The genodermatoses: candidate diseases for gene therapy

Preimplantation Genetic Diagnosis for DEB by Detecting a Novel Family-Specific COL7A1 Mutation in Vietnam

Background

Epidermolysis bullosa (EB) is a disorder characterized by the appearance of blisters, erosions and wounds in response to minimal trauma. The disease manifests with noticeable symptoms ranging from mild to severe, classified into four major types: epidermolysis bullosa simplex (EBS), junctional epidermolysis bullosa (JEB), dystrophic epidermolysis bullosa (DEB) and Kindler syndrome. Preimplantation genetic diagnosis for the disease remains the only available option for families at risk for the recurrence of the disorder without having to terminate an ongoing pregnancy.

Materials and Methods

A novel COL7A1 mutation was used to design primers for the polymerase chain reaction (PCR) to amplify the segment spanning the mutation in the family and their in-vitro fertilization (IVF) embryos. Then, the PCR products were sequenced with Sanger sequencing to detect the alteration in the allele, and some embryos would go through NGS-based preimplantation screening for chromosomal abnormalities.

Results

The established protocol for EB detected mutant allele in 6/9 embryos (66.6%), while the remaining 3 embryos (33.4%) appeared to not carry any mutation. Only one among 3 embryos was recommended to be transferred into the mother's uterus.

Conclusion

The established preimplantation genetic diagnosis procedure is helpful to families affected by epidermolysis bullosa caused by COL7A1 mutations but wish to have healthy children.

Ultrasound mediated transdermal drug delivery

Transdermal drug delivery offers an attractive alternative to the conventional drug delivery methods of oral administration and injections. However, the stratum corneum serves as a barrier that limits the penetration of substances to the skin. Application of ultrasound (US) irradiation to the skin increases its permeability (sonophoresis) and enables the delivery of various substances into and through the skin. This review presents the main findings in the field of sonophoresis in transdermal drug delivery as well as transdermal monitoring and the mathematical models associated with this field. Particular attention is paid to the proposed enhancement mechanisms and future trends in the fields of cutaneous vaccination and gene therapy.

Topical delivery of siRNA into skin using SPACE-peptide carriers

Short-interfering RNAs (siRNAs) offer a potential tool for the treatment of skin disorders. However, applications of siRNA for dermatological conditions are limited by their poor permeation across the stratum corneum of the skin and low penetration into the skin's viable cells. In this study, we report the use of SPACE-peptide in combination with a DOTAP-based ethosomal carrier system to enhance skin delivery of siRNA. A DOTAP-based SPACE Ethosomal System significantly enhanced siRNA penetration into porcine skin in vitro by 6.3±1.7-fold (p<0.01) with an approximately 10-fold (p<0.01) increase in epidermis accumulation of siRNA compared to that from an aqueous solution. Penetration of siRNA was also enhanced at the cellular level. Internalization of SPACE-peptide occurred in a concentration dependent manner marked by a shift in intracellular distribution from punctate spots to diffused cytoplasmic staining at a peptide concentration of 10mg/mL. In vitro delivery of GAPDH siRNA by SPACE peptide led to 83.3±3.0% knockdown relative to the control. In vivo experiments performed using female BALB/C mice also confirmed the efficacy of DOTAP-SES in delivering GAPDH-siRNA into skin. Topical application of DOTAP-SES on mice skin resulted in 63.2%±7.7% of GAPDH knockdown, which was significantly higher than that from GAPDH-siRNA PBS (p<0.05). DOTAP-SES formulation reported here may open new opportunities for cutaneous siRNA delivery.

