Novel insights into the cytoplasmic function of promyelocytic leukaemia (PML) and PML-retinoic acid receptor-[alpha]

The promyelocytic leukaemia protein (PML) is a tumour suppressor initially identified in acute promyelocytic leukaemia (APL). In APL, PML and the retinoic acid receptor alpha (RARalpha) genes are fused as a consequence of the translocation t(15;17). The product of the chimeric gene is the oncogenic PML-RARalpha protein. The PML gene encodes multiple nuclear and cytoplasmic isoforms. PML nuclear isoforms (nPML) are the main components of the PML nuclear bodies (PML-NBs), sub-nuclear structures involved in the modulation of essential cellular players including the tumour suppressor p53. Nuclear PML has been intensively studied, while, the role of cytoplasmic PML remains poorly understood. Increasing evidence indicates that PML could bear cytoplasmic functions in both physiological and pathological settings. This study aims to gain more insights into the function of PML and PML-RARalpha cytoplasmic pool of proteins. Recently, two missense mutations resulting in truncated PML cytoplasmic protein (Mut PML) have been identified in aggressive APL cases. We found that Mut PML alters the structure and the function of the PML-NB mainly through the cytoplasmic relocation of nPML. Remarkably, Mut PML inhibits p53 transcriptional, growth suppressive and apoptotic functions. In the cytoplasm, Mut PML interacts and stabilizes PML-RARalpha, thus potentiating its block of RA-induced transcription and differentiation. A mutant of PML-RARalpha (Delta2) accumulating in the cytoplasm is able to inhibit RA-dependent transcription and differentiation, suggesting that cytoplasmic localization of PML-RARalpha may contribute to transformation. Finally, we found that Delta2 expression blocks G-CSF-dependent myeloid differentiation and causes partial transformation of primary haematopoietic progenitor cells.