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How mitochondrial DNA maketh man

Written by Lauren Robertson, Science Writer.

New research has revealed how mitochondrial DNA can insert itself into our genomes and influence human evolution. In a paper published this week in Nature, scientists found that once in every 4,000 births, DNA from our mitochondria gets inserted into our nuclear genomes. Interestingly, mitochondrial DNA was also found in cancers, suggesting it plays a role in repairing damage to cells.

Mighty mitochondria

For decades, students have learnt that mitochondria are the “powerhouses” of the cell – small organelles that provide our cells with the energy to carry out any necessary biological processes. You might have even learnt how they came to be: “Billions of years ago, a primitive animal cell took in a bacterium that became what we now call mitochondria,” explains Professor Patrick Chinnery, from the Medical Research Council Mitochondrial Biology Unit and Department of Clinical Neurosciences at the University of Cambridge. “These supply energy to the cell to allow it to function normally, while removing oxygen, which is toxic at high levels.”

These organelles contain their own DNA, inherited from our mothers, that is separate from the nuclear DNA encoding most of our genomes. It was previously thought that mitochondrial DNA could not be passed down the paternal line, but a 2018 paper reported evidence to the contrary.  To investigate these claims, a team of researchers from the University of Cambridge and Queen Mary University of London studied the DNA of more than 11,00 families – part of Genomics England’s 100,000 Genomes Project.

Paternal patterns

The team were trawling through all of this DNA to find evidence of paternal inheritance. Surprisingly, they found patterns of mitochondrial DNA in the nuclear DNA of children that were not present in their parents. It is likely that instead of mitochondrial DNA being inherited paternally, it in fact inserts into certain areas of our nuclear DNA.

The team extended this work by sequencing the whole genomes of more than 66,000 people. They found that the insertions are happening with a much higher frequency than previously thought, pointing to an entirely new mechanism that our genomes might harness to evolve.

“Over time, bits of these primitive mitochondria have passed into the cell nucleus, allowing their genomes to talk to each other,” said Chinnery. “This was all thought to have happened a very long time ago, mostly before we had even formed as a species, but we’ve discovered is that’s not true. We can see this happening right now, with bits of our mitochondrial genetic code transferring into the nuclear genome in a measurable way.” The team estimates that each of us carry around 5 of these mitochondrial inserts, but one in seven of us might carry very recent ones.

For better or worse?

The downside is that despite their best attempts to offer a helping hand, these inserts can occasionally lead to rare forms of cancer. By sequencing 12,500 tumour samples, the researchers discovered that mitochondrial DNA was much more common in tumour cells – around 1 in 1,000 tumours contained evidence of mitochondrial DNA. “Mitochondrial DNA appears to act almost like a Band-Aid, a sticking plaster to help the nuclear genetic code repair itself. And sometimes this works, but on rare occasions it might make things worse or even trigger the development of tumours.”

This work suggests that the endosymbiosis between man and bacteria that began around 1.45 billion years ago is not yet complete. Future research will likely focus on the new role of mitochondrial DNA in repairing our genetic code, hopefully aiding our understanding of malignancies.

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