Keratin 13 point mutation underlies the hereditary mucosal epithelial disorder white sponge nevus.

Although pathogenic keratin mutations have been well characterized in inherited epidermal disorders, analogous defects in keratins expressed in non-epidermal epithelia have yet to be described. White sponge nevus (WSN) is a rare autosomal dominant disorder of non-cornifying squamous epithelial differentiation that presents clinically as bilateral white, soft, thick plaques of the oral mucosa. Less frequently the mucous membranes of the nose, esophagus, genitalia and rectum are involved. Histopathological features, including epithelial thickening, parakeratosis, extensive vacuolization of the suprabasal keratinocytes and compact aggregates of keratin intermediate filaments (KIF) in the upper spinous layers, resemble those found in epidermal disorders due to keratin defects. We analysed a multigenerational family with WSN and found cosegregation of the disease with the keratin gene cluster on chromosome 17. We identified a missense mutation in one allele of keratin 13 that leads to proline substitution for a conserved leucine. The mutation occurred within the conserved 1A region of the helical rod domain, which is critical for KIF stability and is the site of most pathogenic keratin mutations. This mutation enlarges the spectrum of keratins with disease-causing defects to include mucosally expressed keratin 13, and extends the known keratin diseases to disorders of non-cornifying stratified squamous epithelia.

Mutations in the gene for transglutaminase 1 in autosomal recessive lamellar ichthyosis.

We recently mapped the disease locus for severe autosomal recessive lamellar ichthyosis (LI) to chromosome 14q11 and showed complete linkage with TGM1, the gene encoding transglutaminase 1. We have now identified point mutations in TGM1 in two of the multiplex LI families used in the linkage study. Each nucleotide change causes a non-conservative amino acid substitution of histidine for one of two adjacent arginine residues in exon 3 of the gene (Arg141His, Arg142His). Within the transglutaminase family, these arginines are invariant within a conserved region, distant from the catalytic site of the enzyme. We hypothesize that these mutations adversely affect formation of crosslinks essential in production of cornified cell envelopes and a normal stratum corneum layer of the skin.

Mutations in the human connexin gene GJB3 cause erythrokeratodermia variabilis.

Erythrokeratodermia variabilis (EKV, OMIM 133200) is an autosomal dominant genodermatosis with considerable intra- and interfamilial variability. It has a disfiguring phenotype characterized by the independent occurrence of two morphologic features: transient figurate red patches and localized or generalized hyperkeratosis. Both features can be triggered by external factors such as trauma to the skin. After initial linkage to the RH locus on 1p, EKV was mapped to an interval of 2.6 cM on 1p34-p35, and a candidate gene (GJA4) encoding the gap junction protein alpha-4 (connexin 31, Cx31) was excluded by sequence analysis. Evidence in mouse suggesting that the EKV region harbours a cluster of epidermally expressed connexin genes led us to characterize the human homologues of GJB3 (encoding Cx31) and GJB5 (encoding Cx31.1). GJB3, GJB5 and GJA4 were localized to a 1.1-Mb YAC in the candidate interval. We detected heterozygous missense mutations in GJB3 in four EKV families leading to substitution of a conserved glycine by charged residues (G12R and G12D), or change of a cysteine (C86S). These mutations are predicted to interfere with normal Cx31 structure and function, possibly due to a dominant inhibitory effect. Our results implicate Cx31 in the pathogenesis of EKV, and provide evidence that intercellular communication mediated by Cx31 is crucial for epidermal differentiation and response to external factors.

Linkage of epidermolytic hyperkeratosis to the type II keratin gene cluster on chromosome 12q.

We investigated the molecular genetics of epidermolytic hyperkeratosis (EHK), a dominant disorder characterized by epidermal blistering, hyperkeratosis, vacuolar degeneration and clumping of keratin filaments. Based on this pathology, we have excluded by linkage analysis several candidate genes for the disease; in contrast, complete linkage was obtained with the type II keratin, K1, on 12q11-q13. Linkage in this region of chromosome 12 was confirmed using several other markers, and multi-locus linkage analyses further supported this location. Keratins are excellent EHK gene candidates since their expression is specific to the suprabasal epidermal layers. In the pedigree studied here, a type II keratin gene, very probably K1, is implicated as the site of the molecular defect causing EHK.

