Retroviral Vector Integration Sites Research Articles

VISMapper: ultra-fast exhaustive cartography of viral insertion sites for gene therapy

BackgroundThe possibility of integrating viral vectors to become a persistent part of the host genome makes them a crucial element of clinical gene therapy. However, viral integration has associated risks, such as the unintentional activation of oncogenes that can result in cancer. Therefore, the analysis of integration sites of retroviral vectors is a crucial step in developing safer vectors for therapeutic use.ResultsHere we present VISMapper, a vector integration site analysis web server, to analyze next-generation sequencing data for retroviral vector integration sites. VISMapper can be found at: http://vismapper.babelomics.org.ConclusionsBecause it uses novel mapping algorithms VISMapper is remarkably faster than previous available programs. It also provides a useful graphical interface to analyze the integration sites found in the genomic context.

Modified Genomic Sequencing PCR Using the MiSeq Platform to Identify Retroviral Integration Sites.

High-throughput mapping of retroviral vector integration sites (RIS) has become an invaluable tool to evaluate novel gene therapy vectors and to track clonal contribution in preclinical and clinical studies. Beard et al. (Methods Mol Biol 2014;1185:321-344) described an improved protocol developed for efficient capture, sequencing, and analysis of RIS that preserves gene-modified clonal contribution information. Here we describe adaptations to the previously published modified genomic sequencing PCR (MGS-PCR) protocol using the Illumina MiSeq paired-end sequencing platform. Lentiviral, gammaretroviral, and foamy virus vector integrations were analyzed. MGS-PCR using the MiSeq platform allows for the use of merged paired-end reads, which allows for efficient localization of RIS to published genomes.

VISA--Vector Integration Site Analysis server: a web-based server to rapidly identify retroviral integration sites from next-generation sequencing.

BackgroundAnalyzing the integration profile of retroviral vectors is a vital step in determining their potential genotoxic effects and developing safer vectors for therapeutic use. Identifying retroviral vector integration sites is also important for retroviral mutagenesis screens.ResultsWe developed VISA, a vector integration site analysis server, to analyze next-generation sequencing data for retroviral vector integration sites. Sequence reads that contain a provirus are mapped to the human genome, sequence reads that cannot be localized to a unique location in the genome are filtered out, and then unique retroviral vector integration sites are determined based on the alignment scores of the remaining sequence reads.ConclusionsVISA offers a simple web interface to upload sequence files and results are returned in a concise tabular format to allow rapid analysis of retroviral vector integration sites.Electronic supplementary materialThe online version of this article (doi:10.1186/s12859-015-0653-6) contains supplementary material, which is available to authorized users.

Gene Therapy Model of X-linked Severe Combined Immunodeficiency Using a Modified Foamy Virus Vector

X-linked severe combined immunodeficiency (SCID-X1) is an inherited genetic immunodeficiency associated with mutations in the common cytokine receptor γ chain (γc) gene, and characterized by a complete defect of T and natural killer (NK) cells. Gene therapy for SCID-X1 using conventional retroviral (RV) vectors carrying the γc gene results in the successful reconstitution of T cell immunity. However, the high incidence of vector-mediated T cell leukemia, caused by vector insertion near or within cancer-related genes has been a serious problem. In this study, we established a gene therapy model of mouse SCID-X1 using a modified foamy virus (FV) vector expressing human γc. Analysis of vector integration in a human T cell line demonstrated that the FV vector integration sites were significantly less likely to be located within or near transcriptional start sites than RV vector integration sites. To evaluate the therapeutic efficacy, bone marrow cells from γc-knockout (γc-KO) mice were infected with the FV vector and transplanted into γc-KO mice. Transplantation of the FV-treated cells resulted in the successful reconstitution of functionally active T and B cells. These data suggest that FV vectors can be effective and may be safer than conventional RV vectors for gene therapy for SCID-X1.

Comparison of gene-trapping efficiency between retroviral and lentiviral vectors in mouse embryonic stem cells

Gene trapping is a method of inserting DNA into the genome at random, generating insertional mutations throughout the genome. The efficiency of retroviral gene trapping is not sufficient in part because of a strong preference for retroviral integration near transcription start sites. In contrast, lentiviral vectors strongly favor integration in the entire region of highly active genes, suggesting that lentiviral vectors would improve the efficiency of gene trapping. In this study, we constructed both lentiviral and retroviral gene-trap vectors and analyzed integration sites in mouse embryonic stem (ES) cells. The frequency of false-positive gene-trap events was about 12-fold higher for the retroviral vector compared to the lentiviral vector. Within intragenic regions, most of the retroviral vector integration sites were found in the 5′ untranslated region, while the lentiviral vector integrated uniformly throughout transcriptional units. The trapping efficiency of unique genes was significantly higher for the lentiviral vector (∼83%) than for the retroviral vector (∼51%). Our data demonstrate that the lentiviral vector can trap the active genes more efficiently than the retroviral vector and will facilitate efficient generation of gene-trap libraries not only in ES cells but also in a wide variety of cell lines and primary cells.

