Systemic delivery of secreted protein by grafts of epidermal keratinocytes: prospects for keratinocyte gene therapy

Xeroderma Pigmentosum: General Aspects and Management

Xeroderma Pigmentosum (XP) is a rare genetic syndrome with a defective DNA nucleotide excision repair. It is characterized by (i) an extreme sensitivity to ultraviolet (UV)-induced damages in the skin and eyes; (ii) high risk to develop multiple skin tumours; and (iii) neurologic alterations in the most severe form. To date, the management of XP patients consists of (i) early diagnosis; (ii) a long-life protection from ultraviolet radiation, including avoidance of unnecessary UV exposure, wearing UV blocking clothing, and use of topical sunscreens; and (iii) surgical resections of skin cancers. No curative treatment is available at present. Thus, in the last decade, in order to prevent or delay the progression of the clinical signs of XP, numerous strategies have been proposed and tested, in some cases, with adverse effects. The present review provides an overview of the molecular mechanisms featuring the development of XP and highlights both advantages and disadvantages of the clinical approaches developed throughout the years. The intention of the authors is to sensitize scientists to the crucial aspects of the pathology that could be differently targeted. In this context, the exploration of the process underlining the conception of liposomal nanocarriers is reported to focus the attention on the potentialities of liposomal technology to optimize the administration of chemoprotective agents in XP patients.

Apolipoprotein E does not cross the blood-cerebrospinal fluid barrier, as revealed by an improved technique for sampling CSF from mice

Apolipoprotein E (apoE) is a 34-kDa glycoprotein that is important in lipoprotein metabolism both peripherally and centrally. Because it is primarily produced in the liver, apoE observed in the brain or cerebrospinal fluid (CSF) could have originated in the periphery; i.e., circulating apoE may cross the blood-brain barrier (BBB) and/or enter CSF and be taken up by brain cells. To determine whether this occurs, a second-generation adenovirus encoding human apoE3 was administered intravenously (iv) to C57BL/6J mice, and the detection of human apoE3 in the CSF was used as a surrogate measure of central availability of this protein utilizing an improved method for sampling CSF from mice. This improved technique collects mouse CSF samples with a 92% success rate and consistently yields relatively large volumes of CSF with a very low rate of blood contamination, as determined by molecular assessment of apolipoprotein B, a plasma-derived protein that is absent in the central nervous system. Through this improved method, we demonstrated that in mice receiving the administered apoE3 adenovirus, human apoE3 was expressed at high levels in the liver, leading to high levels of human apoE3 in mouse plasma. In contrast, human apoE3 levels in the CSF, as assessed by a sensitive ELISA, were essentially undetectable in human apoE3 adenovirus-treated mice, and comparable to levels in LacZ adenovirus-treated control mice. These data indicate that apoE in the CSF cannot be derived from the plasma pool and, therefore, must be synthesized locally in the brain.

A study of the expression of functional human coagulation factor IX in keratinocytes using a nonviral vector regulated by K14 promoter

Ex vivo gene therapy requires a suitable bioreactor for production and delivery of the gene products into a target tissue, and keratinocyte is suitable model in this regard because of its potential for systemic release of proteins. To establish a keratinocyte-specific expression system, a mammalian-based expression plasmid equipped with a 2,240-bp fragment from the human keratin 14 (k14) gene enhancer/promoter region was constructed and used for the insertion of the human coagulation factor IX (hFIX)-cDNA downstream the K14-derived regulatory elements. The human epidermal keratinocytes isolated from neonatal foreskin were cultivated in keratinocyte serum-free media and transfected with the recombinant plasmid. The K14-promoter-driven expression of recombinant hFIX (rhFIX) was evaluated by performing coagulation test as well as enzyme-linked immunosorbent assay on the cultured media collected from the transfected cells at various stages. The rhFIX corresponding transcript and protein were confirmed by performing reverse transcription PCR as well as immunoblotting experiments, respectively. Based on the coagulation activities obtained from the conditioned media of nine isolated clones, the hFIX expression levels vary from 5% to 39% of normal human plasma. Expression levels of the hFIX obtained in this study are comparable to those reported for viral systems. The obtained data supported the potential of keratinocyte for the expression and secretion of biologically active rhFIX and underscore the importance of the examined cis sequences for enhancing gene expression in a mammalian expression system. Besides, it has provided means for further bioengineering strategies to improve the expression efficiency of the hFIX in keratinocytes and other mammalian host cells.

A gene therapy approach for long-term normalization of blood pressure in hypertensive mice by ANP-secreting human skin grafts

The use of bioengineered human skin as a bioreactor to deliver therapeutic factors has a number of advantages including accessibility that allows manipulation and monitoring of genetically modified cells. We demonstrate a skin gene therapy approach that can regulate blood pressure and treat systemic hypertension by expressing atrial natriuretic peptide (ANP), a hormone able to decrease blood pressure, in bioengineered human skin equivalents (HSE). Additionally, the expression of a selectable marker gene, multidrug resistance (MDR) type 1, is linked to ANP expression on a bicistronic vector and was coexpressed in the human keratinocytes and fibroblasts of the HSE that were grafted onto immunocompromised mice. Topical treatments of grafted HSE with the antimitotic agent colchicine select for keratinocyte progenitors that express both MDR and ANP. Significant plasma levels of human ANP were detected in mice grafted with HSE expressing ANP from either keratinocytes or fibroblasts, and topical selection of grafted HSE resulted in persistent high levels of ANP expression in vivo. Mice with elevated plasma levels of human ANP showed lower renin levels and, correspondingly, had lower systemic blood pressure than controls. Furthermore, mice with HSE grafts expressing human ANP did not develop elevated blood pressure when fed a high-salt diet. These findings illustrate the potential of this human skin gene therapy approach to deliver therapeutic molecules systemically for long-term treatment of diverse diseases.

