Promoter Choice for Retroviral Vectors: Transcriptional Strength Versus Trans-Activation Potential

Turning "trashed" genomic loci into treasurable sites for integrating chimeric antigen receptors in T and NK cells

Chimeric antigen receptor (CAR)-based immune cell therapy involves genetically engineering immune cells, such as T cells and natural killer (NK) cells, to express CARs that can specifically recognize target antigens. This modification enables T/NK cells to selectively eliminate tumor cells following adoptive transfer. One common approach to stably integrate CARs into the genome of T/NK cells is through retroviral or lentiviral vectors. However, these vectors mediate semi-random gene integration, posing risks such as oncogenic mutations, gene silencing, and variable CAR expression levels. Targeted integration of CAR genes into the specific genomic locus could overcome these limitations, but identifying the optimal integration sites to maximize the safety and efficacy of CAR-T/NK cell products remains a critical question. Improper integration sites may disturb the endogenous genes surrounding the integration sites, raising safety concerns. Additionally, regulatory elements at the integration sites, such as promoters, can influence the expression level of CAR genes, thus affecting the efficacy of CAR-T/NK cells. In this review, we summarized current strategies for selecting integration sites and promoters in the engineering of CAR-T/NK cells to achieve potent anti-tumor efficacy in preclinical studies.

Anaerobic fungi contain abundant, diverse, and transcriptionally active Long Terminal Repeat retrotransposons

Long Terminal Repeat (LTR) retrotransposons are a class of repetitive elements that are widespread in the genomes of plants and many fungi. LTR retrotransposons have been associated with rapidly evolving gene clusters in plants and virulence factor transfer in fungal-plant parasite-host interactions. We report here the abundance and transcriptional activity of LTR retrotransposons across several species of the early-branching Neocallimastigomycota, otherwise known as the anaerobic gut fungi (AGF). The ubiquity of LTR retrotransposons in these genomes suggests key evolutionary roles in these rumen-dwelling biomass degraders, whose genomes also contain many enzymes that are horizontally transferred from other rumen-dwelling prokaryotes. Up to 10% of anaerobic fungal genomes consist of LTR retrotransposons, and the mapping of sequences from LTR retrotransposons to transcriptomes shows that the majority of clusters are transcribed, with some exhibiting expression greater than 104 reads per kilobase million mapped reads (rpkm). Many LTR retrotransposons are strongly differentially expressed upon heat stress during fungal cultivation, with several exhibiting a nearly three-log10 fold increase in expression, whereas growth substrate variation modulated transcription to a lesser extent. We show that some LTR retrotransposons contain carbohydrate-active enzymes (CAZymes), and the expansion of CAZymes within genomes and among anaerobic fungal species may be linked to retrotransposon activity. We further discuss how these widespread sequences may be a source of promoters and other parts towards the bioengineering of anaerobic fungi.

Restriction-Assembly: A Solution to Construct Novel Adenovirus Vector

Gene therapy and vaccine development need more novel adenovirus vectors. Here, we attempt to provide strategies to construct adenovirus vectors based on restriction-assembly for researchers with little experience in this field. Restriction-assembly is a combined method of restriction digestion and Gibson assembly, by which the major part of the obtained plasmid comes from digested DNA fragments instead of PCR products. We demonstrated the capability of restriction-assembly in manipulating the genome of simian adenovirus 1 (SAdV-1) in this study. A PCR product of the plasmid backbone was combined with SAdV-1 genomic DNA to construct an infectious clone, plasmid pKSAV1, by Gibson assembly. Restriction-assembly was performed repeatedly in the steps of intermediate plasmid isolation, modification, and restoration. The generated adenoviral plasmid was linearized by restriction enzyme digestion and transfected into packaging 293 cells to rescue E3-deleted replication-competent SAdV1XE3-CGA virus. Interestingly, SAdV1XE3-CGA could propagate in human chronic myelogenous leukemia K562 cells. The E1 region was similarly modified to generate E1/E3-deleted replication-defective virus SAdV1-EG. SAdV1-EG had a moderate gene transfer ability to adherent mammalian cells, and it could efficiently transduce suspension cells when compared with the human adenovirus 5 control vector. Restriction-assembly is easy to use and can be performed without special experimental materials and instruments. It is highly effective with verifiable outcomes at each step. More importantly, restriction-assembly makes the established vector system modifiable, upgradable and under sustainable development, and it can serve as the instructive method or strategy for the synthetic biology of adenoviruses.

