RENEGADE chunks of our own DNA have turned out to be serial genetic saboteurs, causing mutations and disease. The trespassing DNA comes from mitochondria 鈥� energy-producing components of cells. Unlike other 鈥渏umping genes鈥� they prefer to insert into active genes.
The finding reveals a potential, but hitherto unknown, source of human disease, and the researchers who have analysed the phenomenon warn that DNA already damaged by pollution or radiation might be especially susceptible to further damage from stray mitochondrial DNA. Working genes are more prone to damage as their DNA is regularly unzipped to churn out proteins.
Miria Ricchetti at the Pasteur Institute in Paris and her team identified 211 insertions of mitochondrial DNA across the genome, 23 of them new: 80 per cent were in genes, but genes only make up 3 per cent of the genome. The targets are widely transcribed genes, probably because they have more double-strand breaks, says Ricchetti.
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If the insertion is harmless and happens in an egg or sperm cell, it can be passed to offspring. One example cited by the team was of a child conceived within the radiation fall out zone of the Chernobyl disaster. Researchers think the mitochondrial insertion took advantage of radiation-damaged DNA in one parent.
Jef Boeke, who studies jumping genes at Johns Hopkins University School of Medicine in Baltimore, Maryland, says mitochondrial insertions occur at a 鈥渨impy鈥� frequency compared to 鈥減rofessional insertion machines鈥� like retrotransposons. These outnumber mitochondrial insertions by 200 to 1.
Mitochondrial DNA is unlikely to be a major source of human gene mutation, he says, but 鈥渢he proposal that there could be a case of this directly related to the Chernobyl nuclear disaster is both fascinating and sobering.鈥�
By comparing the sequences of 21 ethnically diverse volunteers Ricchetti鈥檚 team found six insertions that were not shared by all of them (PLoS Biology, vol 2, e273). These must have occurred after our ancestors split from the ancestors of chimps and thus could give scientists clues about the origins of ethnic groups and historical migrations.