Secretion of mouse growth hormone by transduced primary human keratinocytes: prospects for an animal model of cutaneous gene therapy

BACKGROUND

Keratinocytes are a very attractive vehicle for ex vivo gene transfer and systemic delivery because proteins secreted by these cells may reach the circulation via a mechanism that mimics the natural process.

METHODS

An efficient retroviral vector (LXSN) encoding the mouse growth hormone gene (mGH) was used to transduce primary human keratinocytes. Organotypic raft cultures were prepared with these genetically modified keratinocytes and were grafted onto immunodeficient dwarf mice (lit/scid).

RESULTS

Transduced keratinocytes presented a high and stable in vitro secretion level of up to 11 microg mGH/10(6)cells/day. Conventional epidermal sheets made with these genetically modified keratinocytes, however, showed a drop in secretion rates of > 80% due to detachment of the epithelium from its substratum. Substitution of conventional grafting methodologies with organotypic raft cultures completely overcame this problem. The stable long-term grafting of such cultures onto lit/scid mice could be followed for more than 4 months, and a significant weight increase over the control group was observed in the first 40 days. Circulating mGH levels revealed a peak of 21 ng/ml just 1 h after grafting but, unfortunately, these levels rapidly fell to baseline values.

CONCLUSIONS

mGH-secreting primary human keratinocytes presented the highest in vitro expression and peak circulatory levels reported to date for a form of GH with this type of cells. Together with previous data showing that excised implants can recover a remarkable fraction of their original in vitro mGH secretion efficiency in culture, the factors that might still hamper the success of this promising model of cutaneous gene therapy are discussed.

Gene-modified tissue-engineered skin: the next generation of skin substitutes

Tissue engineering combines the principles of cell biology, engineering and materials science to develop three-dimensional tissues to replace or restore tissue function. Tissue engineered skin is one of most advanced tissue constructs, yet it lacks several important functions including those provided by hair follicles, sebaceous glands, sweat glands and dendritic cells. Although the complexity of skin may be difficult to recapitulate entirely, new or improved functions can be provided by genetic modification of the cells that make up the tissues. Gene therapy can also be used in wound healing to promote tissue regeneration or prevent healing abnormalities such as formation of scars and keloids. Finally, gene-enhanced skin substitutes have great potential as cell-based devices to deliver therapeutics locally or systemically. Although significant progress has been made in the development of gene transfer technologies, several challenges have to be met before clinical application of genetically modified skin tissue. Engineering challenges include methods for improved efficiency and targeted gene delivery; efficient gene transfer to the stem cells that constantly regenerate the dynamic epidermal tissue; and development of novel biomaterials for controlled gene delivery. In addition, advances in regulatable vectors to achieve spatially and temporally controlled gene expression by physiological or exogenous signals may facilitate pharmacological administration of therapeutics through genetically engineered skin. Gene modified skin substitutes are also employed as biological models to understand tissue development or disease progression in a realistic three-dimensional context. In summary, gene therapy has the potential to generate the next generation of skin substitutes with enhanced capacity for treatment of burns, chronic wounds and even systemic diseases.

Engineered skin substitutes: practices and potentials

Wound healing can be problematic in several clinical settings because of massive tissue injury (burns), wound healing deficiencies (chronic wounds), or congenital conditions and diseases. Engineered skin substitutes have been developed to address the medical need for wound coverage and tissue repair. Currently, no engineered skin substitute can replace all of the functions of intact human skin. A variety of biologic dressings and skin substitutes have however contributed to improved outcomes for patients suffering from acute and chronic wounds. These include acellular biomaterials and composite cultured skin analogs containing allogeneic or autologous cultured skin cells.

Gene transfer to epidermal stem cells: implications for tissue engineering

The skin is an attractive target for gene therapy because it is easily accessible and shows great potential as an ectopic site for protein delivery in vivo. Genetically modified epidermal cells can be used to engineer three-dimensional skin substitutes, which when transplanted can act as in vivo 'bioreactors' for delivery of therapeutic proteins locally or systemically. Although some gene transfer technologies have the potential to afford permanent genetic modification, differentiation and eventual loss of genetically modified cells from the epidermis results in temporary transgene expression. Therefore, to achieve stable long-term gene expression, it is critical to deliver genes to epidermal stem cells, which possess unlimited growth potential and self-renewal capacity. This review discusses the recent advances in epidermal stem cell isolation, gene transfer and engineering of skin substitutes. Recent efforts that employ gene therapy and tissue engineering for the treatment of genetic diseases, chronic wounds and systemic disorders, such as leptin deficiency or diabetes, are reviewed. Finally, the use of gene-modified tissue-engineered skin as a biological model for understanding tissue development, wound healing and epithelial carcinogenesis is also discussed.

