Sustained production of human transferrin by transduced fibroblasts implanted into athymic mice: a model for somatic gene therapy

Gene Therapy and its Application in Dermatology

Gene therapy is an experimental technique to treat genetic diseases. It is based on the introduction of nucleic acid with the help of a vector, into a diseased cell or tissue, to correct the gene expression and thus prevent, halt, or reverse a pathological process. It is a promising treatment approach for genetic diseases, inherited diseases, vaccination, cancer, immunomodulation, as well as healing of some refractory ulcers. Both viral and nonviral vectors can be used to deliver the correct gene. An ideal vector should have the ability for sustained gene expression, acceptable coding capacity, high transduction efficiency, and devoid of mutagenicity. There are different techniques of vector delivery, but these techniques are still under research for assessment of their safety and effectiveness. The major challenges of gene therapy are immunogenicity, mutagenicity, and lack of sustainable therapeutic benefit. Despite these constraints, therapeutic success was obtained in a few genetic and inherited skin diseases. Skin being the largest, superficial, easily accessible and assessable organ of the body, may be a promising target for gene therapy research in the recent future.

Endocytic Relay as a Potential Means for Enhancing Ligand Transport through Cellular Tissue Matrices: Analysis and Possible Implications for Drug Delivery

The transport of peptide ligands, such as cytokines, through tissue is complicated by resistances due to cell multilayers and holdup in extracellular matrix. To determine whether it is possible for receptor-mediated endocytic trafficking to enhance ligand transport, we have developed a mathematical model of ligand flux through tissue containing cells possessing complementary receptors. Tissue is considered as two phases: the cell phase and the matrix phase; thus tissue is modeled as analogous to a packed bed reactor. This model allows calculation of steady-state flux of intact and degraded peptide through a one-dimensional cell/tissue matrix. Both environmental and molecular parameters were considered in this study. Results predict that three quantities should have a major influence on growth factor flux: the ratio of matrix diffusivity to intracellular "diffusivity" (D(m)/D(i)), the extracellular matrix proteolysis rate constant (k (prot)), and the fraction of internalized growth factor degraded (f(1)). For basal levels of intracellular degradation (0 < f(1) >/= 0.05) but no extracellular proteolysis, significant enhancement is possible only for D(m)/D(i) >/=1. f(1) increases, enhancement is only possible up to f(1)= 0.07 even for D(m)/D(i) < 1. For significant levels of extracellular proteolysis (k (prot) > 0), the requirements for D(m)/D(i) and f(1) to permit transport enhancement encompass a broader range with the exact values dependent on k (prot). These insights may be helpful for delivery of ligands generated from controlled-release devices or genetically modified autocrine cells, and may also provide better understanding of cytokine transport in embryonic development.

An approach to achieve long-term expression in skin gene therapy

For gene therapy purposes, the skin is an attractive organ to target for systemic delivery of therapeutic proteins to treat systemic diseases, skin diseases, or skin cancer. To achieve long-term stable expression of a therapeutic gene in keratinocytes (KC), we have developed an approach using a bicistronic retroviral vector expressing the desired therapeutic gene linked to a selectable marker (multidrug resistant gene, MDR) that is then introduced into KC and fibroblasts (FB) to create genetically modified human skin equivalent (HSE). After grafting the HSE onto immunocompromised mice, topical colchicine treatment is used to select and enrich for genetically modified keratinocyte stem cells (KSC) that express MDR and are resistant to colchicine's antimitotic effects. Both the apparatus for topical colchicine delivery and the colchicine doses have been optimized for application to human skin. This approach can be validated by systemic delivery of therapeutic factors such as erythropoietin and the antihypertensive atrial natriuretic peptide.

