Inactivation of the Moloney murine leukemia virus long terminal repeat in murine fibroblast cell lines is associated with methylation and dependent on its chromosomal position

Alpharetroviral vectors: from a cancer-causing agent to a useful tool for human gene therapy

Gene therapy using integrating retroviral vectors has proven its effectiveness in several clinical trials for the treatment of inherited diseases and cancer. However, vector-mediated adverse events related to insertional mutagenesis were also observed, emphasizing the need for safer therapeutic vectors. Paradoxically, alpharetroviruses, originally discovered as cancer-causing agents, have a more random and potentially safer integration pattern compared to gammaretro- and lentiviruses. In this review, we provide a short overview of the history of alpharetroviruses and explain how they can be converted into state-of-the-art gene delivery tools with improved safety features. We discuss development of alpharetroviral vectors in compliance with regulatory requirements for clinical translation, and provide an outlook on possible future gene therapy applications. Taken together, this review is a broad overview of alpharetroviral vectors spanning the bridge from their parental virus discovery to their potential applicability in clinical settings.

Dynamics and evolution of β-catenin-dependent Wnt signaling revealed through massively parallel clonogenic screening

Wnt/β-catenin signaling is of significant interest due to the roles it plays in regulating development, tissue regeneration and disease. Transcriptional reporters have been widely employed to study Wnt/β-catenin signal transduction in live cells and whole organisms and have been applied to understanding embryonic development, exploring oncogenesis and developing therapeutics. Polyclonal heterogeneity in reporter cell lines has historically been seen as a challenge to be overcome in the development of novel cell lines and reporter-based assays, and monoclonal reporter cell lines are commonly employed to reduce this variability. A375 cell lines infected with a reporter for Wnt/β-catenin signaling were screened over short (<6) and long (>25) generational timescales. To characterize phenotypic divergence over these time-scales, a microfabricated cell array-based screen was developed enabling characterization of 1119 clonal colonies in parallel. This screen revealed phenotypic divergence after <6 generations at a similar scale to that observed in monoclonal cell lines cultured for >25 generations. Not only were reporter dynamics observed to diverge widely, but monoclonal cell lines were observed with seemingly opposite signaling phenotypes. Additionally, these observations revealed a generational-dependent trend in Wnt signaling in A375 cells that provides insight into the pathway's mechanisms of positive feedback and self-inhibition.

Optimizing reporter constructs for in vivo bioluminescence imaging of interferon-γ stimulated mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) are a promising treatment modality for a variety of diseases. Strategies to investigate the fate of MSCs in vivo are important to unravel their therapeutic mechanisms. However, currently available techniques are hampered by their low sensitivity. We therefore aimed to optimize in vivo bioluminescence imaging of MSCs. We compared MSCs transduced with firefly luciferase (Fluc) and transmembrane-bound Gaussia luciferase driven by the human cytomegalovirus, spleen focus-forming virus (SFFV), and elongation factor 1-α (EF1α) promoters. Although cytomegalovirus-transmembrane-bound Gaussia luciferase-transduced MSCs showed the highest light intensity in vitro, the signal was almost undetectable in vivo. Spleen focus-forming virus-Fluc-transduced MSCs revealed a bright signal in vivo, but transgene expression was silenced upon in vitro stimulation with interferon (IFN)-γ. Therefore, the SFFV promoter was replaced by the EF1α promoter. Light emission of Fluc under the control of EF1α was similar to SFFV-Fluc. Although EF1α-Fluc light emission was decreased tenfold in the presence of IFN-γ when compared with unstimulated MSCs, the bioluminescent signal could still be detected and was clearly distinguishable from untransduced MSCs. Furthermore, stimulation of MSCs with tumor necrosis factor-α hardly affected transgene expression in EF1α-Fluc-transduced MSCs. Thus, the use of the EF1α promoter partially overcomes silencing and allows in vivo bioluminescence imaging of IFN-γ-stimulated MSCs.

Epigenetic changes of lentiviral transgenes in porcine stem cells derived from embryonic origin

Because of the physiological and immunological similarities that exist between pigs and humans, porcine pluripotent cell lines have been identified as important candidates for preliminary studies on human disease as well as a source for generating transgenic animals. Therefore, the establishment and characterization of porcine embryonic stem cells (pESCs), along with the generation of stable transgenic cell lines, is essential. In this study, we attempted to efficiently introduce transgenes into Epiblast stem cell (EpiSC)-like pESCs. Consequently, a pluripotent cell line could be derived from a porcine-hatched blastocyst. Enhanced green fluorescent protein (EGFP) was successfully introduced into the cells via lentiviral vectors under various multiplicities of infection, with pluripotency and differentiation potential unaffected after transfection. However, EGFP expression gradually declined during extended culture. This silencing effect was recovered by in vitro differentiation and treatment with 5-azadeoxycytidine. This phenomenon was related to DNA methylation as determined by bisulfite sequencing. In conclusion, we were able to successfully derive EpiSC-like pESCs and introduce transgenes into these cells using lentiviral vectors. This cell line could potentially be used as a donor cell source for transgenic pigs and may be a useful tool for studies involving EpiSC-like pESCs as well as aid in the understanding of the epigenetic regulation of transgenes.

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.

Correction of murine SCID-X1 by lentiviral gene therapy using a codon-optimized IL2RG gene and minimal pretransplant conditioning

Clinical trials have demonstrated the potential of ex vivo hematopoietic stem cell gene therapy to treat X-linked severe combined immunodeficiency (SCID-X1) using γ-retroviral vectors, leading to immune system functionality in the majority of treated patients without pretransplant conditioning. The success was tempered by insertional oncogenesis in a proportion of the patients. To reduce the genotoxicity risk, a self-inactivating (SIN) lentiviral vector (LV) with improved expression of a codon optimized human interleukin-2 receptor γ gene (IL2RG) cDNA (coγc), regulated by its 1.1 kb promoter region (γcPr), was compared in efficacy to the viral spleen focus forming virus (SF) and the cellular phosphoglycerate kinase (PGK) promoters. Pretransplant conditioning of Il2rg(-/-) mice resulted in long-term reconstitution of T and B lymphocytes, normalized natural antibody titers, humoral immune responses, ConA/IL-2 stimulated spleen cell proliferation, and polyclonal T-cell receptor gene rearrangements with a clear integration preference of the SF vector for proto-oncogenes, contrary to the PGK and γcPr vectors. We conclude that SIN lentiviral gene therapy using coγc driven by the γcPr or PGK promoter corrects the SCID phenotype, potentially with an improved safety profile, and that low-dose conditioning proved essential for immune competence, allowing for a reduced threshold of cell numbers required.

