Identification of two novel mutations in keratin 13 as the cause of white sponge naevus

Keratin 4 regulates the development of human white sponge nevus

BACKGROUND

The aim of this study was to investigate the roles of keratin 4 (KRT4) gene in the development of human white sponge nevus (WSN).

METHODS

Transgenic mice were created using the microinjection method with pcDNA3.1 vectors expressing KRT4 wild-type (WT) gene and E520K mutation. Polymerase chain reaction (PCR) and Western blotting were used to identify the genotype of transgenic founders and their filial generations. Expression of KRT4 in mouse oral mucosa was characterized by immunohistochemistry (IHC), and the whole epithelium layer of transgenic mice was observed using transmission electron microscope (TEM).

RESULTS

The positive rate of KRT4 transgenic mice in F1 generation was 45.5%. Expression level of KRT4 protein was significantly higher in 2-month-old transgenic mice than WT mice. Furthermore, all the epithelial lamina of 3-month-old transgenic mice showed reduced staining of KRT4. The surface and spinous layers were full of hyalocytes and bubble cells, which are similar to the clinical symptoms of WSN. For the ultrastructure, both tonofilaments and Odland bodies increased.

CONCLUSIONS

Our study indicated the mutated KRT4 gene may play important roles in the pathogenesis of WSN.

A novel keratin 13 variant in a four-generation family with white sponge nevus

We report a novel KRT13 germ line variant that causes white sponge nevus (WSN) with mucosal dysplasia. Genital, vaginal, and cervical WSN were observed in four female patients, of whom two had premalignant cervical lesions at young age. Two of the 12 patients with oral WSN developed oral squamous cell carcinoma.

Current approaches to the diagnosis and treatment of white sponge nevus

White sponge nevus (WSN) in the oral mucosa is a rare autosomal dominant genetic disease. The involved mucosa is white or greyish, thickened, folded and spongy. The genes associated with WSN include mutant cytokeratin keratin 4 (KRT4) and keratin 13 (KRT13). In recent years, new cases of WSN and associated mutations have been reported. Here, we summarise the recent progress in our understanding of WSN, including clinical reports, genetics, animal models, treatment, pathogenic mechanisms and future directions. Gene-based diagnosis and gene therapy for WSN may become available in the near future and could provide a reference and instruction for treating other KRT-associated diseases.

Keratin 13 mutations associated with oral white sponge nevus in two Chinese families

White sponge nevus (WSN) is an autosomal dominant hereditary disease. Keratin 4 (KRT4) and Keratin 13 (KRT13) gene mutations were involved in the WSN. We recruited two WSN Chinese families, and oral lesion biopsy with hematoxylin and eosin staining showed that patients had significant pathological characteristics. The mutations of KRT4 and KRT13 gene were detected by PCR and direct sequencing. The multiple alignments of KRT13 from 23 diverse species homology analyses were performed by the ClustalW program. The KRT13 expression was measured by Real-Time RT-PCR and Western blot analysis. Sequencing analysis revealed two mutations of KRT13 gene: one mutation was 332T>C and amino acid change was Leu111Pro. Another mutation was 340C>T and amino acid change was Arg114Cys. The sequence of KRT13 was highly conserved. Real-Time RT-PCR and Western blot analysis results show that KRT13 expression level is lower in patient but keep almost no change in mRNA level. When cells were treated with MG132, KRT13 protein level was increased and kept almost the same in normal and patient cells. We identified two heritable mutations in the KRT13 gene, which were associated with the development of WSN. The abnormal degradation of KRT13 protein of WSN may probably associate with the abnormal ubiquitination process.

Glial fibrillary acidic protein mutations in infantile, juvenile, and adult forms of Alexander disease

Alexander disease is a progressive, usually fatal neurological disorder defined by the widespread and abundant presence in astrocytes of protein aggregates called Rosenthal fibers. The disease most often occurs in infants younger than 2 years and has been labeled a leukodystrophy because of an accompanying severe myelin deficit in the frontal lobes. Later onset forms have also been recognized based on the presence of abundant Rosenthal fibers. In these cases, clinical signs and pathology can be quite different from the infantile form, raising the question whether they share the same underlying cause. Recently, we and others have found pathogenic, de novo missense mutations in the glial fibrillary acidic protein gene in most infantile patients examined and in a few later onset patients. To obtain further information about the role of glial fibrillary acidic protein mutations in Alexander disease, we analyzed 41 new patients and another 3 previously described clinically, including 18 later onset patients. Our results show that dominant missense glial fibrillary acidic protein mutations account for nearly all forms of this disorder. They also significantly expand the catalog of responsible mutations, verify the value of magnetic resonance imaging diagnosis, indicate an unexpected male predominance for the juvenile form, and provide insights into phenotype-genotype relations.

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.

Constitutional mutation of keratin 13 gene in familial white sponge nevus

OBJECTIVE

We sought to investigate a novel mutation in the keratin genes assumed to be responsible for a familial case of oral white sponge nevus.

PATIENTS AND METHODS

The affected family consisted of a 36-year-old woman, her 17-year-old daughter, and her 14-year-old son. Keratin 4 and 13 genes extracted from venous blood lymphocytes were amplified by using the polymerase chain reaction and directly sequenced.

RESULTS

Sequencing analysis of the 3 patients revealed the presence of a novel heterozygous T-to-C transition mutation in exon 1 of the keratin 13 gene, with no abnormalities detected in the keratin 4 gene.

CONCLUSION

We identified a novel heterozygous missense mutation at 332T>C in the keratin 13 gene believed to be related to the development of white sponge nevus.

