Plectin is a cytoskeleton linker protein expressed in a variety of tissues 
including skin, muscle, and nerves. Mutations in its gene are associated with 
epidermolysis bullosa simplex with late-onset muscular dystrophy. Whereas in 
most of these patients the pathogenic events are mediated by nonsense-mediated 
mRNA decay, the consequences of an in-frame mutation are less clear. We analyzed 
a patient with compound heterozygosity for a 3-bp insertion at position 1287 
leading to the insertion of leucine as well as the missense mutation Q1518X 
leading to a stop codon. The presence of plectin mRNA was demonstrated by a 
RNase protection assay. However, a marked reduction of plectin protein was found 
using immunofluorescence microscopy of the patient's skin and Western blot 
analysis of the patient's cultured keratinocytes. The loss of plectin protein 
was associated with morphological alterations in plectin-containing structures 
of the dermo-epidermal junction, in skeletal muscle, and in nerves as detected 
by electron microscopy. In an in vitro overlay assay using recombinant plectin 
peptides spanning exons 2 to 15 the insertion of leucine resulted in markedly 
increased self-aggregation of plectin peptides. These results describe for the 
first time the functional consequences of an in-frame insertion mutation in 
humans.