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

Retinoic Acid (RA) treatment induces disease remission of Acute Promyelocytic Leukaemia (APL) patients by triggering terminal differentiation of neoplastic cells. RA-sensitivity in APL is mediated by its oncogenic protein, which results from the recombination of the PML and the RA receptor (RAR) genes (PML/RAR fusion protein). Ectopic expression of PML/RARinto haemopoietic cell lines results in increased response to RA-induced differentiation. By structure-function analysis of PML/RAR-mediated RA-differentiation, we demonstrated that fusion of PML and RAR sequences and integrity of the PML dimerization domain and of the RAR DNA binding region are required for the effect of PML/RAR on RA-differentiation. Indeed, direct fusion of the PML dimerization domain to the N- or C-terminal extremities of RAR retained full biological activity. All the biologically active PML/RAR mutants formed high molecular weight complexes in vivo. Functional analysis of mutations within the PML dimerization domain revealed that the capacity to form PML/RAR homodimers, but not PML/RAR-PML heterodimers, correlated with the RA-response. These results suggest that targeting of RAR sequences by the PML dimerization domain and formation of nuclear PML/RAR homodimeric complexes are crucial for the ability of PML/RAR to mediate RA-response.