Biosafety challenges for use of lentiviral vectors in gene therapy

Lentiviral vectors are promising tools for the genetic modification of cells in biomedical research and gene therapy. Their use in recent clinical trials for the treatment of adrenoleukodystrophy, β-thalassemia, Wiskott-Aldrich- Syndrome and metachromatic leukodystrophy underlined their efficacy for therapies especially in case of hereditary diseases. In comparison to gammaretroviral LTR-driven vectors, which were employed in the first clinical trials, lentiviral vectors present with some favorable features like the ability to transduce also non-dividing cells and a potentially safer insertion profile. However, genetic modification with viral vectors in general and stable integration of the therapeutic gene into the host cell genome bear concerns with respect to different levels of personal or environmental safety. Among them, insertional mutagenesis by enhancer mediated dysregulation of neighboring genes or aberrant splicing is still the biggest concern. However, also risks like immunogenicity of vector particles, the phenotoxicity of the transgene and potential vertical or horizontal transmission by replication competent retroviruses need to be taken into account. This review will give an overview on biosafety aspects that are relevant to the use of lentiviral vectors for genetic modification and gene therapy. Furthermore, assay systems aiming at evaluating biosafety in preclinical settings and recent promising clinical trials including efforts of monitoring of patients after gene therapy will be discussed.

Generation of iPSCs from genetically corrected Brca2 hypomorphic cells: implications in cell reprogramming and stem cell therapy

Fanconi anemia (FA) is a complex genetic disease associated with a defective DNA repair pathway known as the FA pathway. In contrast to many other FA proteins, BRCA2 participates downstream in this pathway and has a critical role in homology-directed recombination (HDR). In our current studies, we have observed an extremely low reprogramming efficiency in cells with a hypomorphic mutation in Brca2 (Brca2(Δ) (27/) (Δ27)), that was associated with increased apoptosis and defective generation of nuclear RAD51 foci during the reprogramming process. Gene complementation facilitated the generation of Brca2(Δ) (27/) (Δ27) induced pluripotent stem cells (iPSCs) with a disease-free FA phenotype. Karyotype analyses and comparative genome hybridization arrays of complemented Brca2(Δ) (27/) (Δ27) iPSCs showed, however, the presence of different genetic alterations in these cells, most of which were not evident in their parental Brca2(Δ) (27/) (Δ27) mouse embryonic fibroblasts. Gene-corrected Brca2(Δ) (27/) (Δ27) iPSCs could be differentiated in vitro toward the hematopoietic lineage, although with a more limited efficacy than WT iPSCs or mouse embryonic stem cells, and did not engraft in irradiated Brca2(Δ) (27/) (Δ27) recipients. Our results are consistent with previous studies proposing that HDR is critical for cell reprogramming and demonstrate that reprogramming defects characteristic of Brca2 mutant cells can be efficiently overcome by gene complementation. Finally, based on analysis of the phenotype, genetic stability, and hematopoietic differentiation potential of gene-corrected Brca2(Δ) (27/) (Δ) (27) iPSCs, achievements and limitations in the application of current reprogramming approaches in hematopoietic stem cell therapy are also discussed.

Gene therapy cures the anemia and lethal bone marrow failure in a mouse model of RPS19-deficient Diamond-Blackfan anemia

Diamond-Blackfan anemia is a congenital erythroid hypoplasia caused by functional haploinsufficiency of genes encoding ribosomal proteins. Mutations involving the ribosomal protein S19 gene are detected in 25% of patients. Enforced expression of ribosomal protein S19 improves the overall proliferative capacity, erythroid colony-forming potential and erythroid differentiation of hematopoietic progenitors from ribosomal protein S19-deficient patients in vitro and in vivo following xenotransplantation. However, studies using animal models are needed to assess the therapeutic efficacy and safety of the viral vectors. In the present study we have validated the therapeutic potential of gene therapy using mouse models of ribosomal protein S19-deficient Diamond-Blackfan anemia. Using lentiviral gene transfer we demonstrated that enforced expression of ribosomal protein S19 cures the anemia and lethal bone marrow failure in recipients transplanted with ribosomal protein S19-deficient cells. Furthermore, gene-corrected ribosomal protein S19-deficient cells showed an increased pan-hematopoietic contribution over time compared to untransduced cells without signs of vector-mediated toxicity. Our study provides a proof of principle for the development of clinical gene therapy to cure ribosomal protein 19-deficient Diamond-Blackfan anemia.

