Keratin diseases

Detrimental Effects of IFN-γ on an Epidermolysis Bullosa Simplex Cell Model and Protection by a Humanized Anti-IFN-γ Monoclonal Antibody

Epidermolysis bullosa is a group of severe skin blistering disorders, which currently have no cure. The pathology of epidermolysis bullosa is recognized as having an inflammatory component, but the role of inflammation in different epidermolysis bullosa disorders is unclear. Epidermolysis bullosa simplex (EBS) is primarily caused by sequence variants in keratin genes; its most severe form, EBS generalized severe, is characterized by aggregates of keratin proteins, and cell models of EBS generalized severe show constitutively elevated stress. IFN-γ is a major mediator of inflammation, and we show that the addition of IFN-γ alone to disease model keratinocytes promotes keratin aggregation, decreases cell-cell junctions, delays wound closure, and reduces cell proliferation. IFN-γ exposure weakens the intercellular cohesion of monolayers on mechanical stress, with IFN-γ-treated EBS monolayers more fragmented than IFN-γ-treated wild-type monolayers. A humanized monoclonal antibody to IFN-γ neutralized the detrimental effects on keratinocytes, restoring cell proliferation, increasing cell-cell adhesion, accelerating wound closure in the presence of IFN-γ, and reducing IFN-γ-mediated keratin aggregation in EBS cells. These suggest that treatment with IFN-γ blocking antibodies may constitute a promising new therapeutic strategy for patients with EBS and may also have ameliorating effects on other inflammatory skin diseases.

A Scarless Healing Tale: Comparing Homeostasis and Wound Healing of Oral Mucosa With Skin and Oesophagus

Epithelial tissues are the most rapidly dividing tissues in the body, holding a natural ability for renewal and regeneration. This ability is crucial for survival as epithelia are essential to provide the ultimate barrier against the external environment, protecting the underlying tissues. Tissue stem and progenitor cells are responsible for self-renewal and repair during homeostasis and following injury. Upon wounding, epithelial tissues undergo different phases of haemostasis, inflammation, proliferation and remodelling, often resulting in fibrosis and scarring. In this review, we explore the phenotypic differences between the skin, the oesophagus and the oral mucosa. We discuss the plasticity of these epithelial stem cells and contribution of different fibroblast subpopulations for tissue regeneration and wound healing. While these epithelial tissues share global mechanisms of stem cell behaviour for tissue renewal and regeneration, the oral mucosa is known for its outstanding healing potential with minimal scarring. We aim to provide an updated review of recent studies that combined cell therapy with bioengineering exporting the unique scarless properties of the oral mucosa to improve skin and oesophageal wound healing and to reduce fibrotic tissue formation. These advances open new avenues toward the ultimate goal of achieving scarless wound healing.

Keratin 14 is a novel interaction partner of keratinocyte differentiation regulator: receptor-interacting protein kinase 4

The epidermis, the outer layer of the skin, is formed by stratified keratinocyte layers. The self-renewal of the epidermis is provided by sustained proliferation and differentiation of the keratinocyte stem cells localized to the basal layer of the epidermis. Receptor-interacting protein kinase 4 (RIPK4) is an important regulator of keratinocyte differentiation, mutations of which are associated with congenital ectodermal malformations. In an attempt to identify the molecular basis of RIPK4’s function, we applied yeast two-hybrid screen (Y2H) and found basal layer-specific keratin filament component keratin 14 (KRT14) as a novel RIPK4-interacting partner. During keratinocyte differentiation, layer-specific keratin composition is tightly regulated. Likewise, the basal layer specific KRT14/keratin 5 (KRT5) heterodimers are replaced by keratin 1 (KRT1)/keratin 10 (KRT10) in suprabasal layers. The regulation of keratin turnover is under the control of signaling associated with posttranslational modifications in which phosphorylation plays a major role. In this study, we verified the KRT14-RIPK4 interaction, which was identified with Y2H, in mammalian cells and showed that the interaction was direct by using proteins expressed in bacteria. According to our results, the N-terminal kinase domain of RIPK4 is responsible for KRT14-RIPK4 interaction; however, the RIPK4 kinase activity is dispensable for the interaction. In accordance with their interaction, RIPK4 and KRT14 colocalize within the cells, particularly at keratin filaments associated with perinuclear ring-like structures. Moreover, RIPK4 did not show any effect on KRT14/KRT5 heterodimer formation. Our results suggest that RIPK4 may regulate the keratin turnover required for keratinocyte differentiation through interacting with KRT14.

Assays to Study Consequences of Cytoplasmic Intermediate Filament Mutations: The Case of Epidermal Keratins

The discovery of the causative link between keratin mutations and a growing number of human diseases opened the way for a better understanding of the function of the whole intermediate filament families of cytoskeleton proteins. This chapter describes analytical approaches to identification and interpretation of the consequences of keratin mutations, from the clinical and diagnostic level to cells in tissue culture. Intermediate filament pathologies can be accurately diagnosed from skin biopsies and DNA samples. The Human Intermediate Filament Database collates reported mutations in intermediate filament genes and their diseases, and can help clinicians to establish accurate diagnoses, leading to disease stratification for genetic counseling, optimal care delivery, and future mutation-aligned new therapies. Looking at the best-studied keratinopathy, epidermolysis bullosa simplex, the generation of cell lines mimicking keratinopathies is described, in which tagged mutant keratins facilitate live-cell imaging to make use of today's powerful enhanced light microscopy modalities. Cell stress assays such as cell spreading and cell migration in scratch wound assays can interrogate the consequences of the compromised cytoskeletal network. Application of extrinsic stresses, such as heat, osmotic, or mechanical stress, can enhance the differentiation of mutant keratin cells from wild-type cells. To bring the experiments to the next level, 3D organotypic human cultures can be generated, and even grafted onto the backs of immunodeficient mice for greater in vivo relevance. While development of these assays has focused on mutant K5/K14 cells, the approaches are often applicable to mutations in other intermediate filaments, reinforcing fundamental commonalities in spite of diverse clinical pathologies.

Severe Meesmann's epithelial corneal dystrophy phenotype due to a missense mutation in the helix-initiation motif of keratin 12

PURPOSE

To describe a severe phenotype of Meesmann's epithelial corneal dystrophy (MECD) and to determine the underlying molecular cause.

METHODS

We identified a 30-member family affected by MECD and examined 11 of the 14 affected individuals. Excised corneal tissue from one affected individual was examined histologically. We used PCR and direct sequencing to identify mutation of the coding regions of the KRT3 and KRT12 genes.

