Patient Information Genetic Testing for Childhood Hearing Loss
Introduction This document describes the most common genetic cause of childhood hearing loss and explains the role of genetic testing. Childhood hearing loss is relatively common. Approximately 3-4 in 1000 newborn babies have congenital (present at birth) severe to profound hearing loss. Another 1 to 3 in 1000 children develop hearing loss before age 18. About 60% or a little over one-half of the time the hearing loss is believed to be genetic; that is caused by a genetic change (mutation). Although some children born with hearing loss have other medical problems. most children with a genetic hearing loss have no unusual features. They have what is called isolated or non-syndromic hearing loss. This document discusses sensorineural hearing loss or hearing loss resulting from a problem in the inner ear. This is different from hearing loss associated with ear infections, which is usually caused by fluid in the middle ear (conductive hearing loss). Changes in Genes Can Cause Changes in Proteins Genes are made up of DNA, the chemical that stores all the hereditary information in a cell. DNA contains the instructions that cells use to make proteins. Proteins are the building blocks of our cells and have many functions. Some make up structural components of cells while others act as enzymes to speed up chemical reactions in our bodies. Through the action of proteins, genes determine such traits as the strength of our bones, skin color, how well we process foods and detoxify poisons, and our ability to fight infections. A change or mutation in a gene can result in an incorrectly made protein. Some gene changes have no effect at all while others cause the protein not to work. Non-functional proteins often cause disability or disease. Changes in the GJB2 Gene Can Cause Hearing Loss Although changes in many different genes can cause hearing loss, about 50% of children with non-syndromic, recessively inherited genetic hearing loss demonstrate a change in a gene called GJB2, also called the Connexin 26 gene. The most common change within this gene known to cause hearing loss is called 35delG because it involves the loss (deletion) of a small piece of genetic material. Although 35delG is the most common mutation, many other changes in the GJB2 gene have been identified. These changes interfere with the accurate production of a protein called Connexin 26, which forms channels between cells and allows the cells of the inner ear to communicate with each other.
It is believed that we all carry some altered genes that can cause health problems in ourselves or our children. The chance of being a carrier of a particular altered gene can be estimated by our family history and ethnic background. When there is no family history of childhood hearing loss, an individual in the general Caucasian population has about a 2 to 3% (2 to 3 out of 100) chance of carrying an altered GJB2 gene. Those of Ashkenazi Jewish ancestry have about a 4% (4 out of 100) chance of being a carrier. How Can My Child s Hearing Loss Be Genetic When No One Else in the Family Is Affected? A child s hearing loss can be genetic even if no one else in the family is affected. To understand why, it is important to know that genes occur in pairs. One member of each pair is inherited from each of our parents. The members of the gene pair are called alleles. One GJB2 allele is inherited from the mother while the other allele is inherited from the father. Hearing loss caused by CJB2 mutations is an autosomal recessive trait. Autosomal means that the disease occurs at equal frequency in girls and boys. Recessive means that both alleles have to be altered in order for the child to have hearing loss. The parents of a child with hearing loss caused by mutations in this gene have one altered GJB2 allele ( R, and one unaltered allele, r ). They usually have normal hearing. In fact, most individuals do not know they carry an altered allele until they have an affected child. Children whose parents are both carriers, can inherit both altered alleles. Many genetic conditions, for example sickle cell anemia and cystic fibrosis are inherited in this way.
