Novel missense mutation in a patient with recessive pretibial epidermolysis bullosa and a mild phenotype

Editor A 44-year-old man presented with a greater than 20-year history of persistent red patches on his shins. He described occasional blistering in the area but was otherwise asymptomatic. He had no relevant medical or family history. On examination, he had large violaceous plaques on his anterior shins with surface atrophy. (Fig. 1a) A bulla was evident at the peripheral edge of the involved skin on the left shin. (Fig. 1b) No milia were observed. He was also noted to have dystrophic toenails. (Fig. 2) Full skin examination was otherwise unremarkable. Skin biopsy showed subepidermal bulla formation with minimal inflammation. (Fig. 3) Direct immunofluorescence was negative. Given these findings and clinical localization to the shins, the presumptive diagnosis was pretibial epidermolysis bullosa (EB). To confirm the diagnosis, Sanger sequencing of COL7A1, encoding type VII collagen was preformed. Two heterozygous mutations were identified: c.3840delC (p.Gly1281Valfs*44) and c.8780G>A (p.Arg2927His) in exon 31 and exon 117 of this 118 exon gene respectively. The first mutation is a recurrent lossof-function mutation identified previously in other unrelated individuals with recessive dystrophic EB. The second mutation is a new missense mutation occurring within the NC-2 domain of type VII collagen and appears to be the most distal amino acid substitution discovered to date. Pretibial EB is a rare variant of the inherited immunobullous disorder, dystrophic EB. Dystrophic EB includes all subtypes where blistering occurs just beneath the lamina densa of the basement membrane zone and results from mutations in COL7A1. Type VII collagen is composed of three identical a chains, each with a central collagenous triple-helical segment with a large NC-1 domain at one end and a smaller NC-2 domain at the other. Individual molecules form anti-parallel dimers which then aggregate to form anchoring fibrils (AFs). It has been postulated that the NC-2 domain within one a chain may bind to an appropriate site on the triple-helical domain of a second molecule and align cysteine residues, which subsequently become stabilized by intermolecular disulphide bonds. Mutations in the NC-2 domain have been shown to affect dimer formation (NC2 is normally cleaved during this process converting the procollagen collagen). The sites for cleavage and disulphide bonding are proposed to be localized within a 57amino acid segment between residues 2780 and 2837. Christiano et al. reported on a family with recessive DEB where two affected siblings had homozygous methionine to lysine mutations. This substitution was localized to position 2789 within the NC2 domain and these patients had a complete lack of AFs. Another patient with localized DEB had a deletion mutation leading to in-frame skipping of exon 115 (encoding amino acids 2814-2843) resulted in a lack of peptide processing. Anchoring fibrils were reduced with diffuse cross-banding but no other gross morphological alterations suggesting defective lateral aggregation of type VII collagen dimers. Our patient’s mutations, located outside this region (including at the terminus), likely have a diminished impact on the structure and function of type VII collagen and AFs, thus explaining his mild clinical phenotype. Pretibial EB is characterized by blisters, milia and atrophic scarring localized to the pretibial skin associated with