Generating Transgenic Mice by Lentiviral Transduction of Spermatozoa Followed by In Vitro Fertilization and Embryo Transfer

Most transgenic technologies rely on the oocyte as a substrate for genetic modification. Transgenics animals are usually generated by the injection of the gene constructs (including lentiviruses encoding gene constructs or modified embryonic stem cells) into the pronucleus of a fertilized egg followed by the transfer of the injected embryos into the uterus of a foster mother. Male germ cells also have potential as templates for transgenic development. We have previously shown that mature sperm can be utilized as template for lentiviral transduction and as such used to generate transgenic mice efficiently with germ line capabilities. We provide here a detailed protocol that is relatively simple, to establish transgenic mice using lentivirally transduced spermatozoa. This protocol employs a well-established lentiviral gene delivery system (usual for somatic cells) delivering a variety of transgenes to be directly used with sperm, and the subsequent use of these modified sperm in in vitro fertilization studies and embryo transfer into foster female mice, for the establishment of transgenic mice.

Dyskeratosis congenita and the DNA damage response

Dyskeratosis congenita (DC) is a heterogeneous bone marrow failure disorder with known mutations in components of telomerase and telomere shelterin. Recent work in a mouse model with a dyskerin mutation has implicated an increased DNA damage response as part of the cellular pathology, while mouse models with Terc and Tert mutations displayed a normal response. To clarify how these contradictory results might apply to DC pathology in humans, we studied the cellular phenotype in primary cells from DC patients of several genetic subtypes, focussing on T lymphocytes to remain close to the haematopoietic system. We observed novel cell cycle abnormalities in conjunction with impaired growth and an increase in apoptosis. Using flow cytometry and confocal microscopy we examined induction of the DNA damage proteins γ-H2AX and 53BP1 and the cell cycle protein TP53 (p53). We found an increase in damage foci at telomeres in lymphocytes and an increase in the basal level of DNA damage in fibroblasts, but crucially no increased response to DNA damaging agents in either cell type. As the response to induced DNA damage was normal and levels of global DNA damage were inconsistent between cell types, DNA damage may contribute differently to the pathology in different tissues.

Exogenous TERC alone can enhance proliferative potential, telomerase activity and telomere length in lymphocytes from dyskeratosis congenita patients

Dyskeratosis congenita (DC) is an inherited multi-system disorder characterised by muco-cutaneous abnormalities, bone marrow failure and a predisposition to malignancy. Bone marrow failure is the principal cause of mortality and is thought to be the result of premature cell death in the haematopoietic compartment because DC cells age prematurely and tend to have short telomeres. DC is genetically heterogeneous and patients have mutations in genes that encode components of the telomerase complex (DKC1, TERC, TERT, NOP10 and NHP2), and telomere shelterin complex (TINF2), both important in telomere maintenance. Here, we transduced primary T lymphocytes and B lymphocyte lines established from patients with TERC and DKC1 mutations with wild type TERC-bearing lentiviral vectors. We found that transduction with exogenous TERC alone was capable of increasing telomerase activity in mutant T lymphocytes and B lymphocyte lines and improved the survival and thus overall growth of B-lymphocyte lines over a prolonged period, regardless of their disease mutation. Telomeres in TERC-treated lines were longer than in the untreated cultures. This is the first study of its kind in DC lymphocytes and the first to demonstrate that transduction with TERC alone can improve cell survival and telomere length without the need for exogenous TERT.

Targeted retroviral transduction of c-kit+ hematopoietic cells using novel ligand display technology

Gene therapy for a wide variety of disorders would be greatly enhanced by the development of vectors that could be targeted for gene delivery to specific populations of cells. We describe here high-efficiency targeted transduction based on a novel targeting strategy that exploits the ability of retroviruses to incorporate host cell proteins into the surface of the viral particle as they bud through the plasma membrane. Ecotropic retroviral particles produced in cells engineered to express the membrane-bound form of stem cell factor (mbSCF) transduce both human cell lines and primary cells with high efficiency in a strictly c-kit (SCF receptor)-dependent fashion. The availability of efficient targeted vectors provides a platform for the development of a new generation of therapies using in vivo gene delivery. (Blood. 2004;104: 2697-2703)

Enhancer-deleted retroviral vectors restore high levels of superoxide generation in a mouse model of CGD

BACKGROUND

Retroviral vectors possess many advantages for use in gene therapy protocols, especially within the haematopoietic system. A number of attendant problems, however, still limit their safety in clinical application. The effects of the enhancer present in the retroviral long terminal repeat (LTR) are a major concern for the clinical usage of such vectors, as they can exert a powerful regulatory influence on the genes that surround them.

METHODS

To improve the safety and widen the applicability of retroviral vectors for use in gene therapy we have developed an enhancer-deleted (Delta-LTR) retroviral vector that retained high titre and demonstrated transcriptional activity in myeloid cells.

