There are two recessively inherited subtypes of DEB: severe-generalized (HDEB-sev gen; previously named Hallopeau-Siemens type) and generalized-other (HDEB-O; previously named non–Hallopeau-Siemens type); and a dominantly inherited form, dominant dystrophic epidermolysis bullosa (DDEB). The clinical manifestations demonstrate a continuum of severity that complicates definitive diagnosis, especially early in life. The severe generalized subtype, associated with formation of pseudosyndactyly (a mitten-like deformity secondary to fusion of interdigital webbing) in early childhood, carries a SCC risk of up to 85% by the age of 45 years.[98,99] These cancers arise in nonhealing wounds and usually metastasize to cause death within 5 years of the diagnosis of SCC. In one case series, SCC was the leading cause of death for the 15 patients with the severe generalized subtype. Early mortality also has been observed in this disorder, with a mortality rate of up to 40% by the age of 30 years. Extracutaneous manifestations of HDEB-severe generalized include short stature, anemia, strictures of the gastrointestinal and genitourinary tracts, and corneal scarring that may result in blindness.
Diagnosis of epidermolysis bullosa may be accomplished by immunofluorescence or electron microscopy. A list of recommended diagnostic antibodies and their suppliers is available at the Web site of the Dystrophic EB Research Association. Mutation testing is generally used for prenatal diagnosis rather than for the primary diagnosis of epidermolysis bullosa.[103,104]
The rate of de novo mutation for DDEB is approximately 30%; maternal germline mosaicism has also been reported.[105,106] Glycine substitutions in exons 73 to 75 are the most common mutations in DDEB. G2034R and G2043R account for half of these mutations. Less frequently, splice junction mutations and substitutions of glycine and other amino acids may cause the dominant form of dystrophic epidermolysis bullosa. In contrast, more than 400 mutations have been described for the two types of recessive epidermolysis bullosa. The recessive form of the disease is caused primarily by null mutations, although amino acid substitutions, splice junction mutations, and missense mutations have also been reported. In-frame exon skipping may generate a partially functional protein in recessive disease. A founder mutation, c.6527insC (p.R525X), has been observed in 27 of 49 Spanish individuals with recessive DEB. Genotype-phenotype correlations suggest an inverse correlation between the amount of functional protein and severity.
Mutations in COL7A1 result in abnormal triple helical coiling and decreased function, which causes increased skin fragility and blistering. In studies of Ras-driven carcinogenesis in HDEB-severe generalized keratinocytes, retention of the amino-terminal NC1, the first noncollagenous fragment of type VII collagen, is tumorigenic in mice. This retained sequence may mediate tumor-stroma interactions that promote carcinogenesis.
Junctional epidermolysis bullosa
Junctional epidermolysis bullosa (JEB) is an autosomal recessive type of epidermolysis bullosa. JEB results in considerable mortality with approximately 50% of cases dying within the first year of life. Mutations in any of the genes encoding the three basic subunits of laminin 332, previously known as laminin 5 (LAMA3, LAMB3, LAMC2), or mutations in COL17A1 can result in this syndrome.[110,111,112] Individuals with the Herlitz type (a severe clinical form) of JEB are at increased risk of SCC, with a cumulative risk of 18% by the age of 25 years. A mutational study of COL17A1 in individuals with a milder subtype of JEB, called JEB-other, identified mutations in 85 of 86 alleles from 43 individuals. Total loss of COL17A1 protein staining correlated with a more severe phenotype.