Increases in weight of growth hormone‐deficient and immunodeficient (lit/scid) dwarf mice after grafting of hGH‐ secreting, primary human keratinocytes

Primary human keratinocytes, stably transduced with the human growth hormone (hGH) gene (under control of the retroviral LTR promoter) and selected via geneticin secreted as much as 7 microg hGH/106 cells/day. Their grafting onto immunodeficient dwarf mice (lit/scid) led to hGH levels in the circulation that did not go below 0.2-0.3 ng/ml during a 12 day period (peak value, 1.5 ng/ml at 4 h). This phenomenon was associated with a body weight increase of the grafted animals (0.060 g/animal/day) significantly higher (P<0.01) than that of controls (0.023 g/animal/day). This is the first report describing successful utilization of immunodeficient dwarf mice (lit/scid) in keratinocyte-based hGH gene therapy.

Retroviral gene transfer to human epidermal keratinocytes correlates with integrin expression and is significantly enhanced on fibronectin

Human epidermal keratinocytes are an important target for gene therapy because they can be easily expanded in culture and used to generate skin substitutes for the treatment of wounds, genetic diseases of the skin, and for delivery of proteins to the systemic circulation. Although retroviral transduction results in permanent genetic modification, differentiation and loss of transduced cells from the epidermis results in temporary transgene expression. To ensure permanent genetic modification, epidermal stem cells must be transduced with high efficiency. We evaluated gene transfer on two different substrates and found that the efficiency of gene transfer is substantially higher on a substrate of recombinant fibronectin (FN), when compared to tissue culture plastic (TCP). The rate of retroviral transduction on FN is four times faster than transduction on tissue culture plates and is independent of polybrene (PB). The transduction efficiency correlates with the levels of expression of integrin subunits alpha5, alpha2, and beta1, which have been shown to correlate with stem cell phenotype. Notably, cells that adhere rapidly to FN are transduced more efficiently than slowly adherent cells. In addition, integrin-blocking antibodies decrease the efficiency of gene transfer in a dose-dependent manner. Our results suggest that FN may enhance retroviral gene transfer to the least differentiated cells, thereby increasing the potential of genetically modified keratinocytes to treat short- and long-term disease states.

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.

The epidermis as a bioreactor: topically regulated cutaneous delivery into the circulation

Previous studies have documented that the skin can be used as a bioreactor to produce proteins for systemic release to treat diseases. A gene-switch system has been developed that allows regulated expression of therapeutic genes. To determine whether this system could be used in the skin, we developed a transgenic mouse model in which expression of a therapeutic gene could be topically induced in epidermal keratinocytes. After a single induction, high levels of the therapeutic protein, human growth hormone (hGH), were released from keratinocytes into the circulation. The serum levels of hGH were dependent on the amount of inducer applied, and repeated induction resulted in increased weight gain by transgenic versus control mice. Furthermore, physiological levels of hGH were detected in the serum of nude mice after topical induction of small transgenic skin grafts. These results clearly demonstrate the feasibility of using the gene-switch system to regulate the delivery of therapeutic proteins into the circulation via genetically modified keratinocytes.

Principles and practices for treatment of cutaneous wounds with cultured skin substitutes

BACKGROUND

Skin substitutes prepared from cultured skin cells and biopolymers may reduce requirements for donor skin autograft, and have been shown to be effective in treatment of excised burns, burn scars, and congenital skin lesions.

DATA SOURCES

Cultured skin substitutes (CSS) generate skin phenotypes (epidermal barrier, basement membrane) in the laboratory, and restore tissue function and systemic homeostasis. Healed skin is smooth, soft and strong, but develops irregular degrees of pigmentation. Quantitative analysis demonstrates that CSS closes 67 times the area of the donor skin, compared to less than 4 times for split-thickness skin autograft.

CONCLUSIONS

CSS reduce requirements for donor skin autograft for closure of excised, full-thickness cutaneous wounds, and demonstrate qualitative outcome that is not different from meshed, split-thickness autograft. These results offer reductions in morbidity and mortality for the treatment of burns and chronic wounds, and for cutaneous reconstruction.

Skin as a potential organ for ectopic monoclonal antibody production

The therapeutic potential of monoclonal antibodies for treating a variety of severe or life-threatening diseases is high. Although intravenous infusion appears the simplest and most obvious mode of administration, it is not applicable to many long-term treatments. It might be advantageously replaced by gene/cell therapies, however, rendering treatments cost-effective and eliminating the short- and long-term side-effects associated with injection of massive doses of antibodies. We have tested whether skin can potentially be used as an organ for production and systemic delivery of ectopic antibodies. Normal human primary keratinocytes were shown to be capable of synthesis and secretion of a model monoclonal antibody directed against human thyroglobulin upon retroviral gene transduction in vitro. Neo- epidermis reconstructed in vitro, either in cell culture inserts or on dermal substrates, from such modified keratinocytes also produced the monoclonal antibody. Interestingly, the latter could cross the epidermis basal layer and be released in culture fluids. Finally, grafting of epidermis reconstituted in vitro on dermal substrates to SCID mice permitted sustained monoclonal antibody delivery into the bloodstream to be achieved. Our data thus show that genetically engineered keratinocytes can potentially be used for genetic antibody-based immunotherapies. They also indicate that proteins as big as 150 kDa, after release by engineered keratinocytes into skin intercellular spaces, can migrate to the general circulation, which is potentially important for a number of other gene-based therapies.