Mutation of a new gene causes a unique form of Hermansky-Pudlak syndrome in a genetic isolate of central Puerto Rico.

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder characterized by oculocutaneous albinism and a storage pool deficiency due to an absence of platelet dense bodies. Lysosomal ceroid lipofuscinosis, pulmonary fibrosis and granulomatous colitis are occasional manifestations of the disease. HPS occurs with a frequency of one in 1800 in north-west Puerto Rico due to a founder effect. Several non-Puerto Rican patients also have mutations in HPS1, which produces a protein of unknown function. Another gene, ADTB3A, causes HPS in the pearl mouse and in two brothers with HPS-2 (refs. 11,12). ADTB3A encodes a coat protein involved in vesicle formation, implicating HPS as a disorder of membrane trafficking. We sought to identify other HPS-causing genes. Using homozygosity mapping on pooled DNA of 6 families from central Puerto Rico, we localized a new HPS susceptibility gene to a 1.6-cM interval on chromosome 3q24. The gene, HPS3, has 17 exons, and a putative 113.7-kD product expected to reveal how new vesicles form in specialized cells. The homozygous, disease-causing mutation is a large deletion and represents the second example of a founder mutation causing HPS on the small island of Puerto Rico. We also present an allele-specific assay for diagnosing individuals heterozygous or homozygous for this mutation.

Novel transglutaminase-1 mutations and genotype-phenotype investigations of 104 patients with autosomal recessive congenital ichthyosis in the USA.

BACKGROUND: Autosomal recessive congenital ichthyosis (ARCI) is a rare hereditary disorder of cornification. Mutations in the transglutaminase-1 (TGM1) gene, which encodes for the epidermal enzyme transglutaminase-1 (TGase-1), are one of the causes of ARCI. METHODS: The TGM1 mutation spectrum was characterised and genotype-phenotype correlations investigated in 104 patients with ARCI ascertained through the National Registry for Ichthyosis and Related Disorders in the USA. Methods: Germline mutations in TGM1 were identified in 55% (57/104) of patients with ARCI. Arginine residues in TGase-1 were mutated in 39% (22/57) of patients overall and 54% (20/37) of those with missense mutations. In total, 55% (12/22) of missense mutations were within CpG dinucleotides and 92% (11/12) of these mutations were C-->T or G-->A transitions. The genotype-phenotype investigation found that ARCI with TGM1 mutations was significantly associated with presence of collodion membrane at birth (p = 0.006), ectropion (p = 0.001), plate-like scales (p = 0.005) and alopecia (p = 0.001). Patients who had at least one mutation predicted to truncate TGase-1 were more likely to have more severe hypohidrosis (p = 0.001) and overheating (p = 0.0007) at onset of symptoms than were those with exclusively TGM1 missense mutations. A logistic model was developed, which predicted that individuals with collodion membrane, alopecia and/or eye problems are about four times more likely to have TGM1 mutations than patients without these findings. CONCLUSION: This is the largest investigation of patients with ARCI to date. It expands the TGM1 mutation spectrum and confirms that despite genetic and phenotypic heterogeneity in ARCI, TGM1 is the main causative gene for this disorder. The high frequency of mutated arginine codons in TGM1 may be due to the deamination of CpG dinucleotides.

Genetic skin diseases caused by mutations in keratin intermediate filaments.

Keratin intermediate filaments are the major differentiation products of epithelial cells such as the epidermis. The filaments are highly dynamic entities involved in the maintenance of the structural integrity of both the individual cells and the entire tissue. Recent biochemical studies suggest that the keratin proteins overlap each other in several key locations when packed together in filaments. Interestingly, mutations that introduce inappropriate amino acid substitutions in at least some of these overlap regions cause defective keratin filaments that result in at least three classes of autosomal dominant skin disease.

Mutations in the human homologue of the Drosophila patched gene in Caucasian and African-American nevoid basal cell carcinoma syndrome patients.

