The Aztecs worshipped them, breeders charge upward of $1000 for them, and they just plain freak some people out. So what makes hairless dogs so different from their shaggy counterparts? A mere seven letters of genetic code, new research shows.
The American Kennel Club lists three breeds of hairless dogs: the Chinese crested; the Xoloitzcuintli (pronounced show-low-eats-keent-lee), also known as the Mexican hairless dog; and the Peruvian Inca Orchid dog. These canines typically live a normal life span, though they lack the full set of 42 teeth common to other adult dogs. Researchers have long known that a dominant gene causes this set of abnormalities, called canine ectodermal dysplasia (CED). That means dogs need only inherit one copy of it to be bald; with this particular gene, two copies are lethal. But the gene itself remained a mystery.
Tosso Leeb, an animal geneticist at the University of Berne in Switzerland, researched the genetics of hairlessness in cattle before directing his team's attention to the phenomenon in dogs. They collected blood samples from 20 hairless and 19 coated, or Powderpuff, Chinese crested dogs that have normal fur coats. They then scanned the dogs' whole genomes, looking for regions in which the hairless dog genes diverged from the regular code. The technique, called genomewide association mapping, turned up a suspicious-looking region on chromosome 17.
To investigate further, the Leeb and his team collected additional DNA samples from a wider array of hairless and coated dogs. In all, they looked at 93 hairless and 49 coated Chinese crested dogs, 39 hairless and six coated Peruvian hairless dogs, and eight Mexican hairless dogs (there is no coated counterpart). Mapping the chromosome 17 sequences revealed an insertion of seven letters, or base pairs, of genetic code in a gene called FOXI3 in all of the hairless dogs. Although the exact function of the FOXI3 gene is unknown, other genes in the FOX family control embryonic development in mammals. When the researchers investigated the equivalent gene in mice, they traced its activity to developing teeth and fur cells, they report tomorrow in Science. Leeb says the mutation most likely interferes with the genetic instructions for hair and teeth proteins, causing CED in dogs.
Isolating the mutation in dogs could provide a starting point for research on new baldness remedies in humans, says Jeremy Taylor, an animal geneticist at the University of Missouri, Columbia. But don't expect any therapies for dogs. CED is harmless, Taylor says, and dog aficionados prize the condition too much to want to get rid of it.