The genetics of deafness

Deafness is an etiologically heterogeneous trait with many known genetic and environmental causes. Genetic factors account for at least half of all cases of profound congenital deafness, and can be classified by the mode of inheritance and the presence or absence of characteristic clinical features that may permit the diagnosis of a specific form of syndromic deafness. The identification of more than 120 independent genes for deafness has provided profound new insights into the pathophysiology of hearing, as well as many unexpected surprises. Although a large number of genes can clearly cause deafness, recessive mutations at a single locus, GJB2 or Connexin 26, account for more than half of all genetic cases in some, but not all populations. The high frequency may well be related to the greatly improved social, educational, and economic circumstances of the deaf that began with the introduction of sign language 300-400 years ago, along with a high frequency of marriages among the deaf in many countries. Similar mechanisms may account for the rapid fixation of genes for speech after the first mutations appeared 50,000-100,000 years ago. Molecular studies have shown that mutations involving several different loci may be the cause for the same form of syndromic deafness. Even within a single locus, different mutations can have profoundly different effects, leading to a different pattern of inheritance in some cases, or isolated hearing loss without the characteristic syndromic features in others. Most cases of genetic deafness result from mutations at a single locus, but an increasing number of examples are being recognized in which recessive mutations at two loci are involved. For example, digenic interactions are now known to be an important cause of deafness in individuals who carry a single mutation at the Connexin 26 locus along with a deletion involving the functionally related Connexin 30 locus. This mechanism complicates genetic evaluation and counseling, but provides a satisfying explanation for Connexin 26 heterozygotes who, for previously unknown reasons, are deaf. A specific genetic diagnosis can sometimes be of great clinical importance, as in the case of the mitochondrial A1555G mutation which causes gene carriers to be exquisitely sensitive to the ototoxic effects of aminoglycosides. This potentially preventable genetic-environmental interaction was the most common cause of genetic deafness in countries where these antibiotics were used indiscriminately in the past. Advances in genetic knowledge along with the use of cochlear implants have posed unique ethical dilemmas for society as well as the deaf community. Since most deaf children are born to hearing parents, it seems likely that deaf culture, and intermarriages among those born with deafness will recede during this century. Will future critics view this as one of the medical triumphs of the 21(st) Century, or as an egregious example of cultural genocide? On the other hand, genetics can provide empowering knowledge to the deaf community that for the first time can allow many deaf couples to know whether their children will be hearing or deaf even before they are conceived.