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We report here two unrelated families in Japan and Korea having patients with a unique type of epidermolysis bullosa simplex and a novel mutation in the keratin gene KRT5, i.e., a frameshift and delayed stop codon inconsistent with any subtype described before. The patients showed migratory circinate erythema and multiple vesicles on the circular belt-like areas affected by erythema. Electron microscopy of skin biopsies showed a reduction in the number of keratin intermediate filaments in the basal cells without tonofilament clumping. We identified a novel heterozygous deletion mutation (1649delG of KRT5) in both cases. This deletion is predicted to produce a mutant keratin 5 protein with a frameshift of its terminal 41 amino acids and 35 amino acids longer than the wild-type keratin 5 protein due to a delayed termination codon. As the same abnormal elongated mutant KRT5 gene was found in the independent families, the predicted abnormal elongated keratin protein is likely to lead to an atypical clinical phenotype that has never been reported, possibly by interfering with the functional interaction between keratin and its associated proteins.

Hair follicles cycle between stages of growth (anagen) and metabolic quiescence (telogen) throughout life. In mature follicles, transition from telogen back into anagen involves the activation, proliferation, and differentiation of epithelial stem cells located in the bulge, a specialization of the outer root sheath. Recent studies identified keratin 6a (K6a) transcripts as enriched in bulge epithelial stem cells in mouse skin. We used messenger RNA probes, antibodies, a LacZ reporter mouse model, and whole-mount staining assays to investigate the regulation of mK6a during mouse postnatal hair cycling, and compare it to mK75, a companion layer (Cl) marker. We find that mK75 regulation parallels that of inner root sheath (IRS) markers, with expression onset at anagen IIIa above the new hair bulb and subsequent spreading towards the bulge. Although also occurring in the Cl, mK6a expression begins at anagen IIIb in differentiating cells located proximal to the bulge, and subsequently spreads towards the hair bulb. mK6a and mK75 thus exhibit temporally distinct, and spatially opposed, expression patterns in the Cl during postnatal anagen. These findings provide novel insight into the morphogenesis and properties of the Cl, and raise the distinct possibility that it is an integral part of the IRS compartment.

Inherited mutations in the intermediate filament (IF) proteins keratin 5 (K5) or keratin 14 (K14) cause epidermolysis bullosa simplex (EBS), in which basal layer keratinocytes rupture upon trauma to the epidermis. Most mutations are missense alleles affecting amino acids located in the central ␣-helical rod domain of K5 and K14. Here, we study the properties of an unusual EBS-causing mutation in which a nucleotide deletion (1649delG) alters the last 41 amino acids and adds 35 residues to the C terminus of K5. Relative to wild type, filaments coassembled in vitro from purified K5-1649delG and K14 proteins are shorter and exhibit weak viscoelastic properties when placed under strain. Loss of the C-terminal 41 residues contributes to these alterations. When transfected in cultured epithelial cells, K5-1649delG incorporates into preexisting keratin IFs and also forms multiple small aggregates that often colocalize with hsp70 in the cytoplasm. Aggregation is purely a function of the K5-1649delG tail domain; in contrast, the cloned 109 residue-long tail domain from wild type K5 is distributed throughout the cytoplasm and colocalizes partly with keratin IFs. These data provide a mechanistic basis for the cell fragility seen in individuals bearing the K5-1649delG allele, and point to the role of the C-terminal 41 residues in determining K5's assembly properties. INTRODUCTIONIntermediate filaments (IFs) are encoded by a large family of genes comprising Ͼ67 members in the human and mouse genomes Hesse et al., 2001). These 10-to 12-nm wide filaments are prominent structural constituents of the cytoplasm and nucleus in multicellular eukaryotes, but they are distinct from F-actin and microtubules in several key respects. Among them is their association with disease. Inherited mutations in IF proteins are associated with nearly 30 human diseases (Omary et al., 2004). These conditions are dominantly inherited with rare exceptions (Fuchs and Cleveland, 1998;Cassidy et al., 2002;Omary et al., 2004), consistent with the complexity of the assembly pathway and of the architecture of mature IF polymers (Herrmann and Aebi, 2004). Cell and tissue fragility underlies lesion pathogenesis in many of these disorders, reflecting the major function of structural scaffolding fulfilled by IFs in both the cytoplasm and nucleus (Omary et al., 2004;Worman and Courvalin, 2004). Types I and II keratin genes together represent ϳ75% of IF genes and are specifically expressed in epithelial cells Hesse et al., 2001). In part because of a strict heteropolymerization requirement at the protein level (Herrmann and Aebi, 2004), type I and type II keratin genes are tightly regulated in a pairwise and differentiation-related manner in epithelial tissues (Moll et al., 1982;O'Guin et al., 1990;Fuchs, 1995). Correlating with frequent exposure to mechanical stress, epithelia lining up the skin and oral mucosa are keratin rich and sensitive to mutations compromising the structural scaffolding function of keratin IFs. Accordingly, a large fraction of the known IF...