535. Increasing Accuracy and Precision of Vector Integration Site Identification of Sequencing Reads With a New Bioinformatics Framework

In hematopoietic stem cell (HSC) gene therapy (GT) applications patients are transplanted with autologuos HSCs that have been ex-vivo genetically modified with integration competent vectors to express a therapeutic transgene. Specific PCR techniques coupled to next generation sequencing and bioinformatics analysis allow the high throughput retrieval, sequencing and mapping of proviral/genomic DNA junctions present in the blood and bone marrow derived cell populations sampled at different time points after therapy. The increase in sequences available for IS mapping is accompanied by an increase in false positives derived by sequencing errors or sequencing read parsing and mapping on the reference genome. In particular, by analyzing IS datasets form vector marked human and mouse tumor cells, clones with defined integration sites and GT patients, we observed that when multiple sequences arising from the same IS are aligned on the reference genome >10% mapped near (+/- 4 bases) the true insertion site. Without correction, these misaligned sequences not only result in an overestimation of the overall number of IS but in some cases also in the generation of false common insertion sites, worrisome hallmarks of insertional mutagenesis. To mitigate this issue we and others, based on empirical observations, merge sequencing reads mapping within +/- 3 bp into a single IS. Although this adjustment reduces the impact of the "wobbling" around the true ISs, a dedicated method and model is still missing.To further increase the accuracy of genomic positioning of sequencing reads we developed a new bioinformatics framework as post-processing plugin for pipelines that correctly partitions sequencing reads in a given genomic position by considering the relative abundance and distribution of each sequence cluster using local modes and Gaussian scores through an adaptive approach that varies the parameters of the Gaussian curve and proposes different solutions. To chose the best solution, the algorithm first evaluates each solution by exploiting 100 simulations of the input reads and then selects the resulting best solution using the Kolmogorov-Smirnov test. The simulation step is designed to test the mappability of the IS genomic interval and to quantify the impact of the observed nucleotide variations of the reads with respect to the reference genome (PCR artifacts or real genomic differences) that may lead to different mapping results that justify a larger span of the mapped reads surrounding the putative IS. The algorithm returns the list of IS and relative number of reads with the p-value of the best solution.We performed 3 ad-hoc in vitro experiments on a cell clone with 6 known IS in which we measured the precision of IS placement obtaining an average of 100% with our new method whereas <30% using our previous method based on a rigid sliding window approach of 4 bp. We applied our new approach to our clinical trial datasets obtaining improvements in IS genomic placement and overestimation with a reduction of potential false IS of 3% without changing the biological results

Efficient Tet-dependent expression of human factor IX in vivo by a new self-regulating lentiviral vector.

Regulation of gene expression represents a long-sought goal of gene therapy. However, most viral vectors pose constraints on the incorporation of drug-dependent transcriptional regulatory systems. Here, by optimizing the design of self-regulating lentiviral vectors based on the tetracycline system, we have been able to overcome the limitations of previously reported constructs and to reach both robust expression and efficient regulation from a single vector. The improved performance allows us to report for the first time effective long-term in vivo regulation of a human clotting Factor IX (hF.IX) transgene upon systemic administration of a single vector to SCID mice. We showed that hF.IX expression in the plasma could be expressed to therapeutically significant concentrations, adjusted to different set levels by varying the tetracycline dose, rapidly turned off and on, and completely recovered after each treatment cycle. The new vector design was versatile, as it successfully incorporated a tissue-specific promoter that selectively targeted regulated expression to hepatocytes. Robust transgene expression in the systemic circulation coupled to the ability to switch off and even adjust the expression level may open the way to safer gene-based delivery of therapeutics

