Abstract 1306: p63 functions as an essential oncogene in squamous cell carcinoma in vivo and is required for tumor maintenance

Transcription factors mediate diverse functions in normal development and tumorigenesis. The p53-related transcription factor p63 (TP63) plays an essential role in development of stratified epithelial tissues, as germline loss of p63 leads to abnormal proliferation and differentiation of ectoderm-derived precursors. The role of p63 in human cancer, however, is controversial. Loss of p63 is associated with neoplastic progression in some animal models and in a subset of human tumors. Nevertheless, p63 is rarely targeted for somatic mutational inactivation and is indeed overexpressed in squamous cell carcinomas (SCCs), where is has been postulated to function either as a key tumor maintenance factor or as a lineage marker not essential to the malignant process. Here, we show that SCCs are profoundly dependent on p63 expression in vivo, and we identify a key, therapeutically relevant pathway through which p63 mediates tumor maintenance. Conditional genetic ablation of p63 in an endogenous SCC model induces rapid and dramatic tumor regression owing to apoptosis of p63-expressing cells. We identify the FGFR2 oncogene as a direct transcriptional target of p63 that signals through AKT to mediate survival of SCC cells. Finally, we employ the clinical FGFR2 inhibitor AZD4547 to demonstrate the potential therapeutic effect of extinguishing FGFR2 signaling in p63-expressing SCCs. These results define p63 as a key tumor maintenance factor through regulation of FGFR2 signaling in SCC. More broadly, this demonstrates the potential for targeting oncogenic transcription factor pathways in tumors such as SCC that rarely exhibit somatic oncogene-activating mutations.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1306. doi:1538-7445.AM2012-1306

Throwing the cancer switch: reciprocal roles of polycomb and trithorax proteins

The discovery that cancer can be governed above and beyond the level of our DNA presents a new era for designing therapies that reverse the epigenetic state of a tumour cell. Understanding how altered chromatin dynamics leads to malignancy is essential for controlling tumour cells while sparing normal cells. Polycomb and trithorax group proteins are evolutionarily conserved and maintain chromatin in the 'off' or 'on' states, thereby preventing or promoting gene expression, respectively. Recent work highlights the dynamic interplay between these opposing classes of proteins, providing new avenues for understanding how these epigenetic regulators function in tumorigenesis.

Abstract B26: A requirement for p63 function in squamous cell carcinoma in vivo: Can the pathway be targeted for therapy?

Squamous cell carcinoma (SCC) is a common form of human cancer that can arise in various epithelial tissues such as the lungs, skin, and head and neck region. Morbidity and mortality rates for SCC are high compared to many other cancers, and relatively little progress has been made in treatment due to a lack of molecular understanding of the disease. The p53-related protein p63 is essential for the formation of various epithelial tissues, such as the skin, and has many reported roles including suppression of apoptosis, maintenance of stem cell regenerative potential, regulation of differentiation, and maintenance of cellular adhesion. In SCC, p63 is overexpressed and has found to be the target of genomic amplification suggesting an important role in this disease. Previous in vitro studies have shown that p63 is required for survival of SCC cells in culture, however, cell lines lack the complex microenvironment found in human cancer, and thus the in vivo relevance of these results is unclear. To determine the role of p63 in vivo, we have generated a murine model of SCC, which faithfully recapitulates many of the molecular and pathological features of human SCC including invasion and metastasis. To test the role of p63 in tumor maintenance we crossed this model to a conditional p63-null allele (p63flox) and a tamoxifen-inducible Cre transgene, K14-CreER. We have found that genetic excision of both copies of p63 in established SCC tumors results in rapid and dramatic regression of tumors. In addition, even loss of a single copy of the p63 gene results in a reduced rate of growth, suggesting that SCC tumors are exquisitely sensitive to levels of p63 protein. These results support the idea that the p63 pathway is critical for the survival of SCC tumors in vivo, and targeting this pathway in SCC may result in more effective therapy for this highly refractory tumor type.

Citation Information: Clin Cancer Res 2010;16(14 Suppl):B26.

