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We are focusing on the roles of p53 in human and mouse embryonic stem (ES) cells, and have identified chromatin interactions of p53 by genome wide, deep sequencing of chromatin immunoprecipitates. There are major differences between human and mouse stem cell response to DNA damage. To address mechanisms of endogenous p53 regulation in stem cells, we created a new mouse and stem cell model by knock-in of a tandem-affinity-purification (TAP) epitope at the endogenous Trp53 locus. With this approach, we identified Tripartite-motif protein 24 (Trim24), a previously unknown partner of p53. We found that Trim24 is an RING-finger, E3-ligase that ubiquitylates and negatively regulates p53 levels. TRIM24 is degraded and p53 is induced during differentiation of human ES cells, to effect a transient G1/S arrest in response to retinoic acid treatment. Depletion of TRIM24 in human ES cells causes spontaneous differentiation, in contrast to human breast cancer cells where TRIM24 depletion induces spontaneous apoptosis. TRIM24 has PHD and Bromo structural domains, and regulates p53 and p53-dependent functions by chromatin-dependent and–independent mechanisms. TRIM24 is aberrantly expressed in a number of human cancers and may be a useful therapeutic target.
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