Abstract 5732: p53 deficiency causes Myc dysregulation through a novel TGFβ-facilitated promoter switching mechanism to instigate osteosarcoma development

Abstract

Abstract p53 deficiency and Myc dysregulation, driven by genetic and epigenetic alterations, respectively, are frequently associated with cancer. Previously, we reported that the combination of p53 inactivation and Myc overexpression, mediated by Runx transcription factors (TFs), leads to osteosarcoma (OS), the most common primary bone cancer. However, the mechanisms leading to Myc upregulation by Runx in a p53-deficient microenvironment remain unclear. TGFβ is a main driver of the epithelial-to-mesenchymal transition and its signal-dependent TFs, Smads, are known to associate with lineage-determining TFs including Runx. As TGFβ paracrine signalling is reportedly increased during human OS development, we immunodetected increased co-expression of TGFβ and phospho-Smad2 in the bone marrow of osteoprogenitor-specific p53 knockout mice, Osterix (Osx)/Sp7-Cre; p53fl/fl mice, which are widely used as an animal model of human OS. Heterozygous deletion of the type II TGFβ receptor in those mice weakened their susceptibility to OS, phenocopying the overall life-extending effect of Myc depletion in the same model. p53 disruption caused TGFβ to aberrantly induce Myc in osteoprogenitors, rendering the cells tumorigenic upon transplantation into immunocompromised mice. Within the 3-Mb Myc regulatory region, we identified a TGFβ-responsive element dubbed m340, which contains binding sites for multiple TFs including Runx. The m340 enhancer caused Myc upregulation upon p53 disruption by switching its associated promoter from the lncRNA Pvt1 to Myc. In support of this promoter switching mechanism, a specific disruption of the evolutionarily conserved p53 site within the Pvt1 promoter recapitulated TGFβ-induced Myc overexpression. At m340, TGFβ promoted its occupancy by a transcriptional complex consisting of Runx, its cooperative factors Smads and AP1, and CBP, further activating the enhancer by increasing H3K27ac deposition. Runx inhibition not only repressed the aberrant upregulation of Myc by reducing Smad occupancy at m340, but also suppressed OS development in vivo. These results suggest that p53 deficiency causes Myc dysregulation through a novel promoter switching programme that is facilitated by TGFβ signalling and reveals a novel epigenetic mechanism underlying the interaction between cancer-predisposing genetic alterations (p53 deficiency) and environmental cues (TGFβ) in tumorigenesis. Considering the mediating role Runx plays in m340 activation, our study may provide a clinical rationale for targeting Runx in p53-deficient malignancies. Citation Format: Yuki Date, Tomoya Ueno, Kosei Ito. p53 deficiency causes Myc dysregulation through a novel TGFβ-facilitated promoter switching mechanism to instigate osteosarcoma development [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5732.