Gene therapy of inherited skin adhesion disorders: a critical overview

Pathogenetic Therapy of Epidermolysis Bullosa: Current State and Prospects

Epidermolysis bullosa is a severe hereditary disease caused by mutations in genes encoding cutaneous basement membrane proteins. These mutations lead to dermal-epidermal junction failure and, as a result, to disturbances in the morphological integrity of the skin. Clinically, it manifests in the formation of blisters on the skin or mucosa that in some cases can turn into non-healing chronic wounds, which not only impairs patient's quality of life, but also is a live-threatening condition. Now, the main approaches in the treatment of epidermolysis bullosa are symptomatic therapy and palliative care, though they are little effective and are aimed at reducing the pain, but not to complete recovery. In light of this, the development of new treatment approaches aimed at correction of genetic defects is in progress. Various methods based on genetic engineering technologies, transplantation of autologous skin cells, progenitor skin cells, as well as hematopoietic and mesenchymal stem cells are studied. This review analyzes the pathogenetic methods developed for epidermolysis bullosa treatment based on the latest achievements of molecular genetics and cellular technologies, and discusses the prospects for the use of these technologies for the therapy of epidermolysis bullosa.

Phase 1/2a clinical trial of gene-corrected autologous cell therapy for recessive dystrophic epidermolysis bullosa

BACKGROUNDRecessive dystrophic epidermolysis bullosa (RDEB) patients have mutations in the COL7A1 gene and thus lack functional type VII collagen (C7) protein; they have marked skin fragility and blistering. This single-center phase 1/2a open-label study evaluated the long-term efficacy, safety, and patient-reported outcomes in RDEB patients treated with gene-corrected autologous cell therapy.METHODSAutologous keratinocytes were isolated from participant skin biopsies. Epidermal sheets were prepared from cells transduced with a retrovirus carrying the full-length human COL7A1 gene. These gene-corrected autologous epidermal sheets measured 5 × 7 cm (35 cm2) and were transplanted onto 6 wound sites in each of 7 adult participants (n = 42 sites total) from 2013 to 2017. Participants were followed for 2 to 5 years.RESULTSNo participants experienced any serious related adverse events. Wound healing of 50% or greater by Investigator Global Assessment was present in 95% (36 of 38) of treated wounds versus 0% (0 of 6) of untreated control wounds at 6 months (P < 0.0001). At year 1, 68% (26 of 38) of treated wounds had 50% or greater healing compared with 17% (1 of 6) of control wounds (P = 0.025). At year 2, 71% (27 of 38) of treated wounds had 50% or greater healing compared with 17% (1 of 6) of control wounds (P = 0.019).CONCLUSIONC7 expression persisted up to 2 years after treatment in 2 participants. Treated wounds with 50% or greater healing demonstrated improvement in patient-reported pain, itch, and wound durability. This study provides additional data to support the clinically meaningful benefit of treating chronic RDEB wounds with ex vivo, C7 gene-corrected autologous cell therapy. This approach was safe and promoted wound healing that was associated with improved patient-reported outcomes.TRIAL REGISTRATIONClinicaltrials.gov identifier: NCT01263379.FUNDINGEpidermolysis Bullosa Research Partnership, Epidermolysis Bullosa Medical Research Foundation, NIH R01 AR055914, Office of Research and Development at the Palo Alto Veteran's Affairs Medical Center, and the Dermatology Foundation.

Functional therapies for cutaneous wound repair in epidermolysis bullosa

Chronic wounding as a result of recurrent skin blistering in the painful genetic skin disease epidermolysis bullosa, may lead to life-threatening infections, increased risk of tumor formation, and other serious medical complications. Therefore, epidermolysis bullosa patients have an urgent need for optimal wound care and tissue regeneration. Therapeutic strategies using gene-, protein-, and cell-therapies are being developed to improve clinical symptoms, and some of them have already been investigated in early clinical trials. The most favorable options of functional therapies include gene replacement, gene editing, RNA targeting, and harnessing natural gene therapy. This review describes the current progress of the different approaches targeting autologous skin cells, and will discuss the benefits and challenges of their application.

Current and Future Perspectives of Stem Cell Therapy in Dermatology

Stem cells are undifferentiated cells capable of generating, sustaining, and replacing terminally differentiated cells and tissues. They can be isolated from embryonic as well as almost all adult tissues including skin, but are also generated through genetic reprogramming of differentiated cells. Preclinical and clinical research has recently tremendously improved stem cell therapy, being a promising treatment option for various diseases in which current medical therapies fail to cure, prevent progression or relieve symptoms. With the main goal of regeneration or sustained genetic correction of damaged tissue, advanced tissue-engineering techniques are especially applicable for many dermatological diseases including wound healing, genodermatoses (like the severe blistering disorder epidermolysis bullosa) and chronic (auto-)inflammatory diseases. This review summarizes general aspects as well as current and future perspectives of stem cell therapy in dermatology.

