Current approaches and perspectives in human keratinocyte-based gene therapies

Human epidermis organotypic cultures, a reproducible system recapitulating the epidermis in vitro

The translatability of research is highly dependent on models that recapitulate human tissues and organs. Here, we describe a procedure for the generation of human epidermis organotypic cultures (HEOCs) from primary keratinocytes isolated from foreskin and adult skin as well as from an immortalized keratinocyte cell line (KerTr). We tested several media conditions to develop a defined HEOC growing and expansion media. We characterized the HEOCs and show that in optimal culture conditions they express the proliferation marker Ki67, the basement membrane protein collagen 17 (col17) and the epidermal differentiation markers keratin 15 (K15), keratin 14 (K14), keratin 5 (K5), keratin 10 (K10), keratin 1 (K1), transglutaminase 1 (TGM1), transglutaminase 3 (TGM3) and filaggrin (FLG). Thus, they recapitulate the human epidermis and are stratified from the basal layer to the stratum corneum. These HEOC can be generated reproducibly on a large scale, making it an invaluable model for screening therapeutic compounds and also for the study of pathologies affecting the epidermis.

Genome-edited skin epidermal stem cells protect mice from cocaine-seeking behaviour and cocaine overdose

Cocaine addiction is associated with compulsive drug-seeking, and exposure to the drug or to drug-associated cues leads to relapse, even after long periods of abstention. A variety of pharmacological targets and behavioral interventions have been explored to counteract cocaine addiction, but to date no market-approved medications for treating cocaine addiction or relapse exist, and effective interventions for acute emergencies resulting from cocaine overdose are lacking. We recently demonstrated that skin epidermal stem cells can be readily edited by using CRISPR (clustered regularly interspaced short palindromic repeats) and then transplanted back into the donor mice. Here, we show that the transplantation, into mice, of skin cells modified to express an enhanced form of butyrylcholinesterase, an enzyme that hydrolyzes cocaine, enables the long-term release of the enzyme and efficiently protects the mice from cocaine-seeking behavior and cocaine overdose. Cutaneous gene therapy through skin transplants that elicit drug elimination may offer a therapeutic option to address drug abuse.

Engineered Epidermal Progenitor Cells Can Correct Diet-Induced Obesity and Diabetes

Somatic gene therapy is a promising approach for treating otherwise terminal or debilitating diseases. The human skin is a promising conduit for genetic engineering, as it is the largest and most accessible organ, epidermal autografts and tissue-engineered skin equivalents have been successfully deployed in clinical applications, and skin epidermal stem/progenitor cells for generating such grafts are easy to obtain and expand in vitro. Here, we develop skin grafts from mouse and human epidermal progenitors that were engineered by CRISPR-mediated genome editing to controllably release GLP-1 (glucagon-like peptide 1), a critical incretin that regulates blood glucose homeostasis. GLP-1 induction from engineered mouse cells grafted onto immunocompetent hosts increased insulin secretion and reversed high-fat-diet-induced weight gain and insulin resistance. Taken together, these results highlight the clinical potential of developing long-lasting, safe, and versatile gene therapy approaches based on engineering epidermal progenitor cells.

Inhibition of Apoptosis by Expression of Antiapoptotic Proteins in Recombinant Human Keratinocytes

The Fas ligand/Fas interaction plays an important role in the regulation of immune responses. Allografted cells undergo Fas-mediated apoptosis induced by CD8+ T cells. Our objective was to prevent human keratinocytes from immunologically induced apoptosis. We focused on three proteins with inhibitory function on Fas-mediated apoptosis. Human keratinocytes were transfected with either Flip, Faim, or Lifeguard (LFG). The treatment proved to be practicable and efficient. The recombinant keratinocytes with expression of our target proteins were cocultured with CD8+ T cells and the apoptotic activity was then evaluated. Activation of caspase-8 was detectable in control but not in the recombinant cells. Quantitative analysis revealed significant induction of T-cell-induced apoptosis in nontransfected keratinocytes (p = 0.04, n = 12) but not in Flip (p = 0.66), Faim (p = 0.42), or LFG (p = 0.44) expressing cells. Our results suggest that heterotopic expression of antiapoptotic proteins can induce the resistance of keratinocytes to a major mechanism of rejection.

Biological skin substitutes for wound cover and closure

In the past 30 years, the development and use of artificial skin in the treatment of acute and chronic wounds has advanced from an experimental concept to a working reality. However, while there have been an increasing number of artificial skin substitutes licensed for clinical use, they have yet to supplant the current gold standard of an autologous tissue graft for most applications. This article reviews the advantages and disadvantages of the currently available, biologically based substitutes, with special emphasis on their relative efficacy and suitability for treatment of particular wound types. Economic considerations, desirable improvements of currently available materials and the potential impact of future advances in the field will also be discussed.

