Advances in skin gene therapy

Inhibition of CD44 gene expression in human skin models, using self-delivery short interfering RNA administered by dissolvable microneedle arrays

Treatment of skin disorders with short interfering RNA (siRNA)-based therapeutics requires the development of effective delivery methodologies that reach target cells in affected tissues. Successful delivery of functional siRNA to the epidermis requires (1) crossing the stratum corneum, (2) transfer across the keratinocyte membrane, followed by (3) incorporation into the RNA-induced silencing complex. We have previously demonstrated that treatment with microneedle arrays loaded with self-delivery siRNA (sd-siRNA) can achieve inhibition of reporter gene expression in a transgenic mouse model. Furthermore, treatment of human cultured epidermal equivalents with sd-siRNA resulted in inhibition of target gene expression. Here, we demonstrate inhibition of CD44, a gene that is uniformly expressed throughout the epidermis, by sd-siRNA both in vitro (cultured human epidermal skin equivalents) and in vivo (full-thickness human skin equivalents xenografted on immunocompromised mice). Treatment of human skin equivalents with CD44 sd-siRNA markedly decreased CD44 mRNA levels, which led to a reduction of the target protein as confirmed by immunodetection in epidermal equivalent sections with a CD44-specific antibody. Taken together, these results demonstrate that sd-siRNA, delivered by microneedle arrays, can reduce expression of a targeted endogenous gene in a human skin xenograft model.

Cutaneous short-interfering RNA therapy

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

Selectable bicistronic vectors in skin gene therapy

Bicistronic vectors (BCV) are important tools for gene therapy applications allowing selection for increased expression of a desired gene by linking it to a selectable gene, such as the multi-drug resistance (MDR) gene. However, both the design of the BCV and the cell type to be transduced can have a strong impact on the vector performance in the target cells. To analyze which factors might influence the efficiency of BCV in achieving high gene expression levels in skin and to determine the best suited BCV for cutaneous transduction, both keratinocytes (KC) and fibroblasts (FB) were transduced with different BCV constructs, BGIM, BMIG and QGIM. In BGIM, expression of the BCV cassette encompassing the green fluorescent protein (GFP) gene connected to the MDR gene was driven by a retroviral LTR-promoter. In BMIG, the order of the two genes was reversed, while in QGIM the GFP- and MDR-gene were arranged similar as in BGIM, but expressed by a CMV- instead of an LTR-promoter. FACS-analysis revealed that the percentage of genetically modified cells varied substantially with 47.9% QGIM-, 35.5% BMIG- and 17.9% BGIM-transduced KC expressing both genes. For FB the numbers were 56.7% (QGIM), 38.4% (BMIG) and 8.3% (BGIM). Furthermore, the choice of BCV determined the intensity of GFP-expression with the highest levels measured in BGIM-, followed by QGIM- and then BMIG-transduced cells. Interestingly, highly efficient enrichment through colchicine selection was possible for QGIM- (up to 97.1% KC, 97.8% FB) and BMIG- (85.0% KC and 98.0% FB) but not BGIM- (29.9% KC and 18.6% FB) transduced cells. Finally, immunohistochemistry and FACS-analysis demonstrated, that colchicine selection of QGIM-transduced skin equivalents led to increased numbers of GFP-expressing KC (from 51.2% up to 72.3%) and enhanced GFP-intensity in the skin. These results show that BCV present a promising vector system to enhance the expression of a desired gene in skin but important parameters taken into account when employing a selectable BCV for skin gene therapy applications are the retroviral vector backbone, the order in which the genes are arranged, and the target cells to be transduced and selected.

Delivery and Inhibition of Reporter Genes by Small Interfering RNAs in a Mouse Skin Model

RNA interference offers the potential of a novel therapeutic approach for treating skin disorders. To this end, we investigated delivery of nucleic acids, including a plasmid expressing the reporter gene luciferase, to mouse skin by intradermal injection into footpads using in vivo bioluminescence imaging over multiple time points. In order to evaluate the ability of RNA interference to inhibit skin gene expression, reporter gene constructs were co-injected with specific or non-specific siRNAs and the in vivo effects measured. Our results revealed that specific unmodified and modified siRNAs (but not nonspecific matched controls) strongly inhibit reporter gene expression in mice. These results indicate that small interfering RNA, delivered locally as RNA directly or expressed from viral or non-viral vectors, may be effective agents for treating skin disorders.

Histone deacetylase inhibitors preferentially augment transient transgene expression in human dermal fibroblasts

BACKGROUND

Skin is an attractive target for gene therapy. However, low efficiency of gene transfection has been a major problem. Histone deacetylase (HDAC) inhibitors have been reported to increase transgene expression in malignant cells.

OBJECTIVES

We have estimated how much HDAC inhibitors might increase transgene expression in HaCaT cells, normal human epidermal keratinocyte (NHEK) cells, normal human dermal fibroblast (NHDF) cells and also in stratified cultured epidermal sheets that mimic the structure of the skin.

METHODS

After treatment with each HDAC inhibitor [trichostatin A, FK228 and cyclic hydroxamic acid-containing peptide 31 (CHAP31)], transient transgene expression in HaCaT, NHEK and NHDF cells and stratified cultured epidermal sheets was compared with that of respective controls without treatment. Reactivation of transgene expression using HDAC inhibitors in HaCaT cells stably expressing the transgene was also studied.