Design and Characterization of a Human Monoclonal Antibody that Modulates Mutant Connexin 26 Hemichannels Implicated in Deafness and Skin Disorders

Background: Mutations leading to changes in properties, regulation, or expression of connexin-made channels have been implicated in 28 distinct human hereditary diseases. Eight of these result from variants of connexin 26 (Cx26), a protein critically involved in cell-cell signaling in the inner ear and skin. Lack of non-toxic drugs with defined mechanisms of action poses a serious obstacle to therapeutic interventions for diseases caused by mutant connexins. In particular, molecules that specifically modulate connexin hemichannel function without affecting gap junction channels are considered of primary importance for the study of connexin hemichannel role in physiological as well as pathological conditions. Monoclonal antibodies developed in the last three decades have become the most important class of therapeutic biologicals. Recombinant methods permit rapid selection and improvement of monoclonal antibodies from libraries with large diversity.

Methods: By screening a combinatorial library of human single-chain fragment variable (scFv) antibodies expressed in phage, we identified a candidate that binds an extracellular epitope of Cx26. We characterized antibody action using a variety of biochemical and biophysical assays in HeLa cells, organotypic cultures of mouse cochlea and human keratinocyte-derived cells.

Results: We determined that the antibody is a remarkably efficient, non-toxic, and completely reversible inhibitor of hemichannels formed by connexin 26 and does not affect direct cell-cell communication via gap junction channels. Importantly, we also demonstrate that the antibody efficiently inhibits hyperative mutant Cx26 hemichannels implicated in autosomal dominant non-syndromic hearing impairment accompanied by keratitis and hystrix-like ichthyosis-deafness (KID/HID) syndrome. We solved the crystal structure of the antibody, identified residues that are critical for binding and used molecular dynamics to uncover its mechanism of action.

Conclusions: Although further studies will be necessary to validate the effect of the antibody in vivo, the methodology described here can be extended to select antibodies against hemichannels composed by other connexin isoforms and, consequently, to target other pathologies associated with hyperactive hemichannels. Our study highlights the potential of this approach and identifies connexins as therapeutic targets addressable by screening phage display libraries expressing human randomized antibodies.

RNA-based therapies for genodermatoses

Genetic disorders affecting the skin, genodermatoses, constitute a large and heterogeneous group of diseases, for which treatment is generally limited to management of symptoms. RNA-based therapies are emerging as a powerful tool to treat genodermatoses. In this review, we discuss in detail RNA splicing modulation by antisense oligonucleotides and RNA trans-splicing, transcript replacement and genome editing by in vitro-transcribed mRNAs, and gene knockdown by small interfering RNA and antisense oligonucleotides. We present the current state of these therapeutic approaches and critically discuss their opportunities, limitations and the challenges that remain to be solved. The aim of this review was to set the stage for the development of new and better therapies to improve the lives of patients and families affected by a genodermatosis.

Cyclodextrin- and calixarene-based polycationic amphiphiles as gene delivery systems: a structure-activity relationship study

Multi-head/multi-tail facial amphiphiles built on cyclodextrin (CD) and calixarene (CA) scaffolds are paradigmatic examples of monodisperse gene delivery systems. The possibility to precisely control the architectural features at the molecular level offers unprecedented opportunities for conducting structure-activity relationship studies. A major requirement for those channels is the design of a sufficiently diverse ensemble of compounds for parallel evaluation of their capabilities to condense DNA into transfection nanoparticles where the gene material is protected from the environment. Here we have undertaken the preparation of an oriented library of β-cyclodextrin (βCD) and calix[4]arene (CA4) vectors with facial amphiphilic character designed to ascertain the effect of the cationic head nature (aminothiourea-, arginine- or guanidine-type groups) and the macrocyclic platform on the abilities to complex plasmid DNA (pDNA) and in the efficiency of the resulting nanocomplexes to transfect cells in vitro. The hydrophobic domain, formed by hexanoyl or hexyl chains, remains constant in each series, matching the overall structure found to be optimal in previous studies. DLS, TEM and AFM data support that all the compounds self-assemble in the presence of pDNA through a process that involves initially electrostatic interactions followed by formation of βCD or CA4 bilayers between the oligonucleotide filaments. Spherical transfectious nanoparticles that are monomolecular in DNA are thus obtained. Evaluation in epithelial COS-7 and human rhabdomyosarcoma RD-4 cells evidenced the importance of having primary amino groups in the vector to warrant high levels of transfection, probably because of their buffering capacity. The results indicate that the optimal cationic head depends on the macrocyclic core, aminothiourea groups being preferred in the βCD series and arginine groups in the CA4 series. Whereas the transfection efficiency relationships remain essentially unchanged within each series, irrespective of the cell type, the optimal platform (βD or CA4) strongly depends on the cell type. The results illustrate the potential of monodisperse vector prototypes and diversity-oriented strategies on identifying the optimal candidates for gene therapy applications.

