Abstract 922: Delta133p53 represses p53-inducible senescence genes and enhances the generation of human induced pluripotent stem cells

Abstract

Abstract p53 functions to induce cellular senescence and apoptosis, which can be incompatible with self-renewal of pluripotent stem cells such as induced pluripotent stem cells (iPSC) and embryonic stem cells (ESC). On the other hand, p53 regulates DNA damage response and repair and thus plays an essential role in maintaining genomic integrity and suppressing malignant transformation in iPSC and ESC. It remains to be elucidated whether and how p53 and its regulators contribute to balanced regulation between the self-renewing capacity and the genomic and functional integrity in these pluripotent stem cells. We hypothesized the involvement of Δ133p53, a physiological p53 protein isoform that inhibits the activity of full-length p53 (FL-p53), and here examined 12 lines of human iPSC and their original fibroblasts, as well as 3 human ESC lines, for endogenous protein levels of Δ133p53 and FL-p53, and mRNA levels of p53 target genes of different functions. While FL-p53 levels in iPSC and ESC widely ranged from below to above those in the fibroblasts, all iPSC and ESC lines expressed elevated levels of Δ133p53. The p53-inducible genes that mediate cellular senescence (e.g., p21WAF1 and miR-34a), but not those for apoptosis and DNA repair, were downregulated in iPSC and ESC. Consistent with these endogenous expression profiles, overexpression of Δ133p53 in human fibroblasts preferentially repressed the p53-inducible senescence mediators and significantly enhanced their reprogramming to iPSC. The iPSC clones derived from Δ133p53-overexpressing fibroblasts, when injected into immunodeficient mice, formed well-differentiated, benign teratomas, suggesting that Δ133p53 overexpression is non- or less oncogenic than total inhibition of p53 activities. Overexpressed Δ133p53 prevented FL-p53 from binding to the regulatory regions of p21WAF1 and miR-34a, providing a mechanistic basis for its dominant-negative inhibition. This study supports the hypothesis that upregulation of Δ133p53 is an endogenous mechanism that facilitates human somatic cells to become pluripotent without malignant transformation. Citation Format: Izumi Horikawa, Kye-yoon Park, Han Li, Kazunobu Isogaya, Yukiharu Hiyoshi, Katsuhiro Anami, Ana I. Robles, Abdul M. Mondal, Kaori Fujita, Manuel Serrano, Curtis C. Harris. Delta133p53 represses p53-inducible senescence genes and enhances the generation of human induced pluripotent stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 922. doi:10.1158/1538-7445.AM2017-922