? The Dowling-Meara variant of epidermolysis bullosa simplex (EBS-DM) is a severe blistering disease inherited in an autosomal-dominant fashion. Besides symptomatic care, no effective therapeutic treatment is available for EBS. Therefore, gene therapy is the only option for a permanent corrective therapy for these patients. Prior to testing gene therapy approaches for EBS in humans, it is desirable to utilize a pre-clinical animal model to determine the safety and efficacy of these approaches. We have recently generated a transgenic mouse model that mimics EBS-DM at the genetic level. This mouse model differs from the human disease in that expression of the mutant K14 allele, which contains an Arg 131 Cys mutation equivalent to the Arg 125 Cys mutation found in the majority of EBS-DM patients, can be restricted to a small area of the skin. This mouse model has provided an explanation for the lack of mosaic forms of EBS. Patients with mostly normal skin that have patches of diseased skin are referred to as mosaics. Mosaic patients have been described for several skin diseases, but not for EBS. Focal activation of the mutant K14 gene by topical application of an inducer results in blister formation. However, after a few weeks, the blister heals and never reappears. We have demonstrated that the mutant K14 gene was activated in epidermal stern cells. However, the defective EBS stem cells were replaced by normal epidermal stem cells that migrate in from the untreated area surrounding the blister. This mouse model predicts that if a mosaic patch of EBS skin formed during development of an embryo, these defective EBS epidermal stem cells would not survive, but be replaced by normal stem cells. This explains the absence of mosaic forms of EBS. This observation also has important implications for gene therapy approaches for EBS, since it suggests that if EBS stem cells were removed from a patient, genetically corrected and then returned to a blistered area, they would have a selective growth advantage over defective EBS stem cells. Of further interest was the observation that mice which express the mutant K14 allele at levels approximately 50% of wild type K14 fail to exhibit a skin phenotype. This suggests that as long as the ratio of wild type to mutant K14 is above a threshold, possibly as low as 2:1, the skin will have a normal appearance and be fully functional. Thus, successful gene therapy approaches may not require correction or complete suppression of the mutant allele. This proposal will use epidermal stem cells isolated from the EBS-DM mouse model to test new gene therapy strategies that are based on these novel findings. ? ?
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