Stable and unstable transgene integration sites in the human genome: extinction of the Green Fluorescent Protein transgene in K562 cells

Engineering bispecific T-cell engagers to deplete eosinophils for the treatment of severe eosinophilic asthma

Severe eosinophilic asthma (SEA) is characterized by elevated eosinophil counts in the blood and airway mucosa. While monoclonal antibody therapies targeting interleukin-5 (IL-5) and its receptor (IL-5Rα) have improved treatment, some patients remain unresponsive. We propose an alternative approach to eliminate eosinophils using T cells by engineering IL-5Rα × CD3 bispecific T-cell engagers (bsTCEs) that target both IL-5Rα on eosinophils and CD3 on T cells. We designed different formats of IL-5Rα × CD3 bsTCEs, incorporating variations in valency, geometry, and affinity for the target antigen binding. We identified the single-chain variable fragment (scFv)-Fc format with the highest affinity toward the membrane-proximal domain of IL-5Rα in the IL-5Rα-binding arm showed the most potent cytotoxicity against IL-5Rα-expressing peripheral eosinophils by activating autologous primary T cells from healthy donors. This study proposes IL-5Rα × CD3 bsTCEs as potential alternatives for SEA treatment. Importantly, it demonstrates the first application of bsTCEs in eliminating disease-associated cells, including eosinophils, beyond cancer cells.

A Gene Transfer-Positive Cell Sorting System Utilizing Membrane-Anchoring Affinity Tag

Gene delivery efficiency is an essential limit factor in gene study and gene therapy, especially for cells that are hard for gene transfer. Here we develop an affinity cell sorting system that allows efficient enrichment of gene transfer-positive cells. The system expresses an enhanced green fluorescent protein (EGFP) fused with an N-terminal high-affinity Twin-Strep-Tag (TST) that will be anchored to the cell membrane at the out-surface through a glycosylphosphatidylinositol (GPI) membrane-anchoring structure. The EGFP permits microscopy and flow cytometry analysis of the gene transfer-positive cells, and the TST tag at the N terminal of EGFP allows efficient affinity sorting of the positive cells using Strep-Tactin magnetic beads. The cell sorting system enables efficient isolation of gene transfer-positive cells in a simple, convenient, and fast manner. Cell sorting on transfected K-562 cells resulted in a final positive cell percentage of up to 95.0% with a positive cell enrichment fold of 5.8 times. The applications in gene overexpression experiments could dramatically increase the gene overexpression fold from 10 times to 58 times, and in shRNA gene knockdown experiments, cell sorting increased the gene knockdown efficiency from 12% to 53%. In addition, cell sorting in CRISPR/Cas9 genome editing experiments allowed more significant gene modification, with an editing percentage increasing from 20% to 79%. The gene transfer-positive cell sorting system holds great potential for all gene transfer studies, especially on those hard-to-transfect cells.

Identification of Loci Enabling Stable and High-Level Heterologous Gene Expression

Efficient and reliable genome engineering technologies have yet to be developed for diatoms. The delivery of DNA in diatoms results in the random integration of multiple copies, quite often leading to heterogeneous gene activity, as well as host instability. Transgenic diatoms are generally selected on the basis of transgene expression or high enzyme activity, without consideration of the copy number or the integration locus. Here, we propose an integrated pipeline for the diatom, Phaeodactylum tricornutum, that accurately quantifies transgene activity using a β-glucuronidase assay and the number of transgene copies integrated into the genome through Droplet Digital PCR (ddPCR). An exhaustive and systematic analysis performed on 93 strains indicated that 42% of them exhibited high β-glucuronidase activity. Though most were attributed to high transgene copy numbers, we succeeded in isolating single-copy clones, as well as sequencing the integration loci. In addition to demonstrating the impact of the genomic integration site on gene activity, this study identifies integration sites for stable transgene expression in Phaeodactylum tricornutum.

Tissue-specific promoter-based reporter system for monitoring cell differentiation from iPSCs to cardiomyocytes

The possibility of using stem cell-derived cardiomyocytes opens a new platform for modeling cardiac cell differentiation and disease or the development of new drugs. Progress in this field can be accelerated by high-throughput screening (HTS) technology combined with promoter reporter system. The goal of the study was to create and evaluate a responsive promoter reporter system that allows monitoring of iPSC differentiation towards cardiomyocytes. The lentiviral promoter reporter system was based on troponin 2 (TNNT2) and alpha cardiac actin (ACTC) with firefly luciferase and mCherry, respectively. The system was evaluated in two in vitro models. First, system followed the differentiation of TNNT2-luc-T2A-Puro-mCMV-GFP and hACTC-mcherry-WPRE-EF1-Neo from transduced iPSC line towards cardiomyocytes and revealed the significant decrease in both inserts copy number during the prolonged in vitro cell culture (confirmed by I-FISH, ddPCR, qPCR). Second, differentiated and contracting control cardiomyocytes (obtained from control non-reporter transduced iPSCs) were subsequently transduced with TNNT2-luc-T2A-Puro-CMV-GFP and hACTC-mcherry-WPRE-EF1-Neo lentiviruses to observe the functionality of obtained cardiomyocytes. Our results indicated that the reporter modified cell lines can be used for HTS applications, but it is essential to monitor the stability of the reporter sequence during extended cell in vitro culture.