Fiber modifications enable fowl adenovirus 4 vectors to transduce human cells

Background

Pre‐existing immunities hamper the application of human adenovirus (HAdV) vectors in gene therapy or vaccine development. Fowl adenovirus (FAdV)‐based vector might represent an alternative.

Methods

An intermediate plasmid containing FAdV‐4 fiber genes, pMD‐FAV4Fs, was separated from FAdV‐4 adenoviral plasmid pKFAV4GFP. An overlap extension polymerase chain reaction (PCR) was employed for fiber modification in pMD‐FAV4Fs, and the modified fibers were restored to generate new adenoviral plasmids through restriction‐assembly. FAdV‐4 vectors were rescued and amplified in chicken LMH cells. Fluorescence microscopy and flow cytometry were used to evaluate the gene transfer efficiency. The amount of viruses binding to cells was determined by a real‐time PCR. A plaque‐forming assay and one‐step growth curve were used to evaluate virus growth.

Results

Four sites in the CD‐, DE‐, HI‐ and IJ‐loop of fiber1 knob could tolerate the insertion of exogenous peptide. The insertion of RGD4C peptide in the fiber1 knob significantly promoted FAdV‐4 transduction to human adherent cells such as 293, A549 and HEp‐2, and the insertion to the IJ‐loop demonstrated the best performance. The replacement of the fiber2 knob of FAdV‐4 with that of HAdV‐35 improved the gene transfer to human suspension cells such as Jurkat, K562 and U937. Fiber‐modified FAdV‐4 vectors could transduce approximately 80% human cells at an acceptable multiplicity of infection. Enhanced gene transfer mainly resulted from increased virus binding. Fiber modifications did not significantly influence the growth of recombinant FAdV‐4 in packaging cells.

Conclusions

As a proof of principle, it was feasible to enhance gene transduction of FAdV‐4 vectors to human cells by modifying the fibers.

Efficient gene transfer into T lymphocytes by fiber-modified human adenovirus 5

Background

The gene transduction efficiency of adenovirus to hematopoietic cells, especially T lymphocytes, is needed to be improved. The purpose of this study is to improve the transduction efficiency of T lymphocytes by using fiber-modified human adenovirus 5 (HAdV-5) vectors.

Results

Four fiber-modified human adenovirus 5 (HAdV-5) vectors were investigated to transduce hematopoietic cells. F35-EG or F11p-EG were HAdV-35 or HAdV-11p fiber pseudotyped HAdV-5, and HR-EG or CR-EG vectors were generated by incorporating RGD motif to the HI loop or to the C-terminus of F11p-EG fiber. All vectors could transduce more than 90% of K562 or Jurkat cells at an multiplicity of infection (MOI) of 500 viral particle per cell (vp/cell). All vectors except HR-EG could transduce nearly 90% cord blood CD34+ cells or 80% primary human T cells at the MOI of 1000, and F11p-EG showed slight superiority to F35-EG and CR-EG. Adenoviral vectors transduced CD4+ T cells a little more efficiently than they did to CD8+ T cells. These vectors showed no cytotoxicity at an MOI as high as 1000 vp/cell because the infected and uninfected T cells retained the same CD4/CD8 ratio and cell growth rate.

Conclusions

HAdV-11p fiber pseudotyped HAdV-5 could effectively transduce human T cells when human EF1a promoter was used to control the expression of transgene, suggesting its possible application in T cell immunocellular therapy.

