Immunohistochemical and Histopathological Characterization of Spina Bifida Defect Tissues Removed After Prenatal and Postnatal Surgical Repair

Background: Myelomeningocele or spina bifida is an open neural tube defect that is characterized by protrusion of the meninges and the spinal cord through a deformity in the vertebral arch and spinous process. Myelomeningocele of post-natal tissue is well described; however, pre-natal tissue of this defect has no known previous histologic characterization. We compared the histology of different forms of pre-natal myelomeningocele and post-natal myelomeningocele tissue obtained via prenatal intrauterine and postnatal surgical repairs. Methods: Pre-and post-natal tissues from spina bifida repair surgeries were obtained from lipomyelomeningocele, myeloschisis, and myelomeningocele spina bifida defects. Tissue samples were processed for H&E and immunohistochemical staining (KRT14 and p63) to assess epidermal and dermal development. Results: Prenatal skin near the defect site develops with normal epidermal, dermal, and adnexal structures. Within the grossly cystic specimens, histology shows highly dense fibrous connective tissue with complete absence of a normal epidermal development with a lack of p63 and KRT14 expression. Conclusion: Tissues harvested from prenatal and postnatal spina bifida repair surgeries appear as normal skin near the defect site. However, cystic tissues consist of highly dense fibrous connective tissue with complete absence of normal epidermal development.

Effects of TP63 mutations on keratinocyte adhesion and migration

The goal of this study was to investigate the molecular mechanisms responsible for the formation of skin erosions in patients affected by Ankyloblepharon-ectodermal defects-cleft lip/palate syndrome (AEC). This ectodermal dysplasia is caused by mutations in the TP63 gene, which encodes several transcription factors that control epidermal development and homeostasis. We generated induced pluripotent stem cells (iPSC) from AEC patients and corrected the TP63 mutations using genome editing tools. Three pairs of the resulting conisogenic iPSC lines were differentiated into keratinocytes (iPSC-K). We identified a significant downregulation of key components of hemidesmosomes and focal adhesions in AEC iPSC-K compared to their gene-corrected counterparts. Further, we demonstrated reduced AEC iPSC-K migration, suggesting the possibility that a process critical for cutaneous wound healing might be impaired in AEC patients. Next, we generated chimeric mice expressing a TP63-AEC transgene and confirmed a downregulation of these genes in transgene-expressing cells in vivo. Finally, we also observed these abnormalities in AEC patient skin. Our findings suggest that integrin defects in AEC patients might weaken the adhesion of keratinocytes to the basement membrane. We propose that reduced expression of extracellular matrix adhesion receptors, potentially in conjunction with previously identified desmosomal protein defects, contribute to skin erosions in AEC.

Effects of TP63 Mutations on Keratinocyte Adhesion and Migration

The goal of this study was to investigate the molecular mechanisms responsible for the formation of skin erosions in patients affected by Ankyloblepharon-ectodermal defects-cleft lip/palate syndrome (AEC). This ectodermal dysplasia is caused by mutations in the TP63 gene, which encodes several transcription factors that control epidermal development and homeostasis. We generated induced pluripotent stem cells (iPSC) from AEC patients and corrected the TP63 mutations using genome editing tools. Three pairs of the resulting conisogenic iPSC lines were differentiated into keratinocytes (iPSC-K). We identified a significant downregulation of key components of hemidesmosomes and focal adhesions in AEC iPSC-K compared to their gene-corrected counterparts. Further, we demonstrated reduced iPSC-K migration, suggesting the possibility that a process critical for cutaneous wound healing might be impaired in AEC patients. Next, we generated chimeric mice expressing a TP63-AEC transgene and confirmed a downregulation of these genes in transgene-expressing cells in vivo. Finally, we also observed these abnormalities in AEC patient skin. Our findings suggest that integrin defects in AEC patients might weaken the adhesion of keratinocytes to the basement membrane. We propose that reduced expression of extracellular matrix adhesion receptors, potentially in conjunction with previously identified desmosomal protein defects, contribute to skin erosions in AEC.

Rare diseases of ectoderm: Translating discovery to therapy

Heritable conditions known as ectodermal dysplasias are rare and can be associated with marked morbidity, mortality, and a reduced quality of life. The diagnosis and care of individuals affected by one of the many ectodermal dysplasias presents myriad challenges due to their rarity and the diverse phenotypes. These conditions are caused by abnormalities in multiple genes and signaling pathways that are essential for the development and function of ectodermal derivatives. During a 2021 international conference focused on translating discovery to therapy, researchers and clinicians gathered with the goal of advancing the diagnosis and treatment of conditions affecting ectodermal tissues with an emphasis on skin, hair, tooth, and eye phenotypes. Conference participants presented a variety of promising treatment strategies including gene or protein replacement, gene editing, cell therapy, and the identification of druggable targets. Further, barriers that negatively influence the current development of novel therapeutics were identified. These barriers include a lack of accurate prevalence data for rare conditions, absence of an inclusive patient registry with deep phenotyping data, and insufficient animal models and cell lines. Overcoming these barriers will need to be prioritized in order to facilitate the development of novel treatments for genetic disorders of the ectoderm.

Differentiation of Human Induced Pluripotent Stem Cells into Keratinocytes

Investigating basic biological mechanisms underlying human diseases relies on the availability of sufficient quantities of patient cells. As most primary somatic cells have a limited lifespan, obtaining sufficient material for biological studies has been a challenge. The development of induced pluripotent stem cell (iPSC) technology has been a game changer, especially in the field of rare genetic disorders. iPSC are essentially immortal, can be stored indefinitely, and can thus be used to generate defined somatic cells in unlimited quantities. Further, the availability of genome editing technologies, such as CRISPR/CAS, has provided us with the opportunity to create “designer” iPSC lines with defined genetic characteristics. A major advancement in biological research stems from the development of methods to direct iPSC differentiation into defined cell types. In this article, we provide the basic protocol for the generation of human iPSC‐derived keratinocytes (iPSC‐K). These cells have the characteristics of basal epidermal keratinocytes and represent a tool for the investigation of normal epidermal biology, as well as genetic and acquired skin disorders. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC.