Establishment and evaluation of a PRRSV-sensitive porcine endometrial epithelial cell line by transfecting SV40 large T antigen

Background

PRRSV is an infectious illness causing lung injury and abortion in sows. Cells apoptosis in the interface between the endometrium and fetal placenta is a crucial factor causing abortion. Previous study confirmed PRRSV could cause apoptosis of macrophages but rarely produced an obvious change in porcine endometrial epithelial cells (PECs). Recently, PRRSV-induced abortion was attributed to fetal placental and endometrium epithelial cells (Sn⁺ and CD163⁺) apoptosis. However, the mechanism of abortion is still unrevealed because of the limit of porcine endometrium epithelial cells (PEC). The aim of this study was to establish a stable immortalized PECs lines and use it to reveal the abortion mechanism.

Results

In this study, highly purified primary PECs were harvested through differential digestion, and their characteristics were confirmed by CK18, ERɑ and PR staining. Cells were then immortalized by transfecting a lentiviral vector that expressed SV40 large T antigen. PECs lines were obtained after puromycin screening. Proliferation of cell line was evaluated by cell growth curve and cell cycle assays. Cell lines exhibited faster proliferation capacity than primary cells. Biological characteristics of cell line were assessed by Western blot, karyotype analysis and staining, which confirmed that the cell line retained the endometrium characteristics. Finally, PRRSV sensitivity was assessed; expression of Sn and CD163 indicated that primary PECs and cell lines were all potentially sensitive to PRRSV. PRRSV infection tests showed an obvious increase in apoptotic rate in the infected PEC cell line, which suggested its susceptibility.

Conclusion

The newly constructed cell line is a useful tool for studying the mechanism of abortion caused by PRRSV.

Revealing Key Determinants of Clonal Variation in Transgene Expression in Recombinant CHO Cells Using Targeted Genome Editing

Generation of recombinant Chinese hamster ovary (rCHO) cell lines is critical for the production of therapeutic proteins. However, the high degree of phenotypic heterogeneity among generated clones, referred to as clonal variation, makes the rCHO cell line development process inefficient and unpredictable. Here, we investigated the major genomic causes of clonal variation. We found the following: (1) consistent with previous studies, a strong variation in rCHO clones in response to hypothermia (33 vs 37 °C) after random transgene integration; (2) altered DNA sequence of randomly integrated cassettes, which occurred during the integration process, affecting the transgene expression level in response to hypothermia; (3) contrary to random integration, targeted integration of the same expression cassette, without any DNA alteration, into three identified integration sites showed the similar response of transgene expression in response to hypothermia, irrespective of integration site; (4) switching the promoter from CMV to EF1α eliminated the hypothermia response; and (5) deleting the enhancer part of the CMV promoter altered the hypothermia response. Thus, we have revealed the effects of integration methods and cassette design on transgene expression levels, implying that rCHO cell line generation can be standardized through detailed genomic understanding. Further elucidation of such understanding is likely to have a broad impact on diverse fields that use transgene integration, from gene therapy to generation of production cell lines.

Optogenetic Peripheral Nerve Immunogenicity

Optogenetic technologies have been the subject of great excitement within the scientific community for their ability to demystify complex neurophysiological pathways in the central (CNS) and peripheral nervous systems (PNS). The excitement surrounding optogenetics has also extended to the clinic with a trial for ChR2 in the treatment of retinitis pigmentosa currently underway and additional trials anticipated for the near future. In this work, we identify the cause of loss-of-expression in response to transdermal illumination of an optogenetically active peroneal nerve following an anterior compartment (AC) injection of AAV6-hSyn-ChR2(H134R) with and without a fluorescent reporter. Using Sprague Dawley Rag2-/- rats and appropriate controls, we discover optogenetic loss-of-expression is chiefly elicited by ChR2-mediated immunogenicity in the spinal cord, resulting in both CNS motor neuron death and ipsilateral muscle atrophy in both low and high Adeno-Associated Virus (AAV) dosages. We further employ pharmacological immunosuppression using a slow-release tacrolimus pellet to demonstrate sustained transdermal optogenetic expression up to 12 weeks. These results suggest that all dosages of AAV-mediated optogenetic expression within the PNS may be unsafe. Clinical optogenetics for both PNS and CNS applications should take extreme caution when employing opsins to treat disease and may require concurrent immunosuppression. Future work in optogenetics should focus on designing opsins with lesser immunogenicity.

A surrogate reporter system for multiplexable evaluation of CRISPR/Cas9 in targeted mutagenesis

Engineered nucleases in genome editing manifest diverse efficiencies at different targeted loci. There is therefore a constant need to evaluate the mutation rates at given loci. T7 endonuclease 1 (T7E1) and Surveyor mismatch cleavage assays are the most widely used methods, but they are labour and time consuming, especially when one must address multiple samples in parallel. Here, we report a surrogate system, called UDAR (Universal Donor As Reporter), to evaluate the efficiency of CRISPR/Cas9 in targeted mutagenesis. Based on the non-homologous end-joining (NHEJ)-mediated knock-in strategy, the UDAR-based assay allows us to rapidly evaluate the targeting efficiencies of sgRNAs. With one-step transfection and fluorescence-activated cell sorting (FACS) analysis, the UDAR assay can be completed on a large scale within three days. For detecting mutations generated by the CRISPR/Cas9 system, a significant positive correlation was observed between the results from the UDAR and T7E1 assays. Consistently, the UDAR assay could quantitatively assess bleomycin- or ICRF193-induced double-strand breaks (DSBs), which suggests that this novel strategy is broadly applicable to assessing the DSB-inducing capability of various agents. With the increasing impact of genome editing in biomedical studies, the UDAR method can significantly benefit the evaluation of targeted mutagenesis, especially for high-throughput purposes.

