Non-sense mutations on both alleles of either the
type VII collagen gene (COL7A1) or the genes encoding
laminin 5 (LAMA3, LAMB3, or LAMC2) usually result in clinically severe forms of recessive dystrophic or
junctional epidermolysis bullosa, respectively. In this study we assessed two unrelated families whose mutations in genomic
DNA predicted severe recessive
dystrophic epidermolysis bullosa or
junctional epidermolysis bullosa phenotypes but in whom 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 generalized blistering but only mild
scarring. Skin biopsy revealed positive
type VII collagen immunoreactivity and recognizable anchoring fibrils. The
junctional epidermolysis bullosa patients were compound heterozygotes for a frameshift/non-sense combination of mutations in exons 3 and 17 of LAMB3 (29insC/Q834X). These patients did not have the lethal form of
junctional epidermolysis bullosa but, as adults, displayed the milder generalized atrophic benign
epidermolysis bullosa variant. There was undetectable
laminin 5 staining at the dermal-epidermal junction using an antibody to the beta3 chain, but faintly positive alpha3 and gamma2 chain labeling, and there was variable hypoplasia of hemidesmosomes. To explain the milder recessive
dystrophic epidermolysis bullosa and
junctional epidermolysis bullosa phenotypes in these families, reverse transcription-polymerase chain reaction, using
RNA extracted from frozen skin, was able to provide evidence for some rescue of mutant
mRNA transcripts with restoration of the open- reading frame. In the recessive
dystrophic epidermolysis bullosa patients, transcripts containing in-frame skipping of exon 19 of COL7A1 in the
cDNA were detected, and in the
junctional epidermolysis bullosa patients transcripts with in-frame skipping 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 thereby moderating the severity of the skin blistering. This study shows the limitations in predicting phenotype in
epidermolysis bullosa solely based on mutation analysis of genomic
DNA and emphasizes the importance of immunohistochemistry, electron microscopy, and
mRNA assessment as parallel investigations.