Basic Protocol: Directed differentiation of human iPSC into keratinocytes

Support Protocol 1: Coating cell culture dishes or plates with Vitronectin XF^™

Support Protocol 2: Freezing iPSC

Support Protocol 3: Preparing AggreWell^™400 6‐well plates for EB formation

Support Protocol 4: Coating cell culture dishes or plates with Collagen IV

Support Protocol 5: Immunofluorescence staining of cells

Rare Genetic Disorders: Novel Treatment Strategies and Insights Into Human Biology

The last decade has seen a dramatic increase in innovative ideas for the treatment of genetic disorders for which no curative therapies exist. Gene and protein replacement therapies stand out as novel approaches to treat a select group of these diseases, such as certain tissue fragility disorders. Further, the advent of stem cell approaches, such as induced pluripotent stem cells (iPSC) technology, has led to the development of new methods of creating replacement tissues for regenerative medicine. This coincided with the discovery of genome editing techniques, which allow for the correction of disease-causing mutations. The culmination of these discoveries suggests that new and innovative therapies for monogenetic disorders affecting single organs or tissues are on the horizon. Challenges remain, however, especially with diseases that simultaneously affect several tissues and organs during development. Examples of this group of diseases include ectodermal dysplasias, genetic disorders affecting the development of tissues and organs such as the skin, cornea, and epithelial appendages. Gene or protein replacement strategies are unlikely to be successful in addressing the multiorgan phenotype of these diseases. Instead, we believe that a more effective approach will be to focus on correcting phenotypes in the most severely affected tissues. This could include the generation of replacement tissues or the identification of pharmaceutical compounds that correct disease pathways in specific tissues.

Solar Freckles: Long-Term Photochromic Tattoos for Intradermal Ultraviolet Radiometry

While tattooable nanotechnology for in-skin sensing and communication has been a popular concept in science fiction since the 1990s, the first tattooable intradermal nanosensors have only emerged in the past few years, and none have been demonstrated in human skin. We developed a photochromic tattoo that serves as an intradermal ultraviolet (UV) radiometer that provides naked-eye feedback about UV exposure in real time. These small tattoos, or "solar freckles", comprise dermally implanted colorimetric UV sensors in the form of nanoencapsulated leuco dyes that become more blue in color with increasing UV irradiance. We demonstrate the tattoos' functionality for both quantitative and naked-eye UV sensing in porcine skin ex vivo, as well as in human skin in vivo. Solar freckles offer an alternative and complementary approach to self-monitoring UV exposure for the sake of skin cancer prevention. Activated solar freckles provide a visual reminder to protect the skin, and their color disappears rapidly upon removal of UV exposure or application of topical sunscreen. The sensors are implanted in a minimally invasive procedure that lasts only a few seconds, yet remain functional for months to years. These semipermanent tattoos provide an early proof-of-concept for long-term intradermal sensing nanomaterials that provide users with biomedically relevant information in the form of an observable color change.

Melanocyte Precursors in the Hair Follicle Bulge of Repigmented Vitiligo Skin Are Controlled by RHO-GTPase, KCTD10, and CTNNB1 Signaling

In repigmentation of human vitiligo, the melanocyte (MC) precursors in the hair follicle bulge proliferate, migrate, and differentiate to repopulate the depigmented epidermis. Here, we present a comprehensive characterization of pathways and signals in the bulge that control the repigmentation process. Using biopsies from patients with vitiligo, we have selectively harvested, by laser capture microdissection, MC and keratinocyte precursors from the hair follicle bulge of untreated vitiligo skin and vitiligo skin treated with narrow-band UVB. The captured material was subjected to whole transcriptome RNA-sequencing. With this strategy, we found that repigmentation in the bulge MC precursors is driven by KCTD10, a signal with unknown roles in the skin, and CTNNB1 (encoding β-catenin) and RHO guanosine triphosphatase [RHO GTPase, RHO], two signaling pathways previously shown to be involved in pigmentation biology. Knockdown studies in cultured human MCs of RHOJ, the upmost differentially expressed RHO family component, corroborated with our findings in patients with vitiligo, identified RHOJ involvement in UV response and melanization, and confirmed previously identified roles in melanocytic cell migration and apoptosis. A better understanding of mechanisms that govern repigmentation in MC precursors will enable the discovery of molecules that induce robust repigmentation phenotypes in vitiligo.

Loss of TP63 Promotes the Metastasis of Head and Neck Squamous Cell Carcinoma by Activating MAPK and STAT3 Signaling

TP63 is frequently amplified or overexpressed in primary head and neck squamous cell carcinomas (HNSCC). Nevertheless, the role of TP63 in the initiation and progression of HNSCCs is not known. Using archival HNSCC tissue sections, we found that TP63 expression is often downregulated in late-stage human HNSCCs. To establish a causal link between TP63 loss and HNSCC tumorigenesis, we developed a genetically engineered mouse model in which Trp63 (the mouse homolog of human TP63) was ablated from head and neck epithelia. Upon exposure of the mice to a chemical carcinogen, we found that Trp63 ablation accelerated HNSCC initiation and progression. To determine whether these findings are relevant for human HNSCCs, we generated TP63 knockdown HNSCC cell lines. These cells were implanted into the tongue of athymic nude mice to generate orthotopic xenografts. We found that loss of TP63 promoted HNSCC progression and metastasis. Furthermore, we determined that tumor metastasis is dependent on MAPK activation in TP63 knockdown HNSCCs. The significance of these findings is underscored by our finding that pharmacologic inhibition of MAPK activity by trametinib drastically impaired HNSCC metastasis mediated by TP63 loss. In conclusion, our data provide novel mechanistic insights into the role of TP63 loss in HNSCC initiation and progression, and provide a rationale for the development of new therapeutic approaches specifically targeting TP63-dependent tumor pathways. IMPLICATIONS: Our findings uncover a novel functional role for TP63 loss in HNSCC metastasis and identify MAPK signaling as a potential therapeutic target for treating HNSCCs with low TP63 expression.

