As you know, gene therapy aims at correcting a genetic deficiency by the introduction and/or substitution of the mutated gene by a functional copy into relevant cells. The classic examples for gene therapy are normally diseases such as Cystic Fibrosis- monogenic, recessive, autosomal diseases, as these are, for obvious reasons, those that theoretically offer higher chances of success. There are, however, other types of genetic diseases that offer more space for original, unconventional gene therapy approaches.
Mitochondrial DNA diseases are a good example. The human mitochondrial genome is small (16kb) and is housed in the mitochondrial matrix. As most of its genes encode proteins involved in oxidative phosphorylation necessary for cellular respiration and ATP production, is not too surprising that mutations in such genes can cause a variety of diseases, many of which with symptoms associated with progressive defects in muscle and nerve systems.
There are two obvious strategies to tackle this problem using gene therapy. One is to introduce the functional copy of the mutated gene into the nucleus. The protein, with an appropriate mitochondrial targeting sequence, can then be transported to the mitochondria. This is the so-called allotropic expression of the gene, and has been shown to produce promising results in culture. Other option is to target the plasmid containing the functional gene into the mitochondria itself, as these organelles contain all the ribosomal RNA and tRNAs necessary for intra-mitochondrial protein synthesis (assuming that these are not the mutated genes). These strategies have been tried out with some success both using a wild type sequence of the mutated gene, or a gene from another species, known to perform its function in the pathway more efficiently (xenotopic expression).
However, as you might have already noticed, a mitochondrial DNA disease is characterised by the fact that not all the mitochondrial genome copies in a cell are necessarily mutated (the presence of different genome types within the same cell is known as heteroplasmy). This has two main implications: firstly, mtDNA diseases are threshold diseases: only when around 60-95% of copies in a cell are mutated will symptoms appear. This threshold will obviously vary depending on the disease. Secondly, rather than trying to introduce a functional copy into the mutated mitochondria, a much more interesting approach can be to just manipulate the relative amounts of the mutated and the wild type populations of mitochondria.
One approach is to promote specific types of exercise training to promote mitochondrial biogenesis. However, as the geneticist that I am, the alternative strategy- to manipulate the mutated mt genome- is much more interesting. A brilliant idea, in my point of view, is to take advantage of the fact that some mutation types introduce restriction sites. That way, rather than trying to express a functional copy of the mutated gene, it may be more successful to express a specific restriction enzyme that will degrade the mutated genome altogether. This is limited, obviously, to those diseases in which a unique new site is introduced, or in those where the number of restriction sites is higher enough in the mutated population to have a significant effect.
Another clever idea is to methylate the mutated mt genome so that it is not expressed, allowing the wild type population to proliferate and overcome the mutated one with time. The use of mitochondrial targeted PNAs (peptide nucleic acids) were a potential method, but were shown not to cross the inner membrane of the mitochondria. However, the use of zinc-finger peptides with DNA sequence specific binding capacities have been used to successfully methylate differently the mutated mitochondrial genome. If these domains are linked to a nuclease, it may be possible to degrade the mutated genome as well.
These are all new strategies, only attempted in cell culture and their usefulness in in vivo models and ultimately in patients is yet to be determined. Nevertheless, I think these novel approaches just show that even standard problems can be approached in innovative ways.
Kyriakouli, D., Boesch, P., Taylor, R., & Lightowlers, R. (2008). Progress and prospects: gene therapy for mitochondrial DNA disease Gene Therapy, 15 (14), 1017-1023 DOI: 10.1038/gt.2008.91Alexeyev, M., Venediktova, N., Pastukh, V., Shokolenko, I., Bonilla, G., & Wilson, G. (2008). Selective elimination of mutant mitochondrial genomes as therapeutic strategy for the treatment of NARP and MILS syndromes Gene Therapy, 15 (7), 516-523 DOI: 10.1038/gt.2008.11
(the most recent paper I could find in this field, using the restriction enzyme approach in culture)