Gene therapies
One therapeutic strategy to target autosomal recessive disorders is referred to as
gene addition (or supplementation) therapy, where target cells are provided with a correct cDNA copy. This gene therapy strategy has been clinically applied with quite exciting (but still limited) results for treatment of patients with LCA type 2, a very early onset severe retinal dystrophy associated with mutations in RPE65, patients with choroideremia associated with mutations in the Rep1 gene, and other disorders.
However, there are limits to this gene therapy, e.g. in disorders where the mutant gene product has toxic gain of function or dominant negative effects within the cell.
A very promising novel emerging field for the treatment of genetic mutations represent genome edition strategies targeting specific gene loci. Genome editing is based on the innate capacity of cells to repair DNA double strand breaks, which are the most dangerous form of DNA damage that can occur to a cell (Cox et al. 2015). By employing highly specific endonucleases, such as transcription activator like nucleases (TALEN) or RNA based nucleases (CRISPR based systems) and a template DNA containing the correct DNA sequence of the target gene, this technology can be used to repair disease causing mutations, either ex vivo in induced pluripotent stem cells (iPSC) or in vivo following viral vector mediated gene transfer (Yanik et al. 2016).
Gene therapy strategies necessitate the transfer of genes or parts of them into retinal cells by viral or non-viral vector systems. Of all currently available vector systems, the adeno-associated virus (AAV) based vectors have emerged as the gold standard for retinal gene therapy. Vectors based on this virus enable long term and cell type specific transgene expression in retinal pigment epithelium (RPE) cells and/or photoreceptor cells following subretinal injection. An inherent advantage of AAVs is that differect vector serotypes target different retinal cell types with different efficacy enabling a tailored targeting strategy. However, one major draw-back of AAVs is their limited cargo capacity, preventing their use for a large number of genes that are too large to be packaged into AAVs. Strategies using double or even triple AAV vector systems based on the capability of cells to combine overlapping single stranded DNA to one large expression cassette are currently under investigation for solving this problem (Trapani et al. 2013).
The main questions of the research projects of the SPP 2127 are:
- How can we improve efficacy of gene addition approaches in order to restore function to target cells in vivo?
- What is the best way to transfer genes into a large area of the retina? How can realize pan-retinal gene transfer in human patients?
- Is it possible to employ genome editing as a technology to repair disease-causing mutations in vivo?
- Can we sufficiently control cellular DNA repair mechanisms in order to ensure safe modifications to the human genome?