RESULTS

Cases had an unusually severe phenotype with large numbers of intraepithelial cysts present from infancy and they developed subepithelial fibrosis in the second to third decade. In some individuals, the cornea became superficially vascularized, a change accompanied by the loss of clinically obvious epithelial cysts. Visual loss from amblyopia or corneal opacity was common and four individuals were visually impaired (≤6/24 bilaterally) and one was blind (<6/60 bilaterally). In all affected family members, there was a heterozygous missense mutation c. 395T>C (p. L132P) in exon 1 of the KRT12 gene, which codes for the helix-initiation motif of the K12 polypeptide. This sequence change was not found in unaffected family members or in 100 unaffected controls.

CONCLUSIONS

The Leu132Pro missense mutation is within the helix-initiation motif of the keratin and is predicted to result in a significant structural change of the K12 protein. The clinical effects are markedly more severe than the phenotype usually associated with the Arg135Thr mutation within this motif, most frequently seen in European patients with MECD.

Novel function of keratins 5 and 14 in proliferation and differentiation of stratified epithelial cells

Keratin expression in stratified epithelia is tightly regulated during squamous cell differentiation. Keratins 5 and 14 are expressed in mitotically active basal layer cells, but their function is not well defined. Reported here is the possible role of K14 in regulation of cell proliferation/differentiation in stratified epithelial cells.

Keratins are cytoplasmic intermediate filament proteins preferentially expressed by epithelial tissues in a site-specific and differentiation-dependent manner. The complex network of keratin filaments in stratified epithelia is tightly regulated during squamous cell differentiation. Keratin 14 (K14) is expressed in mitotically active basal layer cells, along with its partner keratin 5 (K5), and their expression is down-regulated as cells differentiate. Apart from the cytoprotective functions of K14, very little is known about K14 regulatory functions, since the K14 knockout mice show postnatal lethality. In this study, K14 expression was inhibited using RNA interference in cell lines derived from stratified epithelia to study the K14 functions in epithelial homeostasis. The K14 knockdown clones demonstrated substantial decreases in the levels of the K14 partner K5. These cells showed reduction in cell proliferation and delay in cell cycle progression, along with decreased phosphorylated Akt levels. K14 knockdown cells also exhibited enhanced levels of activated Notch1, involucrin, and K1. In addition, K14 knockdown AW13516 cells showed significant reduction in tumorigenicity. Our results suggest that K5 and K14 may have a role in maintenance of cell proliferation potential in the basal layer of stratified epithelia, modulating phosphatidylinositol 3-kinase/Akt–mediated cell proliferation and/or Notch1-dependent cell differentiation.

Epidermolysis Bullosa Simplex in Scotland Caused by a Spectrum of Keratin Mutations

Epidermolysis bullosa simplex (EBS) is an inherited skin disorder caused by mutations in keratins K5 (keratin 5) and K14 (keratin 14), with fragility of basal keratinocytes leading to epidermal cytolysis and blistering. Patients present with widely varying severity and are classified in three main subtypes: EBS Weber-Cockayne (EBS-WC), EBS Köbner (EBS-K), and EBS Dowling-Meara (EBS-DM), based on distribution and pattern of blisters. We could identify K5/K14 mutations in 20 out of the 43 families registered as affected by dominant EBS in Scotland; with previous studies this covers 70% of all Scottish EBS patients, making this the most comprehensively analyzed EBS population. Nine mutations are novel. All mutations lie within five previously identified rod domain hotspots and the severest blistering was associated with mutations in the helix boundary motifs. In some cases, the same mutation caused symptoms of EBS-WC and/or EBS-K, both within and between families, suggesting a contribution of additional factors to the phenotype. In some patients, no mutations were found in K5, K14, or K15, suggesting involvement of other genes. The results confirm that EBS is best considered as a single disorder with a spectrum of phenotypic variations, from severe EBS-DM at one extreme to mild EBS-WC at the other.

Novel mutations in the helix termination motif of keratin 3 and keratin 12 in 2 Taiwanese families with Meesmann corneal dystrophy

PURPOSE

To analyze mutations of the keratin 3 gene (KRT3) and keratin 12 gene (KRT12) in 2 Taiwanese families with Meesmann corneal dystrophy (MCD).

METHODS

Diagnosis of MCD was confirmed by slit-lamp examination of the cornea in 4 members of family 1 and 6 members of family 2. All exons and flanking intron boundaries of KRT3 and KRT12 were amplified by polymerase chain reaction (PCR), and products were subjected to direct sequencing. Restriction fragment length polymorphism analysis (RFLP) with created mismatch primers, Bst XI and Nsp I, was used to confirm the presence of the mutations in affected individuals in family 1 and family 2, respectively.

RESULTS

A novel heterozygous missense mutation (1508G-->C), predicting the substitution of a proline for an arginine (R503P) was detected in the helix termination motif of the keratin 3 polypeptide in family 1. Another novel heterozygous missense mutation (1286A-->G), predicting the substitution of a cysteine for a tyrosine at codon 429 (Y429C) was detected in the helix termination motif of the keratin 12 polypeptide in family 2. These 2 mutations were excluded from 50 normal controls by RFLP analysis, indicating that they were not common polymorphisms.

CONCLUSIONS

A novel missense mutation (R503P) in KRT3 and another novel missense mutation (Y429C) in KRT12 lead to MCD in 2 unrelated Taiwanese families. The mutant codons in our study are all located in the highly conserved alpha-helix-termination motif, which is essential for keratin filament assembly. Mutation at this area may account for the disruption of keratin filament assembly, leading to MCD.

Progress in epidermolysis bullosa: Genetic classification and clinical implications

Epidermolysis bullosa (EB), a heterogenous group of genodermatoses, is characterized by fragility and blistering of the skin associated with extracutaneous manifestations. Based on clinical severity, constellation of the phenotypic manifestations, and the level of tissue separation within the cutaneous basement membrane zone (BMZ), EB has been divided into distinct subcategories. Traditionally, these include the simplex, junctional, and dystrophic forms of EB, and recently attention has been drawn to hemidesmosomal variants demonstrating tissue separation at the level of the hemidesmosomes. Specific mutations in ten distinct genes expressed within the cutaneous BMZ have been delineated in >500 families with different variants of EB. The types of mutations, their positions along the affected genes, and their consequences at the mRNA and protein levels provide explanation for the phenotypic variability and genetic heterogeneity of this group of genodermatoses. Elucidation of mutations in different forms of EB has direct translational applications for improved diagnosis and molecularly based classification with prognostic implications as well as for genetic counseling and DNA-based prenatal testing in families with EB.