RESULTS

When used to correct a mouse model of autosomal recessive chronic granulomatous disease, the Delta-LTR vectors gave acceptable levels of gene transfer to mouse bone marrow cells. Evidence for a slight preferential expression in myeloid cells was obtained with all the vectors studied. Nitroblue tetrazolium assay of superoxide generation in mouse bone marrow derived haematopoietic colonies revealed that transduction with Delta-LTR vectors could restore functional NADPH oxidase to cells from these animals. Superoxide assay of peripheral blood confirmed that, although relatively low numbers of cells were transduced, the Delta-LTR vector was capable of reconstituting very high levels of oxidase activity, comparable to that obtained from normal cells.

CONCLUSIONS

The Delta-LTR vector described here could provide the basis for a new generation of retroviral vectors with improved safety.

Growth factor displayed on the surface of retroviral particles without manipulation of envelope proteins is biologically active and can enhance transduction

BACKGROUND

The therapeutic potential of retroviruses can be significantly enhanced by display of specific molecules on the retroviral surface. This has been conventionally achieved by the manipulation of retroviral envelope proteins. In this report we have tested whether the natural budding mechanism of the retrovirus could be exploited to incorporate a specific molecule into the retroviral surface.

METHODS

Retroviral packaging cells were engineered to express the membrane-bound form of human stem cell factor (mbSCF). Surface expression of mbSCF on retroviral packaging cells was confirmed by immunofluorescence and flow cytometry. Incorporation of mbSCF into retroviral particles was demonstrated by virus-binding assay and immunomagnetic capture of virus using antibody to SCF. Retroviral supernatants were tested for activity of the incorporated cytokine by proliferation assays on factor-dependent cells. Amphotropic retrovirus displaying surface mbSCF was used to transduce SCF receptor-positive haematopoietic cells.

RESULTS

Retroviruses incorporating surface SCF showed increased levels of binding to cells (MO7e) expressing the SCF receptor, c-kit. mbSCF displayed on the viral surface retained levels of biological activity comparable with those of soluble recombinant growth factor. Transduction of c-kit-positive target cells with viruses displaying mbSCF showed enhanced levels of transduction in comparison with unmodified viruses.

CONCLUSIONS

Expression of the membrane-bound form of human stem cell factor (mbSCF) on the surface of retroviral packaging cells allows its efficient incorporation into retrovirus particles in a biologically active form, opening up the possibility for the use of retroviral display in many therapeutic areas, such as in gene therapy, drug delivery and in the development of novel vaccines.

A functional ISRE is required for myeloid transcription of the p47phox gene

Expression of p47(phox), a component of the phagocytic NADPH oxidase, is both tissue-specific and developmentally regulated. We have investigated transcription from the p47(phox) gene promoter by reporter gene analysis of myeloid PLB985 cells stably transfected with a series of p47(phox) promoter constructs. Stable transfection with constructs containing up to 3100 bp of proximal promoter sequence demonstrated that as little as 144 bp of proximal promoter sequence was able to direct significant reporter gene activity in myeloid cells, but not in HeLa cells. Mutation of a previously uncharacterised interferon-stimulated response element (ISRE) consensus located at positions -104 to-116, or of an established binding site for the Ets family transcription factor, PU.1 (located at positions -39 to -44), abolished transcription in stably transfected myeloid cells. Electrophoretic mobility shift analysis (EMSA) with myeloid cell nuclear extracts demonstrated that an oligonucleotide containing the p47(phox) ISRE consensus was able to compete binding at another bona fide ISRE. Complexes formed on the p47(phox) ISRE itself were competed by other ISRE consensus sequences. We conclude that transcription of p47(phox) in myeloid cells requires a functional ISRE in addition to the previously identified PU.1 binding site.

Haemophilias: advances towards genetic engineering replacement therapy

Both haemophilia A and B are X-linked recessive disorders and therefore occur almost exclusively in males. The genes for both factors VIII and IX have been mapped to the distal end of the long arm of the X chromosome, bands Xq28 and Xq27.1, respectively. The Factor VIII gene comprises 186 kb DNA with 9 kb of exon of DNA which encodes an mRNA of nearly 9 kb. The Factor IX gene is 34 kb in length and the essential genetic information is present in eight exons which encode 1.6 kb mRNA. In gene therapy, genetic modification of the target cells can be either ex vivo or in vivo. The advantage of the ex vivo approach is that the genetic modification is strictly limited to the isolated cells. In the in vivo approach, the integrity of the target tissue is maintained but the major challenge is to deliver the gene to the target tissue. The use of improved retroviral and adenovirus-based vectors for gene therapy has produced clinically relevant levels of human factor VIII in mice and haemophilic dogs. If further improvements can increase the persistence of expression and decrease the immunological responses, phase I clinical trials in patients can be considered.