A regulatory feedback loop involving p63 and IRF6 links the pathogenesis of 2 genetically different human ectodermal dysplasias

The human congenital syndromes ectrodactyly ectodermal dysplasia-cleft lip/palate syndrome, ankyloblepharon ectodermal dysplasia clefting, and split-hand/foot malformation are all characterized by ectodermal dysplasia, limb malformations, and cleft lip/palate. These phenotypic features are a result of an imbalance between the proliferation and differentiation of precursor cells during development of ectoderm-derived structures. Mutations in the p63 and interferon regulatory factor 6 (IRF6) genes have been found in human patients with these syndromes, consistent with phenotypes. Here, we used human and mouse primary keratinocytes and mouse models to investigate the role of p63 and IRF6 in proliferation and differentiation. We report that the DeltaNp63 isoform of p63 activated transcription of IRF6, and this, in turn, induced proteasome-mediated DeltaNp63 degradation. This feedback regulatory loop allowed keratinocytes to exit the cell cycle, thereby limiting their ability to proliferate. Importantly, mutations in either p63 or IRF6 resulted in disruption of this regulatory loop: p63 mutations causing ectodermal dysplasias were unable to activate IRF6 transcription, and mice with mutated or null p63 showed reduced Irf6 expression in their palate and ectoderm. These results identify what we believe to be a novel mechanism that regulates the proliferation-differentiation balance of keratinocytes essential for palate fusion and skin differentiation and links the pathogenesis of 2 genetically different groups of ectodermal dysplasia syndromes into a common molecular pathway.

{Delta}Np73{beta} puts the brakes on DNA repair

Mammalian cells are barraged with endogenous metabolic byproducts and environmental insults that can lead to nearly a million genomic lesions per cell per day. Networks of proteins that repair these lesions are essential for genome maintenance, and a compromise in these pathways propagates mutations that can cause aging and cancer. The p53 tumor suppressor plays a central role in repairing the effects of DNA damage, and has therefore earned the title of "guardian of the genome." In this issue of Genes & Development, Wilhelm and colleagues (pp. 549-560) demonstrate that p73-an older sibling of p53-inhibits pathways that resolve DNA double-strand breaks.

A mouse chromosome 4 balancer ENU-mutagenesis screen isolates eleven lethal lines

BACKGROUND

ENU-mutagenesis is a powerful technique to identify genes regulating mammalian development. To functionally annotate the distal region of mouse chromosome 4, we performed an ENU-mutagenesis screen using a balancer chromosome targeted to this region of the genome.

RESULTS

We isolated 11 lethal lines that map to the region of chromosome 4 between D4Mit117 and D4Mit281. These lines form 10 complementation groups. The majority of lines die during embryonic development between E5.5 and E12.5 and display defects in gastrulation, cardiac development, and craniofacial development. One line displayed postnatal lethality and neurological defects, including ataxia and seizures.

CONCLUSION

These eleven mutants allow us to query gene function within the distal region of mouse chromosome 4 and demonstrate that new mouse models of mammalian developmental defects can easily and quickly be generated and mapped with the use of ENU-mutagenesis in combination with balancer chromosomes. The low number of mutations isolated in this screen compared with other balancer chromosome screens indicates that the functions of genes in different regions of the genome vary widely.

Regulation of Dlx5 and Dlx6 gene expression by p63 is involved in EEC and SHFM congenital limb defects

