Genotype-phenotype correlation in skin fragility-ectodermal dysplasia syndrome resulting from mutations in plakophilin 1

Plakophilin 1 in carcinogenesis

Plakophilin 1 (PKP1) belongs to the desmosome family as an anchoring junction protein in cellular junctions. It localizes at the interface of the cell membrane and cytoplasm. Although PKP1 is a non-transmembrane protein, it may become associated with the cell membrane via transmembrane proteins such as desmocollins and desmogleins. Homozygous deletion of PKP1 results in ectodermal dysplasia-skin fragility syndrome (EDSF) and complete knockout of PKP1 in mice produces comparable symptoms to EDSF in humans, although mice do not survive more than 24 h. PKP1 is not limited to expression in desmosomal structures, but is rather widely expressed in cytoplasm and nucleus, where it assumes important cellular functions. This review will summarize distinct roles of PKP1 in the cell membrane, cytoplasm, and nucleus with an overview of relevant studies on its function in diverse types of cancer.

Mutations in genes encoding desmosomal proteins: spectrum of cutaneous and extracutaneous abnormalities

The desmosome is a type of intercellular junction found in epithelial cells, cardiomyocytes and other specialized cell types. Composed of a network of transmembranous cadherins and intracellular armadillo, plakin and other proteins, desmosomes contribute to cell-cell adhesion, signalling, development and differentiation. Mutations in genes encoding desmosomal proteins result in a spectrum of erosive skin and mucosal phenotypes that also may affect hair or heart. This review summarizes the molecular pathology and phenotypes associated with desmosomal dysfunction with a focus on inherited disorders that involve the skin/hair, as well as associated extracutaneous pathologies. We reviewed the relevant literature to collate studies of pathogenic human mutations in desmosomes that have been reported over the last 25 years. Mutations in 12 different desmosome genes have been documented, with mutations in nine genes affecting the skin/mucous membranes (DSG1, DSG3, DSC2, DSC3, JUP, PKP1, DSP, CDSN, PERP) and eight resulting in hair abnormalities (DSG4, DSC2, DSC3, JUP, PKP1, DSP, CDSN, PERP). Mutations in three genes can result in cardiocutaneous syndromes (DSC2, JUP, DSP), although mutations have been described in five genes in inherited heart disorders that may lack any dermatological manifestations (DSG2, DSC2, JUP, PKP2, DSP). Understanding the diverse nature of these clinical phenotypes, as well as the desmosome gene mutation(s), has clinical value in managing and counselling patients, as well as demonstrating the biological role and activity of specific components of desmosomes in skin and other tissues.

Ectodermal dysplasia-skin fragility syndrome: Two new cases and review of this desmosomal genodermatosis

BACKGROUND

Desmosomes are intercellular cadherin-mediated adhesion complexes that anchor intermediate filaments to the cell membrane and are required for strong adhesion for tissues under mechanical stress. One specific component of desmosomes is plakophilin 1 (PKP1), which is mainly expressed in the spinous layer of the epidermis. Loss-of-function autosomal recessive mutations in PKP1 result in ectodermal dysplasia-skin fragility (EDSF) syndrome, the initial inherited Mendelian disorder of desmosomes first reported in 1997.

METHODS

To investigate two new cases of EDSF syndrome and to perform a literature review of pathogenic PKP1 mutations from 1997 to 2019.

RESULTS

Sanger sequencing of PKP1 identified two new homozygous frameshift mutations: c.409_410insAC (p.Thr137Thrfs*61) and c.1213delA (p.Arg411Glufs*22). Comprehensive analyses were performed for the 18 cases with confirmed bi-allelic PKP1 gene mutations, but not for one mosaic case or 6 additional cases that lacked gene mutation studies. All pathogenic germline mutations were loss-of-function (splice site, frameshift, nonsense) with mutations in the intron 1 consensus acceptor splice site (c.203-1>A or G>T) representing recurrent findings. Skin fragility and nail involvement were present in all affected individuals (18/18), with most cases showing palmoplantar keratoderma (16/18), alopecia/hypotrichosis (16/18) and perioral fissuring/cheilitis (12/15; not commented on in 3 cases). Further observations in some individuals included pruritus, failure to thrive with low height/weight centiles, follicular hyperkeratosis, hypohidrosis, walking difficulties, dysplastic dentition and recurrent chest infections.

