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Biologists at Tsukuba University have produced the first animal model of mitochondrial disease, paving the way for research into human diseases, such as some forms of paralysis and kidney failure, that are caused by mutations in mitochondrial DNA.

Mitochondria are the power plants inside our cells that fuel all biochemical reactions. Most genetic diseases arise from mutations in the human genome, stored in the nucleus of each cell and known as nuclear DNA. Many of these diseases have been studied by inserting the relevant bit of faulty DNA into the nuclear DNA of an animal model, such as a mouse.

Mitochondria, however, have their own DNA. Until now it has not been possible to make animal models of mitochondrial DNA disorders because unlike with nuclear DNA, scientists were unable to insert sections of DNA into mitochondria. It is this breakthrough that has been made by Junichi Hayashi and colleagues and published this week in Nature Genetics.

Mutations in mitochondrial DNA accumulate as mice get older. The scientists fused cells containing 30 percent mutant mitochondrial DNA with fertilized mouse eggs, and created new mice with mutant mitochondria.

It was a lengthy process, taking three years to create a cell line with predominant mitochondrial mutations, and another year of technical wizardry to fuse these cells with mouse eggs. The resultant mouse model has similarities with people suffering from mitochondrial disorders.

Mutations in mitochondrial DNA are known to cause brain and muscle disorders that can lead to heart dysfunction or total paralysis and are sometimes linked to diabetes and hearing loss.

“We didn’t expect that the most severely impaired tissue in our mice would be renal (kidney) tissue,” Hayashi said. “So it possible that some diseases, the causes of which have been unknown, for example those associated with aging, could be due to mitochondrial DNA mutations.”

The mice will be used to study mitochondrial disorders.

They will also be useful for screening drugs and testing new gene therapies whereby missing mitochondrial DNA sequences are inserted.

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