Evidence for the in vivo safety of insulated foamy viral vectors

Retroviral vector mediated stem cell gene therapy is a promising approach for the treatment of hematopoietic disorders. However, genotoxic side effects from integrated vector proviruses are a significant concern for the use of retroviral vectors in the clinic. Insulated foamy viral (FV) vectors are potentially safer retroviral vectors for hematopoietic stem cell gene therapy. We evaluated two newly identified human insulators, A1 and A2 for use in FV vectors. These insulators had moderate insulating capacity and higher titers than previously developed insulated FV vectors. The A1 insulated FV vector was chosen for comparison with the previously described 650cHS4 insulated FV vector in human cord blood CD34⁺ repopulating cells in an immunodeficient mouse model. To maximize the effects of the insulators on the safety of FV vectors, FV vectors containing a highly genotoxic spleen focus forming virus (SFFV) promoter was used to elicit differences in genotoxicity. In vivo, the A1 insulated FV vector showed an approximate 50% reduction in clonal dominance compared to either the 650cHS4 insulated or control FV vectors, although the transduction efficiency of the A1 insulated vector was higher. This data suggests that the A1 insulated FV vector is promising for future pre-clinical and clinical studies.

A human haploid gene trap collection to study lncRNAs with unusual RNA biology

Many thousand long non-coding (lnc) RNAs are mapped in the human genome. Time consuming studies using reverse genetic approaches by post-transcriptional knock-down or genetic modification of the locus demonstrated diverse biological functions for a few of these transcripts. The Human Gene Trap Mutant Collection in haploid KBM7 cells is a ready-to-use tool for studying protein-coding gene function. As lncRNAs show remarkable differences in RNA biology compared to protein-coding genes, it is unclear if this gene trap collection is useful for functional analysis of lncRNAs. Here we use the uncharacterized LOC100288798 lncRNA as a model to answer this question. Using public RNA-seq data we show that LOC100288798 is ubiquitously expressed, but inefficiently spliced. The minor spliced LOC100288798 isoforms are exported to the cytoplasm, whereas the major unspliced isoform is nuclear localized. This shows that LOC100288798 RNA biology differs markedly from typical mRNAs. De novo assembly from RNA-seq data suggests that LOC100288798 extends 289kb beyond its annotated 3' end and overlaps the downstream SLC38A4 gene. Three cell lines with independent gene trap insertions in LOC100288798 were available from the KBM7 gene trap collection. RT-qPCR and RNA-seq confirmed successful lncRNA truncation and its extended length. Expression analysis from RNA-seq data shows significant deregulation of 41 protein-coding genes upon LOC100288798 truncation. Our data shows that gene trap collections in human haploid cell lines are useful tools to study lncRNAs, and identifies the previously uncharacterized LOC100288798 as a potential gene regulator.

Insulators to Improve the Safety of Retroviral Vectors for HIV Gene Therapy

Retroviral vector gene therapy is a promising approach to treating HIV-1. However, integrated vectors are mutagens with the potential to dysregulate nearby genes and cause severe adverse side effects. Leukemia has already been a documented severe adverse event in gene therapy clinical trials for the treatment of primary immunodeficiencies. These side effects will need to be reduced or avoided if retroviral vectors are to be used clinically for HIV-1 treatment. The addition of chromatin insulators to retroviral vectors is a potential strategy for reducing adverse side effects. Insulators have already been effectively used in retroviral vectors to reduce genotoxicity in pre-clinical studies. Here, we will review how insulators function, genotoxicity in gene therapy clinical trials, the design of insulated retroviral vectors, promising results from insulated retroviral vector studies, and considerations for the development of insulated retroviral treatment vectors for HIV-1 gene therapy.

Kidney-selective gene transfection using anionic bubble lipopolyplexes with renal ultrasound irradiation in mice

This study assessed the ability of a new ultrasound (US) responsive gene delivery carrier, bubble lipopolyplexes, to deliver genes to the kidneys. The bubble lipopolyplexes showed highly selective gene expression in kidney tubules, but only after renal irradiation with US. These bubble lipopolyplexes, however, did not increase the expression of biomarkers of kidney injury, including blood urea nitrogen, serum creatinine, kidney injury molecule-1 mRNA, and clusterin mRNA, or induce any histopathological abnormalities in the kidney. Furthermore, pDNA containing CMV early enhancer/chicken beta-actin promoter prolonged gene expression by the bubble lipopolyplexes in the kidney for 42 days. This novel renal gene delivery method, in which transfection of bubble lipopolyplexes was followed by US irradiation of the kidneys, resulting in cell-selective, high, and long-term gene expression without renal injury in mice, may have future applications in patient treatment.

