Usher Syndrome: New Insights Lead to Earlier Treatment
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Children with an inherited hearing and vision disorder known as Usher syndrome can now benefit from better treatment options, thanks to a recent scientific discovery of a method to identify this disorder at birth.
Those with Usher syndrome type 1 are usually born deaf or hearing-impaired in both ears. Vision problems often don't appear, however, until a child is older--around age 10 for those with the most severe form of Usher syndrome and around age 20 for those with a less severe form.
The disorder takes many by surprise. The parents of deaf children often do not know that they both carry the genes that cause Usher syndrome and its inevitable blindness in their children who inherit the Usher mutation from both parents. Approximately 5 percent of children born deaf or hearing-impaired have Usher syndrome, or about one in 25,000 of all people in the United States.
An international team of researchers, zeroing in on one of the several genes thought to be responsible for Usher syndrome, found a telltale mutation--a part of the gene not carrying out its biological orders properly. This mutation (called R245X) appears to account for a large proportion of the most severe form of Usher syndrome in the Ashkenazi Jewish population, a Jewish group descended from eastern European ancestors.
The finding is not a cure for Usher syndrome. Rather, in many cases, a simple blood test can identify whether a baby born with impaired hearing carries a certain faulty gene associated with Usher syndrome, years before vision problems first appear. Early detection of the disorder opens up the full range of treatment options available when children can benefit the most.
For example, some children who have the ability to read lips for about ten years before the onset of blindness could benefit from a cochlear implant, an electronic device implanted behind the ear that can provide a sense of interpretable sound. Children can also "prepare" for their oncoming blindness by learning new communication skills and other life skills while they can still see.
"Knowledge is truly empowerment in this case," said Dr. James Battey, Director of the National Institute on Deafness and Other Communication Disorders (NIDCD), which sponsored the research. "It is as if the door to treatment options for those with Usher syndrome has swung wide open."
The research was a collaboration among the lead scientists at NIDCD and other scientists at the Mount Sinai School of Medicine, the Sackler School of Medicine at Tel Aviv University, and the New York University Medical Center. Drs. Ben-Yosef, Friedman and Griffith at the NIDCD were the lead scientists on this project.
Usher syndrome affects many ethnic groups but was first identified among Jewish people by Dr. Albrecht Von Graefe in Berlin in 1858. Some 50 years later, Dr. Charles Usher of England realized the condition was inherited, or passed, from parents to their children.
There are three types of Usher syndrome. Usher syndrome type 1 (USH1) is marked by profound deafness at birth, balance problems and vision problems around age 10. Usher syndrome type 2 (USH2) is marked by moderate-to-severe hearing loss at birth, but no balance problems and a slower onset of vision loss, usually by around age 20. Usher syndrome type 3 (USH3) is characterized by progressive hearing loss that is post-lingual, or after a child learns to talk. With USH3, there are usually no balance problems, and vision loss occurs either in the first or second decade of life.
The vision loss in Usher syndrome is caused by retinitis pigmentosa. This disease first causes night-blindness and peripheral (side) vision loss due to a steady decline in the retina's ability to detect light. The retina is the light-sensitive tissue at the back of the eye that turns images into nerve impulses for the brain to interpret.
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Usher syndrome is an autosomal recessive disorder. Autosomal means than the disease is not associated with the X or Y sex chromosomes but rather with one of the 22 other chromosomes. Recessive means that a child with Usher syndrome inherits the faulty genes from both parents. A person who inherits the faulty gene from only one parent will be a carrier and will have no symptoms. When two carriers, a man and a woman, have a baby, that child has a 25 percent chance of having Usher syndrome, a 50 percent chance of being only a carrier, and a 25 percent chance of not being a carrier at all.
Dr. Ben-Yosef found a mutation, which the team called R245X, in a gene named PCDH15. This mutation appears to account for USH1 in about 60 percent of Ashkenazi Jewish people. Previously, a group led by Dr. Thomas Friedman of the NIDCD, who was also the head of this recent study, found a mutation carried by 4 percent of Ashkenazi Jewish. That mutation is called 167delT, and is located in a gene called GJB2. (Scientists often study this population, as well as other ethnic populations such as Icelanders or Finns, because it is easier to identify specific genes in populations that show less genetic variability than, say, the "melting pot" populations of Brazil or North America.)