RESULTS

All HDAC inhibitors equally increased transient transgene expression by 2-fold in NHEK cells, 20-fold in NHDF cells and 6-fold in HaCaT cells when compared with untreated cells. This augmented expression continued for 72 h in all cell lines maintained under each HDAC inhibitor. In cells stably expressing the transgene, only CHAP31 reactivated transgene expression. In stratified cultured epidermal sheets, CHAP31 most effectively improved transient transgene expression.

CONCLUSIONS

HDAC inhibitors are most efficient at amplifying transient transgene expression in NHDF cells. This suggests that NHDF cells may be most suitable as transgene targets for transient gene transfection using HDAC inhibitors. Specific HDAC inhibitors may not prove so useful for treating genetic dermatoses requiring cells stably expressing the correct gene, but may be advantageous in treating nonhealing cutaneous wounds or cancer.

A large preclinical animal model to assess ex vivo skin gene therapy applications

Because of its easy accessibility, the skin is a very attractive target for gene therapy purposes. To study potential clinical applications in a preclinical setting, appropriate animal models are needed. Pig skin is very similar to human skin, and a variety of human diseases that are potentially amenable to gene therapy applications also occur in pigs. Only a few studies have analyzed the engraftment of transduced keratinocytes (KC) in pigs, however, with limited success. We describe a porcine model in which pig KC were transduced ex vivo with a retroviral vector encoding a marker gene and subsequently grafted onto the autologous host, utilizing a relatively simple grafting technique. Enhanced transduction efficiency was achieved by an optimized transduction protocol including centrifugation of the retroviral vector at a temperature of 32 degrees C. Transduced KC were then seeded onto acellular dermis, forming a stratified epidermis. Grafting was performed by creating full thickness wounds and placing the skin graft onto the muscle fascia, covered by a protective skin flap for several days. Successful engraftment of transduced KC was demonstrated by immunohistochemistry of biopsies taken at different time points, showing transgene expression in 40-50% of grafted KC. After 4 weeks, KC expressing a foreign marker gene was lost, suggesting a transgene-specific immune response in the immunocompetent pigs and highlighting the potential problems for clinical gene therapy studies when transferring new genetic material into a patient. The model presented here may be used to examine applications of skin gene therapy, where retroviral vectors encoding endogenous pig genes will be expressed in the skin.

Transduced monocyte/macrophages targeted to murine skin by UV light

We have selectively targeted monocyte/macrophages overexpressing an immunomodulatory molecule, latency-associated peptide (LAP), a naturally occurring antagonist for transforming growth factor-beta1, to murine skin utilizing UV light to produce a cutaneous influx of transduced monocyte/macrophages. Bone marrow (BM) cells from BALB/c mice were transduced in vitro with a retroviral construct containing green fluorescent protein (GFP) for detection and human LAP (hLAP) as a test molecule. The transduced BM cells were then cultured in vitro with granulocyte-macrophage colony-stimulating factor (GM-CSF) to produce differentiation to monocyte/macrophages. More than 80% of transduced BM cells were CD11b-positive and MOMA-positive by fluorescence-activated cell-sorter analysis and secreted LAP by ELISA after 10 days of culture in granulocyte-macrophage colony-stimulating factor (GM-CSF). Transduced monocyte/macrophages containing either GFP or hLAP-GFP were then injected intravenously into BALB/c mice. One-half of recipients in each group were exposed to UVB (72 mJ) to induce monocyte/macrophage infiltration into skin. Recipients were sacrificed 60 h after UV irradiation. We found transduced cutaneous macrophages expressing GFP by examining with fluorescence microscopy frozen skin sections of recipient mice immunostained with antibodies to GFP and to macrophage marker F4/80. We identified hLAP sequences by polymerase chain reaction (PCR) of total DNA in recipient blood and UV-irradiated skin but not in unirradiated skin. LAP sequences were also detected at much lower levels in other organs (lung, spleen, and liver) by PCR. Therefore, we have shown that genetically altered monocytic cells can be injected intravenously and targeted to mouse skin by UV exposure. This macrophage-based gene-transfer method may be a potentially useful immunotherapeutic approach for delivering monocyte/macrophage-derived products to skin.

[Carcinogenesis: defect control systems and options for molecular therapy]

DNA mutations lead to multiple defects of different control systems necessary to regulate cell growth, cell death and cell invasion, thus resulting in the development of a malignant tumor. While the loss of single control systems can still be compensated by others, the destruction of the whole network on a molecular level will cause tumor growth. Knowledge about these control systems and their defects during carcinogenesis may it possible to design new therapeutic strategies. Among them, gene therapy offers promising treatment options.

Directing stem cells into the keratinocyte lineage in vitro

A major area of research in regenerative medicine is the potential application of stem cells in skin grafting and tissue engineering. This would require well defined and efficient protocols for directing the commitment and differentiation of stem cells into the keratinocyte lineage, together with their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying skin tissue biology, as well as facilitate the genetic manipulation of stem cells for therapeutic applications. The development of pharmacokinetic and cytotoxicity/genotoxicity screening tests for skin-related biomaterials and drugs could also utilize protocols developed for the commitment and differentiation of stem cells into the keratinocyte lineage. Hence, this review critically examines the various strategies that could be employed to direct the commitment and differentiation of stem cells into the keratinocyte lineage in vitro.

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

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

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

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