Mitochondria surrounded by cytoplasm. Credit: David Furness
Key Points:
- Scientists have shown that in 1 in every 4,000 births, some of the genetic code from mitochondria inserts itself into DNA.
- Mitochondrial DNA appears more often in cancer DNA, suggesting that it acts as a sticking plaster to try and repair damage to the genetic code.
- This defines a new role in DNA repair.
Mitochondrial DNA has always been thought to be passed down the maternal line. Then, in 2018, an American research team thought they found evidence of mitochondrial DNA passed down the paternal line.
Diving deeper into the mystery, a team of researchers from the University of Cambridge and Queen Mary University of London now show that it was not paternal mitochondrial DNA, but rather “inserts” in the nuclear DNA of some children that is not present in that of their parents.
In a new study that extends this work to over 66,000 people, the UK research team showed that the inserts are actually happing all the time, revealing a new way our genome evolves.
The researchers estimate that mitochondrial DNA transfers to nuclear DNA in around 1 in every 4,000 births. If that individual has children of their own, they will pass these inserts on. Most individuals carry five of the new inserts, with one in seven of carrying very recent ones.
When the team looked at sequences taken from 12,500 tumor samples, they found that mitochondrial DNA was even more common, arising in around 1 in 1,000 cancers. In some cases, the mitochondrial DNA inserts actually caused the cancer.
“Our nuclear genetic code is breaking and being repaired all the time,” said Patrick Chinnery from the Medical Research Council Mitochondrial Biology Unit at the University of Cambridge. “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 if might make things worse or even trigger the development of tumors.”
More than half (58%) of the insertions were in regions of the genome that code for proteins. In the majority of cases, the body recognizes the invading mitochondrial DNA and silences it in a process known as methylation, whereby a molecule attaches itself to the insert and switches it off. A similar process occurs when viruses manage to insert themselves into our DNA. However, this method of silencing is not perfect, as some of the mitochondrial DNA inserts go on to be copied and move around the nucleus itself.
The team looked for evidence that the reverse might happen—that mitochondrial DNA absorbs parts of our nuclear DNA—but found none.
Information provided by University of Cambridge.