The finding ultimately will lead to new treatments for this and less severe forms of excessive hair growth as well as baldness, said researchers at the Keck School of Medicine of the University of Southern California (USC).
The researchers made the discovery in cooperation with their counterparts at Peking Union Medical College. Study findings were published in the June issue of the American Journal of Human Genetics.
The initial discovery of the mutation came from a lab at the Peking Union Medical College, which examined the condition, known congenital hypertrichosis syndrome (CGH), in a Chinese family.
Males with this disorder have hair covering their entire face including their eyelids and their upper body, while females have thick patches of hair on their bodies.
Then researchers confirmed the finding in a Mexican family that Pragna Patel, professor in the Keck School, the USC Institute for Genetic Medicine, and the Herman Ostrow School of Dentistry of USC, first began studying in 1993.
"In 1995, we traced the approximate location of the mutation responsible for CGH to a section of the X chromosome in the Mexican family," said Patel.
"We sequenced nearly 100 genes but could not find any mutations. Then recently, Xue Zhang and his colleagues in Beijing were able to pinpoint the exact location, discovering that there was an insertion of chromosome 5 into the X chromosome in the Chinese family," Patel said.
"When the Mexican family was examined, a piece of chromosome 4 was found to be inserted into the same part of the X chromosome, thus confirming that these extremely rare events caused the disorder."
The investigators suggest that insertion of the "extra" DNA sequences into the X chromosome apparently turns on a gene, likely SOX3, located near the insertion site. SOX3 is a strong candidate because other members of this gene family have been shown to play a role in hair growth.
In addition, the insertion has occurred within a block of DNA sequence called a "palindrome," in which the sequence of the four building blocks of DNA (akin to letters of the alphabet) read exactly the same as their complementary sequences, but in the reverse direction. The particular palindrome at the site that the researchers studied is only found in humans.
"We don't yet know the significance of the palindromic sequence in this case," Patel said. "But it appears to be unstable, and can be entirely absent in many individuals with normal hair growth. It 's only when there is insertion of certain chromosome segments at this site that people have extra hair."
Earlier, researchers had theorized that the CGH mutation is " atavistic" -- a trait that reappears after being absent for a long time. One example of an atavistic trait is extra nipples in both men and women.
"It's like the information is there in the genome, but is silenced," Patel said. "Then somehow it's reactivated, and can manifest as the trait. We don't know yet if this is the case with CGH."Further studies will test if this is indeed true. "If in fact the inserted sequences turn on a gene that can trigger hair growth, it may hold promise for treating baldness or hirsutism (excessive hair growth) in the future, especially if we could engineer ways to achieve this with drugs or other means," Patel said.
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