Molecular control of HIV-1 postintegration latency: implications for the development of new therapeutic strategies

The persistence of HIV-1 latent reservoirs represents a major barrier to virus eradication in infected patients under HAART since interruption of the treatment inevitably leads to a rebound of plasma viremia. Latency establishes early after infection notably (but not only) in resting memory CD4⁺ T cells and involves numerous host and viral trans-acting proteins, as well as processes such as transcriptional interference, RNA silencing, epigenetic modifications and chromatin organization. In order to eliminate latent reservoirs, new strategies are envisaged and consist of reactivating HIV-1 transcription in latently-infected cells, while maintaining HAART in order to prevent de novo infection. The difficulty lies in the fact that a single residual latently-infected cell can in theory rekindle the infection. Here, we review our current understanding of the molecular mechanisms involved in the establishment and maintenance of HIV-1 latency and in the transcriptional reactivation from latency. We highlight the potential of new therapeutic strategies based on this understanding of latency. Combinations of various compounds used simultaneously allow for the targeting of transcriptional repression at multiple levels and can facilitate the escape from latency and the clearance of viral reservoirs. We describe the current advantages and limitations of immune T-cell activators, inducers of the NF-κB signaling pathway, and inhibitors of deacetylases and histone- and DNA- methyltransferases, used alone or in combinations. While a solution will not be achieved by tomorrow, the battle against HIV-1 latent reservoirs is well- underway.

Retroviral delivery of ECM genes

The use of recombinant DNA has become a powerful tool in the analysis of functional and structural properties of the extracellular matrix proteins. During last decade, various procedures of plasmid DNA delivery using liposome-based or electroporation-based transfection have been developed. However, in many instances, these procedures were shown to be not effective in DNA transfer or toxic for the mammalian cells. On contrary, retrovirus-mediated infection represents a superior mode of gene delivery with a success rate and viability of the cells approaching 100% in in vitro conditions. The use of the retroviral system also allows permanent insertion of the gene of interest into the chromosome of the infected cell, resulting in efficient gene transfer in which most recipient cells will incorporate and express the transduced gene. In this chapter, we will describe several retrovirus-based systems and provide step-by-step protocols applicable for the production of the recombinant virus and efficient delivery of the ECM genes.

Chromatin, gene silencing and HIV latency

Cellular defence mechanisms against HIV contribute to its persistence.

One of the cellular defenses against virus infection is the silencing of viral gene expression. There is evidence that at least two gene-silencing mechanisms are used against the human immuno-deficiency virus (HIV). Paradoxically, this cellular defense mechanism contributes to viral latency and persistence, and we review here the relationship of viral latency to gene-silencing mechanisms.

The core element of a CpG island protects avian sarcoma and leukosis virus-derived vectors from transcriptional silencing

Unmethylated CpG islands are known to keep adjacent promoters transcriptionally active. In the CpG island adjacent to the adenosine phosphoribosyltransferase gene, the protection against transcriptional silencing can be attributed to the short CpG-rich core element containing Sp1 binding sites. We report here the insertion of this CpG island core element, IE, into the long terminal repeat of a retroviral vector derived from Rous sarcoma virus, which normally suffers from progressive transcriptional silencing in mammalian cells. IE insertion into a specific position between enhancer and promoter sequences led to efficient protection of the integrated vector from silencing and gradual CpG methylation in rodent and human cells. Individual cell clones with IE-modified reporter vectors display high levels of reporter expression for a sustained period and without substantial variegation in the cell culture. The presence of Sp1 binding sites is important for the protective effect of IE, but at least some part of the entire antisilencing capacity is maintained in IE with mutated Sp1 sites. We suggest that this strategy of antisilencing protection by the CpG island core element may prove generally useful in retroviral vectors.

Specific in vivo expression in type II pneumocytes of the Jaagsiekte sheep retrovirus long terminal repeat in transgenic mice

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma, a transmissible lung cancer in sheep. Previous experiments in differentiated murine tissue culture cell lines suggested that the disease specificity of JSRV for secretory lung epithelial cells (type II pneumocytes an Clara cells) reflects transcriptional specificity of the viral long terminal repeat (LTR) for these cells. To test this in vivo, transgenic mice carrying the bacterial beta-galactosidase (beta-Gal) gene driven by the JSRV LTR were generated. Two transgenic lines showed beta-Gal expression in the lungs but not other tissues of F1 animals, although transgene silencing in subsequent generations was a major problem. The cells expressing the transgene were identified by two- and three-color immunofluorescence for marker proteins of type II pneumocytes (surfactant protein C [SPC]) and Clara cells (CC10) as well as for a T7 gene 10 epitope present in the beta-Gal reporter. F1 animals from both lines showed transgene expression in type II pneumocytes, but somewhat surprisingly not in Clara cells. Expression was not detected in bronchiolo-alveolar stem cells (BASCs) either. These results indicate that the JSRV LTR is specifically active in type II pneumocytes in the mouse lung, which is consistent with the fact that JSRV-induced OPA tumors in sheep largely have phenotypic markers of type II pneumocytes.

Gammaretrovirus-mediated correction of SCID-X1 is associated with skewed vector integration site distribution in vivo

We treated 10 children with X-linked SCID (SCID-X1) using gammaretrovirus-mediated gene transfer. Those with sufficient follow-up were found to have recovered substantial immunity in the absence of any serious adverse events up to 5 years after treatment. To determine the influence of vector integration on lymphoid reconstitution, we compared retroviral integration sites (RISs) from peripheral blood CD3(+) T lymphocytes of 5 patients taken between 9 and 30 months after transplantation with transduced CD34(+) progenitor cells derived from 1 further patient and 1 healthy donor. Integration occurred preferentially in gene regions on either side of transcription start sites, was clustered, and correlated with the expression level in CD34(+) progenitors during transduction. In contrast to those in CD34(+) cells, RISs recovered from engrafted CD3(+) T cells were significantly overrepresented within or near genes encoding proteins with kinase or transferase activity or involved in phosphorus metabolism. Although gross patterns of gene expression were unchanged in transduced cells, the divergence of RIS target frequency between transduced progenitor cells and post-thymic T lymphocytes indicates that vector integration influences cell survival, engraftment, or proliferation.

The nested open reading frame in the Epstein-Barr virus nuclear antigen-1 mRNA encodes a protein capable of inhibiting antigen presentation in cis

Herpesviruses employ many mechanisms to evade the immune response, allowing them to persist life-long in their hosts. The Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA-1) and, more recently, the latency-associated nuclear antigen 1 (LANA-1) of the Kaposi Sarcoma Herpesvirus have been shown to function as in cis-acting inhibitors of antigen presentation. In both proteins, long simple repeat elements are responsible for the inhibition, but the sequences of these repeats are strongly dissimilar. Intriguingly, EBNA-1 mRNA contains a large nested open reading frame that codes for a 40.7kDa strongly acidic protein, in addition to the full-length EBNA-1. This protein, here called pGZr, has a 230 amino-acids long glycine, glutamine, and glutamic acid-rich repeat ('GZ' repeat), highly similar (65% amino-acid identity) to the acidic repeat of LANA-1. To evaluate if pGZr, like EBNA-1 and LANA-1, can inhibit antigen presentation in cis, we fused the nested ORF with the E. coli-derived LacZ gene encoding beta-galactosidase. Whereas cells producing the unmodified beta-galactosidase readily present the H-2L(d)-restricted CTL epitope TPHPARIGL, which resides in the C-terminal region of beta-galactosidase, cells producing the pGZr-beta-galactosidase fusion protein do not. Also shorter fragments of the repeat can inhibit peptide presentation. Even though the physiological function of pGZr remains to be elucidated, the GZ-repeat protein may be valuable as inhibitor of presentation of antigenic peptides derived from transgenes in gene therapy.