Keratins and skin disorders

The association of keratin mutations with genetic skin fragility disorders is now one of the best-established examples of cytoskeleton disorders. It has served as a paradigm for many other diseases and has been highly informative for the study of intermediate filaments and their associated components, in helping to understand the functions of this large family of structural proteins. The keratin diseases have shown unequivocally that, at least in the case of the epidermal keratins, a major function of intermediate filaments is to provide physical resilience for epithelial cells. This review article reflects on the variety of phenotypes arising from mutations in keratins and the reasons for this variation.

Generation and characterization of epidermolysis bullosa simplex cell lines: scratch assays show faster migration with disruptive keratin mutations

BACKGROUND

Epidermolysis bullosa simplex (EBS) is an inherited skin fragility disorder caused by mutations in keratin intermediate filament proteins. While discoveries of these mutations have increased understanding of the role of keratins and other intermediate filaments in epithelial tissues, progress towards the development of therapy for these disorders is much slower.

OBJECTIVES

Cell culture model systems that display these structural defects are needed for analysis of the cellular consequences of the mutations and to enable possible therapeutic strategies to be developed. Our aim was to generate immortalized cell lines as such model systems for the study of EBS.

METHODS

We generated a series of stable cell lines expressing EBS-associated keratin mutations, by immortalizing keratinocytes from EBS-affected skin biopsies with either simian virus 40 (SV40) T antigen or human papillomavirus 16 (HPV16) E6/E7, and assessed their keratin expression (by immunofluorescence), proliferation rates and migratory behaviour (in outgrowth and scratch wound assays).

RESULTS

Clonal immortalized keratinocyte cell lines KEB-1, KEB-2, KEB-3 (using SV40 T antigen) and KEB-4, KEB-7 and NEB-1 (using HPV16 E6/E7) were established. These include two lines from a single individual with Weber-Cockayne EBS (i.e. KEB-3 and KEB-4, mutation K14 V270M), and three cell lines from a second family, two from siblings carrying the same mutation (KEB-1, KEB-2 lines from Dowling-Meara EBS, mutation K5 E475G) and one from an unaffected relative (NEB-1). The sixth cell line (KEB-7), with a previously unreported severe mutation (K14 R125P), was the only one to show keratin aggregates in resting conditions. Despite variations in the immortalization procedure, there was no significant difference between cell lines in keratin expression, outgrowth capabilities or response to transient heat shock. However, cell migration, as measured by speed of scratch wound closure, was significantly faster in cells with severe EBS mutations.

CONCLUSIONS

These cell lines provide useful culture systems in which to assess aspects of EBS-induced cell changes. The faster migration after scratch wounding of the EBS keratinocytes may be a consequence of the known upregulation of stress-activated kinase pathways in these cells.

Observation of keratin particles showing fast bidirectional movement colocalized with microtubules

Keratin intermediate filament networks were observed in living cultured epithelial cells using the incorporation of fluorescently tagged keratin from a transfected enhanced green fluorescent protein (EGFP) construct. In steady-state conditions EGFP-keratin exists not only as readily detectable intermediate filaments, but also as small particles, of which there are two types: a less mobile population (slow or static S particles) and a highly dynamic one (fast or F particles). The dynamic F particles move around the cell very fast and in a non-random way. Their movement is composed of a series of steps, giving an overall characteristic zig-zag trajectory. The keratin particles are found all over the cell and their movement is aligned with microtubules; treatment of cells with nocodazole has an inhibitory effect on keratin particle movement, suggesting the involvement of microtubule motor proteins. Double-transfection experiments to visualize tubulin and keratin together suggest that the movement of keratin particles can be bidirectional, as particles are seen moving both towards and away from the centrosome area. Using field emission scanning and transmission electron microscopy combined with immunogold labelling, we also detected particulate keratin structures in untransfected epithelial cells, suggesting that keratin particles may be a natural component of keratin filament dynamics in living cells.

Keratin 17 mutation in pachyonychia congenita type 2 with early onset sebaceous cysts

BACKGROUND

Pachyonychia congenita (PC) is a group of autosomal dominant ectodermal dysplasias caused by mutations in four differentiation-specific keratin genes. Two major clinical subtypes of PC have been generally recognized. Symmetrically thickened fingernails and toenails are the defining characteristic of PC type 2 (PC-2) with onset at infancy. Pilosebaceous cysts are the best hallmark of PC-2, but they usually occur at puberty.

OBJECTIVES

To report a Chinese pedigree of PC-2 with unusually early onset sebaceous cysts and to explore the genetic mutation and its phenotype.

METHODS

Exon 1 of keratin 17 was amplified by polymerase chain reaction (PCR) from genomic DNA from the three patients in the pedigree, the proband, his half-sister and his younger son, two unaffected members in the pedigree and 50 unrelated and unaffected people. PCR products were directly sequenced to detect the mutation.

RESULTS

Direct sequencing of the PCR products revealed a heterozygous 275A-->G mutation in all three affected members. This mutation predicts the substitution of asparagine by serine in codon 92 (N92S) located in the 1A domain of keratin 17.

CONCLUSIONS

Mutation in the 1A domain of keratin 17 underlies the affected members' phenotype, PC-2 with early onset sebaceous cysts and late-onset thickened fingernails and toenails. The onset of the cysts is very early in some people within this family and the age at onset of thickened fingernails and toenails is variable within the family, implying the existence of modifying factors.

Keratin mutations of epidermolysis bullosa simplex alter the kinetics of stress response to osmotic shock

The intermediate filament cytoskeleton is thought to confer physical resilience on tissue cells, on the basis of extrapolations from the phenotype of cell fragility that results from mutations in skin keratins. There is a need for functional cell assays in which the impact of stress on intermediate filaments can be induced and analyzed. Using osmotic shock, we have induced cytoskeleton changes that suggest protective functions for actin and intermediate filament systems. Induction of the resulting stress response has been monitored in keratinocyte cells lines carrying K5 or K14 mutations, which are associated with varying severity of epidermolysis bullosa simplex. Cells with severe mutations were more sensitive to osmotic stress and took longer to recover from it. Their stress-activated response pathways were induced faster, as seen by early activation of JNK, ATF-2 and c-Jun. We demonstrate that the speed of a cell's response to hypotonic stress, by activation of the SAPK/JNK pathway, is correlated with the clinical severity of the mutation carried. The response to hypo-osmotic shock constitutes a discriminating stress assay to distinguish between the effects of different keratin mutations and is a potentially valuable tool in developing therapeutic strategies for keratin-based skin fragility disorders.