Ectodermal dysplasias: Classification and organization by phenotype, genotype and molecular pathway

An international advisory group met at the National Institutes of Health in Bethesda, Maryland in 2017, to discuss a new classification system for the ectodermal dysplasias (EDs) that would integrate both clinical and molecular information. We propose the following, a working definition of the EDs building on previous classification systems and incorporating current approaches to diagnosis: EDs are genetic conditions affecting the development and/or homeostasis of two or more ectodermal derivatives, including hair, teeth, nails, and certain glands. Genetic variations in genes known to be associated with EDs that affect only one derivative of the ectoderm (attenuated phenotype) will be grouped as non-syndromic traits of the causative gene (e.g., non-syndromic hypodontia or missing teeth associated with pathogenic variants of EDA "ectodysplasin"). Information for categorization and cataloging includes the phenotypic features, Online Mendelian Inheritance in Man number, mode of inheritance, genetic alteration, major developmental pathways involved (e.g., EDA, WNT "wingless-type," TP63 "tumor protein p63") or the components of complex molecular structures (e.g., connexins, keratins, cadherins).

Isolating RNA from precursor and mature melanocytes from human vitiligo and normal skin using laser capture microdissection

To characterize the gene expression profile of regenerated melanocytes in the narrow band UVB (NBUVB)-treated vitiligo epidermis and their precursors in the hair follicle, we present here a strategy of RNA isolation from in situ melanocytes using human frozen skin. We developed a rapid immunostaining protocol using the NKI-beteb antibody, which labels differentiated and precursor melanocytes, followed by fluorescent laser capture microdissection. This technique enabled the direct isolation, from melanocyte and adjacent keratinocyte populations, of satisfactory quality RNA that was successfully amplified and analysed by qRT-PCR. The melanocyte-specific gene transcripts TYR, DCT, TYRP1 and PMEL were significantly upregulated in our NBUVB-treated melanocyte samples as compared with the keratinocyte samples, while keratinocyte-specific genes (KRT5 and KRT14) were expressed significantly higher in the keratinocyte samples as compared with the melanocyte samples. Furthermore, in both NBUVB-treated vitiligo skin and normal skin, when bulge melanocytes were compared with epidermal melanocytes, we found significantly lower expression of melanocyte-specific genes and significantly higher expression of three melanocytic stem cell genes (SOX9, WIF1 and SFRP1), while ALCAM and ALDH1A1 transcripts did not show significant variation. We found significantly higher expression of melanocyte-specific genes in the epidermis of NBUVB-treated vitiligo, as compared to the normal skin. When comparing bulge melanocyte samples from untreated vitiligo, NBUVB-treated vitiligo and normal skin, we did not find significant differences in the expression of melanocyte-specific genes or melanocytic stem cell genes. These techniques offer valuable opportunities to study melanocytes and their precursors in vitiligo and other pigmentation disorders.

Repigmentation of Human Vitiligo Skin by NBUVB Is Controlled by Transcription of GLI1 and Activation of the β-Catenin Pathway in the Hair Follicle Bulge Stem Cells

Vitiligo repigmentation is a complex process in which the melanocyte-depleted interfollicular epidermis is repopulated by melanocyte precursors from hair follicle bulge that proliferate, migrate, and differentiate into mature melanocytes on their way to the epidermis. The strongest stimulus for vitiligo repigmentation is narrow-band UVB (NBUVB), but how the hair follicle melanocyte precursors are activated by UV light has not been extensively studied. To better understand this process, we developed an application that combined laser capture microdissection and subsequent whole transcriptome RNA sequencing of hair follicle bulge melanocyte precursors and compared their gene signatures to that of regenerated mature epidermal melanocytes from NBUVB-treated vitiligo skin. Using this strategy, we found up-regulation of TNC, GJB6, and THBS1 in the hair follicle bulge melanocytes and of TYR in the epidermal melanocytes of the NBUVB-treated vitiligo skin. We validated these results by quantitative real-time-PCR using NBUVB-treated vitiligo skin and untreated normal skin. We also identified that GLI1, a candidate stem cell-associated gene, is significantly up-regulated in the melanocytes captured from NBUVB-treated vitiligo bulge compared with untreated vitiligo bulge. These signals are potential key players in the activation of bulge melanocyte precursors during vitiligo repigmentation.

Narrow Band Ultraviolet B Treatment for Human Vitiligo Is Associated with Proliferation, Migration, and Differentiation of Melanocyte Precursors

In vitiligo, the autoimmune destruction of epidermal melanocytes produces white spots that can be repigmented by melanocyte precursors from the hair follicles, following stimulation with UV light. We examined by immunofluorescence the distribution of melanocyte markers (C-KIT, DCT, PAX3, and TYR) coupled with markers of proliferation (KI-67) and migration (MCAM) in precursors and mature melanocytes from the hair follicle and the epidermis of untreated and narrow band UVB (NBUVB)-treated human vitiligo skin. NBUVB was associated with a significant increase in the number of melanocytes in the infundibulum and with restoration of the normal melanocyte population in the epidermis, which was lacking in the untreated vitiligo. We identified several precursor populations (melanocyte stem cells, melanoblasts, and other immature phenotypes), and progressively differentiating melanocytes, some with putative migratory and/or proliferative abilities. The primary melanocyte germ was present in the untreated and treated hair follicle bulge, whereas a possible secondary melanocyte germ composed of C-KIT+ melanocytes was found in the infundibulum and interfollicular epidermis of UV-treated vitiligo. This is an exceptional model for studying the mobilization of melanocyte stem cells in human skin. Improved understanding of this process is essential for designing better treatments for vitiligo, ultimately based on melanocyte stem cell activation and mobilization.

Conflicting Roles for p63 in Skin Development and Carcinogenesis

Epidermal morphogenesis is a complex process that culminates in the formation of a barrier that protects the organism from environmental substances and dehydration. p63, a transcription factor, is essential for normal epidermal morphogenesis as demonstrated by the failure of mice lacking p63 expression to develop an epidermis. However, since two independently generated p63(-/-) mouse models displayed different phenotypes, the role of p63 in epidermal morphogenesis has remained controversial. Furthermore, the tumor susceptibility phenotypes of both p63(-/-) mouse models were strikingly different. In this review, we discuss these conflicting findings and provide evidence for various roles of p63 in the epidermis under normal and pathological conditions.

