WP 2 : Gene therapy

WP coordinator(s): P. Moullier (Node 5) & P. Aubourg /N. Cartier (Node 4).
WPs collaborators: P. Hantraye (Node 1) ; M. Peschanski/P. Moullier (Node 5) ; P. Aubourg (Node 4) ; N.Cartier (Node 4) ; P. Remy (Node 3)

Objectives and perimeter:

The advent of viral gene therapy technology has greatly contributed to the study of human central nervous system (CNS) diseases in animal models, and based on encouraging results on safety and efficacy in several human clinical trials, there is an increasing hope for wider clinical translation of gene therapy for CNS diseases to the clinic. In face of the challenges presented by the complexity and unique biology of the CNS, many vector types have been used, including Lentivirus (LV), Adenovirus, Herpes-Simplex virus and Adeno-Associated Virus (AAV), AAV vectors being the most widely used for their safety profile and ability to drive stable long-term expression of therapeutic transgenes in the brain of rodents, primates, and humans. Therefore the current work package addresses AAV-related issues that are currently still considered as limiting factors for the successful clinical translation of CNS gene therapy for a large range of neurological disorders.

ISSUES: -AAVs have up to now been exclusively administered directly in situ in clinical trials (Parkinson’s disease, for example), but new recently developed AAV serotypes that cross the blood-brain-barrier after peripheral intravascular administration have raised hope for a larger delivery of therapeutic genes into the CNS (1-4). Finally, the intrathecal delivery of AAV has opened the possibility to target therapeutic genes to the spinal cord, dorsal root ganglia, cerebellum and possibly also into brain (4-7). Significant progress has also been made to customize an AAV capsid that allows specific brain cell targeting, such as endothelial cells (8). All these new developments in AAV technology raise important new issues for translational research: a) the optimization of vector delivery whatever the route of administration to achieve safe but also wider diffusion of the vector in the targeted brain structure or into the whole brain (9); b) the prediction of vector diffusion in the human brain (10) from studies performed in large animals, particularly in non-human primates (NHPs) ; c) the development of new AAV vectors targeting therapeutic transgene in specific neuronal and glial (oligodendrocyte, astrocytes, microglia) cell populations (10).

• Importantly, a critical challenge that concerns AAV vectors is to improve the manufacturing of research grade AAV in order to meet the quality regulatory constraints required for Phase I/II trials. The purpose of such high standard is to accelerate the translation from the proof of concept to the clinic, given also that marked differences in terms of neuronal cell targeting/transduction and vector diffusion are often observed between current research grade and GMP-grade AAVs.
• Finally, although the CNS is often considered as an immuno privileged site, there is a key issue to control the host immune response against the therapeutic transgene to increase the safety and efficacy of CNS gene therapy (12).

State of the art:

AAV is a 20-25 nm non-pathogenic parvovirus. More than 12 serotypes are available as recombinant vectors providing multiple transduction patterns in vivo. It has an excellent safety profile and has been evaluated in several Phase I/II trials for neurodegenerative diseases including Parkinson, Canavan, Batten and Alzheimer diseases. Whereas global gene delivery to the CNS can only be achieved to a certain extent by cumbersome procedures, a stereotaxic injection leads essentially to local transduction with limited expression of the therapeutic transgene from injection sites, excepting when the therapeutic transgene encodes for a secreted protein or enzyme. Multiple site injections to address neurodegenerative disorders can potentially increase the occurrence of adverse effects.

• The current leads in the field are: (i) the evaluation of alternative routes for vector delivery; (ii) the engineering of current  AAV vectors to improve diffusion and infectivity as well as selective brain cell tropisms; (iii) the actual behaviour of the brain immune system towards the gene therapy product.