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.

Brain region-specific up-regulation of mouse apolipoprotein E by pharmacological estrogen treatments

Cerebral apolipoprotein E (apoE) has been implicated in neuronal protection and repair. Due to the variable levels and types of estrogen receptors within different brain regions, the effect of estrogen on apoE and the mechanism of this effect may vary within different regions. Ovariectomized female C57BL/6 mice were treated with pharmacological levels of 17 beta-estradiol or placebo for 5 days, resulting in supraphysiological plasma levels of estradiol in the treated mice. ApoE and glial fibrillary acidic protein (GFAP) levels were measured in the cortex, hippocampus and diencephalon. 17 beta-Estradiol up-regulated apoE but not GFAP in the cortex and diencephalon, whereas in the hippocampus, GFAP and apoE were equally up-regulated. Treatment of estrogen receptor (ER) alpha knockout mice with 17 beta-estradiol or treatment of C57BL/6 mice with 17 alpha-estradiol, a poor estrogen receptor agonist, specifically induced apoE in the cortex, but not in the diencephalon. These results indicate that 17 beta-estradiol effects on apoE are either directly or indirectly mediated by ER alpha in the diencephalon, while the effects in the cortex may be mediated by a non-classical mechanism or by ER beta. Measurement of mRNA levels in estrogen versus placebo-treated wild-type mice indicated that the effect of 17 beta-estradiol on apoE was not associated with changes in apoE mRNA levels.

Various cells retrovirally transduced with N-acetylgalactosoamine-6-sulfate sulfatase correct Morquio skin fibroblasts in vitro

Gene therapy may provide a long-term approach to the treatment of mucopolysaccharidoses. As a first step toward the development of an effective gene therapy for mucopolysaccharidosis type IVA (Morquio syndrome), a recombinant retroviral vector, LGSN, derived from the LXSN vector, containing a full-length human wildtype N-acetylgalactosamine-6-sulfate sulfatase (GALNS) cDNA, was produced. Severe Morquio and normal donor fibroblasts were transduced by LGSN. GALNS activity in both Morquio and normal transduced cells was several fold higher than normal values. To measure the variability of GALNS expression among different transduced cells, we transduced normal and Morquio lymphoblastoid B cells and PBLs, human keratinocytes, murine myoblasts C2C12, and rabbit synoviocytes HIG-82 with LGSN. In all cases, an increase of GALNS activity after transduction was measured. In Morquio cells co-cultivated with enzyme-deficient transduced cells, we demonstrated enzyme uptake and persistence of GALNS activity above normal levels for up to 6 days. The uptake was mannose-6-phosphate dependent. Furthermore, we achieved clear evidence that LGSN transduction of Morquio fibroblasts led to correction of the metabolic defect. These results provide the first evidence that GALNS may be delivered either locally or systematically by various cells in an ex vivo gene therapy of MPS IVA.

A cutaneous gene therapy approach to human leptin deficiencies: correction of the murine ob/ob phenotype using leptin-targeted keratinocyte grafts

Leptin deficiency produces a phenotype of obesity, diabetes, and infertility in the ob/ob mouse. In humans, leptin deficiency occurs in some cases of congenital obesity and in lipodystrophic disorders characterized by reduced adipose tissue and insulin resistance. Cutaneous gene therapy is considered an attractive potential method to correct circulating protein deficiencies, since gene-transferred human keratinocytes can produce and secrete gene products with systemic action. However, no studies showing correction of a systemic defect have been reported. We report the successful correction of leptin deficiency using cutaneous gene therapy in the ob/ob mouse model. As a feasibility approach, skin explants from transgenic mice overexpressing leptin were grafted on immunodeficient ob/ob mice. One month later, recipient mice reached body weight values of lean animals. Other biochemical and clinical parameters were also normalized. In a second human gene therapy approach, a retroviral vector encoding both leptin and EGFP cDNAs was used to transduce HK and, epithelial grafts enriched in high leptin-producing HK were transplanted to immunosuppressed ob/ob mice. HK-derived leptin induced body weight reduction after a drop in blood glucose and food intake. Leptin replacement through genetically engineered HK grafts provides a valuable therapeutic alternative for permanent treatment of human leptin deficiency conditions.

Human mesenchymal stem cells maintain transgene expression during expansion and differentiation

Human adult bone marrow contains both hematopoietic stem cells that generate cells of all hematopoietic lineages and human mesenchymal stem cells (hMSCs), which support hematopoiesis and contribute to the regeneration of multiple connective tissues. The goal of the current study was to demonstrate that transduced hMSCs maintain transgene expression after stem cell differentiation in vitro and in vivo. We have introduced genes into cultured hMSCs by retroviral vector transfer and demonstrated long-term in vitro and in vivo expression of human interleukin 3 (hIL-3) and green fluorescent protein (GFP). Protocols were developed to achieve transduction efficiencies of 80-90% in these stem cells. In vitro expression of hIL-3 averaged 350 ng/10(6)cells/24 h over 17 passages (> 6 months) and GFP expression was stable over the same time period. Transduced hMSCs were able to differentiate into osteogenic, adipogenic, and chondrogenic lineages and maintained transgene expression after differentiation. Parallel studies were performed in vivo using NOD/SCID mice. Human MSCs expressing hIL-3 were cultured on several matrices and then delivered by subcutaneous, intravenous, and intraperitoneal routes. Sampling of peripheral blood demonstrated that systemic hIL-3 expression was maintained in the range of 100-800 pg/ml over a period of 3 months. These results illustrate the ability of hMSCs to express genes of therapeutic potential and demonstrate their potential clinical utility as cellular vehicles for systemic gene delivery.