Efficiency and safety of O⁶-methylguanine DNA methyltransferase (MGMT(P140K))-mediated in vivo selection in a humanized mouse model

Efficient O⁶-methylguanine DNA methyltransferase (MGMT(P140K))-mediated myeloprotection and in vivo selection have been demonstrated in numerous animal models and most recently in a phase I clinical study in glioblastoma patients. However, this strategy may augment the genotoxic risk of integrating vectors because of chemotherapy-induced DNA damage and the proliferative stress exerted during the in vivo selection. Thus, to improve the safety of the procedure, we evaluated a self-inactivating lentiviral MGMT(P140K) vector for transduction of human cord blood-derived CD34⁺ cells followed by transplantation of the cells into NOD/LtSz-scid/Il2rγ⁻/⁻ mice. These experiments demonstrated significant and stable enrichment of MGMT(P140K) transgenic human cells in the murine peripheral blood and bone marrow. Clonal inventory analysis utilizing linear amplification-mediated polymerase chain reaction and high-throughput sequencing revealed a characteristic lentiviral integration profile. Among the bone marrow insertions retrieved, we observed considerable overlap to previous MGMT(P140K) preclinical models or the clinical study. However, no significant differences between our chemotherapy-treated and nontreated cohorts were observed. This also hold true when specific cancer gene databases and a functional annotation of hit genes by the Panther Database with respect to molecular function, biological process, or cellular component were assessed. Thus, in summary, our data demonstrate efficient and long-term in vivo selection without overt hematological abnormalities using the lentiviral MGMT(P140K) vector. Furthermore, the study introduces humanized mouse models as a novel tool for the pre-clinical assessment of human gene therapy related toxicity.

From bench to bedside: preclinical evaluation of a self-inactivating gammaretroviral vector for the gene therapy of X-linked chronic granulomatous disease

Chronic granulomatous disease (CGD) is a primary immunodeficiency characterized by impaired antimicrobial activity in phagocytic cells. As a monogenic disease affecting the hematopoietic system, CGD is amenable to gene therapy. Indeed in a phase I/II clinical trial, we demonstrated a transient resolution of bacterial and fungal infections. However, the therapeutic benefit was compromised by the occurrence of clonal dominance and malignant transformation demanding alternative vectors with equal efficacy but safety-improved features. In this work we have developed and tested a self-inactivating (SIN) gammaretroviral vector (SINfes.gp91s) containing a codon-optimized transgene (gp91(phox)) under the transcriptional control of a myeloid promoter for the gene therapy of the X-linked form of CGD (X-CGD). Gene-corrected cells protected X-CGD mice from Aspergillus fumigatus challenge at low vector copy numbers. Moreover, the SINfes.gp91s vector generates substantial amounts of superoxide in human cells transplanted into immunodeficient mice. In vitro genotoxicity assays and longitudinal high-throughput integration site analysis in transplanted mice comprising primary and secondary animals for 11 months revealed a safe integration site profile with no signs of clonal dominance.

Hepatic lentiviral gene transfer is associated with clonal selection, but not with tumor formation in serially transplanted rodents

