Current Gene Therapy - Volume 8, Issue 6, 2008

Vectors based on non-HIV lentiviruses are opening up new approaches for the treatment of human disorders. These vectors efficiently deliver genes into many different types of cells from a broad range of species including man and the resulting gene expression is long-term. These features make them very attractive to be transformed into tools for gene therapy. HIV-1 based lentiviral vectors were initially developed, a process which provided valuable insights into the biology of these vectors allowing progressive improvement of non-HIV vectors. The latest vectors have been refined to a very high level and can be produced safely for the clinic. This review will describe the general features of lentiviral vectors with particular emphasis on vectors derived from the non-HIV lentiviruses such as equine infectious anaemia virus (EIAV), simian immunodeficiency virus (SIV), and feline immunodeficiency virus (FIV). It will then describe some key examples of gene therapy applications in neurological diseases such as Parkinson's disease (PD), motor neuron diseases, lysosomal storage diseases and ocular disorders. Finally, the prospects for clinical application of non-HIV lentiviral vectors for these disorders will also be outlined.

An essential step of the life cycle of retroviruses is the stable insertion of a copy of their DNA genome into the host cell genome, and lentiviruses are no exception. This integration step, catalyzed by the viral-encoded integrase, ensures long-term expression of the viral genes, thus allowing a productive viral replication and rendering retroviral vectors also attractive for the field of gene therapy. At the same time, this ability to integrate into the host genome raises safety concerns regarding the use of retroviral-based gene therapy vectors, due to the genomic locations of integration sites. The availability of the human genome sequence made possible the analysis of the integration site preferences, which revealed to be nonrandom and retrovirus-specific, i.e. all lentiviruses studied so far favor integration in active transcription units, while other retroviruses have a different integration site distribution. Several mechanisms have been proposed that may influence integration targeting, which include (i) chromatin accessibility, (ii) cell cycle effects, and (iii) tethering proteins. Recent data provide evidence that integration site selection can occur via a tethering mechanism, through the recruitment of the lentiviral integrase by the cellular LEDGF/p75 protein, both proteins being the two major players in lentiviral integration targeting.

Lentiviral vectors are among the most efficient gene transfer tools for dividing and non-dividing cells. However, insertional mutagenesis has been observed in clinical trials with oncoretroviral vectors and this has prompted detailed study of genotoxicty of all integrating vectors. For many applications, avoiding integration is the most straightforward approach to overcome this problem and is facilitated by the extensive studies of the integrating mechanisms of lentiviruses. Indeed, non-integrating lentiviral vectors have been developed by mutating the integrase gene or by modifying the attachment sequences of the LTRs. In this review, we first consider on the toxicity associated with integration and on lentivirus integrase biology, and discuss the implications of integrase mutant studies for the development of non-integrating lentiviral vectors. We review published data concerning non-integrating lentiviral vectors with particular focus on their residual integration and transgene expression efficiency. Finally, the latest advances in the development of genetic engineering tools derived from non-integrating lentiviral vectors are presented.

It is generally accepted that active therapeutic immunization approaches hold great promise for treating malignant tumors. In recent years, lentiviral vectors have emerged as promising tools for anti-tumor immunotherapy due to their capacity to transduce a wide range of different dividing and non-dividing cell types, including tumor cells and dendritic cells (DC). The latter are considered to be the key regulators of immunity and are therefore applied as ‘nature’s adjuvant’ in terms of eliciting strong antigen-specific cytotoxic T lymphocyte responses against tumor antigens. Therefore, lentiviral vectors have been carefully examined as gene transfer vehicles, be it for ex vivo or in vivo modification of DC and have been demonstrated to induce potent T cell mediated immune responses that can control tumor growth. Here, we review the use of lentivirally transduced DC and lentiviral vectors - as such - as an anti-tumor immunotherapeutic. Furthermore, we focus on the DC modulatory capacity of lentiviral vectors and the various efforts that have been made to improve the overall performance and safety of in vivo administration of lentiviral vectors. In conclusion, this review highlights the potential of lentiviral vectors as a generally applicable ‘off-the-shelf’ therapeutic for anti-tumor immunotherapy.