Effects of histone acetylation status on the early development of in vitro porcine transgenic cloned embryos

The purpose of this study was to investigate the effects of the histone deacetylase (HDAC) inhibitor trichostatin A (TSA) on transgene expression and development of porcine transgenic cloned embryos, specifically focusing on effects derived from TSA-treated donor cells or TSA-treated reconstructed embryos. The results showed that TSA treatment on reconstructed embryos modified the acetylation status, which significantly improved the development of porcine somatic cell nuclear transfer (SCNT) embryos in vitro, but not donor cells. Furthermore, the treatment of reconstructed embryos with TSA enhanced expression of the pluripotency-related gene POU5F1 and stimulated expression of the anti-apoptotic gene BCL-2. Enhanced green fluorescent protein (EGFP) mRNA expression of every group dropped drastically from donor cells to blastocysts. Interestingly, TSA is likely to prevent a decline in EGFP expression in nuclear reprogramming of porcine SCNT embryos. However DNA hypomethylation induced by modified histone acetylation of donor cells treated with TSA was significantly more effective in increasing EGFP expression in SCNT blastocysts. In conclusion, the acetylation status of both donor cells and reconstructed embryos modified by TSA treatment increased transgene expression and improved nuclear reprogramming and the developmental potential of porcine transgenic SCNT embryos.

The copy number and integration site analysis of IGF-1 transgenic goat

Transgenic animals have been used previously to study gene function, produce important proteins, and generate models for the study of human diseases. As the number of transgenic species increases, reliable detection and molecular characterization of integration sites and copy number are crucial for confirming transgene expression and genetic stability, as well as for safety evaluation and to meet commercial demands. In this study, we generated four transgenic goats by somatic cell nuclear transfer (SCNT). After birth, the cloned goat contained transferred insulin-like growth factor I (IGF-1) gene was initially confirmed using a polymerase chain reaction (PCR)‑based method. The four cloned goats were identified as IGF-1 transgenic goats by southern blotting. The number of copies of the IGF-1 gene in each of the transgenic goats was determined. Additionally, four integration sites of the transgene in the transgenic goats with a modified thermal asymmetric interlaced (TAIL)-PCR method were identified. The four different integration sites were located on chromosomes 2, 11, 16 and 18. The present study identified the copy number and integration sites using quantitative PCR (qPCR) and TAIL-PCR, enabling the bio-safety evaluation of the transgenic goats.

Accelerated repair and reduced mutagenicity of DNA damage induced by cigarette smoke in human bronchial cells transfected with E.coli formamidopyrimidine DNA glycosylase

Cigarette smoke (CS) is associated to a number of pathologies including lung cancer. Its mutagenic and carcinogenic effects are partially linked to the presence of reactive oxygen species and polycyclic aromatic hydrocarbons (PAH) inducing DNA damage. The bacterial DNA repair enzyme formamidopyrimidine DNA glycosylase (FPG) repairs both oxidized bases and different types of bulky DNA adducts. We investigated in vitro whether FPG expression may enhance DNA repair of CS-damaged DNA and counteract the mutagenic effects of CS in human lung cells. NCI-H727 non small cell lung carcinoma cells were transfected with a plasmid vector expressing FPG fused to the Enhanced Green Fluorescent Protein (EGFP). Cells expressing the fusion protein EGFP-FPG displayed accelerated repair of adducts and DNA breaks induced by CS condensate. The mutant frequencies induced by low concentrations of CS condensate to the Na⁺K⁺-ATPase locus (oua^r) were significantly reduced in cells expressing EGFP-FPG. Hence, expression of the bacterial DNA repair protein FPG stably protects human lung cells from the mutagenic effects of CS by improving cells’ capacity to repair damaged DNA.

piggyBac transposon plus insulators overcome epigenetic silencing to provide for stable signaling pathway reporter cell lines

Genetically modified hematopoietic progenitors represent an important testing platform for a variety of cell-based therapies, pharmaceuticals, diagnostics and other applications. Stable expression of a transfected gene of interest in the cells is often obstructed by its silencing. DNA transposons offer an attractive non-viral alternative of transgene integration into the host genome, but their broad applicability to leukocytes and other "transgene unfriendly" cells has not been fully demonstrated. Here we assess stability of piggyBac transposon-based reporter expression in murine prostate adenocarcinoma TRAMP-C2, human monocyte THP-1 and erythroleukemia K562 cell lines, along with macrophages and dendritic cells (DCs) that have differentiated from the THP-1 transfects. The most efficient and stable reporter activity was observed for combinations of the transposon inverted terminal repeats and one 5'- or two cHS4 core insulators flanking a green fluorescent protein reporter construct, with no detectable silencing over 10 months of continuous cell culture in absence of any selective pressure. In monocytic THP-1 cells, the functional activity of luciferase reporters for NF-κB, Nrf2, or HIF-1α has not decreased over time and was retained following differentiation into macrophages and DCs, as well. These results imply pB as a versatile tool for gene integration in monocytic cells in general, and as a convenient access route to DC-based signaling pathway reporters suitable for high-throughput assays, in particular.