FROM THE CLINICAL EDITOR

This study demonstrates a novel gene delivery method to the kidneys, utilizing bubble resulting in highly selective gene expression in renal tubules after ultrasound irradiation. In the studied rodent model, there was no evidence for renal damage using this novel delivery system.

Retrovirus and lentivirus vector design and methods of cell conditioning

Retroviruses are useful tools for the efficient delivery of genes to mammalian cells, owing to their ability to stably integrate into the host cell genome. Over the past few decades, retroviral vectors have been used in gene therapy clinical trials for the treatment of a number of inherited diseases and cancers. The earliest retrovirus vectors were based on simple oncogenic gammaretroviruses such as Moloney murine leukemia virus (MMLV) which, when pseudotyped with envelope proteins from other viruses such as the gibbon ape leukemia virus envelope protein (GALV) or vesicular stomatitis virus G protein (VSV-G), can efficiently introduce genes to a wide range of host cells. However, gammaretroviral vectors have the disadvantage that they are unable to efficiently transduce nondividing or slowly dividing cells. As a result, specific protocols have been developed to activate cells through the use of growth factors and cytokines. In the case of hematopoietic stem cells, activation has to be carefully controlled so that pluripotency is maintained. For many applications, gammaretroviral vectors are being superseded by lentiviral vectors based on human immunodeficiency virus type-1 (HIV-1) which has additional accessory proteins that enable integration in the absence of cell division. In addition, retroviral and lentiviral vector design has evolved to address a number of safety concerns. These include separate expression of the viral genes in trans to prevent recombination events leading to the generation of replication-competent viruses. Further, the development of self-inactivating (SIN) vectors reduces the potential for transactivation of neighboring genes and allows the incorporation of regulatory elements that may target gene expression more physiologically to particular cell types.

Efficacy and safety of Sleeping Beauty transposon-mediated gene transfer in preclinical animal studies

Sleeping Beauty (SB) transposons have been effective in delivering therapeutic genes to treat certain diseases in mice. Hydrodynamic gene delivery of integrating transposons to 5-20% of the hepatocytes in a mouse results in persistent elevated expression of the therapeutic polypeptides that can be secreted into the blood for activity throughout the animal. An alternative route of delivery is ex vivo transformation with SB transposons of hematopoietic cells, which then can be reintroduced into the animal for treatment of cancer. We discuss issues associated with the scale-up of hydrodynamic delivery to the liver of larger animals as well as ex vivo delivery. Based on our and others' experience with inefficient delivery to larger animals, we hypothesize that impulse, rather than pressure, is a critical determinant of the effectiveness of hydrodynamic delivery. Accordingly, we propose some alterations in delivery strategies that may yield efficacious levels of gene delivery in dogs and swine that will be applicable to humans. To ready hydrodynamic delivery for human application we address a second issue facing transposons used for gene delivery regarding their potential to "re-hop" from one site to another and thereby destabilize the genome. The ability to correct genetic diseases through the infusion of DNA plasmids remains an appealing goal.

SIN retroviral vectors expressing COL7A1 under human promoters for ex vivo gene therapy of recessive dystrophic epidermolysis bullosa

Recessive dystrophic epidermolysis bullosa (RDEB) is caused by loss-of-function mutations in COL7A1 encoding type VII collagen which forms key structures (anchoring fibrils) for dermal-epidermal adherence. Patients suffer since birth from skin blistering, and develop severe local and systemic complications resulting in poor prognosis. We lack a specific treatment for RDEB, but ex vivo gene transfer to epidermal stem cells shows a therapeutic potential. To minimize the risk of oncogenic events, we have developed new minimal self-inactivating (SIN) retroviral vectors in which the COL7A1 complementary DNA (cDNA) is under the control of the human elongation factor 1alpha (EF1alpha) or COL7A1 promoters. We show efficient ex vivo genetic correction of primary RDEB keratinocytes and fibroblasts without antibiotic selection, and use either of these genetically corrected cells to generate human skin equivalents (SEs) which were grafted onto immunodeficient mice. We achieved long-term expression of recombinant type VII collagen with restored dermal-epidermal adherence and anchoring fibril formation, demonstrating in vivo functional correction. In few cases, rearranged proviruses were detected, which were probably generated during the retrotranscription process. Despite this observation which should be taken under consideration for clinical application, this preclinical study paves the way for a therapy based on grafting the most severely affected skin areas of patients with fully autologous SEs genetically corrected using a SIN COL7A1 retroviral vector.