Vectors derived from retroviruses such as lentiviruses and onco-retroviruses are probably among the most suitable tools to achieve a long-term gene transfer since they allow stable integration of a transgene and its propagation in daughter cells. Lentiviral vectors should be preferred gene delivery vehicles over vectors derived from onco-retroviruses (MLV) since in contrast to the latter they can transduce non-proliferating target cells. Moreover, lentiviral vectors that have the capacity to deliver transgenes into specific tissues are expected to be of great value for various gene transfer approaches in vivo. Here we provide an overview of innovative approaches to upgrade lentiviral vectors for tissue or cell targeting and which have potential for in vivo gene delivery. In this overview we distinguish between three types of lentiviral vector targeting strategies (Fig 1): 1) targeting of vectors at the level of vector-cell entry through lentiviral vector surface modifications; 2) targeting at the level of transgene transcription by insertion of tissue specific promoters into lentiviral vectors; 3) a novel microRNA technology that rather than targeting the ‘right’ cells will ‘detarget’ transgene expression from non-target cells while achieving high expression in the target-cell. It is clear that each strategy is of enormous value for several gene therapy approaches but combining these three layers of transgene expression control will offer tools to really overcome several drawbacks in the field such as side-effect of off-target expression, clearance of transgene modified cells by immune response to the transgene and lack of biosecurity and efficiency in in vivo approaches.

Recombinant lentiviral vectors (rLV) are powerful tools for gene transfer to the central nervous system (CNS) and hold great potential as a therapeutic gene therapy strategy for neurological disorders. Recent data indicate that rLVs are suitable for functional studies in the CNS by over expression or knock down of specific proteins. Based on a variety of lentiviruses species, different vector systems have been developed. However, the most commonly used rLV vector is based on the human immunodeficiency virus 1 (HIV-1). Here we describe the use of such vectors to achieve cell-specific transgene expression in the brain. In this setting, rLVs are versatile tools both due to their relatively large cloning capacity and their ability to transduce non-dividing cells. Furthermore, we discuss the preclinical development of gene therapy based on enzyme replacement and/or delivery of neurotrophic factors for neurodegenerative diseases and CNS manifestations of lysosomal storage diseases. Neuroprotective strategies that aim to deliver glial cell line-derived neurotrophic factor and ciliary neurotrophic factor for Parkinson's and Huntington's diseases in particular have been documented with success in appropriate animal models. More recently, rLVs were shown to be suitable to express small interfering RNA for treatment in models of Alzheimer's disease and amyotrophic lateral sclerosis. Finally, we present a review of the use of rLVs to model neurodegenerative diseases. rLVs have proven to be a very versatile tool to create genetic models of both Parkinson's and Huntington's diseases and thus provide possibilities to study complex genetic interactions in otherwise wild-type animals evading the necessity to create transgenic mice. Moreover, the potential of these vectors in the development of gene therapy to treat neurological disorders is considerable, which is supported by the fact that clinical trials using rLVs are underway.

Originally developed as a new category of retroviral vectors that are capable of transducing non-dividing cells, vectors based on lentiviruses have been shown to incorporate a number of additional features that are of potential value for clinical gene therapy. These include the utilisation of biological properties of the lentiviral accessory proteins Tat and Rev, which allow conditional mRNA expression and mediate a “stabilisation” of the genomic vector RNA in packaging cells; the integration pattern, which, when compared to gammaretroviral vectors, is less likely to affect promoter-proximal windows or regulatory regions located in DNAse1 hypersensitive sites of cellular genes; and a relatively robust gene expression even in cells that are at relatively high risk of epigenetic transgene silencing. Here, we discuss the mechanisms underlying these potential advantages and their importance for the development of clinical grade gene vectors. We conclude with an overview of clinical trials in which lentiviral vectors have been or are currently being used to counteract advanced forms of HIV infection, treat inherited disorders affecting hematopoietic cells, or transduce neuronal cells of the central nervous system for the treatment of Parkinson disease. As information on most clinical trials is not yet available in the form of peer-reviewed papers, this list may be incomplete. Some additional applications that are expected to lead to the initiation of clinical trials in the near future are also discussed.

Lentiviral vectors are potent gene delivery vehicles that enable stable expression of transgenes in both dividing and post-mitotic cells. Development of lentiviral vectors expressing small hairpin RNAs generates a system that can be used to down regulate specific target genes in vivo and in vitro. In this review, we will discuss two examples of in vivo applications for the use of lentiviral vectors expressing shRNAs: Gene therapy of neurological disorders and generation of transgenic knockdown animals.