SP1 and NF-Y-Dependent Gene Transactivation Defines a Gain-of-Function for the PML-RARα Oncoprotein.

PML-RARa is the product of the chromosomal translocation t(15;17) and is the causative oncogene in more than 98% of the cases of acute promyelocytic leukemia. PML-RARa interferes with the transcription of retinoic acid receptor target genes in a dominant manner, contributing to the transformation of the cells. RARa regulates transcription via heterodimerisation with RXR. PML-RARa can bind as a homodimer to the same sequences as RARa/RXR. At supra-physiological dosis of all-trans retionoic acid (ATRA), the block in differentiation in APL can be overcome. ATRA releases co-repressors from PML-RARa and activates RARa target genes.

The ID1 gene is rapidly induced by ATRA in acute promyelocytic leukemia cells. Promoter analysis showed that the ID1 promoter was activated by PML-RARa but, unexpectedly, not by wild type RARa/RXR. Surprisingly, PML-RARa lacking the DNA binding domain could still transactivate the ID1 promoter indicating that transactivation was mediated without direct DNA binding. Promoter deletion studies showed that adjacent NF-Y and SP1 binding sites were essential for this transactivation. A direct physical interaction was shown between PML-RARa and SP1 in vitro and chromatin immunoprecepitation assays confirmed that a complex of PML-RARa/NF-Y/Sp1 is present on the ID1 promoter in vivo. In addition, we found that ectopic expression of PML-RARa induced expression of the endogenous ID1 gene in response to retinoic acid. We propose a novel, gain-of-function model for PML-RARa in which the fusion protein interferes with the transcription of SP1-NF-Y regulated genes. Interference is mediated without DNA-binding, through direct interaction with SP1. This implicates that apart from the previously described deregulation of retinoic acid receptor target genes, PML-RARa may deregulate an additional class of genes that are nomally not regulated by retinoid receptors.