CONCLUSION

These data expand the molecular basis of EDSF syndrome and help define the spectrum of both the prototypic and variable manifestations of this desmosomal genodermatosis.

Ectodermal Dysplasia-Skin Fragility Syndrome: A Rare Case Report

Ectodermal dysplasia/skin fragility syndrome (ED-SFS) is a newly described autosomal recessive disorder characterized by skin fragility and blistering, palmoplantar keratoderma, abnormal hair growth, nail dystrophy, and occasionally defective sweating. It results from mutations in the PKP1 gene encoding plakophilin 1 (PKP1), which is an important component of stratifying epithelial desmosomes and a nuclear component of many cell types. Only 12 cases of this rare genodermatosis have been reported so far. We present an unusual case of ED-SFS in a 12-year boy who was normal at birth but subsequently developed skin fragility, hair and nail deformities, abnormal dentition, palmoplantar keratoderma, and abnormal sweating but no systemic abnormality.

Inherited desmosomal disorders

Desmosomes serve as intercellular junctions in various tissues including the skin and the heart where they play a crucial role in cell-cell adhesion, signalling and differentiation. The desmosomes connect the cell surface to the keratin cytoskeleton and are composed of a transmembranal part consisting mainly of desmosomal cadherins, armadillo proteins and desmoplakin, which form the intracytoplasmic desmosomal plaque. Desmosomal genodermatoses are caused by mutations in genes encoding the various desmosomal components. They are characterized by skin, hair and cardiac manifestations occurring in diverse combinations. Their classification into a separate and distinct clinical group not only recognizes their common pathogenesis and facilitates their diagnosis but might also in the future form the basis for the design of novel and targeted therapies for these occasionally life-threatening diseases.

A plakophilin-1 gene mutation in an egyptian family with ectodermal dysplasia-skin fragility syndrome

Ectodermal dysplasia-skin fragility syndrome (ED-SFS) is a rare genodermatosis caused by mutations in the PKP1 gene, encoding the desmosomal plaque protein plakophilin-1. Since its initial description in 1997, few individuals with this disorder have been reported to date. Here, we present the first Egyptian cases of ED-SFS, carrying a novel homozygous mutation in the PKP1 gene. Direct sequencing of the amplified DNA from the affected cases disclosed a G-to-T transversion at nucleotide position c.203-1 within intron 1 of PKP1 (c.203-1G>T). To the best of our knowledge, this mutation has not been previously described in the databases.

In VivoFunction of Desmosomes

Desmosomes are morphologically and biochemically defined cell-cell junctions that are required for maintaining the mechanical integrity of skin and the heart in adult mammals. Furthermore, since mice with null mutations in desmosomal plaque proteins (plakoglobin and desmoplakin) die in utero, it is also evident that desmosomes are indispensable for normal embryonic development. This review focuses on the role of desmosomes in vivo. We will summarize the effects of mutations in desmosomal genes on pre- and post-embryonic development of mouse and man and discuss recent findings relating to the specific role of desmosomal cadherins in skin differentiation and homeostasis.

The molecular composition and function of desmosomes

Desmosomes are intercellular adhesive junctions that are particularly prominent in tissues experiencing mechanical stress, such as the heart and epidermis. Whereas the related adherens junction links actin to calcium-dependent adhesion molecules known as classical cadherins, desmosomes link intermediate filaments (IF) to the related subfamily of desmosomal cadherins. By tethering these stress-bearing cytoskeletal filaments to the plasma membrane, desmosomes serve as integrators of the IF cytoskeleton throughout a tissue. Recent evidence suggests that IF attachment in turn strengthens desmosomal adhesion. This collaborative arrangement results in formation of a supracellular network, which is critical for imparting mechanical integrity to tissues. Diseases and animal models targeting desmosomal components highlight the importance of desmosomes in development and tissue integrity, while the downregulation of individual protein components in cancer metastasis and wound healing suggests their importance in cell homeostasis. This chapter will provide an update on desmosome composition, function, and regulation, and will also discuss recent work which raises the possibility that desmosome proteins do more than play a structural role in tissues where they reside.