Reduction of nontarget infection and systemic toxicity by targeted delivery of conditionally replicating viruses transported in mesenchymal stem cells

The fiber-modified adenoviral vector Delta-24-RGD (D24RGD) offers vast therapeutic potential. Direct injection of D24RGD has been used to successfully target ovarian tumors in mice. However, systemic toxicity, especially in the liver, profoundly limits the efficacy of direct viral vector delivery. Mesenchymal stem cells (MSC) have the ability to function as a vector for targeted gene therapy because of their preferential engraftment into solid tumors and participation in tumor stroma formation. We show that MSC-guided delivery of D24RGD is specific and efficient and reduces the overall systemic toxicity in mice to negligible levels compared with D24RGD alone. In our model, we found efficient targeted delivery of MSC-D24RGD to both breast and ovarian cell lines. Furthermore, immunohistochemical staining for adenoviral hexon protein confirmed negligible levels of systemic toxicity in mice that were administered MSC-D24RGD compared with those that were administered D24RGD. These data suggest that delivery of D24RGD through MSC not only increases the targeted delivery efficiency, but also reduces the systemic exposure of the virus, thereby reducing overall systemic toxicity to the host and ultimately enhancing its value as an anti-tumor therapeutic candidate.

Long-term Engraftment of Single Genetically Modified Human Epidermal Holoclones Enables Safety Pre-assessment of Cutaneous Gene Therapy

Predicting the risks of permanent gene therapy approaches involving the use of integrative gene-targeting vectors has become a critical issue after the unfortunate episode of a clinical trial in children with X-linked severe combined immunodeficiency (X-SCID). Safety pre-assessment of single isolated gene-targeted stem cells or their derivative clones able to regenerate their tissue of origin would be a major asset in addressing untoward gene therapy effects in advance. Human epidermal stem cells, which have extensive proliferative potential in vitro, theoretically offer such a possibility as a method of assessment. By means of optimized organotypic culture and grafting methods, we demonstrate the long-term in vivo regenerative capacity of single gene-targeted human epidermal stem cell clones (holoclones). Both histopathological analysis of holoclone-derived grafts in immunodeficient mice and retroviral insertion site mapping performed in the holoclone in vitro and after grafting provide proof of the feasibility of pre-assessing genotoxicity risks in isolated stem cells before transplantation into patients. Our results provide an experimental basis for previously untested assumptions about the in vivo behavior of epidermal stem cells prospectively isolated in vitro and pave the way for a safer approach to cutaneous gene therapy.

Relationship between retroviral vector membrane and vector stability

The present work studies the physico-chemical properties of retroviral vector membrane, in order to provide some explanation for the inactivation kinetics of these vectors and to devise new ways of improving transduction efficiency. For this purpose, vectors with an amphotropic envelope produced by TE Fly A7 cells at two culture temperatures (37 and 32 °C) were characterized by different techniques. Electron paramagnetic resonance (EPR) results showed that vectors produced at 32 °C are more rigid than those produced at 37 °C. Further characterization of vector membrane composition allowed us to conclude that the vector inactivation rate increases with elevated cholesterol to phospholipid ratio. Differential scanning calorimetry (DSC) showed that production temperature also affects the conformation of the membrane proteins. Transduction studies using HCT116 cells and tri-dimensional organ cultures of mouse skin showed that vectors produced at 37 °C have higher stability and thus higher transduction efficiency in gene therapy relevant cells as compared with vectors produced at 32 °C. Overall, vectors produced at 37 °C show an increased stability at temperatures below 4 °C. Since vector membrane physico-chemical properties are affected in response to changes in culture temperature, such changes, along with alterations in medium composition, can be used prospectively to improve the stability and the transduction efficiency of retroviral vectors for therapeutic purposes.

Gene therapy in combination with tissue engineering to treat epidermolysis bullosa

In the last 20 years epidermal stem cells have been extensively used for tissue regeneration of epidermis and other epithelial surfaces. The tremendous progress achieved has led to the development of protocols aimed at the correction of rare genetic disorders such as epidermolysis bullosa (EB), a severe, often lethal, blistering disorder of the skin. Approximately 400,000-500,000 people are affected worldwide and no definitive treatments have yet been developed. Gene therapy might represent an alternative therapeutic approach. This paper reviews the different strategies used to genetically modify keratinocytes from EB patients and addresses issues such as the use of in vivo or ex vivo approaches, how to target keratinocytes with stem cell properties in order to have long-term therapeutic gene expression, and which gene transfer agents should be used. The progress made has led the authors' group to submit a request for a Phase I/II ex vivo therapy clinical trial for patients with junctional EB.

