Clinical and genetic heterogeneity in X-linked deafness

Connexin 26 mutations in nonsyndromic autosomal recessive hearing loss: speech and hearing rehabilitation

OBJECTIVE

Hearing loss is the most common form of sensory impairment, with approximately one infant/1000 born with profound congenital deafness. A pre-lingual bilateral sensorineural hearing impairment poses a substantial problem as it negatively impacts on the subject's ability to conduct a normal social life. The aim of the study was to observe, in a group of children affected by pre-lingual non-syndromic autosomal recessive hearing impairment: (1) the role of the possible mutation of connexin 26 in the pathogenesis of the hearing loss; (2) the audiological and clinical aspects of the hearing impairment; (3) therapy to be adopted for the different patients.

METHODS

The study was carried out on 39 patients, 16 males and 23 females, aged between six and 17 years (mean 12 years), affected by non syndromic congenital deafness, presumably hereditary, referred to the out-patients audiology clinic for children of the Department of Otolaryngology of the Federico II University of Naples.

RESULTS

Our study conducted on 39 children with pre-lingual bilateral sensorineural autosomal recessive deafness showed as follows: (I) from a molecular perspective: an incidence of 41% in the cases studied of mutations in the encoding of the connexin 26 gene; a prevalence in our case study of the 35delG mutation (69%). (II) The characteristics of the hearing impairments in the children studied were homogeneous, regardless of the presence or absence of a connexin 26 mutation: the hearing impairment was pre-lingual bilateral sensorineural, the impairment often involved mainly the high frequencies, but, especially in the severe forms an involvement of all the frequencies was not rare; the hearing impairments were symmetrical and non progressive in time. (III) The results of the application of prosthesis and thereafter rehabilitative language therapy are generally satisfactory but correlated of course to the severity of the hearing loss.

CONCLUSION

In conclusion, we hope that further developments in the research on genetic hearing impairments will promptly result in advances in clinical practice.

Genetics and molecular biology of deafness. Update

This article discusses the latest research in the molecular biology and genetics of hearing impairment and its importance to otolaryngologists. Recent research has led to the discovery of many of the genes and gene products that are responsible for hereditary hearing impairment. State mandated screening of newborn infants for hearing loss ensures that a large number of hearing-impaired children will be detected at a very early age. Additionally, these children often will be referred to the otolaryngologist for evaluation of the hearing impairment. It is the otolaryngologist who must gather a detailed family history and perform a thorough physical examination to fully assess the cause of the hearing impairment. In taking the family history, it is important to note that the diagnosis of a hereditary hearing impairment often involves the evaluation of a large-sized family that has a history of hearing disorders. A history of an affected individual in a small family does not necessarily support a diagnosis of hearing impairment in later affected offspring because of the small sample size. Often, a hearing impairment that is part of a syndrome may not be detected because the physical findings associated with a syndrome are subtle in a young infant. For example, the white forelock seen in patients with Waardenburg's syndrome type I cannot be visualized in the infant who lacks hair. Additionally, some patients with syndromic hearing impairment do not present with physical findings, but rather they exhibit abnormal laboratory studies. Additional points to remember include the following: As infectious iatrogenic causes of hearing impairment decrease, the relative incidence of hereditary hearing impairment will increase. Hereditary hearing impairment can present as an isolated finding, or in association with a number of anomalies recognizable as a syndrome. The study of genetics and molecular biology has led to the identification of genes associated with hearing impairment and will allow for future screening and possible therapy for the hearing-impaired. The screening of newborns for hearing impairment using the techniques of molecular biologists and geneticists will result in early identification and appropriate intervention for those at risk for hereditary hearing impairment. An understanding of the syndromic and nonsyndromic causes of hereditary hearing impairment can help the otolaryngologist make a diagnosis and provide appropriate audiologic and educational management to the patient.

Genetics and molecular biology of deafness

With increased emphasis on early detection of hearing impairment, more babies are likely to be referred at younger ages to otolaryngologists for evaluation. With a diminution in the number of infants who have hearing impairment as a result of such factors as maternal infection, neonatal sepsis, or ototoxicity, the relative importance of detecting a genetic cause of newborn hearing impairment is likely to increase. Therefore, the otolaryngologist must become familiar with common causes of hereditary hearing impairment and the ways in which the newborn should be evaluated for hereditary hearing impairment. Advancements are rapidly being made in the ability to detect genes that cause hearing impairment, and we are now on the threshold of discovering ways to use gene therapy to prevent or treat hereditary deafness.