537. New Graph-Based Algorithm for Comprehensive Identification and Tracking Retroviral Integration Sites

Vector integration sites (IS) in hematopoietic stem cell (HSC) gene therapy (GT) applications are stable genetic marks, distinctive for each independent cell clone and its progeny. The characterization of IS allows to identify each cell clone and individually track its fate in different tissues or cell lineages and during time, and is required for assessing the safety and efficacy of the treatment. Bioinformatics pipelines for IS detection used in GT identify the sequence reads mapping in the same genomic position of the reference genome as a single IS but discard those ambiguously mapped in multiple genomic regions. The loss of such significant portion of patients' IS may hide potential malignant events thus reducing the reliability of IS studies. We developed a novel tool that is able to accurately identify IS in any genomic region even if composed by repetitive genomic sequences. Our approach exploits an initial genome free analysis of sequencing reads by creating an undirected graph in which nodes are the input sequences and edges represent valid alignments (over a specific identity threshold) between pairs of nodes. Through the analysis and decomposition of the graph, the method identifies indivisible subgraphs of sequences (clusters), each of them corresponding to an IS. Once extracted the consensus sequence of the clusters and aligned on the reference genome, we collect the alignment results and the annotation labels from RepeatMasker. By combining the set of genomic coordinates and the annotation labels, the method retraces the initial sequence graph, statistically validates the clusters through permutation test and produces the final list of IS. We tested the reliability of our tool on 3 IS datasets generated from simulated sequencing reads with incremental rate of nucleotide variations (0%, 0.25% and 0.5%) and real data from a cell line with known IS and we compared out tool to VISPA and UClust, used for GT studies. In the simulated datasets our tool demonstrated precision and recall ranging 0.85-0.97 and 0.88-0.99 respectively, producing the aggregate F-score ranging 0.86-0.98 which resulted higher than VISPA and UClust. In the experimental case of sequences from LAM-PCR products, our tool and VISPA were able to identify all the 6 known ISs for >98% of the reads produced, while UClust identified only 5 out 6 ISs. We then used our tool to reanalyze the sequencing reads of our GT clinical trial for Metachromatic Leukodystrophy (MLD) completing the hidden portion of IS. The overall number of ISs, sequencing reads and estimated actively re-populating HSCs was increased by an average fold ~1.5 with respect the previously published data obtained through VISPA whereas the diversity index of the population did not change and no aberrant clones in repeats occurred. Our tool addresses and solves important open issues in retroviral IS identification and clonal tracking, allowing the generation of a comprehensive repertoire of IS

674 insertional mutagenesis to identify mechanisms of cetuximab resistance in colorectal cancer

Anti-cancer drugs designed to target specific molecular pathways have shown an excellent therapeutic potential but also very poor long-term durability of tumor responses, mainly due to the outbreak of resistant clones among the residual neoplastic cell population. For that reason, understanding the molecular mechanisms underlying the onset of anti-cancer drug resistance (ACDR) is one of the major goals of clinical research. ACDR has been widely studied by DNA/RNA sequencing of primary human samples and several culprits identified. We have previously developed an approach based on lentiviral vector (LV)-induced insertional mutagenesis that allowed to identify the genes involved in lapatinib and erlotinib resistance on HER2+ human breast cancer cell lines and EGFR+ pancreatic cell line respectively. Here we took advantage of this platform to investigate ACDR genes in colorectal cancer (CRC). Cetuximab, anti-EGFR monoclonal antibody, is used as first line therapy in metastatic CRC, which results in prolonged survival of treated patients. However, nearly all patients relapse due to ACDR. We thus selected CRC cells sensitive to cetuximab deriving either from five microsatellite stable cell lines or from eight Patient Derived Xenografts (PDX), primary human CRC cells implanted subcutaneously into immunodeficient mice (NSG). To induce insertional mutagenesis we generated a luciferase-expressing LV harboring the SFFV enhancer/promoter in the long terminal repeats able to perturb the expression of genes nearby the integration site. As control, we used a non-genotoxic SIN-LV. We set up a collagenase IV-based disaggregation protocol that allows single-cell suspension and a serum-free culture condition to maintain the stemness of in vitro cultured cells. This protocol allowed to efficiently disaggregate and expand CRC cells in vitro as well as reach a LV copy number per cell ranging from 0.25 to 5.6. Luciferase gene expression was stable and allowed live-animal monitoring for up to 30 weeks after transplant. CRC-0069 and -0077 PDXs and NCI-H508 and HDC82 cell lines were transduced ex vivo and kept in vitro and/or transplanted in NSG mice. After in vitro or in vivo expansion of the transduced CRCs cetuximab treatment was applied. After an initial shrinking of the tumor mass in mice we observed ACDR in 3 out of 10 mice transplanted with NCI-H508 cells transduced with SFFV-LV and in none of the controls. Genomic DNA from resistant cells is being used for insertion site (IS) analysis to identify common IS, ACDR gene candidates. IS obtained from SIN-LV groups will be used to filter LV integration biases, whereas IS from SFFV-LV transduced cells but not treated with cetuximab will be used to filter mutations that provide a proliferative advantage unrelated to cetuximab treatment. We will validate the most promising candidates by LV-mediated overexpression and knockdown techniques. This approach could pave the way to perform insertional mutagenesis-based forward genetics studies on primary human samples

