Jumping genes, once thought to have quieted down, are still actively changing the human genome. "Jumping genes are transposable elements — genetic sequences that can mobilize themselves from one part of the genome to another," says John Moran, a professor at the University of Michigan Medical School. He and his team study long interspersed element-1 retrotransposons. "The way they mediate their mobility from one genetic location to another is that they're first transcribed into RNA, and then that resulting mRNA is reverse transcribed and integrated into a new genomic position," he says.
These retrotransposons and their copy-and-paste manner of reproduction were, at one time, thought to be extinct in humans. Moran's data, recently published in the journal Cell, shows that they're still hopping. Moran and his colleagues started by looking at the genomes of six individuals and were able to identify 68 new full-length -LINE-1s that had not been found before. They tested 67 of them, 37 of which were actively transposing themselves onto other parts of the genome.
Because they are integrating themselves within new genomic locations, Moran says, the retro-transposons are essentially mutagens. "The thought is that these elements are interested in their own self-propagation," he adds. But the number of these elements in the genome, coupled with their current activity in the population, indicates that they've added greatly to genetic diversity in people.
Unfortunately, Moran adds, many researchers disparage transposable element-derived sequences as junk DNA. But knowing that these elements can jump around and may create insertion polymorphisms when they do — especially if they land in genes — means that researchers need to better appreciate the level of diversity that LINE-1 retrotransposons contribute. "About 30 percent of our genome is due either to the direct action of LINE-1 retrotransposition or to the promiscuous mobilization of other cellular RNAs by the LINE-1 retrotransposition machinery," he says. "We've only just begun to identify the extent by which LINE-1 mediated retrotransposition events affect the human genome."