COMPOUND HETEROZYGOSITY FOR A RECESSIVE GLYCINE SUBSTITUTION AND A SPLICE-SITE MUTATION IN THE COL7A1 GENE CAUSES AN UNUSUALLY MILD FORM OFLOCALIZED RECESSIVE DYSTROPHIC EPIDERMOLYSIS-BULLOSA

Type VII collagen is the major component of anchoring fibrils, adhesio n structures of stratified epithelia that span the basement membrane r egion and papillary dermis, Mutations in the gene COL7A1 encoding type VII collagen cause dystrophic epidermolysis bullosa, a clinically het erogeneous autosomal dominant or recessive blistering disorder of the skin and mucous membranes. In this report, we investigate three siblin gs affected by an unusually mild form of localized recessive dystrophi c epidermolysis bullosa who were shown to be compound heterozygotes fo r novel mutations affecting COL7A1. The maternally inherited mutation is a G-->C transversion that converts a codon for glycine to a codon f or arginine (G1347R), The paternal mutation is a neutral G-->A transit ion at the last base of exon 70 (5820G-->A) that alters the correct sp licing of COL7A1 pre-mRNA, giving rise to an aberrant mRNA carrying th e in-frame skipping of exon 70 in addition to the full-length RNA tran script carrying the G-->A substitution. Consistent with the normal lev els of COL7A1 mRNA transcripts detected by northern analysis, immunobl otting and immunofluorescence studies evidenced that the patient kerat inocytes synthesize and secrete normal amounts of stable type VII coll agen, which is correctly deposited at the dermal-epidermal junction. I n addition, mutated type VII collagen molecules assemble to form numer ous, normally shaped anchoring fibrils, as shown by electron microscop ic examination. The combination of a recessive glycine substitution wi th a splice site mutation that permits partially correct splicing ther efore leads to a normal expression of mutated type VII collagen molecu les with marginally altered biologic activity, and to the extremely mi ld phenotype observed in our patients.