High-level genomic integration, epigenetic changes, and expression of sleeping beauty transgene

Sleeping Beauty transposon (SB-Tn) has emerged as an important nonviral vector for integrating transgenes into mammalian genomes. We report here a novel dual fluorescent reporter cis SB-Tn system that permitted nonselective fluorescent-activated cell sorting for SB-Tn-transduced K562 erythroid cells. Using an internal ribosome entry site element, the green fluorescent protein (eGFP) was linked to the SB10 transposase gene as an indirect marker for the robust expression of SB10 transposase. Flourescence-activated cell sorting (FACS) by eGFP resulted in significant enrichment (>60%) of cells exhibiting SB-Tn-mediated genomic insertions and long-term expression of a DsRed transgene. The hybrid erythroid-specific promoter of DsRed transgene was verified in erythroid or megakaryocyte differentiation of K562 cells. Bisulfite-mediated genomic analyses identified different DNA methylation patterns between DsRed(+) and DsRed(-) cell clones, suggesting a critical role in transgene expression. Moreover, although the host genomic copy of the promoter element showed no CpG methylation, the same sequence carried by the transgene was markedly hypermethylated. Additional evidence also suggested a role for histone deacetylation in the regulation of DsRed transgene. The presence of SB transgene affected the expression of neighboring host genes at distances >45 kb. Our data suggested that a fluorescent reporter cis SB-Tn system can be used to enrich mammalian cells harboring SB-mediated transgene insertions. The observed epigenetic changes also demonstrated that transgenes inserted by SB could be selectively modified by endogenous factors. In addition, long-range activation of host genes must now be recognized as a potential consequence of an inserted transgene cassette containing enhancer elements.

Stable transgene expression in primitive human CD34+ hematopoietic stem/progenitor cells, using the Sleeping Beauty transposon system

Sleeping Beauty (SB) transposon-mediated integration has been shown to achieve long-term transgene expression in a wide range of host cells. In this study, we improved the SB transposon-mediated gene transfer system for transduction of human CD34(+) stem/progenitor cells by two approaches: (1) to increase the transposition efficacy, a hyperactive mutant of SB, HSB, was used; (2) to improve the expression of the SB transposase and the transgene cassette carried by the transposon, different viral and cellular promoters were evaluated. SB components were delivered in trans into the target cells by Nucleoporation. The SB transposon-mediated integration efficacy was assessed by integrated transgene (enhanced green fluorescent protein [eGFP]) expression both in vitro and in vivo. In purified human cord blood CD34(+) cells, HSB achieved long-term transgene expression in nearly 7-fold more cells than the original SB transposase. Significantly brighter levels of eGFP expression (5-fold) were achieved with the human elongation factor 1alpha (EF1-alpha) promoter in Jurkat human T cells, compared with that achieved with the modified myeloproliferative sarcoma virus long terminal repeat enhancer-promoter (MNDU3); in contrast, the MNDU3 promoter expressed eGFP at the highest level in K-562 myeloid cells. In human CD34(+) cord blood cells studied under conditions directing myeloid differentiation, the highest transgene integration and expression were achieved using the EF1-alpha promoter to express the SB transposase combined with the MNDU3 promoter to express the eGFP reporter. Stable transgene expression was achieved at levels up to 27% for more than 4 weeks of culture after improved gene transfer to CD34(+) cells (average, 17%; n = 4). In vivo studies evaluating engraftment and differentiation of the SB-modified human CD34(+) cells demonstrated that SB-modified human CD34(+) cells engrafted in NOD/SCID/gamma chain(null) (NSG) mice and differentiated into multilineage cell types with eGFP expression. More importantly, secondary transplantation studies demonstrated that the integrated transgene was stably expressed in more primitive CD34(+) hematopoietic stem cells (HSCs) with long-term repopulating capability. This study demonstrates that an improved HSB gene transfer system can stably integrate genes into primitive human HSCs while maintaining the pluripotency of the stem cells, which shows promise for further advancement of non-virus-based gene therapy using hematopoietic stem cells.