Silencing of reporter gene expression in skin using siRNAs and expression of plasmid DNA delivered by a soluble protrusion array device (PAD)

Despite rapid progress in the development of potent and selective small interfering RNA (siRNA) agents for skin disorders, translation to the clinic has been hampered by the lack of effective, patient-friendly delivery technologies. The stratum corneum poses a formidable barrier to efficient delivery of large and/or charged macromolecules including siRNAs. Intradermal siRNA injection results in effective knockdown of targeted gene expression but is painful and the effects are localized to the injection site. The use of microneedle arrays represents a less painful delivery method and may have utility for the delivery of nucleic acids, including siRNAs. For this purpose, we developed a loadable, dissolvable protrusion array device (PAD) that allows skin barrier penetration. The PAD tips dissolve upon insertion, forming a gel-like plug that releases functional cargo. PAD-mediated delivery of siRNA (modified for enhanced stability and cellular uptake) resulted in effective silencing of reporter gene expression in a transgenic reporter mouse model. PAD delivery of luciferase reporter plasmids resulted in expression in cells of the ear, back, and footpad skin as assayed by intravital bioluminescence imaging. These results support the use of PADs for delivery of functional nucleic acids to cells in the skin with an efficiency that may support clinical translation.

Cutaneous short-interfering RNA therapy

Since the 1990s, RNA interference (RNAi) has become a major subject of interest, not only as a tool for biological research, but also, more importantly, as a therapeutic approach for gene-related diseases. The use of short-interfering RNAs (siRNAs) for the sequence-specific knockdown of disease-causing genes has led to numerous preclinical and even a few clinical studies. Applications for cutaneous delivery of therapeutic siRNA are now emerging owing to a strong demand for effective treatments of various cutaneous disorders. Although successful studies have been performed using several different delivery techniques, most of these techniques encounter limitations for translation to the clinic with regards to patient compliance. This review describes the principal findings and applications in cutaneous RNAi therapy and focuses on the promises and pitfalls of the delivery systems.

Progress in heritable skin diseases: translational implications of mutation analysis and prospects of molecular therapies*

Epidermolysis bullosa, a group of blistering disorders, serves as the paradigm of the tremendous progress made in understanding the molecular genetics of heritable skin diseases. Mutations in 10 distinct genes have been disclosed in the classic forms of epidermolysis bullosa, and the level of expression of the mutated genes within the cutaneous basement membrane zone, the types and combinations of mutations and their consequences at the mRNA and protein levels, when placed in the context of the individual's genetic background and exposure to environmental trauma, all determine the subtype and the phenotypic severity in each case. The translational implications of mutation analysis include improved diagnosis and subclassification, refined genetic counseling of families at risk, and development of DNA-based pre natal and preimplantation genetic diagnosis. The prospects of molecular therapies for epidermolysis bullosa include further development of strategies for gene therapy, protein replacement therapy and cell-based therapies, including stem cell therapy and bone marrow transfer. Collectively, advances in the molecular genetics of heritable skin diseases clearly emphasize the value of basic research for improved diagnostics and patient care for genetic skin diseases.

siRNA silencing of keratinocyte-specific GFP expression in a transgenic mouse skin model

Small interfering RNAs (siRNAs) can be designed to specifically and potently target and silence a mutant allele, with little or no effect on the corresponding wild-type allele expression, presenting an opportunity for therapeutic intervention. Although several siRNAs have entered clinical trials, the development of siRNA therapeutics as a new drug class will require the development of improved delivery technologies. In this study, a reporter mouse model (transgenic click beetle luciferase/humanized monster green fluorescent protein) was developed to enable the study of siRNA delivery to skin; in this transgenic mouse, green fluorescent protein reporter gene expression is confined to the epidermis. Intradermal injection of siRNAs targeting the reporter gene resulted in marked reduction of green fluorescent protein expression in the localized treatment areas as measured by histology, real-time quantitative polymerase chain reaction and intravital imaging using a dual-axes confocal fluorescence microscope. These results indicate that this transgenic mouse skin model, coupled with in vivo imaging, will be useful for development of efficient and 'patient-friendly' siRNA delivery techniques and should facilitate the translation of siRNA-based therapeutics to the clinic for treatment of skin disorders.