Retroviral transduction of quiescent haematopoietic cells using a packaging cell line expressing the membrane-bound form of stem cell factor

Gene therapy vectors based on murine retroviruses are unable to transduce non-dividing cells. This has proven a particular problem in the haematopoietic system where the target cells of choice, the pluripotent stem cells are quiescent. In an attempt to circumvent this difficulty we have constructed a retroviral producer line that expresses the membrane bound form of human recombinant stem cell factor (SCF) on its cell surface. This should enable the retroviral producers to deliver a growth signal to the target cells simultaneous with their exposure to retrovirus. We tested the ability of these modified producers to transduce a growth factor-starved, SCF-dependent cell line (TF-1) and demon- strated that these cells, though quiescent, can still be successfully transduced. This approach was extended to targeting of umbilical cord blood CD34+ cells, a predominantly quiescent population that normally require the addition of cytokines for efficient transduction. Using the SCF-expressing producer line in the absence of exogenously added cytokines, we observed a marked stimulation in transduction efficiency over that achieved using the parent producer line alone. Colonies derived from these cells arising in semi-solid media were also shown to be positive for expression of a retrovirally encoded reporter gene.

Enhanced retroviral transduction of 5-fluorouracil-resistant human bone marrow (stem) cells using a genetically modified packaging cell line

Pluripotent hematopoietic stem cells (PHSC) are rare cells capable of multilineage differentiation, long-term reconstituting activity and extensive self-renewal. Such cells are the logical targets for many forms of corrective gene therapy, but are poor targets for retroviral mediated gene transfer owing to their quiescence, as retroviral transduction requires that the target cells be cycling. To try and surmount this problem we have constructed a retroviral producer line that expresses the membrane-bound form of human stem cell factor (SCF) on its cell surface. These cells are capable, therefore, of delivering a growth signal concomitant with recombinant retroviral vector particles. In this report we describe the use of this cell line to transduce a highly quiescent population of cells isolated from adult human bone marrow using the 5-fluorouracil (FU) resistance technique of Berardi et al. Quiescent cells selected using this technique were transduced by cocultivation with retroviral producers expressing surface bound SCF or with the parent cell line that does not. Following coculture, the cells were plated in long-term bone marrow culture for a further 5 weeks, before plating the nonadherent cells in semisolid media. Colonies forming in the semisolid media over the next 14 days were analyzed by polymerase chain reaction for the presence of the retroviral vector genome. Over six experiments, the transduction frequency of the quiescent 5-FU resistant cells using the SCF-expressing producer line averaged about 20%, whereas those transduced using the parent producer line showed evidence of reduced levels or no transduction.

Restoration of superoxide generation to a chronic granulomatous disease-derived B-cell line by retrovirus mediated gene transfer

Failure of a superoxide generating system, the NADPH oxidase, present in neutrophils and other phagocytes gives rise to chronic granulomatous disease (CGD), a group of single-gene inherited disorders all characterized by an extreme susceptibility to pyogenic infection, with potentially fatal consequences. About 30% of CGD cases are caused by an autosomally inherited deficiency of a 47-Kd cytoplasmic component of the oxidase (p47-phox). Epstein-Barr virus (EBV) immortalized B-lymphocyte lines established from these CGD patients also express this NADPH oxidase defect and consequently are rendered incapable of generating superoxide on stimulation. We have used a p47-phox-deficient EBV-transformed B-cell line as a recipient for retroviral transfer of a functional p47-phox cDNA. The presence and activity of the retrovirally encoded p47-phox in the transduced cells is demonstrated and we show that this restores their capacity to generate superoxide.

Identification of the defective NADPH-oxidase component in chronic granulomatous disease: a study of 57 European families

Chronic Granulomatous Disease (CGD) manifests as a predisposition to infection as a result of defective function of the NADPH oxidase of phagocytic cells. Proteins identified as part of this system include two subunits of a cytochrome b (cytochrome b-245) and two cytosolic factors. The affected oxidase component was determined in 63 CGD patients from 57 families, by Western blotting of extracts of their neutrophils with antibodies to those proteins. 38 (67%) of the families were X-linked with a defect of the beta subunit of the cytochrome. 13 (23%) lacked p47-phox, 3 (5%) p67-phox, and 3 (5%) the alpha subunit of the cytochrome.

Post-transcriptional regulation of the chicken thymidine kinase gene

In attempting to understand the molecular basis of the control of chicken thymidine kinase (cTK) gene expression, we have examined the steady state cTK RNA content, and the patterns of DNA methylation, chromatin structure and endogenous nuclear runoff transcription of this gene in dividing and non-dividing cells. Our results reveal that the steady state level of cTK poly A+ RNA is correlated with the divisional activity of normal avian cells and tissues. However, no differences in the pattern of Hpa II site methylation or chromatin structure are found among cells containing high or undetectable levels of steady state cTK RNA. In addition, no differences in cTK transcription as assayed by nuclear runoff experiments are detectable in isolated nuclei derived from dividing or non-dividing cells containing high or low levels of steady state cTK RNA. These results suggest that the principal control of chicken thymidine kinase gene expression is post-transcriptional in nature.