Cytokine-independent growth and clonal expansion of a primary human CD8+ T-cell clone following retroviral transduction with the IL-15 gene

AbstractMalignancies arising from retrovirally transduced hematopoietic stem cells have been reported in animal models and human gene therapy trials. Whether mature lymphocytes are susceptible to insertional mutagenesis is unknown. We have characterized a primary human CD8+ T-cell clone, which exhibited logarithmic ex vivo growth in the absence of exogenous cytokine support for more than 1 year after transduction with a murine leukemia virus–based vector encoding the T-cell growth factor IL-15. Phenotypically, the clone was CD28−, CD45RA−, CD45RO+, and CD62L−, a profile consistent with effector memory T lymphocytes. After gene transfer with tumor-antigen–specific T-cell receptors, the clone secreted IFN-γ upon encountering tumor targets, providing further evidence that they derived from mature lymphocytes. Gene-expression analyses revealed no evidence of insertional activation of genes flanking the retroviral insertion sites. The clone exhibited constitutive telomerase activity, and the presence of autocrine loop was suggested by impaired cell proliferation following knockdown of IL-15Rα expression. The generation of this cell line suggests that nonphysiologic expression of IL-15 can result in the long-term in vitro growth of mature human T lymphocytes. The cytokine-independent growth of this line was a rare event that has not been observed in other IL-15 vector transduction experiments or with any other integrating vector system. It does not appear that the retroviral vector integration sites played a role in the continuous growth of this cell clone, but this remains under investigation.

Myeloid Expansion after Insertional Activation of MDS1/EVI1, PRDM16 and SETBP1 in a Successful Chronic Granulomatous Gene Therapy Trial.

Successful gene therapy trials of ADA-SCID and SCID-X1 demonstrated the curative potential of oncoretroviral gene transfer. Integration of the retroviral vectors used in these studies has been thought to be a random process but severe side effects in gene therapy and in vitro studies revealed preferred insertion of these vectors mainly around transcription start sites. In SCID patients proliferation advantage of gene corrected cells was one reason for the success of the trials, whereas in the most recent chronic granulomatous disease (CGD) gene therapy trial corrected cells do not have any selective advantage therefore the two patients received mild busulfan treatment before transplantation. High efficiency transduction and conditioning have helped in the successful correction of the patients. Peripheral blood granulocytes show a stable expression (>10%) of the transgene (gp91phox) in patient 1 (15 months post treatment) as well as in patient 2 (11 months post treatment). We reasoned that, unlike T cells, which have the capability to proliferate independent of their bone marrow progenitors, granulocytes more directly reflect the influence of retrovirus insertion, and should therefore allow to closely monitor clonal fate in vivo and its potential relation to vector insertion. To study the clonality of the corrected myelopoiesis, the long term activity of individual cell clones, and the distribution of integration sites in active cells we carried out high sensitive LAM-PCR. The highly polyclonal composition of transduced cells forming myelopoiesis caused the sustained expression of gp91phox. Individual clones carrying the transgene could be detected at multiple time points. To define whether corrected cells have a proliferation advantage due to their vector integration we started large-scale sequencing and mapping of involved insertion sites. We here present >700 unique mappable integration sites of the two treated patients. The distribution of the SFFV based retroviral vector integration sites in this trial turned non random 5 months after transplantation. Corrected long-term myelopoiesis expanded 3- to 5- fold in the two patients due to activating common integration sites (CIS) in the zinc finger transcription factor homologs MDS1/EVI1, PRDM16, or in SETBP1, suggesting that these genes influence regulation of normal long-term hematopoiesis in humans. Our data indicate that the therapeutic benefit in this trial was activated through insertional side effects, therefore our findings have important implications in novel gene therapy approaches.

Collaborative RISC-Score Database: Creation of an International Database for Retroviral Integration Analysis.