Deficient plakophilin-1 expression due to a mutation in PKP1 causes ectodermal dysplasia-skin fragility syndrome in Chesapeake Bay retriever dogs

In humans, congenital and hereditary skin diseases associated with epidermal cell-cell separation (acantholysis) are very rare, and spontaneous animal models of these diseases are exceptional. Our objectives are to report a novel congenital acantholytic dermatosis that developed in Chesapeake Bay retriever dogs. Nine affected puppies in four different litters were born to eight closely related clinically normal dogs. The disease transmission was consistent with an autosomal recessive mode of inheritance. Clinical signs occurred immediately after birth with superficial epidermal layers sloughing upon pressure. At three month of age, dogs exhibited recurrent superficial skin sloughing and erosions at areas of friction and mucocutaneous junctions; their coat was also finer than normal and there were patches of partial hair loss. At birth, histopathology revealed severe suprabasal acantholysis, which became less severe with ageing. Electron microscopy demonstrated a reduced number of partially formed desmosomes with detached and aggregated keratin intermediate filaments. Immunostaining for desmosomal adhesion molecules revealed a complete lack of staining for plakophilin-1 and anomalies in the distribution of desmoplakin and keratins 10 and 14. Sequencing revealed a homozygous splice donor site mutation within the first intron of PKP1 resulting in a premature stop codon, thereby explaining the inability to detect plakophilin-1 in the skin. Altogether, the clinical and pathological findings, along with the PKP1 mutation, were consistent with the diagnosis of ectodermal dysplasia-skin fragility syndrome with plakophilin-1 deficiency. This is the first occurrence of ectodermal dysplasia-skin fragility syndrome in an animal species. Controlled mating of carrier dogs would yield puppies that could, in theory, be tested for gene therapy of this rare but severe skin disease of children.

The desmosomal plaque proteins of the plakophilin family

Three related proteins of the plakophilin family (PKP1_3) have been identified as junctional proteins that are essential for the formation and stabilization of desmosomal cell contacts. Failure of PKP expression can have fatal effects on desmosomal adhesion, leading to abnormal tissue and organ development. Thus, loss of functional PKP 1 in humans leads to ectodermal dysplasia/skin fragility (EDSF) syndrome, a genodermatosis with severe blistering of the epidermis as well as abnormal keratinocytes differentiation. Mutations in the human PKP 2 gene have been linked to severe heart abnormalities that lead to arrhythmogenic right ventricular cardiomyopathy (ARVC). In the past few years it has been shown that junctional adhesion is not the only function of PKPs. These proteins have been implicated in cell signaling, organization of the cytoskeleton, and control of protein biosynthesis under specific cellular circumstances. Clearly, PKPs are more than just cell adhesion proteins. In this paper we will give an overview of our current knowledge on the very distinct roles of plakophilins in the cell.

Disadhesion of epidermal keratinocytes: a histologic clue to palmoplantar keratodermas caused by DSG1 mutations

BACKGROUND

Recent developments in molecular genetics may lead to re-examination of the histopathology of inherited palmoplantar keratodermas (PPKs) based on more precise groupings of the various entities and syndromes.

OBJECTIVE

We sought to characterize the histopathological findings in PPKs associated with mutations in DSG1, which encodes desmoglein 1.

METHODS

We studied the histopathology of 3 cases of keratosis palmoplantaris striata type I and one case of diffuse PPK, all associated with autosomal-dominant mutations in DSG1. Our cases for comparison included 4 cases with Mal de Meleda PPK associated with autosomal-recessive SLURP1 mutations, one case with pachyonychia congenita type II PPK associated with an autosomal-dominant KRT17 mutation, and one case with focal PPK associated with an autosomal-dominant KRT16 mutation.

RESULTS

The distinguishing histopathological features of the 3 keratosis palmoplantaris striata type I cases and the diffuse PPK case associated with DSG1 mutation were: varying degrees of widening of the intercellular spaces and partial disadhesion of keratinocytes in the mid and upper epidermal spinous cell layers, often extending to the granular cell layer. These findings, which are associated with haploinsufficiency of desmoglein 1, were not observed in any of the other 6 PPK cases. Mild perinuclear eosinophilic condensations and cytoplasmic vacuolizations were observed in the spinous cell layer keratinocytes of the pachyonychia congenita type II PPK and the nonspecified focal PPK cases.

LIMITATIONS

There were a limited number of patients and control patients with hereditary PPKs.

CONCLUSION

Widening of the intercellular spaces and disadhesion of epidermal keratinocytes may serve as a histologic clue to PPKs caused by DSG1 mutations.