The nevoid basal cell carcinoma syndrome (NBCCS), or Gorlin syndrome, is a multisystem autosomal dominant disorder. The salient features of this syndrome include multiple basal cell carcinomas, palmar and/or plantar pits, odontogenic keratocysts, skeletal and developmental anomalies, and ectopic calcification. Other features include such tumors as ovarian fibromas and medulloblastomas. There is extensive interfamilial as well as intrafamilial variability with respect to the manifestation and severity of the phenotype. Alterations in the human homologue (PTCH) of the Drosophila segment polarity gene patched have been identified in NBCCS patients as well as tumors associated with this syndrome. We report several mutations in this gene in NBCCS patients and present the clinical phenotypes of the individuals in whom these mutations were identified.

Genetic epidemiologic studies in the etiology of skin diseases.

Genetic epidemiologic studies have provided critical insights into the etiology of both rare and common skin diseases. Designs for these studies are distinct from those generally employed in epidemiologic studies. Here, we review the types of data collected for various genetic epidemiologic designs, inherent strengths and weaknesses, and their similarities to more classic epidemiologic methods. Examples from the study of skin diseases are provided to highlight the successful application of these methods.

Epidermolytic hyperkeratosis: applied molecular genetics.

Epidermolytic hyperkeratosis is an autosomal dominant ichthyosis characterized by blistering, especially at birth and during childhood, and hyperkeratosis. Epidermolytic hyperkeratosis presents striking clinical heterogeneity, particularly between families. Several avenues of research have implicated an abnormality of epidermal differentiation in the pathogenesis of this disease. In a three-generation family with 20 affected individuals, we tested a variety of candidate loci and identified linkage to the type II keratin region on chromosome 12. Further investigation revealed a mutation in the H1 subdomain of the keratin 1 gene as the cause of EHK in this family. Because keratin 10 is the co-expressed partner of keratin 1, it was not surprising when abnormalities in keratin 10 were found in other families with EHK. We have examined 52 patients from 21 families and have identified at least six clinical phenotypes. The most useful distinguishing feature was the presence or absence of severe hyperkeratosis of the palms and soles. We and others are continuing to search for and characterize mutations in keratin 1 and 10 in patients with epidermolytic hyperkeratosis. Correlation of the clinical disease types with the specific mutations should lead to a better understanding of the relationship between keratin structure and function in normal and diseased epidermis.

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.

Congenital recessive ichthyosis unlinked to loci for epidermal transglutaminases.

Congenital recessive ichthyosis has a broad range of clinical presentations, which may be considered a spectrum of phenotypes with classic lamellar ichthyosis at one pole and classic congenital ichthyosiform erythroderma at the other. The identification of mutations in the transglutaminase-1 gene as a cause of lamellar ichthyosis implicates transglutaminases in other congenital recessive ichthyoses. We investigated two multiplex families with clinical manifestations between the two poles for linkage to the transglutaminase-1 locus on chromosome 14. Strongly negative lod scores prompted a search for linkage to two other epidermally expressed transglutaminases, transglutaminase-2 and transglutaminase-3, on chromosome 20. No evidence for linkage was found. These data confirm the hypothesis that the congenital recessive ichthyoses are genetically heterogeneous and in two families exclude two other transglutaminases that could be considered as candidate loci for at least some of the nonlamellar recessive ichthyoses.

Fine mapping of the locus for nevoid basal cell carcinoma syndrome on chromosome 9q.

The nevoid basal cell carcinoma syndrome is an autosomal dominant disorder characterized primarily by multiple basal cell carcinomas, odontogenic keratocysts, and pits of the palms and soles. Tumor deletion studies and linkage analysis in Caucasians have revealed that the gene is on chromosome 9q. To further refine the location of the nevoid basal cell carcinoma syndrome locus, we tested linkage to this region in three families. Evaluation of recombinants suggested that the nevoid basal cell carcinoma syndrome locus lies in the interval defined distally by D9S127. Our data, together with existing published data defining D9S12 as a proximal flanking marker, refine the location of nevoid basal cell carcinoma syndrome to an 8.3-cM interval. Two of the families studied were African-American and show a notable variation in phenotypic expression in which affected individuals developed few skin cancers. However, despite clinical heterogeneity, our data are consistent with the hypothesis that the same locus is involved in these African-American families.

Hailey-Hailey disease maps to a 5 cM interval on chromosome 3q21-q24.