Integrating animal models and in vitro tissue models to elucidate the role of desmosomal proteins in diseases

Desmosomes are intercellular junctions that provide tissues with structural stability. These junctions might also act as signaling centers that transmit environmental clues to the cell, thereby affecting cell differentiation, migration, and proliferation. The importance of desmosomes is underscored by devastating skin and heart diseases caused by mutations in desmosomal genes. Recent observations suggest that abnormal desmosomal protein expression might indirectly contribute to skin disorders previously not linked to these proteins. For example, it has been postulated that reduced desmosomal protein expression occurs in patients affected by Ankyloblepharon-ectodermal defects-cleft lip/palate syndrome (AEC), a skin fragility disorder caused by mutations in the transcription factor TP63. Currently, it is not clear how these changes in desmosomal gene expression contribute to AEC. We will discuss new approaches that combine in vitro and in vivo models to elucidate the role of desmosomal gene deregulation in human skin diseases such as AEC.

Modeling AEC-New approaches to study rare genetic disorders

Ankyloblepharon-ectodermal defects-cleft lip/palate (AEC) syndrome is a rare monogenetic disorder that is characterized by severe abnormalities in ectoderm-derived tissues, such as skin and its appendages. A major cause of morbidity among affected infants is severe and chronic skin erosions. Currently, supportive care is the only available treatment option for AEC patients. Mutations in TP63, a gene that encodes key regulators of epidermal development, are the genetic cause of AEC. However, it is currently not clear how mutations in TP63 lead to the various defects seen in the patients' skin. In this review, we will discuss current knowledge of the AEC disease mechanism obtained by studying patient tissue and genetically engineered mouse models designed to mimic aspects of the disorder. We will then focus on new approaches to model AEC, including the use of patient cells and stem cell technology to replicate the disease in a human tissue culture model. The latter approach will advance our understanding of the disease and will allow for the development of new in vitro systems to identify drugs for the treatment of skin erosions in AEC patients. Further, the use of stem cell technology, in particular induced pluripotent stem cells (iPSC), will enable researchers to develop new therapeutic approaches to treat the disease using the patient's own cells (autologous keratinocyte transplantation) after correction of the disease-causing mutations.

Building models for keratin disorders

Palmoplantar keratoderma is a hallmark of pachyonychia congenita (PC) and focal non-epidermolytic palmoplantar keratoderma (FNEPPK). By generating keratin 16 (Krt16)-deficient mice, Lessard and Coulombe, as described in this issue, have generated a mouse model to replicate these palmoplantar lesions. Studies using this model may provide novel insights into the molecular mechanisms responsible for the formation of palmoplantar lesions in PC and FNEPPK patients.

Mutant p63 causes defective expansion of ectodermal progenitor cells and impaired FGF signalling in AEC syndrome

Ankyloblepharon-ectodermal defects-cleft lip/palate (AEC) syndrome, which is characterized by cleft palate and severe defects of the skin, is an autosomal dominant disorder caused by mutations in the gene encoding transcription factor p63. Here, we report the generation of a knock-in mouse model for AEC syndrome (p63(+/L514F) ) that recapitulates the human disorder. The AEC mutation exerts a selective dominant-negative function on wild-type p63 by affecting progenitor cell expansion during ectodermal development leading to a defective epidermal stem cell compartment. These phenotypes are associated with impairment of fibroblast growth factor (FGF) signalling resulting from reduced expression of Fgfr2 and Fgfr3, direct p63 target genes. In parallel, a defective stem cell compartment is observed in humans affected by AEC syndrome and in Fgfr2b(-/-) mice. Restoring Fgfr2b expression in p63(+/L514F) epithelial cells by treatment with FGF7 reactivates downstream mitogen-activated protein kinase signalling and cell proliferation. These findings establish a functional link between FGF signalling and p63 in the expansion of epithelial progenitor cells and provide mechanistic insights into the pathogenesis of AEC syndrome.

Loss of Desmocollin 3 in skin tumor development and progression

Desmocollin 3 (DSC3) is a desmosomal cadherin that is required for maintaining cell adhesion in the epidermis as demonstrated by the intra-epidermal blistering observed in Dsc3 null skin. Recently, it has been suggested that deregulated expression of DSC3 occurs in certain human tumor types. It is not clear whether DSC3 plays a role in the development or progression of cancers arising in stratified epithelia such as the epidermis. To address this issue, we generated a mouse model in which Dsc3 expression is ablated in K-Ras oncogene-induced skin tumors. Our results demonstrate that loss of Dsc3 leads to an increase in K-Ras-induced skin tumors. We hypothesize that acantholysis-induced epidermal hyperplasia in the Dsc3 null epidermis facilitates Ras-induced tumor development. Further, we demonstrate that spontaneous loss of DSC3 expression is a common occurrence during human and mouse skin tumor progression. This loss occurs in tumor cells invading the dermis. Interestingly, other desmosomal proteins are still expressed in tumor cells that lack DSC3, suggesting a specific function of DSC3 loss in tumor progression. While loss of DSC3 on the skin surface leads to epidermal blistering, it does not appear to induce loss of cell-cell adhesion in tumor cells invading the dermis, most likely due to a protection of these cells within the dermis from mechanical stress. We thus hypothesize that DSC3 can contribute to the progression of tumors both by cell adhesion-dependent (skin surface) and likely by cell adhesion-independent (invading tumor cells) mechanisms.

