Low Levels Of Transgene Expression Research Articles

Using a Human Liver Tissue Equivalent (hLTE) Platform to Define the Functional Impact of Liver-Directed AAV Gene Therapy

Clinical trials employing AAV vectors for hemophilia A have been hindered by unanticipated immunological and/or inflammatory responses in some of the patients. Also, these trials have often yielded lower levels of transgene expression than were expected based upon preclinical studies, highlighting the poor correlation between the transduction efficiency observed in traditional 2D cultures of primary cells in vitro, and that observed in those same cell types in vivo. It has been also recognized that there are marked species-specific differences in AAV-vector tropism, raising the critical question of the accuracy with which various animal models will likely predict tropism/vector transduction efficiency, and eventual treatment success in humans. Human liver tissue equivalents (hLTEs) are comprised of major cell types in the liver in physiologically relevant frequencies and possess the ability to recapitulate the biology and function of native human liver. Here, we hypothesize that hLTEs can be used as a better model to predict the efficacy and safety of AAV gene therapy in humans. We fabricated hLTEs using 75% hepatocytes, 10% stellate cells, 10% Kupffer cells, and 5% liver sinusoid-derived endothelial cells in 96-well Elplasia plates with 79 microwells per well. hLTEs were transduced at an MOI of 10 5vg/cell, on the day of fabrication, with the clinically relevant serotypes AAV5 (hLTE-5) or AAV3b (hLTE-3b), both encoding a GFP reporter. After 4 days of self-aggregation, live/dead assay was performed to confirm viability. Non-transduced hLTEs served as negative controls (hLTE(-)), and hLTEs exposed to 20 mM acetaminophen were used as positive controls for liver inflammation/damage. Incucyte® Live-Cell Imaging system was used to track the aggregation and GFP expression of hLTEs. Over the course of the next 5 days, media was collected to determine hepatic functionality, RNA was isolated to assess dysregulation of genes involved in inflammation and fibrosis, DNA was isolated to determine whether AAV vectors integrate into the genome of human hepatocytes and, if so, to define the frequency at which this occurs and the genomic loci of integration, and hLTEs were fixed and processed at appropriate times for histological analyses and transmission electron microscopy (TEM). TEM analysis revealed that all groups exhibited microvilli and bile-canaliculus-like structures, demonstrating the formation of a rudimentary biliary system and, more importantly, proving that hLTEs resemble native liver structure. Incucyte® imaging showed that AAV5 and AAV3b transduction impaired formation of hLTEs (57.57 ± 2.42 and 24.57 ± 4.01 spheroids/well, respectively) in comparison with hLTE(-) (74.86 ± 3.8 spheroids/well). Quantification of GFP expression demonstrated that AAV5 yielded the most efficient transduction of hLTEs (fold change in GFP expression compared to control: 2.73 ± 0.09 and 1.19 ± 0.03 for hLTE-5 and hLTE-3b, respectively). Chromogenic assays showed decreased urea production in cell culture supernatants of AAV transduced groups compared to the non-transduced hLTEs on days 6 and 10 of culture, demonstrating decreased hepatocyte functionality. However, ALT and AST levels were similar in all groups. On day 10, hLTEs were either used for RNA isolation or fixed in 4% PFA and processed for histology. Masson's Trichrome and Alcian Blue/Sirius Red staining was performed to detect fibrosis, which was then quantified using ImageJ. These analyses showed no significant increase in fibrosis in either hLTE-5 or hLTE-3b compared to hLTE(-). Nevertheless, RT 2 PCR Array for Human Fibrosis detected dysregulation of several genes involved in fibrosis/inflammation in both hLTE-5 and hLTE-3b (16/84 and 26/84, respectively). In conclusion, data collected thus far show successful recapitulation of native liver biology and demonstrate that AAV5 transduces hLTEs more efficiently than AAV3b. However, impaired self-aggregation and decreased hepatocyte functionality was observed in both AAV-transduced groups. Studies to address the incidence and location(s) of AAV integration are ongoing. We have thus shown that the hLTE system can provide critical new knowledge regarding the efficacy and safety of AAV gene therapy in the human liver. DisclosuresNo relevant conflicts of interest to declare.