Hailey-Hailey disease (HHD) is a rare autosomal dominant genodermatosis characterized by disturbed keratinocyte adhesion. The disease has recently been mapped to a 14 cM region on chromosome 3q. We have further refined the location of the HHD gene by linkage analysis in six HHD families from Germany and Italy using 11 polymorphic microsatellite markers and found no evidence for genetic heterogeneity. We observed complete cosegregation between HHD and marker D3S1587, with a maximal lod score of 4.54. Detailed haplotype analyses allowed us to narrow the interval containing the HHD locus to 5 cM, flanked by D3S1589 and D3S1290.

Sjögren-Larsson syndrome is caused by a common mutation in northern European and Swedish patients.

Sjögren-Larsson syndrome (SLS) is an autosomal recessive disorder characterized by congenital ichthyosis, mental retardation, and spastic diplegia or tetraplegia. Patients with SLS have deficient activity of fatty aldehyde dehydrogenase (FALDH), an enzyme involved in long-chain fatty alcohol oxidation. The cDNA encoding FALDH has recently been cloned and several different mutations have been found in SLS patients. We have now identified a point mutation (C943 --> T) in 7 of 19 kindreds of European descent, accounting for 24% of the SLS alleles. The C943T mutation was only found in patients of northern European ancestry from Sweden, the Netherlands, Germany, and Belgium. Haplotype analysis suggested that the patients carrying the C943T allele were distantly related. All four Swedish patients were homozygous for C943T, indicating that this mutation is probably the major cause of SLS in the inbred Swedish families. The mutation leads to the substitution of serine for the highly conserved proline 315 in the FALDH protein, and expression studies confirm that it destroys enzymatic activity. The mutation was readily detected with an MnlI restriction enzyme digestion test. The finding that C943T is a common SLS mutation in northern European and Swedish patients affords a rapid simple method for diagnosing SLS by screening patients for this mutation with DNA-based methods.

A mutation in the V1 end domain of keratin 1 in non-epidermolytic palmar-plantar keratoderma.

Mutations in keratin 9 have been found in families with an epidermolytic form of palmar-plantar keratoderma (PPK). In another form of PPK (Unna-Thost type), epidermolysis is not observed histologically. We studied a pedigree with this non-epidermolytic form of PPK. By gene linkage analysis, the type I keratin locus could be excluded but complete linkage with the type II keratin region was found. Sequence analysis identified a single base change in the amino-terminal V1 variable subdomain of keratin 1, which caused a lysine to isoleucine substitution. This non-conservative mutation completely cosegregated with the disease and was not observed in 50 unrelated unaffected individuals. An examination of keratin amino-terminal sequences revealed a previously unreported 22-residue window in the V1 subdomain that is conserved among most type II keratins. The altered lysine is an invariant residue in this conserved sequence. Previously described keratin mutations affect the central regions important for filament assembly and stability, and cause diseases characterized by cellular degeneration or disruption. This is the first disease mutation in a keratin chain variable end region. The observation that it is not associated with epidermolysis supports the concept that the amino-terminal domain of keratins may be involved in supramolecular interactions of keratin filaments rather than stability. Therefore, hyperkeratosis associated with this mutation may be due to perturbations in the interactions of the keratin end domain with other cellular components.

Mutations in the H1 and 1A domains in the keratin 1 gene in epidermolytic hyperkeratosis.

In the autosomal dominant disorder epidermolytic hyperkeratosis, the structural integrity of the keratin intermediate filaments is altered in the suprabasal layers of the epidermis. We and others have used genetic linkage studies and mutation analysis to establish that single amino acid substitutions in either the keratin 1 or keratin 10 chains can cause epidermolytic hyperkeratosis. However, a larger database of mutations is required to better understand the relationship between specific mutations in these keratin chains and their effect on keratin filament structure. A larger database will also provide a catalog that may be useful for genetic counseling purposes. In this paper, we report the identification of three new mutations of the keratin 1 chain of epidermolytic hyperkeratosis probands in highly conserved residues in the H1 or beginning of the 1A rod domain segments. These correspond to regions involved in molecular overlaps between neighboring molecules in keratin filaments. Using an in vitro assay, synthetic peptides bearing these substitutions show diminished capacity to disassemble preformed filaments in vitro in comparison to the wild type peptides. Moreover, analyses of all mutations in epidermolytic hyperkeratosis known to date demonstrate remarkable clustering in the molecular overlap region. We conclude that non-conservative substitutions in the overlap region are likely to interfere with normal keratin filament structure and function, leading to pathology.