The effect of c-myc on stem cell fate influences skin tumor phenotype

Nonmelanoma skin cancers (NMSCs) consist of a variety of tumor types including basal cell carcinoma, squamous cell carcinoma, a variety of hair follicle tumors, and sebaceous gland tumors. Genetic alterations that alter the fate of multipotent stem cells are believed to influence NMSC phenotype. We previously generated a transgenic mouse line which constitutively expressed c-myc under the control of the K14 promoter (K14.MYC2). These mice exhibited an increase in size and number of sebaceous glands, suggesting that c-myc diverted multipotential stem cells to a sebaceous lineage. Our goal in the current study was to determine if alterations in the commitment of multipotent stem cells to different cell fates would influence tumor phenotype. To this end, we exposed K14.MYC2 mice to a chemical carcinogenesis protocol and discovered that these mice were predisposed to develop sebaceous adenomas. Our data demonstrate that genetic alterations that alter the fate of multipotent stem cells during embryonic development can markedly influence the phenotype of NMSC that develop following exposure to carcinogens.

p63 in skin development and ectodermal dysplasias

The transcription factor p63 is critically important for skin development and maintenance. Processes that require p63 include epidermal lineage commitment, epidermal differentiation, cell adhesion, and basement membrane formation. Not surprisingly, alterations in the p63 pathway underlie a subset of ectodermal dysplasias, developmental syndromes in which the skin and skin appendages do not develop normally. This review summarizes the current understanding of the role of p63 in normal development and ectodermal dysplasias.

p63 directly induces expression of Alox12, a regulator of epidermal barrier formation

Epidermal development and differentiation are tightly controlled processes that culminate in the formation of the epidermal barrier. A critical regulator of different stages of epidermal development and differentiation is the transcription factor p63. More specifically, we previously demonstrated elsewhere that p63 is required for both the commitment to stratification and the commitment to terminal differentiation. We now demonstrate that DeltaNp63alpha, the predominantly expressed p63 isoform in postnatal epidermis, also plays a role in the final stages of epidermal differentiation, namely the formation of the epidermal barrier. We found that DeltaNp63alpha contributes to epidermal barrier formation by directly inducing expression of ALOX12, a lipoxygenase which contributes to epidermal barrier function. Our data demonstrate that DeltaNp63alpha directly interacts with the promoter of Alox12 in the developing epidermis. Furthermore, we found that the induction of Alox12 expression by DeltaNp63alpha depends on intact p63 binding sites in the Alox12 promoter. Finally, we found that DeltaNp63alpha can induce Alox12 expression only in differentiating keratinocytes, consistent with the role of ALOX12 in epidermal barrier formation.

DeltaNp63 knockdown mice: A mouse model for AEC syndrome

Dominant mutations in TP63 cause ankyloblepharon ectodermal dysplasia and clefting (AEC), an ectodermal dysplasia characterized by skin fragility. Since DeltaNp63alpha is the predominantly expressed TP63 isoform in postnatal skin, we hypothesized that mutant DeltaNp63alpha proteins are primarily responsible for skin fragility in AEC patients. We found that mutant DeltaNp63alpha proteins expressed in AEC patients function as dominant-negative molecules, suggesting that the human AEC skin phenotype could be mimicked in mouse skin by downregulating DeltaNp63alpha. Indeed, downregulating DeltaNp63 expression in mouse epidermis caused severe skin erosions, which resembled lesions that develop in AEC patients. In both cases, lesions were characterized by suprabasal epidermal proliferation, delayed terminal differentiation, and basement membrane abnormalities. By failing to provide structural stability to the epidermis, these defects likely contribute to the observed skin fragility. The development of a mouse model for AEC will allow us to further unravel the genetic pathways that are normally regulated by DeltaNp63 and that may be perturbed in AEC patients. Ultimately, these studies will not only contribute to our understanding of the molecular mechanisms that cause skin fragility in AEC patients, but may also result in the identification of targets for novel therapeutic approaches aimed at treating skin erosions. (c) 2009 Wiley-Liss, Inc.

Differential PERP regulation by TP63 mutants provides insight into AEC pathogenesis

Ankyloblepharon Ectodermal Dysplasia and Cleft Lip/Palate (AEC) or Hay-Wells Syndrome is an autosomal dominant disorder characterized by a variety of phenotypes in ectodermal derivatives, including severe skin erosions, ankyloblepharon, coarse and wiry hair, scalp dermatitis, and dystrophic nails. AEC is caused by mutations in the gene encoding the TP63 transcription factor, specifically in the Sterile Alpha Motif (SAM) domain. The exact mechanism, however, by which these specific TP63 mutations lead to the observed spectrum of phenotypes is unclear. Analysis of individual TP63 target genes provides a means to understand specific aspects of the phenotypes associated with AEC. PERP is a TP63 target critical for cell-cell adhesion due to its participation in desmosomal adhesion complexes. As PERP null mice display symptoms characteristic of ectodermal dysplasia syndromes, we hypothesized that PERP dysfunction might contribute to AEC. Using luciferase reporter assays, we demonstrate here that PERP induction is in fact compromised with some, but not all, AEC-patient derived TP63 mutants. Through analysis of skin biopsies from AEC patients, we show further that a subset of these display aberrant PERP expression, suggesting the possibility that PERP dysregulation is involved in the pathogenesis of this disease. These findings demonstrate that distinct AEC TP63 mutants can differentially compromise expression of downstream targets, providing a rationale for the variable spectra of symptoms seen in AEC patients. Elucidating how specific TP63 target genes contribute to the pathogenesis of AEC will ultimately help design novel approaches to diagnose and treat AEC.

Making an epidermis

The skin functions as a barrier protecting the body from dehydration and environmental insults. This barrier function is mainly provided by the outermost layer of the skin, the epidermis. The epidermis is maintained by epidermal stem cells which reside in the basal layer and which generate daughter cells that move upward toward the surface of the skin. During this journey, keratinocytes undergo a series of biochemical and morphological changes that result in the formation of the various layers of the epidermis. Eventually, these cells turn into the outermost layer of dead cornified cells that are sloughed into the environment. This review summarizes our current understanding of the mechanisms that control proliferation and differentiation of epidermal stem cells, and thus addresses fundamental processes that control epidermal morphogenesis and function.

