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Bullous pemphigoid antigen 2 (BP180; COL17A1) collagen gene mutations typically result in nonlethal junctional epidermolysis bullosa. We have identified a patient, who had phenotypic features of mainly epidermolysis bullosa simplex and evidence for both intraepidermal and junctional blister formation. Mutation analysis disclosed compound heterozygous mutations in the COL17A1 gene, leading to deletion of Ile-18 to Asn-407 from the intracellular domain of BP180, BP180 Delta 18-407. To gain insight into the mechanisms underlying the phenotype, we have investigated the functional consequences of this truncation in BP180. The results demonstrate that: (1) in cultured keratinocytes of the patient, the assembly of hemidesmosomes, and their linkage with intermediate filaments are impaired; (2) BP180 Delta 18-407 is not capable of binding to the hemidesmosomal components BP230, plectin, and the beta 4 subunit of the alpha 6 beta 4 integrin in yeast two-hybrid assays; (3) BP180 Delta 18-407 is recruited into hemidesmosome-like structures in both normal and BP180-deficient transfected keratinocytes when ectopically expressed, suggesting that the extracellular domain of BP180 Delta 18-407 determines its topogenic fate; and, finally (4) the proteolytic shedding of the extracellular domain of BP180 Delta 18-407 is not impaired in transfected COS-7 cells. Collectively, the data demonstrate that the truncation of the intracellular domain of BP180 impairs the organization of hemidesmosomes, affecting both the mechanical stability of basal keratinocytes and dermoepidermal cohesion.

Bullous pemphigoid (BP) is an autoimmune skin disease characterized by autoantibodies against the hemidesmosomal protein BP180. In addition to IgG autoantibodies, IgE class autoantibodies have been reported in BP patients. Because animal models utilizing only IgG antibodies do not totally replicate human BP, we examined the specificity and potential relevance of IgE autoantibodies in this disease. Thirty BP patients participated in these studies. Serum IgE was measured and the IgE specificity was determined by immunoblotting. Double labeling Immunofluorescence was performed using combinations of specific antibodies to human mast cell tryptase, IgE and BP180. BP180-stimulated histamine release was measured from basophils of untreated BP patients (n=9), BP patients undergoing immunosuppressive therapy (n=9) and controls (n=16). Elevated IgE levels were found In 70% of untreated BP patients. IgE autoantibodies directed against BP180 were detected in 86% of untreated patients and in all but one of these patients the IgE reacted with the NC16A domain of BP180. IgE-coated mast cells were detected in perilesional skin of the BP patients. Moreover, BP180 peptides were detected on these mast cells. BP180-stimulated histamine release was significantly higher in basophils obtained from untreated BP patients compared with control basophils (p=0.006) or from treated BP patients (p=0.01). These findings support the hypothesis that IgE autoantibodies are involved in the pathogenesis of BP. IgE and IgG BP autoantibodies share the same antigenic specificity. Antigen-specific degranulation of basophils and/or mast cells from BP patients suggests a mechanism by which IgE may contribute to lesion development.