Genomic analysis of Sleeping Beauty transposon integration in human somatic cells

The Sleeping Beauty (SB) transposon is a non-viral integrating vector system with proven efficacy for gene transfer and functional genomics. However, integration efficiency is negatively affected by the length of the transposon. To optimize the SB transposon machinery, the inverted repeats and the transposase gene underwent several modifications, resulting in the generation of the hyperactive SB100X transposase and of the high-capacity “sandwich” (SA) transposon. In this study, we report a side-by-side comparison of the SA and the widely used T2 arrangement of transposon vectors carrying increasing DNA cargoes, up to 18 kb. Clonal analysis of SA integrants in human epithelial cells and in immortalized keratinocytes demonstrates stability and integrity of the transposon independently from the cargo size and copy number-dependent expression of the cargo cassette. A genome-wide analysis of unambiguously mapped SA integrations in keratinocytes showed an almost random distribution, with an overrepresentation in repetitive elements (satellite, LINE and small RNAs) compared to a library representing insertions of the first-generation transposon vector and to gammaretroviral and lentiviral libraries. The SA transposon/SB100X integrating system therefore shows important features as a system for delivering large gene constructs for gene therapy applications.

Induced pluripotent stem cells in dermatology: potentials, advances, and limitations

The discovery of methods for reprogramming adult somatic cells into induced pluripotent stem cells (iPSCs) has raised the possibility of producing truly personalized treatment options for numerous diseases. Similar to embryonic stem cells (ESCs), iPSCs can give rise to any cell type in the body and are amenable to genetic correction by homologous recombination. These ESC properties of iPSCs allow for the development of permanent corrective therapies for many currently incurable disorders, including inherited skin diseases, without using embryonic tissues or oocytes. Here, we review recent progress and limitations of iPSC research with a focus on clinical applications of iPSCs and using iPSCs to model human diseases for drug discovery in the field of dermatology.

Synthesis of Tat tagged and folate modified N-succinyl-chitosan self-assembly nanoparticles as a novel gene vector

The purpose of this research was to prepare a novel type of Tat tagged and folate modified N-succinyl-chitosan (Tat-Suc-FA) self-assembly nanoparticles, to provide a new vector for tumor gene therapy. In this study, Tat-Suc-FA polymers was synthesized and characterized using (1)H NMR and FT-IR. The copolymer had a mean diameter of 65 ± 22.6 nm, a zeta potential of 40 ± 0.2 mV. The cytotoxicity assay showed that Tat-Suc-FA polymers were less toxic than chitosan in the tested concentration range (from 2 to 500 μg/ml). Tat-Suc-FA/DNA complexes at various weight ratios were formulated and characterized. Particle sizes of Tat-Suc-FA/DNA complexes were between 54 and 106 nm as determined by dynamic light scattering. Accordingly, Transmission electron microscope photo of Tat-Suc-FA/DNA complexes exhibited a spherical and compact morphology. Zeta potentials of these complexes changed as the weight ratio varied (from 3 to 44 mV). Agarose gel electrophoresis assay showed that Tat-Suc-FA could efficiently condense the DNA, when the weight ratio was above 1.5/1. Together, these results suggest that the low toxic Tat-Suc-FA cationic polymers could be considered for use as a novel type of gene delivery vectors.

Gene therapy for skin diseases

The skin possesses qualities that make it desirable for gene therapy, and studies have focused on gene therapy for multiple cutaneous diseases. Gene therapy uses a vector to introduce genetic material into cells to alter gene expression, negating a pathological process. This can be accomplished with a variety of viral vectors or nonviral administrations. Although results are promising, there are several potential pitfalls that must be addressed to improve the safety profile to make gene therapy widely available clinically.

Tropism-modified AAV vectors overcome barriers to successful cutaneous therapy

Autologous human keratinocytes (HK) forming sheet grafts are approved as skin substitutes. Genetic engineering of HK represents a promising technique to improve engraftment and survival of transplants. Although efficacious in keratinocyte-directed gene transfer, retro-/lentiviral vectors may raise safety concerns when applied in regenerative medicine. We therefore optimized adeno-associated viral (AAV) vectors of the serotype 2, characterized by an excellent safety profile, but lacking natural tropism for HK, through capsid engineering. Peptides, selected by AAV peptide display, engaged novel receptors that increased cell entry efficiency by up to 2,500-fold. The novel targeting vectors transduced HK with high efficiency and a remarkable specificity even in mixed cultures of HK and feeder cells. Moreover, differentiated keratinocytes in organotypic airlifted three-dimensional cultures were transduced following topical vector application. By exploiting comparative gene analysis we further succeeded in identifying αvβ8 integrin as a target receptor thus solving a major challenge of directed evolution approaches and describing a promising candidate receptor for cutaneous gene therapy.