A novel, immortal, and multipotent human neural stem cell line generating functional neurons and oligodendrocytes

The discovery and study of neural stem cells have revolutionized our understanding of the neurogenetic process, and their inherent ability to adopt expansive growth behavior in vitro is of paramount importance for the development of novel therapeutics based on neural cell replacement. Recent advances in high-throughput assays for drug development and gene discovery dictate the need for rapid, reproducible, long-term expansion of human neural stem cells (hNSCs). In this view, the complement of wild-type cell lines currently available is insufficient. Here we report the establishment of a stable human neural stem cell line (immortalized human NSCs [IhNSCs]) by v-myc-mediated immortalization of previously derived wild-type hNSCs. These cells demonstrate three- to fourfold faster proliferation than wild-type cells in response to growth factors but retain rather similar properties, including multipotentiality. By molecular biology, biochemistry, immunocytochemistry, fluorescence microscopy, and electrophysiology, we show that upon growth factor removal, IhNSCs completely downregulate v-myc expression, cease proliferation, and differentiate terminally into three major neural lineages: astrocytes, oligodendrocytes, and neurons. The latter are functional, mature cells displaying clear-cut morphological and physiological features of terminally differentiated neurons, encompassing mostly the GABAergic, glutamatergic, and cholinergic phenotypes. Finally, IhNSCs produce bona fide oligodendrocytes in fractions up to 20% of total cell number. This is in contrast to the negligible propensity of hNSCs to generate oligodendroglia reported so far. Thus, we describe an immortalized hNSC line endowed with the properties of normal hNSCs and suitable for developing the novel, reliable assays and reproducible high-throughput gene and drug screening that are essential in both diagnostics and cell therapy studies.

Lentiviral vectors containing an enhancer-less ubiquitously acting chromatin opening element (UCOE) provide highly reproducible and stable transgene expression in hematopoietic cells

Ubiquitously acting chromatin opening elements (UCOEs) consist of methylation-free CpG islands encompassing dual divergently transcribed promoters of housekeeping genes that have been shown to confer resistance to transcriptional silencing and to produce consistent and stable transgene expression in tissue culture systems. To develop improved strategies for hematopoietic cell gene therapy, we have assessed the potential of the novel human HNRPA2B1-CBX3 UCOE (A2UCOE) within the context of a self-inactivating (SIN) lentiviral vector. Unlike viral promoters, the enhancer-less A2UCOE gave rise to populations of cells that expressed a reporter transgene at a highly reproducible level. The efficiency of expression per vector genome was also markedly increased in vivo compared with vectors incorporating either spleen focus-forming virus (SFFV) or cytomegalovirus (CMV) promoters, suggesting a relative resistance to silencing. Furthermore, an A2UCOE-IL2RG vector fully restored the IL-2 signaling pathway within IL2RG-deficient human cells in vitro and successfully rescued the X-linked severe combined immunodeficiency (SCID-X1) phenotype in a mouse model of this disease. These data indicate that the A2UCOE displays highly reliable transcriptional activity within a lentiviral vector, largely overcoming insertion-site position effects and giving rise to therapeutically relevant levels of gene expression. These properties are achieved in the absence of classic enhancer activity and therefore may confer a high safety profile.

Genetic modification of hematopoietic stem cells with nonviral systems: past progress and future prospects

Serious unwanted complications provoked by retroviral gene transfer into hematopoietic stem cells (HSCs) have recently raised the need for the development and assessment of alternative gene transfer vectors. Within this context, nonviral gene transfer systems are attracting increasing interest. Their main advantages include low cost, ease of handling and large-scale production, large packaging capacity and, most importantly, biosafety. While nonviral gene transfer into HSCs has been restricted in the past by poor transfection efficiency and transient maintenance, in recent years, biotechnological developments are converting nonviral transfer into a realistic approach for genetic modification of cells of hematopoietic origin. Herein we provide an overview of past accomplishments in the field of nonviral gene transfer into hematopoietic progenitor/stem cells and we point at future challenges. We argue that episomally maintained self-replicating vectors combined with physical methods of delivery show the greatest promise among nonviral gene transfer strategies for the treatment of disorders of the hematopoietic system.

DNA methylation of Sleeping Beauty with transposition into the mouse genome

The Sleeping Beauty transposon is a recently developed non-viral vector that can mediate insertion of transgenes into the mammalian genome. Foreign DNA elements that are introduced tend to invoke a host-defense mechanism resulting in epigenetic changes, such as DNA methylation, which may induce transcriptional inactivation of mammalian genes. To assess potential epigenetic modifications associated with Sleeping Beauty transposition, we investigated the DNA methylation pattern of transgenes inserted into the mouse genome as well as genomic regions flanking the insertion sites with bisulfite-mediated genomic sequencing. Transgenic mouse lines were created with two different Sleeping Beauty transposons carrying either the Agouti or eGFP transgene. Our results showed that DNA methylation in the keratin-14 promoter and Agouti transgene were negligible. In addition, two different genomic loci flanking the Agouti insertion site exhibited patterns of DNA methylation similar to wild-type mice. In contrast, high levels of DNA methylation were observed in the eGFP transgene and its ROSA26 promoter. These results indicate that transposition via Sleeping Beauty into the mouse genome may result in a significant level of de novo DNA methylation. This may depend on a number of different factors including the cargo DNA sequence, chromosomal context of the insertion site, and/or host genetic background.

Versatility of gene therapy vectors through viruses

Several viruses have been engineered for gene therapy applications, and the specific properties of each viral vector have been exploited to target a variety of inherited and acquired diseases. Preclinical and clinical studies demonstrated that viral vectors are highly versatile tools capable of efficient transfer of foreign genetic information into almost all cell types and tissues. Gene therapy applications depend on vector characteristics, such as host range, cell- or tissue-specific targeting, genome integration, efficiency and duration of transgene expression, packaging capacity, and suitability for scale-up production. This review discusses the advances in the development of viral vectors, with particular emphasis on how knowledge of virus biology has been exploited to design a variety of vectors with improved safety characteristics and efficiency, potentially suitable for a large number of gene therapy applications.

Current status of gene therapy strategies to treat HIV/AIDS

Progress in developing effective gene transfer approaches to treat HIV-1 infection has been steady. Many different transgenes have been reported to inhibit HIV-1 in vitro. However, effective translation of such results to clinical practice, or even to animal models of AIDS, has been challenging. Among the reasons for this failure are uncertainty as to the most effective cell population(s) to target, the diffuseness of these target cells in the body, and ineffective or insufficiently durable gene delivery. Better understanding of the HIV-1 replicative cycle, host factors involved in HIV-1 infection, vector biology and application, transgene technology, animal models, and clinical study design have all contributed vastly to planning current and future strategies for application of gene therapeutic approaches to the treatment of AIDS. This review focuses on the newest developments in these areas and provides a strong basis for renewed optimism that gene therapy will have an important role to play in treating people infected with HIV-1.

