Transfer of mitochondrial DNA (mtDNA) into the nuclear (germline) genome occurs in one in every 4000 births.
Scientists, at the University of Cambridge and Queen Mary University of London, examining the genomes of approximately 66,000 individuals, found that more than 99 percent of these had detectable mtDNA segments embedded in them. More than 90 percent had inserted into the 'nuclear' genome after, rather than before, humans diverged from apes. This finding is in contrast to what had previously been believed.
'Billions of years ago, a primitive animal cell took in a bacterium that became what we now call mitochondria. These supply energy to the cell to allow it to function normally, while removing oxygen, which is toxic at high levels. Over time, bits of these primitive mitochondria have passed into the cell nucleus, allowing their genomes to talk to each other,' Professor Patrick Chinnery, from the University of Cambridge and lead author on the study, said. 'This was all thought to have happened a very long time ago, mostly before we had even formed as a species, but what we've discovered is that 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.'
Previously, the scientists studied 11,000 genomes sequenced as part of the 100,000 genomes project, and publishing their research in Nature Communications. They found that mtDNA had inserted into the nuclear DNA of children that was not present in their parents.
Expanding on this work, increasing their sample size to over 66,000 people, they estimate that mtDNA transfer occurs once in every 4000 births probably via a process within the mother's egg cell. These inserts can then be passed down to subsequent generations, highlighting a new way our genome evolves.
Publishing their findings in Nature, the scientists found that 58 percent of the mtDNA inserts were found to occur in protein-coding genome regions. They further found that sequences from 12,500 tumour samples showed that mtDNA inserts are more common in tumour DNA, being present in one in every 1000 cancers. In some cases, the mtDNA inserts actually cause the cancer.
'Our nuclear genetic code is breaking and being repaired all the time,' said Professor Chinnery. 'mtDNA 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 if might make things worse or even trigger the development of tumours.'
Professor Sir Mark Caulfield from Queen Mary University of London, who was also involved in the study, concluded: 'I am so delighted that the 100,000 Genomes Project has unlocked the dynamic interplay between mtDNA and our genome in the cell's nucleus. This defines a new role in DNA repair, but also one that could occasionally trigger rare disease, or even malignancy.
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