3. Safety Assessment of SIN LVs Harboring Chromatin Insulators in the Sensitive Cdkn2a-/- In Vivo Genotoxicity Assay Show Enhancer-Blocking Activity of Specific Insulator Sequences

Chromatin insulators (CI) have been proposed as safety features to increase the safety of self-inactivating (SIN) lentiviral vectors (LV) for gene therapy applications.By taking advantage of an in vivo genotoxicity assay based on the systemic injection of LVs in newborn tumor-prone Cdkn2a-/- mice we were able to measure vector-induced genotoxicity as an accelerated tumor onset that was proportional to the genotoxic potential of the tested LV. Importantly, we took advantage of integration sites (IS) analysis to qualitatively characterize CI that were shown by other in vitro and ex vivo studies to function as insulators. Recently we showed for the first time that a CAAT-box binding Nuclear factor 1 (CTF/NF1)-based CI, when cloned in the LTRs of a SIN.LV with a strong SFFV enhancer-promoter in internal position, significantly reduced the frequency of tumors harboring integrations activating Map3k8 oncogene accompanied by a marked skewing towards tumors harboring inactivating insertions targeting Pten.Here by using this stringent in vivo genotoxicity assay and IS analysis in tumors we expanded our studies towards other CI sequences whose function is regulated by the binding of the CCCTC-binding factor (CTCF), the best characterized insulator protein in vertebrates.Each CTCF-based insulating cassette was cloned in the LTRs of a LV construct containing the SFFV promoter in internal position (CTCF.SIN.LVs) and injected in Cdkn2a-/- mice. Interestingly, mice treated with some of the CTCF.SIN.LVs tested displayed an increased median survival time (ranging from 193.5 to 214 days) compared to mice treated with the uninsulated parental SIN.LV (186 days). Importantly, our preliminary IS analysis in tumors (881 IS) showed that two CTCF.SIN.LVs did not target Map3k8 oncogene while Pten was often disrupted by exonic insertions, an escape genotoxicity mechanism on which CI cannot act.These data confirm that the inclusion of two novel CTCF-based CIs of human origin completely abrogated the formation of tumors caused by enhancer-mediated activation of an oncogene in vivo.The ability of these two new insulator elements to block the crosstalk between powerful vector enhancers and cellular regulatory elements increase the safety of SIN LVs and justify their prompt adoption in future gene therapy applications

Torque loss after miniscrew placement: An in-vitro study followed by a clinical trial

7noTo evaluate torque loss a week after insertion, both in an in vivo and an in vitro experimental setup were designed. In the in vivo setup a total of 29 miniscrews were placed in 20 patients who underwent orthodontic treatment. Maximum insertion torque (MIT) was evaluated at insertion time (T1). A week later, insertion torque was measured again by applying a quarter turn (T2); no load was applied on the screw during the first week. In the in vitro setup a total of 20 miniscrews were placed in pig rib bone samples. MIT was evaluated at insertion time (T1). Bone samples were kept in saline solution and controlled environment for a week during which the solution was refreshed every day. Afterwards, torque was measured again by applying a quarter turn (T2). The comparison of MIT over time was done calculating the percentage difference of the torque values between pre- and post-treatment and using the parametric two independent samples t-test or the non-parametric Mann–Whitney test. After a week unloaded miniscrews showed a mean loss of rotational torque of 36.3% and 40.9% in in vitro and in in vivo conditions, respectively. No statistical differences were found between the two different setups. Torque loss was observed after the first week in both study models; in vitro experimental setup provided a reliable study model for studying torque variation during the first week after insertion.openopenMigliorati, Marco; Drago, Sara; Barberis, Fabrizio; Schiavetti, Irene; Dalessandri, Domenico; Benedicenti, Stefano; Biavati, Armando SilvestriniMigliorati, Marco; Drago, Sara; Barberis, Fabrizio; Schiavetti, Irene; Dalessandri, Domenico; Benedicenti, Stefano; Biavati, Armando Silvestrin