Efficient transcriptional targeting of human hematopoietic stem cells and blood cell lineages by lentiviral vectors containing the regulatory element of the Wiskott-Aldrich syndrome gene

The ability to effectively transduce human hematopoietic stem cells (HSCs) and to ensure adequate but "physiological" levels of transgene expression in different hematopoietic lineages represents some primary features of a gene-transfer vector. The ability to carry, integrate, and efficiently sustain transgene expression in HSCs strongly depends on the vector. We have constructed lentiviral vectors (LV) containing fragments of different lengths of the hematopoietic-specific regulatory element of the Wiskott-Aldrich syndrome (WAS) gene-spanning approximately 1,600 and 170 bp-that direct enhanced green fluorescent protein (EGFP) expression. The performance of vectors carrying the 1,600 and 170 bp fragments of the WAS gene promoter was compared with that of a vector carrying the UbiquitinC promoter in human cord blood CD34(+) cells and their differentiated progeny both in vitro and in vivo in non-obese diabetic mice with severe combined immunodeficiency. All vectors displayed a similar transduction efficiency in CD34(+) cells and promoted long-term EGFP expression in different hematopoietic lineages, with an efficiency comparable to, and in some instances (for example, the 170-bp promoter) superior to, that of the UbiquitinC promoter. Our results clearly demonstrate that LV containing fragments of the WAS gene promoter/enhancer region can promote long-term transgene expression in different hematopoietic lineages in vitro and in vivo and represent suitable and highly efficient vectors for gene transfer in gene-therapy applications for different hematological diseases and for research purposes. In particular, the 170-bp carrying vector, for its reduced size, could significantly improve the transduction/expression of large-size genes.

Epigenetic activation of unintegrated HIV-1 genomes by gut-associated short chain fatty acids and its implications for HIV infection

Integration of HIV-1 linear DNA into the host chromatin is an essential step in the viral life cycle. However, the majority of reverse-transcribed, nuclear-imported viral genomes remain episomal, either as linear or circular DNA. To date, these nonintegrated viral genomes are largely considered "dead-end products" of reverse transcription. Indeed, limited gene expression from nonintegrated HIV-1 has been reported, although the mechanism that renders nonintegrating HIV-1 genomes incapable of supporting efficient viral replication has not been fully elucidated. Here, we demonstrate that nonintegrating HIV-1 and HIV-1-based vector genomes are organized into chromatin structures and enriched with histone modifications typical of transcriptionally silenced chromatin. Gene expression and replication of nonintegrating HIV-1 was notably increased in vitro upon exposure to histone deacetylase inhibitors (HDACi) in the form of various short-chain fatty acids (SCFAs) known to be endogenously produced by normal microbial-gut flora. Furthermore, we demonstrated genetic and functional crosstalk between episomal and integrated vector/viral genomes, resulting in recombination between integrated and nonintegrated HIV-1, as well as mobilization of episomal vector genomes by productive viral particles encoded by integrated viral genomes. Finally, we propose a mechanism describing the role of episomal HIV-1 forms in the viral life cycle in a SCFA-rich gut environment.

A combination of transposable elements and magnetic cell sorting provides a very efficient transgenesis system for chicken primary erythroid progenitors

Background

Stable transgenesis is an undeniable key to understanding any genetic system. Retrovirus-based insertional strategies, which feature several technical challenges when they are used, are often limited to one particular species, and even sometimes to a particular cell type as the infection depends on certain cellular receptors. A universal-like system, which would allow both stable transgene expression independent of the cell type and an efficient sorting of transfected cells, is required when handling cellular models that are incompatible with retroviral strategies.

