Lentiviral MGMT(P140K)-mediated in vivo selection employing a ubiquitous chromatin opening element (A2UCOE) linked to a cellular promoter

Lentiviral-Mediated Systemic RNA Interference In Vivo

The shRNA-encoding lentivirus has been widely used for gene manipulation in preclinical studies. It is a powerful tool for gene transfer and shows promise in its ability to efficiently transduce immune cells and hematopoietic stems cells, which are the initial therapeutic target of autoimmune diseases, and considering that gene manipulation of these cells is usually difficult to achieve using other techniques. In previous chapters, we have described how to produce concentrated shRNA-encoding lentiviral particles. Here, systemic in vivo application of lentivirus, including viral quantification prior to injection, intraperitoneal injection, and quantification of integrated provirus, is introduced.

Sustained and regulated gene expression by Tet-inducible "all-in-one" retroviral vectors containing the HNRPA2B1-CBX3 UCOE®

Genetic modification of induced pluripotent stem (iPS) cells may be necessary for the generation of effector cells for cellular therapies. Hereby, it can be important to induce transgene expression at restricted and defined time windows, especially if it interferes with pluripotency or differentiation. To achieve this, inducible expression systems can be used such as the tetracycline-inducible retroviral vector system, however, retroviral expression can be subjected to epigenetic silencing or to position-effect variegation. One strategy to overcome this is the incorporation of ubiquitous chromatin opening elements (UCOE^®'s) into retroviral vectors to maintain a transcriptionally permissive chromatin state at the integration site. In this study, we developed Tet-inducible all-in-one gammaretroviral vectors carrying different sized UCOE^®'s derived from the A2UCOE. The ability to prevent vector silencing by preserving the Tet-regulatory potential was investigated in different cell lines, and in murine and human iPS cells. A 670-bp fragment spanning the CBX3 promoter region of A2UCOE (U670) was the most potent element in preventing silencing, and conferred the strongest expression from the vector in the induced state. While longer fragments of A2UCOEs also sustained expression, vector titers and induction efficiencies were impaired. Finally, we demonstrate that U670 can be used for constitutive expression of the transactivator in the all-in-one vector for faithful regulation of transgenes by doxycycline, including the thrombopoietin receptor Mpl conferring cytokine-dependent cell growth.

Lentiviral-Mediated Systemic RNA Interference In Vivo

The shRNA-encoding lentivirus has been widely used for gene manipulation in preclinical studies. It is a powerful tool for gene transfer and shows promise in its ability to efficiently transduce immune cells and hematopoietic stem cells, which are the initial therapeutic target of autoimmune diseases, considering that gene manipulation of these cells is usually difficult to achieve using other techniques. In previous chapters, we have described how to produce concentrated shRNA-encoding lentiviral particles. Here, systemic in vivo application of lentivirus, including viral quantification prior to injection, intraperitoneal injection, and quantification of integrated provirus, is introduced.

The CpG-sites of the CBX3 ubiquitous chromatin opening element are critical structural determinants for the anti-silencing function

Suppression of therapeutic transgene expression from retroviral gene therapy vectors by epigenetic defence mechanisms represents a problem that is particularly encountered in pluripotent stem cells (PSCs) and their differentiated progeny. Transgene expression in these cells, however, can be stabilised by CpG-rich ubiquitous chromatin opening elements (UCOEs). In this context we recently demonstrated profound anti-silencing properties for the small (679 bp) CBX3-UCO element and we now confirmed this observation in the context of the defined murine chromosomal loci ROSA26 and TIGRE. Moreover, since the structural basis for the anti-silencing activity of UCOEs has remained poorly defined, we interrogated various CBX3 subfragments in the context of lentiviral vectors and murine PSCs. We demonstrated marked though distinct anti-silencing activity in the pluripotent state and during PSC-differentiation for several of the CBX3 subfragments. This activity was significantly correlated with CpG content as well as endogenous transcriptional activity. Interestingly, also a scrambled CBX3 version with preserved CpG-sites retained the anti-silencing activity despite the lack of endogenous promoter activity. Our data therefore highlight the importance of CpG-sites and transcriptional activity for UCOE functionality and suggest contributions from different mechanisms to the overall anti-silencing function of the CBX3 element.

Lentivector Knockdown of CCR5 in Hematopoietic Stem and Progenitor Cells Confers Functional and Persistent HIV-1 Resistance in Humanized Mice

Gene-engineered CD34(+) hematopoietic stem and progenitor cells (HSPCs) can be used to generate an HIV-1-resistant immune system. However, a certain threshold of transduced HSPCs might be required for transplantation into mice for creating an HIV-resistant immune system. In this study, we combined CCR5 knockdown by a highly efficient microRNA (miRNA) lentivector with pretransplantation selection of transduced HSPCs to obtain a rather pure population of gene engineered CD34(+) cells. Low-level transduction of HSPCs and subsequent sorting by flow cytometry yielded >70% transduced cells. Mice transplanted with these cells showed functional and persistent resistance to a CCR5-tropic HIV strain: viral load was significantly decreased over months, and human CD4(+) T cells were preserved. In one mouse, viral mutations, resulting presumably in a CXCR4-tropic strain, overcame HIV resistance. Our results suggest that HSPC-based CCR5 knockdown may lead to efficient control of HIV in vivo. We overcame a major limitation of previous HIV gene therapy in humanized mice in which only a proportion of the cells in chimeric mice in vivo are anti-HIV engineered. Our strategy underlines the promising future of gene engineering HIV-resistant CD34(+) cells that produce a constant supply of HIV-resistant progeny.

IMPORTANCE

Major issues in experimental long-term in vivo HIV gene therapy have been (i) low efficacy of cell transduction at the time of transplantation and (ii) transduction resulting in multiple copies of heterologous DNA in target cells. In this study, we demonstrated the efficacy of a transplantation approach with a selection step for transduced cells that allows transplantation of an enriched population of HSPCs expressing a single (low) copy of a CCR5 miRNA. Efficient maintenance of CD4(+) T cells and a low viral titer resulted only when at least 70% of the HIV target cells were genetically modified. These findings imply that clinical protocols of HIV gene therapy require a selective enrichment of genetically targeted cells because positive selection of modified cells is likely to be insufficient below this threshold. This selection approach may be beneficial not only for HIV patients but also for other patients requiring transplantation of genetically modified cells.

Recent trends in the gene therapy of β-thalassemia

The β-thalassemias are a group of hereditary hematological diseases caused by over 300 mutations of the adult β-globin gene. Together with sickle cell anemia, thalassemia syndromes are among the most impactful diseases in developing countries, in which the lack of genetic counseling and prenatal diagnosis have contributed to the maintenance of a very high frequency of these genetic diseases in the population. Gene therapy for β-thalassemia has recently seen steadily accelerating progress and has reached a crossroads in its development. Presently, data from past and ongoing clinical trials guide the design of further clinical and preclinical studies based on gene augmentation, while fundamental insights into globin switching and new technology developments have inspired the investigation of novel gene-therapy approaches. Moreover, human erythropoietic stem cells from β-thalassemia patients have been the cellular targets of choice to date whereas future gene-therapy studies might increasingly draw on induced pluripotent stem cells. Herein, we summarize the most significant developments in β-thalassemia gene therapy over the last decade, with a strong emphasis on the most recent findings, for β-thalassemia model systems; for β-, γ-, and anti-sickling β-globin gene addition and combinatorial approaches including the latest results of clinical trials; and for novel approaches, such as transgene-mediated activation of γ-globin and genome editing using designer nucleases.