A glimpse into the evolutionary history of a gene linked to Crohn’s disease has revealed a bizarre example of just how dynamic genomes can be.

The gene, called Immunity-related GTPase M (IRGM), seems to have been destroyed in the ancestor of Old and New World monkeys millions of years ago, only to be brought back to life again in the common ancestor of humans and great apes when a retrovirus lurking in the genome inserted itself into the gene.

It is probably the first example of a resurrected gene. “It’s like lightning striking twice in the same spot,” says Evan Eichler, a human geneticist at the University of Washington in Seattle.

In most mammals, IRGM is a member of a large family of related genes thought to help eliminate pathogens, such as the bacterium responsible for tuberculosis, that invade host cells and then thrive within them. But in humans and some primates, the family has withered away to only two members, IRGM and IRGC. It is not known how humans survive the loss of so many IRGM genes; mice that lack family members are more vulnerable to infection1.

There is reason to suspect that IRGM may have an important function in humans, however. Researchers have found that some people carry a deletion in front of the IRGM gene that may alter expression of the gene and increase the risk of Crohn’s disease, a painful autoimmune disease of the digestive system.

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Share |