Acute muscle injury and physiological stress from chronic muscle diseases and aging lead to impairment of skeletal muscle function. This raises the question of whether p53, a cellular stress sensor, regulates muscle tissue repair under stress conditions. By investigating muscle differentiation in the presence of genotoxic stress, we discovered that p53 binds directly to the myogenin promoter and represses transcription of myogenin, a member of the MyoD family of transcription factors that plays a critical role in driving terminal muscle differentiation. This reduction of myogenin protein is observed in G1-arrested cells and leads to decreased expression of late but not early differentiation markers. In response to acute genotoxic stress, p53-mediated repression of myogenin reduces post-mitotic nuclear abnormalities in terminally differentiated cells. This study reveals a mechanistic link previously unknown between p53 and muscle differentiation, and suggests new avenues for managing p53-mediated stress responses in chronic muscle diseases or during muscle aging. Cell Death and Differentiation (2015) 22, 560-573; doi:10.1038/cdd.2014; published online 12 December 2014The tumor suppressor p53 promotes cell cycle arrest or apoptosis in response to diverse stress signals such as DNA damage, thus preventing propagation of genetically compromised cells. [1][2][3] Among the diverse functions attributed to p53, a growing body of evidence supports its role in regulation of differentiation and maintenance of cellular function and integrity. 1,[4][5][6][7] For example, p53 represses Nanog to maintain genetic stability of the stem cell pool by promoting differentiation of mouse embryonic stem cells (mESCs) after DNA damage. 6 Skeletal muscle differentiation, a key step during muscle tissue formation, is orchestrated by the MyoD family of myogenic regulatory factors (MRFs). MyoD determines the myogenic lineage, whereas myogenin, a member of the MRF family, functions downstream of MyoD and plays a critical role in driving terminal differentiation as myogenin-null mice show a lethal deficiency of differentiated skeletal muscle. [8][9][10][11][12][13] The dynamic differentiation program of skeletal muscle is characterized by the orderly expression of genes and structural changes that can be recapitulated in vitro, as myogenic cells undergo cell cycle withdrawal and express early and then late differentiation genes with the formation of mononucleated myocytes to elongated multinucleated myotubes. 8,9,14 Under normal unstressed conditions, p53 has been shown to promote muscle differentiation in vitro. [15][16][17][18][19][20] In contrast, under stress-associated conditions such as inflammation, chronic exposure to double-strand DNA breaks, and aging, enhanced p53 activity has been shown to correlate with skeletal muscle atrophy in vivo. [21][22][23][24][25] In addition, it has been shown that p53 and its downstream effectors are required for an inflammatory cytokine-mediated inhibition of myogenic differentiation in vitro. 22 Int...