A Method of Destroying Only Damaged Mitochondrial DNA

Mitochondrial DNA (mtDNA) damage is an important cause of degenerative aging. Via a complicated chain of events it leads to a small population of malfunctioning cells overtaken by malfunctioning mitochondria that export harmful reactive compounds into surrounding tissue.

There are a number of possible approaches to fix this issue, reversing its contribution to aging and age-related disease. One of them is to deliver undamaged, replacement mitochondrial DNA to all cells in the body, such as via protofection. The issue with this approach is that mitochondria are essentially like bacteria in the way they reproduce. Certain types of damage to their DNA produce mitochondria that evade cellular quality control mechanisms and outcompete their undamaged peers despite the fact that they are dysfunctional. Delivering fresh undamaged mitochondrial DNA into that cell doesn't get rid of the damaged copies, and the damaged copies have already demonstrated an ability to thrive. The suspicion is that the benefits of such a treatment would be temporary at best.

But what if this delivery of new mitochondrial DNA could be paired up with a means to selectively remove the damaged mitochondrial DNA? Given such a technology it might even be possible to skip the delivery entirely and just remove damaged DNA. This would sacrifice a small number of cells, those in a state of dysfunction that lack any remaining undamaged mitochondrial DNA to recreate a population of working mitochondria. Here is an example of such research; like most work on mitochondrial repair it is focused on inherited mitochondrial disease rather than aging, but could produce a technology platform applicable to aging:

Delivery and selection of mtDNA in mitochondria in a heritable manner is yet to be achieved, so alternative approaches to genetic therapy of primary mitochondrial diseases are being sought. One of these approaches is based on pathogenic mtDNA mutations being generally heteroplasmic, with observable pathology only present when the ratio of mutated mtDNA exceeds a certain threshold. The selective elimination of mutated mtDNA allows a cell to repopulate with wild-type mtDNA molecules by a yet uncharacterized mechanism of mtDNA copy number maintenance, alleviating the defective mitochondrial function that underlies mtDNA disease.

We designed and engineered mitochondrially targeted obligate heterodimeric zinc finger nucleases (mtZFNs) for site-specific elimination of pathogenic human mitochondrial DNA (mtDNA). Expression of mtZFNs led to a reduction in mutant mtDNA haplotype load, and subsequent repopulation of wild-type mtDNA restored mitochondrial respiratory function in a [cell model of mtDNA damage]. This study constitutes proof-of-principle that, through heteroplasmy manipulation, delivery of site-specific nuclease activity to mitochondria can alleviate a severe biochemical phenotype in primary mitochondrial disease arising from deleted mtDNA species.

Link: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3992073/


So this research was only in vitro? Lets hope that it holds up in mice and that the researches have the funding to carry out a study in mice.

Posted by: Jim at June 3rd, 2014 6:08 AM

I think that due to the stochastic nature of aging-related mtDNA damage, it may not be possible to target that damage with ZFNs.

How then to ensure the dominance of fresh, undamaged mtDNA over old, possibly dysfunctional mtDNA? One strategy that occurrs to me is to include a gene for some kind of mitochondrial inhibitor compound in the freshly delivered mtDNA to which the new phenotype is resistant. The inhibitor would be expressed only in cells where the new mtDNA has been successfully delivered, and those cells necessarily have the new and improved mitochondira present to pick up the slack. It's a strategy like that of the black walnut tree, which chemically inhibits the growth of other plants in its vicinity to ensure its competitive success.

Posted by: José at June 3rd, 2014 3:21 PM

How do you alter the mtDNA in all of the cells of an adult organism? Is that even possible?

Posted by: AwesomePossum at June 4th, 2014 4:32 PM

@AwesomePossum: Forms of gene therapy. Delivery rather than alteration has been the focus, as the mainstream goal is to supply copies of damaged genes to fix inherited diseases, and in that context it doesn't matter that the old DNA is still around, so long as the new DNA is there to supply the genetic blueprint.

So you might look at protofection as a way to do it:


There are other varied means of getting to the end goal of having the proteins available, none of which actually involve fixing the mitochondrial DNA or delivering new DNA to mitochondria. There is the adoption of RNA mechanisms such as those used by tropical parasites:


And there is the SENS approach of allotopic expression that moves the problem to become one of whole body gene therapy for nuclear DNA instead:


Posted by: Reason at June 4th, 2014 4:50 PM
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