Lentiviral (LV) vectors are promising tools for long-term genetic correction of hereditary diseases. In hematopoietic stem cell gene therapies adverse events in patients due to vector integration-associated genotoxicity have been observed. Only a few studies have explored the potential risks of LV gene therapy targeting the liver. To analyze hepatic genotoxicity in vivo, we transferred the fumarylacetoacetate hydrolase (FAH) gene by LV vectors into FAH((-/-)) mice (n = 97) and performed serial hepatocyte transplantations (four generations). The integration profile (4,349 mapped insertions) of the LV vectors was assessed by ligation-mediated polymerase chain reaction and deep sequencing. We tested whether the polyclonality of vector insertions was maintained in serially transplanted mice, linked the integration sites to global hepatocyte gene expression, and investigated the effects of LV liver gene therapy on the survival of the animals. The lifespan of in vivo gene-corrected mice was increased compared to 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) control animals and unchanged in serially transplanted animals. The integration profile (4,349 mapped insertions) remained polyclonal through all mouse generations with only mild clonal expansion. Genes close to the integration sites of expanding clones may be associated with enhanced hepatocyte proliferation capacity.

CONCLUSION

We did not find evidence for vector-induced tumors. LV hepatic gene therapy showed a favorable risk profile for stable and long-term therapeutic gene expression. Polyclonality of hepatocyte regeneration was maintained even in an environment of enforced proliferation.

Evaluating a ligation-mediated PCR and pyrosequencing method for the detection of clonal contribution in polyclonal retrovirally transduced samples

Retroviral gene transfer has proven therapeutic potential in clinical gene therapy trials but may also cause abnormal cell growth via perturbation of gene expression in the locus surrounding the insertion site. By establishing clonal marks, retroviral insertions are also used to describe the regenerative potential of individual cells. Deep sequencing approaches have become the method of choice to study insertion profiles in preclinical models and clinical trials. We used a protocol combining ligation-mediated polymerase chain reaction (LM-PCR) and pyrosequencing for insertion profiling and quantification in cells of various tissues transduced with various retroviral vectors. The presented method allows simultaneous analysis of a multitude of DNA-barcoded samples per pyrosequencing run, thereby allowing cost-effective insertion screening in studies with multiple samples. In addition, we investigated whether the number of pyrosequencing reads can be used to quantify clonal abundance. By comparing pyrosequencing reads against site-specific quantitative PCR and by performing spike-in experiments, we show that considerable variation exists in the quantification of insertion sites even when present in the same clone. Our results suggest that the protocol used here and similar approaches might misinterpret abundance clones defined by insertion sites, unless careful calibration measures are taken. The crucial variables causing this variation need to be defined and methodological improvements are required to establish pyrosequencing reads as a quantification measure in polyclonal situations.

Identity, potency, in vivo viability, and scaling up production of lentiviral vector-induced dendritic cells for melanoma immunotherapy

SmartDCs (Self-differentiated Myeloid-derived Antigen-presenting-cells Reactive against Tumors) consist of highly viable dendritic cells (DCs) induced to differentiate with lentiviral vectors (LVs) after an overnight ex vivo transduction. Tricistronic vectors co-expressing cytokines (granulocyte-macrophage-colony stimulating factor [GM-CSF], interleukin [IL]-4) and a melanoma antigen (tyrosine related protein 2 [TRP2]) were used to transduce mouse bone marrow cells or human monocytes. Sixteen hours after transduction, the cells were dispensed in aliquots and cryopreserved for identity, potency, and safety analyses. Thawed SmartDCs readily differentiated into highly viable cells with a DC immunophenotype. Prime/boost subcutaneous administration of 1×10(6) thawed murine SmartDCs into C57BL/6 mice resulted into TRP2-specific CD8(+) T-cell responses and protection against lethal melanoma challenge. Human SmartDC-TRP2 generated with monocytes obtained from melanoma patients secreted endogenous cytokines associated with DC activation and stimulated TRP2-specific autologous T-cell expansion in vitro. Thawed human SmartDCs injected subcutaneously in NOD.Rag1(-/-).IL2rγ(-/-) mice maintained DC characteristics and viability for 1 month in vivo and did not cause any signs of pathology. For development of good manufacturing practices, CD14(+) monocytes selected by magnetic-activated cell separation were transduced in a closed bag system (multiplicity of infection of 5), washed, and cryopreserved. Fifty percent of the monocytes used for transduction were recovered for cryopreservation. Thawed SmartDCs produced in two independent runs expressed the endogenous cytokines GM-CSF and IL-4, and the resulting homogeneous SmartDCs that self-differentiated in vitro contained approximately 1.5-3.0 copies of integrated LVs per cell. Thus, this method facilitates logistics, standardization, and high recovery for the generation of viable genetically reprogrammed DCs for clinical applications.

