For most people, it's trivially easy to reach for an object with one hand and keep the other hand still. But in people with a rare inherited condition, when the brain orders one hand to move, the other hand performs the same movement at the same time. Now scientists think they've found a gene mutation that's responsible for this "mirror movement" disorder. The find could yield insights into how the brain gets wired during development.
The discovery is exciting, says Susan Ackerman, a neurogeneticist at the Jackson Laboratory in Bar Harbor, Maine, who was not involved in the research. The mutation affects a receptor for a signaling molecule called netrin that is involved in one of the best-studied pathways in developmental neuroscience, she says, but all of that work has been done in animals such as mice, worms, and fruit flies. "To my knowledge, this is the first indication that it's really important in humans too."
Mirror movements are a rare and puzzling phenomenon. Babies often exhibit mirror movements in which, for example, an intentional grasping movement with one hand or a kick with one leg is accompanied by a similar involuntary movement by the other side. But these anomalies almost never persist into adulthood, says Guy Rouleau, a neurologist at the University of Montreal in Canada. However, Rouleau and colleagues have recently identified two families—one in Iran and one in Canada—in which some individuals exhibit mirror movements as adults.
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In tomorrow's issue of Science, Rouleau and colleagues report that people from both families who exhibit mirror movements have defects in one of their two copies of a gene called DCC. This gene encodes a receptor for netrin. Although the DCC mutation differs between the Iranian and Canadian families, both mutations result in a truncated receptor protein that can't bind with netrin, the researchers found. No such mutations turned up in a sample of 760 people without mirror movement symptoms.
Animal studies have suggested that during brain development, netrin tells growing nerve fibers to cross over to the other side of the brain, and Rouleau speculates that the message might not always get through in people with only one working copy of the DCC receptor. Normally, the left side of the brain controls the right side of the body and vice versa, but in people with a faulty DCC gene, a subset of the fibers from the brain's motor command center may stay on the same side instead of crossing over. As a result, when the left motor cortex sends out a command to, say, wave the right arm, the left arm gets the message and waves too.