Hearing Gene; Sounding out the Heredity of Deafness
Ezzell, Carol, Science News
Nineteenth-century naturalist Charles Darwin was probably not the first person to observe that many blue-eyed cats are also born deaf, although he might have been the first to record the observation. Over the years, farmers and city folk alike have noticed that some blue-eyed, all-white cats are poor mousers and tend to get mauled in fights -- good tip-offs that they can hear neither prey nor predator. Generations of pet owners have also noted that these cats usually startle easily and never learn to respond to their names.
Geneticists studying the inherited causes of deafness have now uncovered an explanation for why blue eyes and white hair go hand in hand with deafness in species as disparate as cats, mice, mink, horses and humans.
Two independent groups of researchers have found that people with a pigmentation and hearing disorder called Waardenburg's syndrome bear mutations in a gene thought to direct a critical phase of early embryonic development. The researchers suspect that defects in an extremely similar gene cause a comparable syndrome in most mammals -- including the common white house cat and a pedigreed domestic feline called the Turkish Van.
Waardenburg's syndrome accounts for 2 to 3 percent of the 1 million cases of congenital deafness in the United States, according to the National Institute on Deafness and Other Communication Disorders in Bethesda, Md. But many researchers studying for a much larger proportion of cases.
"I think it's very, very underdiagnosed," says Aubrey Milunsky of the Boston University School of Medicine, who led one of the two groups reporting the new findings. "It's so variable; the manifestations can be very mild."
Waardenburg's syndrome -- first described in 1951 by Dutch physician Petrus J. Waardenburg -- has several hallmarks, including widely spaced eyes that are pinched at the inner corners, deafness, light blue or mismatched eye color, fused eyebrows, and a white forelock of hair. The syndrome is a dominant genetic disorder, which means that a defect in only one of the two copies of a necessary gene causes
However, because of genetic phenomena known as incomplete penetrance and variable expressivity, most Waardenburg's patients do not exhibit all of the syndrome's characteristic traits. A hearing mother with the Waardenburg's traits of a white forelock and widely spaced eyes, for example, can have a deaf son with widely spaced eyes and fused eyebrows, but no white forelock.
For the past few years, geneticists at six laboratories in three countries have sought the gene responsible for Waardenburg's. Many have studied so-called "Splotch" mice, named for the white splotches that appear on the animal's bellies, heads and tails.
Splotch mice have several Waardenburg's traits, including white forelocks and small, widely spaced eyes. A group led by Douglas J. Epstein of McGill University in Montreal reported in the Nov. 15, 1991 CELL that Splotch mice have a defect in one copy of a gene named Pax-3, which prods the ectoderm of the embryo to develop into the skin and nervous system. The chemical structure of Pax-3 resembles that of another gene family, the homeobox genes, which play a role in overall body development in the early embryos of everything from mice and fruitflies to humans (SN: 4/20/91, p.255).
Encouraged by the Canadian finding, Milunsky's group and a separate team of European researchers led by Andrew P. Read of St. Mary's Hospital in Manchester, England, began looking for similar mutations among families with Waardenburg's. Working independently, the two groups found mutations in the human equivalent of the Pax-3 gene in four families. …