AAV Vector Dose Determines TLR9 Dependence of CD8+ T Cell Response to Transgene Product

Adeno-associated viral (AAV) vectors are currently evaluated in multiple Phase III clinical trial for the treatment of hemophilia and neuromuscular disorders. A major concern is the potential for immune responses. Viral vectors are initially sensed by the innate immune system, which shapes subsequent adaptive immune responses. Particularly, toll-like receptors (TLRs) have been reported as major sensors of pathogens during innate immune response. TLRs recognize pathogen-associated molecular patterns (PAMPs). Our previous studies found that cross-priming of AAV capsid-specific CD8+ T cells depended on TLR9-MyD88 pathway. TLR9 is an endosomal DNA receptor that responds most potently to unmethylated CpG motifs as found in bacterial and viral DNA. Similarly, others documented TLR9-dependent CD8+ T cell responses against non-secreted transgene products such as LacZ and hemagglutinin upon muscle-directed AAV gene transfer. Similarly, we published that CD8+ T cell responses to a secreted ovalbumin (ova) transgene product were substantially reduced (although not entirely eliminated) upon muscle gene transfer in TLR9-deficient mice [J Innate Immun. 7:302-14]. For those studies, we had used a self-complementary scAAV genomes, which we found to more strongly activate TLR9 than conventional single-stranded ssAAV vectors. Here, we performed intramuscular injections of 3 doses of ssAAV1-CMV-ova vector (2X1010, 2X1011 and1X1012 vg) in wild-type (WT), TLR9-/-, or MYD88-/- C57BL/6 mice. Using MHC tetramer (H2-Kb -SIINFEKL), ova-specific CD8+ T cell frequencies were monitored in peripheral blood for up to 6 weeks. As expected from prior studies, TLR9-/- mice showed a substantially reduced response (1.2% tetramer+ of CD8) at the low dose when compared to WT (12% tetramer+ of CD8) animals (p<0.0001, n=5/group). To our surprise, CD8+ T cell responses were similar in TLR9-/- and WT mice at the 2 higher doses. TLR9-/- mice displayed 16% and 3.3% tetramer+ of CD8 frequencies at the median and the high doses, respectively; which was comparable to WT mice, where 15% and 4.8% tetramer+ of CD8 frequencies were observed (n=5/group). Therefore, sensing of the AAV genome by TLR9 is more critical for the CD8+ T cell response to the secreted transgene product at lower vector doses (possibly related to the lower levels of transgene expression). Interestingly, transgene product-specific CD8+ T cell responses were much reduced in MyD88-/- mice, in which 0.2% and 1.7% tetramer+ of CD8 frequencies were found for low and median doses. Therefore, an alternative signaling pathway that includes the MyD88 adaptor molecule likely exists that is more critical than TLR9 above a certain level of expression. The reduced strength of the CD8+ T cell response seen at the highest vector dose compared to the medium dose may be explained by a transient increase in FoxP3+ Treg and in PD-1+ T cells that we observed 1 week after gene transfer and that was significantly greater at the highest vector dose. In related experiments, we performed intramuscular gene transfer using a ssAAV1-EF1a-FIX vector in hemophilia B mice (C3H/HeJ F9-/-, 1x1011 vg/mouse). Here, we used either a vector with native sequences or with an expression cassette that was entirely devoid of CpG motifs (and there stimulates TLR9 less effectively). CpG depletion did not have substantial effects on antibody formation against human FIX or the viral capsid. However, CD8+ T cell infiltrates in skeletal muscle were markedly reduced but not entirely eliminated when tissue sections were examined 1 month after gene transfer. In conclusion, TLR9 signaling is one important factor in the activation of transgene product-specific CD8+ T cells in AAV gene transfer, but other pathways exist that may be more critical depending on vector dose or levels of expression. Disclosures Herzog: Takeda Pharmaceuticals: Patents & Royalties.