Beyond structure: do intermediate filaments modulate cell signalling?

Intermediate filament (IF) proteins form the largest family of cytoskeletal proteins in mammalian cells. The function of these proteins has long been thought to be only structural. However, this single function does not explain their diverse tissue- and differentiation-specific expression patterns. Evidence is now emerging that IF also act as an important framework for the modulation and control of essential cell processes, in particular, signal transduction events. Here, we review the most recent developments in this growing and exciting new field.

Characterization of early assembly intermediates of recombinant human keratins

The intermediate filaments (IFs) form major structural elements of the cytoskeleton. In vitro analyses of these fibrous proteins reveal very different assembly properties for the nuclear and cytoplasmic IF proteins. However, keratins in particular, the largest and most heterogenous group of cytoplasmic IF proteins, have been difficult to analyze due to their rapid assembly dynamics under the near-physiological conditions used for other IF proteins. We show here that keratins, like other cytoplasmic IF proteins, go through a stage of assembling into full-width soluble complexes, i.e., "unit-length filaments" (ULFs). In contrast to other IF proteins, however, longitudinal annealing of keratin ULFs into long filaments quasi-coincides with their formation. In vitro assembly of IF proteins into filaments can be initiated by an increase of the ionic strength and/or lowering of the pH of the assembly buffer. We now document that 23-mer peptides from the head domains of various IF proteins can induce filament formation even under conditions of low salt and high pH. This suggests that the "heads" are involved in the formation and longitudinal association of the ULFs. Using a Tris-buffering protocol that causes formation of soluble oligomers at pH 9, the epidermal keratins K5/14 form less regular filaments and less efficiently than the simple epithelial keratins K8/18. In sodium phosphate buffers (pH 7.5), however, K5/14 were able to form long partially unraveled filaments which compacted into extended, regular filaments upon addition of 20 mM KCl. Applying the same assembly regimen to mutant K14 R125H demonstrated that mutations causing a severe disease phenotype and morphological filament abnormalities can form long, regular filaments with surprising efficiency in vitro.

A novel keratin 5 mutation (K5V186L) in a family with EBS-K: a conservative substitution can lead to development of different disease phenotypes

Epidermolysis bullosa simplex is a hereditary skin blistering disorder caused by mutations in the KRT5 or KRT14 genes. More than 50 different mutations have been described so far. These, and reports of other keratin gene mutations, have highlighted the existence of mutation "hotspots" in keratin proteins at which sequence changes are most likely to be detrimental to protein function. Pathogenic mutations that occur outside these hotspots are usually associated with less severe disease phenotypes. We describe a novel K5 mutation (V186L) that produces a conservative amino acid change (valine to leucine) at position 18 of the 1A helix. The phenotype of this case is unexpectedly severe for the location of the mutation, which lies outside the consensus helix initiation motif mutation hotspot, and other mutations at this position have been associated in Weber--Cockayne (mild) epidermolysis bullosa simplex only. The mutation was confirmed by mismatch-allele-specific polymerase chain reaction and the entire KRT5 coding region was sequenced, but no other changes were identified. De novo K5/K14 (mutant and wild-type) filament assembly in cultured cells was studied to determine the effect of this mutation on filament polymerization and stability. A computer model of the 1A region of the K5/K14 coiled-coil was generated to visualize the structural impact of this mutation and to compare it with an analogous mutation causing mild disease. The results show a high level of concordance between genetic, cell culture and molecular modeling data, suggesting that even a conservative substitution can cause severe dysfunction in a structural protein, depending on the size and structure of the amino acid involved.

Assignment of the gene for a new hereditary nail disorder, isolated congenital nail dysplasia, to chromosome 17p13

Isolated congenital nail dysplasia is an autosomal dominant disorder recently observed in a large family from southern Germany. The disorder is characterized by longitudinal streaks, thinning, and impaired formation of the nail plates leading to increased vulnerability of the free nail margins. In most cases, all fingernails and toenails are similarly involved with some accentuation of the thumb and great toenails. Histologic changes include hypergranulosis of the nail matrix and epithelial outgrowths from the nail bed. Patients do not show any alterations of hair growth and dentition, no malfunction of sweat glands and sensory organs, and no skeletal abnormalities. Isolated congenital nail dysplasia manifests from the first year of life with variable expressivity. In order to localize chromosomally the gene underlying isolated congenital nail dysplasia, linkage to the known keratin gene cluster regions on chromosomes 12q12 and 17q21 was ruled out first. The analysis of 150 microsatellite markers on various chromosomes mapped the isolated congenital nail dysplasia gene to the 6 cM interval between markers at D17S926 and D17S1528 on chromosome 17p13. Markers at D17S849, D17S 1840, and D17S1529 co-segregated completely with the isolated congenital nail dysplasia locus. The maximum two-point LOD score was found for the marker at D17S 1840 (Zmax = 6.72 at Thetamax = 0.00). The identified region harbors no currently known genes involved in skin or nail abnormalities. Isolated congenital nail dysplasia probably represents a novel isolated defect of nail development. The localization of this gene is, therefore, the first step towards the identification of a new factor in nail formation.

Hepatocyte cytokeratins are hyperphosphorylated at multiple sites in human alcoholic hepatitis and in a mallory body mouse model

Alcoholic hepatitis (AH) is associated with cytokeratin 8 and 18 (CK8/18) accumulation as cytoplasmic inclusion bodies, termed Mallory bodies (MBs). Studies with MB mouse models and cultured hepatocytes suggested that CK8/18 hyperphosphorylation might be involved in MB formation. However, no data exist on phosphorylation of CK8/18 in human AH. In this study, antibodies that selectively recognize phosphorylated epitopes of CK8 or CK18 were used to analyze CK8/18 phosphorylation states in normal human and murine livers, human AH biopsies, and livers of 3,5-diethoxycarbonyl-1, 4-dihydrocollidine (DDC)-intoxicated mice, the last serving as model for MB induction. Hepatocyte cytokeratins become hyperphosphorylated at multiple sites in AH and in DDC-intoxicated mice. Hyperphosphorylation of CK8/18 occurred rapidly, after 1 day of DDC intoxication and preceded architectural changes of the cytoskeleton. In long-term DDC-intoxicated mice as well as in human AH, MBs preferentially contain hyperphosphorylated CK8/18 as compared with the cytoplasmic cytokeratin intermediate filament network suggesting that CK8/18 hyperphosphorylation may play a contributing role in MB pathogenesis. Furthermore, the site-specific phosphorylation of cytokeratin in different stages of MB induction provides indirect evidence for the involvement of a variety of protein kinases known to be activated in stress responses, mitosis, and apoptosis.

