A novel keratin 5 mutation (K5V186L) in a family with EBS-K: a conservative substitution can lead to development of different disease phenotypes

Epidermolysis bullosa simplex is a hereditary skin blistering disorder caus ed by mutations in the KRT5 or KRT14 genes. More than 50 different mutation s have been described so far, These, and reports of other keratin gene muta tions, have highlighted the existence of mutation "hotspots" in keratin pro teins at which sequence changes are most likely to be detrimental to protei n function. Pathogenic mutations that occur outside these hotspots are usua lly associated with less severe disease phenotypes. We describe a novel K5 mutation (V186L) that produces a conservative amino acid change (valine to leucine) at position 18 of the 1A helix. The phenotype of this case is unex pectedly severe for the location of the mutation, which lies outside the co nsensus helix initiation motif mutation hotspot, and other mutations at thi s position have been associated in Weber-Cockayne (mild) epidermolysis bull osa simplex only. The mutation was confirmed by mismatch-allele-specific po lymerase chain reaction and the entire KRT5 coding region was sequenced, bu t no other changes were identified. De novo K5/K14 (mutant and wild-type) f ilament assembly in cultured cells was studied to determine the effect of t his mutation on filament polymerization and stability. A computer model of the 1A region of the K5/K14 coiled-coil was generated to visualize the stru ctural impact of this mutation and to compare it with an analogous mutation causing mild disease. The results show a high level of concordance between genetic, cell culture and molecular modeling data, suggesting that even a conservative substitution can cause severe dysfunction in a structural prot ein, depending on the size and structure of the amino acid involved.