Selection for Evi1 activation in myelomonocytic leukemia induced by hyperactive signaling through wild-type NRas

Activation of NRas signaling is frequently found in human myeloid leukemia and can be induced by activating mutations as well as by mutations in receptors or signaling molecules upstream of NRas. To study NRas-induced leukemogenesis, we retrovirally overexpressed wild-type NRas in a murine bone marrow transplantation (BMT) model in C57BL/6J mice. Overexpression of wild-type NRas caused myelomonocytic leukemias ∼3 months after BMT in the majority of mice. A subset of mice (30%) developed malignant histiocytosis similar to mice that received mutationally activated NRas(G12D)-expressing bone marrow. Aberrant Ras signaling was demonstrated in cells expressing mutationally active or wild-type NRas, as increased activation of Erk and Akt was observed in both models. However, more NRas(G12D) were found to be in the activated, GTP-bound state in comparison with wild-type NRas. Consistent with observations reported for primary human myelomonocytic leukemia cells, Stat5 activation was also detected in murine leukemic cells. Furthermore, clonal evolution was detected in NRas wild-type-induced leukemias, including expansion of clones containing activating vector insertions in known oncogenes, such as Evi1 and Prdm16. In vitro cooperation of NRas and Evi1 improved long-term expansion of primary murine bone marrow cells. Evi1-positive cells upregulated Bcl-2 and may, therefore, provide anti-apoptotic signals that collaborate with the NRas-induced proliferative effects. As activation of Evi1 has been shown to coincide with NRAS mutations in human acute myeloid leukemia, our murine model recapitulates crucial events in human leukemogenesis.

Efficient in vivo regulation of cytidine deaminase expression in the haematopoietic system using a doxycycline-inducible lentiviral vector system

Regulated transgene expression may reduce transgene-specific and genotoxic risks associated with gene therapy. To prove this concept, we have investigated the suitability of doxycycline (Dox)-inducible human cytidine deaminase (hCDD) overexpression from lentiviral vectors to mediate effective myeloprotection while circumventing the lymphotoxicity observed with constitutive CDD activity. Rapid Dox-mediated transgene induction associated with a 6-17-fold increase in drug resistance was observed in 32D and primary murine bone marrow (BM) cells. Moreover, robust Dox-regulated transgene expression in the entire haematopoietic system was demonstrated for primary and secondary recipients of hCDD-transduced R26-M2rtTA transgenic BM cells. Furthermore, mice were significantly protected from myelosuppressive chemotherapy as evidenced by accelerated recovery of granulocytes (1.9±0.6 vs 1.3±0.3, P=0.034) and platelets (883±194 vs 584±160 10(3) per μl, P=0.011). Minimal transgene expression in the non-induced state and no overt cellular toxicities including lymphotoxicity were detected. Thus, using a relevant murine transplant model our data provide conclusive evidence that drug-resistance transgenes can be expressed in a regulated fashion in the lymphohaematopoietic system, and that Dox-inducible systems may be used to reduce myelotoxic side effect of anticancer chemotherapy or to avoid side effects of high constitutive transgene expression.

Lentiviral vector induced insertional haploinsufficiency of Ebf1 causes murine leukemia