The molecular genetics of keratin disorders

Keratins are the type I and II intermediate filament proteins which form a cytoskeletal network within all epithelial cells. They are expressed in pairs in a tissue- and differentiation-specific fashion. Epidermolysis bullosa simplex (EBS) was the first human disorder to be associated with keratin mutations. The abnormal keratin filament aggregates observed in basal cell keratinocytes of some EBS patients are composed of keratins K5 and K14. Dominant mutations in the genes encoding these proteins were shown to disrupt the keratin filament cytoskeleton resulting in cells that are less resilient and blister with mild physical trauma. Identification of mutations in other keratin genes soon followed with attention focussed on disorders showing abnormal clumping of keratin filaments in specific cells. For example, in bullous congenital ichthyosiform erythroderma, clumping of filaments in the suprabasal cells led to the identification of mutations in the suprabasal keratins, K1 and K10. Mutations have now been identified in 18 keratins, all of which produce a fragile cell phenotype. These include ichthyosis bullosa of Siemens (K2e), epidermolytic palmoplantar keratoderma (K1, K9), pachyonychia congenita (K6a, K6b, K16, K17), white sponge nevus (K4, K13), Meesmann's corneal dystrophy (K3, K12), cryptogenic cirrhosis (K8, K18) and monilethrix (hHb6, hHb1).In general, these disorders are inherited as autosomal dominant traits and the mutations act in a dominant-negative manner. Therefore, treatment in the form of gene therapy is difficult, as the mutant gene needs to be inactivated. Ways of achieving this are actively being studied. Reliable mutation detection methods from genomic DNA are now available. This enables rapid screening of patients for keratin mutations. For some of the more severe phenotypes, prenatal diagnosis may be requested and this can now be performed from chorionic villus samples at an early stage of the pregnancy. This review article describes the discovery of, to date, mutations in 18 keratin genes associated with inherited human diseases.

A novel mutation in the keratin 4 gene causing white sponge naevus

BACKGROUND

White sponge naevus (WSN) is a rare, autosomal dominant disorder that predominantly affects noncornified stratified squamous epithelia, most commonly the buccal mucosa. Clinically, WSN manifests as thickened spongy mucosa with a white opalescent tint in the mouth and may be confused with other disorders that cause white lesions on oral mucosa. Recent studies have identified pathogenic mutations in KRT4 and KRT13, the genes encoding mucosa-specific keratins, in WSN.

OBJECTIVES

To search for possible mutations in KRT4 and KRT13.

METHODS

We report a case of WSN in a young man who presented with diffuse irregular whitish plaques involving the buccal and gingival mucosae and the tongue. Results Pathologically, the affected mucosa showed epithelial thickening, parakeratosis and extensive vacuolization of the suprabasal keratinocytes. Mutation analysis revealed a heterozygous missense mutation 1345G-->A in KRT4, predicting an amino acid change, E449K, in the 2B domain of the K4 polypeptide.

CONCLUSIONS

We report the first mutation analysis of a Taiwanese patient with WSN. Potentially this novel mutation could disrupt the stability of keratin filaments and result in WSN.

GFAP mutations in Alexander disease

Alexander disease is a rare but often fatal disease of the central nervous system. Infantile, juvenile and adult forms have been described that present with different clinical signs, but are unified by the characteristic presence in astrocytes of Rosenthal fibers-protein aggregates that contain glial fibrillary acidic protein (GFAP) and small stress proteins. The chance discovery that mice expressing a human GFAP transgene formed abundant Rosenthal fibers suggested that mutations in the GFAP gene are a cause of Alexander disease. Sequencing results from several laboratories have indeed now identified GFAP coding mutations in most cases of the disease, including both the infantile and juvenile forms. These mutations have been found in the 1A, 2A and 2B segments of the conserved central rod domain of GFAP, and also in the variable tail region. All changes detected are heterozygous missense mutations, and none has been found in any parent of a patient that has been tested. This indicates that most cases of Alexander disease arise through de novo, dominant, GFAP mutations. Many of these mutations are homologous to ones described in other intermediate filament diseases. These other diseases have been attributed to a dominant loss of function, as the intermediate filament network is usually disrupted and a similar phenotype is observed in mice in which the corresponding intermediate filament gene has been inactivated. However, astrocytes of Alexander disease patients have normal appearing intermediate filaments, and GFAP null mice do not display the symptoms or pathology of Alexander disease. Thus, Alexander disease likely results from a dominant gain of function. Drawing upon the homology of many of the Alexander disease mutations to those found in other intermediate filament diseases, it is suggested that the gain of function is due to a partial block of filament assembly that leads to accumulation of an intermediate that participates in toxic interactions.

A novel mutation in the keratin 13 gene causing oral white sponge nevus

White sponge nevus (WSN) is an autosomal-dominantly inherited form of mucosal leukokeratosis. Defects in keratins, proteins that form the stress-bearing cytoskeleton in epithelia, have been shown to cause several epithelial fragility disorders. Recently, mutations in the genes encoding mucosal-specific keratins K4 and K13 were shown to be the underlying cause of WSN. We have studied a large Scottish family with 19 persons affected by WSN in four generations. The K4 locus was excluded by genetic linkage analysis; however, genetic linkage consistent with a K13 defect was obtained. Subsequently, a heterozygous missense mutation 335A>G was detected in exon 1 of the KRT13 gene, predicting the amino acid change N112S in the 1A domain of the K13 polypeptide. The mutation was confirmed in affected family members and was excluded from 50 unaffected people by restriction enzyme analysis. These results confirm that mucosal keratin defects are the cause of WSN.