Patient-specific naturally gene-reverted induced pluripotent stem cells in recessive dystrophic epidermolysis bullosa

Spontaneous reversion of disease-causing mutations has been observed in some genetic disorders. In our clinical observations of severe generalized recessive dystrophic epidermolysis bullosa (RDEB), a currently incurable blistering genodermatosis caused by loss-of-function mutations in COL7A1 that results in a deficit of type VII collagen (C7), we have observed patches of healthy-appearing skin on some individuals. When biopsied, this skin revealed somatic mosaicism resulting from the self-correction of C7 deficiency. We believe this source of cells could represent an opportunity for translational “natural” gene therapy. We show that revertant RDEB keratinocytes expressing functional C7 can be reprogrammed into induced pluripotent stem cells (iPSCs) and that self-corrected RDEB iPSCs can be induced to differentiate into either epidermal or hematopoietic cell populations. Our results give proof in principle that an inexhaustible supply of functional patient-specific revertant cells can be obtained—potentially relevant to local wound therapy and systemic hematopoietic cell transplantation. This technology may also avoid some of the major limitations of other cell therapy strategies, e.g., immune rejection and insertional mutagenesis, which are associated with viral- and nonviral- mediated gene therapy. We believe this approach should be the starting point for autologous cellular therapies using “natural” gene therapy in RDEB and other diseases.

Superficial dermal fibroblasts enhance basement membrane and epidermal barrier formation in tissue-engineered skin: implications for treatment of skin basement membrane disorders

Basement membrane is a highly specialized structure that binds the dermis and the epidermis of the skin, and is mainly composed of laminins, nidogen, collagen types IV and VII, and the proteoglycans, collagen type XVIII and perlecan, all of which play critical roles in the function and resilience of skin. Both dermal fibroblasts and epidermal keratinocytes contribute to the development of the basement membrane, and in turn the basement membrane and underlying dermis influence the development and function of the epidermal barrier. Disruption of the basement membrane results in skin fragility, extensive painful blistering, and severe recurring wounds as seen in skin basement membrane disorders such as epidermolysis bullosa, a family of life-threatening congenital skin disorders. Currently, there are no successful strategies for treatment of these disorders; we propose the use of tissue-engineered skin as a promising approach for effective wound coverage and to enhance healing. Fibroblasts and keratinocytes isolated from superficial and deep dermis and epidermis, respectively, of tissue from abdominoplasty patients were independently cocultured on collagen-glycosaminoglycan matrices, and the resulting tissue-engineered skin was assessed for functional differences based on the underlying specific dermal fibroblast subpopulation. Tissue-engineered skin with superficial fibroblasts and keratinocytes formed a continuous epidermis with increased epidermal barrier function and expressed higher levels of epidermal proteins, keratin-5, and E-cadherin, compared to that with deep fibroblasts and keratinocytes, which had an intermittent epidermis. Further, tissue-engineered skin with superficial fibroblasts and keratinocytes formed better basement membrane, and produced more laminin-5, nidogen, collagen type VII, compared to that with deep fibroblasts and keratinocytes. Overall, our results demonstrate that tissue-engineered skin with superficial fibroblasts and keratinocytes forms significantly better basement membrane with higher expression of dermo-epidermal adhesive and anchoring proteins, and superior epidermis with enhanced barrier function compared to that with deep fibroblasts and keratinocytes, or with superficial fibroblasts, deep fibroblasts, and keratinocytes. The specific use of superficial fibroblasts in tissue-engineered skin may thus be more beneficial to promote adhesion of newly formed skin and wound healing, and is therefore promising for the treatment of patients with basement membrane disorders and other skin blistering diseases.

Allogeneic blood and bone marrow cells for the treatment of severe epidermolysis bullosa: repair of the extracellular matrix

Contrary to the prevailing professional opinion of the past few decades, recent experimental and clinical data support the fact that protein replacement therapy by allogeneic blood and marrow transplantation is not limited to freely diffusible molecules such as enzymes, but also large structural proteins such as collagens. A prime example is the cross-correction of type VII collagen deficiency in generalised severe recessive dystrophic epidermolysis bullosa, in which blood and marrow transplantation can attenuate the mucocutaneous manifestations of the disease and improve patients' quality of life. Although allogeneic blood and marrow transplantation can improve the integrity of the skin and mucous membranes, today's accomplishments are only the first steps on the long pathway to cure. Future strategies will be built on the lessons learned from these first transplant studies.