Directed Evolution of AAV Serotype 5 for Increased Hepatocyte Transduction and Retained Low Humoral Seroreactivity

Most recombinant adeno-associated virus (AAV) capsids utilized in liver gene therapy have significant levels of pre-existing neutralizing antibodies in the human population. These neutralizing factors limit the patient pools eligible for receiving AAV-mediated therapies. AAV serotype 5 (AAV5) does not face the same barrier of humoral immunity as most AAV serotypes due to its low seroprevalence. However, AAV5 can only facilitate a low level of transgene expression in the liver, constraining its application to a small number of liver diseases. To improve the liver transduction of AAV5 while retaining its low seroprevalence, we constructed a library of AAV5 mutants via random mutagenesis and screened in Huh7 cells. Two molecularly evolved AAV5 variants, MV50 and MV53, demonstrated significantly increased transduction efficiency in Huh7 cells (∼12×) and primary human hepatocytes (∼10×). All variants had retained low seroreactivity toward pooled human immunoglobulin G (IgG) when compared to AAV5, which was significantly less seroreactive than AAV9. Functional characterization of the mutants also revealed insights into the functions of various domains, especially the VR-I, in the AAV5 capsid. The result is AAV5 variant capsids with much enhanced human hepatocyte transduction, potentially useful for liver-directed gene therapy.

In vitro evolution of enhanced RNA replicons for immunotherapy

Self-replicating (replicon) RNA is a promising new platform for gene therapy, but applications are still limited by short persistence of expression in most cell types and low levels of transgene expression in vivo. To address these shortcomings, we developed an in vitro evolution strategy and identified six mutations in nonstructural proteins (nsPs) of Venezuelan equine encephalitis (VEE) replicon that promoted subgenome expression in cells. Two mutations in nsP2 and nsP3 enhanced transgene expression, while three mutations in nsP3 regulated this expression. Replicons containing the most effective mutation combinations showed enhanced duration and cargo gene expression in vivo. In comparison to wildtype replicon, mutants expressing IL-2 injected into murine B16F10 melanoma showed 5.5-fold increase in intratumoral IL-2 and 2.1-fold increase in infiltrating CD8 T cells, resulting in significantly slowed tumor growth. Thus, these mutant replicons may be useful for improving RNA therapeutics for vaccination, cancer immunotherapy, and gene therapy.

Effect of promoter, promoter mutation and enhancer on transgene expression mediated by episomal vectors in transfected HEK293, Chang liver and primary cells

ABSTRACT The episomal vector cannot integrate into the host cell chromosome, which has no potential risk in gene therapy. However, the low level of transgene expression driven by episomal vectors needs to be solved. In this study, we investigated the effects of enhancers, promoters and promoter variants on transgene expression levels driven by episomal vectors in HEK293, Chang liver and primary cells. Results showed that all eight cis-acting elements used could increase transfection efficiency and transient eGFP expression in transfected HEK293 and Chang liver cells. In stably transfected mammalian cells, the elongation factor-1 alpha (EF-1α) promoter and mutant-404 showed high and stable transgene expression. The mechanisms might be related to the type and quantity of transcription factor regulatory elements. Moreover, quantitative reverse transcription polymerase chain reaction analysis showed that mRNA expression levels were not directly proportional to protein expression levels. Furthermore, the EF-1α promoter conferred high transgene expression levels in primary cells, and the plasmid was also present in the episomal state. Taken together, these results provided valuable information for improving transgene expression with episomal vectors in mammalian cells.

A chimeric HS4 insulator-scaffold attachment region enhances transgene expression in transfected Chinese hamster ovary cells.

