Revertant mosaicism: partial correction of a germ-line mutation in COL17A1by a frame-restoring mutation

Generalized atrophic benign epidermolysis bullosa is an autosomal recessive subepidermal blistering disease typified by null mutations in COL17A1. In 1 large kindred, affected individuals were homozygous for a 2-bp deletion i n COL17A1, 4003delTC, which resulted in a downstream premature termination codon, nonsense-mediated mRNA decay, and abrogation of type XVII collagen s ynthesis. Interestingly, 1 of these patients, although phenotypically ident ical to her affected siblings, showed focal expression of type XVII collage n in epidermal basement membrane in a pattern suggestive of revertant mosai cism. When studies of randomly obtained epidermal, oromucosal, and peripher al blood cells failed to identify the genetic basis of this apparent mosaic ism, microscopic subpopulations of potentially revertant epidermal cells (i .e., those overlying basement membrane containing type XVII collagen) were selectively isolated using laser capture microdissection. Analysis of DNA a nd RNA from these cells revealed a second mutation, 4080insGG, on 1 allele of COL17A1. This 2-bp insertion corrected the reading frame just proximal t o the premature termination codon, countered nonsense-mediated mRNA decay, and allowed protein production by patient keratinocytes in vivo and in vitr o. These studies elucidate the molecular basis of a novel form of revertant mosaicism in humans.