The researchers said that the Ashkenazi Jewish population isn't more susceptible to Usher syndrome than the general population. The mutation R245X, however, might be unique to the Ashkenazi. All told, scientists know of seven genes associated with Usher syndrome: for USH1, the genes are MY07A, USH1C, CDH23, PCDH15, and SANS; for USH2 and USH3, the genes are USH2A and USH3A, respectively.
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There is still no cure for Usher syndrome, so scientists say that early identification is the best means to minimize the social isolation often associated with being both deaf and blind.
The researchers recommend that Ashkenazi Jewish infants with profound hearing loss should be screened, or checked, for the R245X mutation. This can be done through a blood test. Testing positive for the mutation means that the child likely has Usher syndrome, although this is not a certainty. Remember that many genes and, thus, many mutations are implicated in this disorder. Likewise, testing negative (that is, no R245X mutation present) does not mean that a Jewish child absolutely does not have Usher syndrome. The testing is merely an indication of risk.
Children who test positive (one or two copies of R245X), however, should have their eyes examined regularly for retinitis pigmentosa. An eye care professional can often spot this disease before vision problems start through the detection of minute pigmentation and nerve damage. An early sign of retinitis pigmentosa, compounded by deafness, is a near-certain indication of Usher syndrome.
A man and woman considering conceiving a child can also be screened to determine if they are carriers of Usher syndrome. If both prospective parents are carriers, a genetic counselor can prepare them for the social and education needs of a child with Usher syndrome.
Dr. Battey said that, in the search for the cause and cure of Usher syndrome, more mutations may be discovered and, thus, more testing options will come available. "This is a highly important discovery," he said. "When we can identify a mutation carried by many people in a population, basic science translates directly to clinical possibilities for children with Usher syndrome." That is, more research leads to more hope.
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Where can I get additional information?
American Association of the Deaf-Blind 814 Thayer Avenue, Suite 302
Silver Spring, MD 20910
TTY: (301) 588-6545
Fax: (301) 588-8705
E-mail:
aadb@erols.comInternet:
www.aadb.orgBetter Hearing Institute515 King Street, Suite 420
Alexandria, VA 22314
Voice/TTY: (703) 684-3391
Toll-free Voice/TTY: (800) EAR-WELL
Fax: (703) 684-3394
E-mail:
mail@betterhearing.orgInternet:
www.betterhearing.orgBoys Town National Research HospitalGenetics Department, Usher Syndrome Project
555 North 30th Street
Omaha, NE 68131
Toll-free Voice/TTY: (800) 835-1468
Fax: (402) 498-6331
Internet:
www.boystownhospital.org/UsherSyndrome/index.aspThe Foundation Fighting Blindness, Inc.11435 Cronhill Drive
Owings Mills, MD 21117-2220
Voice: (410) 568-0150
Toll-free Voice: (800) 683-5555
TTY: (410) 363-7139
Toll-free TTY: (800) 683-5551
Fax: (410) 363-2393
E-mail:
info@blindness.orgInternet:
www.blindness.orgHelen Keller National Center for Deaf-Blind Youth & Adults111 Middle Neck Road
Sands Point, NY 11050
Voice: (516) 944-8900
TTY: (516) 944-8637
Fax: (516) 944-7302
Internet:
www.hknc.org National Information Clearinghouse on Children Who Are Deaf-Blind (DB-LINK)345 North Monmouth Avenue
Monmouth, OR 97361
Toll-free Voice: (800) 438-9376
Toll-free TTY: (800) 854-7013
Fax: (503) 838-8150
E-mail:
dblink@tr.wou.eduInternet:
www.tr.wou.edu/dblinkNational Organization for Rare Disorders (NORD)55 Kenosia Avenue
PO Box 1968
Danbury, CT 06813
Voice: (203) 744-0100
Toll-free Voice: (800) 999-6673
Fax: (203) 798-2291
E-mail:
orphan@rarediseases.orgInternet:
www.rarediseases.orgSelf Help for Hard of Hearing People, Inc. (SHHH)7910 Woodmont Avenue, Suite 1200
Bethesda, MD 20814
Voice: (301) 657-2248
TTY: (301) 657-2249
Fax: (301) 913-9413
E-mail:
national@shhh.orgInternet:
www.hearingloss.org/Harvard Medical School Center for Hereditary DeafnessUnderstanding the Genetics of Deafness: A Guide for Patients and Families Internet:
http://hearing.harvard.eduNIH Publication No. 98-4291A
December 2003
For more information, contact the NIDCD Information Clearinghouse.
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