response.