Transgene-specific host responses in cutaneous gene therapy: the role of cells expressing the transgene

A major issue in long-term gene therapy is host immune responses to therapeutic cells when transgene encodes a potential antigen. The nature of these responses depends on several factors including the type of cell and tissue expressing the transgene. Keratinocytes and fibroblasts, which are known to display distinct immunogenic profiles, are both potential targets for transgene expression in cutaneous gene therapy. However, whether there is an immunological advantage in targeting one cell type over the other is not known. To study the effect of cell type on transgene-specific host responses independent of antigen levels or methods of gene transfer and transplantation, we used a skin transplantation model in which transgene expression can be targeted transgene to either keratinocytes or fibroblasts. Although targeting an antigen to either cell type resulted in the induction of immune responses, these responses differed significantly. Transgenic keratinocytes were rejected acutely by a dominant Th2 response, while in the majority of grafted animals transgenic fibroblasts failed to induce acute rejection despite the induction of Th1 type inflammation in the graft. In a small number of mice, transgenic fibroblasts persisted for at least 20 weeks despite elicitation of antigen-specific responses. Therefore, fibroblasts may be an immunologically preferred target over keratinocytes for cutaneous gene therapy.

Cutaneous DNA delivery and gene expression in ex vivo human skin explants via wet-etch microfabricated microneedles

Micro-needle arrays increase skin permeability by forming channels through the outer physical barrier, without stimulating pain receptors populating the underlying dermis. It was postulated that micro-needle arrays could facilitate transfer of DNA to human skin epidermis for cutaneous gene therapy applications. Platinum-coated "wet-etch" silicon micro-needles were shown to be of appropriate dimensions to create micro-conduits, approximately 50 microm in diameter, extending through the stratum corneum (SC) and viable epidermis. Following optimisation of skin explant culturing techniques and confirmation of tissue viability, the ability of the micro-needles to mediate gene expression was demonstrated using the beta-galactosidase reporter gene. Preliminary studies confirmed localised delivery, cellular internalisation and subsequent gene expression of pDNA following micro-needle disruption of skin. A combination of this innovative gene delivery platform and the ex vivo skin culture model will be further exploited to optimise cutaneous DNA delivery and address fundamental questions regarding gene expression in skin.

Epidermolysis bullosa: prospects for cell-based therapies

Heritable forms of epidermolysis bullosa (EB) are characterized by chronic, lifelong blistering and erosions due to mutations in 10 distinct genes expressed at the cutaneous basement membrane zone. No specific treatment for this group of intractable diseases is currently available. Recent progress in molecular therapies has indicated that cell-based approaches may potentially offer amelioration--and perhaps even cure--for afflicted individuals. In this issue, Wong et al. (2008) demonstrate the feasibility of direct intradermal injection of allogeneic fibroblasts to the lesional skin of patients with recessive dystrophic EB, with improvement in skin fragility.

Efficient in vivo targeting of epidermal stem cells by early gestational intraamniotic injection of lentiviral vector driven by the keratin 5 promoter

At the present time, no efficient in vivo method for gene transfer to skin stem cells exists. In this study, we hypothesized that early in gestation, specific epidermal stem cell populations may be accessible for gene transfer. To test this hypothesis, we injected lentiviral vectors encoding the green fluorescence protein marker gene driven by either the cytomegalovirus promoter or the keratin 5 (K5) promoter into the murine amniotic space at early developmental stages between embryonic days 8 and 12. This resulted in sustained green fluorescent protein (GFP) expression in both basal epidermal stem cells and bulge cells in the hair follicles of the skin. Transduction of stem cell populations was dependent on the developmental stage, and confirmed by the prolonged duration of GFP expression in all skin elements into adulthood. In addition, transduced stem cell populations responded to regenerative signals after wounding and actively participated in wound healing. Finally, we quantified the fraction of epidermal stem cells transduced, and the distribution of transduction related to the promoters utilized, confirming improved efficiency with the K5 promoter. This simple approach has possible biological applications in our study of gene functions in skin, and perhaps future clinical applications for treatment of skin based disorders.