Chinese hamster ovary (CHO) cells are one of the most commonly used expression systems for the production of recombinant proteins but low levels of transgene expression and transgene silencing are frequently encountered. Epigenetic regulatory elements such as the chicken β‐globin locus control region hypersensitive site 4 (HS4) and scaffold/matrix attachment regions (S/MARs) have positive effects on transgene expression. In this study, a chimeric HS4‐SAR was cloned upstream or downstream of an enhanced green fluorescent protein (eGFP) expression cassette in a eukaryotic vector, and the resulting vectors were transfected into CHO cells. eGFP was detected by flow cytometry. Real‐time quantitative PCR (qPCR) was used to determine copy numbers of the stably transfected cells. And fluorescence in situ hybridization (FISH) was used to detect the status of vector in the host cell chromosome. The results showed that HS4‐SAR positioned downstream of the expression cassette could enhance eGFP expression by 4.83‐fold compared with the control vector. There may not be a relationship between transgene copy number and gene expression level. HS4‐SAR did not appear to alter the integration of the transgene into the host cell chromosome or its position in the chromosome. We found a synthetic chimeric HS4‐SAR positively increased transgene expression in CHO cells.

Increased transgene expression level of rabies virus vector for transsynaptic tracing.

Viral vectors that can infect neurons transsynaptically and can strongly express foreign genes are useful for investigating the organization of neural circuits. We previously developed a propagation-competent rabies virus (RV) vector based on a highly attenuated HEP-Flury strain (rHEP5.0-CVSG), which selectively infects neurons and propagates between synaptically connected neurons in a retrograde direction. Its relatively low level of transgene expression, however, makes immunostaining necessary to visualize the morphological features of infected neurons. To increase the transgene expression level of this RV vector, in this study we focused on two viral proteins: the large protein (L) and matrix protein (M). We first attempted to enhance the expression of L, which is a viral RNA polymerase, by deleting the extra transcription unit and shortening the intergenic region between the G and L genes. This viral vector (rHEP5.0-GctL) showed increased transgene expression level with efficient transsynaptic transport. We next constructed an RV vector with a rearranged gene order (rHEP5.0-GML) with the aim to suppress the expression of M, which plays a regulatory role in virus RNA synthesis. Although this vector showed high transgene expression level, the efficiency of transsynaptic transport was low. To further evaluate the usability of rHEP5.0-GctL as a transsynaptic tracer, we inserted a fluorescent timer as a transgene, which changes the color of its fluorescence from blue to red over time. This viral vector enabled us the differentiation of primary infected neurons from secondary infected neurons in terms of the fluorescence wavelength. We expect this propagation-competent RV vector to be useful for elucidating the complex organization of the central nervous system.

Robust Lentiviral Gene Delivery But Limited Transduction Capacity of Commonly Used Adeno-Associated Viral Serotypes in Xenotransplanted Human Skin.

Skin is an easily accessible organ, and therapeutic gene transfer to skin remains an attractive alternative for the treatment of skin diseases. Although we have previously documented potent lentiviral gene delivery to human skin, vectors based on adeno-associated virus (AAV) rank among the most promising gene delivery tools for in vivo purposes. Thus, we compared the potential usefulness of various serotypes of recombinant AAV vectors and lentiviral vectors for gene transfer to human skin in a xenotransplanted mouse model. Vector constructs encoding firefly luciferase were packaged in AAV capsids of serotype 1, 2, 5, 6, 8, and 9 and separately administered by intradermal injection in human skin transplants. For all serotypes, live bioimaging demonstrated low levels of transgene expression in the human skin graft, and firefly luciferase expression was observed primarily in neighboring tissue outside of the graft. In contrast, gene delivery by intradermally injected lentiviral vectors was efficient and led to extensive and persistent firefly luciferase expression within the human skin graft only. The study demonstrates the limited capacity of single-stranded AAV vectors of six commonly used serotypes for gene delivery to human skin in vivo.

