By David N. Leff
On the fabled island of Bali, in the village of Bengkala, dwell 2,200 men, women andchildren. Forty-five of them were born profoundly and permanently deaf.
Their inherited malady goes by the name of nonsyndromic sensorineural recessive deafness(NSRD). "This," explained molecular geneticist Sally Camper, at the University ofMichigan Medical School, in Ann Arbor, "means deafness not associated with any otherobvious inborn anomalies, such as widely spaced-out eyes or a white forelock."
NSRD accounts for about 80 percent of hereditary hearing loss in the world today. "Theremaining 20 percent," Camper observed, "would include dominantly inherited deafness,and syndromes in which hearing deficit was only one of several symptoms. NSRD," sheadded, "would include some of the deafness types for which Karen Avraham in Israel andKaren Steel in the U.K. have researched the molecular basis." (See BioWorldToday, March 20, 1998, p. 1.)
Camper is senior author of a research article in today's issue of Science, datedMay 29, 1998, and titled: "Correction of deafness in shaker-2 mice by anunconventional myosin in a BAC transgene."
These cryptic code words reflect "the first permanent correction of a deafness-relatedgenetic mutation," Camper told BioWorld Today, "and the fifth time that theidentification of a deafness gene in mice helped scientists find a similar gene in humans."
Her story begins just 70 years ago, in 1928, "when a scientist working in France wasexploring the effects of radiation, and how X-rays caused genetic damage. In 1928," sheobserved, "people didn't really know how dangerous X-rays might be. So they exposedmice to the radiation," she continued, "then mated them and bred their progeny, to seewhat happened. What happened was a lot of murine mutations. Deafness, later tracked toa mutation on mouse chromosome 11, was one of them."
Transfected Mice Mimic Human Hearing Deficit
The hearing-minus mice in a litter acquired the name shaker; it became a modelringer for human NSRD.
"We were assuming, from genetic mapping," Camper recounted, "that the shaker-2 mutant mouse was going to be a model for the NSRD gene region, denoted DFNB3on the human genome. Because throughout evolution, the genes that neighbor one anothertend to stay neighbors."
She and her co-authors identified a mouse region with "somewhere between one and threecentiMorgans of DNA distance. That's critical," Camper observed. "That could contain anumber of genes; we had to find the one responsible for normal hearing.
"So we used transgenic technology as a shortcut," she went on, "and began injectingvarious artificial chromosome clones from bacteria that we knew were in the region thatcould contain the gene. So imagine at least a dozen bacterial artificial chromosomes(BACs), each about 100 kilobases long, that might have the gene. We injected them intoeggs from deaf mice, and transferred the fertilized eggs to surrogate mothers, to be borntransgenic.
"Then we looked for mice in the litter that could hear."
To tell a mouse that can detect sound from one who can't, Camper continued, "there arelow-tech and high-tech ways. The low-tech way is to snap your fingers. If it hears, itjumps."
One newborn mouse in a litter of eight jumped, cured of his inherited deafness.
"The high-tech way," she noted, "is an electrical auditory brain-stem response sensor,which we carried out on our transgenic mice to assess how completely their hearing wasrestored."
Second Team Finds Deafness Gene
Meanwhile, a multinational consortium of scientists led by molecular geneticist ThomasFriedman had done linkage analysis on one NSRD family in that Bali village, plus two inIndia. Friedman heads the molecular genetics lab at the National Institute on Deafness andOther Communication Disorders, and is a co-senior author of Camper's article.
His findings appear back-to-back with Camper's in today's Science under the title:"Association of unconventional myosin MYO15 mutations with humannonsyndromic deafness DNFB3." Friedman and Camper are co-senior authors ofthat paper.
They report that DNFB3, a gene locus for NSRD, maps to a 3 centiMorganstretch of human chromosome 17's short arm. In it, they pinpointed a maverick myosingene, MYO15. Sequence analysis of its presence in the three families turned uptwo missense mutations and one nonsense mutation in the genomes of all deaf familymembers.
"This discovery," Friedman said, "suggests that MYO15 may account for asignificant proportion of hereditary hearing loss." He added, "The MYO15 gene isvirtually the same as the mouse Myo gene recently discovered by Camper and hercollaborators at Michigan."
Cell biologist and anatomist Yehoash Raphael, at the University of Michigan, is a co-author of Camper's paper. His primary goal, and that of his lab, is to correct hearing lossin people born deaf, or with acquired hearing loss.
"We have been doing gene transfer, which is our main focus, for four years," Raphael toldBioWorld Today. "We can introduce viral vectors into inner ear cells and expressdifferent proteins from different genes for different purposes, but the one cell that has beenvery difficult to transfect with a vector was the hair cell. These are the cells that transducethe sound into electrical nerve impulses. So we are working on that now."
Raphael sees only one strategy to accomplish such correction of deafness: "to have avector that will introduce the correct copy of the gene directly into the hair cells.
"In families born with the defect," he added, "we don't have data yet as to whether the haircells are there, but maybe damaged or degenerated. If the cells degenerated, it's likely toolate to hope for correction in that family.
"But even with these families described here, if they're born with the hair cells, it is likelythat we could still correct the deficit by introducing the correct gene and having it expressthe correct gene product."
But beyond such NSRD candidates, Raphael foresees, "the general indication forcorrection is that in some other mutations, where people lose their hearing in mid-life oreven later, then we could go ahead and intervene.
"The major technical obstacle, which we are working on now," he went on, "is to developthe right vectors to accomplish the gene transfer into the hair cells of a potential patient."
Camper said, "In the inner ear of a hearing person, fibers, or stereocilia, on the surface ofhair cells, move in response to changes in sound frequency like a field of wheat moving inresponse to wind speed and direction. Their movement sends electrical signals to auditorynerves, which the brain translates into sound."
To which Raphael added: "Hair cell stereocilia in shaker-2 mice look as if they'vebeen mowed down. Their cells are alive, but the stereocilia have been stunted by themutation." *