Results

We report here on the combination of a stable insertional transgenesis technique, based on the Tol2 transposon system together with the magnetic cell sorting (MACS) technique, which allows specific selection of cells carrying the transgene in an efficient, reliable and rapid way.

Conclusion

This new Tol2/MACS system leads to stable expression in a culture of primary chicken erythroid cells highly enriched in cells expressing the transgene of interest. This system could be used in a wide variety of vertebrate species.

Sonoporation of the minicircle-VEGF(165) for wound healing of diabetic mice

PURPOSE

The purpose of this study is to examine the efficiency of sonoporation with minicircle DNA for the skin wound healing in diabetic mice.

METHODS

Minicircle DNA containing the human VEGF(165) was constructed and tested in vitro. Diabetes was induced in 2-week old male C57BL/6J mice via streptozotocin (STZ) injection. 6 mm circular skin wounds were made on the mice back. After the subcutaneous injection of the minicircle DNA at the edge of the wound, the mice were exposed to the ultrasound irradiation for the sonoporation. Wound areas were analyzed until the day 12. Blood perfusion and angiogenesis were evaluated using a laser Doppler imaging and CD31 immunostaining, respectively. Re-epithelialization was assessed by histochemical analysis using hematoxylin and eosin staining.

RESULTS

Accelerated wound closure was observed in the mice receiving sonoporation of minicircle-VEGF(165), which corresponds to the markedly increased skin blood perfusion and CD31 expression. Histological analysis revealed that the minicircle treated wound tissues showed fully restored normal architectures as compared with the non-treated diabetic controls with the markedly edematous and chaotic morphologies.

CONCLUSIONS

Ultrasound mediated gene therapy with the minicircle-VEGF(165) is effective for the healing of the skin wound of the diabetic mice.

Gene transfer: the challenge of regulated gene expression

Gene therapy is expected to have a major impact on human healthcare in the future. However, precise regulation of therapeutic gene expression in vivo is still a challenge. Natural and synthetic enhancer-promoters (EPs) can be utilized to drive gene transcription in a temporal, spatial or environmental signal-inducible manner in response to heat shock, hypoxia, radiation, chemotherapy, epigenetic agents or viral infection. To allow tightly regulated expression, a regulatable gene-expression system can also be implemented. Most of these systems are based on small molecule (drug)-responsive artificial transactivators. In this review, we aim to provide a brief overview of the classes of EPs and regulatable systems, along with lessons learned from these studies. We highlight the potential applications in gene transfer, gene therapy for cancer and genetic disease and the future challenges for clinical applications.

Physiological promoters reduce the genotoxic risk of integrating gene vectors

The possible activation of cellular proto-oncogenes as a result of clonal transformation is a potential limitation in a therapeutic approach involving random integration of gene vectors. Given that enhancer promiscuity represents an important mechanism of insertional transformation, we assessed the enhancer activities of various cellular and retroviral promoters in transient transfection assays, and also in a novel experimental system designed to measure the activation of a minigene cassette contained in stably integrating retroviral vectors. Retroviral enhancer-promoters showed a significantly greater potential to activate neighboring promoters than did cellular promoters derived from human genes, elongation factor-1alpha (EF1alpha) and phosphoglycerate kinase (PGK). Self-inactivating (SIN) vector design reduced but did not abolish enhancer interactions. Using a recently established cell culture assay that detects insertional transformation by serial replating of primary hematopoietic cells, we found that SIN vectors containing the EF1alpha promoter greatly decrease the risk of insertional transformation. Despite integration of multiple copies per cell, activation of the crucial proto-oncogene Evi1 was not detectable when using SIN-EF1alpha vectors. On the basis of several quantitative indicators, the decrease in transforming activity was highly significant (more than tenfold, P < 0.01) when compared with similarly designed vectors containing a retroviral enhancer-promoter with or without a well-characterized genetic insulator core element. In this manner, the insertional biosafety of therapeutic gene vectors can be greatly enhanced and proactively evaluated in sensitive cell-based assays.