Deafness

Abstract

Recent statistical data indicate that in western countries, at least 1:1000 suffer of congenital hearing impairment. Genetic factors are responsible for more than 60% of the congenital cases. Sometimes, however, hearing loss is a multifactorial disorder caused by both genetic and environmental factors. Molecular genetics of deafness has experienced remarkable progress in the last decade. Genes responsible for hereditary hearing impairment are being mapped and cloned progressively. This article, while reporting some syndromic conditions related to deafness, focuses on nonsyndromic hearing loss. Genes involved in this type of pathologic condition have only recently begun to be identified. Owing to genetic heterogeneity, private nature of most mutations and large size of most deafness genes, the molecular diagnostic possibilities in recessive deafness are today limited only for a few genes. Facility and benefits of genetic tracking, in particular, should make it an important public health issue so that determinations of early diagnosis of hearing loss can be properly established.

Key concepts:

  • Deafness is the most common sensory deficit in humans, affecting 1 in 1000 new‐born, and to date more than 23 different DFNB proteins have been discovered.

  • DFNB proteins can be classified into two classes: (1) genes regulating potassium homeostasis and (2) genes producing proteins that are important in the formation of stereocilia.

  • Owing to genetic heterogeneity, the molecular diagnostic possibilities in recessive deafness are still limited. However, facility and benefits of genetic tracking should make them an important public health issue.

Keywords: hereditary hearing loss; myosin; connexin 26; ear; sensory hair cells; organ of Corti

Figure 1.

The structure of the human ear. Sound waves are captured by the auricle and conveyed through the external acoustic duct to the tympanic membrane, causing this membrane to vibrate. These vibrations are transmitted through the auditory ossicles of the middle ear to the footplate of the stapes, which is anchored in the oval window of the vestibule of the cochlea (Kandel et al., ).

Figure 2.

Schematic representation of the inner ear and a cross section of the detailed structure of the organ of Corti. (a) The cochlea consists of three fluid‐filled compartments throughout its entire length of 33 mm. A cross section of the cochlea shows the arrangement of the three ducts. The oval window, against which the stapes pushes in response to sound, communicates with the scale vestibuli. The scale tympani is closed at its base by the round window, with a thin, flexible membrane. Between these two compartments lies the scale media, an endolymph‐filled tube whose epithelial lining includes the 16 000 hair cells surmounting the basilar membrane (Kandel et al., ). (b) The hair bundle of each inner cell is a linear arrangement of the cell's stereocilia, whereas the hair bundle of each outer hair cell is a more elaborate, V‐shaped palisade of stereocilia. The hair cells are separated and supported by phalangeal and pillar cells. One hair cell has been removed from the middle row of the outer hair cells so that three‐dimensional aspects of the relationship between supporting cells and hair cells can be seen (Kandel et al., ).

Figure 3.

Localization of Cx26 in mouse cochlea. In (a) and (b) panels are represented two immunofluorescence of cross sections of a mouse cochlea, with stria vascularis in evidence. Cx26 is marked in green.

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Further Reading

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Terrinoni, Alessandro, Melino, Gerry, Serra, Valeria, Alessandrini, Marco, Napolitano, Bianca, and Bruno, Ernesto(Sep 2009) Deafness. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001453.pub2]