One goal, different strategies--molecular and cellular approaches for the treatment of inherited skin fragility disorders

Epidermolysis bullosa (EB) is a heterogeneous group of inherited diseases characterized by the formation of blisters in the skin and mucosa. There is no cure or effective treatment for these potentially severe and fatal diseases. Over the past few years, several reports have proposed different molecular strategies as new therapeutic options for the management of EB. From classical vector-based gene therapy to cell-based strategies such as systemic application of bone marrow stem cells or local application of fibroblasts, a broad range of molecular approaches have been explored. This array also includes novel methods, such as protein replacement therapy, gene silencing and the use of induced pluripotent stem cells (iPCs). In this review, we summarize current concepts of how inherited blistering diseases might be treated in the future and discuss the opportunities, promises, concerns and risks of these innovative approaches.

Cell-based therapies for epidermolysis bullosa - from bench to bedside

Significant progress has been made over the past two decades in molecular genetics of epidermolysis bullosa (EB), a group of heritable blistering disorders, with diagnostic and prognostic implications. More recently, novel molecular approaches have been developed towards potential treatment of EB, with emphasis on gene-, protein-, and cell-based strategies. This overview highlights cell-based approaches that have recently been tested in pilot clinical trials, attesting to the potential of regenerative medicine for blistering skin diseases.

Disorders of the cutaneous basement membrane zone--the paradigm of epidermolysis bullosa

The cutaneous basement membrane zone (BMZ) is a highly specialized functional complex that provides the skin with structural adhesion and resistance to shearing forces. Its regulatory functions include control of epithelial-mesenchymal interactions under physiological and pathological conditions. Mutations in genes encoding components of the BMZ are associated with inherited skin disorders of the epidermolysis bullosa (EB) group, characterized by skin fragility, mechanically induced blisters and erosions of the skin and mucous membranes. Although most disease-associated genes are known, the genetic basis of new EB subtypes linked to mutations in genes for focal adhesion proteins was uncovered only recently. The molecular mechanisms leading to blistering, abnormal wound healing, predisposition to skin cancer, and other complications in EB have been elucidated using animal models and disease proteomics. The rapid progress in understanding the molecular basis of EB has enabled the development of strategies for biologically valid causal therapies.

Laminin 332 in junctional epidermolysis bullosa

Laminin 332 is an essential component of the dermal-epidermal junction, a highly specialized basement membrane zone that attaches the epidermis to the dermis and thereby provides skin integrity and resistance to external mechanical forces. Mutations in the LAMA3, LAMB3 and LAMC2 genes that encode the three constituent polypeptide chains, α3, β3 and γ2, abrogate or perturb the functions of laminin 332. The phenotypic consequences are diminished dermal-epidermal adhesion and, as clinical symptoms, skin fragility and mechanically induced blistering. The disorder is designated as junctional epidermolysis bullosa (JEB). This article delineates the signs and symptoms of the different forms of JEB, the mutational spectrum, genotype-phenotype correlations as well as perspectives for future molecular therapies.

Topically applied flightless I neutralizing antibodies improve healing of blistered skin in a murine model of epidermolysis bullosa acquisita

Epidermolysis bullosa (EB) is a chronic inheritable disease that leads to severe blistering and fibrosis. Previous studies have shown that the actin cytoskeletal protein flightless I (Flii) impairs wound healing associated with EB. Using a mouse model of EB acquisita (EBA), the effect of "mopping up" Flii using Flii-neutralizing antibodies (FnAbs) before, during, and after blister formation was determined. FnAbs, incorporated into a cream vehicle and applied topically to the skin, penetrated into the basal epidermis and upper papillary dermis but were not detected in serum or other organs and did not alter neutrophil or macrophage infiltration into the blistered skin. Histological assessment of blister severity showed that treatment of early-stage blisters with FnAb cream reduced their severity and improved their rate of healing. Treatment of established blisters with FnAb cream also improved healing and restored the skin's tensile strength toward that of normal skin. Repeated application of FnAbs to EBA skin before the onset of blistering reduced the severity of skin blistering. Independent of when the FnAbs were applied, skin barrier function and wound healing were improved and skin fragility was reduced, suggesting that FnAbs could potentially improve healing of patients with EB.

Management of severe epidermolysis bullosa by haematopoietic transplant: principles, perspectives and pitfalls

People with severe forms of epidermolysis bullosa (EB) develop widespread blistering and progressively debilitating multisystem complications that may result in a shortened lifespan. As some wounds in EB individuals are difficult or impossible to access with topical therapy, we examined the potential of systemic therapy with normal haematopoietic stem cells. In both animal models and children with EB, healthy donor cells from the haematopoietic graft migrated to the injured skin; simultaneously, there was an increase in the production of skin-specific structural proteins deficient in EB, increased skin integrity and reduced tendency to blister formation. Even though the majority of evaluable individuals have had a positive response in skin healing, frequently changing their quality of life, the improvement in lifestyle has been varied and the overall clinical response incomplete. To change the current amelioration of disease into a full cure, we propose to (i) increase safety as well as efficacy of haematopoietic cell transplant (HCT) using co-infusion of mesenchymal stromal/stem cells with haematopoietic stem cells and non-myeloablative conditioning for transplant; (ii) optimize homing of donor cells into the skin erosions in animal models of EB; and (iii) discover and test new drugs for EB therapy using patient-specific induced pluripotent stem cells. We conclude that although HCT has always been a risky treatment restricted to those with serious life-threatening or debilitating diseases, by most benchmarks, the results of HCT in EB have shown that HCT has the potential of being a durable, systemic therapy for people with severe forms of EB.