Primary over-expression of AβPP in muscle does not lead to the development of inclusion body myositis in a new lineage of the MCK-AβPP transgenic mouse

The aim of this study is to determine whether primary over-expression of AβPP in skeletal muscle results in the development of features of inclusion body myositis (IBM) in a new lineage of the MCK-AβPP transgenic mouse. Quantitative histological, immunohistochemical and western blotting studies were performed on muscles from 3 to 18 month old transgenic and wild-type C57BL6/SJL mice. Electron microscopy was also performed on muscle sections from selected animals. Although western blotting confirmed that there was over-expression of full length AβPP in transgenic mouse muscles, deposition of amyloid-β and fibrillar amyloid could not be demonstrated histochemically or with electron microscopy. Additionally, other changes typical of IBM such as rimmed vacuoles, cytochrome C oxidase-deficient fibres, upregulation of MHC antigens, lymphocytic inflammatory infiltration and T cell fibre invasion were absent. The most prominent finding in both transgenic and wild-type animals was the presence of tubular aggregates which was age-related and largely restricted to male animals. Expression of full length AβPP in this MCK-AβPP mouse lineage did not reach the levels required for immunodetection or deposition of amyloid-β as in the original transgenic strains, and was not associated with the development of pathological features of IBM. These negative results emphasise the potential pitfalls of re-deriving transgenic mouse strains in different laboratories.

Generation of transgenic Wuzhishan miniature pigs expressing monomeric red fluorescent protein by somatic cell nuclear transfer

Red fluorescent protein and its variants enable researchers to study gene expression, localization, and protein-protein interactions in vitro in real-time. Fluorophores with higher wavelengths are usually preferred since they efficiently penetrate tissues and produce less toxic emissions. A recently developed fluorescent protein marker, monomeric red fluorescent protein (mRFP1), is particularly useful because of its rapid maturation and minimal interference with green fluorescent protein (GFP) and GFP-derived markers. We generated a pCX-mRFP1-pgk-neoR construct and evaluated the ability of mRFP1 to function as a fluorescent marker in transgenic Wuzhishan miniature pigs. Transgenic embryos were generated by somatic cell nuclear transfer (SCNT) of nuclei isolated from ear fibroblasts expressing mRFP1. Embryos generated by SCNT developed into blastocysts in vitro (11.65%; 31/266). Thereafter, a total of 685 transgenic embryos were transferred into the oviducts of three recipients, two of which became pregnant. Of these, one recipient had six aborted fetuses, whereas the other recipient gave birth to four offspring. All offspring expressed the pCX-mRFP1-pgk-neoR gene as shown by PCR and fluorescence in situ hybridization analysis. The transgenic pigs expressed mRFP1 in all organs and tissues at high levels. These results demonstrate that Wuzhishan miniature pigs can express mRFP1. To conclude, this transgenic animal represents an excellent model with widespread applications in medicine and agriculture.

Synthetic zinc finger repressors reduce mutant huntingtin expression in the brain of R6/2 mice

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder caused by expanded CAG repeats in the huntingtin (HTT) gene. Although several palliative treatments are available, there is currently no cure and patients generally die 10-15 y after diagnosis. Several promising approaches for HD therapy are currently in development, including RNAi and antisense analogs. We developed a complementary strategy to test repression of mutant HTT with zinc finger proteins (ZFPs) in an HD model. We tested a "molecular tape measure" approach, using long artificial ZFP chains, designed to bind longer CAG repeats more strongly than shorter repeats. After optimization, stable ZFP expression in a model HD cell line reduced chromosomal expression of the mutant gene at both the protein and mRNA levels (95% and 78% reduction, respectively). This was achieved chromosomally in the context of endogenous mouse HTT genes, with variable CAG-repeat lengths. Shorter wild-type alleles, other genomic CAG-repeat genes, and neighboring genes were unaffected. In vivo, striatal adeno-associated virus viral delivery in R6/2 mice was efficient and revealed dose-dependent repression of mutant HTT in the brain (up to 60%). Furthermore, zinc finger repression was tested at several levels, resulting in protein aggregate reduction, reduced decline in rotarod performance, and alleviation of clasping in R6/2 mice, establishing a proof-of-principle for synthetic transcription factor repressors in the brain.

Reactivation of Silenced WT1 Transgene by Hypomethylating Agents - Implications for in vitro Modeling of Chemoimmunotherapy

BACKGROUND

A cell line with transfected Wilms' tumor protein 1 (WT1) is has been used for the preclinical evaluation of novel treatment strategies of WT1 immunotherapy for leukemia due to the lack of appropriate murine leukemia cell line with endogenous WT1. However, silencing of the transgene occurs. Regarding the effects of hypomethylating agents (HMAs) on reactivation of silenced genes, HMAs are considered to be immune enhancers.

METHODS

We treated murine WT1- transfected C1498 (mWT1-C1498) with increasing doses of decitabine (DAC) and azacitidine (AZA) to analyze their effects on transgene reactivation.