Integrating vectors developed on the basis of various retroviruses have demonstrated therapeutic potential following genetic modification of long-lived hematopoietic stem and progenitor cells. Lentiviral vectors (LV) are assumed to circumvent genotoxic events previously observed with γ-retroviral vectors, due to their integration bias to transcription units in comparison to the γ-retroviral preference for promoter regions and CpG islands. However, recently several studies have revealed the potential for gene activation by LV insertions. Here, we report a murine acute B-lymphoblastic leukemia (B-ALL) triggered by insertional gene inactivation. LV integration occurred into the 8th intron of Ebf1, a major regulator of B-lymphopoiesis. Various aberrant splice variants could be detected that involved splice donor and acceptor sites of the lentiviral construct, inducing downregulation of Ebf1 full-length message. The transcriptome signature was compatible with loss of this major determinant of B-cell differentiation, with partial acquisition of myeloid markers, including Csf1r (macrophage colony-stimulating factor (M-CSF) receptor). This was accompanied by receptor phosphorylation and STAT5 activation, both most likely contributing to leukemic progression. Our results highlight the risk of intragenic vector integration to initiate leukemia by inducing haploinsufficiency of a tumor suppressor gene. We propose to address this risk in future vector design.

Concise review: managing genotoxicity in the therapeutic modification of stem cells

The therapeutic use of procedures for genetic stem cell modification is limited by potential adverse events related to uncontrolled mutagenesis. Prominent findings have been made in hematopoietic gene therapy, demonstrating the risk of clonal, potentially malignant outgrowth on the basis of mutations acquired during or after therapeutic genome modification. The incidence and the growth rate of insertional mutants have been linked to the "stemness" of the target cells and vector-related features such as the integration pattern, the architecture, and the exact content of transgene cassettes. Milieu factors supporting the survival and expansion of mutants may eventually allow oncogenic progression. Similar concerns apply for medicinal products based on pluripotent stem cells. Focusing on the genetic stress induced by insertional mutagenesis and culture adaptation, we propose four conclusions. (a) Mutations occurring in the production of stem cell-based medicines may be unavoidable and need to be classified according to their risk to trigger the formation of clones that are sufficiently long-lived and mitotically active to acquire secondary transforming mutations. (b) The development of rational prevention strategies depends upon the identification of the specific mutations forming such "dominant clones" (which can also be addressed as cancer stem cell precursors) and a better knowledge of the mechanisms underlying their creation, expansion, and homeostatic control. (c) Quantitative assay systems are required to assess the practical value of preventive actions. (d) Improved approaches for the genetic modification of stem cells can address all critical steps in the origin and growth control of mutants.

Development of novel efficient SIN vectors with improved safety features for Wiskott-Aldrich syndrome stem cell based gene therapy

Gene therapy is a promising therapeutic approach to treat primary immunodeficiencies. Indeed, the clinical trial for the Wiskott-Aldrich Syndrome (WAS) that is currently ongoing at the Hannover Medical School (Germany) has recently reported the correction of all affected cell lineages of the hematopoietic system in the first treated patients. However, an extensive study of the clonal inventory of those patients reveals that LMO2, CCND2 and MDS1/EVI1 were preferentially prevalent. Moreover, a first leukemia case was observed in this study, thus reinforcing the need of developing safer vectors for gene transfer into HSC in general. Here we present a novel self-inactivating (SIN) vector for the gene therapy of WAS that combines improved safety features. We used the elongation factor 1 alpha (EFS) promoter, which has been extensively evaluated in terms of safety profile, to drive a codon-optimized human WASP cDNA. To test vector performance in a more clinically relevant setting, we transduced murine HSPC as well as human CD34+ cells and also analyzed vector efficacy in their differentiated myeloid progeny. Our results show that our novel vector generates comparable WAS protein levels and is as effective as the clinically used LTR-driven vector. Therefore, the described SIN vectors appear to be good candidates for potential use in a safer new gene therapy protocol for WAS, with decreased risk of insertional mutagenesis.

