Molecular genetics of X-linked hearing impairment

Temporal bone histopathologic and genetic studies in Mohr-Tranebjaerg syndrome (DFN-1)

OBJECTIVE

To describe the temporal bone histopathologic and genetic abnormalities in a case of Mohr-Tranebjaerg syndrome.

BACKGROUND

Mohr-Tranebjaezrg syndrome (DFN-1) is an X-linked, recessive, syndromic hearing loss, characterized by postlingual sensorineural hearing loss with onset in childhood, followed in adult life by progressive dystonia, spasticity, dysphagia, and optic atrophy. The syndrome is caused by mutations in the DDP (deafness/dystonia peptide) gene, which are thought to result in mitochondrial dysfunction with subsequent neurodegeneration. The temporal bone pathologic changes in this syndrome have not been reported.

METHODS

Hearing loss developed in the patient at age 4, blindness at age 48, and dystonia at age 57. Genetic studies on peripheral blood showed a l51delT mutation in his DDP gene. He died at age 66. The right temporal bone was subjected to light microscopy and polymerase chain reaction-based analysis of the DDP gene sequence.

RESULTS

There was near complete loss of spiral ganglion cells with loss of nearly all peripheral and central processes. Only 1,765 spiral ganglion cells remained (8.5% of mean normal for age). The organ of Corti (including hair cells), stria vascularis, and spiral ligament were preserved. There was also a severe loss of Scarpa's ganglion cells with preservation of vestibular hair cells. The population of geniculate and trigeminal ganglion cells appeared normal. Sequence analysis from temporal bone DNA showed the 15ldelT DDP gene mutation.

CONCLUSION

Sensorineural hearing loss in Mohr-Tranebjaerg syndrome is the result of a postnatal, progressive, severe auditory neuropathy.

The clinical spectrum of type IV collagen mutations

Clinical manifestations of type IV collagen mutations can vary from the severe, clinically and genetically heterogeneous renal disorder, Alport syndrome, to autosomal dominant familial benign hematuria. The predominant form of Alport syndrome is X-linked; more than 160 different mutations have yet been identified in the type IV collagen alpha 5 chain (COL4A5) gene, located at Xq22-24 head to head to the COL4A6 gene. The autosomal recessive form of Alport syndrome is caused by mutations in the COL4A3 and COL4A4 genes, located at 2q35-37. Recently, the first mutation in the COL4A4 gene was identified in familial benign hematuria. This paper presents an overview of type IV collagen mutations, including eight novel COL4A5 mutations from our own group in patients with Alport syndrome. The spectrum of mutations is broad and provides insight into the clinical heterogeneity of Alport syndrome with respect to age at renal failure and accompanying features such as deafness, leiomyomatosis, and anti-GBM nephritis.

Non-syndromic dominant sensorineural hearing loss: from a few phenotypes to many genotypes

Sensorineural hearing loss affects approximately 1 in 2 persons at about 80 years of age and 1 in 750 in childhood. The best known forms of hearing loss with an autosomal dominant pattern of inheritance are the syndromic-mediated ones. At present, the non-syndromic autosomal dominant inherited forms can only be distinguished by the shape of the tone-audiogram. Based on gene linkage studies twelve different genotypes for autosomal dominant hereditary non-syndromic forms of sensorineural hearing loss have been recognized in a period of almost 2 years. In view of the great diversity of types that have been recognized in such a short period, it can be expected that over the next 10 years, several dozens genetically-mediated forms of autosomal dominant inherited sensorineural hearing loss will be detected. Similar developments are taking place in the non-syndromic autosomal recessive hereditary forms of sensorineural hearing loss and deafness. The above indicates clearly that before too long, new genetic investigation techniques will enable us to distinguish between forms of sensorineural hearing loss that could not be distinguished in the past.

New Methods in the Molecular Genetic Study and Treatment of Hearing Loss

In genetics, both discovery and methodology are advancing at an accelerating rate. As a result, the role of clinicians, and in this case clinical audiologists, must be re-examined from time to time to assure the sort of cooperation that will maximize results for both the investigators and the patients. This article will briefly review the current state of molecular genetic methodology and relate the assumptions inherent in such methods to the character of the clinical data needed from the audiologist inthese cases. The basic assumption of most molecular biologic investigations of ear disease is that a binary (yes or no) diagnosis can be made by the audiologists and physicians as to the disease status of each member of an affected family. The binary assumption gives rise to a number of clinical diagnosis issues not easily understood by molecular biologists, and these issues are discussed in order to facilitate progress in genetic cases.

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.

X-linked ataxia, weakness, deafness, and loss of vision in early childhood with a fatal course

An X-linked recessive disease with, in almost all patients, a fatal course in early childhood, occurring in a five-generation family is described. The 12 affected boys had early-onset floppiness, ataxia, liability to infections especially of the upper respiratory tract, deafness, and later, a flaccid tetraplegia and areflexia. Eleven boys died before the age of 5 years. One boy is still alive at the age of 12 years, but in addition to the above-mentioned signs, he must be ventilated at night and is nearly blind due to optic atrophy. In the only patient whose central nervous system could be examined at the time of autopsy, an almost complete absence of myelin in the posterior columns of the spinal cord was found. This may be the main pathological substrate for the neurological findings. No biochemical or immunological defects were detected. The family also counted 16 healthy male siblings and 13 definite of 28 possible female carriers. Some carriers developed a hearing impairment in early adulthood. As far as is known now, this disease has not been described before.