Adenoviral gene delivery to primary human cutaneous cells and burn wounds

The adenoviral transfer of therapeutic genes into epidermal and dermal cells is an interesting approach to treat skin diseases and to promote wound healing. The aim of this study was to assess the in vitro and in vivo transfection efficacy in skin and burn wounds after adenoviral gene delivery. Primary keratinocytes (HKC), fibroblasts (HFB), and HaCaT cells were transfected using different concentrations of an adenoviral construct (eGFP). Transfection efficiency and cytotoxicity was determined up to 30 days. Expression was quantified by FACS analysis and fluorimeter. Cytotoxicity was measured using the trypan blue exclusion method. 45 male Sprague Dawley rats received 2x10(8) pfu of Ad5-CMV-LacZ or carrier control intradermally into either superficial partial thickness scald burn or unburned skin. Animals were euthanized after 48 h, 7 or 14 days posttreatment. Transgene expression was assessed using immunohistochemistry and bioluminescent assays. The highest transfection rate was observed 48 h posttransfection: 79% for HKC, 70% for HFB, and 48% for HaCaT. The eGFP expression was detectable in all groups over 30 days (P>0.05). Cytotoxic effects of the adenoviral vector were observed for HFB after 10 days and HaCaT after 30 days. Reporter gene expression in vivo was significantly higher in burned skin compared with unburned skin (P=0,004). Gene expression decreases from 2 to 7 days with no significant expression after 14 days. This study demonstrates that effective adenoviral-mediated gene transfer of epidermal primary cells and cell-lines is feasible. Ex vivo gene transfer in epithelial cells might have promise for the use in severely burned patients who receive autologous keratinocyte sheets. Transient cutaneous gene delivery in burn wounds using adenoviral vectors causes significant concentrations in the wound tissue for at least 1 week. Based on these findings, we hypothesize that transient cutaneous adenoviral gene delivery of wound healing promoting factors has potential for clinical application.

Injection of genetically engineered fibroblasts corrects regenerated human epidermolysis bullosa skin tissue

Current therapeutic strategies for genetic skin disorders rely on the complex process of grafting genetically engineered tissue to recipient wound beds. Because fibroblasts synthesize and secrete extracellular matrix, we explored their utility in recessive dystrophic epidermolysis bullosa (RDEB), a blistering disease due to defective extracellular type VII collagen. Intradermal injection of RDEB fibroblasts overexpressing type VII collagen into intact RDEB skin stably restored correctly localized type VII collagen expression in vivo and normalized hallmark RDEB disease features, including subepidermal blistering and anchoring fibril defects. This article was published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

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.

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.

Virus-mediated gene transfer for cutaneous gene therapy

Cutaneous gene therapy offers unique opportunities and limitations in the use of viral vectors for corrective gene transfer. Skin presents a formidable barrier to microbial invasion and is nourished by small blood vessels, thus ruling out the possibility of directed virus delivery through cannulated blood vessels. However, skin is physically accessible and its resident keratinocyte stem cell population is susceptible to direct in vivo transduction with retroviral vectors. Furthermore, keratinocyte stem cells transduced in culture have been shown to persist and to express the encoded transgene when grafted to immunocompromised mice. Cutaneous gene therapy trials are likely to involve virus-mediated transduction as a principal means of gene transfer.

Collagen-based biomaterials as 3D scaffold for cell cultures: applications for tissue engineering and gene therapy

Many substances are used in the production of biomaterials: metals (titanium), ceramics (alumina), synthetic polymers (polyurethanes, silicones, polyglycolic acid (PGA), polylactic acid (PLA), copolymers of lactic and glycolic acids (PLGA), polyanhydrides, polyorthoesters) and natural polymers (chitosan, glycosaminoglycans, collagen). With the rapid development in tissue engineering, these different biomaterials have been used as three-dimensional scaffolds and cell transplant devices. The principal biochemical and biological characteristics of the collagen-based biomaterials are presented, including their interactions with cells (fibroblasts), distinct from those of synthetic polymers, and their potential use in gene therapy through the formation of neo-organs or organoids.

Gene therapy and dermatology: more than just skin deep

BACKGROUND

Recent advances in the molecular characterization of dermatologic disease have substantively augmented the understanding of the pathogenetic processes underlying disorders of the skin. This new knowledge coupled with progress in gene delivery technologies has paved the way for introducing cutaneous gene therapy into the dermatologic therapeutic armamentorium.

OBJECTIVE

This review article includes an overview of the current strategies for delivery of gene therapy with an emphasis on the potential role of cutaneous gene delivery in the treatment of skin and systemic diseases.

CONCLUSIONS

Accessibility for gene delivery, clinical evaluation, and topical modulation of gene expression render the skin a very attractive tissue for therapeutic gene delivery. However, there are several key hurdles to be overcome before cutaneous gene therapy becomes a viable clinical option. These include difficulties in inducing sustained expression of the desired gene in vivo, the challenge of targeting genes to long-lived stem cells, and the difficulty in achieving specific and uniform transfer to different compartments of the skin. However, these problems are not insurmountable and will likely be resolved in conjunction with ongoing advances in delineating gene expression profiles and other molecular properties of the skin, strategies for stem cell isolation, and improved approaches to regulating gene delivery and expression. These advances should create the framework for translating the enormous potential of cutaneous gene therapy into the clinical arena and, thereby, substantively improving the management of both cutaneous and systemic disease.

