Studies in tissue culture cells have implicated p300 and CBP acetyltransferases in myogenic regulatory factor (MRF) mediated transcription and terminal differentiation of skeletal muscle cells. However, in vivo data placing p300 and CBP on myogenic differentiation pathways are not yet available. In this report we provide genetic evidence that p300 but not CBP acetyltransferase (AT) activity is required for myogenesis in the mouse and in embryonic stem (ES) cells. A fraction of embryos carrying a single p300 AT- deficient allele exhibit impaired MRF expression, delayed terminal differentiation and a reduced muscle mass. In mouse embryos lacking p300 protein, Myf-5 induction is severely attenuated. Similarly, ES cells homozygous for a p300 AT or a p300 null mutation fail to activate Myf5 and MyoD transcription efficiently, while Pax3, acting genetically upstream of these MRFs, is expressed. In contrast, ES cells lacking CBP AT activity express MyoD and Myf5 and undergo myogenic differentiation. These data reveal a specific requirement for p300 and its AT activity in the induction of MRF gene expression and myogenic cell fate determination in vivo.

p300 and CBP are large nuclear acetyltransferases exhibiting a complex multi‐domain structure. Mouse embryos nullizygous for either p300 or Cbp die at midgestation, while heterozygotes are viable but in part display defects in neurulation or bone morphogenesis. To directly examine the contribution of the acetyltransferase (AT) activity to mouse development, we have abrogated this function by a knock‐in approach. Remarkably, a single AT‐deficient allele of p300 or Cbp leads to embryonic or neonatal lethality, indicating that the mutant alleles are dominant. Formation of the cardiovascular system, the lung and the small intestine are strongly impaired in p300 AT and to a much lesser extent in Cbp AT mutant embryos, a difference that is also reflected by the defects in gene expression. Embryonic stem cells homozygous for either the p300 AT or a p300 null mutation respond differently to BMP2 stimulation, indicating that the two alleles are not equivalent. Unexpectedly, the p300 AT‐mutant cells upregulate BMP‐inducible genes to levels similar or even higher than observed in wild‐type cells.

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