Remodeling of the dermal-epidermal junction in bilayered skin constructs after silencing the expression of the p.R2622Q and p.G2623C collagen VII mutants

The integrity of skin depends on a complex system of extracellular matrix molecules that form a biological scaffold. One of its elements is the dermal basement membrane that provides a link between the epidermis and the dermis. Mutations in collagen VII, a key component of the dermal membrane zone, are associated with dystrophic epidermolysis bullosa. Although it has been proposed that silencing the mutated COL7A1 allele is a promising approach to restore the dermal basement membrane zone formed in the presence of collagen VII mutants, limitations exist to testing this proposal. Here, we employed a model that utilized skin-like constructs in which engineered collagen VII mutant chains harboring the R2622Q or G2623C substitution were expressed conditionally, but the wild-type chains were expressed unconditionally. We demonstrated that switching off the production of the mutant collagen VII chains in skin constructs restores the organization of collagen VII and laminin 332 deposits in the dermal-epidermal junction to the level of control. We also demonstrated that remodeling of collagen IV deposits was not fully effective after silencing the expression of collagen VII mutants. Thus, our study suggests that while silencing mutant alleles of COL7A1 may repair critical elements of the affected dermal basement membrane, it may not be sufficient to fully remodel its entire architecture initially formed in the presence of the mutant collagen VII chains.

Distinct strategies are required to suppress antigen-specific responses to genetically modified keratinocytes and fibroblasts

Keratinocytes and fibroblasts are potential targets of gene/cell therapy for genodermatoses. Immune elimination of genetically modified cells, however, presents a major impediment to effective therapy. Using ex vivo approaches to gene transfer, we have previously shown that expression of an antigen by either cell type in skin induces immune rejection of transplanted cells, although the nature of immune responses induced by these two cell types are distinct. In this study, we explore the efficacy of local immunosuppressive strategies to divert destructive immune responses from genetically modified fibroblast and keratinocytes. Expression of CTLA4Ig and, to a lesser extent, PDL1, by antigenic fibroblasts protected them from immune rejection resulting in long-term graft survival (>18 weeks). Similar treatment was not effective for antigenic keratinocytes. Long-term protection of transgenic keratinocytes was achieved through transient blockade of CD40/CD154 interactions during the first 2 weeks of cell transplantation. Although neither of these strategies induced antigen-specific tolerance, they were sufficient to prevent rejection of genetically modified cells. These results indicate that different strategies are required to protect antigenic cell types even within the same tissue. Moreover, induction of antigen-specific tolerance is not a necessary requirement for long-term survival of genetically modified skin cells.

Gammaretroviral vectors: biology, technology and application

Retroviruses are evolutionary optimized gene carriers that have naturally adapted to their hosts to efficiently deliver their nucleic acids into the target cell chromatin, thereby overcoming natural cellular barriers. Here we will review—starting with a deeper look into retroviral biology—how Murine Leukemia Virus (MLV), a simple gammaretrovirus, can be converted into an efficient vehicle of genetic therapeutics. Furthermore, we will describe how more rational vector backbones can be designed and how these so-called self-inactivating vectors can be pseudotyped and produced. Finally, we will provide an overview on existing clinical trials and how biosafety can be improved.

Risk assessment in skin gene therapy: viral-cellular fusion transcripts generated by proviral transcriptional read-through in keratinocytes transduced with self-inactivating lentiviral vectors

Cutaneous gene therapy can be envisioned through the use of keratinocyte stem cell clones in which retroviral genotoxic risks can be pre-assessed. While transactivation of cellular genes by the retroviral long terminal repeat enhancer has been proven in experimental and clinical settings, the formation of chimeric viral-cellular transcripts originated by the inefficient termination (read-through) of retroviral transcripts remains to be studied in depth. We now demonstrate the widespread presence of viral-cellular fusion transcripts derived from integrated proviruses in keratinocytes transduced with self-inactivating (SIN) retroviral vectors. We have detected high molecular weight RNAs in northern blot analysis of retroviral vector expression in individual cell clones. Characterization of some of these transcripts revealed that they originate from genes located at the proviral integration sites. One class of transcripts corresponds to fusions of the viral vectors with intronic sequences, terminating at cryptic polyadenylation sites located in introns. A second class comprises fusion transcripts with coding sequences of genes at the integration sites. These are generated through splicing from a cryptic, not previously described donor site in the lentiviral vectors to exons of cellular genes, and have the potential to encode unintended open reading frames, although they are downregulated by cellular mechanisms. Our data contribute to a better understanding of the impact of SIN lentiviral vector integration on cellular gene transcription, and will be helpful in improving the design of this type of vectors.

