Deletion of a cytoplasmic domain of integrin beta4 causes epidermolysis bullosa simplex

Abstract

Integrin alpha6beta4 is a hemidesmosomal transmembrane molecule involved in maintaining basal cell-matrix adhesion through interaction of the large intracytoplasmic tail of the beta4 subunit with the keratin intermediate filament network, at least in part through its binding with plectin and BP180/type XVII collagen. Here we report a patient with predominant features of epidermolysis bullosa simplex due to a mutation in the integrin beta4 gene. The patient, a 49-y-old female, had mild blistering of hands and feet from birth on, dystrophy of the nails with onychogryposis, and enamel hypoplasia. She had no alopecia and no history of pyloric atresia. Electron microscopy and antigen mapping of a skin blister revealed that the level of separation was intraepidermal, low in the basal keratinocytes through the attachment plaque of the hemidesmosome. Immuno-fluorescence microscopy revealed absent binding of monoclonal antibody 450-11 A against the third fibronectin III repeat on the intracellular domain of integrin beta4, whereas binding was reduced with monoclonal antibodies recognizing epitopes on amino-terminal and carboxy-terminal ends of the polypeptide. At the molecular level the phenotype was caused by a novel 2 bp deletion 4733delCT in ITGB4, resulting in in-frame skipping of exon 36 and a deduced 50 amino acid deletion (1450-1499) within the third fibronectin type III repeat in the cytoplasmic domain of the integrin beta4 polypeptide. Immunoblot analysis demonstrated a 5 kDa shorter beta4 polypeptide. The 4733delCT mutation was heterozygously present in the DNA. The patient is also expected to be heterozygous for a null allele, as no full-size protein was detected in vitro and the epitope 450-11 A was absent in vivo. These data show that deletion of the third fibronectin type III repeat in the cytoplasmic domain of integrin beta4, which is thought to interact with BP180/type XVII collagen, is clinically pathogenic and results in a mild phenotype with predominant features of epidermolysis bullosa simplex.