Defective mitochondria — the ‘batteries’ that power the cells of our bodies — could in future be repaired using gene-editing techniques. Scientists have now shown that it is possible to modify the mitochondrial genome in live mice, paving the way for new treatments for incurable mitochondrial disorders.

Our cells contain mitochondria, which provide the energy for our cells to function. Each of these mitochondria is coded for by a tiny amount of mitochondrial DNA. Mitochondrial DNA makes up only 0.1% of the overall human genome and is passed down exclusively from mother to child.

Faults in our mitochondrial DNA can affect how well the mitochondria operate, leading to mitochondrial diseases, serious and often fatal conditions that affect around 1 in 5,000 people. The diseases are incurable and largely untreatable.

There are typically around 1,000 copies of mitochondrial DNA in each cell, and the percentage of these that are damaged, or mutated, will determine whether a person will suffer from mitochondrial disease or not. Usually, more than 60% of the mitochondria in a cell need to be faulty for the disease to emerge, and the more defective mitochondria a person has, the more severe their disease will be. If the percentage of defective DNA could be reduced, the disease could potentially be treated.

A cell that contains a mixture of healthy and faulty mitochondrial DNA is described as ‘heteroplasmic’. If a cell contains no healthy mitochondrial DNA, it is ‘homoplasmic’.

In 2018, a team from the MRC Mitochondrial Biology Unit at the University of Cambridge applied an experimental gene therapy treatment in mice and were able to successfully target and eliminate the damaged mitochondria DNA in heteroplasmic cells, allowing mitochondria with healthy DNA to take their place.

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