308. Inclusion of Heterologous ITRs in Dual AAV Vectors for Retinal Gene Therapy

AAV cargo capacity limited to about 5 kb prevents their applications from treatment of inherited blinding conditions which require the transfer of genes with larger coding sequences. However, dual AAV vectors, each containing one of the two halves of a large gene expression cassette, are emerging as promising tools to overcome this limitation. However, dual AAV show lower levels of transgene expression than a single AAV vector and are associated with production of proteins shorter than expected from either the 5’- or 3’-half AAV These limitations can be in part overcome by improving the productive tail-to-head AAV genome concatemerization by including heterologous inverted terminal repeats (ITRs) at both ends of each dual AAV vector genome.We have generated dual AAV hybrid vectors with heterologous ITRs from AAV2 (ITR2) and AAV5 (ITR5) which are the most divergent among the various AAV serotypes. However, we found a significant reduction in the yields of AAV vectors with heterologous ITR2 and 5 when compared to those of vectors with homologous ITR2. In addition, the genome titer performed on ITR2 sequences results lower than that on a different part of the genome suggesting that ITRs might not be intact in AAV vectors with heterologous ITRs. Finally, the amount of full length protein relative to shorter was similar between vector with homologous and heterologous ITRs. We are currently testing dual AAV hybrid vectors with ITRs derived from other serotypes divergent from AAV2 which could overcome some of the limitations we have found using ITR5.

309. Optimization of Dual AAV Vectors for Gene Therapy of Inherited Retinal Diseases

Retinal gene therapy with adeno-associated viral (AAV) vectors is safe and effective in humans. However, AAV limited cargo capacity prevents its application to therapies of inherited retinal diseases (IRD) due to mutations in genes over 5 kb. Dual AAV vectors, each containing one of the two halves of a large gene expression cassette, are emerging as promising tools to overcome this limitation. Dual AAV trans-splicing and hybrid vectors transduce efficiently the mouse and pig retina and are effective in animal models of IRD. However, some of dual AAV limitations include lower levels of transgene expression compared to a single AAV vector and the production of proteins shorter than expected from either the 5’- or 3’-half AAV. Thus, further development of dual AAV vectors is required before their clinical translation. To increase dual AAV recombination we have exploited various regions of homology while to mediate the degradation of the proteins shorter than expected we have tested the ability of various degradation signals. We found that the levels of transgene expression achieved with the alternative regions of homology are similar to those achieved with dual AAV vectors carrying the AK region of homology we have previously shown to be effective. Notably, we have identified a degradation signal which mediates the degradation of proteins shorter than expected from dual AAV vectors. In conclusion, our study outlines optimized features of dual AAV vectors that improve their safety and efficacy. This represents a step towards the clinical translation of dual AAV for retinal gene therapy.

Survivin promoter-regulated oncolytic adenovirus with Hsp70 gene exerts effective antitumor efficacy in gastric cancer immunotherapy

Gene therapy is a promising adjuvant therapeutic strategy for cancer treatment. To overcome the limitations of current gene therapy, such as poor transfection efficiency of vectors, low levels of transgene expression and lack of tumor targeting, the Survivin promoter was used to regulate the selective replication of oncolytic adenovirus in tumor cells, and the heat shock protein 70 (Hsp70) gene was loaded as the anticancer transgene to generate an AdSurp-Hsp70 viral therapy system. The efficacy of this targeted immunotherapy was examined in gastric cancer. The experiments showed that the oncolytic adenovirus can selectively replicate in and lyse the Survivin-positive gastric cancer cells, without significant toxicity to normal cells. AdSurp-Hsp70 reduced viability of cancer cells and inhibited tumor growth of gastric cancer xenografts in immuno-deficient and immuno-reconstruction mouse models. AdSurp-Hsp70 produced dual antitumor effects due to viral replication and high Hsp70 expression. This therapeutic system used the Survivin promoter-regulated oncolytic adenovirus vector to mediate targeted expression of the Hsp70 gene and ensure safety and efficacy for subsequent gene therapy programs against a variety of cancers.

