Abstract
White sponge nevus (WSN) is a rare autosomal dominant disorder characterized by white plaques of oral mucosa; it is benign condition with no effective treatment. The disorder usually manifests during early childhood or adolescence. Mutations of keratin 4 or 13 gene have been identified as causing WSN. The aim of this study is to determine whether keratin 4 or 13 gene mutation was the molecular basis of WSN in a Japanese family. The proband in this family was an 11-year-old boy, with three other people affected by WSN. Genomic DNA was extracted from two affected members and an unaffected member. Segments of keratin 4 and 13 genes were amplified by PCR, and direct DNA sequencing was carried out. Sequence analysis revealed a heterozygous 3 bp deletion (N160Del) localized in the helix-initiation motif at the beginning of alpha-helical domain 1A of keratin 4 gene from affected members. One member lacking the phenotype was genetically tested normal. The authors identified a mutation of the keratin 4 gene recurrent in a family affected by WSN. Further investigation of the multifunctional role of keratin genes is warranted in the group of inherited epithelial disorders that may result in identification of effective treatment for this genetic disease.
White sponge nevus (WSN; OMIM 193900 ) is a rare autosomal dominant disorder characterized by painless, white, and ‘spongy’ plaques of oral mucosa. This disorder predominantly affects the oral mucosa, and less frequently, other mucosal sites including the nasal, esophageal, laryngeal, vaginal and anal mucosa. The most commonly affected site of the oral cavity is the buccal mucosa. Clinically, it is characterized by bilateral white plaques of the buccal mucosa, and the cornified surface may peel away from the underlying tissue. In many cases, it is a painless disorder and may remain asymptomatic throughout life. Painful and burning symptoms may occur due to secondary infections following microbial colonization of the surface epithelium. It is thought to be a benign disorder, and there is no effective treatment. The disorder usually manifests during infancy or childhood, and it may be difficult to differentiate from mucosal yeast infections or other mucosal diseases. Recent studies have reported that mutations of the keratin 4 or keratin 13 genes cause WSN. Keratins are differentially expressed during epidermal differentiation, in a stratum-specific and tissue-specific manner. The suprabasal keratinocytes of the buccal, nasal, esophageal mucosa, and anogenital epithelia specifically express keratin 4 and keratin 13.
The objective of this study is to determine whether the keratin 4 or 13 gene mutation was the molecular basis of WSN in three generations of a Japanese family.
Case report
The proband in this family was a 11-year-old Japanese boy, who was affected with white plaques of the right and left buccal mucosa ( Figs 1-III:1 and 2A ). His mother (39 years) was provisionally diagnosed with white sponge nevus in the Oral and Maxillofacial unit at Okazaki City Hospital, Japan at the age of 28 years ( Figs 1-II:4 and 2B ). The onset of her white lesions occurred when she was a junior high school student. She could recollect receiving several topical treatments but none was effective in resolving the disorder. She noticed similar lesions in her two children, at the age of 4 years in the proband and at 2 years in the younger son, and referred them to the Okazaki City Hospital. Based on the clinical and family history, their lesions were diagnosed as WSN. The authors observed this family regularly, but there was no evidence of the disease moving to another site such as the nails or conjunctiva. The proband had more prominent white lesions bilaterally on the buccal mucosa as well as on the lateral margin of the tongue. His brother had a typical white sponge like lesions on the buccal mucosa bilaterally, but his tongue was not affected ( Figs 1-III:2 and 2C ). His grandmother had episodes of oral mucosal lesions during adulthood (data not shown).
DNA extraction and direct sequencing
Venous blood samples were obtained for DNA studies from two affected members and one unaffected member of the family ( Fig. 1-II:3, II:4 and III:1 ). Ethical approval for genetic studies was obtained from the Review Board at the Okazaki City Hospital, Japan, and written consent was obtained from the members of the family. Genomic DNA was extracted from peripheral blood lymphocytes. DNA samples were amplified at segments of keratin 4 and keratin 13 genes by the polymerase chain reaction (PCR) method. The products were resolved by electrophoresis on 1.2% agarose gel and purified by ethanol precipitation. The products were directly sequenced using the Big Dye Termination Reaction Kit on an ABI PRISM 370 DNA sequencer (Applied Biosystems, Foster, CA, USA).
Results
The presence of 4 affected members in the family of 8 was consistent with a typical autosomal dominant transmitted disorder ( Fig. 1 ). A biopsy specimen taken from the white plaque regions of the buccal mucosa of the proband’s mother revealed a marked epithelial hyperplasia, parakeratosis, intracellular oedema and vacuolization of the spinous cells, and thickening of cell membranes of the upper third of the epithelium ( Fig. 3 ). The diagnosis of WSN was supported by the family history and the clinical and histopathological findings.
The authors found a heterozygous 3 bp deletion (N160Del) localized in the exon 1 of the keratin 4 gene ( Fig. 4 ), which results in the removal of an asparagine residue from the predicted amino-acid sequence in the helix-initiation motif at the beginning of the alpha-helical domain 1A. An identical deleted variant has been reported in WSN patients and showed a remarkable degree of conservation among different keratins of the same type, indicating its functional importance. In contrast, no mutation was found in the keratin 13 gene. The mutation was also not found in the DNA of one of the unaffected members ( Fig. 1 -II:3) examined, suggesting the causative nature of the mutation detected in the proband and other affected members of the family.