Alopecia in epidermolysis bullosa

Hair abnormalities observed in epidermolysis bullosa (EB) are of variable severity and include mild hair shaft abnormalities, patchy cicatricial alopecia, cicatricial alopecia with a male pattern distribution, and alopecia universalis. Alopecia is usually secondary to blistering, and scalp areas more exposed to friction, such as the occipital area, are involved more frequently. This article reviews the hair abnormalities reported in the different subtypes of EB.

Progress in heritable skin diseases: translational implications of mutation analysis and prospects of molecular therapies*

Epidermolysis bullosa, a group of blistering disorders, serves as the paradigm of the tremendous progress made in understanding the molecular genetics of heritable skin diseases. Mutations in 10 distinct genes have been disclosed in the classic forms of epidermolysis bullosa, and the level of expression of the mutated genes within the cutaneous basement membrane zone, the types and combinations of mutations and their consequences at the mRNA and protein levels, when placed in the context of the individual's genetic background and exposure to environmental trauma, all determine the subtype and the phenotypic severity in each case. The translational implications of mutation analysis include improved diagnosis and subclassification, refined genetic counseling of families at risk, and development of DNA-based pre natal and preimplantation genetic diagnosis. The prospects of molecular therapies for epidermolysis bullosa include further development of strategies for gene therapy, protein replacement therapy and cell-based therapies, including stem cell therapy and bone marrow transfer. Collectively, advances in the molecular genetics of heritable skin diseases clearly emphasize the value of basic research for improved diagnostics and patient care for genetic skin diseases.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Genetic diseases of junctions

Tight junctions, gap junctions, adherens junctions, and desmosomes represent intricate structural intercellular channels and bridges that are present in several tissues, including epidermis. Clues to the important function of these units in epithelial cell biology have been gleaned from a variety of studies including naturally occurring and engineered mutations, animal models and other in vitro experiments. In this review, we focus on mutations that have been detected in human diseases. These observations provide intriguing insight into the biological complexities of cell-cell contact and intercellular communication as well as demonstrating the spectrum of inherited human diseases that are associated with mutations in genes encoding the component proteins. Over the last decade or so, human gene mutations have been reported in four tight junction proteins (claudin 1, 14, 16, and zona occludens 2), nine gap junction proteins (connexin 26, 30, 30.3, 31, 32, 40, 43, 46, and 50), one adherens junction protein (P-cadherin) and eight components of desmosomes (plakophilin (PKP) 1 and 2, desmoplakin, plakoglobin--which is also present in adherens junctions, desmoglein (DSG) 1, 2, 4, and corneodesmosin). These discoveries have often highlighted novel or unusual phenotypes, including abnormal skin barrier function, alterations in epidermal differentiation, and developmental anomalies of various ectodermal appendages, especially hair, as well as a range of extracutaneous pathologies. However, this review focuses mainly on inherited disorders of junctions that have an abnormal skin phenotype.

Dyskeratosis as a histologic feature in epidermolysis bullosa simplex-Dowling Meara

BACKGROUND

Intracellular keratin aggregation and clumping is a characteristic ultrastructural feature in epidermolysis bullosa simplex (EBS)-Dowling Meara (DM) yet without histologic correlates in routinely stained specimens.

OBJECTIVE

We sought to detect histologic clues to keratin aggregation and clumping in the involved epidermis of EBS-DM.

METHODS

Four cases of EBS-DM caused by dominant keratin (KRT)5 and KRT14 mutations were studied histologically and ultrastructurally. The histologic slides of 11 additional EBS cases (9 Weber-Cockayne subtypes and two Koebner subtypes) were also reviewed histologically.

RESULTS

Intracytoplasmic aggregation and clumping of tonofilaments were observed ultrastructurally in all 4 EBS-DM cases. Intracytoplasmic eosinophilic homogenizations and inclusions (ie, dyskeratosis) in individual keratinocytes were detected histologically in 3 of the 4 EBS-DM cases, but in none of the 9 EBS-Weber-Cockayne cases or the two EBS-Koebner cases.

LIMITATIONS

This was a relatively small studied group.

CONCLUSION

The histopathological detection of dyskeratosis in individual keratinocytes may provide a valuable clue to keratin aggregation and clumping, and to the diagnosis in EBS-DM.