Cutaneous gene transfer and therapy: the present and the future

The easy accessibility of the skin as a therapeutic target provides an exciting potential for this organ for the development of gene therapy protocols for cutaneous diseases and a variety of metabolic disorders. Thus far, full phenotypic reversion of a diseased phenotype has been achieved in vivo for junctional epidermolysis bullosa and X-linked or lamellar ichthyosis and in vitro for xeroderma pigmentosum. These recessive skin diseases are characterized by skin blistering, abnormalities in epidermal differentiation and increased development of skin cancers, respectively. Corrective gene delivery at both molecular and functional levels was achieved by transduction of cultured skin cells using retroviral vectors carrying the specific curative cDNA. These positive results should prompt clinical trials based on transplantation of artificial epithelia reconstructed ex vivo using genetically modified keratinocytes. Promising results have also been obtained in phenotypic reversion of cells isolated from patients suffering from a number of metabolic diseases such as gyrate atrophy, familial hypercholesterolemia or phenylketonuria. In these diseases transplantation of autologous artificial epithelia expressing the transgenes of interest or direct transfer of the DNA to the skin represents a potential therapeutic approach for the systemic delivery of active molecules. Successful cutaneous gene therapy trials, however, require development of protocols for efficient gene transfer to epidermal stem cells, and information about the host immune response to the recombinant polypeptides produced by the implanted keratinocytes. The availability of spontaneous animal models for genodermatoses will validate the gene therapy approach in preclinical trials.

Treatment of severe hypercholesterolemia in apolipoprotein E-deficient mice by intramuscular injection of plasmid DNA

We report on systemic delivery and long-term biological effects of apolipoprotein E (apoE) obtained by intramuscular (i.m.) plasmid DNA injection. ApoE plays an important role in lipoprotein catabolism and apoE knock-out mice develop severe hypercholesterolemia and diffuse atherosclerosis. We have injected apoE-deficient mice with 80 microg of a plasmid vector (pCMV-E3) encoding the human apoE3 cDNA under the control of the CMV promoter-enhancer in both posterior legs. Local expression of the transgene was demonstrated throughout 16 weeks. Human apoE3 recombinant protein reached 0.6 ng/ml serum level. After i.m. injection of pCMV-E3 expression vector the mean serum cholesterol concentrations decreased from 439 +/- 57 mg/dl to 253 +/- 99 mg/dl (P < 0.05) 2 weeks after injection and persisted at a significantly reduced level throughout the 16 weeks observation period (P < 0.005). Serum cholesterol was unaffected and reached an absolute level of 636 +/- 67 mg/dl in control groups. Finally, injection of pCMV-E3 into apoE-deficient mice resulted in a redistribution of cholesterol content between lipoprotein fractions, with a marked decrease in VLDL, IDL and LDL cholesterol content and an increase in HDL cholesterol. These results demonstrate that severe hypercholesterolemia in apoE-deficient mice can be effectively reversed by i.m. DNA injection, and indicate that this approach could represent a useful tool to correct several hyperlipidemic conditions resulting in atherosclerosis.

Regulated cutaneous gene delivery: the skin as a bioreactor

Epidermal keratinocytes can secrete polypeptides into the bloodstream, and they can be easily expanded in culture and genetically modified. It is thus possible to use epidermal keratinocytes for the systemic delivery of transgene products. Here we review the development of epidermal secretory systems, from cultured keratinocytes to skin grafts and transgenic mouse models. We also discuss a gene-switch approach for regulated cutaneous gene delivery.

Advances in skin gene therapy

Specific anatomical and biological properties make the skin a very interesting target organ for gene therapy approaches. Different cell types of the epidermis, such as keratinocytes, melanocytes, or dendritic cells, can be genetically modified to treat a broad spectrum of diseases, including genetically inherited skin disorders, tumour diseases, metabolic disorders and infectious diseases. The easy accessibility of skin suggests that different methods for gene delivery can be pursued, depending on the desired application. The approach used to deliver DNA to the skin will influence not only the efficiency of DNA delivery, but also the level and duration of transgene expression. Furthermore, the desired biological effect will also influence the decision of which gene transfer method is the best choice. Among the current challenges of cutaneous gene therapy are: optimising the efficiency of direct in vivo gene delivery; targeting specific epidermal cells, including keratinocyte stem cells; achieving sustained gene expression and regulating gene expression in vivo. This review summarises recent advances in the field of skin gene therapy and evaluates possible strategies to overcome obstacles and achieve successful clinical applications of skin gene therapy.

