Your brain may be mutating in a way that was thought to be very rare

Mutations involving the incorrect addition of DNA from energy-producing mitochondria to the cell’s main genome were thought to be extremely rare. Now, a study of brain tissue suggests that these mutations occur in all of us, and their numbers may be a factor in aging.

“Not only were they there, but they were more present in the dorsolateral prefrontal cortex, an area of ​​the brain associated with cognitive abilities,” Ryan Mills At the University of Michigan.

In every human cell, nearly all of the DNA—about 6 billion letters—is located in the nucleus. But the energy-producing organelles called mitochondria have their own tiny genome, about 16,600 letters long.

That’s because mitochondria were once free-living bacteria with much larger genomes. In the roughly 2 billion years since these bacteria formed a symbiotic relationship with our ancient ancestors, most of the original bacterial genomes have either been lost or moved into the main genome in the nucleus.

Because of the evidence for this transfer, biologists have long known that it must be possible for snippets of mitochondrial DNA to somehow make their way into the cell nucleus and be added to the main genome. However, Mills said, people thought such mutations were extremely rare. Over the past few years, studies by his group and others have shown that it is not as rare as we thought, At least in cancer cells.

Now, Mills and his colleagues have shown that this mutation also occurs in noncancerous cells. The team analyzed DNA sequences from brain tissue samples taken at autopsies from 1,200 people.

The sampling and sequencing was done by another team, but Mills and his colleagues looked for any mutations that involved the addition of mitochondrial DNA to the nuclear genome. “We were just curious,” Mills said.

Not only did they find such a mutation, they also discovered that it was, on average, more common in people who died younger.

Mills said it’s not clear whether the mutations are a sign of aging or a cause of it. “The jury is still out,” he said. “But I have a hard time believing that if you put the entire mitochondrial sequence somewhere in the genome, it might not have an effect.”