HIV-mediated Expression of Btk in Hematopoietic Stem Cells is not Sufficient to Restore B Cell Function in X-linked Immunodeficient Mice

Mutations of Bruton's tyrosine kinase (Btk), which is critical for B cell development and function, cause X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. Although the severity of the clinical phenotype differs between the two species, xid mice are considered useful for evaluating treatment strategies for XLA patients. Hematopoietic stem cells (HSCs; 1 approximately 3 x 10(5))from xid mice were transduced with an HIV vector containing the human Btk (hBtk) gene under the control of the internal murine stem cell virus (MSCV) promoter and injected into 4-week-old xid mice. Thirty weeks later, the copy number of the integrated HIV vector was over 0.2 per cell in both bone marrow and spleen, but serum concentrations of IgM and IgG3 and the antibody response to nitrophenol (NP)-Ficoll challenge were not restored. The number of differentiated B cells (IgM(low)IgD(high)) was increased, while the peritoneal B1 cell count remained low. These results indicate that HIV-mediated expression of hBtk in bone marrow stem cells partially promotes B cell development, but is not sufficient for the restoration of B cell function in xid mice.

An Erythroid-Specific Chromatin Opening Element Increases β-Globin Gene Expression from Integrated Retroviral Gene Transfer Vectors

Gene therapy strategies requiring long-term high-level expression from integrated genes are currently limited by inconsistent levels of expression. This may be observed as variegated, silenced or position-dependent gene expression. Each of these phenomena involve suppressive chromatin structures. We hypothesized that by actively conferring an open chromatin structure on integrated vectors would increase transgene expression. To test this idea we used a 100bp element from the β-globin locus control region (LCR) which is able to independently open local chromatin structure in erythroid tissues. This element includes binding sites for GATA-1, NF-E2, EKLF and Sp-1 and is evolutionarily conserved. We constructed a series of MSCV-based vectors containing the β-globin gene driven by a minimal β-globin promoter with combinations of the HSFE and LCR derived enhancer elements. Pools of MEL clones containing integrated vectors were analyzed for chromatin structure and β-globin gene expression. The HSFE increased the extent of nuclease sensitive chromatin over the promoters of the constructs. The most effective vector included tandem copies of the HSFE and produced a 5-fold increase in expression compared to the promoter alone. These results indicate that the HSFE is able to augment the opening of β-globin promoter chromatin structure and significantly increase gene expression in the context of an integrated retroviral vector.

Evidence that the mouse 3' kappa light chain enhancer confers position-independent transgene expression in T- and B-lineage cells

One of the major obstacles for successful application of murine leukemia virus (MLV) vectors to genetic therapy of lymphocyte disorders is low levels of transgene expression or the eventual loss of expression. To overcome this problem, an improved retroviral vector was constructed utilizing the myeloproliferative sarcoma virus (MPSV) long terminal repeat (LTR), which provided a significantly higher level of transgene expression in human lymphoid cells than did MLV vectors. Nevertheless, transgene expression remained low in a large percentage of transduced cells. To address whether lymphocyte enhancer elements might improve transgene expression mediated by retroviral vectors in lymphocytes, we cloned the mouse immunoglobulin 3' kappa light chain enhancer gene (mE3') into the MPSV vector. We found that the mE3' conferred a higher, more uniform and sustained level of expression in transduced T- and B-cell lines, and in primary T cells, than did the control vector lacking this element. Integration sites were diverse and a single copy of the proviral genome was present in all examined transduced cells. The mE3' failed to enhance transgene expression in most nonlymphoid cells, indicating it is relatively lineage-specific. Taken together, these results provide strong evidence that the mE3' functions as a locus control region (LCR) in conferring enhanced integration-site-independent expression of a retroviral transgene.

Demethylation of host-cell DNA at the site of avian retrovirus integration

The transcriptional activity of an integrated retroviral copy strongly depends on the adjacent host-cell DNA at the site of integration. Transcribed DNA loci as well as cis-acting sequences like enhancers or CpG islands usually permit expression of nearby integrated proviruses. In contrast, proviruses residing close to cellular silencers tend to transcriptional silencing and CpG methylation. Little is known, however, about the influence of provirus integration on the target sequence in the host genome. Here, we report interesting features of a simplified Rous sarcoma virus integrated into a non-transcribed hypermethylated DNA sequence in the Syrian hamster genome. After integration, CpG methylation of this sequence has been lost almost completely and hypomethylated DNA permits proviral transcription and hamster cell transformation by the proviral v-src oncogene. This, however, is not a stable state, and non-transformed revertants bearing transcriptionally silenced proviruses segregate with a high rate. The provirus silencing is followed by DNA methylation of both provirus regulatory regions and adjacent cellular sequences. This CpG methylation is very dense and resistant to the demethylation effects of 5-aza-2(')-deoxycytidine and/or trichostatin A. Our description exemplifies the capacity of retroviruses/retroviral vectors to overcome, at least transiently, negative position effects of DNA methylation at the site of integration.

Creation of immune ‘stealth’ genes for gene therapy through fusion with the Gly-Ala repeat of EBNA-1

A major obstacle in gene-therapy protocols is T-cell-mediated destruction of transgene-expressing cells. Therefore new approaches are needed to prevent rapid clearance of transduced cells. We exploited the Gly-Ala repeat (GAr) domain of the Epstein-Barr virus nuclear antigen-1, since the GAr prevents cytotoxic T-lymphocyte-epitope generation. Here we show that three different enzymes (viz. the E. coli LacZ gene encoded beta-galactosidase, firefly luciferase, and HSV1 thymidine kinase) fused with the GAr retained their function. Moreover, linking GAr with beta-galactosidase successfully prevented recognition of GAr-LacZ-expressing cells by beta-galactosidase-specific CTL. Nonetheless, vaccination with a GAr-LacZ adenovirus or with an allogeneic cell line expressing GAr-LacZ resulted in the induction of beta-gal-specific CTL. This demonstrates that the GAr domain does not inhibit cross presentation of antigens, but only affects breakdown of endogenously synthesized proteins. These data demonstrate how the GAr domain can be exploited to create immuno'stealth' genes by hiding transgene products from CTL-mediated immune attack.

Mutation of CpGs in the murine stem cell virus retroviral vector long terminal repeat represses silencing in embryonic stem cells

Although DNA methylation and transcriptional repression are generally associated, a causal role for DNA methylation in silencing of retroviral vectors has not been established. The newer generation murine stem cell virus retroviral vector (MSCV) lacks many of the repressive cis-acting DNA sequences identified in Moloney murine leukemia virus but remains sensitive to transcriptional silencing in various cell types. To determine the contribution of cytosine methylation to MSCV silencing, we mutated CpG dinucleotides located in the MSCV long terminal repeat (LTR) that are clustered in the U3 region and directly spanning the transcription start site in the R region. Effects of the CpG mutations on MSCV silencing were assessed in murine embryonic stem cells. An analysis of numerous clonal proviral integrants showed that mutation of CpGs in both clusters eliminated proviral integrants that were completely silenced. Variegated expression was shown to represent a substantial component of intraclonal silencing and was independent of the presence of CpGs in the LTR. Treatment of transduced cells with 5-azadeoxycytidine delayed establishment of the silenced state but had only a modest effect on expression of some proviral integrants at late times post-transduction. These results are direct evidence for a causal contribution of DNA methylation in the LTR to MSCV silencing and define the promoter region CpGs as a repressive element in embryonic stem cells. Furthermore, distinct mechanisms are suggested for establishment and maintenance of the silenced proviral state.