RESULTS

DAC and AZA decreased the number of viable cells in a dose- or time-dependent manner. Quantification of WT1 mRNA level was analyzed by real-time polymerase chain reaction after mWT1-C1498 treated with increasing dose of HMA. DAC treatment for 48 h induced 1.4-, 14.6-, and 15.5-fold increment of WT1 mRNA level, compared to untreated sample, at 0.1, 1, and 10µM, respectively. Further increment of WT1 expression in the presence of 1 and 10µM DAC was evident at 72 h. AZA treatment also induced up-regulation of mRNA, but not to the same degree as with DAC treatment. The correlation between the incremental increases in WT1 mRNA by DAC was confirmed by Western blot and concomitant down-regulation of WT1 promoter methylation was revealed.

CONCLUSION

The in vitro data show that HMA can induce reactivation of WT1 transgene and that DAC is more effective, at least in mWT1-C1498 cells, which suggests that the combination of DAC and mWT1-C1498 can be used for the development of the experimental model of HMA-combined WT1 immunotherapy targeting leukemia.

Transfecting the hard-to-transfect lymphoma/leukemia cells using a simple cationic polymer nanocomplex

Although the development of gene delivery systems via non-viral-mediated methods is advancing rapidly, it remains a challenge to deliver plasmids into hard-to-transfect cells, such as lymphoma/leukemia cells. To develop an efficient transfection method, we formulated a simple nanocomplex by incorporating poly β-amino ester (PBAE) polymers with plasmid DNAs containing a GFP reporter gene. The formed PBAE-plasmid nanocomplexes are approximately 200nm in diameter and stable under physiological conditions, but become rapidly biodegradable when pH decreases <7.0. Cultured lymphoma/leukemia cells were used for transfection assays and resultant gene delivery rates were determined by quantifying GFP expression. Exposure of cells to the nanocomplexes composed of fractioned PBAE (>7kDa) resulted in GFP expression in 3% of cells, similar to that mediated by the standard Lipofectamine method. However, with polybrene pre-treatment, the nanocomplex could achieve GFP expression in up to 32% of lymphoma/leukemia cells, an 8-fold increase over that mediated by Lipofectamine. These findings demonstrated a simple, efficient method for in vitro gene delivery into hard-to-transfect cells. The nanocomplexes are biodegradable and have minimal cytotoxicity, suggesting the potential use for in vivo gene delivery.

A biobrick library for cloning custom eukaryotic plasmids

Researchers often require customised variations of plasmids that are not commercially available. Here we demonstrate the applicability and versatility of standard synthetic biological parts (biobricks) to build custom plasmids. For this purpose we have built a collection of 52 parts that include multiple cloning sites (MCS) and common protein tags, protein reporters and selection markers, amongst others. Importantly, most of the parts are designed in a format to allow fusions that maintain the reading frame. We illustrate the collection by building several model contructs, including concatemers of protein binding-site motifs, and a variety of plasmids for eukaryotic stable cloning and chromosomal insertion. For example, in 3 biobrick iterations, we make a cerulean-reporter plasmid for cloning fluorescent protein fusions. Furthermore, we use the collection to implement a recombinase-mediated DNA insertion (RMDI), allowing chromosomal site-directed exchange of genes. By making one recipient stable cell line, many standardised cell lines can subsequently be generated, by fluorescent fusion-gene exchange. We propose that this biobrick collection may be distributed peer-to-peer as a stand-alone library, in addition to its distribution through the Registry of Standard Biological Parts (http://partsregistry.org/).

A mechanistic understanding of production instability in CHO cell lines expressing recombinant monoclonal antibodies

One of the most significant problems in industrial bioprocessing of recombinant proteins using engineered mammalian cells is the phenomenon of cell line instability, where a production cell line suffers a loss of specific productivity (qP). This phenomenon occurs with unpredictable kinetics and has been widely observed in Chinese hamster ovary (CHO) cell lines and with all commonly used gene expression systems. The underlying causes (both genetic and physiological) and the precise molecular mechanisms underpinning cell line instability have yet to be fully elucidated, although recombinant gene silencing and loss of recombinant gene copies have been shown to cause qP loss. In this work we have investigated the molecular mechanisms underpinning qP instability over long-term sub-culture in CHO cell lines producing recombinant IgG1 and IgG2 monoclonal antibodies (Mab's). We demonstrate that production instability derives from two primary mechanisms: (i) epigenetic--methylation-induced transcriptional silencing of the CMV promoter driving Mab gene transcription and (ii) genetic--progressive loss of recombinant Mab gene copies in a proliferating CHO cell population. We suggest that qP decline resulting from loss of recombinant genes is a consequence of the inherent genetic instability of recombinant CHO cell lines.

Transgene expression is associated with copy number and cytomegalovirus promoter methylation in transgenic pigs

Transgenic animals have been used for years to study gene function, produce important proteins, and generate models for the study of human diseases. However, inheritance and expression instability of the transgene in transgenic animals is a major limitation. Copy number and promoter methylation are known to regulate gene expression, but no report has systematically examined their effect on transgene expression. In the study, we generated two transgenic pigs by somatic cell nuclear transfer (SCNT) that express green fluorescent protein (GFP) driven by cytomegalovirus (CMV). Absolute quantitative real-time PCR and bisulfite sequencing were performed to determine transgene copy number and promoter methylation level. The correlation of transgene expression with copy number and promoter methylation was analyzed in individual development, fibroblast cells, various tissues, and offspring of the transgenic pigs. Our results demonstrate that transgene expression is associated with copy number and CMV promoter methylation in transgenic pigs.

