Search

The dystrophic forms of epidermolysis bullosa (DEB) are due to mutations in the type VII collagen gene (COL7A1). In dominant DEB, a characteristic genetic lesion is a glycine substitution mutation within the collagenous domain of the protein. In this study, we have examined the molecular basis of six new families in which the proband has clinical features and/or ultrastructural findings consistent with DEB. The results revealed a glycine substitution mutation in all six families, four of which are novel and previously unpublished. In three families with clinically unaffected parents, de novo mutations G2043R and G2040V were found. These results emphasize the predominance of glycine substitution mutations in dominant DEB, and indicate that in some cases the phenotype is due to de novo dominant mutations.

Generalized atrophic benign epidermolysis bullosa (GABEB; OMIM no. 226650) is a rare hemidesmosomal variant of EB, inherited in an autosomal recessive fashion. In previous studies, mutations in the gene (COL17A1) encoding the type XVII collagen, a transmembrane component of hemidesmosomes, were detected in most patients with GABEB. However, evidence for genetic defects in the laminin 5 genes has also been presented. In the present investigation, we examined three patients, representing two families with GABEB, for mutations in the LAMBS gene. Heteroduplex scanning of the gene, followed by direct automated sequencing, revealed that Patient 1 was a compound heterozygote for a missense mutation (C293S) and a premature termination codon-causing mutation (1367delAC). The latter mutation resulted in accelerated mRNA decay, which rendered the corresponding mRNA transcript undetectable by reverse transcriptase-PCR. Patients 2 and 3, siblings with slightly different clinical presentations, were homozygous for a G-->A transition affecting the last nucleotide of exon 7 (628G-->A). This mutation resulted in amino acid substitution (E210K), as well as in multiple aberrant splice variants affecting exons 6 to 8. These observations expand the repertoire of LAMBS mutations in nonlethal variants of EB, and they illustrate the consequences of the mutations at the mRNA and protein levels.

Epidermolysis bullosa with pyloric atresia (EB-PA; OMIM 226730) is a clinically and genetically heterogeneous autosomal recessive blistering disorder, including lethal and nonlethal variants. Recently, expression of alpha6beta4 integrin, a transmembrane protein of the epithelial basement membranes, has been shown to be altered in these patients. In this work, we have explored the molecular pathology of the lethal form of EB-PA, and we describe novel ITGB4 mutations in five alleles of three patients. The mutation detection strategy included polymerase chain reaction amplification of each exon of ITGB4, followed by heteroduplex analysis and direct nucleotide sequencing. The novel mutations included a homozygous 2-bp deletion in exon 34 (4501delTC), compound heterozygosity for a 2-bp deletion within the paternal allele (120delTG) within exon 3 and a cysteine substitution in the maternal allele (C245G) within exon 7, and the paternal nonsense mutation within exon 4 (Q73X). Thus, three of four distinct mutations predicted truncated polypeptide chains, whereas the missense mutation in the extracellular domain of beta4 integrin may affect ligand binding or dimerization of alpha6 and beta4 integrin subunits. These mutations emphasize the critical importance of the alpha6beta4 integrin in providing stability to the association of epidermis to the underlying dermis at the cutaneous basement membrane zone.

Generalized atrophic benign epidermolysis bullosa (GABEB) is a nonlethal variant of junctional epidermolysis bullosa (JEB). Previous findings have suggested that type XVII collagen is the candidate gene for mutations in this disease. We now have cloned the entire human type XVII collagen gene (COL17A1) and have elucidated its intron-exon organization. The gene comprises 56 distinct exons, which span approximately 52 kb of the genome, on the long arm of chromosome 10. It encodes a polypeptide, the alpha1(XVII) chain, consisting of an intracellular globular domain, a transmembrane segment, and an extracellular domain that contains 15 separate collagenous subdomains, the largest consisting of 242 amino acids. We also have developed a strategy to identify mutations in COL17A1 by use of PCR amplification of genomic DNA, using primers placed on the flanking introns. The PCR products are scanned for sequence variants by heteroduplex analysis using conformation-sensitive gel electrophoresis and then are subjected to direct automated sequencing. We have identified several intragenic polymorphisms in COL17A1, as well as mutations, in both alleles, in two Finnish families with GABEB. The probands in both families showed negative immunofluorescence staining with an anti-type XVII collagen antibody. In one family, the proband was homozygous for a 5-bp deletion, 2944del5, which resulted in frameshift and a premature termination codon of translation. The proband in the other family was a compound heterozygote, with one allele containing the 2944del5 mutation and the other containing a nonsense mutation, Q1023X. These results expand the mutation database in different variants of JEB, and they attest to the functional importance of type XVII collagen as a transmembrane component of the hemidesmosomes at the dermal/epidermal junction.