The congenital malformation Split Hand-Foot Malformation (SHFM, or ectrodactyly) is characterized by a medial cleft of hands and feet, and missing central fingers. Five genetically distinct forms are known in humans; the most common (type-I) is linked to deletions of DSS1 and the distalless-related homeogenes DLX5 and DLX6. As Dlx5;Dlx6 double-knockout mice show a SHFM-like phenotype, the human orthologs are believed to be the disease genes. SHFM-IV and Ectrodactyly-Ectodermal dysplasia-Cleft lip (EEC) are caused by mutations in p63, an ectoderm-specific p53-related transcription factor. The similarity in the limb phenotype of different forms of SHFM may underlie the existence of a regulatory cascade involving the disease genes. Here, we show that p63 and Dlx proteins colocalize in the nuclei of the apical ectodermal ridge (AER). In homozygous p63- (null) and p63EEC (R279H) mutant limbs, the AER fails to stratify and the expression of four Dlx genes is strongly reduced; interestingly, the p63+/EEC and p63+/- hindlimbs, which develop normally and have a normally stratified AER, show reduced Dlx gene expression. The p63+/EEC mutation combined with an incomplete loss of Dlx5 and Dlx6 alleles leads to severe limb phenotypes, which are not observed in mice with either mutation alone. In vitro, DeltaNp63alpha induces transcription from the Dlx5 and Dlx6 promoters, an activity abolished by EEC and SHFM-IV mutations, but not by Ankyloblepharon-Ectodermal defects-Cleft lip/palate (AEC) mutations. ChIP analysis shows that p63 is directly associated with the Dlx5 and Dlx6 promoters. Thus, our data strongly implicate p63 and the Dlx5-Dlx6 locus in a pathway relevant in the aetio-pathogenesis of SHFM.

The quest for the 1p36 tumor suppressor

Genomic analyses of late-stage human cancers have uncovered deletions encompassing 1p36, thereby providing an extensive body of literature supporting the idea that a potent tumor suppressor resides in this interval. Although several genes have been proposed as 1p36 candidate tumor suppressors, convincing evidence that their encoded products protect from cancer has been scanty. A recent functional study identified chromodomain helicase DNA-binding protein 5 (CHD5) as a novel tumor suppressor mapping to 1p36. Here, we discuss evidence supporting the tumor-suppressive role of CHD5. Together, these findings suggest that strategies designed to enhance CHD5 activity could provide novel approaches for treating a broad range of human malignancies.

p53: at the crossroad between cancer and ageing

The p53 tumour suppressor plays an undisputed role in cancer. p53's tumour suppressive activity stems from its ability to respond to a variety of stresses to trigger cell cycle arrest, apoptosis or senescence, thereby protecting against malignant transformation. An increasing body of evidence suggests that p53 also drives organismal ageing. Although genetic models with altered p53 function display age-related phenotypes and thus provide in vivo evidence that p53 contributes to the ageing process, p53's role in organismal ageing remains controversial. Anti-cancer therapies that target p53 and reactivate or enhance its activity are considered good alternatives for treating various neoplasms. Therefore, it is important to determine whether these clinical approaches compromise tissue homeostasis and contribute to ageing. This review presents a number of models with altered p53 function and discusses how these models implicate p53 as part of a molecular network that integrates tumour suppression and ageing.

Abnormal urethra formation in mouse models of split-hand/split-foot malformation type 1 and type 4

Urogenital birth defects are one of the common phenotypes observed in hereditary human disorders. In particular, limb malformations are often associated with urogenital developmental abnormalities, as the case for Hand-foot-genital syndrome displaying similar hypoplasia/agenesis of limbs and external genitalia. Split-hand/split-foot malformation (SHFM) is a syndromic limb disorder affecting the central rays of the autopod with median clefts of the hands and feet, missing central fingers and often fusion of the remaining ones. SHFM type 1 (SHFM1) is linked to genomic deletions or rearrangements, which includes the distal-less-related homeogenes DLX5 and DLX6 as well as DSS1. SHFM type 4 (SHFM4) is associated with mutations in p63, which encodes a p53-related transcription factor. To understand that SHFM is associated with urogenital birth defects, we performed gene expression analysis and gene knockout mouse model analyses. We show here that Dlx5, Dlx6, p63 and Bmp7, one of the p63 downstream candidate genes, are all expressed in the developing urethral plate (UP) and that targeted inactivation of these genes in the mouse results in UP defects leading to abnormal urethra formation. These results suggested that different set of transcription factors and growth factor genes play similar developmental functions during embryonic urethra formation. Human SHFM syndromes display multiple phenotypes with variations in addition to split hand foot limb phenotype. These results suggest that different genes associated with human SHFM could also be involved in the aetiogenesis of hypospadias pointing toward a common molecular origin of these congenital malformations.