Gene transfer in human skin with different pseudotyped HIV-based vectors

Pseudotyping lentiviral vector with other viral surface proteins could be applied for treating genetic anomalies in human skin. In this study, the modification of HIV vector tropism by pseudotyping with the envelope glycoprotein from vesicular stomatitis virus (VSV), the Zaire Ebola (EboZ) virus, murine leukemia virus (MuLV), lymphocytic choriomeningitis virus (LCMV), Rabies or the rabies-related Mokola virus encoding LacZ as a reporter gene was evaluated qualitatively and quantitatively in human skin xenografts. High transgene expression was detected in dermal fibroblasts transduced with VSV-G-, EboZ- or MuLV-pseudotyped HIV vector with tissue irregularities in the dermal compartments following repeated injections of EboZ- or LCMV-pseudotyped vectors. Four weeks after transduction, double-labeling immunofluorescence of beta-galactosidase and involucrin or integrin beta1 demonstrated that VSV-G-, EboZ- or MuLV-pseudotyped HIV vector effectively targeted quiescent epidermal stem cells which underwent terminal differentiation resulting in transgene expression in their progenies. Among the six different pseudotyped HIV-based vectors evaluated, VSV-G-pseudotyped vector was found to be the most efficient viral glycoprotein for cutaneous transduction as demonstrated by the highest level of beta-galactosidase expression and genome copy number evaluated by TaqMan PCR.

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.

Host immune responses in ex vivo approaches to cutaneous gene therapy targeted to keratinocytes

Epidermal gene therapy may benefit a variety of inherited skin disorders and certain systemic diseases. Both in vivo and ex vivo approaches of gene transfer have been used to target human epidermal stem cells and achieve long-term transgene expression in immunodeficient mouse/human chimera models. Immunological responses however, especially in situations where a neoantigen is expressed, are likely to curtail expression and thereby limit the therapy. In vivo gene transfer to skin has been shown to induce transgene-specific immune responses. Ex vivo gene transfer approaches, where keratinocytes are transduced in culture and transplanted back to patient, however, may avoid signals provided to the immune system by in vivo administration of vectors. In the current study, we have developed a stable epidermal graft platform in immunocompetent mice to analyze host responses in ex vivo epidermal gene therapy. Using green fluorescent protein (GFP) as a neoantigen and an ex vivo retrovirus-mediated gene transfer to mouse primary epidermal cultures depleted of antigen-presenting cells (APCs), we show induction of GFP-specific immune responses leading to the clearance of transduced cells. Similar approach in immunocompetent mice tolerant to GFP resulted in permanent engraftment of transduced cells and continued GFP expression. Activation of transgene-specific immune responses in ex vivo gene transfer targeted to keratinocytes require cross-presentation of transgene product to APCs, a process that is most amenable to immune modulation. This model may be used to explore strategies to divert transgene-specific immune responses to less destructive or tolerogenic ones.

Preservation of phenotype in an organotypic cell culture model of a recessive keratinization defect of Norfolk terrier dogs

The purpose of this study is to reproduce in vitro a recessive keratinization defect of Norfolk terrier dogs characterized by a lack of keratin 10 (K10) production. Keratinocytes from skin biopsy samples of four normal dogs and two affected dogs were cultured organotypically with growth factor-supplemented media in order to stimulate cornification. The cultured epidermis from the normal dogs closely resembled the normal epidermis in vivo and cornified. The cultured epidermis from the affected dogs displayed many phenotypic alterations identified in skin biopsies from dogs with this heritable defect. Immunohistochemistry and immunoblotting showed a marked decrease in K10 from the cultures of the affected keratinocytes, compared to that in K10 from the cultures of the normal keratinocytes. Real-time reverse transcription polymerase chain reaction quantitation showed a 31-fold decrease in K10, a 1.75-fold increase in K1 and a 136-fold increase in K2e between the affected and the normal epidermis. Organotypic keratinocytes showed a 241-fold decrease in K10, a 31-fold decrease in K1 and a 1467-fold decrease in K2e between the affected and normal cultures. Although in vitro keratin expression did not precisely simulate in vivo, the morphology of the normal and the affected epidermis was largely preserved; thus, this culture system may provide an alternative to in vivo investigations for cutaneous research involving cornification.