Novel proline substitution mutations in keratin 16 in two cases of pachyonychia congenita type 1

Pachyonychia congenita (PC) is a group of inherited ectodermal dysplasias, the characteristic phenotype being hypertrophic nail dystrophy. Two main clinical subtypes, PC-1 and PC-2, are inherited as autosomal dominant disorders, but other less well characterized clinical forms also exist. The PC-1 phenotype may be distinguished by the absence of the epidermal cysts found in PC-2, and it has been shown to be caused by mutations in either keratin K16 or its expression partner, the K6a isoform of K6. Mutations in K16 have also been shown to cause a milder related phenotype, focal non-epidermolytic palmoplantar keratoderma. Recently, we have developed a long-range polymerase chain reaction (PCR) strategy which allows specific amplification of the entire functional K16 gene (KRT16A), without amplification of the two K16 pseudogenes (psiKRT16B and psiKRT16C), enabling mutation analysis based on genomic DNA. Here, using this methodology, we describe novel mutations R127P and Q122P in the helix 1A domain of K16 in two families presenting with PC-1. Both mutations were excluded from 50 normal unrelated individuals by restriction enzyme analysis of K16 PCR fragments. In one family, ultrastructural analysis was performed, revealing distinctive tonofilament abnormalities. Specifically, keratin filament bundles were greatly condensed, but did not form the dense amorphous aggregates seen in a number of other keratin disorders. In the second kindred, autosomal dominant cataract was present in some but not all members affected by PC. As the cataract phenotype did not fully cosegregate with the K16 mutation, and given that K16 is not expressed in the lens, these two phenotypes may be coincidental.

Ontogeny and regional variability of keratin 2e (K2e) in developing human fetal skin: a unique spatial and temporal pattern of keratin expression in development

Keratin 2e (K2e) is expressed in the upper spinous and granular cells of adult epidermis. A highly specific polyclonal antibody was made against a C-terminal peptide of K2e and used to observe K2e expression at different developmental stages. At 12.5 weeks estimated gestational age (EGA) K2e was detected in trunk skin in scattered cells in the intermediate layer. At 13.5 weeks EGA, greater numbers of intermediate cells were stained with variable intensity, and staining in this pattern increased with age. Epidermal sheets from 14 weeks EGA showed that K2e + cells were excluded from developing hair follicles. At 135 days EGA, the following regional patterns were observed: in cheek, trunk, dorsal and ventral knee, elbow and dorsal hand there was moderate to intense staining of upper intermediate keratinocytes excluding cells of the hair canals and sweat ducts. The periumbilical region distinctly lacked K2e staining, while more distal areas showed increasing numbers of K2e + cells. The earliest expression of K2e was at 10 weeks EGA in the presumptive nail bed of developing digits. By 13.5 weeks EGA this pattern had shifted to the proximal nail fold, and K2e was absent in the nail bed. K2e was excluded from developing sweat glands and ducts and from developing hair follicles at the hair germ and early peg stages. By 15 weeks EGA in the fetal hair follicle small groups of cells were K2e + and by 19 weeks K2e + cells were seen at the level of the matrix. Some overlap in staining was detected for K2e with K10, and in palmar skin with K9; however, mostly the filamentous staining patterns for these keratins were distinctive. This study shows that the complex patterns of temporal and regional expression of K2e differ from known patterns for other epidermal keratins and suggest different regulation and function for this epidermal keratin.

A role for phosphorylation in the dynamics of keratin intermediate filaments

Keratins undergo highly dynamic events in the epithelial cells that express them. These dynamic changes have been associated with important cell processes. We have studied the possible role of keratin phosphorylation-dephosphorylation processes in the control of these dynamic events. Drugs that affect the protein phosphorylation metabolism (activators or inhibitors of protein kinases or protein phosphatases) have been used in two different dynamic experimental systems. First, the behaviour of keratins after the formation of cell heterokaryons, and second, the assembly of a newly synthesised keratin after transfection into the pre-existing keratin cytoskeleton. The main difference between these two systems stems on the alteration of the amount of keratin polypeptides present in the cells, since in heterokaryons this amount was unaltered whilst in transfection experiments there is an increase due to the presence of the transfected protein. We observed in both systems that the inhibition of protein kinases led to a delayed dynamic behaviour of the keratin polypeptides. On the contrary, the inhibition of protein phosphatases by okadaic acid or the activation of protein kinases by phorbol esters promoted a substantial increase in the kinetics of these processes. Biochemical studies demonstrate that this behavioural changes can be correlated with changes in the phosphorylation state of the keratin polypeptides. As a whole, present results indicate that the highly dynamic properties of the keratin polypeptides can be modulated by phosphorylation.

Characterization of a 190-kilobase pair domain of human type I hair keratin genes

Polymerase chain reaction-based screening of an arrayed human P1 artificial chromosome (PAC) library using primer pairs specific for the human type I hair keratins hHa3-II or hHa6, led to the isolation of two PAC clones, which covered 190 kilobase pairs (kbp) of genomic DNA and contained nine human type I hair keratin genes, one transcribed hair keratin pseudogene, as well as one orphan exon. The hair keratin genes are 4-7 kbp in size, exhibit intergenic distances of 5-8 kbp, and display the same direction of transcription. With one exception, all hair keratin genes are organized into 7 exons and 6 positionally conserved introns. On the basis of sequence homologies, the genes can be grouped into three subclusters of tandemly arranged genes. One subcluster harbors the highly related genes hHa1, hHa3-I, hHa3-II, and hHa4. A second subcluster of highly related genes comprises the novel genes hHa7 and hHa8, as well as pseudogene PsihHaA, while the structurally less related genes hHa6, hHa5, and hHa2 are constituents of the third subcluster. As shown by reverse transcription-polymerase chain reaction, all hair keratin genes, including the pseudogene, are expressed in the human hair follicle. The transcribed pseudogene PsihHaA contains a premature stop codon in exon 4 and exhibits aberrant pre-mRNA splicing. Evolutionary tree construction reveals an early divergence of hair keratin genes from cytokeratin genes, followed by the segregation of the genes into the three subclusters. We suspect that the 190-kbp domain contains the entire complement of human type I hair keratin genes.

