MitoSENS
Preventing damage from mitochondrial mutations.
The mitochondrion is a machine within the cell that acts as the "power plant" of the cell. Mitochondria take oxygen and chemically combine it with energy-rich nutrients from our food, to make carbon dioxide and water (which we exhale) and ATP, the "energy currency" of the cell.
The mitochondrion is therefore a really essential part of the cell. Lots of other parts of the cell are essential too, though, so why have a whole SENS strand devoted to it? The answer is that, unlike any other part of the cell, mitochondria have their own DNA (mtDNA), separate from the nucleus. Being at the site of cellular respiration, the mtDNA is vulnerable to its reactive by-products. Worse yet, the mitochondria's capacity for repairing DNA damage is much more limited than that of the nucleus. Thus we need a different system to combat the inevitable accumulation of such mutations.
The Solution
As usual, we're lucky: evolution has done the hardest part of this for us already. Mitochondria are very complex; they contain about a thousand different kinds of protein, each encoded by a different gene. But nearly all of those genes are not in the mitochondrial DNA at all – they are in the nucleus! The proteins are constructed in the cell, outside the mitochondrion, just like all non-mitochondrial proteins. Then, a complicated apparatus called the TIM/TOM complex hauls the proteins into the mitochondrion, through the membranes that make up its surface. Only 13 of the mitochondrion's component proteins are still encoded by its own DNA, and it's therefore only these 13 genes that remain vulnerable to the constant assault from free radicals produced during respiration (the life-giving reaction of oxygen and food by the mitochondria).
This gives us a wonderful opportunity: rather than fixing mitochondrial mutations, we can make them harmless to us. By putting "backup copies" of these few remaining genes into the nucleus, we can prevent the harm caused by any mutations that may occur of the original versions. Even without the mitochondrial DNA, the proteins that it encodes will still be produced and incorporated into the mitochondrial machinery, allowing the cellular power plants to continue humming along normally. Doing this requires making a few minor modifications in the genes so that the proteins that they produce will be easier for the TIM/TOM machinery to handle, such as changing a few of the amino acid "building blocks" that make them up, or inserting disposable molecular "braces" called inteins into them that temporarily hold them straighter to keep them from getting tangled up as they cross the membranes. Since genes in our chromosomes are very, very much better protected from mutations than the mitochondrial DNA is, we can rely on the chromosomal copies carrying on working in very nearly all our cells for much longer than a currently normal lifetime.
The cells that most severely accumulate mutant mitochondria are non-dividing ones like muscle fibres and neurons. This is a shame, because it means we will need really good gene therapy to get these supplementary genes into the cells that need them. But gene therapy is improving all the time - and also, even if we could only fix half the affected cells, we'd still be achieving a rejuvenation, which we could progressively improve upon with new gene therapy advances.
Even though the above therapy sounds simple, this project needs a lot of work. The 13 proteins of interest are actually quite difficult for the TIM/TOM machinery to process even when we "tell" it to do so, so we still need to work on making that part easier. But there has been good progress in this area in the past couple of years. Scientists have shown that they can restore more normal function to cells with mutated genes for several different mitochondrial proteins using this technique (which is called "allotopic expression"), and even further progress was reported in January of 2007. Researchers at the University of Florida College of Medicine announced that they had been able to put the gene for either the normal human protein whose mutation leads to blindness in humans, or for the mutated human protein itself, into the nerves of living rodents, and that the proteins were successfully imported into their cells' mitochondria1. Excitingly, the proteins functioned just as predicted, with the normal version allowing for normal nerve function, and the mutated version reproducing many of the key features of the human disease.
MitoSENS Progress
MitoSENS research began at Cambridge University, in the MRC-Dunn Human Nutrition Unit. Ian Holt, Ph.D., head of the Mitochondrial Diseases research at the Dunn - who supervised the first MitoSENS projects - commented, "For over 30 years mutations in mitochondrial DNA have been suspected to be important contributors to aging. If we can incorporate working copies of that mtDNA into our nuclear DNA, the mtDNA will be rendered superfluous and any mutations it suffers will be inconsequential. Researchers have tried to do this for many years, with only limited success. The work that Mark [Hamalainen] will perform in my lab is the most systematic attempt yet to get this technology to work."
In March 2009, the MitoSENS Research program was transferred to the lab of Dr Marisol Corral-Debrinski in the newly opened Institut de la Vision in Paris. Dr Corral-Debrinski's lab has demonstrated efficient allotopic expression in cultured human fibroblasts carrying mtDNA mutations, and has recently begun in vivo studies. We are collaborating with her to hasten the development of gene therapies that may obviate mitochondrial DNA mutations.
In September 2010 Dr. Matthew "Oki" O'Connor joined the SENS Research Center to head up a new drive to expand allotypic expression of all 13 mitochondrially encoded genes.
Our Work
Resources
Talks on this topic at IABG 10: King
At SENS2: Lightowlers, Smigrodzki, Weiner, Yagi
At SENS3: Adhya, Corral-Debrinski, Hamalainen, Weissig