Toward epidermal stem cell-mediated ex vivo gene therapy of junctional epidermolysis bullosa

Junctional epidermolysis bullosa (JEB) is a group of severe, inherited skin diseases caused by mutations in the genes encoding laminin 5 or other comp onents of the hemidesmosome. Since human epidermis is a self-renewing tissu e, gene therapy of JEB requires the stable integration of the transgene int o the genome of the epidermal stem cell. Human epidermal stem cells can ind eed be cultivated and stably transduced with replication-defective retrovir al vectors, allowing full phenotypic correction of the adhesion properties of JEB keratinocytes, Epidermal stem cells generate cohesive sheets of stra tified epithelium suitable for the permanent coverage of massive skin defec ts, and genetically modified epidermal sheets maintain long-term expression of the transgene after transplantation on immunodeficient animals, Moreove r, we have developed a clinical procedure that allows transplantation of cu ltured epidermal sheets on large body areas under local anesthesia and with out cicatricial outcomes. Thus, (1) the possibility of cultivating lining e pithelia, (2) the availability of noninvasive surgical procedures that allo w the grafting of large skin areas, and (3) the demonstration of sustained transgene expression in vitro and in vivo by epidermal stem cells, prompt u s to propose the implementation of a phase I/II clinical trial aimed at the ex viva gene therapy of selected JEB patients. The aim of the trial is to validate the ex vivo procedure in a clinical setting, to prove its overall safety, and to analyze critical issues such as long-term survival of the ge netically modified implant, immune response against the transgene product, and persistence of transgene expression at therapeutic levels.