Homogeneity and persistence of transgene expression by omitting antibiotic selection in cell line isolation

Nonuniform, mosaic expression patterns of transgenes are often linked to transcriptional silencing, triggered by epigenetic modifications of the exogenous DNA. Such phenotypes are common phenomena in genetically engineered cells and organisms. They are widely attributed to features of transgenic transcription units distinct from endogenous genes, rendering them particularly susceptible to epigenetic downregulation. Contrary to this assumption we show that the method used for the isolation of stably transfected cells has the most profound impact on transgene expression patterns. Standard antibiotic selection was directly compared to cell sorting for the establishment of stable cells. Only the latter procedure could warrant a high degree of uniformity and stability in gene expression. Marker genes useful for the essential cell sorting step encode mostly fluorescent proteins. However, by combining this approach with site-specific recombination, it can be applied to isolate stable cell lines with the desired expression characteristics for any gene of interest.

Spontaneous switch from Agamma- to beta-globin promoter activity in a stable transfected dual reporter vector

Here it is analyzed the expression of a mini locus dual reporter construct composed by a micro-LCR and by the promoters for (A)gamma- and beta-globin gene, each one linked to a different Luciferase, in stably transfected GM979 cells for as long as 1-2 years from transfection. The transfected GM979 cells rapidly (within 1 month) evolved into a stable population which expresses constant levels of reporters for more than a year of continuous bulk culture. No silencing of the inserted construct was observed over time. In contrast, after 1 month, the reporter activity (both from (A)gamma- and beta-promoter) expressed per cell increased over time. The analysis of the Luciferase contained in single cell clones indicated that the higher reporter activity was due to increased gene expression per cell rather than to clonal selection of the most expressing clones. Since the activity driven by the beta-promoter increased 10-fold more than that driven by the (A)gamma one, the ratio between (A)gamma-driven/((A)gamma-driven + beta-driven) reporter activity in the cells decreased after 1 month and became similar to the gamma/(gamma + beta) globin mRNA ratio expressed by adult erythroid cells. Moreover, although both cells from early and late bulk culture responded to incubation with butyric acid, a known inducer of fetal globin gene expression, by increasing the reporter activity driven by the (A)gamma-promoter, only cells from late bulk culture decreased, as normal primary erythroblasts do, the activity of the reporter driven by the beta-promoter. These results suggest that the rapid changes in activity driven by the (A)gamma- and beta-globin promoters occurring during the first month after transfection may represent a novel in vitro model to study epigenetic regulation of the (A)gamma- and beta-promoter during the fetal to adult hemoglobin switch and confirm GM979 cells stably transfected with the dual reporter construct as a reliable assay for automated screening of chemical inducers of fetal globin gene activation.

Reversal of the silencing of tetracycline-controlled genes requires the coordinate action of distinctly acting transcription factors

BACKGROUND

Regulation of genes transferred to eukaryotic organisms is often limited by the lack of consistent expression levels in all transduced cells, which may result in part from epigenetic gene silencing effects. This reduces the efficacy of ligand-controlled gene switches designed for somatic gene transfers such as gene therapy.

METHODS

A doxycycline-controlled transgene was stably introduced in human cells, and clones were screened for epigenetic silencing of the transgene. Various regulatory proteins were targeted to the silent transgene, to identify those that would mediate regulation by doxycycline.

RESULTS

A doxycycline-controlled minimal promoter was found to be prone to gene silencing, which prevents activation by a fusion of the bacterial TetR DNA-binding domain with the VP16 activator. DNA modification studies indicated that the silenced transgene adopts a poorly accessible chromatin structure. Several cellular transcriptional activators were found to restore an accessible DNA structure when targeted to the silent transgene, and they cooperated with Tet-VP16 to mediate regulation by doxycycline.

CONCLUSIONS

Reversal of the silencing of a tetracycline-regulated minimal promoter requires a chromatin-remodeling activity for subsequent promoter activation by the Tet-VP16 fusion protein. Thus, distinct regulatory elements may be combined to obtain long-term regulation and persistent expression of exogenous genes in eukaryotic cells.

Production of stably transfected cell lines using immunoporation

Previous work from this laboratory has shown that immunoporation has the potential for the selective transfection of a range of different animal cells based on their immunological identity. The unique ability of immunoporation to target cells for transfection combined with the high efficiency of transfection and the high viability of cells make this method extremely promising for scientific and medical research. The experiments reported here show that not only can immunoporation produce transient transfection but also stably transfected cells are produced and such types of cells will be essential for the use of this method for gene therapy.