A new mutation Rim3 resembling Re(den) is mapped close to retinoic acid receptor alpha (Rara) gene on mouse chromosome 11

A new mouse mutation, recombination-induced mutation 3 (Rim3), arose spontaneously in our mouse facility. This mutation exhibits corneal opacity as well as abnormal skin and hair development resembling rex denuded (Re(den)) and bareskin (Bsk). Large-scale linkage analysis with two kinds of intersubspecific backcrosses revealed that Rim3 is mapped to the distal portion of Chromosome (Chr) 11, in which Re(den) and Bsk have been located, and is very close to the retinoic acid receptor, alpha (Rara). The genes, keratin gene complex-1, acidic, gene 10, 12 (Krt1-10, 12), granulin (Grn), junctional plakoglobin (Jup) and Rara, all of which regulate growth and differentiation of epithelial cells, are genetically excluded as candidate genes for Rim3, but are clustered in the short segment on mouse Chr 11.

Isolation and chromosomal localization of a cornea-specific human keratin 12 gene and detection of four mutations in Meesmann corneal epithelial dystrophy

Keratin 12 (K12) is an intermediate-filament protein expressed specifically in corneal epithelium. Recently, we isolated K12 cDNA from a human corneal epithelial cDNA library and determined its full sequence. Herein, we present the exon-intron boundary structure and chromosomal localization of human K12. In addition, we report four K12 mutations in Meesmann corneal epithelial dystrophy (MCD), an autosomal dominant disorder characterized by intraepithelial microcysts and corneal epithelial fragility in which mutations in keratin 3 (K3) and K12 have recently been implicated. In the human K12 gene, we identified seven introns, defining eight individual exons that cover the coding sequence. Together the exons and introns span approximately 6 kb of genomic DNA. Using FISH, we found that the K12 gene mapped to 17q12, where a type I keratin cluster exists. In this study, four new K12 mutations (Arg135Gly, Arg135Ile, Tyr429Asp, and Leu140Arg) were identified in three unrelated MCD pedigrees and in one individual with MCD. All mutations were either in the highly conserved alpha-helix-initiation motif of rod domain 1A or in the alpha-helix-termination motif of rod domain 2B. These sites are essential for keratin filament assembly, suggesting that the mutations described above may be causative for MCD. Of particular interest, one of these mutations (Tyr429Asp), detected in both affected individuals in one of our pedigrees, is the first mutation to be identified within the alpha-helix-termination motif in type I keratin.

Evidence for genetic heterogeneity in monilethrix

Monilethrix is a rare inherited defect of the hair shaft resulting in hair fragility and dystrophic alopecia. In contrast to recent reports mapping monilethrix to the type II epithelial and trichocyte keratin gene cluster on 12q13, we strongly excluded these candidate genes in another family with autosomal dominant monilethrix. Moreover, there was no evidence for linkage of the disease to the keratin gene cluster on chromosome 17q12-q21, thus excluding defects in all known trichocyte and epithelial keratins as the cause of monilethrix in this family. Likewise, several other genes known to play an important role in hair shaft formation (trichohyalin and involucrin, ultra-high sulfur matrix proteins, and transglutaminases 1, 2, and 3) did not provide any evidence for linkage. Our results indicate genetic heterogeneity in monilethrix and suggest that aberrations in at least one other gene result in a similar phenotype.

Oncogenic regulation and function of keratins 8 and 18

Keratin 8 (K8) and keratin 18 (K18) are the most common and characteristic members of the large intermediate filament gene family expressed in 'simple' or single layer epithelial tissues of the body. Their persistent expression in tumor cells derived from these epithelia has led to the wide spread use of keratin monoclonal antibodies as aids in the detection and identification of carcinomas. Oncogenes which activate ras signal transduction pathways stimulate expression of the K18 gene through transcription factors including members of the AP-1 (jun and fos) and ETS families. The persistent expression of K8 and K18 may reflect the integrated transcriptional activation of such transcription factors and, in the cases of ectopic expression, an escape from the suppressive epigenetic mechanisms of DNA methylation and chromatin condensation. Comparison of the mechanisms of transcriptional control of K18 expression with expression patterns documented in both normal and pathological conditions leads to the proposal that persistent K8 and K18 expression is a reflection of the action of multiple different oncogenes converging on the nucleus through a limited number of transcription factors to then influence the expression of a large number of genes including these keratins. Furthermore, correlation of various tumor cell characteristics including invasive behavior and drug sensitivity with K8 and K18 expression has stimulated consideration of the possible functions of these proteins in both normal development and in tumorigenesis. Recent developments in the analysis of the functions of these intermediate filament proteins provide new insights into diverse functions influenced by K8 and K18.

Plectin deficiency results in muscular dystrophy with epidermolysis bullosa

We report that mutation in the gene for plectin, a cytoskeleton-membrane anchorage protein, is a cause of autosomal recessive muscular dystrophy associated with skin blistering (epidermolysis bullosa simplex). The evidence comes from absence of plectin by antibody staining in affected individuals from four families, supportive genetic analysis (localization of the human plectin gene to chromosome 8q24.13-qter and evidence for disease segregation with markers in this region) and finally the identification of a homozygous frameshift mutation detected in plectin cDNA. Absence of the large multifunctional cytoskeleton protein plectin can simultaneously account for structural failure in both muscle and skin.

The region coding for the helix termination motif and the adjacent intron 6 of the human type I hair keratin gene hHa2 contains three natural, closely spaced polymorphic sites

Mutations in distinct sites of epidermal keratins, in particular in the helix initiation and termination regions, cause human genodermatoses due to faulty intermediate filament formation. Extension of this observation to human hereditary hair and nail diseases includes population analyses of human hair keratin genes for natural sequence variations in the corresponding sites. Here we report on a large-scale genotyping of the short helix termination region (HTR) of the human type I cortical hair keratins hHa1, a3-I, and a3-II, and the cuticular hair keratin hHa2. We describe two polymorphic loci, P1 and P2, exclusively in the cuticular hHa2 gene, both creating dimorphic protein variants. P1 is due to a C to T mutation in a CpG element leading to a threonine to methionine substitution; P2 concerns a serine codon AGT that also occurs as an asparagine coding variant AAC. A third polymorphism, P3, is linked with a C to T point mutation located at the very beginning of intron 6. The three polymorphic sites are clustered in a 39-nucleotide sequence of the hHa2 gene. Both allelic frequency calculations in individuals of different races and pedigree studies indicate that the two-allelic hHa2 variants resulting from P1 and P2 occur ubiquitously in a ratio of about 1:1 (P1) and 2:1 (P2) respectively in our survey, and are clearly inherited as Mendelian traits. A genotype carrying both mutations simultaneously on one allele could not be detected in our sampling, and there was no association of a distinct allelic hHa2 variant with the known ethnic form variations of hairs. Sequence comparisons of the HTR of hHa2 with those of other type I hair keratins including the hHa2-ortholog from chimpanzee provide evidence that the P1- and P2-linked mutations must have occurred very early in human evolution and that the two P2-associated codon variants may be the result of two independent point mutations in an ancestral AGC serine codon. These data describe natural polymorphisms in the HTR of a member of the keratin multigene family.