Molecular and diagnostic aspects of genetic skin fragility

Genetic syndromes with skin fragility represent a heterogeneous group of very rare disorders caused by mutations in genes encoding proteins or protein subunits important for the mechanical resistance of keratinocytes and for cell-cell or cell-extracellular matrix adhesion. The common symptoms are skin blistering or peeling, with various degrees of severity and distribution, ranging from localized to generalized forms. Associated features include involvement of skin annexes, mucous membranes, teeth, muscles or the digestive tract. Morphological investigation of skin samples provides evidence for the tissue level of blister formation, while immunostainings may reveal defective proteins, providing clues concerning the genetic origin of the disease. Extensive mutation analysis and subsequent identification of new gene defects provide accurate diagnostics, and lead to better understanding of the functions of the respective proteins, with the potential for new therapeutic strategies.

Preimplantation genetic diagnosis of skin fragility-ectodermal dysplasia syndrome

Skin fragility-ectodermal dysplasia syndrome is an autosomal recessive disorder caused by loss-of-function mutations in the desmosomal protein, plakophilin 1. Clinically, there may be considerable morbidity from extensive skin erosions and painful fissures on the palms and soles. In the absence of any specific treatment, prenatal diagnosis is an option for couples at reproductive risk of recurrence. In 2000, we developed and applied a single cell nested polymerase chain reaction protocol to test one couple for compound heterozygous plakophilin 1 gene mutations by preimplantation genetic diagnosis (PGD). Although pregnancy was established, an unrelated trisomy 22 led to a spontaneous abortion. However, eight embryos of known genetic status were cryopreserved at that stage, and we planned to undertake subsequent frozen embryo replacement cycles that might lead to the birth of an unaffected child in this family. Embryo cryopreservation was carried out in June 2000 using standard protocols in a three-step freezing procedure. Four embryos were thawed in March 2003, one of which was viable and was used in a frozen embryo replacement cycle, but pregnancy did not occur. The remaining four embryos were thawed in February 2004, two of which were viable (both carriers of the paternal mutation) and these were used in a second frozen embryo replacement cycle, and a singleton pregnancy was established. The child's plakophilin 1 genotype was assessed by direct nucleotide sequencing across the site of both potential mutations. Following two frozen embryo replacement cycles, and almost 4 years after the initial embryo biopsy and mutation analysis, a pregnancy was achieved that progressed to term with the birth of a healthy baby girl. Nucleotide sequencing of cord blood DNA, taken immediately after delivery, showed that the child was a heterozygous carrier of the paternal mutation but not of the maternal mutation. This case demonstrates the value of embryo cryopreservation, which can increase the number of embryo replacement procedures and hence the cumulative pregnancy rate per retrieval cycle. Moreover, this is the first report of successful full-term pregnancy and birth of a healthy baby following exclusion of a severe genodermatosis by PGD. The successful outcome of PGD in this case illustrates what is technically possible for couples at risk of recurrence of a severe inherited skin disease.

Ectodermal dysplasia–skin fragility syndrome resulting from a new homozygous mutation, 888delC, in the desmosomal protein plakophilin 1

We report an unusual case of an inherited disorder of the desmosomal protein plakophilin 1, resulting in ectodermal dysplasia-skin fragility syndrome. The affected 6-year-old boy had red skin at birth and subsequently developed skin fragility, progressive plantar keratoderma, nail dystrophy, and alopecia. Skin biopsy revealed widening of intercellular spaces in the epidermis and a reduced number of small, poorly formed desmosomes. Mutation analysis of the plakophilin 1 gene PKP1 revealed a homozygous deletion of C at nucleotide 888 within exon 5. This mutation differs from the PKP1 gene pathology reported in 8 previously published individuals with this rare genodermatosis. However, all cases show similar clinical features, highlighting the importance of functional plakophilin 1 in maintaining desmosomal adhesion in skin, as well as the role of this protein in aspects of ectodermal development.

Focal palmoplantar keratoderma caused by an autosomal dominant inherited mutation in the desmoglein 1 gene

BACKGROUND

Palmoplantar keratodermas (PPK) encompass a large genetically heterogeneous group of diseases associated with hyperkeratosis of the soles and/or palms that occur either isolated or in association with other cutaneous and extracutaneous manifestations. Pathogenic mutations in the desmoglein 1 gene (DSG1) have recently been identified in a subset of patients with the striate type of PPK.