Increasing use of hematopoietic stem cells for retroviral vector-mediated gene therapy and recent reports on leukemogenesis in mice and humans have created intense interest to characterize vector integrations on the genomic level. As techniques to determine insertion sites are more commonly applied in gene therapy laboratories there is a need to systematically collect and analyze the data arising from such studies in a vector insertion database. This will allow determining factors responsible for preferential integration of various vector types in specific chromosomal regions, genes or gene sections. The information derived from a vector insertion data base will be useful to recognize more “dangerous” vector types and may provide useful information for vector design.We have set up an automatic sequence analysis tool (ensuring quality criteria e.g. verification of LTR- and adapter sequence, score >40, e-value >10e-40, hit RefSeq, next RefSeq etc.) which simplifies data input enormously while ensuring high quality standards. Our group is establishing the "collaborative RISC (retroviral insertion estimation into chromosome) -Score Database (CRSD)"- assessment project, based on the M-CHIPS (Multi-Conditional Hybridisation Intensity Processing System) microarray data warehouse and analysis software (K. Fellenberg et al. 2001, 2002). The data obtained from the sequence analysis tool were automatically fed in the data base. A total of 287 retroviral vector integration sites were isolated and sequence analysis was performed with the above describe analysis tool. In human bone marrow repopulating cells they occurred with significantly increased frequency into chromosomes 17 and 19 (n=189). Analysis of targeted RefSeq genes showed a favored integration (48%) within the first intron. In comparison, retroviral vector integrations in T-cells (n=98) showed an entirely different chromosomal distribution pattern while the percentage of the targeted RefSeq genes was similar (46%). Further, more than 1200 sequences were submitted to the data base, originating from different vectors (SF-MDR-, MoLV-based TK/neoR-Mo3TIN-, Moloney-MGMT-, Harvey-based Neo-, Harvey-based MDR-, and lentiviral GFP-SIN-vectors) and different transduced cells (mouse hematopoietic cells, mouse fibroblasts, rhesus hematopoietic cells, human hematopoietic cells, human T-cells). The set-up and internal structure of the data base will be presented.Collaborations have been forged to include further groups and vector types.Bioinformatical analysis will allow recognizing even complex vector integration patterns and will broaden our understanding for the determinants of vector integration into the genome. This in turn can lead to the construction of "favorable" vectors and help to reduce the genotoxicity of retroviral or lentiviral vector-mediated gene transfer.

Insertion of Retroviral Vectors in NOD/SCID Repopulating Human Peripheral Blood Progenitor Cells Occurs Preferentially in the Vicinity of Transcription Start Regions and in Introns

Reports on insertional “genotoxicity” in patients have created intense interest in characterizing retroviral vector integrations on the genomic level. The retroviral vector SF91m3 was used for transduction of human peripheral blood progenitor cells (PBPC). These PBPC were transplanted into nonobese diabetic/severe combined immunodeficient mice. A total of 186 retroviral vector integration sites were isolated by ligation-mediated PCR from chimeric mouse bone marrow of five PBPC donors, sequenced, and blasted against the human genome. Preferred integration near the transcription start regions, within CpG islands, and within Alu regions was observed. Detailed analysis of targeted RefSeq genes showed a favored integration within the first intron. Integrations were most common in genes coding for signaling proteins, transcription factors, and kinases. In all genes targeted independently multiple times the respective orientation of the provirus within the gene was identical, indicating integration hot spot regions and similar steric determinants for integration sites. Possible explanations for these findings could be nonrandom vector integration, clonal selection due to gene expression interference, or engraftment issues related to gene insertion in signaling and cell cycle genes. The low frequency of integrations in exons may be reassuring as to the safety of retroviral gene therapy with normal human PBPC.

Rapid detection of retroviral vector integration sites in colony-forming human peripheral blood progenitor cells using PCR with arbitrary primers.

We have developed a highly sensitive polymerase chain reaction (PCR)-based technique termed two-step PCR, which uses arbitrary primers to identify proviral integration sites in retrovirally marked human colony-forming cells. The two-step PCR was established on cell line clones transduced with the SF1m retroviral vector and independently validated by demonstrating identical integration sites with ligation-mediated PCR, a different technique requiring restriction enzyme digestion and adapter ligation for amplifying unknown DNA flanking the provirus. Two-step PCR was performed on peripheral blood progenitor cell (PBPC) colonies that contained as few as 75 cells, which was estimated by quantitative real-time PCR. We were able to amplify and directly sequence proviral integration sites in 35 % of PBPC colonies (25/72, five donors). Identity to the vector long-terminal repeat was confirmed and flanking DNA was found to match with human database sequences, reaffirming specificity. Two-step PCR is a valuable new tool for rapid analysis of genomic target sites for viral vectors, and will aid significantly in understanding clonal development of hematopoiesis and other cell types. Our protocol has the potential for general applicability as the arbitrary primers described here bind to genomic DNA and are thus independent of the vector backbone used.