Lentivirus-mediated gene transfer to human epidermis

For long-term cutaneous gene therapy, the therapeutic gene must be targeted to stem cells and be stably transmitted to and expressed in descendant cells. Retroviral vectors are highly efficient in gene transfer to human keratinocyte stem cells in culture; however, they cannot transduce quiescent stem cells in vivo. As lentiviral vectors (LVV) transduce non-proliferating cells, their ability to target human epidermal stem cells was evaluated. LVV were highly efficient in gene transfer to clonogenic keratinocytes in vitro. Despite higher transgene DNA content and comparable levels of transgene mRNA, levels of transgene product directed by lentivectors were 3-folds lower than that of retrovectors. When transduced keratinocytes were grafted onto mice, transgene expression persisted for at least 20 wk; however, transgene product was detected primarily in the uppermost layers of epidermis. Inclusion of an element that is known to facilitate nuclear export of intron-less transcripts, resulted in enhanced transgene expression in keratinocytes. In vivo transduction of xenografted human skin with these vectors resulted in efficient gene transfer to epidermal progenitor cells. These results demonstrate stem cell transduction by LVV and point out the utility of using these vectors for direct gene transfer to and sustained expression in human epidermis.

Ultrasound and transdermal drug delivery

Transdermal drug delivery offers an attractive alternative to the conventional drug delivery methods of oral administration and injection. However, the stratum corneum acts as a barrier that limits the penetration of substances through the skin. Application of ultrasound to the skin increases its permeability (sonophoresis) and enables the delivery of various substances into and through the skin. This review presents the main findings in the field of sonophoresis, namely transdermal drug delivery and transdermal monitoring. Particular attention is paid to proposed enhancement mechanisms and future trends in the field of cutaneous vaccination and gene delivery.

Suppression of the immune response against exogenous desmoglein 3 in desmoglein 3 knockout mice: an implication for gene therapy

Gene therapies for recessive genetic diseases may provoke unwanted immune responses against the introduced gene product because patients, especially those with null mutation of a certain protein, have no tolerance for the protein of interest. This study used desmoglein 3 knockout (Dsg3-/-) mice as a disease model for a genetic defect in DSG3, to investigate whether nonviral gene therapy induces an immune response against Dsg3 and whether the reaction against Dsg3 can be prevented. When mouse Dsg3 cDNA was injected in the skin of Dsg3-/- mice, 50% of treated Dsg3-/- mice developed anti-Dsg3 IgG, which can bind native Dsg3 in vivo. To prevent this response, we used an anti-CD40L monoclonal antibody, MR1, which blocks the costimulatory interaction between CD40 and CD40L. To evaluate the effect of MR1, we grafted Dsg3+/+skin on Dsg3-/- mice, to mimic stable gene transfer of Dsg3. After skin grafting, all the recipient Dsg3-/- mice were treated with either MR1 (n=8) or control hamster IgG (n=8). All of the control IgG-treated mice developed circulating anti-Dsg3 IgG about 2 wk after grafting, and IgG deposition was observed on the surfaces of keratinocytes in the grafted Dsg3+/+skin. Such anti-Dsg3 IgG production was significantly prevented, however, when the recipient mice were treated with MR1. These findings suggested that gene therapies for recessive diseases may provoke an immune response against the transgene product, and that the CD40-CD40L interaction might be a reasonable target for effective prevention of such undesirable immune responses, leading, in turn, to a successful gene therapy.

Immune-mediated loss of transgene expression in skin: implications for cutaneous gene therapy

A clearer understanding of the immune-mediated loss of transgene from cutaneous epithelium is necessary for development of effective clinical gene therapy protocols for patients who carry null mutations in the target gene. We have used retrovirus-mediated transfer of lacZ to mouse skin as a model to investigate the mechanism of immune-mediated transgene loss in skin. Transduction of C57Bl/6 mouse skin resulted in elicitation of both humoral and cellular immune responses. Antibody responses did not play a major role in the loss of transgene. Infiltration of the transduced skin with CD4(+) and CD8(+) cells and induction of transgene-specific cytotoxic T lymphocytes implied a role for T-cell-mediated responses. Transduction of mice deficient in either major histocompatibility complex (MHC) class I or class II molecules resulted in transient transgene expression. Only in MHC(-/-) mice lacking expression of both class I and class II MHC molecules was persistent transgene expression seen. These data indicate a primary role for T-cell-mediated responses in the immune-mediated loss of transgene expression. Furthermore, CD4 and CD8 T cells have overlapping roles and either population can effectively eliminate transduced cells. Therefore, long-term cutaneous gene therapy may require development of strategies to interfere with activation or function of both T cell populations.

