Cancer-causing gene 'jumped' from plants to humans

A 'jumping gene' known to cause cancer and neurological disorders may have been transferred to humans from plants, scientists say.

Scientists from the University of Adelaide in Australia have shown that widespread transfer of genes between species has radically changed the genomes of today's mammals, and been an important driver of evolution.

In the world's largest study of so-called "jumping genes", researchers have traced two particular jumping genes across 759 species of plants, animals and fungi. These jumping genes are actually small pieces of DNA that can copy themselves throughout a genome and are known as transposable elements.

They have found that cross-species transfers, even between plants and animals, have occurred frequently throughout evolution.

Both of the transposable elements they traced - L1 and BovB - entered mammals as foreign DNA. This is the first time anyone has shown that the L1 element, important in humans, has jumped between species.

"Jumping genes, properly called retrotransposons, copy and paste themselves around genomes, and in genomes of other species. How they do this is not yet known although insects like ticks or mosquitoes or possibly viruses may be involved - it's still a big puzzle," said David Adelson, from the University of Adelaide.

"This process is called horizontal transfer, differing from the normal parent-offspring transfer, and it's had an enormous impact on mammalian evolution," said Adelson.

For example, 25 per cent of the genome of cows and sheep is derived from jumping genes.

"Think of a jumping gene as a parasite. What's in the DNA is not so important - it's the fact that they introduce themselves into other genomes and cause disruption of genes and how they are regulated," said Adelson.

The study, published in the journal Genome Biology, found horizontal gene transfer was much more widespread than had been thought.

"L1 elements were thought to be inherited only from parent to offspring," said Atma Ivancevic, a postdoctoral researcher at the University of Adelaide.

"Most studies have only looked at a handful of species and found no evidence of transfer. We looked at as many species as we could," said Ivancevic.

L1 elements in humans have been associated with cancer and neurological disorders. Understanding the inheritance of this element is important for understanding the evolution of diseases, researchers said.

They found L1s are abundant in plants and animals, although only appearing sporadically in fungi. However, the most surprising result was the lack of L1s in two key mammal species - the Australian monotremes (platypus and echidna) - showing that the gene entered the mammalian evolutionary pathway after the divergence from monotremes.

"We think the entry of L1s into the mammalian genome was a key driver of the rapid evolution of mammals over the past 100 million years," said Adelson.

The team also looked at the transfer of BovB elements between species. BovB is a much younger jumping gene: it was first discovered in cows but has since been shown to jump between a bizarre array of animals including reptiles, elephants and marsupials.

Previous research, led by Adelson, found that ticks were the most likely facilitators of cross-species BovB transfer.

The team believes that studying insect species will help find more evidence of cross-species transfer. They also aim to study other jumping genes and explore the possibility of aquatic vectors, such as sea worms and nematodes.