Vascular endothelial growth factor expression promotes the growth of breast cancer brain metastases in nude mice

Patients with breast cancer brain metastases cannot be cured and have a poor prognosis, with a median survival time of six months after diagnosis, despite developments in diagnostic and therapeutic modalities. In large part the progress in understanding the biology of breast cancer brain metastasis has been limited by the lack of suitable cell lines and experimental models. The objective of this study was to develop a reliable experimental model to study the pathogenesis of breast cancer brain metastases, using intra-internal carotid artery injection of breast cancer cells into nude mice. Brain metastasis-selected variant cells were recovered after three cycles of injection into the internal carotid artery of nude mice and harvest of brain metastases, resulting in variants termed MDA-231 BR1, -BR2 and -BR3. The metastasis-selected cells had increased potential for experimental brain metastasis and mice injected with these cells had significantly shorter mean survival than mice injected with the original cell line. Brain metastatic lesions of the selected variants contained significantly more CD31-positive blood vessels than metastases of the non-selected cell line. The variants selected from brain metastases released significantly more VEGF-A and IL-8 into culture supernatants than the original cell line, and more VEGF-A RNA when cultured in normoxic conditions. Mice injected with MDA-231 BR3 into the carotid artery were treated with the VEGF-receptor tyrosine kinase inhibitor PTK787/Z 222584. Oral administration of the inhibitor resulted in a significant decrease in brain tumor burden, reduced CD31-positive vessels in the brain lesions and incidence of PCNA positive tumor cells, and increased apoptosis in the tumor, as measured by TUNEL labeling. We conclude that elevated VEGF expression contributes to the ability of breast cancer cells to form brain metastases. Targeting endothelial cells with a VEGF-receptor specific tyrosine kinase inhibitor reduced angiogenesis and restricted the growth of the brain metastases.

Novel non-viral method for transfection of primary leukemia cells and cell lines

BACKGROUND: Tumor cells such as leukemia and lymphoma cells are possible targets for gene therapy. However, previously leukemia and lymphoma cells have been demonstrated to be resistant to most of non-viral gene transfer methods. METHODS: The aim of this study was to analyze various methods for transfection of primary leukemia cells and leukemia cell lines and to improve the efficiency of gene delivery. Here, we evaluated a novel electroporation based technique called nucleofection. This novel technique uses a combination of special electrical parameters and specific solutions to deliver the DNA directly to the cell nucleus under mild conditions. RESULTS: Using this technique for gene transfer up to 75% of primary cells derived from three acute myeloid leukemia (AML) patients and K562 cells were transfected with the green flourescent protein (GFP) reporter gene with low cytotoxicity. In addition, 49(+/- 9.7%) of HL60 leukemia cells showed expression of GFP. CONCLUSION: The non-viral transfection method described here may have an impact on the use of primary leukemia cells and leukemia cell lines in cancer gene therapy.

Illegitimate DNA integration in mammalian cells

Foreign DNA integration is one of the most widely exploited cellular processes in molecular biology. Its technical use permits us to alter a cellular genome by incorporating a fragment of foreign DNA into the chromosomal DNA. This process employs the cell's own endogenous DNA modification and repair machinery. Two main classes of integration mechanisms exist: those that draw on sequence similarity between the foreign and genomic sequences to carry out homology-directed modifications, and the nonhomologous or 'illegitimate' insertion of foreign DNA into the genome. Gene therapy procedures can result in illegitimate integration of introduced sequences and thus pose a risk of unforeseeable genomic alterations. The choice of insertion site, the degree to which the foreign DNA and endogenous locus are modified before or during integration, and the resulting impact on structure, expression, and stability of the genome are all factors of illegitimate DNA integration that must be considered, in particular when designing genetic therapies.

Platelet derived growth factor (PDGF) responsive epidermis formed from human keratinocytes transduced with the PDGF beta receptor gene

Platelet-derived growth factor is a major proliferative and migratory stimulus for connective tissue cells during the initiation of skin repair processes. In response to injury, locally produced platelet-derived growth factor is secreted by a diversity of cutaneous cell types whereas target activity is confined to cells of mesenchymal origin, e.g. dermal fibroblasts and smooth muscle cells. Although epidermal cells contribute to cutaneous platelet-derived growth factor activity by their ample capacity to secrete platelet-derived growth factor ligand, normal epidermal keratinocytes are not known to express any member of the platelet-derived growth factor receptor family. In order to study if epidermis may be genetically transformed to a platelet-derived growth factor sensitive compartment we aimed to introduce the gene encoding human platelet-derived growth factor receptor beta (PDGF beta R) into epidermal keratinocytes using a retrovirus-derived vector. Successful gene transfer to primary cells was confirmed by immunofluorescence staining, southern blotting, and ligand-induced receptor autophosphorylation. By culturing a mixture of PDGF beta R-transduced and unmodified keratinocytes at the air-liquid interface on devitalized dermis, we were able to establish a multilayered epithelium showing histologic similarities to that evolved from native keratinocytes or keratinocytes transduced with the reporter gene encoding enhanced green fluorescent protein. Receptor-modified epidermal tissue cultured for 6 days and examined by immunofluorescence microscopy was shown to contain PDGF beta R-expressing keratinocytes distributed in all layers of living epidermis. By continued tissue culture in serum-containing medium, the epidermis became increasingly cornified although receptor-positive cells were still observed within the viable basal compartment. Stimulation of PDGF beta R-transduced epidermis with recombinant platelet-derived growth factor BB had a mitogenic effect as reflected by an increased frequency of Ki-67 positive keratinocytes. The study demonstrates that transgene expression of human PDGF beta R can be achieved in epidermal keratinocytes by retroviral transduction, and that ligand activation of such gene-modified skin equivalent enhances cell proliferation. In perspective, viral PDGF beta R gene transfer to keratinocytes may be a useful approach in studies of receptor tyrosine kinase mediated skin repair and epithelialization.