27. Aberrant Expression of the Stem Cell microRNA-126 Induces B Cell Malignancy

MicroRNAs are essential regulators of normal and malignant hematopoiesis. miRNAs are relevant for gene therapy, since they can be exploited to fine-tune the expression profile of vector constructs or to alter viral tropism (GentnerN Chiriaco et al, 2014; Escobar et al, 2014) and described the function of miR-126 in HSC where it regulates the balance between quiescence and self-renewal (Lechman et al, 2012). We here report a novel role for miR-126 in the induction and maintenance of high-grade B cell malignancies. By ectopically expressing miR-126 in transplanted BM cells, we observed that up to 60% of mice (n=71) developed B cell malignancies. LV insertion site (IS) analysis revealed that all tumors were monoclonal. We then tracked back leukemic clone to different hematopoietic lineages prospectively purified from the mice 2-6 months before disease onset. IS sharing between normal lineages and leukemic clone suggests stem or multipotent progenitor cell as origin for most tumors. Importantly, we show that miR-126 is the direct cause of genesis and maintenance of leukemia, since leukemogenesis is abolished when miRNA expression is inhibited by doxycycline (doxy) using a tetracycline-repressible miR-126 cassette, and established symptomatic leukemia completely regresses when miR-126 is switched off by doxy through induction of apoptosis. Transcriptional profiling indicated that miR-126 regulates multiple genes in p53 pathway both in murine blasts and in normal human CD34+ cells. Previous work suggested expression of miR-126 in acute lymphoblastic leukemia (ALL) and germinal center lymphoma. To further establish the relevance of miR-126 in human disease, we measured miR-126 expression in blasts from 16 adult patients with ALL. miR-126 was highly expressed in most studied ALL cases (Phil+: n=11, Phil-: n=5), at similar levels as CD34+ cells. We then down-regulated miR-126 in primary blasts from human B-ALL patients (n=5), and we observed increased apoptosis and impaired engraftment in xenograft models after primary and secondary transplantation (miR-126/KD: n=32 mice; Ctrl: n=37 mice), demonstrating the relevance of miR-126 in human B-ALL. In conclusion, we present a novel spontaneous mouse model for high grade B cell malignancies which are addicted to miR-126 expression, provide insight into the dynamic process of leukemogenesis by clonal IS tracking and unveil key tumor signaling pathways controlled by miR-126. Down-regulation of miR-126 could be exploited as therapeutic strategy in ALL, since it would deplete leukemic cells while expanding normal HSC, two ways to restore normal hematopoieis

731. Hematopoietic Stem Cell Gene Transfer and Integration Site Analysis in Tumor-Prone Mice Uncovers Low Genotoxicity of Lentiviral Vector Integration

Insertional mutagenesis represents a major hurdle to successful gene therapy and mandates for sensitive pre-clinical assays of genotoxicity. Cdkn2a|[minus]|/|[minus]| mice are defective for p53 and Rb pathways, and are susceptible to a broad range of cancer-triggering genetic lesions. We exploited the sensitivity of these tumor-prone mice to develop an in-vivo genotoxicity assay, based on transplantation of Cdkn2a|[minus]|/|[minus]| hematopoietic stem cells (HSC), treated or not with prototypical retroviral (RV) and lentiviral (LV) vectors. In our rationale if RV or LV treatment is genotoxic, then transplanted mice will show a significantly earlier tumor onset. The sensitivity of the model was shown by the ability to detect a vector dose-dependent acceleration in tumor onset in mice transplanted with RV-treated cells. Such acceleration, as in previous studies, is consequent to genetic lesions, produced by vector integration, that cooperate with the germ-line mutation, and is contingent on LTR activity

Promoter trapping reveals significant differences in integration site selection between MLV and HIV vectors in primary hematopoietic cells

Recent reports have indicated that human immunodeficiency virus (HIV) and murine leukemia virus (MLV) vectors preferentially integrate into active genes. Here, we used a novel approach based on genetic trapping to rapidly score several thousand integration sites and found that MLV vectors trapped cellular promoters more efficiently than HIV vectors. Remarkably, 1 in 5 MLV integrations trapped an active promoter in different cell lines and primary hematopoietic cells. Such frequency was even higher in growth-stimulated lymphocytes. We show that the different behavior of MLV and HIV vectors was dependent on a different integration pattern within transcribed genes. Whereas MLV-based traps showed a strong bias for promoter-proximal integration leading to efficient reporter expression, HIV-based traps integrated throughout transcriptional units and were limited for expression by the distance from the promoter and the reading frame of the targeted gene. Our results indicate a strong propensity of MLV to establish transcriptional interactions with cellular promoters, a behavior that may have evolved to enhance proviral expression and may increase the insertional mutagenesis risk. Promoter trapping efficiency provides a convenient readout to assess transcriptional interactions between the vector and its flanking genes at the integration site and to compare integration site selection among different cell types and in different growth conditions