A mutation in the mucosal keratin K4 is associated with oral white sponge nevus

White sponge nevus (WSN) is a benign autosomal dominant disorder which affects non-cornifying stratified squamous epithelia (MIM 193900) (ref. 1). Phenotypically it presents as white 'spongy' plaques (oral leukokeratoses), most commonly in the mouth but also reported in the esophagus and anogenital mucosa. Histologically, the plaques show evidence of hyperproliferation, acanthosis and tonofilament aggregation. These types of pathogenic changes are characteristic of many of the epidermal keratin disorders. Keratins are expressed in pairs by epithelial cells in a tissue and cell specific manner. The major differentiation specific keratins of the buccal mucosa, nasal, esophageal and anogenital epithelia are K4 and K13 (ref. 7). The tissue distribution and nature of the lesions in patients affected by WSN suggested that mutations in K4 and/or K13 might be responsible for this disorder. We have now confirmed this hypothesis and report here a three base-pair (bp) deletion in the helix initiation peptide of K4 in affected members from two families with this condition.

Temperature sensitivity of the keratin cytoskeleton and delayed spreading of keratinocyte lines derived from EBS patients

Point mutations in the keratin intermediate filament genes for keratin 5 or keratin 14 are known to cause hereditary skin blistering disorders such as epidermolysis bullosa simplex, in which epidermal keratinocytes are extremely fragile and the skin blisters on mild trauma. We show that in 2 phenotypically diverse cases of epidermolysis bullosa simplex, the keratin mutations result in a thermoinstability of the intermediate filament cytoskeleton which can be reproducibly demonstrated even in the presence of tissue culture-induced keratins and in conditions where filament fragility is not otherwise obvious. SV40-T antigen and HPV16 (E6--E7) immortalised keratinocyte cell lines were examined, established from control and epidermolysis bullosa simplex-affected individuals with either severe (Dowling-Meara) or mild (Weber-Cockayne) forms of the disease. In standard tissue culture conditions no significant and consistent abnormality of the keratin cytoskeleton could be demonstrated. However after thermal stress a reduced stability of the keratin filaments was demonstrable in the epidermolysis bullosa simplex cell lines, with filaments breaking into aggregates similar to those seen in skin from EBS patients. These aggregates were maximal at 15 minutes after heat shock and the filament network structure was substantially reversed by 60 minutes. Differences were also seen in the cells during respreading after replating: cells containing mutant keratins were slower to respread than controls and fine aggregates were seen at the cell margins in the Dowling-Meara derived cell line. Such delays in restoring the normal intermediate filament network after physiological processes involving cytoskeleton remodelling may render the cells vulnerable to cytolysis in vivo if physically challenged during this time window. The steady reduction in the mitotic index of the epidermis during the first few years of life could then explain the clinical improvement which is frequently observed in growing children.

Keratin 14 gene mutations in patients with epidermolysis bullosa simplex

Mutations in genes encoding the keratin intermediate filaments expressed in basal cells have been identified in some families with epidermolysis bullosa simplex as the proximate cause of the fragility. We have systematically scanned genomic sequences of one of these keratins, keratin 14, for mutations in patients from 49 apparently independent kindreds using single-strand conformation polymorphism analysis. The ten mutations identified are clustered at three sites--the ends of the helices and the L12 linker region, mutation sites that have been identified in past, more limited studies. Early onset of blistering in these ten families is correlated with more widespread distribution of lesions.

Cloning and characterization of multiple human genes and cDNAs encoding highly related type II keratin 6 isoforms

The human type II keratin 6 (K6; 56 kDa) is expressed in a heterogeneous array of epithelial tissues under normal conditions, but is better known for its strong induction in stratified epithelia that feature an enhanced cell proliferation rate or abnormal differentiation. Previous work has established the existence of two functional genes encoding K6 protein isoforms in the human genome, although only a partial cDNA clone is available for K6a, the dominant human K6 isoform in skin epithelial tissues (Tyner, A., and Fuchs, E. (1986) J. Cell Biol. 103, 1945-1955). We screened human genomic and skin cDNA libraries with probes derived from the K6b gene, and isolated clones containing the full-length gene and cDNA predicted to encode K6a. A thorough characterization of a large number of genomic (57) as well as cDNA (64) clones further revealed the existence of as many as six different human K6 protein isoforms that are highly related at the gene structure, nucleotide sequence, and predicted amino acid sequence levels. Based on the information accumulated to date we propose an evolutionary model in which the multiplicity of human K6 genes is explained by successive gene duplication events. We further demonstrate that K6a is clearly the dominant K6 isoform in skin tissue samples and cultured epithelial cell lines and that the various isoforms are differentially regulated within and between epithelial tissue types. Our findings have direct implications for an understanding of the regulation and function of K6 during hyperproliferation in stratified epithelia and the search for disease-causing mutations in K6 sequences in the human population.

Ultrastructural changes resulting from keratin-9 gene mutations in two families with epidermolytic palmoplantar keratoderma

Palmoplantar keratoderma of Voerner type (or epidermolytic palmoplantar keratoderma) is an autosomal dominant inherited disorder of keratinization with histologic features of epidermolytic hyperkeratosis. We studied members of two large unrelated kindreds with epidermolytic palmoplantar keratoderma, and biopsy specimens of lesional palmar skin from both families confirmed the histologic changes of epidermolytic hyperkeratosis. Whorls of abnormally aggregated keratin filaments were seen ultrastructurally to be associated with signs of cellular disintegration in spinous and granular cells. Direct sequencing of genomic DNA samples obtained from several members of each family established the substitution of a highly conserved arginine by tryptophan (R162W) in the 1A region of the alpha-helical rod domain of keratin 9. This arginine residue in a highly conserved region of keratins 1 and 10 is affected by disruptive missense point mutations in many patients with bullous ichthyosiform erythroderma. An equivalent position in the sole and palm restricted keratin 9 appears to be the mutation hot spot in epidermolytic palmoplantar keratoderma. To date, R162W is the most prevalent genetic defect reported in this genodermatosis.