What is the Chance that Two Carriers Will Have A Child with Hearing Loss? A couple who are both carriers, that is, have one R allele and one r allele, have a one in four (25%) chance that a child they conceive will inherit both altered alleles ( r ) and have hearing loss. This also means that they have a three in four (75%) chance that a child will inherit at least one unaltered allele, R, from one parent and not have hearing loss. For each child there was a 50% chance of inheriting one unaltered R allele and one altered roo allele at the time of his or her conception. These children will be carriers like their parents and are not expected to have hearing loss. The chances are the same for each pregnancy the parents have together. Not all genetic causes of hearing loss are inherited in this way. What Type of Changes Can Be Detected in the GJB2Gene? Although changes in the GJB2 gene cause up to 40% of all genetic hearing loss, most children with hearing loss will not have an alteration in this gene, and the cause of hearing loss will not be explained by this testing. When children with hearing loss are tested for changes in the GJB2 gene, some will have two altered copies or alleles known to be the cause of the hearing loss. The most common of these is the 35delG allele, but other changes can be found as well. In individuals who are of Ashkenazi-Jewish ancestry, a different genetic change. called 167delT, is more common. In Japanese families, an allele associated with hearing loss called 235delC is more common. Informing the laboratory about your ancestry helps to increase the accuracy of the test because of these differences. In addition, numerous less common changes can be detected by this testing. Some of these changes are known to cause hearing loss, others are normal variants, while others have unknown significance. The Results of Genetic Testing Can Be Inconclusive When only one copy of a change in the GJB2 gene causing hearing loss is detected, the interpretation of the results can be ambiguous. A child with hearing loss can have a change on only one allele but no detectable change on the other allele. In fact, up to 25% of children with hearing loss who have GJE2 testing will have only one detectable abnormal allele. One possible explanation is that there is some other change in the gene that is not detectable by current tests. In some families, additional studies may be ordered to try to detect other genetic changes. On the other hand, it is also possible that the child truly has only one copy of the genetic change and is a carrier. This can
mean that the child s hearing loss is due to a different genetic or environmental cause and unrelated to the GJB2 gene. Ambiguous or inconclusive results also occur when the child has the 35delG or other hearing loss associated change on one allele but the other allele has a detectable change that has not been proven to cause hearing loss. Some genetic changes are believed to have no effect and are called polymorphisms or variants. These are not considered to cause hearing loss or any other disease. Other changes are rare and their possible effect is not yet known. As we learn more about the effect of rare changes in the GJE2 gene, additional information should become available to answer these questions. It is important to remember that a child with hearing loss who does not have two detectable changes within the GJE2 gene may still have a genetic form of hearing loss. What Are the Benefits of Genetic Testing? The main benefit of GJB2testing is the ability to determine the cause and inheritance of hearing loss in some families. Identifying a child s change in the GJEJ gene may eventually help doctors predict the progression of the hearing loss. It may allow surgical interventions such as cochlear implants to be done at an optimal time. When a child with hearing loss caused by a change in the GJE2 gene is identified, we cannot. at this time, predict if the hearing loss will progress and at what rate. The severity of loss can vary from mild to moderate to profound; some have more loss as they get older, some do not. It is hoped that by identifying the group of children with hearing loss due to alterations in the GJB2 gene. These questions can be answered. As more is learned about the changes in the GJB2gene, more information will become available to guide professionals and families as they make decisions about the management of a child s hearing loss.
Knowing the specific cause of hearing loss can give families very accurate information about their chances of having another child with hearing loss. In those families in which the specific genetic cause of the hearing loss has been identified, it is possible to test a new sibling for this genetic change soon after birth. Early detection of hearing loss allows early intervention and can prevent or reduce language delays. There are hundreds of genes that, when altered, can cause hearing loss. Some of these genetic changes are associated with other health problems, including blindness, heart arrhythmias and kidney problems. Changes in the GJB2 gene have not been shown to cause any of these other medical problems. Implications for Other Relatives Determining that a child s hearing loss is due to changes in the GJB2 gene has implications for other relatives. It means that both parents are carriers and that they have a 25% chance that any future child they have together will also have hearing loss. It also means that the full siblings (brothers and sisters) of the child with hearing loss have a 2 in 3 (66%) chance of being a carrier like the parents. Siblings can be tested to determine whether or not they are carriers, although being a carrier does not seem to be related with any health problems. Carrier testing can provide information about a couple s chance of having a child with hearing loss. We recommend that carrier testing should be delayed until the child reaches an age when the decision whether or not to have testing can be made by the individual themselves. Other relatives (aunts, uncles, cousins) of a child with hearing loss caused by changes in the GJB2 gene have an increased chance of being a carrier over the chance of individuals in the general population. Some of these relatives may wish to undergo carrier testing. There is currently no evidence that being a carrier of this change causes health problems or later hearing loss. However, there is some concern that a positive carrier test result may be a cause for health and/or life insurance discrimination or other types of discrimination and should be done only after the person has been fully informed of the benefits and disadvantages of testing. When the 35delG or another change in the GJB2 gene is determined to be the cause of hearing loss in a child, the families have many issues they need to discuss. For that reason, a referral is made to the Division of Genetics and Metabolism in the Department of Pediatrics at UNC Hospitals for a full discussion with a Medical Geneticist and Certified Genetic Counselor. Genetics professionals support the idea of nondirective counseling, which means that genetic counseling is meant to be informative and supportive. It is not meant to tell people what to do or whether or not to have children.
