Obsessive-compulsive mice, which were once pulling their hair out from too much grooming, are now sitting pretty. Their cure? A bone marrow transplant. In a study published today in Cell, scientists show an unsuspected link between a psychological disorder and the immune system.

Here’s how they did it:

Step 1 – Finding the Problem

Since excessive cleaning is a behavior, scientists first thought to look for defects in the mouse brain. They noticed that mice with a mutant version of the gene Hoxb8 were the ones cleaning themselves bald. Hoxb8 is important for creating microglia–nervous system repair cells that search for damage in the brain.

Although some microglia start out in the brain, others are born in the bone marrow and move in. Overall, adult mice with faulty Hoxb8 harbored about 15% fewer microglia in the brain than normal. [ScienceNow]

Since many microglia move from bone marrow to brain, the scientists decided to give the compulsive mice, with the mutant Hoxb8 gene, a marrow transplant.

Step 2 — Treatment

They took marrow from mice with regular Hoxb8 and gave it to the compulsive mice mice. Within four weeks, the mice stopped their obsessive cleaning. Within about three months, they had a full coat of hair.

“A lot of people are going to find it amazing,” said Mario Capecchi at the University of Utah, who won the Nobel prize for medicine in 2007 for his work on mouse genetics [and was a co-author of the paper]. “That’s the surprise: bone marrow can correct a behavioral defect.” [Guardian]

Step 3 — Prognosis

Scientists aren’t sure why a gene controlling immune cells (the microglia) appears to cure a psychological disorder, but they have some suspicions.

“Why couple behavior such as grooming to the host’s immune system?” the researchers ask in [the paper's] conclusion. “From an evolutionary perspective it may make perfect sense to couple a behavior such as grooming, whose purpose is to reduce pathogen count, with the cellular machinery–the innate and adaptive immune systems–used to eliminate pathogens.” [e! Science News]

Humans have a psychological disorder that mirrors this grooming and hair loss compulsion in mice–the obsessive-compulsive spectrum disorder trichotillomania in which people pull out their own hair. But before jumping on this apparent marrow transplant cure, scientists would need to find the particular human gene responsible.

Capecchi warns that bone marrow transplants are too risky to be commonly used against, for example, OCD. Rather, a fuller understanding of the immune system-mental illness connection should produce new treatments. [Scientific American]

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Image: flickr/Bascom Hogue