Neural potential of a stem cell population in the hair follicle

The bulge region of the hair follicle serves as a repository for epithelial stem cells that can regenerate the follicle in each hair growth cycle and contribute to epidermis regeneration upon injury. Here we describe a population of multipotential stem cells in the hair follicle bulge region; these cells can be identified by fluorescence in transgenic nestin-GFP mice. The morphological features of these cells suggest that they maintain close associations with each other and with the surrounding niche. Upon explantation, these cells can give rise to neurosphere-like structures in vitro. When these cells are permitted to differentiate, they produce several cell types, including cells with neuronal, astrocytic, oligodendrocytic, smooth muscle, adipocytic, and other phenotypes. Furthermore, upon implantation into the developing nervous system of chick, these cells generate neuronal cells in vivo. We used transcriptional profiling to assess the relationship between these cells and embryonic and postnatal neural stem cells and to compare them with other stem cell populations of the bulge. Our results show that nestin-expressing cells in the bulge region of the hair follicle have stem cell-like properties, are multipotent, and can effectively generate cells of neural lineage in vitro and in vivo.

CHD5 is a tumor suppressor at human 1p36

Cancer gene discovery has relied extensively on analyzing tumors for gains and losses to reveal the location of oncogenes and tumor suppressor genes, respectively. Deletions of 1p36 are extremely common genetic lesions in human cancer, occurring in malignancies of epithelial, neural, and hematopoietic origin. Although this suggests that 1p36 harbors a gene that drives tumorigenesis when inactivated, the identity of this tumor suppressor has remained elusive. Here we use chromosome engineering to generate mouse models with gain and loss of a region corresponding to human 1p36. This approach functionally identifies chromodomain helicase DNA binding domain 5 (Chd5) as a tumor suppressor that controls proliferation, apoptosis, and senescence via the p19(Arf)/p53 pathway. We demonstrate that Chd5 functions as a tumor suppressor in vivo and implicate deletion of CHD5 in human cancer. Identification of this tumor suppressor provides new avenues for exploring innovative clinical interventions for cancer.

p63, cellular senescence and tumor development

Deficiency of p63, a p53-related protein, causes severe defects in epithelial morphogenesis. Studies of p63-compromised mouse models reveal that p63 deficiency induces cellular senescence both in cultured cells and in vivo, through regulation p19(Arf)/p53 and p16(Ink4a)/Rb pathways. An extensive tumor study of p63-compromised mice demonstrated that p63 deficiency does not predispose to, but rather protects from, tumor development. These findings further implicate p63 as a negative regulator of the tumor suppressive mechanism of cellular senescence.

DeltaNp63 plays an anti-apoptotic role in ventral bladder development

The bladder, the largest smooth-muscle organ in the human body, is responsible for urine storage and micturition. P63, a homolog of the p53 tumor-suppressor gene, is essential for the development of all stratified epithelia, including the bladder urothelium. The N-terminal truncated isoform of p63, DeltaNp63, is known to have anti-apoptotic characteristics. We have established that DeltaNp63 is not only the predominant isoform expressed throughout the bladder, but is also preferentially expressed in the ventral bladder urothelium during early development. We observed a host of ventral defects in p63-/- embryos, including the absence of the abdominal and ventral bladder walls. This number of ventral defects is identical to bladder exstrophy, a congenital anomaly exhibited in human neonates. In the absence of p63, the ventral urothelium was neither committed nor differentiated, whereas the dorsal urothelium was both committed and differentiated. Furthermore, in p63-/- bladders, apoptosis in the ventral urothelium was significantly increased. This was accompanied by the upregulation of mitochondrial apoptotic mediators Bax and Apaf1, and concurrent upregulation of p53. Overexpression of DeltaNp63gamma and DeltaNp63beta in p63-/- bladder primary cell cultures resulted in a rescue, evidenced by significantly reduced expressions of Bax and Apaf1. We conclude that DeltaNp63 plays a crucial anti-apoptotic role in normal bladder development.