What Does It Mean When There Are No Changes Identified in the GJB2 Gene? When a child with hearing loss is tested for changes in the GJB2 gene, it is possible that neither allele will have the 35delG or other changes known to cause hearing loss. This could mean that the hearing loss is not due to a change in the GJB2 gene at all but rather to changes in a different gene. There are many genes that, when altered, can cause hearing loss. GJB2 is only one of these genes. If the change is in another gene that is inherited in the same way as the CJB2 gene, that is, in an autosomal recessive pattern, the chance that the parents could have another child with childhood hearing loss remains 25% (1 in 4). Changes in other genes causing hearing loss can be inherited in other ways and the parents chances for future affected children will differ. Another possible explanation is that the child s hearing loss is not genetic at all. In these situations, the assessments of the chance of the parents having another child with hearing loss are complicated. If you would like to learn more about your chance of having additional children with hearing loss, an appointment can be made in the Genetics and Metabolism Clinic for a genetics evaluation of the child and discussion about these issues. How is Hearing Loss Caused by Changes in the 6JB2 Gene Treated? Currently, hearing loss caused by genetic changes is treated the same way no matter what the specific cause. Treatment depends on the severity of hearing loss, and ranges from hearing aids to cochlear implantation. With further research, we hope to determine which interventions are more appropriate based on specific genetic diagnoses. The Carolina Children s Communicative Disorders Program (CCCDP) is funded by a grant through the North Carolina Department of Health and Human Services, Division of Public Health, and is based at the Department of Otolaryngology/ Head and Neck Surgery at UNC. The purpose of the grant is to financially assist North Carolinians under 21 years of age with hearing loss and other communicative disorders. Participants are accepted into the Program based on family size, income, other medical expenses, and the limitations of insurance and other resources such as Medicaid. The financial criteria are flexible so that we may serve as many children as possible. Requests for CCCDP applications and brochures may be made directly by families or indirectly to families through professionals or by calling (919) 419-1449.
Summary Affected children who are the only ones in their family with hearing loss or who have affected brothers and/or sisters can have detectable changes in the GJB2 gene. This is sometimes referred to as Connexin 26 related hearing loss. The most common change causing hearing loss is called 35delG within the GJB2 gene. There is a wide variation in the degree and kind of hearing loss in individuals with this change. Other changes within the gene can be responsible for hearing loss in people of different ancestries. Hearing loss results from a wide range of causes. There are genes other than the GJB2 gene that, when altered, also cause hearing loss. Testing a child with hearing loss to detect changes within the GJB2 gene may be negative but this does not necessarily mean that the child s hearing loss is not genetic. Genetic evaluation and counseling is available through the Genetics and Metabolism Clinic. Appointments can be made by calling (919) 966-4202. Evaluation and treatment of hearing loss is available through the Department of Otolaryngology/Head and Neck Surgery. Appointments can be made by calling (919) 966-6484. For more information please contact: UNC Neuroscience Hospital 101 Manning Drive Chapel Hill, NC 27599 Phone: 919.966.6490 Fax: 919.843.9361 Carolina Pointe 5915 Farrington Road, Suite 102 Chapel Hill, NC 27517 Phone: 919.490.3738 Fax: 919.490.3745 med.unc.edu/ent