Keratin intermediate filaments are important cytoskeletal structural
proteins involved in maintaining cell shape and function. Mutations in the epidermal
keratin genes,
keratin 5 or
keratin 14 lead to the disruption of
keratin filament assembly, resulting in an autosomal dominant inherited blistering
skin disease,
epidermolysis bullosa simplex (EBS). We investigated a large EBS kindred who exhibited a markedly heterogeneous clinical presentation and detected two distinct
keratin 5 mutations in the proband, the most severely affected. One missense mutation (E170K) in the highly conserved helix initiation
peptide sequence of the 1A rod domain was found in all the affected family members. In contrast, the other missense mutation (E418K) was found only in the proband. The E418K mutation was located in the stutter region, an interruption in the heptad repeat regularity, whose function as yet remains unclear. We hypothesized that this mutated stutter allele was clinically silent when combined with the wild type allele but aggravates the clinical severity of EBS caused by the E170K mutation on the other allele. To confirm this in vitro, we transfected mutant
keratin 5 cDNA into cultured cells. Although only 12.7% of the cells transfected with the E170K mutation alone showed disrupted
keratin filament aggregations, significantly more cells (30.0%) cotransfected with both E170K and E418K mutations demonstrated
keratin aggregation (p < 0.05). These transfection assay results corresponded to the heterogeneous clinical findings of the EBS patient in this kindred. We have identified the first case of both compound heterozygous dominant (E170K) and recessive (E418K) mutations in any
keratin gene and confirmed the significant involvement of the stutter region in the assembly and organization of the
keratin intermediate filament network in vitro.