Ex-vivo gene therapy restores LEKTI activity and corrects the architecture of Netherton syndrome-derived skin grafts

Netherton syndrome (NS) is a debilitating congenital skin disorder caused by mutations in the SPINK5 gene encoding the lymphoepithelial Kazal-type-related inhibitor (LEKTI). It is characterized by defective keratinization, recurrent infections, and hypernatraemic dehydration with a mortality rate of about 10% in the first year of life. Currently, there are no curative treatments for NS. We have developed a HIV-1 based, self-inactivating lentiviral vector to express SPINK5 in keratinocytes as part of an ex-vivo gene therapy strategy for NS. High transduction efficiency was achieved in NS keratinocytes and reconstitution of LEKTI expression was confirmed in previously deficient cells. These genetically corrected keratinocytes were further tested in an in vitro organotypic culture (OTC) system and in vivo mouse/human skin engraftment model. Results showed correction of epidermal architecture in both OTCs and regenerated skin grafts. Importantly, the results from corrected skin grafts indicated that even where detectable LEKTI expression was restored to a limited numbers of cells, a wider bystander benefit occurred around these small populations. As LEKTI is a secreted protein, the genetically modified graft may provide not only an immediate local protective barrier, but also act as a source of secreted LEKTI providing a generalized benefit following ex-vivo gene therapy.

Artificial skin in perspective: concepts and applications

Skin, the largest organ of the human body, is organized into an elaborate layered structure consisting mainly of the outermost epidermis and the underlying dermis. A subcutaneous adipose-storing hypodermis layer and various appendages such as hair follicles, sweat glands, sebaceous glands, nerves, lymphatics, and blood vessels are also present in the skin. These multiple components of the skin ensure survival by carrying out critical functions such as protection, thermoregulation, excretion, absorption, metabolic functions, sensation, evaporation management, and aesthetics. The study of how these biological functions are performed is critical to our understanding of basic skin biology such as regulation of pigmentation and wound repair. Impairment of any of these functions may lead to pathogenic alterations, including skin cancers. Therefore, the development of genetically controlled and well characterized skin models can have important implications, not only for scientists and physicians, but also for manufacturers, consumers, governing regulatory boards and animal welfare organizations. As cells making up human skin tissue grow within an organized three-dimensional (3D) matrix surrounded by neighboring cells, standard monolayer (2D) cell cultures do not recapitulate the physiological architecture of the skin. Several types of human skin recombinants, also called artificial skin, that provide this critical 3D structure have now been reconstructed in vitro. This review contemplates the use of these organotypic skin models in different applications, including substitutes to animal testing.

Differentiation of mouse induced pluripotent stem cells into a multipotent keratinocyte lineage

Recent breakthroughs in the generation of induced pluripotent stem cells (iPSCs) have provided a novel renewable source of cells with embryonic stem cell-like properties, which may potentially be used for gene therapy and tissue engineering. Although iPSCs have been differentiated into various cell types, iPSC-derived keratinocytes have not yet been obtained. In this study, we report the in vitro differentiation of mouse iPSCs into a keratinocyte lineage through sequential applications of retinoic acid and bone-morphogenetic protein-4 and growth on collagen IV-coated plates. We show that iPSCs can be differentiated into functional keratinocytes capable of regenerating a fully differentiated epidermis, hair follicles, and sebaceous glands in an in vivo environment. Keratinocytes derived from iPSCs displayed characteristics similar to those of primary keratinocytes with respect to gene and protein expression, as well as their ability to differentiate in vitro and to reconstitute normal skin and its appendages in an in vivo assay. At present, no effective therapeutic treatments are available for many genetic skin diseases. The development of methods for the efficient differentiation of iPSCs into a keratinocyte lineage will enable us to determine whether genetically corrected autologous iPSCs can be used to generate a permanent corrective therapy for these diseases.

