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ΔNp63α is primarily expressed in the epithelial tissue, including the mammary gland and epidermis, where it is indispensable to maintain the high proliferative potential of somatic stem cells.1 Although mutations of p63 are extremely rare in human cancers, several tumors, including primary head and neck squamous cell carcinomas (HNSCCs), squamous cell epithelial lung malignancies, non-small-cell lung cancers and basal-like subtypes of breast cancer,2,3,4,5,6 often display elevated levels of ΔNp63, which is associated with poor prognosis. However, the mechanism of action of ΔNp63α in tumors remains mostly unknown. ΔNp63 isoforms, the amino-deleted isoforms encoded by TP63, lack the N-terminal transactivation (TA) domain, but are still able to transcriptionally regulate a distinct subset of genes due to the presence of a second TA domain (TA2). Thus, ΔNp63α has been shown to function both as a transcriptional activator and as a transcriptional repressor. Although the ΔNp63 transcriptional profile has been extensively characterized in normal epithelial cells and in cancer cell lines, little is known about how ΔNp63α directly transactivates genes, and which co-activators are required at enhancer and promoter sites. Furthermore, detailed information precisely mapping the TA2 domain is still missing. In contrast, much more information is available on the mechanisms of ΔNp63α-mediated transcriptional repression. ΔNp63α represses transcription by directly antagonizing p53 family members or by modulating the chromatin landscape near target genes (Figure 1). In the past several years, one prevalent hypothesis in the literature has been that ΔNp63α represses TAp73/p53 target genes simply by acting as dominant-negative to prevent TAp73/p53 occupancy at the shared DNA responsive elements (Figure 1a). For example, p63 knockdown in HNSCC cell lines results in TAp73-dependent apoptosis via PUMA and NOXA upregulation. In this system, ΔNp63α forms hetero-tetramers with TAp73, preventing the binding of TAp73 to PUMA enhancers.7 Although this notion is still valid, it did not explain several results obtained in HNSCC2 and in other cancer types suggesting that alternative TAp73/p53-independent mechanisms, employed by ΔNp63α, are engaged. Indeed, in keratinocytes and in HNSCC cell lines, ΔNp63α physically interacts with the histone deacetylases HDAC1 and HDAC2, and recruits these enzymes to p63 and p53 enhancer sites, thus mediating histone H3 and H4 deacetylation and consequent transcriptional inhibition (Figure 1b).8 Another ΔNp63α-dependent mechanism of repression is the recruitment of the SRCAP chromatin remodelling complex, via a physical interaction with the SAMD9L subunit.2 SRCAP complex is involved in H2A/H2A.Z exchange, mediating H2A.Z deposition near p63 response elements, thus creating a chromatin environment that is in a repressed conformation; this has been demonstrated in keratinocytes, lung SCC and HNSCC cell lines (Figure 1c).
Recently, using a yeast two-hybrid assay, Regina et al.9 showed that ΔNp63α interacts with SETDB1, a histone lysine methyl transferase (HMT) that is important in epigenetic regulation (Figure 1d). SETDB1 belongs to the SET (Suppression of variegation, Enhancer of zeste, Trithorax)-domain containing enzymes. HMTs catalyze the transfer of one to three methyl groups from S-adenosyl-methionine to specific lysine residues on histone proteins.10 Depending on the site and degree of methylation, this modification can have various effects, including regulation of chromatin organization and gene transcription. Among the different HMTs, SETDB1 has been of increasing interest due to its involvement in melanoma, where it is located in a recurrently amplified chromosome fragment.11 SETDB1 amplification has been also described in lung tumors.12 Regina et al.9 demonstrated that SETB1 is also overexpressed in different breast cancer cell lines and in primary tumors. Knockdown of SETDB1 resulted in growth-inhibitory effects. The authors also identified a list of 30 genes possibly repressed by ΔNp63 in a SETDB1-dependent manner, some of which correlated with the survival of breast cancer patients, suggesting that the ΔNp63α−SETDB1 interaction has a relevant and functional role in breast tumorigenesis.
These findings indicate a third mechanism through which ΔNp63α represses transcription, demonstrating that ΔNp63α uses different partners in a combinatorial fashion and in a cell-type-specific manner. Understanding mechanistically how ΔNp63α recruits chromatin remodelers, and identifying repressed target genes in different cells and cancer types, could be important in the future to modulate senescence/proliferation in epithelial cells and to block rapid cancer expansion.
Memmi EMet al.Proc Natl Acad Sci USA 2015; 112: 3499–3504.
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