A team of scientists from the University of Maryland School of Medicine has found the strongest evidence yet that bacteria occasionally transfer their genes into human genomes, finding bacterial DNA sequences in about a third of healthy human genomes and in a far greater percentage of cancer cells. The results, published today (20 June) in PLOS Computational Biology, suggest that gene transfer from bacteria to humans is not only possible, but also somehow linked to over-proliferation: either cancer cells are prone to these intrusions or the incoming bacterial genes help to kick-start the transformation from healthy cells into cancerous ones.
“It really does seem that human genome sequence data from somatic cells show signs of LGT events from bacteria, and so do cancer cells,” said Jonathan Eisen from University of California, Davis, who coordinated the peer review of the new study but was not involved in the work. “Wild stuff does happen.”
The trillions of bacteria in our bodies regularly exchange DNA with each other, but the idea that their genes could end up in human DNA has been very controversial. In 2001, the team that sequenced the first human genome claimed to have found 113 cases of such lateral gene transfers (LGT), but their conclusion was later refuted. This high-profile error “had a chilling effect on the field,” according to Julie Dunning Hotopp who led the new study. Although her team has since found several cases of LGT between bacteria and invertebrates, “it’s still difficult to convince people that it may be happening in the human genome,” she said.
Rather than looking for bacterial genes that had become permanent parts of the human genome, Dunning Hotopp’s team searched for traces of microbial DNA in somatic cells—the cells of the body that do not form gametes.
Lab members David Riley and Karsten Sieber scanned publicly available data from the 1000 Genomes Project and found more than 7,000 instances of LGT from bacteria, affecting around a third of the people they studied. When they analyzed sequences from the Cancer Genome Atlas, they discovered 691,000 more instances of LGT 99.9 percent of these came from tumor samples rather than normal tissues. Via Bacterial DNA in Human Genomes