Genetic therapy for the nervous system

Abstract

Genetic therapy is undergoing a renaissance with expansion of viral and synthetic vectors, use of oligonucleotides (RNA and DNA) and sequence-targeted regulatory molecules, as well as genetically modified cells, including induced pluripotent stem cells from the patients themselves. Several clinical trials for neurologic syndromes appear quite promising. This review covers genetic strategies to ameliorate neurologic syndromes of different etiologies, including lysosomal storage diseases, Alzheimer's disease and other amyloidopathies, Parkinson's disease, spinal muscular atrophy, amyotrophic lateral sclerosis and brain tumors. This field has been propelled by genetic technologies, including identifying disease genes and disruptive mutations, design of genomic interacting elements to regulate transcription and splicing of specific precursor mRNAs and use of novel non-coding regulatory RNAs. These versatile new tools for manipulation of genetic elements provide the ability to tailor the mode of genetic intervention to specific aspects of a disease state.

Figure 1.

Therapeutic strategies based on genetic etiology. (A) Recessive disease. In the case of a recessively inherited disease where both alleles (or one on the X chromosome in males) of a gene are mutated, the goal is to replace the defective gene with a functional counterpart or to correct the defect. Typically, a vector is used to deliver a promoter–cDNA expression cassette which either integrates into the genome (1), e.g. lentivirus vector, or resides as an extrachromosomal element (4), e.g. AAV vector. Other approaches include attempts to achieve homologous recombination with the resident allele (4) or, if appropriate, to correct the transcript through transplicing of the precursor mRNA (3). (B) Dominant disease. In the case of dominantly inherited diseases with only one defective neurotoxic gene, a common strategy is to block expression of the mutant message through RNAi (1) which can be delivered either as an shRNA encoded in a vector or as an oligonucleotide. Another approach is to try to neutralize damage caused by the mutant protein through the use of targeted antibodies (2). (C) Multifactorial dysfunction. For many neurodegenerative diseases, there is no one gene that can be targeted to alleviate the condition. Approaches include trying to supplement the neurotransmitter capacity of ‘sick' neurons by providing them with enzymes to generate more of their normal neurotransmitter for release at synapses (1). Alternately, the neurotransmitter profile of interacting neurons can be altered to change their output from excitatory to inhibitory, or vice versa (2). In another common scenario, cells in the vicinity are genetically modified to produce a growth factor which is supportive for the sick neurons (3).

Figure 2.

Routes of gene delivery to the CNS. CED of viral vectors into the brain improves considerably their distribution in target structures and hence 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 (top left diagram). Infusion of recombinant proteins or oligonucleotides into the brain ventricular system, or intrathecal 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 (top right diagram). The BBB with its many constituents has thwarted most gene transfer vehicles from entering the brain from the vasculature. In recent years, PILs and a new generation of viral vectors (AAV and SV40) have been shown to mediate efficient CNS gene transfer after i.v. infusion in newborn and adult animals (bottom right diagram).

Figure 3.

Schematic representation of existing cellular therapies, including iPS cells, which require vector-based strategies to generate neuroprecursor cells and neurons to ultimately treat neurodegenerative diseases. A variety of cellular therapies have been devised to repair and replace degenerated neuronal networks within the CNS. These strategies utilize multiple sources of neurons and neuroprecursor cells, including fetal brain, pre-implantation embryos, mesenchymal stem cells and iPS cells. The generation of iPS cells requires the use of viral vectors to deliver multiple genes key for the molecular reprogramming of patient fibroblasts. These iPS cells can be differentiated into neurons or neuroprecursor cells that are subsequently transplanted into the diseased brain.