RETRACTED: Progress and prospects of skin gene therapy: a ten year history

Selectable bicistronic vectors in skin gene therapy

Bicistronic vectors (BCV) are important tools for gene therapy applications allowing selection for increased expression of a desired gene by linking it to a selectable gene, such as the multi-drug resistance (MDR) gene. However, both the design of the BCV and the cell type to be transduced can have a strong impact on the vector performance in the target cells. To analyze which factors might influence the efficiency of BCV in achieving high gene expression levels in skin and to determine the best suited BCV for cutaneous transduction, both keratinocytes (KC) and fibroblasts (FB) were transduced with different BCV constructs, BGIM, BMIG and QGIM. In BGIM, expression of the BCV cassette encompassing the green fluorescent protein (GFP) gene connected to the MDR gene was driven by a retroviral LTR-promoter. In BMIG, the order of the two genes was reversed, while in QGIM the GFP- and MDR-gene were arranged similar as in BGIM, but expressed by a CMV- instead of an LTR-promoter. FACS-analysis revealed that the percentage of genetically modified cells varied substantially with 47.9% QGIM-, 35.5% BMIG- and 17.9% BGIM-transduced KC expressing both genes. For FB the numbers were 56.7% (QGIM), 38.4% (BMIG) and 8.3% (BGIM). Furthermore, the choice of BCV determined the intensity of GFP-expression with the highest levels measured in BGIM-, followed by QGIM- and then BMIG-transduced cells. Interestingly, highly efficient enrichment through colchicine selection was possible for QGIM- (up to 97.1% KC, 97.8% FB) and BMIG- (85.0% KC and 98.0% FB) but not BGIM- (29.9% KC and 18.6% FB) transduced cells. Finally, immunohistochemistry and FACS-analysis demonstrated, that colchicine selection of QGIM-transduced skin equivalents led to increased numbers of GFP-expressing KC (from 51.2% up to 72.3%) and enhanced GFP-intensity in the skin. These results show that BCV present a promising vector system to enhance the expression of a desired gene in skin but important parameters taken into account when employing a selectable BCV for skin gene therapy applications are the retroviral vector backbone, the order in which the genes are arranged, and the target cells to be transduced and selected.

Cutaneous gene therapy

Possibilities of using the skin for somatic gene therapy have been investigated for more than 20 years. Strategies have included both direct gene transfer into the skin and indirect gene transfer utilizing cultured cells as an intermediate step for gene manipulation. Viral as well as nonviral vectors have been used, and both gene addition and gene editing have been performed. Although cutaneous gene therapy has now begun translating into clinical medicine (as seen by the first clinical gene therapy project of an inherited skin disorder) further developments are still required.

A large preclinical animal model to assess ex vivo skin gene therapy applications

Because of its easy accessibility, the skin is a very attractive target for gene therapy purposes. To study potential clinical applications in a preclinical setting, appropriate animal models are needed. Pig skin is very similar to human skin, and a variety of human diseases that are potentially amenable to gene therapy applications also occur in pigs. Only a few studies have analyzed the engraftment of transduced keratinocytes (KC) in pigs, however, with limited success. We describe a porcine model in which pig KC were transduced ex vivo with a retroviral vector encoding a marker gene and subsequently grafted onto the autologous host, utilizing a relatively simple grafting technique. Enhanced transduction efficiency was achieved by an optimized transduction protocol including centrifugation of the retroviral vector at a temperature of 32 degrees C. Transduced KC were then seeded onto acellular dermis, forming a stratified epidermis. Grafting was performed by creating full thickness wounds and placing the skin graft onto the muscle fascia, covered by a protective skin flap for several days. Successful engraftment of transduced KC was demonstrated by immunohistochemistry of biopsies taken at different time points, showing transgene expression in 40-50% of grafted KC. After 4 weeks, KC expressing a foreign marker gene was lost, suggesting a transgene-specific immune response in the immunocompetent pigs and highlighting the potential problems for clinical gene therapy studies when transferring new genetic material into a patient. The model presented here may be used to examine applications of skin gene therapy, where retroviral vectors encoding endogenous pig genes will be expressed in the skin.

Target proteins in inherited and acquired blistering skin disorders

Maintenance of an intact epidermis depends on secure adhesion between adjacent keratinocytes, and between basal keratinocytes and the underlying epidermal basement membrane. The major adhesion units that achieve this are the hemidesmosomes and desmosomes, but when these structures are disrupted, e.g., by gene mutations or autoantibodies, the resilience of the epidermis is lost and blisters develop. Recently, there have been considerable advances in our knowledge of the proteins and glycoproteins that contribute to maintaining keratinocyte adhesion via hemidesmosomes and desmosomes, as well as new insights into the molecular pathogenesis of several inherited and autoimmune blistering skin diseases. These new basic scientific data are clinically relevant, helping to improve patient management and to provide a rationale for developing better and more specific treatments for patients with inherited or acquired blistering skin diseases. In addition, there have also been improvements in our understanding of the organization and assembly of these adhesion structures, and their involvement in signalling pathways, intricately linked to skin development, wound healing and tumour invasion. This review provides an update on the structure and organization of hemidesmosomes and desmosomes, and on the molecular pathology of their various components that result in bullous skin diseases.