Induced pluripotent stem cells from individuals with recessive dystrophic epidermolysis bullosa

Recessive dystrophic epidermolysis bullosa (RDEB) is an inherited blistering skin disorder caused by mutations in the COL7A1 gene-encoding type VII collagen (Col7), the major component of anchoring fibrils at the dermal-epidermal junction. Individuals with RDEB develop painful blisters and mucosal erosions, and currently, there are no effective forms of therapy. Nevertheless, some advances in patient therapy are being made, and cell-based therapies with mesenchymal and hematopoietic cells have shown promise in early clinical trials. To establish a foundation for personalized, gene-corrected, patient-specific cell transfer, we generated induced pluripotent stem (iPS) cells from three subjects with RDEB (RDEB iPS cells). We found that Col7 was not required for stem cell renewal and that RDEB iPS cells could be differentiated into both hematopoietic and nonhematopoietic lineages. The specific epigenetic profile associated with de-differentiation of RDEB fibroblasts and keratinocytes into RDEB iPS cells was similar to that observed in wild-type (WT) iPS cells. Importantly, human WT and RDEB iPS cells differentiated in vivo into structures resembling the skin. Gene-corrected RDEB iPS cells expressed Col7. These data identify the potential of RDEB iPS cells to generate autologous hematopoietic grafts and skin cells with the inherent capacity to treat skin and mucosal erosions that typify this genodermatosis.

Factors affecting quality of life in epidermolysis bullosa

Epidermolysis bullosa is a rare group of inherited disorders that manifests as blistering or erosion of the skin in response to little or no apparent trauma. Patients suffer from long-term physical, social and economic consequences. Despite the fact that quality of life assessment has rapidly become a significant outcome variable in dermatology research, only a few studies regarding epidermolysis bullosa patients are available. This paper reviews studies evaluating the effect of epidermolysis bullosa on quality of life in patients and caregivers in order to assess which factors can best describe their health-related quality of life. Currently, considerable deficits prevail in both the diagnosis and treatment of rare skin diseases. Moreover, the psychological and social morbidities very easily evade the eyes of clinicians. Comparison of the impact of epidermolysis bullosa with that of other conditions is clearly valuable for making public policy decisions. This review indicates the need for more rigorous exploration of the impact of such diseases to further guide treatment and to monitor quality of care.

Bone marrow transplantation for recessive dystrophic epidermolysis bullosa

BACKGROUND

Recessive dystrophic epidermolysis bullosa is an incurable, often fatal mucocutaneous blistering disease caused by mutations in COL7A1, the gene encoding type VII collagen (C7). On the basis of preclinical data showing biochemical correction and prolonged survival in col7 −/− mice, we hypothesized that allogeneic marrow contains stem cells capable of ameliorating the manifestations of recessive dystrophic epidermolysis bullosa in humans.

METHODS

Between October 2007 and August 2009, we treated seven children who had recessive dystrophic epidermolysis bullosa with immunomyeloablative chemotherapy and allogeneic stem-cell transplantation. We assessed C7 expression by means of immunofluorescence staining and used transmission electron microscopy to visualize anchoring fibrils. We measured chimerism by means of competitive polymerase-chain-reaction assay, and documented blister formation and wound healing with the use of digital photography.

RESULTS

One patient died of cardiomyopathy before transplantation. Of the remaining six patients, one had severe regimen-related cutaneous toxicity, with all having improved wound healing and a reduction in blister formation between 30 and 130 days after transplantation. We observed increased C7 deposition at the dermal-epidermal junction in five of the six recipients, albeit without normalization of anchoring fibrils. Five recipients were alive 130 to 799 days after transplantation; one died at 183 days as a consequence of graft rejection and infection. The six recipients had substantial proportions of donor cells in the skin, and none had detectable anti-C7 antibodies.

CONCLUSIONS

Increased C7 deposition and a sustained presence of donor cells were found in the skin of children with recessive dystrophic epidermolysis bullosa after allogeneic bone marrow transplantation. Further studies are needed to assess the long-term risks and benefits of such therapy in patients with this disorder. (Funded by the National Institutes of Health; ClinicalTrials.gov number, NCT00478244.)

Targeted approaches for gene therapy and the emergence of engineered meganucleases

BACKGROUND

In spite of significant advances in gene transfer strategies in the field of gene therapy, there is a strong emphasis on the development of alternative methods, providing better control of transgene expression and insertion patterns.

OBJECTIVE

Several new approaches consist of targeting a desired transgene or gene modification in a well defined locus, and we collectively refer to them as 'targeted approaches'. The use of redesigned meganucleases is one of these emerging technologies. Here we try to define the potential of this method, in the larger scope of targeted strategies.

METHODS

We survey the different types of targeted strategies, presenting the achievements and the potential applications, with a special emphasis on the use of redesigned endonucleases.

CONCLUSION

redesigned endonucleases represent one of the most promising tools for targeted approaches, and the opening of a clinical trial for AIDS patients has recently shown the maturity of these strategies. However, there is still a 'quest' for the best reagents, that is the endonucleases providing the best efficacy:toxicity ratio. New advances in protein design have allowed the engineering of new scaffolds, such as meganucleases, and the landscape of existing methods is likely to change over the next few years.