International Research Symposium on Ankyloblepharon-Ectodermal Defects-Cleft Lip/Palate (AEC) syndrome

Ankyloblepharon-ectodermal defects-cleft lip/palate (AEC) syndrome (Hay-Wells syndrome, MIM #106220) is a rare autosomal dominant ectodermal dysplasia syndrome. It is due to mutations in the TP63 gene, known to be a regulatory gene with many downstream gene targets. TP63 is important in the differentiation and proliferation of the epidermis, as well as many other processes including limb and facial development. It is also known that mutations in TP63 lead to skin erosions. These erosions, especially on the scalp, are defining features of AEC syndrome and cause significant morbidity and mortality in these patients. It was this fact that led to the 2003 AEC Skin Erosion Workshop. That conference laid the groundwork for the International Research Symposium for AEC Syndrome held at Texas Children's Hospital in 2006. The conference brought together the largest cohort of individuals with AEC syndrome, along with a multitude of physicians and scientists. The overarching goals were to define the clinical and pathologic findings for improved diagnostic criteria, to obtain tissue samples for further study and to define future research directions. The symposium was successful in accomplishing these aims as detailed in this conference report. Following our report, we also present 11 manuscripts within this special section that outline the collective clinical, pathologic, and mutational data from 18 individuals enrolled in the concurrent Baylor College of Medicine IRB-approved protocol: Characterization of AEC syndrome. These collaborative findings will hopefully provide a stepping-stone to future translational projects of TP63 and TP63-related syndromes.

IKKalpha is a p63 transcriptional target involved in the pathogenesis of ectodermal dysplasias

The transcription factor p63 plays a pivotal role in the development and differentiation of the epidermis and epithelial appendages. Indeed, mutations in p63 are associated with a group of ectodermal dysplasias characterized by skin, limb, and craniofacial defects. It was hypothesized that p63 exerts its functions by activating specific genes during epidermal development, which in turn regulate epidermal stratification and differentiation. We have identified I-kappaB kinase alpha (IKKalpha) as a direct transcriptional target of p63 that is induced at early phases of terminal differentiation of primary keratinocytes. We show that the DeltaNp63 isoform is required for IKKalpha expression in differentiating keratinocytes and that mutant p63 proteins expressed in ectodermal dysplasia patients exhibit defects in inducing IKKalpha. Furthermore, we observed reduced IKKalpha expression in the epidermis of an ankyloblepharon ectodermal dysplasia clefting patient. Our data demonstrate that a failure to properly express IKKalpha may play a role in the development of ectodermal dysplasias.

Sorting out the p63 signaling network

p63 is a transcription factor required for normal epidermal development and differentiation. Because of the complexity of these processes, p63 is expected to regulate a myriad of target genes, providing impetus to many laboratories to identify these genes. p63 target genes have been shown to encode a diverse group of proteins, including structural proteins, proteins that control cell cycle withdrawal, and proteins that regulate the epidermal differentiation program. In this issue, Antonini et al., describe a novel p63 target gene whose evolutionary conservation suggests a critical role for this gene in the epidermis.

Disruption of epidermal specific gene expression and delayed skin development in AP-2 gamma mutant mice

Summary Sentence: Conditional ablation of AP-2 gamma results in a delay in skin development and abnormal expression of p63, K14, K1, filaggrin, repetin and secreted Ly6/Plaur domain containing 1, key genes required for epidermal development and differentiation. The development of the epidermis, a stratified squamous epithelium, is dependent on the regulated differentiation of keratinocytes. Differentiation begins with the initiation of stratification, a process tightly controlled through proper gene expression. AP-2 gamma is expressed in skin and previous research suggested a pathway where p63 gene induction results in increased expression of AP-2 gamma, which in turn is responsible for induction of K14. This study uses a conditional gene ablation model to further explore the role of AP-2 gamma in skin development. Mice deficient for AP-2 gamma exhibited delayed expression of p63, K14, and K1, key genes required for development and differentiation of the epidermis. In addition, microarray analysis of E16.5 skin revealed delayed expression of additional late epidermal differentiation genes: filaggrin, repetin and secreted Ly6/Plaur domain containing 1, in mutant mice. The genetic delay in skin development was further confirmed by a functional delay in the formation of an epidermal barrier. These results document an important role for AP-2 gamma in skin development, and reveal the existence of regulatory factors that can compensate for AP-2 gamma in its absence.

Double-inducible gene activation system for caspase 3 and 9 in epidermis

Expression of genes with tight and precise temporal and spatial control is desired in a wide variety of applications ranging from cultured cells and transgenic animals to gene therapy. While current inducible systems, such as RU486 and chemical inducers of dimerization (CID), have improved earlier inducible models (Gossen et al., 1995, Science. 268:1766-1769; Wang et al., 1994, Proc Natl Acad Sci USA 91:8180-8184), no single system is perfect at present. One potential drawback of these systems is leakage of transgene expression, causing limitations of each system. We have developed an inducible model containing both RU486 and CID systems, which in addition to inducing caspase activation, has potential applicability specifically to other genes encoding proteins that require a dimerization event for activation. This Double-Inducible Gene Activation System generates two barriers for the target gene expression and protein activation thereby minimizing leakage.

Mechanisms regulating epithelial stratification

The epidermis is a stratified epithelium that functions as a barrier protecting the organism from dehydration, mechanical trauma, and microbial insults. This barrier function is established during embryogenesis through a complex and tightly controlled stratification program. Whereas the morphological changes that occur during epidermal development have been extensively studied, the molecular mechanisms that govern this process remain poorly understood. In this review we summarize the current advances that have been made in understanding the molecular mechanisms that regulate epidermal morphogenesis.

p63 induces key target genes required for epidermal morphogenesis

Mice lacking p63, a single gene that encodes a group of transcription factors that either contain (TA) or lack (DeltaN) a transactivation domain, fail to develop stratified epithelia as well as epithelial appendages and limbs. DeltaNp63 isoforms are predominantly expressed during late embryonic and postnatal epidermal development, however, the function of these proteins remains elusive. Using an epidermal-specific inducible knockdown mouse model, we demonstrate that DeltaNp63 proteins are essential for maintaining basement membrane integrity and terminal differentiation of keratinocytes. Furthermore, we have identified two DeltaNp63alpha target genes that mediate these processes. We propose that DeltaNp63alpha initially induces expression of the extracellular matrix component Fras1, which is required for maintaining the integrity of the epidermal-dermal interface at the basement membrane. Subsequently, induction of IkappaB kinase-alpha by DeltaNp63alpha initiates epidermal terminal differentiation resulting in the formation of the spinous layer. Our data provide insights into the role of DeltaNp63alpha in epidermal morphogenesis and homeostasis, and may contribute to our understanding of the pathogenic mechanisms underlying disorders caused by p63 mutations.

