Recent Developments in Gene Therapy Research Targeted to Cerebellar Disorders

The cerebellum plays an important role in coordinated movement, motor learning and vestibular function. Cerebellar damage results in impaired body balance and disturbance in gait and posture. The cerebellum is impaired by various genetic diseases, such as spinocerebellar ataxia and mucopolysaccharidosis, and these diseases could be good candidates for gene therapy. There are at least two challenges that should be overcome before the clinical application of gene therapy is used to treat cerebellar disorders. The first challenge results from its large size, as the cerebellum is the second largest component of the central nervous system. The cerebellum can be subdivided into three main parts: the cerebrocerebellum, the vestibulocerebellum and the spinocerebellum (see Section 2 for details). Each subdivision in the cerebellum plays a distinct role and, to attain a satisfactory rescue of the cerebellar function by gene therapy, a therapeutic gene should be delivered efficiently and extensively into the large cerebellum. The second challenge is to deliver a gene into specific target cell populations in the cerebellum. In Parkinson's disease, which is caused by degeneration of nigra-striatal dopaminergic neurons, cell type-specific delivery of a therapeutic gene is a secondary matter, as the supply of sufficient amounts of dopamine, irrespective of neurons or glia, in the striatum is most critical for the functional recovery of the basal ganglia. By contrast, specific cell populations within the cerebellum, such as cerebellar Purkinje cells, Bergman glia or neurons in the deep cerebellar nuclei, are selectively impaired in most cerebellar diseases, such as spinocerebellar ataxia. Thus, affected cell types differ depending on the disease type, and the selective delivery of a therapeutic gene to a subset of affected cell types could be a key advance for rescuing cells that are degenerating from progressive damage, restoring cerebellar function and, ultimately, helping patients to recover from cerebellar ataxia. To this end, we have developed methods that allow for Purkinje cell-specific and Bergmann glia-specific gene expression in mice by modifying the culture conditions of lentiviral vector-producing human embryonic kidney (HEK) 293FT cells in combination with celltype-specific promoters in lentiviral vectors. Moreover, a new injection technique for efficient and widespread gene delivery into the cerebellar cortex has been devised, which takes advantage of the anatomical location of the cerebellum. Using the newly developed gene transfer method for the cerebellum, we aimed to restore the abnormal phenotypes of two types of ataxic mice, both of which are affected in Purkinje cells by different