A new X linked recessive deafness syndrome with blindness, dystonia, fractures, and mental deficiency is linked to Xq22

X linked recessive deafness accounts for only 1.7% of all childhood deafness. Only a few of the at least 28 different X linked syndromes associated with hearing impairment have been characterised at the molecular level. In 1960, a large Norwegian family was reported with early onset progressive sensorineural deafness, which was indexed in McKusick as DFN-1, McKusick 304700. No associated symptoms were described at that time. This family has been restudied clinically. Extensive neurological, neurophysiological, neuroradiological, and biochemical, as well as molecular techniques, have been applied to characterise the X linked recessive syndrome. The family history and extensive characterisation of 16 affected males in five generations confirmed the X linked recessive inheritance and the postlingual progressive nature of the sensorineural deafness. Some obligate carrier females showed signs of minor neuropathy and mild hearing impairment. Restudy of the original DFN-1 family showed that the deafness is part of a progressive X linked recessive syndrome, which includes visual disability leading to cortical blindness, dystonia, fractures, and mental deficiency. Linkage analysis indicated that the gene was linked to locus DXS101 in Xq22 with a lod score of 5.37 (zero recombination). Based on lod-1 support interval of the multipoint analysis, the gene is located in a region spanning from 5 cM proximal to 3 cM distal to this locus. As the proteolipid protein gene (PLP) is within this region and mutations have been shown to be associated with non-classical PMD (Pelizaeus-Merzbacher disease), such as complex X linked hereditary spastic paraplegia, PLP may represent a candidate gene for this disorder. This family represents a new syndrome (Mohr-Tranebjaerg syndrome, MTS) and provides significant new information about a new X linked recessive sydromic type of deafness which was previously thought to be isolated deafness.

Nonsyndromic hearing loss: an analysis of audiograms

We examined features of the audiograms of 136 individuals, from 28 families, affected by nonsyndromic genetic hearing loss. There were 83 (12 families) with autosomal dominant (AD) loss, 50 (15 families) with autosomal recessive (AR) loss, and 3 (1 family) with X-linked recessive loss. The main audiogram shapes found were sloping (50.3%), residual (26.5%), and flat (21.0%). Specific shapes (ascending and U-shaped) only occurred in 3.7% of AD cases. Audiogram shapes were found to be significantly different between AD and AR families, and showed intrafamilial and interfamilial variability. In the AR group, the main shapes were residual and sharply sloping, and in the AD group, sharply sloping, flat, and gently sloping. There was a significant difference in the degree of hearing loss between AD and AR types, with AD being milder than AR. It has been shown that there is more marked intrafamilial variation in the degree of hearing loss in AD families than in AR ones. The results suggest that the audiograms of nonsyndromic hearing loss are usually nonspecific and that counseling of family members would be better based on the specific family's condition rather than on group information.

Neuro-otological function in X-linked hearing loss: a multipedigree assessment and correlation with other clinical parameters

Auditory and vestibular investigations were carried out in 19 affected men and 13 obligate female carriers of 7 pedigrees with nonsyndromic hearing loss segregating as an X-linked trait. In addition, high resolution computerised tomographic scanning was carried out in 24 affected males and 12 obligate female carriers. The neuro-otological results confirm that non syndromic X-linked hearing loss is a clinically heterogeneous condition, but radiological assessment of the cochlea revealed two distinct groups: a normal group, and an abnormal group characterised by a bulbous internal auditory meatus, a dilated facial nerve canal and incomplete separation of the basal coil of the cochlea from the internal auditory meatus. Within a given pedigree there was marked consistency of the presence or absence of the CT scan abnormality in the affected males. One third of the obligate female carriers of the radiologically abnormal pedigrees were shown to have a similar abnormal finding, but as two thirds were normal, radiological examination did not predict carrier status. In the affected men, pure tone audiometric data did not correlate with the radiological abnormality, whereas vestibular function was strikingly correlated, being normal in all but one case in pedigrees with normal radiology and absent, or grossly impaired, in the pedigrees with abnormal radiology. Neuro-otological abnormalities were documented in approximately two thirds of the obligate female carriers, but were insufficiently frequent in occurrence or specific in type to be of predictive value.