Preventing Damage from Mitochondrial Mutations
Mitochondria are the living machines within cells that act as their “power plants,” converting the energy-rich nutrients in our food into ATP, the form of energy that directly powers biochemical reactions in the cell. Unlike any other part of the cell, mitochondria have their own DNA (mtDNA), separate from the DNA in the cell’s nucleus, where all the rest of our genes are kept.
Just like real power plants, mitochondria generate toxic waste products in the process of “burning” food energy as fuel – in this case, spewing out highly-reactive molecules called free radicals, which can damage cellular structures. And the mtDNA is especially vulnerable to these free radicals, because it is located so close to the center of its production. At worst, a free radical “hit” to the mtDNA can cause major deletions in its genetic code, eliminating the mitochondria’s ability to use the instructions to make proteins that are critical components of their energy-generating system.
Lacking the components needed to produce cellular energy the normal way, these mutant mitochondria enter into an abnormal metabolic state to keep going — a state that produces little energy, while generating large amounts of waste that the cell is not equipped to metabolize. Perversely, the cell tends to hang onto these defective, mutant mitochondria, while sending normal ones to the recycling center, so if just one mitochondrion suffers a deletion, its progeny quickly take over the entire cell. Although this happens to just a few cells in our body, those few cells wind up doing disproportionate damage to the body as a whole, because they dump the waste that their mutant mitochondria generate into the circulation, poisoning their environment by causing oxidative stress to rise all over the body.
It would be ideal if we could prevent mitochondrial deletions from happening, or fix them after they’ve occurred before they can do harm; unfortunately, the state of the science is nowhere near the point where this would be a realistic goal. Instead, the MitoSENS strategy is to accept that mitochondrial mutations will occasionally happen, but engineer a system to prevent the harm they cause to the cell. We can do this by putting “backup copies” of the mitochondrial genes into the nucleus, where they cannot be damaged by free radicals generated in the mitochondria. That way, even if the original genes in the mitochondrial are deleted, the backup copies will be able to supply the proteins needed to keep normal energy production going, allowing the cellular power plants to continue humming along normally and preventing them from entering into the toxic, mutant metabolic state. This approach is called allotopic expression of the proteins.
In this case, we’re lucky: evolution has actually done the hardest part of this for us already, and given suggestions on how to finish the job. Today, the mitochondrial DNA contains instructions for building just 13 proteins needed by the mitochondria. But deep back in evolutionary history, there were literally about a thousand such genes. Those genes still exist, and they are all still needed to keep the mitochondria running normally — but over hundreds of thousands of years, evolutionary forces drove them out of the mitochondria, and relocated them in the nucleus. This process was driven by the same problem that the MitoSENS strategy aims to fix: that having all of those genes so close to the cell’s “ground zero” of free radical production was wreaking havoc, and the cell would be much better off if they were relocated to the safe harbor in the nucleus. The proteins encoded by these genes are constructed in the main body of the cell, outside of the mitochondria, and then imported into the mitochondria through specialized transport docks in their membranes.
One of the challenges to executing this plan is that the 13 remaining proteins would actually be quite difficult for the mitochondria to import, because once they are formed in the main body of the cell, they quickly fold up on themselves, making it difficult or impossible for them to be threaded through the pores in the mitochondrial membrane. This is probably why evolutionary pressure hasn’t already forced the genes to relocate.
But there are several potential ways around this problem. One is to look to solutions discovered by evolution in other organisms. In several cases, evolution has favored the retention of modified versions of the genes for the very same proteins that our mitochondria still encode in their own DNA, because the proteins made by these modified genes have less tendency to snarl up in the cell body. This resistance to snarling means that when the genes for the modified version of the proteins are encoded in the nucleus, they can be produced in the main body of the cell, and still thread their way into the mitochondria. It’s possible that suitably-modified versions of these organisms’ nuclear-encoded mitochondrial genes could work for us, too.
Alternatively, we may be able to insert disposable molecular “braces” called inteins into the sequence of the proteins, that would temporarily hold them straight enough to let them pass through the membranes.
A third approach, which SENS Research Foundation is now most actively researching, was pioneered by Professor Marisol Corral-Debrinski at the Institut de la Vision at Pierre and Marie Curie University, Paris. She altered the genes for the proteins that need to be moved to the nucleus so that the protein would be “decoded” from its instructions very close to the mitochondrial surface, instead of far away in the cell body. This approach allows the allotopically-expressed proteins to be threaded directly into the mitochondria before they have the chance to twist up too much.