Modulating polymer chemistry to enhance non-viral gene delivery inside hydrogels with tunable matrix stiffness

Non-viral gene delivery holds great promise for promoting tissue regeneration, and offers a potentially safer alternative than viral vectors. Great progress has been made to develop biodegradable polymeric vectors for non-viral gene delivery in 2D culture, which generally involves isolating and modifying cells in vitro, followed by subsequent transplantation in vivo. Scaffold-mediated gene delivery may eliminate the need for the multiple-step process in vitro, and allows sustained release of nucleic acids in situ. Hydrogels are widely used tissue engineering scaffolds given their tissue-like water content, injectability and tunable biochemical and biophysical properties. However, previous attempts on developing hydrogel-mediated non-viral gene delivery have generally resulted in low levels of transgene expression inside 3D hydrogels, and increasing hydrogel stiffness further decreased such transfection efficiency. Here we report the development of biodegradable polymeric vectors that led to efficient gene delivery inside poly(ethylene glycol) (PEG)-based hydrogels with tunable matrix stiffness. Photocrosslinkable gelatin was maintained constant in the hydrogel network to allow cell adhesion. We identified a lead biodegradable polymeric vector, E6, which resulted in increased polyplex stability, DNA protection and achieved sustained high levels of transgene expression inside 3D PEG-DMA hydrogels for at least 12 days. Furthermore, we demonstrated that E6-based polyplexes allowed efficient gene delivery inside hydrogels with tunable stiffness ranging from 2 to 175 kPa, with the peak transfection efficiency observed in hydrogels with intermediate stiffness (28 kPa). The reported hydrogel-mediated gene delivery platform using biodegradable polyplexes may serve as a local depot for sustained transgene expression in situ to enhance tissue engineering across broad tissue types.

Overexpression of the ABA-Dependent AREB1 Transcription Factor from Arabidopsis thaliana Improves Soybean Tolerance to Water Deficit

Among current strategies for the development of drought-tolerant plants, engineering transcription factors that regulate the expression of genes related to abiotic stress is promising. Soybean plants overexpressing the transcription factor AtAREB1, which is involved in abscisic acid (ABA)-dependent stress responses, were generated using biolistics. Embryos of cultivar BR16, which is considered drought sensitive, were co-transformed with two plant expression vectors, 35S::AtAREB1 and 35S::ahas. Among the lines obtained, the drought tolerance of A24.10 and A2889.12 was assessed because these lines overexpressed the AtAREB1 gene and had a low copy number of the transgene. Another line, A2057.03, was also selected because it had a high copy number of the transgene (more than 100 copies) and low levels of transgene expression. Among the two low-copy lines, the A24.10 plants exhibited a slightly lower number of leaves and a shortening of the internode length; however, no growth retardation was observed for the line A2057.03. Water stress tolerance was particularly improved in the low-copy lines A24.10 and A2889.12. Plants of these lines were able to survive a water stress period of 5 days and exhibited no leaf damage, i.e., dried areas in the leaves 3 days after rewatering. Furthermore, these lines exhibited better growth and physiological performance under water-deficit (higher relative rate of shoot length, stomatal conductance, and photosynthesis) when compared to the wild type. Our results show the potential for improving drought tolerance by overexpressing AtAREB1 in soybean.

Expression of erythropoietin in Indian tetraploid potato variety

With the advent of protein-based biotech drugs in the market, the quest for the “perfect” protein expression system, which is both economical and effective, has come into focus. Currently bacteria, yeast, insect cells, mammalian cells, transgenic animal and transgenic plants are widely used for the expression of therapeutic proteins. Among these, transgenic plants provide advantages in terms of low production cost, lower capital investment in infrastructure, and suitable post-translational modifications. The major limitation of plants as an expression host is the low level of transgene expression. To increase the expression of heterologous proteins in plants, a number of approaches have been used. One of the approaches is to increase the transgene expression by using tissue-specific promoter(s) which can concentrate the protein of interest in targeted tissues and, thus, prove advantageous in downstream purification. In the present report, a protocol for expression of heterologous protein erythropoietin in potato tuber using patatin, the tuber-tissue-specific promoter, was standardized. Expression vectors for production of the erythropoietin gene under tissue-specific promoter were successfully constructed. For production of a transgenic plant, tissue culture techniques for regeneration of the whole plant from single explants were standardized. Polymerase chain reaction (PCR) analysis was performed to confirm the stable integration of the erythropoietin gene in the potato plant by using sequence-specific primers.