Cross-regulation between Notch and p63 in keratinocyte commitment to differentiation

Notch signaling promotes commitment of keratinocytes to differentiation and suppresses tumorigenesis. p63, a p53 family member, has been implicated in establishment of the keratinocyte cell fate and/or maintenance of epithelial self-renewal. Here we show that p63 expression is suppressed by Notch1 activation in both mouse and human keratinocytes through a mechanism independent of cell cycle withdrawal and requiring down-modulation of selected interferon-responsive genes, including IRF7 and/or IRF3. In turn, elevated p63 expression counteracts the ability of Notch1 to restrict growth and promote differentiation. p63 functions as a selective modulator of Notch1-dependent transcription and function, with the Hes-1 gene as one of its direct negative targets. Thus, a complex cross-talk between Notch and p63 is involved in the balance between keratinocyte self-renewal and differentiation.

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.

Reactivation of Developmentally Expressed p63 Isoforms Predisposes to Tumor Development and Progression

Genes that are active during normal development are frequently reactivated during neoplastic transformation. We now report that developmentally expressed TAp63 isoforms are frequently reactivated in human squamous cell carcinomas. To determine the consequences of TAp63 reactivation, we induced TAp63alpha expression during chemically-induced skin carcinogenesis. Deregulated TAp63alpha expression dramatically accelerated tumor development and progression, frequently resulting in epithelial-mesenchymal transitions to spindle cell carcinomas and lung metastases. Consistent with this observation, we detected high levels of Twist and N-cadherin in tumors overexpressing TAp63alpha. Thus, as observed for other developmental pathways, aberrant reactivation of TAp63 predisposes to tumor development and progression.

Deregulated minichromosomal maintenance protein MCM7 contributes to oncogene driven tumorigenesis

Minichromosomal maintenance protein 7 (MCM7) is an essential component of the replication helicase complex (MCM2-7) required for DNA replication. Although this function is highly conserved among eukaryotes, additional functions for the MCM molecules continue to be described. Minichromosomal maintenance protein 7 is a marker for proliferation and is upregulated in a variety of tumors including neuroblastoma, prostate, cervical and hypopharyngeal carcinomas. To further investigate the general role of MCM7 in tumorigenesis, we generated a mouse model with deregulated MCM7 expression targeted to the basal layer of the epidermis using the keratin 14 (K14) promoter (K14.MCM7). When subjected to a two-stage chemical carcinogenesis protocol (dimethylbenz[alpha]anthracene (DMBA) initiation with 12-ortho-tetradecanoylphorbol-13-acetate promotion), K14.MCM7 mice showed significantly increased incidence and prevalence of tumor development relative to controls. Furthermore, within 40 weeks of treatment over 45% K14.MCM7 mice exhibited tumors that had converted to squamous cell carcinomas versus none in the control group. As predicted from previous skin carcinogenesis studies using DMBA as the initiating agent, Ras mutations where found in more than 90% of tumors isolated from K14.MCM7 mice. Whereas previous studies have shown that MCM7 is useful as a proliferation marker, our data suggest that deregulated MCM7 expression actively contributes to tumor formation, progression and malignant conversion.

TAp63alpha induces AP-2gamma as an early event in epidermal morphogenesis

Epidermal morphogenesis begins with the commitment of the single-layered surface ectoderm to initiate a stratification program, a process that requires the expression of the transcription factor TAp63alpha. To determine the molecular mechanism by which TAp63alpha induces genes associated with the commitment to stratification, such as K14, we have used a combination of in vitro and in vivo approaches. Our initial gene expression profiling studies suggested that TAp63alpha could regulate one or more AP-2 genes, which have been implicated in development and maintenance of the epidermis. We now demonstrate that TAp63alpha directly induces AP-2gamma expression in embryonic epidermis, when commitment to stratification occurs. Furthermore, we show that, in the absence of AP-2gamma, TAp63alpha fails to induce K14 expression in vitro. Our data identify AP-2gamma as the first in vivo target gene of TAp63alpha, and provide novel insights into the molecular mechanisms associated with early events in epidermal morphogenesis.

P63 deficiency: a failure of lineage commitment or stem cell maintenance?

A critical role for p63 in the development of stratified epithelia, such as the epidermis, has been recognized since the generation of mice lacking p63 expression. The molecular role of p63 in epidermal morphogenesis, however, remained controversial. The epidermal phenotype of p63-/- mice, which are born with a single-layered surface epithelium instead of a fully stratified epidermis, suggested that p63 could have a role in stem cell maintenance or in the commitment to stratification. In this review, we discuss evidence suggesting that p63 is required for the commitment to stratification, making p63 the earliest known gene expressed in the developing epidermis that is specific for the keratinocyte lineage.

Asymmetric Cell Division in Skin Development: A New Look at an Old Observation

During skin development, the single-layered surface ectoderm covering the mouse embryo must initiate stratification and terminal differentiation to develop a functional epidermis. A recent article by Lechler and Fuchs in Nature (Lechler and Fuchs, 2005) suggests that these events are triggered by asymmetric cell division.

