Gene transfer to the central nervous system: Current state of the art of the viral vectors

Neurons in the CNS establish exceedingly complex and precise networks organised via specific synaptic connections that ultimately determine the cellular basis of cognitive processes and behaviour. This fragile and intricate circuitry presents a challenging barrier for fundamental neurobiology studies or clinical gene therapy where long-term genetic modification is wanted. Small volumes, low toxicity, minimal immune reaction, slow delivery times, and preferential targeting of specific cell types in selected subregions are often sine qua non for vector-mediated gene transfer. This review addresses the state-of-the-art of gene transfer to the CNS, in particular the use of adenovirus, herpes simplex virus, adeno-associated virus, simian virus 40 (SV40), lentivirus and alphavirus vectors. The advantages and drawbacks of these molecular tools with respect to their tropism; ability to traffic via axoplasmic retrograde transport; duration of transgene expression; innate, adaptive and memory immunity; and toxicity are discussed.