A deletion mutation in COL17A1 in five Austrian families with generalized atrophic benign epidermolysis bullosa represents propagation of an ancestral allele.

Patients with generalized atrophic benign epidermolysis bullosa, a usually nonlethal form of junctional epidermolysis bullosa, have generalized blistering, nail dystrophy, patchy alopecia, and dental abnormalities. Skin fragility in most cases is due to mutations in the gene encoding type XVII collagen (COL17A1). Recently, we reported five Austrian families with generalized atrophic benign epidermolysis bullosa who share the same COL17A1 mutation. Affected individuals in three families are homozygous for 4003delTC, whereas those in two others are compound heterozygotes. To determine if the occurrence of 4003delTC in these unrelated families signifies propagation of an ancestral allele or a mutational hot spot, haplotypes were determined for polymorphisms both within and flanking COL17A1. Five intragenic polymorphisms were chosen based on their informativeness. One of these, not previously reported, was 2988 A or C that introduces a new restriction site for Eco0109 I. All the 4003delTC alleles showed the same haplotype for these five polymorphic markers. Fourteen microsatellite polymorphisms were selected based on their high heterozygosity and their location within 10q23-q25 near COL17A1. Three families shared microsatellite polymorphisms covering at most 19 cM, whereas the others shared smaller regions consistent with cross-over events during passage of this mutation through several generations. These results indicate that 4003delTC occurs on a single ancestral allele.

Linkage studies in erythrokeratodermias: fine mapping, genetic heterogeneity and analysis of candidate genes.

Erythrokeratodermias are a clinically heterogeneous group of rare autosomal dominant disorders of cornification with overlapping features including hyperkeratosis and erythema. We ascertained five extended pedigrees with different phenotypes for a linkage study. Three families presented with localized erythrokeratodermia variabilis, and one with erythrokeratodermia and ataxia. Another family had Greither disease associated with variable hyperkeratotic plaques. Despite their phenotypic differences, both erythrokeratodermia variabilis and erythrokeratodermia with ataxia map to a common region in 1p34-p35. Multipoint linkage and haplotype analyses place erythrokeratodermia variabilis between the marker D1S496 and D1S186 with a maximum LOD score of 12.88. Our linkage results provide compelling evidence for genetic homogeneity among families of mixed European and French-Canadian origin. In contrast, results excluded Greither's disease from the established erythrokeratodermia variabilis gene region indicating genetic heterogeneity of erythrokeratodermias. Based on recombinations, two genes assigned to 1p34-p35 were excluded: cartilage matrix protein and avian myelocytosis viral oncogene. Connexin-37 (GJA4), a member of the connexin gene family, maps within the erythrokeratodermia variabilis region and is an attractive candidate gene. Direct sequencing of the coding region of GJA4 in four patients revealed several variations, including a novel polymorphism within the 5' cytoplasmic domain, but no pathogenic mutations were found, thus excluding Connexin-37 as a candidate. There is evidence, however, that other epidermally expressed connexins cluster in this region, and one may yet be determined to play a role in the pathogenesis of erythrokeratodermia variabilis.

Fine mapping of the Darier's disease locus on chromosome 12q.

Darier's disease (DD) is an autosomal dominant genodermatosis characterized by epidermal acantholysis and dyskeratosis. We have performed genetic linkage studies in 10 families with DD (34 affected) by analyzing 14 polymorphic microsatellite markers. Our results confirm recent reports mapping the DD gene to chromosome 12q23-q24.1. Haplotype analysis of recombinant chromosomes in our families, along with previously reported data, narrow the location of the DD gene to a 5 cM interval flanked by the loci D12S354 and D12S84/D12S105. This localization allowed exclusion of two known genes, PLA2A and PAH, as candidate loci for DD. Three other gene loci (PPP1C, PMCH, PMCA1), mapping in 12q21-q24, remain potential candidates.