Expression of erythropoietin in Indian tetraploid potato variety

With the advent of protein-based biotech drugs in the market, the quest for the “perfect” protein expression system, which is both economical and effective, has come into focus. Currently bacteria, yeast, insect cells, mammalian cells, transgenic animal and transgenic plants are widely used for the expression of therapeutic proteins. Among these, transgenic plants provide advantages in terms of low production cost, lower capital investment in infrastructure, and suitable post-translational modifications. The major limitation of plants as an expression host is the low level of transgene expression. To increase the expression of heterologous proteins in plants, a number of approaches have been used. One of the approaches is to increase the transgene expression by using tissue-specific promoter(s) which can concentrate the protein of interest in targeted tissues and, thus, prove advantageous in downstream purification. In the present report, a protocol for expression of heterologous protein erythropoietin in potato tuber using patatin, the tuber-tissue-specific promoter, was standardized. Expression vectors for production of the erythropoietin gene under tissue-specific promoter were successfully constructed. For production of a transgenic plant, tissue culture techniques for regeneration of the whole plant from single explants were standardized. Polymerase chain reaction (PCR) analysis was performed to confirm the stable integration of the erythropoietin gene in the potato plant by using sequence-specific primers.

Position effect variegation and epigenetic modification of a transgene in a pig model

Sequences proximal to transgene integration sites are able to regulate transgene expression, resulting in complex position effect variegation. Position effect variegation can cause differences in epigenetic modifications, such as DNA methylation and histone acetylation. However, it is not known which factor, position effect or epigenetic modification, plays a more important role in the regulation of transgene expression. We analyzed transgene expression patterns and epigenetic modifications of transgenic pigs expressing green fluorescent protein, driven by the cytomegalovirus (CMV) promoter. DNA hypermethylation and loss of acetylation of specific histone H3 and H4 lysines, except H4K16 acetylation in the CMV promoter, were consistent with a low level of transgene expression. Moreover, the degree of DNA methylation and histone H3/H4 acetylation in the promoter region depended on the integration site; consequently, position effect variegation caused variations in epigenetic modifications. The transgenic pig fibroblast cell lines were treated with DNA methyltransferase inhibitor 5-Aza-2'-deoxycytidine and/or histone deacetylase inhibitor trichostatin A. Transgene expression was promoted by reversing the DNA hypermethylation and histone hypoacetylation status. The differences in DNA methylation and histone acetylation in the CMV promoter region in these cell lines were not significant; however, significant differences in transgene expression were detected, demonstrating that variegation of transgene expression is affected by the integration site. We conclude that in this pig model, position effect variegation affects transgene expression.

Physiological and histological changes in skeletal muscle following in vivo gene transfer by electroporation

Electroporation (EP) is used to transfect skeletal muscle fibers in vivo, but its effects on the structure and function of skeletal muscle tissue have not yet been documented in detail. Methods We studied the changes in contractile function and histology after EP to determine the mechanism of recovery, the extent of myofiber damage, and the efficiency of expression of a green fluorescent protein (GFP) transgene in the tibialis anterior (TA) muscle of adult male C57Bl/6J mice. Results Immediately after EP, contractile torque decreased by ~ 80% from pre-EP levels. Within 3 hr, torque recovered to ~ 50% but stayed low until day 3. Functional recovery progressed slowly and was complete at day 28. In muscles depleted of satellite cells by X-irradiation, torque remained low after day 3, suggesting that myogenesis is necessary for complete recovery. In unirradiated muscle, myogenic activity after EP was confirmed by an increase in fibers with central nuclei or developmental myosin. Damage after EP was confirmed by the presence of necrotic myofibers infiltrated by CD68+ macrophages, which persisted in electroporated muscle for 42 days. Expression of GFP was detected at day 3 after EP and peaked on day 7, with ~ 25% of fibers transfected. The number of fibers expressing GFP, the distribution of GFP+ fibers, and the intensity of fluorescence in GFP+ fibers were highly variable. We conclude that EP of murine TA muscles produces highly variable and low levels of transgene expression, and causes extensive damage that requires myogenesis for recovery.