Targeted overexpression of leptin to keratinocytes in transgenic mice results in lack of skin phenotype but induction of early leptin resistance

The epidermis has a great potential as a bioreactor to produce proteins with systemic action. However, the consequences of ectopic epidermal protein overexpression need to be carefully addressed to avoid both local and systemic adverse effects. Thus, the long-term effects of leptin on skin physiology have not been studied, and the metabolic consequences of sustained keratinocyte-derived leptin overexpression are unknown. Herein we describe that very high serum leptin levels can be achieved from a cutaneous source in transgenic mice in which leptin cDNA overexpression was driven by the keratin K5 gene regulatory sequences. Histopathological analysis including the study of skin differentiation and proliferation markers in these transgenic mice revealed that keratinocyte-derived leptin overexpression appears not to have any impact on cutaneous homeostasis. Although young K5-leptin transgenic mice showed remarkable thinness and high glucose metabolism as shown in other leptin transgenic mouse models, a marked leptin insensitivity become apparent as early as 3-4 months of age as demonstrated by increased weight gain and insulin resistance development. Other signs of leptin/insulin resistance included increased bone mass, organomegaly, and wound healing impairment. In addition, to provide evidence for the lack of untoward effects of leptin on epidermis, this transgenic mouse helps us to establish the safe ranges of keratinocyte-derived leptin overexpression and may be useful as a model to study leptin resistance.

Recombinant adeno-associated virus type 2-mediated gene transfer into human keratinocytes is influenced by both the ubiquitin/proteasome pathway and epidermal growth factor receptor tyrosine kinase

Efficient gene delivery into keratinocytes is a prerequisite for successful skin gene therapy. Vectors based on recombinant adeno-associated virus type 2 (rAAV-2) offer several promising features that make them attractive for cutaneous applications. However, highly efficient gene delivery may be hampered by different cellular factors, including lack of viral receptors, impairment of cytoplasmic trafficking or limitations in viral second-strand synthesis. This study was undertaken to find factors that influence rAAV-2-mediated in vitro gene transfer into human keratinocytes and, consequently, ways to optimize gene delivery. Transduction experiments using rAAV-2 vectors expressing green fluorescent protein (GFP) demonstrated that impaired cellular trafficking of vector particles and high levels of autophosphorylation at epidermal growth factor receptor tyrosine kinase (EGF-R TK) have a negative influence on gene transfer into keratinocytes. Treatment of keratinocytes with proteasome inhibitor MG132 resulted in a transient augmentation of GFP expression in up to 37% of cells. Treatment with EGF-R TK inhibitors (quinazoline type) enhanced transgene expression in 10-14.5% of the cells. Gene expression was stable for more than 10 weeks and persisted until proliferative senescence occurred. This stable gene expression allows speculation that keratinocyte stem cells have initially been transduced. These findings might have relevance for the use of rAAV-2 vectors in skin gene therapy: transient enhancement of rAAV-2 transduction with proteasome inhibitors might be useful for genetic promotion of wound healing or skin-directed vaccination. Treatment with quinazolines may increase rAAV-2 transduction of keratinocyte stem cells, which is important for gene therapy approaches to inherited diseases.

Assessment of optimal transduction of primary human skin keratinocytes by viral vectors

BACKGROUND

Genetically modified keratinocytes generate transplantable self-renewing epithelia suitable for delivery of therapeutic polypeptides. However, the variety of viral vectors and experimental conditions currently used make fragmented or contradictory the information on the transduction efficiency of the human primary keratinocytes. To compare the suitability of the most currently used viral vectors for efficient gene transfer to human keratinocytes, we have performed a comparative study using a panel of recombinant constructs.

METHODS

For each vector, the transduction efficiency and the persistence of the transgene expression were quantified by fluorescence microscopy and flow cytometry analysis of the infected cells.

RESULTS

We show that: (1) canine and human adenoviral vectors achieve a highly efficient but transient transduction of both primary and immortalized keratinocytes; (2) the adenovirus-associated virus (AAV) vectors transduce immortalized keratinocytes, albeit with a short-lived gene expression (<4 days), but fail to infect primary keratinocytes; and (3) under appropriate conditions, the oncoretroviral and lentiviral vectors can permanently transduce up to 100% of primary keratinocytes, but the highly clonogenic keratinocytes are more efficiently targeted by lentiviral vectors.

CONCLUSIONS

Therefore, AAV vectors are unsuitable to transduce primary keratinocytes, while human and canine adenoviral vectors appears to be appropriate to achieve short-term delivery of therapeutic products. Recombinant retroviruses provide sustained expression of the transgene, but the lentiviral vectors are the most suitable for ex vivo gene therapy because of their ability to transduce clonogenic primary keratinocytes.