“Sensorineural” hearing loss is caused by problems in the inner ear or in the hearing nerve, or both. “Mixed” hearing loss is a result of both kinds of problems. 4 What Causes Hearing Loss? Hearing loss can be caused by changes in genes or by outside factors (like injuries, illness or certain medications), or both. For many children with hearing loss, the cause is unknown. Genes are passed from parents to children and cause things to run in families. There are many genes that are involved in hearing. Sometimes, a gene does not form in the way it should. When this happens, it is called a mutation. Some mutations cause syndromic hearing loss and others cause nonsyndromic hearing loss. Even among some families with genetic hearing loss, a loss is not due to mutations in any of the known genes. Scientists are working to find all of the genes involved in hearing loss. Genes are described in more detail in the next section. There are also a number of things that are not genetic that can cause hearing loss. For example, babies who are born too early and babies who need help breathing (for example, using a ventilator) are more likely to develop hearing loss than are other babies. Some infections (such as cytomegalovirus) the mother has during her pregnancy can cause the baby to have hearing loss. Also, some infections (such as meningitis) that babies and children have can cause hearing loss. Sometimes, both genes and environment work together to cause hearing loss. For example, there are some medicines that can cause hearing loss, but only among people who have certain mutations in their genes. 5 How Do Doctors Figure Out What Caused a Person’s Hearing Loss? Doctors begin by looking at a person’s physical features, medical history, and family history. Based on this, they classify the hearing loss in the ways described earlier (congenital or acquired, prelingual or postlingual, progressive or nonprogressive, conductive or sensorineural, syndromic or nonsyndromic, and familial or sporadic). The classifications often point to certain causes. The doctors might ask for more medical tests to look for signs of syndromic hearing loss, and they might ask for genetic tests. 6 About Genes & Mutations What Are Genes? Genes tell the cells of people’s bodies how to grow and work. For example, the instructions in certain genes control what color a person’s eyes will be. Each person gets half of his or her genes from each parent, and that is why a person tends to look like his or her parents. Genes are stored in a chemical called deoxyribonucleic acid (DNA). DNA is a chemical chain made up of “bases”. There are four bases: A, C, T and G. The specific order, or sequence, of these bases determines the exact information carried in each gene, like the way that a specific pattern of letters makes up the words in a sentence. The DNA is packaged into small units called chromosomes. Each cell in a person’s body contains a set of chromosomes, and thus a set of genetic instructions. Every person should have 23 pairs of chromosomes. One chromosome from each pair is from the person’s mother and one chromosome of each pair is from the father. The first 22 pairs of chromosomes are a matching pair in everybody. The 23rd pair is called the sex chromosomes, and they help determine if a person is male or female. A female has two X chromosomes, and a male has one X and one Y chromosome. A mother will give one of her two X chromosomes to each of her children. A father will give either his X or his Y chromosome. A child who gets the father’s Y chromosome will be a boy. A child who gets the father’s X chromosome will be a girl. A mutation is any change in the usual DNA sequence. For example, suppose part of a gene usually has the sequence GTAC. A mutation can change the sequence to GTTC in some people. This change in sequence can change the way that the gene works so that people with this mutation can have a particular condition. Not all mutations affect the person who has them. Genetic conditions often are described in terms of the chromosome that contains the gene. If the gene is on one of the first 22 pairs of chromosomes, called autosomes, the genetic condition is called an “autosomal” condition. If the gene is on the X chromosome, the condition is called “X-linked”, or “sex-linked”. Genetic conditions also are grouped by how they run in families. Different mutations cause conditions to run in families in different patterns. Mutations are called “dominant” or “recessive” depending on how they cause conditions and how they run in families. 10 Autosomal Dominant Conditions “Autosomal” conditions affect both males and females equally. In “dominant” conditions, the condition is passed from parent to child. If one parent has the condition, each child has a 50% (1 in 2) chance of having it as well. “Autosomal” genes are genes that are found on one of the first 22 pairs of chromosomes, the pairs that are the same for both males and females. In the case of “dominant” conditions, a child will have the condition if he or she has one copy of the gene with the dominant mutation. It takes only one copy of a dominant mutation to cause the condition. Figure 4 shows how children get dominant conditions from their parents. In this example shown, the usual copy is symbolized by and the copy with the dominant mutation is symbolized by . When one parent has the dominant condition, he or she has a usual copy ( ) and a copy with the dominant mutation ( ). He or she will give each child one or the other. Therefore each child has a 50% (1 in 2) chance of getting the gene with the dominant mutation and of having the condition. If the other parent has two usual copies, and therefore does not have the condition, he or she will give each child one of the two usual copies ( ). Even though a child gets one of the usual copies from the parent who does not have the condition, if he or she gets a dominant copy from the parent with the condition, the child also will have the condition. 11 An Example of Autosomal Dominant Inheritance Deaf Father Deaf Daughter Hearing Son Deaf Son Hearing Daughter Hearing Mother copy with a dominant mutation. Figure 4 An Example of Autosomal Recessive Inheritance 12 Hearing