The discovery of p63 and of p73, two proteins with high sequence identity to the famous tumor suppressor p53, has sparked speculations that the surveillance of the genetic stability of cells is maintained by an entire network of p53-like tumor suppressors. Surprisingly, however, knock-out mouse studies of all three family members resulted in very different phenotypes. While p53 knock-out mice are mainly characterized by their high frequency of developing tumors at very early age, both the p63 and the p73 knock-out mice show severe developmental defects. Inactivation by mutating the DNA binding domain of p73 results in hippocampal dysgenesis, hydrocephalus, chronic infections and inflammation, as well as abnormalities in pheromone sensory pathways. In addition to these developmental functions of p73, recent analysis of heterozygote knock-out mice (p73+/-) as well as isoform-specific knock-out mouse studies (TAp73-isoform specific knock-out) have suggested that p73 can also act as a tumor suppressor.
The p63 knock-out mice show even more pronounced developmental defects, such as severe limb truncations, lack of a multi-layered skin and other epithelial structures. The p63 knock-out mouse studies led to the identification of mutations in the human p63 gene as the cause of six different human syndromes that are characterized by limb truncations, cleft lip and palates and/or skin abnormalities. The combination of these patient data, the knock-out mice studies and the observation that p63 is highly expressed in the basal layer of stratified epithelial tissues including skin resulted in the model that p63 plays a major role in maintaining epithelial stem cells.
Recently, a second function of p63 was identified: it serves as a quality control factor in female oocytes where p63 is highly expressed. Detection of DNA double strand breaks results in the activation of p63 by phosphorylation and the induction of cell death. In combination with the identification of p63-like genes in nematodes and other invertebrates that are normally not threatened by the development of tumors, this quality control function of germ cells seems to emerge as the ancient function of the entire p53 family. The discovery has also medical relevance: Women treated for cancer with DNA damaging chemo therapy often become infertile. This infertility can now be linked to the activation of p63 which results in the elimination of all oocytes.
Despite the high expression level of p63 in resting oocytes, cell death is not induced without the detection of DNA damage, suggesting that the activity of p63 must be tightly regulated. We have started to characterize the mechanism of the regulation of TAp63α by a combination of biochemical, structural, cell culture and mouse experiments. Our data have revealed that TAp63α in oocytes is kept in a close, transcriptional inhibited and only dimeric conformation. Detection of DNA damage results in the phosphorylation of TAp63α which leads to the opening of the closed dimeric state followed by the formation of active tetramers. This active form has a 20-fold higher DNA binding affinity triggering the expression of PUMA and NOXA which induce apoptosis in the oocytes.