The use of herpes simplex virus vectors for gene therapy in neurological diseases

Herpes simplex virus type 1 (HSV-1) vectors have now been developed and enable the efficient delivery of foreign genes under the control of appropriate promoter elements into non-dividing neurons in vitro and in vivo. Their use is based on the natural ability of HSV-1 to spread throughout the nervous system and to establish a lifelong latent infection in neurons. HSV is present in an episomal form in the neuronal nucleus, and normal neuronal functions remain unaltered. A wide variety of foreign genes can theoretically be packaged into the large HSV genome. A number of technical problems will need to be overcome to ensure the stable expression of the foreign gene products, adequate control of the levels of their expression, the safety of the vectors and the correct targeting of the vectors to the appropriate neuronal cell populations. Such vectors have the potential to replace missing gene products in neurons in patients with a variety of metabolic and neurodegenerative diseases, and also to insert growth factors or enzymes into the local vicinity of neurological lesions to promote neuronal repair. HSV-1 vectors also have the potential to define the genetic basis of various neurophysiological functions which may prove to be useful in evaluating altered neuronal function encountered in disease.