Genotoxic potential of lineage-specific lentivirus vectors carrying the beta-globin locus control region

Insertional mutagenesis by long terminal repeat (LTR) enhancers in gamma-retrovirus-based vectors (GVs) in clinical trials has prompted deeper investigations into vector genotoxicity. Experimentally, self-inactivating (SIN) lentivirus vectors (LVs) and GV containing internal promoters/enhancers show reduced genotoxicity, although strong ubiquitously-active enhancers dysregulate genes independent of vector type/design. Herein, we explored the genotoxicity of beta-globin (BG) locus control region (LCR), a strong long-range lineage-specific-enhancer, with/without insulator (Ins) elements in LV using primary hematopoietic progenitors to generate in vitro immortalization (IVIM) assay mutants. LCR-containing LV had approximately 200-fold lower transforming potential, compared to the conventional GV. The LCR perturbed expression of few genes in a 300 kilobase (kb) proviral vicinity but no upregulation of genes associated with cancer, including an erythroid-specific transcription factor occurred. A further twofold reduction in transforming activity was observed with insulated LCR-containing LV. Our data indicate that toxicology studies of LCR-containing LV in mice will likely not yield any insertional oncogenesis with the numbers of animals that can be practically studied.

Gene therapy of MPL deficiency: challenging balance between leukemia and pancytopenia

Signaling of the thrombopoietin (THPO) receptor MPL is critical for the maintenance of hematopoietic stem cells (HSCs) and megakaryocytic differentiation. Inherited loss-of-function mutations of MPL cause severe thrombocytopenia and aplastic anemia, a syndrome called congenital amegakaryocytic thrombocytopenia (CAMT). With the aim to assess the toxicity of retroviral expression of Mpl as a basis for further development of a gene therapy for this disorder, we expressed Mpl in a murine bone marrow transplantation (BMT) model. Treated mice developed a profound yet transient elevation of multilineage hematopoiesis, which showed morphologic features of a chronic myeloproliferative disorder (CMPD) with progressive pancytopenia. Ten percent of mice (3/27) developed erythroleukemia, associated with insertional activation of Sfpi1 and Fli1. The majority of transplanted mice developed a progressive pancytopenia with histopathological features of a myelodysplastic syndrome (MDS)-like disorder. To avoid these adverse reactions, improved retroviral vectors were designed that mediate reduced and more physiological Mpl expression. Self-inactivating gamma-retroviral vectors were constructed that expressed Mpl from the phosphoglycerate kinase (PGK) or the murine Mpl promoter. Mice that received BM cells expressing Mpl from the Mpl promoter were free of any previously observed adverse reactions.

Insertional transformation of hematopoietic cells by self-inactivating lentiviral and gammaretroviral vectors

Gene transfer vectors may cause clonal imbalance and even malignant cell transformation by insertional upregulation of proto-oncogenes. Lentiviral vectors (LV) with their preferred integration in transcribed genes are considered less genotoxic than gammaretroviral vectors (GV) with their preference for integration next to transcriptional start sites and regulatory gene regions. Using a sensitive cell culture assay and a series of self-inactivating (SIN) vectors, we found that the lentiviral insertion pattern was approximately threefold less likely than the gammaretroviral to trigger transformation of primary hematopoietic cells. However, lentivirally induced mutants also showed robust replating, in line with the selection for common insertion sites (CIS) in the first intron of the Evi1 proto-oncogene. This potent proto-oncogene thus represents a CIS for both GV and LV, despite major differences in their integration mechanisms. Altering the vectors' enhancer-promoter elements had a greater effect on safety than the retroviral insertion pattern. Clinical grade LV expressing the Wiskott-Aldrich syndrome (WAS) protein under control of its own promoter had no transforming potential. Mechanistic studies support the conclusion that enhancer-mediated gene activation is the major cause for insertional transformation of hematopoietic cells, opening rational strategies for risk prevention.

Preventing and exploiting the oncogenic potential of integrating gene vectors

Gene therapy requires efficient gene delivery to cure or prevent disease by modifying the genome of somatic cells. However, gene vectors, which insert themselves into the host genome in order to achieve persistent protein expression, can trigger oncogenesis by upregulating cellular protooncogenes. This adverse event, known as insertional mutagenesis, has become a major hurdle in the field. Vectors developed on the basis of lentiviruses are considered to be less genotoxic than the hitherto used gamma-retroviral vectors. For their report in this issue of the JCI, Montini et al. utilized a tumor-prone mouse model to identify the genetic determinants of insertional mutagenesis (see the related article beginning on page 964). They report that the lentiviral integration pattern and additional improvements in vector design reduce the genotoxic risk. These findings will inform future vector design with the goal of limiting genotoxicity for gene therapy or increasing genotoxicity for protooncogene discovery.