Adeno-associated virus expresses transgenes in hair follicles and epidermis

Adeno-associated virus (AAV) vectors are nonpathogenic, integrating DNA vectors capable of transducing dividing and nondividing cells with the potential of long-term expression. Evaluating this interesting vector system in the skin for the first time, we found that an AAV vector containing the lacZ gene (AAVlacZ) led to the expression of beta-galactosidase for more than 6 weeks following in vivo injection. Interestingly, expression was present not only in dividing and postmitotic epidermal keratinocytes but also in hair follicle epithelial cells and eccrine sweat glands. However, expression upon readministration was limited. Functional studies in swine using human erythropoietin were hampered by immunogenicity. Thus, AAV seems to be the only vector to date that efficiently targets hair follicle epithelial cells. It may also be useful when longer term expression in keratinocytes than that achievable by direct injection of plasmid DNA is desired.

Correction of the coagulation defect in hemophilia A mice through factor VIII expression in skin

To test the hypothesis that factor VIII expressed in the epidermis can correct hemophilia A, we generated transgenic mice in a factor VIII–deficient background that express human factor VIII under control of the involucrin promoter. Mice from 5 transgenic lines had both phenotypic correction and plasma factor VIII activity. In addition to the skin, however, some factor VIII expression was detected in other tissues that have stratified squamous epithelia. To determine whether an exclusively cutaneous source of factor VIII could correct factor VIII deficiency, we grafted skin explants from transgenic mice onto mice that are double knockouts for the factor VIII and RAG-1 genes. Two graft recipients had plasma factor VIII activity of 4% to 20% of normal and improved whole blood clotting compared with factor VIII–deficient mice. Thus, expression of factor VIII from the epidermis can correct hemophilia A mice, thereby supporting the feasibility of cutaneous gene therapy for systemic disease.

Adenoviral-mediated gene transfer in wound healing: acute inflammatory response in human skin in the SCID mouse model

The use of an adenoviral vector as a means of therapeutic protein delivery for the treatment of impaired wound healing is a potentially effective application of current gene transfer techniques. This study was designed to investigate the ability of adenovirus to mediate gene transfer in healing wounds in human skin in vivo. The human skin/severe combined immunodeficient mouse chimera model was used to study both the response of human tissue to adenoviral infection and the nature of the acute inflammatory response. The effects of adenoviral infection and transgene expression on the rate and quality of human wound healing were then investigated. Cell- and species-specific monoclonal antibodies were used to characterize the resident skin cell types participating in wound repair, the inflammatory response, and the proliferative potential of adenovirus-treated compared to control skin. Our studies show that, following wounding, normal skin architecture is restored in the presence of adenoviral infection equivalent to noninfected controls. Despite an increased acute inflammatory response after adenovirus injection, no difference in the healing capabilities of wounded skin was observed, suggesting that adenovirus-mediated gene transfer for growth factor-mediated acceleration of wound healing may be feasible.

Polarized secretion of transgene products from salivary glands in vivo

Previously (Kagami et al. Hum. Gene Ther. 1996;7:2177-2184) we have shown that salivary glands are able to secrete a transgene-encoded protein into serum as well as saliva. This result and other published data suggest that salivary glands may be a useful target site for vectors encoding therapeutic proteins for systemic delivery. The aim of the present study was to assess in vivo if transgene-encoded secretory proteins follow distinct, polarized sorting pathways as has been shown to occur "classically" in cell biological studies in vitro. Four first-generation, E1-, type 5 recombinant adenoviruses were used to deliver different transgenes to a rat submandibular cell line in vitro or to rat submandibular glands in vivo. Subsequently, the secretory distribution of the encoded proteins was determined. Luciferase, which has no signal peptide, served as a cell-associated, negative control and was used to correct for any nonspecific secretory protein release from cells. The three remaining transgene products tested, human tissue kallikrein (hK1), human growth hormone (hGH), and human alpha1-antitrypsin (halpha1AT), were predominantly secreted (>96%) in vitro. Most importantly, in vivo, after a parasympathomimetic secretory stimulus, both hK1 and hGH were secreted primarily in an exocrine manner into saliva. Conversely, halpha1AT was predominantly secreted into the bloodstream, i.e., in an endocrine manner. The aggregate results are consistent with the recognition of signals encoded within the transgenes that result in specific patterns of polarized protein secretion from rat submandibular gland cells in vivo.

Selection of keratinocytes transduced with the multidrug resistance gene in an in vitro skin model presents a strategy for enhancing gene expression in vivo

In gene therapy studies, achieving prolonged, high-level gene expression in a significant percentage of cells has been difficult. One solution to enhance expression would be to select for cells expressing both the desired gene and a linked selectable marker gene in a bicistronic vector. As a potential target tissue, the skin is easily accessible for safe topical application of a selecting agent that could lead to significant gene expression in a high percentage of keratinocytes. To test the feasibility of such an approach, a skin raft culture model was developed. Human keratinocytes were transduced with the multidrug resistance (MDR) gene, which confers resistance to a variety of cytostatic and antimitotic compounds, such as colchicine. While growing on acellular dermis, transduced keratinocytes were treated with various doses of colchicine (10-50 ng/ml). Colchicine treatment increased the percentage of keratinocytes expressing MDR to almost 100% in raft cultures, Significantly, keratinocytes in colchicine-treated, MDR-transduced raft cultures were able to proliferate normally and form a stratified, differentiated epidermis. This model suggests that topical selection for MDR-expressing keratinocytes in vivo should be feasible without hampering the biologic integrity of skin. Thus, topical selection leading to enhanced expression of a desired gene, linked to a resistance gene, holds future promise for skin gene therapy.

