Multiple copies of a mystery gene may make us human
From mice to monkeys to chimps to people, a brain-protein gene pumps up.
A newly discovered mystery gene may have helped build the modern human brain, researchers report today.
Scientists don't know what the gene does. But they do know that humans have more copies of it than chimpanzees, monkeys, rats and mice. And they know that the gene makes a protein that is found in the human brain. That suggests that it may help to give the human brain its unique ability to think and reason, they say.
"This really is a remarkable discovery," says Thomas Insel, director of the US National Institute of Mental Health in Bethesda, Maryland, which was not involved in the research and did not fund it. "People are going to argue about it and be fascinated by this, because it takes us in a couple of directions we haven't really been in before."
The work is part of the emerging tide of new discoveries made possible by the sequencing of several genomes closely related to humans, including the chimpanzee and the macaque monkey. Both of these species are in the branch of the evolutionary tree that gave rise to humans: the primate lineage. By comparing DNA among primates to DNA from more distant mammals, scientists are gaining clues to what makes primates, and people, unique (see ' Homing in on the genes for humanity').
The new work may also confirm a hot idea among genome scientists: that the number of copies of genes is an important source of variation that may be driving evolution. Until recently, scientists thought that most genetic variations between people and between species were due to small changes in the sequence of DNA lettering. But researchers are now discovering the importance of DNA variations that occur on a larger scale, including areas of repeated identical sequences that code for multiple copies of genes.
These large-scale variations are less well understood; places in a genome where DNA repeats itself are notoriously difficult to stitch together in sequencing projects. But such sequences are increasingly being appreciated as important and more work is being called for in this area.
It was by searching out such repeats that James Sikela, at the University of Colorado at Denver and Health Sciences Center, Aurora, and his colleagues pinpointed the gene they report today in the journal Science1.
The team compared the DNA sequences of humans, chimpanzees and monkeys, and looked for genes that were repeated more often in human DNA than in the other primate genomes. One gene that codes for a piece of protein called DUF1220 stood out. Humans carry 212 copies of DUF1220, whereas chimps have 37 copies, and monkeys have only 30 copies, the researchers found. Mice and rats each had a paltry single copy of the protein-coding region. When the team looked for the protein in the human body, they found it in many places, including in neurons in the brain.
That may mean that the protein does something related to brain function something that becomes more crucial as one travels along the evolutionary path from mice to monkeys to chimps to people. Such a protein could have helped to define what makes our brains human, the scientists say, although they caution that that has not yet been proven, and probably won't be until they know what it does.
Working out the protein's task won't be simple. With mice, researchers simply knock out genes to determine what they do. This cannot be done with primates.
"These genes may be very good candidates to be involved in some type of cognitive function, and that's very tantalizing," Sikela says. "But that's about as far as we are right now."
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- Popesco M. C., et al. Science, 313. 1304 - 1307 (2006).
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