What do mitochondrial mutations mean for how quickly you age?

Each cell contains thousands of copies of mitochondrial DNA, the small piece of genetic material found in the energy factories of your cells. Because there are so many copies, it takes a long time for a single error to become dominant enough to cause damage. Only when a mutation expands clonally, meaning it is copied over and over until it makes up the majority within a cell, does a measurable energy deficit arise. This likely helps explain why diseases such as Parkinson's disease and cancer tend to appear late in life and then worsen gradually.

The classical assumption was that the cause lay in accumulated damage from free radicals. That picture has since become more nuanced. The mutator mouse, an animal model that ages more rapidly due to a higher rate of mtDNA errors, showed no increase in free radical production. More recent insight suggests that most mutations arise early in development as copying errors and subsequently expand slowly, rather than through oxidative attack. Other mechanisms, such as a disrupted energy balance, appear to be at least as important.

Beyond mtDNA mutations themselves, the broader condition of mitochondria plays a role. With ageing, energy production declines, the structure and clean-up systems of mitochondria become dysregulated, and damaged mitochondria send out signalling molecules that activate the immune system. This can lead to chronic low-grade inflammation, a phenomenon researchers link to a vicious cycle: inflammation damages mitochondria, and damaged mitochondria fuel the inflammation. How large a contribution that cycle makes to actual human ageing has not yet been sufficiently investigated.

A complication for anyone hoping to intervene: animal research in the worm C. elegans showed that the timing of mitochondrial disruption can completely reverse its effect. Early in development it led to a longer lifespan, while later in life it led to a shorter one. Interventions that appear 'lifespan-extending' in broad screening studies may therefore only work when applied at the right moment. This makes translating animal findings to humans particularly difficult.

Systems that recycle damaged mitochondria are a promising avenue for intervention, but it has not yet been demonstrated in humans that these systems can be targeted and safely adjusted. The link between mitochondrial quality control and ageing is plausible; effective interventions for humans are not yet available.