Quantification of human Alu sequences by real-time PCR--an improved method to measure therapeutic efficacy of anti-metastatic drugs in human xenotransplants

For measuring the efficacy of new anti-metastatic drugs in preclinical models, macroscopical analysis or classical histology of secondary organs are established methods. However, macroscopical evaluation does not take into consideration intra-organ metastasis. Histological analysis is often performed in few sections of the relevant organs, and this may be misleading, since equal distribution of tumor cells within an organ is unlikely. In addition, recent studies have demonstrated that anti-tumorigenic drugs are able to promote metastasis and to change the metastatic pattern. Therefore, extensive analysis of metastasis is mandatory for the evaluation of new compounds. A feasibility study was conducted to find out if the quantification of human Alu sequences could be applied as a surrogate marker for metastasis in xenografts. Alu PCR was performed by using the LightCycler system, which allows PCR reaction and subsequent quantification of the PCR products in less than 30 min. We found that i) the equivalent of one human tumor cell in 1 x 10(6) murine cells could be detected; ii) in tumor-carrying mice, Alu signal increased over time in secondary organs; iii) this increase was more prominent using highly metastatic tumor cells; iv) Alu signal intensity in DNA extracted from tissue slides correlated with the expression of histological tumor markers; v) in three different tumor models (colon, breast and lung), treatment with Taxol or 5-fluorouracil reduced the amount of Alu in different organs. In contrast, reduction of Alu by the matrix metalloproteinase inhibitor RO 28-2653 was not significant. Taken together, quantification of Alu sequences is a fast and accurate method to evaluate the therapeutic efficacy of anti-metastatic drugs in xenografts.

A novel enhanced green fluorescent protein (EGFP)-K562 flow cytometric method for measuring natural killer (NK) cell cytotoxic activity

Enhanced green fluorescent protein (EGFP) was stably expressed in human erythroleukaemia K562 cells (EGFP-K562) and used as target cells for measurement of natural killer (NK) cell cytotoxicity by flow cytometry. The compromised EGFP-K562 target cells were stained with propidium iodide (PI) and showed dual (green-red) fluorescent. Although the kinetic study demonstrated that the optimal incubation time for the assay was 4 h, a 2-h incubation period also gave comparable results. This new technique correlated strongly with the standard chromium (51Cr) release assay at the correlation coefficients of 0.87 and 0.89 at p-value <0.001 for 2- and 4-h incubation times, respectively. The EGFP-K562 stable cell line provides a novel method to measure NK cytotoxicity by flow cytometry without pre-staining or pre-labeling target cells.

In vitro and in vivo hematopoietic potential of human stem cells residing in muscle tissue

OBJECTIVE

We studied the in vitro and in vivo hematopoietic potential of human stem cells residing in muscle tissue collected from adults with head and neck cancer.

MATERIALS AND METHODS

Adherent muscle cells were cultured in F12 medium with 10% fetal bovine serum and transplanted into immunodeficient mice.

RESULTS

On day 12 we obtained a median of 500,000 adherent cells per gram muscle sample. Thy-1, endoglin, HER2/neu, and P1H12 were expressed in the majority of cells. CD34, VEGFR2, c-kit, VCAM-1, and CXCR4 were expressed in 0.5-1.5%, 1-5%, 1-15%, 9-15%, and 30% of cells, respectively. Immunodeficient mice transplanted with fresh muscle cells or less than 500,000 cultured cells showed little or no human engraftment. In mice transplanted with more than 500,000 cultured cells, up to 14% human CD45(+) hematopoietic cells (including myeloid and lymphoid subsets) were detected by flow cytometry. Engraftment was confirmed by polymerase chain reaction, Southern blotting, and DNA sequencing. Liver, muscle, and spleen evaluated for human DNA were positive in the majority of mice showing hematopoietic engraftment in the bone marrow. In vivo hematopoietic engraftment potential was maintained in cultured CD45(-) muscle cells transduced with the green fluorescence protein gene.

CONCLUSIONS

Human stem cells residing in muscle tissue can generate multilineage hematopoiesis in immunodeficient mice. Surprisingly, this hematopoietic potential increased in cultured versus fresh cells from muscle tissue.

Kinetics of fluorescence expression in nonhuman primates transplanted with GFP retrovirus-modified CD34 cells

Downregulation and loss of proviral expression have been demonstrated to occur in a variety of in vitro studies and in mouse models. Here we evaluated the kinetics of proviral expression after transplantation in a competitive repopulating model in the baboon. Transgene persistence and green fluorescent protein (GFP) expression in peripheral blood leukocytes (PBL) were analyzed in four animals by semiquantitative PCR and flow cytometry for up to 80 weeks (range 17-80). All animals were transplanted with cells transduced with EGFP or EYFP reporters driven by Moloney murine leukemia virus (MoMuLV) or a modified promoter/enhancer, (MND) respectively. Simultaneous dual-color analysis of fluorescence levels in granulocyte and lymphocyte subsets following hematopoietic reconstitution demonstrated progressive loss of fluorescence intensity occurring predominantly early after transplant in cells transduced with both retrovirus backbones and at serial time points. In addition, we carried out PCR analysis of DNA extracted from sorted EGFP(-)/EYFP(-) cells and confirmed the presence of cells genetically marked by either vector in this population, indicating the persistence of cells that have downregulated or lost retroviral gene expression. In comparison to mouse studies, however, we did not detect substantial differences between MND and MoMuLV backbones.