Recent advances in the molecular basis of inherited skin diseases

From pemphix to desmogleins

In the not so distant past, the word pemphigus or pemphix was common for describing various diseases characterized by blistering as well as various disorders that do not originate from a blistering pathology. Patients with these conditions were grouped in "other" skin diseases. Step by step, during the past, we were introduced to these severe conditions. First, we learned from sporadic case reports, then new differentiations were reported according to histology, later immunopathology was developed, and now there are discoveries of new molecules. Immense progress with new approaches to therapy has been achieved, but much improvement is still needed. The modern definition of pemphigus undoubtedly represents a group of rare, intraepidermal autoimmune bullous diseases characterized by intraepidermal blisters and circulating autoantibodies desmogleins against the keratinocytes cell surface.

Skin immune surveillance by T cells--a new order?

Although studies of the skin have provided fundamental models for innate and adaptive immune surveillance of body surfaces, there remains relatively little understanding of the role that epithelial cells play in sensing infection and/or organ dysfunction, and the pathways available to them to communicate with local and systemic immune cells. In particular, evidence is emerging for a novel stress response initiated by local lymphocytes, rather than dendritic cells, and based on their recognition of epithelial stress-induced antigens. Its consequences are to sustain tissue integrity by providing immunoprotection and novel modes of immunoregulation, whereas its dysregulation may promote body surface immunopathologies.

[Hereditary blistering disorders]

Epidermolysis bullosa (EB) is a group of genetic skin disorders whose common feature is the formation of blisters following minor trauma. They present with a wide clinical spectrum of manifestations because of a variety of molecular defects. In patients with mild phenotypes, only skin is affected. The most severe EB forms are multiorgan disorders with a poor prognosis. EB arises from abnormalities in proteins of the dermal-epidermal junction. These specialized protein components aggregate to form anchoring complexes, which attach the epidermis to the dermis. Three major EB-forms can be distinguished on the basis of ultrastructural blistering level: EB simplex--epidermolytic, junctional EB--in the lamina lucida and dystrophic EB--dermolytic. To establish a provisional diagnosis for an EB patient, clinical data, family history and morphologic examination of the skin, e.g. by antigen-mapping, are needed. Complete knowledge of the genetic defect provides the basis to a rational genetic counseling and prenatal testing. Treatment of EB is based on wound care; multidisciplinary management of cases with severe course is required.

The Caenorhabditis elegans vab-10 spectraplakin isoforms protect the epidermis against internal and external forces

Morphogenesis of the Caenorhabditis elegans embryo is driven by actin microfilaments in the epidermis and by sarcomeres in body wall muscles. Both tissues are mechanically coupled, most likely through specialized attachment structures called fibrous organelles (FOs) that connect muscles to the cuticle across the epidermis. Here, we report the identification of new mutations in a gene known as vab-10, which lead to severe morphogenesis defects, and show that vab-10 corresponds to the C. elegans spectraplakin locus. Our analysis of vab-10 reveals novel insights into the role of this plakin subfamily. vab-10 generates isoforms related either to plectin (termed VAB-10A) or to microtubule actin cross-linking factor plakins (termed VAB-10B). Using specific antibodies and mutations, we show that VAB-10A and VAB-10B have distinct distributions and functions in the epidermis. Loss of VAB-10A impairs the integrity of FOs, leading to epidermal detachment from the cuticle and muscles, hence demonstrating that FOs are functionally and molecularly related to hemidesmosomes. We suggest that this isoform protects against forces external to the epidermis. In contrast, lack of VAB-10B leads to increased epidermal thickness during embryonic morphogenesis when epidermal cells change shape. We suggest that this isoform protects cells against tension that builds up within the epidermis.

Integrins in development: moving on, responding to, and sticking to the extracellular matrix

Integrins are cell surface receptors of the extracellular matrix present in all animals. Genetic analysis in worms, flies, and vertebrates has revealed integrin involvement in key developmental processes, and we focus here on examples of integrin functions that are comparable across these model organisms. Integrins contribute to cell movement by providing traction to migrating cells, through assembly of extracellular matrices that can serve as tracks for migration, and by transmitting guidance signals that direct cells or cell processes to their targets. Integrins also participate in signaling events that govern tissue differentiation and organogenesis. Finally, adhesion by integrin-mediated junctions allows tissues to withstand mechanical load and is essential for tissue integrity.