Silencing of retroviral vector transduced LacZ reporter gene by frameshift mutation

Moloney murine leukemia virus-based vector expressing Escherichia coli beta-galactosidase (lacZ) as reporter gene and the transposon Tn5 neomycin resistance (neo) gene was transduced at low-multiplicity of infections into NIH 3T3 cells. Geneticin (G418)-resistant cells were recloned and cell lines containing beta-galactosidase positive or beta-galactosidase negative cells were obtained. Both positive and negative cell lines contained a single proviral copy at distinct integration sites. RNA complementary to lacZ was detected in beta-galactosidase positive as well as in one of three investigated beta-galactosidase negative cell lines. DNA sequence analysis of proviral LacZ gene in beta-galactosidase negative cell line C6 showed a single nucleotide insertion at position 1567 resulting in reading frame shift and translational stop codon at position 1629. This mutation explains the enzyme inactivation. The absence of beta-galactosidase after retroviral transduction of LacZ reproter gene may be a consequence of definite mutation but not a consequence of ineffective transduction or transcriptional inactivation of transgene.

Transcriptional suppression of in vitro-integrated human immunodeficiency virus type 1 does not correlate with proviral DNA methylation

Persistence of human immunodeficiency virus type 1 (HIV-1) constitutes a major obstacle in the control of HIV-1 infection. Here we investigated whether the CpG methylation of the HIV-1 promoter can directly influence the expression of the HIV-1 genome and thereby contribute to the persistence and latency of HIV-1. The levels of CpG methylation in the promoter of HIV-1 were studied after bisulfite-induced modification of DNA in five Jurkat clonal cell lines transduced by an HIV-1 long terminal repeat (LTR)-driven retroviral vector and expressing enhanced green fluorescent protein (GFP) and in primary resting memory T cells challenged with HIV-1 or with an HIV-1-derived retroviral vector. Basal HIV-1 promoter activities were low or undetectable in three tested HIV-1 LTR-GFP clones, one of which encoded the Tat protein, and they reached medium or high levels in two other clones. The CpG dinucleotide that occurred in a latently infected clonal cell line 240 nucleotides upstream of the transcription start remained methylated after reactivation of HIV-1 transcription with 10 nM phorbol-12-myristate-13-acetate. In two clones showing a medium promoter activity and in resting memory T cells, the HIV-1 LTR was generally not methylated. Our results show that the methylation profiles of the HIV-1 LTR, including those present in latently infected cells, are low and do not correlate with the transcriptional activity. We suggest that, in a noncloned cellular population in which the HIV-1 proviruses are randomly integrated in the human genome, HIV-1 latency is imperfectly controlled by CpG methylation and is inherently accompanied by residual replication.

SWI/SNF chromatin remodelling complex and retroviral gene silencing

Because of the unique infectious cycle of retroviruses which involves the integration of the retroviral genome into the host chromosome, many cellular chromosomal proteins are used by the virus to maintain its gene expression. At the same time, cellular mechanisms for the surveillance and exclusion of non-self expression by such intragenomic parasites operate as an important host defence system in the cellular nuclei. Retroviruses have strategies for escaping from host defence systems, such as by maintaining or reactivating viral expression in specific host cell types. Understanding such epigenetical regulation would be essential for progress in retroviral virology. In this review, we emphasise the importance of the chromatin remodelling factor SWI/SNF complex as one of the key players in epigenetic regulation of host and viral gene expression. An understanding of these mechanisms will surely lead to new ideas on the pathogenicity of this virus, on the latent infection observed in many other viruses, and further forward the design of unique retroviral vectors for long-term transgene expression, providing strong tools for human gene therapy and regenerative medicine.

Retroviral gene transfer to human epidermal keratinocytes correlates with integrin expression and is significantly enhanced on fibronectin

Human epidermal keratinocytes are an important target for gene therapy because they can be easily expanded in culture and used to generate skin substitutes for the treatment of wounds, genetic diseases of the skin, and for delivery of proteins to the systemic circulation. Although retroviral transduction results in permanent genetic modification, differentiation and loss of transduced cells from the epidermis results in temporary transgene expression. To ensure permanent genetic modification, epidermal stem cells must be transduced with high efficiency. We evaluated gene transfer on two different substrates and found that the efficiency of gene transfer is substantially higher on a substrate of recombinant fibronectin (FN), when compared to tissue culture plastic (TCP). The rate of retroviral transduction on FN is four times faster than transduction on tissue culture plates and is independent of polybrene (PB). The transduction efficiency correlates with the levels of expression of integrin subunits alpha5, alpha2, and beta1, which have been shown to correlate with stem cell phenotype. Notably, cells that adhere rapidly to FN are transduced more efficiently than slowly adherent cells. In addition, integrin-blocking antibodies decrease the efficiency of gene transfer in a dose-dependent manner. Our results suggest that FN may enhance retroviral gene transfer to the least differentiated cells, thereby increasing the potential of genetically modified keratinocytes to treat short- and long-term disease states.

Mechanisms that regulate silencing of gene expression from retroviral vectors

The propensity of retroviruses toward transcriptional silencing limits their value as gene therapy vectors. Silencing has been shown to be particularly robust when stem cells are used for transduction, posing a significant problem for gene therapy of hematologic diseases. Stability of proviral expression with newer generation vectors is significantly improved over that obtainable with original vectors based on Moloney murine leukemia virus (MoMLV). However, strategies to increase resistance further to retroviral silencing are needed, because newer generation vectors have been shown to remain prone to a significant degree of silencing that could limit their efficacy as gene therapy vectors. Proviral silencing has been attributed to known mechanisms of cellular gene repression, such as DNA methylation and histone modification, as well as uncharacterized mechanisms that act independently of DNA methylation. A further understanding of transcriptional silencing that occurs in stem cells and during hematopoietic development is needed for design of effective vectors for gene therapy of hematologic diseases.

Maintenance of integrated proviral gene expression requires Brm, a catalytic subunit of SWI/SNF complex

We show here that murine leukemia virus-based retrovirus vector transgene expression is rapidly silenced in human tumor cell lines lacking expression of Brm, a catalytic subunit of the SWI/SNF chromatin remodeling complex, even though these vectors can successfully enter, integrate, and initiate transcription. We detected this gene silencing as a reduction in the ratio of cells expressing the exogenous gene rather than a reduction in the average expression levels, indicating that down-regulation occurs in an all-or-none manner. Retroviral gene expression was protected from silencing and maintained in Brm-deficient host cells by exogenous expression of Brm but not BRG1, an alternative ATPase subunit in the SWI/SNF complex. Introduction of exogenous Brm to these cells suppressed recruitment of protein complexes containing YY1 and histone deacetylase (HDAC) 1 and 2 to the 5'-long terminal repeat region of the integrated provirus, leading to the enhancement of acetylation of specific lysine residues in histone H4 located in this region. Consistent with these observations, treatment of Brm-deficient cells with HDAC inhibitors but not DNA methylation inhibitors suppressed retroviral gene silencing. These results suggest that the Brm-containing SWI/SNF complex subfamily (trithorax-G) and a complex including YY1 and HDACs (Polycomb-G) counteract each other to maintain transcription of exogenously introduced genes.

