A keratin 14 'knockout' mutation in recessive epidermolysis bullosa simplex resulting in less severe disease

Epidermolysis bullosa simplex (EBS) is a blistering skin disease caused in most cases by mis-sense mutations in genes encoding the basal epidermal ker atin (K) 5 and K14. The inheritance is usually autosomal dominant and the m utant keratin proteins appear to exert a dominant negative effect on the ke ratin intermediate filament cytoskeleton in basal keratinocytes. We report a child with a homozygous K14 mutation resulting in the complete absence of K14 protein in the epidermis; remarkably! he only had mild to moderate dis ease. Electron microscopy of a skin biopsy showed a marked reduction in num bers of keratin intermediate filaments in the basal keratinocytes. Immunofl uorescence microscopy using monoclonal antibody LL001 against K14 showed no staining, suggesting a functional knockout of K14. Sequence analysis of ge nomic DNA revealed a homozygous mutation in codon 31 of K14 that resulted i n a premature stop codon further downstream in exon 1. The child's mother, who is unaffected by the disease, is heterozygous for the mutation. The con sanguineous father was unaffected and unavailable for testing, The resultin g mRNA is predicted to encode a protein of 116 amino acids, of which the fi rst 30 are identical to the normal K14 sequence, and the remaining 86 resid ues are mis-sense sequence, Four previously reported cases of autosomal rec essive EBS with functional knockout of K14 were severely affected by bliste ring. in contrast to our patient in whom the predicted protein has only the first 30 amino acids of K14 and is therefore the closest to a true knockou t of K14 protein yet identified.