Senescence impairs successful reprogramming to pluripotent stem cells

Somatic cells can be reprogrammed into induced pluripotent stem (iPS) cells by overexpression of combinations of transcription factors such as Oct4, Sox2, Klf4 and c-Myc. However, reprogramming is slow and stochastic, suggesting the existence of mechanisms that limit its efficiency. Senescence is an irreversible G1 cell cycle arrest elicited by replicative exhaustion or in response to stresses such as DNA damage, or aberrant expression of oncogenes. The arrest observed during senescence is implemented mainly through activation of p53 and the upregulation of the cyclin-dependent kinase (CDK) inhibitors, p16INK4a and p21Cip1. In this work the relation between the process of reprogramming to iPS cells and senescence was investigated. The expression of the 4 reprogramming factors from a polycistronic vector in human fibroblasts (IMR90) was shown to induce cell cycle arrest and upregulation of p53, p16INK4a and p21Cip1. Reprogramming-induced senescence (RIS) results from the activation of a DNA damage response, and chromatin remodeling of the INK4a/ARF locus, as shown by a decrease in the levels of the H3K27me3 modification following expression of the reprogramming factors. RIS resembles a stress response, which parallels oncogene-induced senescence, however context-dependent differences may also contribute as shown by the fact that ES cells-specific miRNAs can partially bypass RIS. Additionally when expressed individually, each reprogramming factor was shown to have a negative effect over the proliferation of somatic cells. Since reprogramming initially triggers a stress response with characteristics of senescence it may act as an initial barrier limiting the efficiency of the process. Indeed, ablation of different senescence effectors improved the efficiency of reprogramming, both in mouse and human cells. Additionally, the polycomb protein CBX7 was also shown to increase reprogramming efficiency in a process that may partially dependent on repression of the INK4a/ARF locus. The senescence response to expression of reprogramming factors uncovers an important barrier to induced pluripotency but also highlights the importance of tumour suppressor pathways in preventing dedifferentiation during tumorigenesis. Identification of RIS mediators may help to understand this connection and provide safer approaches to increase reprogramming efficiency