Drug selection with paclitaxel restores expression of linked IL-2 receptor gamma -chain and multidrug resistance (MDR1) transgenes in canine bone marrow

Unstable expression of transferred genes is a major obstacle to successful gene therapy of hematopoietic diseases. We have investigated in a canine large-animal model whether expression of transduced genes can be recovered in vivo. Mixed-breed dogs had undergone autologous bone marrow transplantation (BMT) with stem cell factor and granulocyte-colony-stimulating factor-mobilized retrovirally marked hematopoietic cells. The bicistronic retroviral vector construct allowed for coexpression of MDR1 and human IL-2 receptor common gamma-chain cDNAs. The latter gene is deficient in X-linked severe combined immunodeficiency. After initial high-level expression, P-glycoprotein and the gamma-chain were undetectable in blood and bone marrow 17 months post-BMT. Six months later, one dog was treated i.v. with 125 mg/m2 paclitaxel. Three administrations restored expression of the two linked genes to high levels in blood and bone marrow. Two dogs treated with higher paclitaxel doses died from myelosuppression after the first administration. As determined by flow cytometry, both genes were expressed in granulocytes, monocytes, and lymphocytes of the surviving animal. PCR analysis of DNA from peripheral blood confirmed that the retroviral cDNA was increased after paclitaxel treatment, suggesting enrichment of transduced cells. P-glycoprotein was detectable for more than 1 year after cessation of paclitaxel. Repeated analyses of blood and bone marrow aspirates gave no indication of hematopoietic disturbance after BMT with transduced cells and paclitaxel treatment. In summary, we have shown that with the use of a drug-selectable marker gene, chemotherapy can select for cells that express an otherwise nonselected therapeutic gene in blood and bone marrow.

Therapeutic levels of human factor VIII in mice implanted with encapsulated cells: potential for gene therapy of haemophilia A

BACKGROUND

A gene therapy delivery system based on microcapsules enclosing recombinant cells engineered to secrete a therapeutic protein has been evaluated. The microcapsules are implanted intraperitoneally. In order to prevent cell immune rejection, cells are enclosed in non-antigenic biocompatible alginate microcapsules prior to their implantation into mice. It has been shown that encapsulated myoblasts can deliver therapeutic levels of Factor IX (FIX) in mice. The delivery of human Factor VIII (hFVIII) in mice using microcapsules was evaluated in this study.

METHODS

Mouse C2C12 myoblasts and canine MDCK epithelial kidney cells were transduced with MFG-FVIII (B-domain deleted) vector. Selected recombinant clones were enclosed in alginate microcapsules. Encapsulated recombinant clones were subsequently implanted intraperitoneally into C57BL/6 and immunodeficient SCID mice.

RESULTS

Plasma of mice receiving C2C12 and encapsulated MDCK cells had transient therapeutic levels of FVIII in immunocompetent C57BL/6 mice (up to 20% and 7% of physiological levels, respectively). In addition, FVIII delivery in SCID mice was also transient, suggesting that a non-immune mechanism must have contributed to the decline of hFVIII in plasma. Quantitative RT-PCR analysis confirmed directly that the decline of hFVIII is due to a reduction in steady-state hFVIII mRNA, consistent with transcriptional repression. Furthermore, encapsulated cells retrieved from implanted mice were viable, but secreted FVIII ex vivo at three-fold lower levels than the pre-implantation levels. In addition, antibodies to hFVIII were detected in immunocompetent C57BL/6 mice.

CONCLUSIONS

Implantable microcapsules can deliver therapeutic levels of FVIII in mice, suggesting the potential of this gene therapy approach for haemophilia A. The findings suggest vector down-regulation in vivo.

Robust and efficient regulation of transgene expression in vivo by improved tetracycline-dependent lentiviral vectors

We developed a panel of lentiviral vectors that displayed tetracycline-regulated transgene expression over two orders of magnitude in bulk, non-selected populations of transduced cells in vitro and in vivo. The robust expression and homogeneous response indicated that most transduced vector genomes were transcription competent and responsive to regulation, providing the lentiviral vector with a novel competitive advantage for gene transfer. After ex vivo transduction and transplantation of cord blood CD34+ cells into NOD/SCID mice, reporter gene expression could be switched "on" and "off" in human hematopoietic cells in vivo for prolonged times, proving integration of the regulated expression system into long-term repopulating cells. By vector injection into established tumor grafts, we achieved efficient delivery and quantitative regulation of transgene expression in vivo. By these approaches, gene function studies can now be performed in in vivo models of human hematopoiesis and cancer. In the future, regulated lentiviral vectors will improve the safety and efficacy of gene therapy.

Lentiviral vector-mediated transduction of murine CD34(-) hematopoietic stem cells

OBJECTIVE

Efficient gene transfer into murine hematopoietic stem cells (HSCs) provides a powerful tool for exploring hematopoietic stem cell biology. In this study, we evaluated the efficiency of lentiviral vector-mediated gene transfer into murine CD34(-/low)c-Kit(+)Sca-1(+)Lin(-) (CD34(-) KSL) cells that are highly enriched for HSCs.

MATERIALS AND METHODS

FACS-sorted CD34(-) KSL cells were transduced with the vesicular stomatitis virus G glycoprotein-pseudotyped HIV-1-based lentiviral vector containing the green fluorescent protein (GFP) gene under the control of the cytomegalovirus promoter, and then 50 transduced cells were transplanted into lethally irradiated mice. Transduction efficiency was assessed by FACS analysis for GFP expression in peripheral blood (PB) cells. FACS-sorted GFP(+) KSL bone marrow (BM) cells from primary recipients were used for secondary transplantation, and GFP expression in PB cells of reconstituted mice was analyzed by FACS.

RESULTS

GFP expression was detected in PB cells of all primary recipients (n = 10) at an average of 40% (range 26-58%) when the lentiviral vector containing the woodchuck hepatitis virus posttranscriptional regulatory element was used. GFP(+) cells were found in multilineage cells in PB, BM, spleen, and thymus for at least 8 months posttransplantation. In secondary recipients, donor-derived GFP(+) KSL BM cells could reconstitute and GFP expression was detected in both myeloid and lymphoid cells in PB.

CONCLUSION

Our results indicate that lentiviral vectors can efficiently transduce highly enriched murine HSCs and sustain long-term expression of the transgene in the multilineage differentiated progeny in reconstituted mice.