Biologic aspects of expression of stably integrated transgenes in cells of the skin in vitro and in vivo

The observation that transgenes can be stably integrated into the genome of fibroblasts using recombinant retroviruses enhanced interest in using these cells as a vector for gene therapy. This enthusiasm has lessened during the past 8 years, not because skin has lost the features that make it attractive for gene therapy, but rather because stable transgene expression in vivo has not been achieved. All investigators who have used genetically modified fibroblasts to study in vivo aspects of gene therapy have shown a decrease in transgene expression with time. This contrasts with transgene expression in similarly transduced fibroblasts in vitro, where expression is not lost or is lost very slowly. We have initiated an approach to bring further understanding to the biology of transgene expression by fibroblasts carrying stably integrated transgenes in an in vivo setting. Experiments described permit the following conclusions. Expression by and survival of genetically modified fibroblasts a) requires a persistent matrix scaffold in in vivo settings; b) is prolonged if the matrix is allowed to mature in vitro; c) is enhanced if the matrix is partially sequestered behind a coating of normal fibroblasts; and d) can be substantively prolonged in vivo by immortalizing the cells. These observations support the notion that prolonged expression of transgenes by fibroblasts can be achieved in vivo and that gene therapy utilizing fibroblasts and other cells of the skin has clinical utility.

Persistent transgene expression and normal differentiation of immortalized human keratinocytes in vivo

Cells transduced ex vivo with transgenes encoded on retroviruses have constant and prolonged expression in vitro; however, in vivo expression is quickly lost. Much attention has been directed at methods to circumvent this problem. We have shown that loss of transgene expression does not occur when transduced immortalized 3T3 cells are transplanted to the in vivo setting of athymic mice. Ease of acquisition and potential for clinical application led us to assess the potential of using immortalized human keratinocytes for expression of transgenes in vivo. Human keratinocytes were immortalized with a HPV16-E6/E7 retrovirus, transduced with a lacZ retrovirus, cloned by limiting dilution, seeded onto a physiologic dermal substrate, and transplanted to athymic mice. Six weeks after transplantation, the immortalized transgene expressing keratinocytes had formed an epidermis that was indistinguishable from one formed by nonimmortalized keratinocytes; furthermore, there was no loss of expression of the lacZ gene. These observations show that methods to extend cell survival are an alternative approach to achieving stable and prolonged expression of transgenes in vivo and that HPV16-E6/ E7 immortalized keratinocytes generate an epidermis with normal morphology.

Evidence for keratinocyte stem cells in vitro: long term engraftment and persistence of transgene expression from retrovirus-transduced keratinocytes

Epidermis is renewed by a population of stem cells that have been defined in vivo by slow turnover, label retention, position in the epidermis, and enrichment in beta1 integrin, and in vitro by clonogenic growth, prolonged serial passage, and rapid adherence to extracellular matrix. The goal of this study is to determine whether clonogenic cells with long-term growth potential in vitro persist in vivo and give rise to a fully differentiated epidermis. Human keratinocytes were genetically labeled in culture by transduction with a retrovirus encoding the lacZ gene and grafted to athymic mice. Analysis of the cultures before grafting showed that 21.1-27.8% of clonogenic cells with the capacity for >30 generations were successfully transduced. In vivo, beta-galactosidase (beta-gal) positive cells participated in the formation of a fully differentiated epithelium and were detected throughout the 40-week postgraft period, initially as loosely scattered clusters and later as distinct vertical columns. Viable cells recovered from excised grafts were seeded at clonal densities and 23.3-33.3% of the colonies thus formed were beta-gal positive. In addition, no evidence of transgene inactivation was obtained: all keratinocyte colonies recovered from grafted tissue that were beta-gal negative also lacked the lacZ transgene. These results show that cells with long-term growth properties in vitro do indeed persist in vivo and form a fully differentiated epidermis, thereby exhibiting the properties of stem cells.

Therapeutic gene delivery to the skin

The skin represents a site for treatment of cutaneous and systemic disease and is the most accessible somatic tissue for therapeutic gene transfer in humans. Monogenic hereditary skin diseases, such as ichthyosis and epidermolysis bullosa subtypes, and disorders characterized by low levels of polypeptides in the systemic circulation, are current central foci of efforts in cutaneous-gene transfer. Additional efforts center on the treatment of wounds and malignancies. Recent developments in models of gene delivery to the skin underscore key challenges that must be met before successful treatment of human disease by cutaneous gene delivery can be achieved.

