Gene-specific repression of the p53 target gene PUMA via intragenic CTCF-Cohesin binding.

The p53 transcriptional program orchestrates alternative responses to stress, including cell cycle arrest and apoptosis, but the mechanism of cell fate choice upon p53 activation is not fully understood. Here we report that PUMA (p53 up-regulated modulator of apoptosis), a key mediator of p53-dependent cell death, is regulated by a noncanonical, gene-specific mechanism. Using chromatin immunoprecipitation assays, we found that the first half of the PUMA locus (approximately 6 kb) is constitutively occupied by RNA polymerase II and general transcription factors regardless of p53 activity. Using various RNA analyses, we found that this region is constitutively transcribed to generate a long unprocessed RNA with no known coding capacity. This permissive intragenic domain is constrained by sharp chromatin boundaries, as illustrated by histone marks of active transcription (histone H3 Lys9 trimethylation [H3K4me3] and H3K9 acetylation [H3K9Ac]) that precipitously transition into repressive marks (H3K9me3). Interestingly, the insulator protein CTCF (CCCTC-binding factor) and the Cohesin complex occupy these intragenic chromatin boundaries. CTCF knockdown leads to increased basal expression of PUMA concomitant with a reduction in chromatin boundary signatures. Importantly, derepression of PUMA upon CTCF depletion occurs without p53 activation or activation of other p53 target genes. Therefore, CTCF plays a pivotal role in dampening the p53 apoptotic response by acting as a gene-specific repressor.