Moderation of phenotypic severity in dystrophic and junctional forms of epidermolysis bullosa through in-frame skipping of exons containing non-senseor frameshift mutations

Non-sense mutations on both alleles of either the type vn collagen gene (CO L7A1) or the genes encoding laminin 5 (LAMA3, LAMB3, or LAMC2) usually resu lt in clinically severe forms of recessive dystrophic or junctional epiderm olysis bullosa, respectively. In this study we assessed two unrelated famil ies whose mutations in genomic DNA predicted severe recessive dystrophic ep idermolysis bullosa or junctional epidermolysis bullosa phenotypes but in w hom the manifestations were milder than expected, The recessive dystrophic epidermolysis bullosa patients had a homozygous single base-pair frameshift mutation in exon 19 of COL7A1 (2470insG). Clinically, there was generalize d blistering but only mild scarring. Skin biopsy revealed positive type VII collagen immunoreactivity and recognizable anchoring fibrils, The junction al epidermolysis bullosa patients were compound heterozygotes for a framesh ift/non-sense combination of mutations in exons 3 and 17 of LAMB3 (29insC/Q 834X), These patients did not have the lethal form of junctional epidermoly sis bullosa but, as adults, displayed the milder generalized atrophic benig n epidermolysis bullosa variant. There was undetectable laminin 5 staining at the dermal-epidermal junction using an antibody to the beta 3 chain, but faintly positive alpha 3 and gamma 2 chain labeling, and there was variabl e hypoplasia of hemidesmosomes. To explain the milder recessive dystrophic epidermolysis bullosa and junctional epidermolysis bullosa phenotypes in th ese families, reverse transcription-polymerase chain reaction, using RNA ex tracted from frozen skin, was able to provide evidence for some rescue of m utant mRNA transcripts with restoration of the open-reading frame. In the r ecessive dystrophic epidermolysis bullosa patients, transcripts containing in-frame skipping of exon 19 of COL7A1 in the cDNA were detected, and in th e junctional epidermolysis bullosa patients transcripts with in-frame skipp ing of exon 17 of LAMB3 were identified. The truncated proteins encoded by these transcripts are expected to lack certain critical domains involved in cell-matrix attachment, but may still be able to contribute to adhesion th ereby moderating the severity of the skin blistering. This study shows the limitations in predicting phenotype in epidermolysis bullosa solely based o n mutation analysis of genomic DNA and emphasizes the importance of immunoh istochemistry, electron microscopy, and mRNA assessment as parallel investi gations.