Development of spliceosome-mediated RNA trans-splicing (SMaRT) for the correction of inherited skin diseases

Gene therapy of large genes (e.g. plectin and collagen genes) is hampered by size limitations for insertions of the currently used viral vectors. To reduce the size of these insertions spliceosome-mediated RNA trans-splicing (SMaRT), which provides intron-specific gene-correction at the pre-RNA level, can be an alternative approach. To test its applicability in skin gene therapy, SMaRT was used in the context of the 4003delTC mutation in the collagen XVII gene (COL17A1) causing generalized atrophic benign junctional epidermolysis bullosa. A beta-galactosidase (beta-gal) trans-splicing assay system was established using intron 51 of COL17A1 as the target for trans-splicing. In this system, intron 51 is flanked by the 5'exon and the 3'exon of the beta-gal gene, the latter containing two in-frame stop codons. Cotransfection of a pre-trans-splicing molecule consisting of the binding domain of intron 51 and the 3'exon of beta-gal without the stop codons resulted in a 300-fold increase of beta-gal activity compared to controls. A 2-3-fold increase in efficiency was obtained through an elongation of the binding domains. Replacement of the complete 3'end of the COL17A1 gene was shown using a collagen XVII mini-gene construct. The beta-gal assay was used in human keratinocytes to evaluate the influence of a keratinocyte-specific spliceosome background. Reverse transcription polymerase chain reaction and beta-gal activity assay showed functional correction of the stop-codons in cultured human keratinocytes and in an immortalized GABEB cell line harbouring the 4003delTC mutation. These results demonstrate that SMaRT is feasible in a keratinocyte-specific context and therefore may be applied in skin gene therapy.

Epidermolysis bullosa simplex keratinocytes with extended lifespan established by ectopic expression of telomerase

As part of a strategy to develop somatic gene therapy of epidermolysis bullosa simplex (EBS) we have established patient keratinocytes with expanded lifespan by ectopic expression of the human telomerase gene (hTert). The presence of an active telomerase enzyme was demonstrated by the telomerase repeat amplification protocol (TRAP). The hTert(+) cells have a normal karyotype and the cells have, until now, undergone more than 80 population doublings (PDs) after hTert retroviral transduction while control cells exhibited senescence-associated proliferation arrest after 8 PDs. In organotypic culture the hTert(+) cells are capable of forming a stratified epidermis illustrating their preserved ability to differentiate.

Progress in molecular genetics of heritable skin diseases: the paradigms of epidermolysis bullosa and pseudoxanthoma elasticum

The 42nd Annual Symposium on the Biology of the Skin, entitled "The Genetics of Skin Disease", was held in Snowmass Village, Colorado, in July 1993. That meeting presented the opportunity to discuss how modern approaches to molecular genetics and molecular biology could be applied to understanding the mechanisms of skin diseases. The published proceedings of this meeting stated that "It is an opportune time to examine the genetics of skin disease" (Norris et al, 1994). Indeed, this meeting just caught the wave of early pioneering studies that have helped us to understand the molecular basis of a large number of genodermatoses. This overview presented in the 50th Annual Symposium on the biology of the skin, highlights the progress made in the molecular genetics of heritable skin diseases over the past decade.

Genetic reversion of inherited skin disorders

Human epidermis is a squamous stratified epithelium whose integrity relies on balanced processes of cell attachment, proliferation, and differentiation. In monogenic skin dermatoses, such as mecano-bullous diseases, or DNA repair deficiencies such as the xeroderma pigmentosum (XP), alterations of skin integrity may have devastating consequences as illustrated by the extremely high epidermal cancer proneness of XP patients. The lack of efficient pharmacological treatments, the easy accessibility of skin, and the possibility of long term culture and genetic manipulations ex vivo of epidermal keratinocytes, have encouraged approaches toward gene transfer and skin therapy prospects. We review here some of the human genetic disorders that exhibit major traits in skin, as well as requirements and difficulties inherent to approaches aimed at stable phenotypic correction.