OBSERVATION

We have identified a patient with a focal non-striated form of PPK associated with discrete troubles of keratinisation at sites exposed to mechanical trauma, such as the knees, ankles or finger knuckles, and with mild nail dystrophy. Genetic analyses disclosed a novel dominantly inherited heterozygous single base insertion in exon 3 of DSG1, 121insT, leading to a premature termination codon. The mutation was also present in the father and in a sister.

CONCLUSION

Our observation extends the spectrum of clinical features associated with genetic defects in DSG1 and provides further evidence that perturbation of desmoglein 1 expression has a critical impact on the integrity of tissues experiencing strong mechanical stress.

Histopathological and ultrastructural study of ectodermal dysplasia/skin fragility syndrome

Ectodermal dysplasia/skin fragility syndrome (EDSFS) (MIM604536) is a newly described autosomal recessive disorder characterized by skin fragility and blistering, palmoplantar keratoderma, abnormal hair growth, nail dystrophy, and occasionally defective sweating. It results from mutations in the PKP1 gene encoding plakophilin 1 (PKP1), which is an important component of stratifying epithelial desmosomes and a nuclear component of many cell types. Our study was performed to further characterize the histopathology of EDSFS in different cutaneous sites with a special emphasis on the hypotrichosis and keratoderma. A total of 4 biopsies were obtained from 2 EDSFS female patients, aged 9 days to 4 years. The biopsies were taken from the blistering skin of the leg and trunk, the hyperkeratotic skin of the sole, and the hypotrichotic scalp. The observed histopathologic features included: widened intercellular spaces, suprabasal intraepidermal clefts and blisters with acantholytic keratinocytes, detachments of the upper epidermal layers due to disadhesion, varying degrees of dyskeratosis that were much more pronounced in the plantar hyperkeratotic skin, and increased number of catagen-telogen hair follicles. The electron-microscopic observations attributed the disadhesion and acantholysis to reduced numbers of small hypoplastic desmosomes, and the dyskeratosis to the detachment of intracellular keratin filaments from the desmosomes with perinuclear condensation, which might also underlie the plantar keratoderma. The hair follicle findings suggest disturbance in the hair cycle, which might be attributed to disturbed nuclear PKP1 function or result from aberrant desmosomal signaling.

Inherited disorders of desmosomes

Desmosomes are highly organized intercellular junctions that provide mechanical integrity to tissues by anchoring intermediate filaments to sites of strong adhesion. These cell-cell adhesion junctions are found in skin, heart, lymph nodes and meninges. Over the last 8 years, several naturally occurring human gene mutations in structural components of desmosomes have been reported. These comprise autosomal dominant or recessive mutations in plakophilin 1, plakophilin 2, desmoplakin, plakoglobin, desmoglein 1, desmoglein 4 and corneodesmosin. These discoveries have often highlighted novel or unusual phenotypes, including abnormal skin fragility and differentiation, and developmental anomalies of various ectodermal appendages, especially hair. Some desmosomal gene mutations may also result in cardiac disease, notably cardiomyopathy. This article describes the spectrum of clinical features that may be found in the inherited disorders of desmosomes and highlights the key functions of several of the desmosomal proteins in tissue adhesion and cell biology.

Human hair abnormalities resulting from inherited desmosome gene mutations

Over the last eight years, several naturally occurring human gene mutations in structural components of desmosomes, cell-cell adhesion junctions found in skin, heart and meninges, have been reported. These comprise dominant or recessive mutations in plakophilin 1, plakophilin 2, desmoplakin, desmoglein 1, desmoglein 4, plakoglobin and corneodesmosin. Of note, as well as compromising tissue integrity, many of the resulting phenotypes have been associated with visible changes in hair. This article describes the particular hair abnormalities resulting from these desmosome gene mutations. Collectively, the data demonstrate the surprising effects inherited desmosome gene/protein pathology may have on hair growth and development. Further analysis of these and other desmosome genes is likely to resolve more hair disease mysteries and provides several further intriguing new discoveries in years to come.