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.

A direct in vivo approach for skin gene therapy

In vivo gene therapy is a direct and effective way to express genes in the epidermis. Plasmid DNA that contains the desired gene can be injected intradermally, and it is rapidly absorbed and expressed by the epidermis. Because gene expression following plasmid injection is transient, the two principal therapeutic uses of this approach are genetic immunization and the expression of biological response modifiers to treat skin disease.

Systemic replacement therapy from genetically modified epidermal keratinocytes

Epidermal keratinocytes are a potential vehicle for gene transfer and systemic delivery. We review data showing that epidermis-secreted protein does indeed reach the circulation, and we discuss factors that bear upon the issue of how much protein epidermal keratinocytes can deliver to the circulation.

Epidermal expression of apolipoprotein E gene during fin and scale development and fin regeneration in zebrafish

Apolipoprotein E (apoE) plays an important role in systemic and local lipid homeostasis. We have examined the expression of apoE during morphogenesis and regeneration of paired and unpaired fins and during scale development in zebrafish (Danio rerio). In situ hybridization analysis revealed that, during embryogenesis, apoE is expressed in the epithelial cells of the median fin fold and of the pectoral fin buds. ApoE remains expressed in the elongating fin folds throughout development of the fins. During the larval to juvenile transition, apoE transcripts were present in the distal, interray and lateral epidermis of developing fins. Furthermore, as scale buds started to form, apoE was expressed in large scale domains which later, became restricted to the external posterior epidermal part of scales. A low level of transcripts could be observed at later developmental stages at these locations probably because fins and scales continue to grow throughout the animal's life. During regeneration of both pectoral and caudal fins, a marked increase in apoE expression is observed as early as 12 hours after amputation in the wound epidermis. High levels of apoE transcripts are then localized primarily in the basal cell layer of the apical epidermis. The levels of apoE expression were maximum between the second to fourth days and then progressively declined to basal level by day 14. ApoE transcripts were also observed in putative macrophages infiltrated in the mesenchymal compartment of regenerating fins a few hours after amputation. In conclusion, apoE is highly expressed in the epidermis of developing fins and scales and during fin regeneration while no expression can be detected in the skin of the trunk. ApoE may play a specific role in fin and scale differentiation at sites where important epidermo-dermal interactions occur for the elaboration of the dermal skeleton and/or for lipid uptake and redistribution within these rapidly growing structures.

Topical gene delivery to murine skin

We topically applied naked plasmid DNA containing the luciferase or chloramphenicol acetyltransferase cDNA directly to mouse skin. Gene expression was detected in skin samples as early as 4 h after DNA application, plateaued from 16 to 72 h post-application, and had decreased significantly by 7 d post-application. Reporter gene activity following topical DNA delivery was comparable with that produced by intradermal injection of DNA. Plasmid DNA at concentrations > or =0.25 microg per microl were required to achieve maximal expression levels. Reporter gene expression following topical administration was largely confined to the superficial layers of the epidermis and to hair follicles. Surprisingly, certain cationic liposomes inhibited the efficiency of cutaneous gene transfer. This technique provides a simple, clinically relevant approach to deliver genes to the skin, with potential application in treating a variety of cutaneous disorders.

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.

Skin substitutes from cultured cells and collagen-GAG polymers

Engineering skin substitutes provides a potential source of advanced therapies for the treatment of acute and chronic wounds. Cultured skin substitutes (CSS) consisting of human keratinocytes and fibroblasts attached to collagen-glycosaminoglycan substrates have been designed and tested in preclinical and clinical studies. Cell culture techniques follow general principles of primary culture and cryopreservation in liquid nitrogen for long-term storage. Biopolymer substrates are fabricated from xenogeneic (bovine) collagen and glycosaminoglycan that are lyophilised for storage until use. At maturity in air-exposed culture, CSS develop an epidermal barrier that is not statistically different from native human skin, as measured by surface electrical capacitance. Preclinical studies in athymic mice show rapid healing, expression of cytokines and regulation of pigmentation. Clinical studies in burn patients demonstrate a qualitative outcome with autologous skin that is not different from 1:4 meshed, split-thickness autograft skin, and with a quantitative advantage over autograft skin in the ratio of healed skin to biopsy areas. Chronic wounds resulting from diabetes or venous stasis have been closed successfully with allogeneic CSS prepared from cryopreserved skin cells. These results define the therapeutic benefits of cultured skin substitutes prepared with skin cells from the patient or from cadaver donors. Future directions include genetic modification of transplanted cells to improve wound healing transiently or to deliver gene products systemically.