Topical colchicine selection of keratinocytes transduced with the multidrug resistance gene (MDR1) can sustain and enhance transgene expression in vivo

For skin gene therapy, achieving prolonged high-level gene expression in a significant percentage of keratinocytes (KC) is difficult because we cannot selectively target KC stem cells. We now demonstrate that topical colchicine treatment can be used to select, in vivo, KC progenitor cells transduced with the multidrug resistance gene (MDR1). When human skin equivalents containing MDR1-transduced KC were grafted onto immunocompromised mice, topical colchicine treatments significantly increased (7-fold) the percentage of KC expressing MDR1, compared to vehicle-treated controls, for up to 24 wk. Topical colchicine treatment also significantly enhanced the amount of MDR1 protein expressed in individual KC. Furthermore, quantitative real-time PCR analysis of MDR1 transgene copy number demonstrates that topical colchicine treatment selects and enriches for KC progenitor cells in the skin that contain and express MDR1. For clinical skin gene therapy applications, this in vivo selection approach promises to enhance both the duration and expression level of a desired therapeutic gene in KC, by linking its expression to the MDR1 selectable marker gene.

Cutaneous gene transfer for skin and systemic diseases

Recent progress in molecular genetics has illuminated the basis for a wide variety of inherited and acquired diseases. Gene therapy offers an attractive therapeutic approach capitalizing upon these new mechanistic insights. The skin is a uniquely attractive tissue site for development of new genetic therapeutic approaches both for its accessibility as well as for the large number of diseases that are amenable in principle to cutaneous gene transfer. Amongst these opportunities are primary monogenic skin diseases, chronic wounds and systemic disorders characterized by low or absent levels of circulating polypeptides. For cutaneous gene therapy to be effective, however, significant progress is required in a number of domains. Recent advances in vector design, administration, immune modulation, and regulation of gene expression have brought the field much nearer to clinical utility.

Tissue-engineered skin substitutes

The last two years have seen new tissue-engineered skin substitutes come onto the market and begin to resolve the various roles to which each is best suited. It is becoming evident that some of the very expensive cell-based products have cost-benefit advantage despite their high price and are valuable within the restricted applications for which they are intended. The use of skin substitutes for testing purposes has extended from epidermal keratinocytes to other integumentary epithelia and into preparations containing multiple cell types in which reactions resulting from paracrine interactions can be examined. Challenges remain in the application of gene therapy techniques to skin substitutes, both the control of transgene expression and in the selection of suitable genes to transfect. A coming challenge is the production of tissue-engineered products without the use of animal products other than human cells. A challenge that may be diminishing is the importance of acute rejection of allogeneic tissue-engineered skin substitutes.

Sustainable systemic delivery via a single injection of lentivirus into human skin tissue

The skin offers a tissue site accessible for delivery of gene-based therapeutics. To develop the capability for sustained systemic polypeptide delivery via cutaneous gene transfer, we generated and injected pseudotyped HIV-1 lentiviral vectors intradermally at a range of doses into human skin grafted on immune-deficient mice. Unlike Moloney murine leukemia virus (MLV)-based retrovectors, which failed to achieve detectable cutaneous gene transfer by this approach, lentivectors effectively targeted all major cell types within human skin tissue, including fibroblasts, endothelial cells, keratinocytes, and macrophages. After a single injection, lentivectors encoding human erythropoietin (EPO) produced dose-dependent increases in serum human EPO levels and hematocrit that increased rapidly within one month and remained stable subsequently. Delivered gene expression was confined locally at the injection site. Excision of engineered skin led to rapid and complete loss of human EPO in the bloodstream, confirming that systemic EPO delivery was entirely due to lentiviral targeting of cells within skin rather than via spread of the injected vector to visceral tissues. These findings indicate that the skin can sustain dosed systemic delivery of therapeutic polypeptides via direct lentivector injection and thus provide an accessible and reversible approach for gene-based delivery to the bloodstream.