Gene therapy for tissue repair: approaches and prospects

Recent advances in molecular biology have resulted in the development of new technologies for the introduction and expression of genes in human somatic cells. This emerging field, known as gene therapy, is broadly defined as the transfer of genetic material to cells/tissues in order to achieve a therapeutic effect for inherited as well as acquired diseases. We and others are exploring the potential application of this technology to tissue repair. One primary focus has been to transfer genes encoding wound healing growth factors, a broad class of proteins which control local events in tissues such as cell proliferation, cell migration and the formation of extracellular matrix. Using several different strategies for gene transfer, wound healing growth factor genes have been introduced and expressed in cells and tissues in vitro as well as in vivo. Various experimental models of wound healing and tissue repair have been used to evaluate the efficacy of this new and exciting approach to tissue repair.

Alteration in the organ distribution of iron by truncated transferrin: implications for iron chelation therapy

The ability of the partial molecule of transferrin, truncated transferrin (t-Tf), to act as an excretable biologic iron chelator was examined. We confirmed the observations of Zak and Aisen (Zak O, Aisen P. Biochem Biophys Acta 1985;1952:24-8) that thermolysin treatment of human transferrin produces half molecules that retain iron-binding capacity. These molecules are poorly recognized by surface receptors on either human or murine cells. Although the plasma half-life of human transferrin in mice is moderately long (40 hours), injection of t-Tf into mice results in its rapid clearance (half-life = 10 minutes). Injection of iron 59-labeled transferrin results in the deposition of iron in the major hematopoetic organs of mice such as the spleen, bone marrow, and liver. Injection of 59Fe-labeled t-Tf results in the quantitative recovery of iron in the kidneys: 59Fe is retained in the kidney for substantial periods of time with little evidence of its excretion into urine. Injection of iodine 125-labeled t-Tf also results in the deposition of radioactivity in the kidneys, but 125I is rapidly excreted into the urine, where it is detected as free iodine. These results indicate that although t-Tf is directed to the kidney and filtered by the glomerulus, the molecule is reabsorbed and degraded, and iron is retained. These results have implications in the design of iron chelators.

Use of cloned genetically modified human fibroblasts to assess long-term survival in vivo

Because human fibroblasts are easily brought to tissue culture conditions and can be stably transduced with retroviral vectors encoding transgenes ex vivo, genetically modified fibroblasts are frequently considered in strategies to correct disease with gene therapy. This enthusiasm has been dampened by studies showing that transgene expression by genetically modified fibroblasts diminishes with time in vivo, but not in vitro, for reasons that are unclear. We elected to study this problem using cloned human fibroblasts that had been cloned by limiting dilution and stably transduced with a retroviral vector encoding lacZ ex vivo. These were seeded onto a nonbiodegradable nylon matrix that was transplanted to nude mice. Transgene expression was followed prospectively by histologic exam. Data show that human fibroblasts can withstand the pressure of cloning by limiting dilution. In addition, they can be passaged from 10 to > 20 times, and > 1 x 10(20) of genetically modified fibroblasts can be generated as progeny of one cell. Loss of transgene expression by the cloned genetically modified fibroblasts in vivo occurs in an orderly and progressive fashion, but is not complete by 4 months. Neither the loss nor the persistence of expression appear to be random. These observations are most compatible with the thesis that a major cause of the loss of transgene expression in vivo is secondary to apoptosis of the genetically modified fibroblast. Loss of expression of transgenes in senescent genetically modified fibroblasts occurs more rapidly than in their presenescent counterparts in the age-neutral, in vivo setting of the nude mouse.

Cutaneous gene therapy

Gene therapy efforts in a variety of tissues have foundered on fundamental technologic barriers, such as difficulties in achieving high-efficiency gene transfer to diseased tissues and in sustaining delivered transgene production. The skin offers an attractive tissue for development of approaches to therapeutic gene delivery by virtue of its accessibility for regulation by topical agents, the ease of gene transfer into cutaneous tissues, and the ready ability to monitor the impact of somatic gene transfer. With the ability of the skin to deliver therapeutic polypeptides to the systemic circulation and the recent molecular characterization of monogenic skin diseases, efforts to target genes to the skin are expected to accelerate. The current status of gene therapy efforts is reviewed, with a special focus on the skin.