Genetic pathways required for epidermal morphogenesis

The epidermis is composed of keratinocytes which undergo a highly reproducible terminal differentiation program resulting in the formation of a protective barrier, which is established during embryogenesis. Significant progress has recently been made in understanding the genetic pathways associated with the earliest event characteristic of epidermal morphogenesis, commitment to stratification. This process depends on the expression of p63, a transcription factor which is transcribed into isoforms that contain (TA) or lack (AN) a transactivation domain. In the absence of p63 expression, epithelia remain single-layered, while ectopic TAp63alpha expression in single-layered epithelia initiates stratification. Later events during epidermal morphogenesis require withdrawal from the cell cycle and commitment to terminal differentiation. Some of the genetic pathways underlying these events are beginning to be elucidated, however, the exact molecular events remain to be determined. In this review, we summarize the involvement of several signaling pathways in different stages of epidermal morphogenesis.

p63 and epithelial appendage development

Abstract Epithelial appendages share a common developmental program that relies on extensive interactions between epithelia and adjacent mesenchyme. The transcription factor p63 has a critical role in epithelial appendage development in both vertebrates and non-vertebrates. Both mice and zebrafish lacking p63 expression fail to develop epithelial appendages and other structures that develop as a result of epithelial-mesenchymal interactions. Furthermore, dominantly inherited mutations in p63 are the cause of a subset of human ectodermal dysplasias, which are characterized by developmental abnormalities in epithelia and epithelial appendages. While the importance of p63 for epithelial appendage development is evident, the molecular mechanisms by which p63 functions are largely unknown. In this review, we will discuss the current knowledge of the developmental role of p63 and the implications for epithelial appendage development.

The role of p63 in development and differentiation of the epidermis

Expression of p63, a transcription factor that is transcribed into six isoforms, is required for proper development of stratified epithelia, such as the epidermis. In the absence of p63, epithelia remain single-layered. The molecular role of p63 in development and differentiation of stratified epithelia, however, remains controversial. Based on recent studies, we now believe that p63 has a dual role and is essential for development as well as maintenance of the epidermis. During embryogenesis, p63 may be the molecular switch required for initiation of epithelial stratification. This is based on our recent data demonstrating that ectopic expression of a p63 isoform in single-layered epithelia results in the induction of a stratification program. Furthermore, in the mature epidermis, p63 may maintain the proliferative potential of basal keratinocytes. This is suggested by the observation that p63 is primarily expressed in the basal compartment of the epidermis, that p63 expression induces hyperproliferation, and that its expression needs to be downregulated for terminal differentiation to take place. In this review, we discuss recent evidence supporting this dual role for p63 and place it in the context of our increasing knowledge of epidermal development and differentiation.

Genes involved in stem cell fate decisions and commitment to differentiation play a role in skin disease

Multipotent stem cells residing in the bulge region of the hair follicle give rise to cells of different fates including those forming hair follicles, interfollicular epidermis, and associated glands. Stem cell fate determination is regulated by genes involved in both proliferation and differentiation, which are tightly regulated processes. Understanding the molecular mechanisms by which proliferation and differentiation are regulated will provide useful insight into treating human diseases caused by the deregulation of these processes. Two genes involved in regulating proliferation and differentiation are c-Myc and p63, both of which have been found to be deregulated/mutated in several human diseases. Accelerating proliferation leads to neoplastic human diseases and deregulated c-Myc has been implicated in a variety of cancers. Evidence indicates that c-Myc also diverts stem cells to an epidermal and sebaceous gland fate at the expense of the hair follicle fate. Therefore, deregulation of c-Myc has the potential to not only accelerate tumorigenesis, but also influence skin tumor phenotype. In addition, the inhibition of differentiation may also predispose to the development of skin cancer. Recent evidence suggests that the transcription factor p63, is not only responsible for the initiation of an epithelial stratification program during development, but also the maintenance of the proliferative potential of basal keratinocytes in mature epidermis. Mutations in the p63 gene have been shown to cause ectodermal dysplasias and deregulated expression of p63 has been observed in squamous cell carcinomas. In this review, we will discuss recent data implicating a role for both c-Myc and p63 in human skin diseases.

Transgenic mouse models provide new insights into the role of p63 in epidermal development

The epidermis is a stratified epithelium which provides a barrier between the organism and the environment protecting it from dehydration and pathogenic insult. A gene essential for development of the epidermis and other stratified epithelia is the transcription factor p63. The p63 gene is transcribed into isoforms that contain (TA) or lack (DeltaN) a transactivation domain. Of these isoforms, only TAp63 isoforms are expressed in the uncommitted surface ectoderm, while DeltaNp63 isoforms are expressed after the surface ectoderm has committed to a stratification program. Consistent with these embryonic expression profiles, we found that TAp63alpha functions as the master switch for initiation of epithelial stratification. Furthermore, TAp63alpha induces proliferation and inhibits terminal differentiation. This inhibition is overcome by the subsequent expression of DeltaNp63alpha which, in this context, acts as a dominant-negative molecule and allows basal keratinocytes to withdraw from the cell cycle and commit to terminal differentiation. These data demonstrate that TA- and DeltaNp63 isoforms have fundamentally different roles during epidermal development and provide new insight into the molecular events required for normal epidermal morphogenesis.

p63 is the molecular switch for initiation of an epithelial stratification program

Development of stratified epithelia, such as the epidermis, requires p63 expression. The p63 gene encodes isoforms that contain (TA) or lack (DeltaN) a transactivation domain. We demonstrate that TAp63 isoforms are the first to be expressed during embryogenesis and are required for initiation of epithelial stratification. In addition, TAp63 isoforms inhibit terminal differentiation, suggesting that TAp63 isoforms must be counterbalanced by DeltaNp63 isoforms to allow cells to respond to signals required for maturation of embryonic epidermis. Our data demonstrate that p63 plays a dual role: initiating epithelial stratification during development and maintaining proliferative potential of basal keratinocytes in mature epidermis.

Roles of TGFbeta signaling in epidermal/appendage development

The transforming growth factor beta (TGFbeta) superfamily encompasses a number of structurally related proteins that can be divided into several subfamilies including TGFbetas, activins/inhibins and bone morphogenetic proteins (BMPs). The Smads are major intracellular mediators in transducing the signals of TGFbeta superfamily members, and are abundantly expressed in the developing epidermis and epidermal appendages. Moreover, the phenotypes of transgenic/knockout mice with altered components of the TGFbeta superfamily signaling pathway suggest that TGFbeta superfamily signaling is required for epidermal/appendage development. TGFbeta superfamily members are involved in most events during epidermal/appendage development through the TGFbeta signal transduction pathway and through cross talk with other signaling pathways. Future studies will be instrumental in defining the precise roles for TGFbeta superfamily signaling in epidermal/appendage development.