Keratin 16 and keratin 17 mutations cause pachyonychia congenita

Pachyonychia congenita (PC) is a group of autosomal dominant disorders characterized by dystrophic nails and other ectodermal aberrations. A gene for Jackson-Lawler PC was recently mapped to the type I keratin cluster on 17q. Here, we show that a heterozygous missense mutation in the helix initiation motif of K17 (Asn92Asp) co-segregates with the disease in this kindred. We also show that Jadassohn-Lewandowsky PC is caused by a heterozygous missense mutation in the helix initiation peptide of K16 (Leu130Pro). The known expression patterns of these keratins in epidermal structures correlates with the specific abnormalities observed in each form of PC.

Intermediate filaments in disease

Intermediate filaments are major structural proteins encoded by a large multigene family. Their tissue-specific expression makes them important in studies of development, differentiation and pathology. Most intermediate filaments are keratins; recent discoveries of keratin mutations in a range of genetic skin disorders have clarified their role as providing essential structural support for cells in different physical settings.

Altered distribution of keratinization markers in epidermolytic hyperkeratosis

Epidermolytic hyperkeratosis (EH) is a genetic disorder of keratins associated with epidermal differentiation. Affected individuals carry gene mutations for conserved sequences of keratins K1 or K10. The structural alterations of tonofilaments in EH seem to be a direct consequence of the keratin gene mutations. EH epidermis, however, shows many other unexplained abnormalities including acanthosis, hypergranulosis, and hyperkeratosis. To further elucidate the pathogenetic mechanism of EH, we studied distribution patterns of other keratinization-associated molecules including involucrin, small proline-rich protein (SPRR) 1, loricrin and trichohyalin in the skin of four patients by light and electron microscopic immunohistochemistry in conjunction with conventional transmission electron microscopy. The middle to upper epidermal cells showed moderate to strong immunoreactivities to involucrin, SPRR1 and loricrin antibodies. Both intracellular staining and cell peripheral staining was seen for involucrin and SPRR1 antibodies. Loricrin labelling was prematurely associated with the plasma membrane of granular cells, possibly relating to abnormal keratin filament aggregation and cellular vacuolization. Some loricrin labelling was localized on the keratin aggregates, suggesting intermolecular associations between keratin and loricrin. Trichohyalin, hardly detectable in normal epidermis, was present in some granular and cornified cells in EH in association with keratin filaments, suggesting that it may function as an intermediate filament-associated protein. While cornified cell envelopes were intensely labelled only with loricrin antibodies in normal skin, they were immunoreactive to involucrin, SPRR1 and loricrin antibodies in EH. Sequential change in electron density of the cornified cell envelopes, a constant feature in normal skin, was often absent in EH. These results suggest an altered assembly process of cornified cell envelopes in EH.

Epidermolysis bullosa: hereditary skin fragility diseases as paradigms in cell biology

Recent research into the molecular basis of epidermolysis bullosa has provided a unique insight into a variety of mechanisms in normal cell biology, such as cell-matrix interactions, and has uncovered an excellent model for studies on keratin intermediate filaments. The simplex forms of epidermolysis bullosa are caused by mutations in the genes for the basal epidermal keratins, K5 and K14. Most mutations affect highly conserved parts of the molecules, illustrating their importance in normal keratin filament assembly and integrity. Mutations in corresponding regions of the differentiation-associated keratins, K1 and K10 can also occur in epidermolytic ichthyosis. Both recessive and dominant forms of dystrophic epidermolysis bullosa result from mutations in an anchoring fibril collagen gene, COL7A1. Junctional epidermolysis bullosa is caused by mutations in the genes encoding different chains of the novel laminin isoform, nicein/kalinin, also known as laminin 5, which is associated with the anchoring filament-hemidesmosome complex of the basement membrane zone. These recent findings strengthen the evidence for the role of nicein/kalinin and type VII collagen in adherence and stabilization of the dermo-epidermal junction.

A functional "knockout" of human keratin 14

The importance of keratins and other intermediate filaments in the maintenance of tissue structure is emphasized by the discovery that many hereditary skin-blistering diseases are caused by mutations in keratin genes. Here, we describe a situation in which keratin 14 (K14) is missing altogether in the epidermis: A homozygous 2-nucleotide deletion in exon I of the K14 gene causes premature termination of the mRNA transcripts upstream from the start of the rod domain and results in a K14 null phenotype. In this individual no keratin intermediate filaments are visible in basal epidermal cells, although filaments are present in the upper layers of the epidermis. No compensating keratin expression is detected in vivo, and K14 mRNA is down-regulated. The individual, diagnosed as Köbner (generalized) EBS, suffers from severe widespread keratinocyte fragility and blistering at many body sites, but although the phenotype is severe, it is not lethal. This K14-/- phenotype confirms that only one K14 gene is expressed in human epidermis and provides an important model system for examining the interdependence of different keratin filament systems and their associated structures in the skin.

Ichthyosis bullosa of Siemens--a disease involving keratin 2e

Ichthyosis bullosa of Siemens (IBS) is a congenital bullous ichthyosis without erythroderma. In contrast to bullous congenital ichthyosiform erythroderma (BCIE), there is a relatively mild involvement of the skin and epidermolytic hyperkeratosis (EHK) is restricted to the upper suprabasal layers of the epidermis. Tonofilament aggregation was observed by EM in suprabasal cells from affected patients in the two families under study, indicative of a keratin abnormality. Keratin 2e is a differentiation specific type II keratin expressed suprabasally in the epidermis. Part of the K2e gene was amplified by polymerase chain reaction using genomic DNA from affected and unaffected individuals from two IBS families. Direct sequencing of polymerase chain reaction products revealed a point mutation in the highly conserved helix termination motif, producing the protein sequence change LLEGEE-LLEGKE. This mutation was found in all affected members of a five-generation kindred and also in a sporadic case in a second unrelated family. No mutation was seen in unaffected individuals. The mutation destroys a MnlI restriction site, which allowed exclusion of the mutation from a population of 50 unaffected unrelated individuals by restriction fragment analysis of K2e PCR products. This is the sixth keratin gene found to be involved in an inherited epidermal disorder.