Epithelial barrier function: assembly and structural features of the cornified cell envelope

Terminally differentiating stratified squamous epithelial cells assemble a specialized protective barrier structure on their periphery termed the cornified cell envelope (CE). It is composed of numerous structural proteins that become cross-linked by several transglutaminase enzymes into an insoluble macromolecular assembly. Several proteins are involved in the initial stages of CE assembly, but only certain proteins from a choice of more than 20 different proteins are used in the final stages of CE reinforcement, apparently to meet tissue-specific requirements. In addition, a variable selection of proteins may be upregulated in response to genetic defects of one of the CE proteins or tissue injury, in an effort to maintain an effective barrier. Additionally, in the epidermis and hair fiber cuticle, a layer of lipids is covalently attached to the proteins, which provides essential water barrier properties. Here we describe our current understanding of CE structure, a possible mechanism of its assembly, and various disorders that cause a defective barrier.

Dermatological applications of DNA array technology

The Human Genome Project and other large scale sequencing consortia continue to generate huge amounts of DNA sequence data. Despite the identification of specific disease-related genes and genetic markers, we still appear to know little about how diverse gene products actually interact with each other or respond to other chemical or biological stimuli. Such information is of course fundamental to understanding complex disease pathways and biochemical processes and, as such, has spawned new fields of investigative genetics, that of functional genomics and proteomics. DNA array technology is emerging as a powerful, high-throughput and versatile tool that can be applied to the study of functional genomics. This article reviews the methodology involved in array analysis and provides insight into how, as an investigative tool, DNA arrays are becoming increasingly useful in understanding fundamental abnormalities in dermatological disease and also in refining the management of patients with certain skin disorders.

A novel nonsense mutation at E106 of the 2B rod domain of keratin 14 causes dominant epidermolysis bullosa simplex

Epidermolysis bullosa simplex (EBS) is classified into three main types and is caused, in most cases, by missense mutations in the genes encoding keratin (K) 5 and K14. In this study, we clinically, ultrastructurally, immunohistochemically, and molecularly studied a patient with a dominant EBS, Köbner type. Using sequence analysis of genomic DNA, a novel K14 nonsense mutation was identified. A heterozygous mutation G1231T of KRT14 was found to be associated with the disease in the patient. The mutation created a premature stop codon (amino acid codon 411, residue 106 of the 2B helix) in the K14 molecule. This residue lies in a highly conserved region and was recently found to be absolutely required for molecular stability and intermediate filament assembly in K5 and K14. The E411X (E106X) heterozygous ablation, missing the last 16 amino acid residues of the 2B and the entire tail domain of K14, led to disease but did not result in clumping of keratin filaments. It is the first premature stop codon mutation of K14 found in dominant EBS.

Hornerin, a novel profilaggrin-like protein and differentiation-specific marker isolated from mouse skin

A novel mouse cDNA named hornerin was isolated by RNA differential display applied to developing mouse skin. Hornerin, which has 2,496 amino acids, comprises EF-hand domains at the N terminus followed by a spacer sequence and a large repetitive domain, indicating that hornerin is a novel member of the "fused gene"-type cornified envelope precursor protein family. The repetitive domain of hornerin was found to be rich in glycine, serine, and glutamine. Hornerin was expressed in the tongue, esophagus, forestomach, and skin among the adult mouse tissues examined, all of them cornifying stratified epithelium. In the embryonic mouse skin, hornerin mRNA was first detected on gestational day 15.5 in the epidermis coincidentally with the formation of a granular layer. In accordance with this, hornerin was detected in the granular and cornified layers of the mature epidermis. In the granular cells of the epidermis, the hornerin protein was detected in keratohyalin granules together with profilaggrin. Furthermore, Western blot analysis of the mouse skin showed that the hornerin protein was cleaved during the process of epidermal differentiation, indicating possible posttranslational proteolytic processing as is observed in profilaggrin. Differentiation of primary mouse epidermal keratinocytes with 0.12 mm Ca(2+) resulted in the induction of hornerin. These results indicate that hornerin is structurally as well as functionally most similar to profilaggrin among the family members and possibly plays pleiotropic roles, including a role in cornification.

Keratinocyte adhesion and the missing link: from Dowling-Meara to Hay-Wells. St John's Hospital Dermatological Society Annual Oration 2000

Maintaining a protective barrier against the environment is an essential function of normal skin. Critical to this role are several structural proteins and glycoproteins that contribute to adhesive junctions linking adjacent keratinocytes and basal keratinocytes to the underlying dermis, as well as other regulatory proteins involved in aspects of epidermal development, differentiation and proliferation. Inherited abnormalities in the genes that encode these components may give rise to a range of genodermatoses, many of which are characterized structurally by a 'missing' or perturbed adhesive link and clinically by congenital skin blistering. This oration reviews some of the original clinical descriptions and observations made in this field, as well as providing an update on the corresponding recent molecular discoveries. The emphasis is on contributions made by past and present members of the St John's Hospital Dermatological Society.