Retrovirus Vectors: Toward the Plentivirus?

Targeted genome modifications using integrase-deficient lentiviral vectors

Gene correction aims at repairing a defective gene directly in the cellular genome, which warrants tissue-specific and sustained expression of the repaired gene through its endogenous promoter. We have developed a novel system based on integrase-deficient lentiviral vectors (IDLVs) that allows us to correct an endogenous mutation using a strategy based on homologous recombination (HR). In a proof-of-concept approach, an IDLV encoding a repair template was co-delivered with an I-SceI nuclease expression vector to rescue a defective enhanced green fluorescent protein (EGFP) gene. Expression of the nuclease created a double-strand break within the target locus, which was crucial for stimulating IDLV-based gene repair. Stable gene correction was realized in up to 12% of the cells, depending on the vector dose, the nuclease expression levels, and the cell type. Genotypic analyses confirmed that gene correction was the result of genuine HR between the target locus and the IDLV repair template. This study presents IDLVs as valuable tools for introducing precise and permanent genetic modifications in human cells.

T-cell receptor gene therapy for cancer: the progress to date and future objectives

In the last decade research has begun into the use of T-cell receptor (TCR) gene therapy as a means to control and eradicate malignancies. There is now a large body of evidence to demonstrate that through the use of this technology one can redirect T-cell antigen specificity to produce both cytotoxic and helper T cells, which are functionally competent both in vitro and in vivo and show promising antitumour effects in humans. This review focuses on the means by which TCR gene transfer is achieved and the recent advances to modify the TCRs and vector delivery systems which aim to enhance the efficiency and safety of TCR gene transfer protocols.

Live and let die: in vivo selection of gene-modified hematopoietic stem cells via MGMT-mediated chemoprotection

Gene transfer into hematopoietic stem cells (HSC) provides a potential means of correcting monogenic defects and altering drug sensitivity of normal bone marrow to cytotoxic agents. These applications have significant therapeutic potential but the translation of successful murine studies into human therapies has been hindered by low gene transfer in large animals (including humans), and recent serious side effects in a human immunodeficiency trial related to insertional mutagenesis. The latter trial, along with other subsequent trials, while bringing into focus the potential risks of integrating vector systems, also clearly demonstrate the potential usefulness of in vivo selection as it relates to inefficient stem cell transduction. Developing from initial studies by our group and other investigators in which drug resistance was utilized to demonstrate the feasibility of using gene transfer to effect protection from myelotoxicity of chemotherapeutic agents, expression of mutant forms of O(6)-methyguanine-DNA-methytransferase (MGMT) coupled with the simultaneous use of pharmacologic inhibitors and chemotherapeutic agents has been shown to provide a powerful method to select HSC in vivo. While stem and progenitor cell protection and resulting selection in vivo has potential applications for the treatment of selected cancers (allowing dose escalation) and for correction of monogenic disease (allowing an iatrogenic survival advantage of transduced cells in vivo), such an in vivo selection may have untoward effects on stem cell behavior. These deleterious effects may include stem cell exhaustion; lineage skewing; accumulation of genotoxic lesions; and clonal dominance driven towards a pro-leukemic phenotype. Knowledge of the likelihood of such deleterious events occurring as well as their potential implications will be critical to future clinical applications and may also enhance our understanding of both normal stem cell behavior and the evolution of hematopoietic malignancies.

RNAi therapy for HIV infection: principles and practicalities

Inside eukaryotic cells, small RNA duplexes, called small interfering RNAs (siRNAs), activate a conserved RNA interference (RNAi) pathway which leads to specific degradation of complementary target mRNAs through base-pairing recognition. As with other viruses, studies have shown that replication of the HIV-1 in cultured cells can be targeted and inhibited by synthetic siRNAs. The relative ease of siRNA design and the versatility of RNAi to target a broad spectrum of mRNAs have led to the promise that drug discovery in the RNAi pathway could be effective against pathogens.

This review discusses the current experimental principles that guide the application of RNAi against HIV and describes challenges and limitations that need to be surmounted in order for siRNAs to become practical antiviral drugs. The practical use of RNAi therapy for HIV infection will depend on overcoming several challenges, including the ability to establish long-term expression of siRNA without off-target effects and the capacity to counteract mutant escape viruses.

The Genetic Engineering of Hematopoietic Stem Cells: the Rise of Lentiviral Vectors, the Conundrum of the LTR, and the Promise of Lineage-restricted Vectors

Recent studies on the integration patterns of different categories of retroviral vectors, the genotoxicity of long-terminal repeats (LTRs) and other genetic elements, the rise of lentiviral technology and the emergence of regulated vector systems providing tissue-restricted transgene expression and RNA interference, are profoundly changing the landscape of stem cell-based therapies. New developments in vector design and an increasing understanding of the mechanisms underlying insertional oncogenesis are ushering in a new phase in hematopoietic stem cell (HSC) engineering, thus bringing the hitherto exclusive reliance on LTR-driven, gamma-retroviral vectors to an end. Based on their ability to transduce non-dividing cells and their genomic stability, lentiviral vectors offer new prospects for the manipulation of HSCs. Tissue-specific vectors, as exemplified by globin vectors, not only provide therapeutic efficacy, but may also enhance safety, insofar that they restrict transgene expression in stem cells, progenitor cells and blood cells in all but the transcriptionally targeted lineage. This review provides a survey of these advances as well as several remaining challenges, focusing in particular on the importance of achieving adequate levels of protein expression from a limited number of vector copies per cell-ideally one to two.

Replication-competent Vectors and Empty Virus-like Particles: New Retroviral Vector Designs for Cancer Gene Therapy or Vaccines

Replication-defective vectors based on murine oncoretroviruses were the first gene transfer vectors to be used in successful gene therapies. Despite this achievement, they have two major drawbacks: insufficient efficacy for in vivo gene transfer and insertional mutagenesis. Attempts to overcome these problems have led to two retroviral vector designs of principally opposite character: replication-competent vectors transducing largely intact genomes and genome-free vectors. Replication-competent retroviral vectors have achieved dramatically improved efficacy for in vivo cancer gene therapy and genome-free retroviral vectors expressing different kinds of antigens have proven excellent as immunogens. Current developments aim to improve the safety of the replication-competent vectors and to augment the production efficiency of the genome-free vectors by expression from heterologous viral or non-viral vectors. Together with the continuous advances of classical defective retroviral vectors for ex vivo gene therapy, these developments illustrate that, due to their tremendous design versatility, retroviral vectors remain important vectors for gene therapy applications.