Leukemia diagnosis in murine bone marrow transplantation models

The mouse is the most commonly used experimental animal, and a wide range of tumor types can arise in their hematopoietic system. Therefore, for research scientists and graduate students working in the field of experimental hematology, immunology, and cancer research, there is an urgent need for well-established protocols for the preparation of histology and cytology for leukemia diagnosis. Moreover, the criteria for the classification of hematopoietic neoplasms often vary between different laboratories. In this chapter, we describe diagnosis and analysis of leukemia in murine bone marrow transplantation models based primarily on the findings of the histology and cytology of hematopoietic and infiltrated tissues, peripheral blood smear, and immunophenotyping by FACS analysis.

Murine hematopoietic stem cell transduction using retroviral vectors

Hematopoietic stem cells (HSCs) represent an important target cell population in bone marrow transplantation and gene therapy applications. Their progeny cells carry the genetic information of the HSCs and replenish the blood and immune system. Therefore, in the setting of inherited diseases, transduction of HSCs with retroviral vectors (including gammaretro- and lentiviral vectors) offers the possibility to correct the phenotype in all blood lineages as demonstrated in clinical trials for immunodeficiencies (e.g., X-SCID). In the process of developing gene therapy strategies for patient applications, suitable mouse models for the human gene therapy are important to validate the concept. Stem-cell-enriched populations such as lineage negative cells as the functional equivalent of human CD34(+) cells can be isolated from murine bone marrow and efficiently transduced using retroviral vectors. This chapter provides a step-by-step protocol for retroviral transduction of murine lineage negative cells.

Retroviral insertion site analysis in dominant haematopoietic clones

Identification of retroviral vector insertion sites in single, dominating cell clones has become an important tool for the investigation of cellular signalling pathways involved in clonal expansion and malignant transformation. Also, recent severe adverse events in clinical trials resulting from retroviral vector-mediated insertional mutagenesis underline the need of well-designed safety studies including integration site analyses to estimate cost/benefit ratios in gene therapy. We have recently described a modified ligation-mediated PCR (LM PCR) method allowing preferential retrieval of insertion sites causally linked to clonal dominance of an affected clone. In the first part of the given work we focus on particularities of the LM PCR procedure to be taken into account when working with self-inactivating as compared to 'classical' retrovectors. In the following sections we focus on data acquisition, processing, organisation, and analysis. Thus the protocol presented here should be helpful in establishing and utilising databases of retroviral integration sites.

Leukemia induction after a single retroviral vector insertion in Evi1 or Prdm16

Insertional activation of cellular proto-oncogenes by replication-defective retroviral vectors can trigger clonal dominance and leukemogenesis in animal models and clinical trials. Here, we addressed the leukemogenic potential of vectors expressing interleukin-2 receptor common gamma-chain (IL2RG), the coding sequence required for correction of X-linked severe combined immunodeficiency. Similar to conventional gamma-retroviral vectors, self-inactivating (SIN) vectors with strong internal enhancers also triggered profound clonal imbalance, yet with a characteristic insertion preference for a window located downstream of the transcriptional start site. Controls including lentivirally transduced cells revealed that ectopic IL2RG expression was not sufficient to trigger leukemia. After serial bone marrow transplantation involving 106 C57Bl6/J mice monitored for up to 18 months, we observed leukemic progression of six distinct clones harboring gamma-retroviral long terminal repeat (LTR) or SIN vector insertions in Evi1 or Prdm16, two functionally related genes. Three leukemic clones had single vector integrations, and identical clones manifested with a remarkably similar latency and phenotype in independent recipients. We conclude that upregulation of Evi1 or Prdm16 was sufficient to initiate a leukemogenic cascade with consistent intrinsic dynamics. Our study also shows that insertional mutagenesis is required for leukemia induction by IL2RG vectors, a risk to be addressed by improved vector design.