Evaluation of retroviral vector design in defined chromosomal loci by Flp-mediated cassette replacement

Successful retroviral vector construction is still empirical. Test systems for vector efficiency are based on statistical comparison of numerous infectants with single proviral integrates, since their expression depends on the chromosomal surroundings. More reliable data would be obtained if different vector constructs were studied in an identical chromosomal context. Here, we demonstrate the use of a new method, in which chromosomal sites are provirally tagged in such a way that they can be targeted with other expression cassettes. The original tagging integrate is replaced in one step by the targeting element. This permits a reliable comparison of different retroviral vector configurations, eliminating the influence of neighboring chromosomal elements. We compared different retroviral vector types for coexpression of two genes: a vector containing an internal promoter and a vector with an internal ribosome entry site (IRES) element. In contrast to bicistronic retroviral vectors, dual-promoter proviruses exhibited rapid inactivation of the long terminal repeat (LTR)-driven gene expression. Targeted exchange of the dual-promoter provirus with a bicistronic retroviral cassette resulted in gain of expression stability. The reverse experiment confirmed this promoter interaction phenomenon since initial expression stability from a single-promoter bicistronic provirus was lost by targeted exchange with a dual-promoter cassette. In addition, targeting exchange of the dual-promoter provirus, replacing the LTR with an artificial (Tet) promoter restored expression stability. These observations, valid for various integration sites, prove the strong interaction between the LTR and the internal promoter. Our results have implications for retroviral vector design and suggest that retroviral coexpression of two genes is more predictable in the bicistronic configuration.

Locus control region of the human CD2 gene in a lentivirus vector confers position-independent transgene expression

Vectors derived from murine leukemia virus (MLV) have been used in many human gene therapy clinical trials. However, insertion of the locus control regions (LCRs) derived from the beta-globin gene locus or the CD2 gene into MLV vectors frequently led to vector rearrangement. Since the human immunodeficiency virus (HIV) sequence diverges significantly from the MLV sequence, we tested whether the LCR sequence is more stable in the context of an HIV vector. Clones derived from human fibrosarcoma line HT1080 cells transduced with an HIV vector containing the T-cell-specific CD2 LCR exhibit the same wide range of transgene expression as clones lacking the LCR. In contrast, Jurkat and primary T-cell clones derived from the transduction of the LCR-containing vector show, on average, a three- to fourfold increase in transgene expression relative to that of the control vector. This is consistent with previous observations that the CD2 LCR contains a T-cell-specific enhancer. In addition, the clones derived from the LCR-containing vector have a much lower clonal variation in transgene expression than those derived from the control vector. We also demonstrate that the level of transgene expression is proportional to the vector copy number. These results suggest that the human CD2 LCR sequence is compatible with HIV vector sequences and confers enhanced integration site-independent and copy number-dependent expression of the transgene. Thus, HIV vectors may represent the ideal vehicle to deliver genes controlled by various cis-acting elements such as LCRs.

The site of HIV-1 integration in the human genome determines basal transcriptional activity and response to Tat transactivation

Because of the heterogeneity of chromatin, the site of integration of human immunodeficiency virus (HIV) in the genome could have dramatic effects on its transcriptional activity. We have used an HIV-1-derived retroviral vector, in which the green fluorescent protein is under the control of the HIV promoter, to generate by infection 34 Jurkat clonal cell lines each containing a single integration of the HIV-1 vector. In the absence of Tat, a 75-fold difference in expression level between the highest and lowest expressing clones was observed. Basal promoter activity was low in 80% of the clones and moderate to high in the remaining 20% of clones. We found that differences in expression levels are due to the integration site and are not controlled by DNA methylation or histone acetylation. Tat activated transcription in each clone, and an inverse correlation was observed between basal transcriptional activity and inducibility by Tat. These observations demonstrate that the chromatin environment influences basal HIV gene expression and that the HIV Tat protein activates transcription independently of the chromatin environment.

In vivo demethylation of a MoMuLV retroviral vector expressing the herpes simplex thymidine kinase suicide gene by 5' azacytidine

We constructed a functional MoMuLV-based bicistronic retroviral vector encoding the herpes simplex virus type I thymidine kinase gene, which induces sensitivity to the prodrug ganciclovir (gcv), and the reporter beta-galactosidase gene (MFG-tk-IRES-lacZ). The U937 histiocytic cell line was transduced with this vector, and a clone (VB71) with high-level transgene expression was selected. Severe combined immunodeficient (SCID) mice were injected with VB71 cells to evaluate the role of long terminal repeat methylation in transgene silencing in vivo and to see whether 5-azacytidine (5' aza-C) demethylating agent prevented it. We found 5' aza-C maintained gene expression at high level in vitro. In vivo, time to tumor onset was significantly longer in SCID mice receiving the VB71 cells, 5' aza-C, and gcv compared with animals treated with either 5' aza-C or gcv alone. The number of injected tumor cells influences tumor onset time and the efficacy of 5' aza-C and gcv treatment. The standard gcv treatment schedule (10 mg/kg from d + 1 until the onset of tumor) controlled tumor onset better than short-term treatment with high doses. In conclusion, the results extend our previous findings that transgene methylation in vivo may be prevented with an appropriate schedule of 5' aza-C and gcv.

Gene therapy for hemophilia

Hemophilia A and B are X-chromosome linked recessive bleeding disorders that result from a deficiency in factor VIII (FVIII) and factor IX (FIX) respectively. Though factor substitution therapy has greatly improved the lives of hemophiliac patients, there are still limitations to the current treatment that have triggered interest in alternative treatments by gene therapy. Significant progress has recently been made in the development of gene therapy for the treatment of hemophilia A and B. These advances parallel the technical improvements of existing vector systems including MoMLV-based retroviral, adenoviral and AAV vectors, and the development of new delivery methods such as lentiviral vectors, helper-dependent adenoviral vectors and improved non-viral gene delivery methods. Therapeutic and physiologic levels of FVIII and FIX could be achieved in FVIII- and FIX-deficient mice and hemophilia dogs by different gene therapy approaches. Long-term correction of the bleeding disorders and in some cases a permanent cure has been realized in these preclinical studies. However, the induction of neutralizing antibodies often precludes stable phenotypic correction. Another complication is that certain promoters are prone to transcriptional inactivation in vivo, precluding long-term FVIII or FIX expression. Several gene therapy phase I clinical trials are currently ongoing in patients suffering from severe hemophilia A or B. No significant adverse side-effects were reported, and semen samples were negative for vector sequences by sensitive PCR assays. Most importantly, some subjects report fewer bleeding episodes and occasionally have very low levels of clotting factor activity detected. The results from the extensive preclinical studies in normal and hemophilic animal models and encouraging preliminary clinical data indicate that the simultaneous development of different strategies is likely to bring a permanent cure for hemophilia one step closer to reality.

T cell-specific expression from Mo-MLV retroviral vectors containing a CD4 mini-promoter/enhancer

BACKGROUND

Gene therapy of various immunological disorders will greatly benefit from improved retroviral vectors (RVs) with T cell specificity. Such vectors can be designed by placing a gene of therapeutic interest under the control of tissue-specific transcriptional elements. However, low titers and loss of specificity are frequently encountered with tissue-specific vectors. The aim of the present study was to develop a T cell-specific RV.

METHODS

We constructed a series of Moloney murine leukemia virus (Mo-MLV)-based RVs expressing enhanced green fluorescent protein (EGFP) under the control of a mini-promoter/enhancer cassette derived from the CD4 gene (CD4pmE) and tested them in cell lines and peripheral blood lymphocytes. Expression of EGFP was monitored by fluorescence microscopy and analyzed by flow cytometry.

RESULTS

The CD4pmE cassette was inserted between the viral long terminal repeats (LTRs) in self-inactivating vectors (SIN vectors) or was substituted to the 3' U3 viral promoter/enhancer (hybrid vectors). High vector titers but poor specific expression of EGFP were achieved when CD4pmE was inserted in sense orientation in SIN vectors. Low titers but high specificity were observed when the CD4pmE cassette was in anti-sense orientation. In contrast, high titers and good T cell specificity were obtained with hybrid vectors.

