by on p63 shows high sequence homology to the most important tumor suppressor protein p53 and constitutes together with the protein p73 the p53 family of transcription factors. For p63, two different main functions have been identified: p63 is responsible for the development and maintenance of stratified epithelial tissues such as skin, and it is the main quality control factor that monitors the genetic integrity in oocytes.
This quality control function seems to be the evolutionary oldest function of the entire p53 protein family, as even evolutionary distant species such as Caenorhabditis elegans, Drosophila melanogaster, Ciona intestinalis, or sea anemones express a protein in their germ cells that resembles p63.
p63 is expressed as several different isoforms created by the use of different promotors and RNA splicing. The genetic quality control is carried out by a specific isoform, called TAp63α that is expressed exclusively in female oocytes. This isoform gets expressed at the end of the process of homologous recombination when oocytes have repaired the DNA double strand breaks inflicted by the type II topoisomerase-like DNA transesterase Spo11. If DNA double strand breaks are still present TAp63α initiates an apoptotic program via the expression of PUMA and NOXA that eliminates damaged oocytes. Following this check point oocytes enter dictyate arrest and remain arrested in prophase of meiosis I until they get recruited for ovulation. During the entire time—which in humans can last for ~50 years—TAp63α is highly expressed. To prevent the initiation of apoptosis in non-damaged oocytes, TAp63α adopts a closed, inactive, and only dimeric conformation. Detection of DNA damage results in the phosphorylation of S582 of TAp63α by the kinase Chk2. While this modification does not activate TAp63α, it recruits another kinase, CK1, which in general requires pre-phosphorylated substrates. The addition of four more phosphate groups switches TAp63α to its active and tetrameric conformation.
This activation is irreversible and de-phosphorylation does not convert the protein back to its inactive dimeric state. Oocyte death resulting in Premature Ovarian Insufficiency (POI) can be triggered, for example, by chemotherapy and irradiation as used for the treatment of cancer patients or by mutations in p63 that abrogate the function of the C-terminal Transactivation inhibitory domain (TID).
We are investigating the regulation of the activity of TAp63α by a combination of cell culture experiments, structure determination and ribosome profiling for studying the co-translational folding process.
The maintenance of stratified epithelial tissues is carried out by a different p63 isoform, ΔNp63α which lacks the N-terminal transactivation domain. This isoform which fomrs tetramers constitutively organizes the chromatin landscape of epithelial cells to orchestrate an epithelial specific transcriptional program. p63 mutations are associated with distinct developmental disorders in humans that feature at least one of three phenotypical hallmarks: ectodermal dysplasia (ED), limb defects, and orofacial clefting (OFC). These are associated with mutations in different domains of ΔNp63α. Mutations in the DNA binding domain mainly result in the EEC/LMS/ADULT syndrome, characterized by all three hallmarks. Mutations in the SAM domain cause the AEC/RHS syndrome. Patients exhibit no limb defects but suffer from skin erosions and ankyloblepharon instead. Finally, the non-syndromic disorder split hand/feet malformation 4 has no well-defined genetic basis as mutations are found throughout the p63 gene.
We are investigating the function of p63 in organizing the chromatin landscape and the molecular mechanisms of the syndromes caused by p63 mutations.
