Viral vectors for therapy of neurologic diseases

Abstract

Neurological disorders - disorders of the brain, spine and associated nerves - are a leading contributor to global disease burden with a shockingly large associated economic cost. Various treatment approaches - pharmaceutical medication, device-based therapy, physiotherapy, surgical intervention, among others - have been explored to alleviate the resulting extent of human suffering. In recent years, gene therapy using viral vectors - encoding a therapeutic gene or inhibitory RNA into a "gutted" viral capsid and supplying it to the nervous system - has emerged as a clinically viable option for therapy of brain disorders. In this Review, we provide an overview of the current state and advances in the field of viral vector-mediated gene therapy for neurological disorders. Vector tools and delivery methods have evolved considerably over recent years, with the goal of providing greater and safer genetic access to the central nervous system. Better etiological understanding of brain disorders has concurrently led to identification of improved therapeutic targets. We focus on the vector technology, as well as preclinical and clinical progress made thus far for brain cancer and various neurodegenerative and neurometabolic disorders, and point out the challenges and limitations that accompany this new medical modality. Finally, we explore the directions that neurological gene therapy is likely to evolve towards in the future. This article is part of the Special Issue entitled "Beyond small molecules for neurological disorders".

Keywords: Brain tumors; Gene therapy; Lysosomal storage diseases; Metabolic diseases; Neurodegeneration; Viral vectors.

Fig. 1

Mechanism of receptor-mediated host cell entry and subsequent gene transfer by viral vectors. A. Retroviral and lentiviral vectors. The viral attachment glycoproteins on the envelope surface of retroviral and lentiviral vectors bind to the cognate cell surface receptors and co-receptors and subsequently fuse to the cell membrane to enter the host cell. Once in the cytoplasm, the virion partially uncoats and the RNA transgene is reverse transcribed to the cDNA form. Nuclear entry is followed by integration of the viral cDNA into the host genome. B. Adenoviral vectors. These vectors bind to the CAR receptor on the host cell surface, through the fiber knob on the capsid. This facilitates cell entry by endocytosis through clathrin-coated pit pathway and subsequent conversion to an endosome. A reduction in pH in the endosome triggers the release of the degraded virion into the cytoplasm. The escaped virion releases its viral double stranded DNA into the nucleus through the nuclear pore. C. Herpes simplex virus (HSV) vectors. Glycoproteins gB and gC on the HSV virion envelope mediate the initial attachment of virus particles to HSPG on the host cell surface. gB binding to PILRα and gD binding to HVEM, nectin-1 or nectin-2 then triggers membrane fusion, followed by release of the capsids into the host-cell cytoplasm. Post cytoplasmic entry, the intact viral capsid travels to the nucleus via microtubule-dynein mediated transport, where the viral DNA enters the nucleus through the nuclear pore. D. Adeno associated virus (AAV) vectors. Receptor ligands on the surface of AAV capsids vary with serotype, and allow capsids to bind to a variety of glycan receptors on the host cell surface (HSPG, N- and O-linked sialic acids, galactose). Once bound, the virion gets internalized via clathrin-coated vesicles/endosomes and trafficked to the nuclear area while still associated with the endosome. Endosomal acidification leads to release of the damaged capsid, which is transported into the nucleus. Capsid uncoating in the nucleus is followed by conversion of the single-stranded vector genome to double-stranded DNA. The double-stranded vector genome persists as stable circular and/or concatemeric extrachromosomal episomes. Abbreviations: CAR – coxsackievirus and adenovirus receptor, Env - envelope glycoprotein, HSPG – heparin sulfate glycoprotein, HVEM – herpesvirus entry mediator, LTR – long terminal repeats, PI3K – phosphatidylinositol-3 kinase, PILRα – paired immunoglobulin-like type 2 receptor-α.

Fig. 2

Modes of genetic modification and gene delivery in the nervous system. A: Genetic modification. Gene composition or expression in cells can be modified by introduction of DNA (e.g. expression plasmids), RNA oligonucleotides (e.g. siRNA, microRNA) and genes encoded in virus vectors. These are typically delivered by direct injection into the brain parenchyma (P) or ventricles (Ve), the spinal cord or the eye. These DNA/RNA sequences can also be used to genetically modify (GM) cells for delivery through direct injection or vascular infusion. These different vehicles: genes (G), vectors (V), oligonucleotides (O) and cells (C) can be combined in different modalities, including “jump starting” primary and stem cells to take on specific phenotypes through transcriptional regulation. B: Routes of gene delivery to the CNS. Vectors or cells can be introduced into a specific region of the brain by stereotactic surgery. In cases where more extensive vector distribution is desired, CED of viral vectors into the brain improves considerably their range in target structures by increasing transduction volumes. This technique can yield volumes of transduced cell distribution 3–3.5-fold larger than the infused volume, which is highly significant for human applications. Viral vectors or secreted transgene products (growth factors, lysosomal enzymes) can be further distributed from the primary target structure by axonal transport of vectors and/or by products and by release of products into the extracellular space. Infusion of vectors or oligonucleotides into the brain ventricular system, or intrathecal space of the spinal canal or subarachnoid space, leads to widespread CNS distribution via CSF flow. An alternative strategy is to use viral vectors to engineer ependymal cells lining the ventricles or choroid plexus cells to secrete therapeutic proteins into CSF. The BBB with its many constituents has thwarted most gene transfer vehicles from entering the brain from the vasculature. However some AAV serotypes, e.g. AAV9, AAVrh8 and AAVrh10, as well as vesicles containing AAV particles are able to transit this barrier quite efficiently, although the exact mechanism of that transit is not known. Fig. 2 and associated figure legend modified from Breakefield and Sena-Esteves (2010) and Bowers et al. (2011).