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Familial deafness

Congenital deafness occurs at a frequency of 1:1000 newborns. This is a heterogeneous group - there are many causes of deafness of which 2/3 are genetic and 1/3 are not.

Non-genetic causes usually manifest as unilateral, asymmetric hearing impairment and are usually caused by environmental factors such as frequent use of intravenous aminoglycoside-type antibiotics, premature birth, prolonged intensive care, viral infection in the first months following birth (particularly mumps and CMV), intrauterine infection with CMV or other viruses, brain damage at birth, meningitis, etc.

Genetic causes are also diverse. The main types of inheritance patterns are ">autosomal recessive - 83%, autosomal dominant - 10%, and X-linked - 5%. Recently, however, it has been shown that deafness can also be transmitted by mitochondrial inheritance - individuals with specific mutations in the mitochondrial genome can have an extreme sensitivity to several medications, especially aminoglycoside antibiotics, which can cause deafness following the administration of a single low dose. It is estimated that mitochondrial inheritance accounts for approximately 2% of all cases of hereditary deafness.

In cases that have a genetic cause, the hearing impediment is usually bilateral and symmetrical.

In 83% of cases the inheritance is autosomal recessive, and the parents of an affected child have a 25% risk in each pregnancy for having another child with a similar hearing disorder.

In all cases where there is a family history, it is important for the geneticist to establish the inheritance pattern and hence the risk to the various family members of having a child with a hearing impediment.

In some cases the hearing impairment is part of a syndrome, with other organs also being involved. In retinitis pigmentosa (a disorder of vision) both the eye and the ear are involved, and there are other syndromes where hearing impairment is associated with white hair tips, mental retardation, skin spots, kidney diseases, etc. The presence of such signs and symptoms will assist the geneticist in making an accurate diagnosis, based on which he will be able to determine the risk of recurrence and decide which tests should be performed before and during a subsequent pregnancy.

In recent years there has been great progress in identifying the genes that are responsible for deafness. There are very many genetic causes, and within each type of inheritance pattern there are a large number of different genes that cause hearing impairment. For example, more than 20 genes are currently known to be responsible for autosomal recessive deafness, so that if two people with this type of deafness marry, as long as the genes carried by each of the parents are different, they can have children with perfect hearing. This is also the case for other inheritance patterns.

Genetic testing

Molecular testing is as yet not available for all of the genes concerned. However, about 40% of autosomal recessive hearing defects are a result of known, recognized mutations in the Connexin 26 gene.

There is one specific mutation that is common among Ashkenazi Jews - this is 167delT, a deletion of a single nucleotide T at position 167 of the gene. More recently it has been found that another common mutation (a deletion of part of the gene) in a nearby gene called Connexin 30, can also cause a malfunction of the Connexin 26 gene. Therefore a couple where both partners carry a mutation in Connexin 26, or where one partner carries a mutation in Connexin 26 and the other a mutation in Connexin 30, can have a child who is deaf, with a risk in each case of 25%. Because of this, both the testing of deaf individuals of Ashkenazi origin and the screening of healthy Ashkenazi Jews for carrier status should include at least these 2 mutations in Connexin 26 and Connexin 30.

In Connexin 26 there is another mutation that is common in all ethnic groups - 35delG (a deletion of a single nucleotide G at position 35 of the gene).

Carrier testing

The cause of the deafness can be established in at least 40% of families with autosomal recessive hearing defects by performing a relatively simple test. In such cases the parents are given appropriate counseling and are offered prenatal diagnosis or diagnosis right after birth.

It is important to test for mutations in the Connexin 26 gene (and also in the Connexin 30 gene in Ashkenazim) in individuals with hearing defects, and if one is found, the carrier status of the parents can be tested. However, because of the good prognosis, this is not mandatory. This is also the reason for not recommending general screening for non-syndromic hearing loss, although by means of a relatively easy test, 1:30 individuals will be found to be a carrier of a mutation in Connexin 26 and/or Connexin 30.

Tests are also available nowadays for many other syndromes associated with deafness, including for the specific mitochondrial mutation that predisposes to drug (aminoglycoside) induced deafness, etc.

The importance of early diagnosis

In recent years there have been significant advances in the management and treatment of hearing-impaired children. A cochlear implant can be inserted, and this is very effective, especially if carried out in early childhood. This is why early diagnosis is important.

Hearing loss in adulthood

Hearing impairment in adulthood is common - over a certain age, 1/3 of the population have hearing impairments. Appropriate tests for these individuals are on the whole not available, and, of course, prenatal testing in such cases is of lesser importance.

The geneticist's role

In genetic counseling, the presence or absence of a syndrome is ascertained, the probability for a genetic or environmental cause is determined, and if a genetic cause is identified, assessment of the risk of having an infant with a problem and decision as to which tests should be performed before and during a subsequent pregnancy.
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