A COMBINATION OF A COMMON SPLICE-SITE MUTATION AND A FRAMESHIFT MUTATION IN THE COL7A1 GENE - ABSENCE OF FUNCTIONAL COLLAGEN-VII IN KERATINOCYTES AND SKIN

We describe a patient with severe generalized dystrophic epidermolysis bullosa (EBD) and a novel combination of compound heterozygous mutati ons in the COL7A1 gene, The maternal mutation was an A-to-G transition (425-A --> G) at position -2 of the donor splice site within exon 3 t hat causes aberrant splicing of two abnormal transcripts. One includes intron 3, and one excludes both exon 3 and intron 3. Both splice vari ants contained a premature termination Of the translation. The paterna l mutation is a 25-bp deletion in exon 20 (2638del25) that leads to a frameshift and a premature termination codon 133 bp downstream from th e site of deletion. This combination of mutations allowed expression o f collagen VII mRNA. Immunofluorescence staining of the patient's skin and cultured keratinocytes with domain-specific collagen VII antibodi es, however, demonstrated markedly reduced levels of alpha 1(VII) poly peptides, and no stable collagen VII protein could be extracted from t he patient's cells. Electron microscopy showed severely hypoplastic fi brils below the lamina densa, without evidence of normal anchoring fib rils, The clinically unaffected parents were heterozygous for the muta tions, suggesting that both COL7A1 gene defects were recessively inher ited disease-causing mutations that are ''silent'' in heterozygous car riers but in combination can severely interfere with the dermal-epider mal adhesion and lead to severe EBD.