Apolipoprotein E is present in primary localized cutaneous amyloidosis

Apolipoprotein E (apoE) is one of the amyloid associated proteins that is found in the amyloid plaque of Alzheimer's disease and systemic amyloidosis. ApoE might play an important part in the etiology of Alzheimer's disease by functioning as a "pathologic chaperone" to promote the formation of amyloid filaments. In this study, we investigated whether apoE is associated with amyloid deposits of primary localized cutaneous amyloidosis using immunohistochemistry, immunogold electron microscopy, and immunoblotting. The subjects consisted of 12 patients with lichen amyloidosus and one patient with macular amyloidosis. Light microscopically, amyloid deposits in the dermal papillae were round in shape and stained with Congo red. Immunohistochemically, apoE was detected in amyloid deposits in all the cases examined. Immunogold electron microscopy showed apoE immunoreactivity on the amyloid deposition. Immunoblots of amyloid-positive skin showed 35K and 14K proteins, which were taken to be apoE and its fragment, respectively. In normal skin extract, only the 35K protein was detected by the anti-human apoE. Moreover, the intensity of the amyloid-positive skin sample was stronger than that of the normal skin sample. Monoclonal anti-cytokeratin antibody reacted with the 45K protein of the amyloid-positive skin extract. These results indicate that apoE is a component of primary localized cutaneous amyloidosis, and that it might play an important role in primary localized cutaneous amyloidosis.

Development of engineered cells for implantation in gene therapy

Human gene therapy is based on the technology of genetic engineering of cells, either through ex vivo or in vivo methods of gene transfer. Many autologous cell types have been successfully modified to deliver recombinant gene products. An alternate form of gene therapy based on genetic modification of non-autologous cells is described. Protection within immuno-isolating devices would allow implantation of well-established recombinant cell lines in different allogeneic hosts, potentially offering a more cost-effective approach to gene therapy. Implantation with microencapsulated fibroblasts and myoblasts has resulted in successful recombinant product delivery in vivo. Correction of disease phenotypes in animal models of human genetic diseases has also been achieved. Cell types such as myoblasts which can differentiate terminally within the implantation device are particularly promising for the future development of this method of gene therapy.

Long-term expression of human clotting factor IX from retrovirally transduced primary human keratinocytes in vivo

A persistent obstacle that has hampered gene transfer experiments is the short-term nature of transgene expression in vivo. In this article we present evidence for sustained expression from primary human keratinocytes, using the retroviral vector MFG. Primary keratinocytes were transduced in culture with the MFG retroviral vector containing the coding region from factor IX cDNA. Transduced keratinocytes, which secreted on average 830 ng of factor IX/10(6) cells/24 hr in tissue culture, were used to form a bilayered skin equivalent and grafted onto nude mice under a silicone transplantation chamber. Between 0.1 and 2.75 ng of human factor IX per milliliter was found in mouse plasma for more than 1 year, suggesting that keratinocyte stem cells were both transduced and grafted. The results show, for the first time, that long-term expression is obtainable in retrovirally transduced keratinocytes after transplantation.

Frameshift mutations in the type VII collagen gene (COL7A1) in five Mexican cousins with recessive dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is caused by mutations in the type VII collagen gene (COL7A1). In this study, we assessed the molecular basis of recessive DEB in five affected individuals from two Mexican families. Both fathers of the affected children were first cousins. Genomic DNA was extracted from peripheral blood samples and assessed for COL7A1 mutations by polymerase chain reaction (PCR) amplification, heteroduplex analysis and direct automated sequencing of PCR products displaying heteroduplex bandshifts. In one family, we identified a homozygous 1 bp insertion of a G nucleotide in exon 19 of COL7A1, designated 2470insG, in three affected sisters. This mutation causes a frameshift and a premature termination codon on both alleles 178 bp downstream from the insertion; both parents were shown to be heterozygous carriers of this mutation. In the second family, the father of the other two affected children was also found to be a heterozygous carrier of this frameshift mutation. In addition, his unrelated partner was shown to be a heterozygous carrier of a different COL7A1 frameshift mutation, an insertion of a T nucleotide in exon 32, designated 3948insT. This mutation also results in a premature termination codon, 126 bp downstream from the insertion. Both affected children were compound heterozygotes for the 2470insG/3948insT mutations in COL7A1. Overall, these molecular findings offer a genetic explanation for the skin fragility in these related Mexican patients with recessive DEB. Immediate benefits from elucidation of the mutations include assessment of carrier status in other members of the family and the feasibility of DNA-based prenatal testing in subsequent pregnancies.

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.

A model of corrective gene transfer in X-linked ichthyosis

Single gene recessive genetic skin disorders offer attractive prototypes for the development of therapeutic cutaneous gene delivery. We have utilized X-linked ichthyosis (XLI), characterized by loss of function of the steroid sulfatase arylsulfatase C (STS), to develop a model of corrective gene delivery to human skin in vivo. A new retroviral expression vector was produced and utilized to effect STS gene transfer to primary keratinocytes from XLI patients. Transduction was associated with restoration of full-length STS protein expression as well as steroid sulfatase enzymatic activity in proportion to the number of proviral integrations in XLI cells. Transduced and uncorrected XLI keratinocytes, along with normal controls, were then grafted onto immunodeficient mice to regenerate full thickness human epidermis. Unmodified XLI keratinocytes regenerated a hyperkeratotic epidermis lacking STS expression with defective skin barrier function, effectively recapitulating the human disease in vivo. Transduced XLI keratinocytes from the same patients, however, regenerated epidermis histologically indistinguishable from that formed by keratinocytes from patients with normal skin. Transduced XLI epidermis demonstrated STS expression in vivo by immunostaining as well as a normalization of histologic appearance at 5 weeks post-grafting. In addition, transduced XLI epidermis demonstrated a return of barrier function parameters to normal. These findings demonstrate corrective gene delivery in human XLI patient skin tissue at both molecular and functional levels and provide a model of human cutaneous gene therapy.