p63 regulates an adhesion programme and cell survival in epithelial cells

p63 is critical for epithelial development yet little is known about the transcriptional programmes it regulates. By characterising transcriptional changes and cellular effects following modulation of p63 expression, we have defined a vital role for p63 in cellular adhesion. Knockdown of p63 expression caused downregulation of cell adhesion-associated genes, cell detachment and anoikis in mammary epithelial cells and keratinocytes. Conversely, overexpression of the TAp63gamma or deltaNp63alpha isoforms of p63 upregulated cell adhesion molecules, increased cellular adhesion and conferred resistance to anoikis. Apoptosis induced by loss of p63 was rescued by signalling downstream of beta4 integrin. Our results implicate p63 as a key regulator of cellular adhesion and survival in basal cells of the mammary gland and other stratified epithelial tissues.

p63 heterozygous mutant mice are not prone to spontaneous or chemically induced tumors

Homology between p63 and p53 has suggested that these proteins might function similarly. However, the majority of data from human tumors have not supported a similar role for p63 in tumor suppression. To investigate this issue, we studied spontaneous tumorigenesis in p63+/- mice in both WT and p53-compromised backgrounds. We found that p63+/- mice were not tumor prone and mice heterozygous for both p63 and p53 had fewer tumors than p53+/- mice. The rare tumors that developed in mice with compromised p63 were also distinct from those of p53+/- mice. Furthermore, p63+/- mice were not prone to chemically induced tumorigenesis, and p63 expression was maintained in carcinomas. These findings demonstrate that, in agreement with data from human tumors, p63 plays a markedly different biological role in cancer than p53.

p63 regulates multiple signalling pathways required for ectodermal organogenesis and differentiation

Heterozygous germline mutations in p63, a transcription factor of the p53 family, result in abnormal morphogenesis of the skin and its associated structures, including hair follicles and teeth. In mice lacking p63, all ectodermal organs fail to develop, and stratification of the epidermis is absent. We show that the ectodermal placodes that mark early tooth and hair follicle morphogenesis do not form in p63-deficient embryos, although the multilayered dental lamina that precedes tooth placode formation develops normally. The N-terminally truncated isoform of p63 (DeltaNp63) was expressed at high levels in embryonic ectoderm at all stages of tooth and hair development, and it was already dominant over the transactivating TAp63 isoform prior to epidermal stratification. Bmp7, Fgfr2b, Jag1 and Notch1 transcripts were co-expressed with DeltaNp63 in wild-type embryos, but were not detectable in the ectoderm of p63 mutants. In addition, beta-catenin and Edar transcripts were significantly reduced in skin ectoderm. We also demonstrate that BMP2, BMP7 and FGF10 are potent inducers of p63 in cultured tissue explants. Hence, we suggest that p63 regulates the morphogenesis of surface ectoderm and its derivatives via multiple signalling pathways.

p63: a new link between senescence and aging

Cellular senescence is a distinctive form of cell cycle arrest that has been suggested to modulate the processes of tumor suppression and aging. Though a detailed understanding of the cellular machinery regulating this process is emerging, a more thorough understanding of the key players linking senescence to organismal aging is needed. The recent discovery that loss of the p53-related protein p63 induces cellular senescence and causes features of accelerated aging provides further evidence that cellular senescence is intimately linked with organismal aging, and identifies p63 as a key regulator of both of these processes.

p63 is an essential proapoptotic protein during neural development

The p53 family member p63 is required for nonneural development, but has no known role in the nervous system. Here, we define an essential proapoptotic role for p63 during naturally occurring neuronal death. Sympathetic neurons express full-length TAp63 during the developmental death period, and TAp63 levels increase following NGF withdrawal. Overexpression of TAp63 causes neuronal apoptosis in the presence of NGF, while cultured p63-/- neurons are resistant to apoptosis following NGF withdrawal. TAp63 is also essential in vivo, since embryonic p63-/- mice display a deficit in naturally occurring sympathetic neuron death. While both TAp63 and p53 induce similar apoptotic signaling proteins and require BAX expression and function for their effects, TAp63 induces neuronal death in the absence of p53, but p53 requires coincident p63 expression for its proapoptotic actions. Thus, p63 is essential for developmental neuronal death, likely functioning both on its own, and as an obligate proapoptotic partner for p53.

p63: oncogene or tumor suppressor?

p53, the original member of the family of genes now known to include p63 and p73, was first heralded as an oncogene because of its potent transformation capabilities and its robust expression in human tumors. However, it was later discovered that only mutant p53 was oncogenic, and that wild type p53 functioned as a tumor suppressor. Decades later, p63, the newest member of this gene family, is involved in a similar controversy: is p63 an oncogene or a tumor suppressor? Recent progress on understanding the in vivo role of p63 in cancer has focused primarily on investigating its involvement in the tumor-suppressive mechanism of apoptosis, by analyzing mouse models to assess its tumor-suppressive capabilities, and by assessing its expression in human cancers.

