Gene transactivation without direct DNA binding defines a novel gain-of-function for PML-RARα

Abstract

IntroductionAcute promyelocytic leukemia (APL) is characterized by an excess of immature promyelocytes in the bone marrow that fail to differentiate toward mature granulocytes. In approximately 98% of the cases, the retinoic acid receptor-␣ (RAR␣) gene is fused to the promyelocytic leukemia (PML) gene resulting in a PML-RAR␣ fusion protein. The PML-RAR␣ chimeric protein contains most of the PML sequence and a large part of RAR␣, including its DNAand nuclear hormone-binding domains. APL blasts can be forced to terminally differentiate using pharmacological doses of all-trans retinoic acid (ATRA). When treated with chemotherapy, APL patients can be cured in approximately 40% of the cases. The combination of ATRA with chemotherapy leads to a remarkably high cure rate of approximately 90%, 1,2 and APL currently represents the best prognostic group among the different forms of leukemia. This treatment constitutes one of the first examples of successful induction of differentiation of malignant cells yielding significant clinical results.The role of PML-RAR␣ in transformation and terminal differentiation has been studied intensively in the past decade. PML-RAR␣ was shown to act as a dominant oncogene, interfering with the normal function of the unrearranged PML as well as the unrearranged RAR␣ protein. Expression of the fusion protein in immature hematopoietic cells induced a maturation block at the promyelocytic stage. Inoculation of PML-RAR␣-transduced bone marrow cells into irradiated syngenic mice resulted in the development of retinoic acid-sensitive leukemia. 3,4 Furthermore, PML-RAR␣ transgenic mice developed a myeloproliferative syndrome that progressed to overt leukemia in 30% to 90% of the animals after 6 to 12 months, depending on the promoter that was used. [5][6][7] PML is a ubiquitously expressed protein that localizes to nuclear substructures termed nuclear bodies. More than 50 protein partners with various biologic functions colocalize with PML. 8 In APL cells, these nuclear bodies are disrupted and dispersed into numerous small microspeckles. 9 PML has multiple tumorsuppressor functions and is involved in growth control, replicative senescence, and apoptosis. 10 PML Ϫ/Ϫ mice are prone to develop tumors in response to various forms of stress. In addition, PML-RAR␣ transgenic mice develop leukemia much faster in a PML Ϫ/Ϫ background. 11 Retinoic acid receptors are transcription factors that activate genes in a ligand-dependent manner. RAR␣ binds to DNA as a heterodimer with RXR proteins. In the absence of ligand, both RAR␣/RXR and PML-RAR␣ bind corepressors such as N-Cor and SMRT and recruit histone deacetylases leading to gene silencing. In the presence of ligand, the corepressors are replaced by coactivators, leading to transcriptional activation. However, PML-RAR␣ releases the corepressors at much higher concentrations of ligand compared with the unrearranged receptors. Since PML-RAR␣ competes with unrearranged receptors for the same DNAbinding sites, the presence of the fusion protein results in dom...

Identification of a myeloid committed progenitor as the cancer-initiating cell in acute promyelocytic leukemia

“…PML-RAR␣ recruitment to C/EBP␣ regulatory elements could also occur through an indirect mechanism. For instance, PML-RAR␣ has been described to interact with Sp1 55 and to repress target genes without a canonical retinoic acid response element within their promoter. 56 The C/EBP␣ proximal promoter contains 2 sites that bind to Sp1 by electrophoretic mobility shift assay (data not shown) that could enable recruitment of PML-RAR␣, and 2 potential Sp1 binding sites in the previously described methylated region upstream of the promoter.…”

The CEBPA gene is down-regulated in acute promyelocytic leukemia and its upstream promoter, but not the core promoter, is highly methylated

“…PML-RARA recruitment to C/EBPA regulatory elements could also occur through an indirect mechanism. Wageningen et al 19 show that PML-RARA physically interacts with Sp1 in the absence of DNA and proposed a model in which PML-RARA binds to a DNA-bound Sp1/NF-Y complex, thus modulating gene expression. The CEBPA upstream methylated region (-1423 to -1121 bp from TSS) contains two potential SP1 and USF binding sites 11 that could enable recruitment of PML-RARA.…”

Therapy-induced PML/RARA Proteolysis and Acute Promyelocytic Leukemia Cure

“…Recent studies have shown that the differentiation block and the transcriptional repression induced by PML/ RARA involve not only RARA homodimerization (22), but also PML sumoylation, RXR binding, and recruitment of the polycomb complex (23)(24)(25)(26)(27). Other studies have shown that PML/ RARA may be tethered onto DNA through specific interactions with other transcription factors, identifying yet another way for the fusion to alter gene expression (28). Apart from transcriptional regulation, PML/RARA also disrupts PML nuclear bodies (29)(30)(31)(32), which regulate stem cell self-renewal (14,15,33,34), raising the tantalizing prospect that other molecular mechanisms than transcriptional repression may be implicated in APL leukemogenesis.…”