Intron‐mediated enhancement as a method for increasing transgene expression levels in barley

It is desirable to produce transgenic plants which have optimized and stable levels of transgene expression. Low levels of transgene expression may lead to an insufficient quantity of transgenic protein being produced for a particular purpose. This report demonstrates a means of enhancing transgene expression in barley beyond that conferred by the Ubi1 promoter, via the inclusion of an intron at a specific position within the transgene coding sequence. We independently cloned two different introns (RpoT-i4 from maize and UBQ10-i1 from Arabidopsis) into the same position within the firefly luciferase (luc) coding sequence. The constructs produced were transformed into barley (Hordeum vulgare) via Agrobacterium-mediated transformation, and the resulting transformant populations (of between 119 and 123 independent plants for each construct) were assayed for luciferase activity. Both introns significantly increased luciferase activity, and a quantitative reverse-transcription polymerase chain reaction assay revealed that the introns increased the accumulation of luciferase mRNA transcripts. The enhanced transgene expression levels were maintained in the T(1) and T(2) progenies. These findings show that intron-mediated enhancement is a valuable additional tool for achieving high and stable levels of transgene expression in crop plants.

Development and Validation of an Improved Inducer–Regulator Protein Complex in the pBRES-Regulated Expression System

Widespread adaptation of small molecule-regulated expression systems requires the development of selective inducer molecules that do not have any significant side effects on the endogenous receptors from which the regulated expression system is derived. Here we report the identification and in vitro validation of a novel inducer-receptor pair for the single-plasmid regulated expression system termed pBRES, which contains the ligand-binding domain from the human progesterone receptor (hPR). A small molecule inducer, BLX-913, has been identified as having a 30-fold lower IC(50) for the human progesterone receptor than mifepristone (MFP), the previously best characterized inducer for pBRES. Using modeling-guided protein engineering, compensatory mutations were installed at positions W755 and V729 (hPR numbering) in the ligand-binding pocket of the pBRES regulator protein (pBRES RP) to accommodate the new inducer and allow induction of transgene expression to levels previously seen with MFP. The improved inducer-pBRES RP complex was validated in vitro by monitoring the induction of luciferase, murine secreted alkaline phosphatase, and human interferon beta transgenes in mouse skeletal muscle cells. The engineered pBRES demonstrated low levels of transgene expression in the absence, and high expression levels in the presence, of the new BLX-913 inducer. Findings presented here allow induction of the pBRES-regulated gene expression system by a compound with markedly lower anti-hPR activity than MFP, the previously best characterized inducer.

Quantitative analysis of retroviral and lentiviral gene transfer to murine embryonic stem cells

The effectiveness of retrovirus or lentivirus transduction of embryonic stem (ES) cells is often limited because transgene expression is silenced or variegated. We wondered if other steps of transduction, in addition to gene expression, were restricted in ES cells. We quantitatively compared (1) the amount of virus binding, (2) the number of integrated transgenes, and (3) the resulting level of gene expression. We found that three- to fourfold fewer retroviruses and lentiviruses bound to R1 mES cells than to NIH 3T3 cells, suggesting that both types of viruses bind less efficiently to mES cells. Retroviruses and lentiviruses differed in the efficiency with which they completed post-binding steps of transduction. In R1 mES cells, we detected 3-fold fewer integrated retrovirus transgenes and 11-fold lower expression levels than in NIH 3T3 cells, which suggests that the primary limitation to retrovirus transduction may be low levels of transgene expression. In contrast, we detected 10-fold fewer integrated lentivirus transgenes and 8-fold lower expression levels in R1 mES cells than in NIH 3T3 cells, which suggests that lentivirus transduction may be limited by inefficient intracellular post-binding steps of transduction. The implications of our findings for developing improved viral vectors for transducing mES cells are discussed.