Compound Heterozygosity for New Splice Site Mutations in the Plakophilin 1 Gene ( PKP1) in a Chinese Case of Ectodermal Dysplasia–Skin Fragility Syndrome

Ectodermal dysplasia-skin fragility syndrome is a rare autosomal recessive inherited disease characterized by skin fragility, nail dystrophy and hyperkeratosis of palms and soles. Skin biopsy shows the loss of cell adhesion and the decrease of desmosomes in number and size. Mutations in PKP1 have been found to be the underlying cause of the syndrome. We report here a Chinese case of ectodermal dysplasia-skin fragility syndrome. Mutation analysis revealed compound heterozygosity for mutations in PKP1 of the proband. A new splice site mutation (c.1053 T>A+c.1054+1 G>T) near the 3' end of exon 5 and at the donor end of intron 5 on one allele was transmitted from the proband's mother. Another new splice site mutation (c.1835-2 A>G) near the acceptor end of intron 10 originated from her father. The absence of the mutant mRNA and plakophilin 1 protein in the proband's skin may result from the mechanism of nonsense-mediated mRNA decay induced by premature stop codons in PKP1 transcripts due to the two splice site mutations.

A role for plakophilin-1 in the initiation of desmosome assembly

Plakophilins (pkp-1, -2, and -3) comprise a family of armadillo-repeat containing proteins that are found in the desmosomal plaque and in the nucleus. Plakophilin-1 is most highly expressed in the suprabasal layers of the epidermis and loss of plakophilin-1 expression results in skin fragility-ectodermal dysplasia syndrome, which is characterized by a reduction in the number and size of desmosomes in the epithelia of affected individuals. To investigate the role of plakophilin-1 during desmosome formation, we fused plakophilin-1 to the hormone-binding domain of the estrogen receptor to create a fusion protein (plakophilin-1/ER) that can be activated in cell culture by the addition of 4-hydroxytamoxifen. When plakophilin-1/ER was expressed in A431 cells it was incorporated into endogenous desmosomes and did not disrupt desmosome formation. A derivative of A431 cells (A431D) do not form desmosomes, even though they express all the components believed to be necessary for desmosome assembly. Expression and activation of plakophilin-1/ER in A431D cells resulted in punctate desmoplakin staining on the cell surface. Co-expression of a classical cadherin (N-cadherin) and plakophilin-1/ER in A431D cells resulted in punctate desmoplakin staining at cell-cell borders. These data suggest that plakophilin-1 can induce assembly of desmosomal components in A431D cells in the absence of a classical cadherin; however a classical cadherin (N-cadherin) is required to direct assembly of desmosomes between adjacent cells. The activatable plakophilin-1/ER system provides a unique culture system to study the assembly of the desmosomal plaque in culture.

Requirement of plakophilin 2 for heart morphogenesis and cardiac junction formation

Plakophilins are proteins of the armadillo family that function in embryonic development and in the adult, and when mutated can cause disease. We have ablated the plakophilin 2 gene in mice. The resulting mutant mice exhibit lethal alterations in heart morphogenesis and stability at mid-gestation (E10.5–E11), characterized by reduced trabeculation, disarrayed cytoskeleton, ruptures of cardiac walls, and blood leakage into the pericardiac cavity. In the absence of plakophilin 2, the cytoskeletal linker protein desmoplakin dissociates from the plaques of the adhering junctions that connect the cardiomyocytes and forms granular aggregates in the cytoplasm. By contrast, embryonic epithelia show normal junctions. Thus, we conclude that plakophilin 2 is important for the assembly of junctional proteins and represents an essential morphogenic factor and architectural component of the heart.

Plakophilin 1: an important stabilizer of desmosomes

Desmosomes are cell-cell junctions found primarily in epithelial tissues and particularly in those that frequently undergo mechanical stress. Desmosomal cadherins provide the adhesion for opposing cell membranes and desmosomal plaque proteins link cytoskeletal intermediate filaments to these cadherins. Plakophilin 1 is a desmosomal plaque protein and a member of the armadillo family of structural and signalling proteins. Expressed primarily in the suprabasal layers of stratifying epithelia, plakophilin 1 is absent in patients with the rare autosomal recessive disorder, ectodermal-dysplasia skin-fragility syndrome (OMIM 604536). These patients exhibit skin fragility with trauma induced blistering, especially on the weight-bearing parts of the soles, and thus provide a vivid illustration of the clinical importance of PKP1 in skin physiology. Recent in vitro data also implicate plakophilin 1 in regulating desmosome integrity in response to low calcium concentrations and in altering the migratory properties of wounded keratinocyte sheets in culture. These findings further underscore the significant role of PKP1 in keratinocyte cell biology.