CONCLUSION

An efficient T cell-specific retroviral vector was obtained.

Fibroblasts and dermal gene therapy: a minireview

This review highlights our current understanding of the biology of, survival of, and transgene expression by genetically modified fibroblasts (GMFb) carrying stably integrated transgenes in vivo. Experimental data demonstrate that three elements will enhance expression by and survival of GMFb in vivo: a matrix scaffolding to take the place of the existing dermis, the presence of elements of the extracellular matrix in the construct used to move GMFb to the in vivo setting, and the utilization of immortalized fibroblasts to carry the transgenes. Although moving GMFb to an in vivo setting is an invasive procedure, there are a number of clinical settings where GMFb appear to be the suitable cell for gene therapy.

Retroviral expression in embryonic stem cells and hematopoietic stem cells

Achieving long-term retroviral expression in primary cells has been problematic. De novo DNA methylation of infecting proviruses has been proposed as a major cause of this transcriptional repression. Here we report the development of a mouse stem cell virus (MSCV) long terminal repeat-based retroviral vector that is expressed in both embryonic stem (ES) cells and hematopoietic stem (HS) cells. Infected HS cells and their differentiated descendants maintained long-term and stable retroviral expression after serial adoptive transfers. In addition, retrovirally infected ES cells showed detectable expression level of the green fluorescent protein (GFP). Moreover, GFP expression of integrated proviruses was maintained after in vitro differentiation of infected ES cells. Long-term passage of infected ES cells resulted in methylation-mediated silencing, while short-term expression was methylation independent. Tissues of transgenic animals, which we derived from ES cells carrying the MSCV-based provirus, did not express GFP. However, treatment with the demethylating agent 5-azadeoxycytidine reactivated the silent provirus, demonstrating that DNA methylation is involved in the maintenance of retroviral repression. Our results indicate that retroviral expression in ES cells is repressed by methylation-dependent as well as methylation-independent mechanisms.

Variability of transcriptional regulation after gene transfer with the retroviral tetracycline system

Inducible transcription and position-independent expression are critical issues after gene transfer. To gain insight into the amount of variability of transcriptional regulation due to random proviral integration, we analyzed a total of 200 C6 glioma and rat-1 fibroblast clones retrovirally infected with the conventional and reverse tet systems where a luciferase reporter gene was placed under control of a tetracycline-responsive promoter. Repressed luciferase activities differed by up to 81000-fold among individual clones. Repressed activities close to baseline levels were observed in eight clones, all of them transduced with the conventional tet system. Regulation factors ranged from less than two-fold (indicating absence of regulation), observed in 17 clones to 90-fold. Regulation was higher with the conventional tet system as compared with the reverse tet system. Our data show that even under these standardized conditions there was a very high variability in absolute expression levels and regulability between individual clones, and they suggest that homogeneous transcriptional regulation in a cellular population remains a challenge for research in biotechnology.

Inactivation of a GFP retrovirus occurs at multiple levels in long-term repopulating stem cells and their differentiated progeny

Hematopoietic stem cell gene therapy holds promise for the treatment of many hematologic disorders. One major variable that has limited the overall success of gene therapy to date is the lack of sustained gene expression from viral vectors in transduced stem cell populations. To understand the basis for reduced gene expression at a single-cell level, we have used a murine retroviral vector, MFG, that expresses the green fluorescent protein (GFP) to transduce purified populations of long-term self-renewing hematopoietic stem cells (LT-HSC) isolated using the fluorescence-activated cell sorter. Limiting dilution reconstitution of lethally irradiated recipient mice with 100% transduced, GFP+ LT-HSC showed that silencing of gene expression occurred rapidly in most integration events at the LT-HSC level, irrespective of the initial levels of GFP expression. When inactivation occurred at the LT-HSC level, there was no GFP expression in any hematopoietic lineage clonally derived from silenced LT-HSC. Inactivation downstream of LT-HSC that stably expressed GFPin long-term reconstituted animals was restricted primarily to lymphoid cells. These observations suggest at least 2 distinct mechanisms of silencing retrovirally expressed genes in hematopoietic cells.

Inactivation of a GFP retrovirus occurs at multiple levels in long-term repopulating stem cells and their differentiated progeny

Hematopoietic stem cell gene therapy holds promise for the treatment of many hematologic disorders. One major variable that has limited the overall success of gene therapy to date is the lack of sustained gene expression from viral vectors in transduced stem cell populations. To understand the basis for reduced gene expression at a single-cell level, we have used a murine retroviral vector, MFG, that expresses the green fluorescent protein (GFP) to transduce purified populations of long-term self-renewing hematopoietic stem cells (LT-HSC) isolated using the fluorescence-activated cell sorter. Limiting dilution reconstitution of lethally irradiated recipient mice with 100% transduced, GFP+ LT-HSC showed that silencing of gene expression occurred rapidly in most integration events at the LT-HSC level, irrespective of the initial levels of GFP expression. When inactivation occurred at the LT-HSC level, there was no GFP expression in any hematopoietic lineage clonally derived from silenced LT-HSC. Inactivation downstream of LT-HSC that stably expressed GFPin long-term reconstituted animals was restricted primarily to lymphoid cells. These observations suggest at least 2 distinct mechanisms of silencing retrovirally expressed genes in hematopoietic cells.

Transcriptional properties of genomic transgene integration sites marked by electroporation or retroviral infection

As a possible consequence of their survival strategy, proviruses are predominantly found in transcription-promoting genomic sites. For certain applications, these findings have led to the preferential use of retroviral vectors for the stable integration of transgenes. This study demonstrates that transcription levels of single-copy proviruses, which have been established either by infection or by single-copy transfection (electroporation), are rather comparable. Therefore, electroporation is suggested as an alternative gene transfer route in cases where the use of infectious retroviral vehicles is to be avoided due to safety considerations. A difference between clones derived from these two gene transfer routes concerns the inactivation pattern which, for electroporated clones, is an exclusive property of the low expressers. This difference may be due to the nature of the illegitimate recombination event which is thought to be less invasive if catalyzed by the retroviral integrase. Substantial differences between infection and Ca phosphate-mediated transfection that have been reported earlier are explained by the respective transfection parameters.

Optimization of regulated LTR-mediated expression

Retroviral vectors are ideally suited to the study of gene function, allowing efficient, stable expression. Many biological systems (e.g., cell cycle, apoptosis) require the use of regulated expression systems. We therefore developed a regulated retroviral vector system, TRA99, based on a tetracycline transactivator-dependent LTR, where the MMLV enhancer was replaced with a tetracycline-response element. Using fluorescence-activated flow cytometric analysis of a destabilized green fluorescent protein to monitor expression levels, we optimized the minimal promoter configuration with respect to both activated and repressed transcription. The TRA99 vectors demonstrate regulated expression with activated levels comparable to those of standard retroviral vectors and repressed levels indistinguishable from background. This was achieved without using an internal promoter cassette, thus retaining the cis-packaging elements requisite for helper-mediated transfer.