Differential expression of p63 isoforms in female reproductive organs

p63 is the identity switch for uterine/vaginal epithelial cell fate, and disruption of p63 expression by diethylstilbestrol (DES) induces cervical/vaginal adenosis in mice. In this article, we report the expression patterns of p63 isoforms (TA, DeltaN, alpha, beta and gamma) in mice, focusing on the reproductive tract. We also present the reproductive tract phenotype of female p63-/- mice. Finally, to better evaluate the potential role of p63 in human development of DES-induced cervical/vaginal adenosis, we describe the ontogeny of p63 in human female fetuses. In adult mice, the DeltaN isoforms of p63 were expressed only in squamous/basal/myoepithelial cells of epithelial tissues, while TA isoforms of p63 were highly expressed in germ cells of the ovary and testis. In fetal mice, the DeltaN and alpha forms of p63 were expressed in the cloacal and urogenital sinus epithelia. In the female p63-/- mice, the sinus vagina developed, but p63-/- sinus vaginal epithelium failed to undergo squamous differentiation confirming an essential role of p63 in squamous epithelial differentiation. Although TAp63 was highly expressed in developing primordial germ cells/oocytes, p63-/- ovaries and oocytes developed normally. The ontogeny of p63 in female reproductive organs was essentially identical in mouse and human. In the human fetus at the susceptible stage for DES-induced cervical/vaginal adenosis, most cervical/vaginal epithelial cells were columnar and negative for p63. Therefore, inhibition of p63 expression by DES should change the cell fate of human Müllerian duct epithelial cells and cause cervical/vaginal adenosis as previously demonstrated in mouse.

p63 deficiency activates a program of cellular senescence and leads to accelerated aging

The p53 tumor suppressor plays a key role in organismal aging. A cellular mechanism postulated to drive the aging process is cellular senescence, mediated in part by p53. Although senescent cells accumulate in elderly individuals, most studies have relied on correlating in vitro senescence assays with in vivo phenotypes of aging. Here, using two different mouse models in which the p53-related protein p63 is compromised, we demonstrate that cellular senescence and organismal aging are intimately linked and that these processes are mediated by p63 loss. We found that p63(+/-) mice have a shortened life span and display features of accelerated aging. Both germline and somatically induced p63 deficiency activates widespread cellular senescence with enhanced expression of senescent markers SA-beta-gal, PML, and p16(INK4a). Using an inducible tissue-specific p63 conditional model, we further show that p63 deficiency induces cellular senescence and causes accelerated aging phenotypes in the adult. Our results thus suggest a causative link between cellular senescence and aging in vivo, and demonstrate that p63 deficiency accelerates this process.

Perp Is a p63-Regulated Gene Essential for Epithelial Integrity

p63 is a master regulator of stratified epithelial development that is both necessary and sufficient for specifying this multifaceted program. We show here that Perp, a tetraspan membrane protein originally identified as an apoptosis-associated target of the p53 tumor suppressor, is the first direct target of p63 clearly involved in mediating this developmental program in vivo. During embryogenesis, Perp is expressed in an epithelial pattern, and its expression depends on p63. Perp-/- mice die postnatally, with dramatic blistering in stratified epithelia symptomatic of compromised adhesion. Perp localizes specifically to desmosomes, adhesion junctions important for tissue integrity, and numerous structural defects in desmosomes are observed in Perp-deficient skin, suggesting a role for Perp in promoting the stable assembly of desmosomal adhesive complexes. These findings demonstrate that Perp is a key effector in the p63 developmental program, playing an essential role in an adhesion subprogram central to epithelial integrity and homeostasis.