Homozygous splice site mutations in PKP1 result in loss of epidermal plakophilin 1 expression and underlie ectodermal dysplasia/skin fragility syndrome in two consanguineous families

During the last years, a growing number of inherited skin disorders have been recognized to be caused by abnormal function of desmosomal proteins. In the present study, we describe the first female individuals affected with the ectodermal dysplasia/skin fragility syndrome (MIM604536), a rare autosomal recessive disease due to mutations in the PKP1 gene encoding plakophilin 1, a critical component of desmosomal plaque. One patient was shown to carry a homozygous splice site mutation in intron 4. The second patient displayed a homozygous recurrent mutation affecting the acceptor splice site of intron 1. Both mutations were associated with intraepidermal separation, widening of intercellular spaces, and abnormal desmosome ultrastructure, and were found to result in the absence of immunoreactive plakophilin 1 in the epidermis of the affected individuals. These two cases emphasize the role of molecular genetics in the assessment of congenital blistering in newborns and illustrate the importance of proper desmosomal activity for normal epidermis development and function.

Alterations in desmosome size and number coincide with the loss of keratinocyte cohesion in skin with homozygous and heterozygous defects in the desmosomal protein plakophilin 1

Recessive mutations in the desmosomal plaque protein plakophilin 1 (PkP1) underlie ectodermal dysplasia/skin fragility syndrome (MIM 604536). We undertook an immunohistochemical and quantitative electron microscopic examination of suprabasal desmosomes from 4 skin samples from 3 PkP1 deficient patients, an unaffected carrier with a PKP1 heterozygous acceptor splice site mutation and 5 healthy control subjects. Desmosomal plaque size (>50 desmosomes per individual) and frequency (>20 high power fields, HPF) were assessed. Compared with controls, desmosomes were reduced dramatically both in size (49%) and frequency (61%) in the lower suprabasal layers (LSB) in PkP1 null patients (P<0.01). In the LSB compartment of the heterozygous carrier, corresponding reductions were 37% and 20%, respectively (P<0.01). Surprisingly, the PkP1 null patient's upper suprabasal layer, (USB), desmosome size was larger (59%, P<0.01) than the control value, and showed increased desmoglein 1 and PkP2 USB staining. The USB desmosome frequency in PKP1 null patients was similar to the LSB compartment (but reduced by 43% compared to USB controls). The carrier showed no difference in the USB desmosome size and frequency compared with the controls (P>0.05). The PKP1 null patients showed poorly developed inner and outer desmosomal plaques. Thus, both the patients and unaffected carrier showed reductions in the LSB desmosome size and number; despite only PkP1 null patients exhibiting any phenotype. These findings attest to the molecular recruiting and stabilizing roles of PkP1 in desmosome formation, particularly in the LSB compartment.

Genetic evidence for a novel human desmosomal cadherin, desmoglein 4

Desmosomes are essential adhesion structures in most epithelia that link the intermediate filament network of one cell to its neighbor, thereby forming a strong bond. The molecular components of desmosomes belong to the cadherin superfamily, the plakin family, and the armadillo repeat protein family. The desmosomal cadherins are calcium-dependent transmembrane adhesion molecules and comprise the desmogleins and desmocollins. To date, three human desmoglein isoforms have been characterized, namely desmogleins 1, 2, and 3 that are expressed in a tissue- and differentiation-specific manner. Here we have identified and characterized, at the genetic level, a novel human desmoglein cDNA sharing homology with desmogleins 1, 2, 3 and we name this desmoglein 4. The human desmoglein 4 cDNA (3.6 kb) contains an open reading frame of 3120 bp that encodes a precursor protein of 1040 amino acids. The predicted mature protein comprises 991 amino acids with a molecular weight of 107822 Da at pI 4.38. Human desmoglein 4 shares 41% identity with human desmoglein 1, 37% with human desmoglein 2, and 50% with human desmoglein 3. Analysis of the exon/intron organization of the human desmoglein 4 gene (DSG4) demonstrates that it is composed of 16 exons spanning approximately 37 kb of 18q12 and is situated between DSG1 and DSG3. We have demonstrated using RT-PCR on multiple tissue cDNA samples that desmoglein 4 has very specific tissue expression in salivary gland, testis, prostate, and skin.