Genetic causes of nonsyndromic hearing loss

Chronic prenatal hypoxia impairs cochlear development, a mechanism involving connexin26 expression and promoter methylation

Chronic prenatal hypoxia is a damaging to fetal development and may have various consequences, including hearing loss. Connexin 26 (Cx26) is one of the major protein subunits required for gap junction formation, and has an important role in maintaining homeostasis in the cochlea and normal hearing. Cx26 mutation and expression abnormality are closely associated with inherited nonsyndromic deafness, but the association between Cx26 and prenatal hypoxia is less established. The present study aimed to examine Cx26 expression and aberrant methylation the Cx26 promoter region in the cochlea from rats exposed to chronic prenatal hypoxia. Hematoxylin and eosin staining demonstrated that the number of hair cells in the organ of Corti were less in the hypoxia group. Reverse transcription-quantitative polymerase chain reaction and western blot analysis revealed that protein and mRNA levels of Cx26 were decreased in the hypoxia group compared with the control group. Further bisulfite sequencing analysis revealed that prenatal hypoxia significantly increased the methylation status of the promoter region of the Cx26 gene. These results demonstrate that chronic prenatal hypoxia caused hearing impairment, and suggest that promoter region hypermethylation and expression downregulation of Cx26 underlie the mechanism of action.

Correlation between audiometric data and the 35delG mutation in ten patients

Mutations in the connexin 26 gene seem to be extremely common in non-syndromic hereditary deafness genesis, especially the 35delG, but there are still only a few studies that describe the audiometric characteristics of patients with these mutations.

Aim

to analyze the audiometric characteristics of patients with mutations in the connexin 26 gene in order to outline genotype-phenotype correlation.

Materials and Methods

Tonal audiometries of 33 index cases of non-syndromic sensorineural hearing loss were evaluated and eight affected relatives. Specific molecular tests were carried out to analyze mutations in the connexin 26 gene. Experiment Design: Retrospective, cross-sectional study.

Results

A 27.3% prevalence of mutation 35delG was found in the index cases and 12.5% among the relatives affected. In relation to hearing loss degree, 41.5% of the patients were found with profound hearing loss, 39% with severe HL and 19.5% with moderate HL with homozygote and heterozygote patients for the 35delG predominating in the severe-moderate hearing losses.

Conclusion

Our results suggest that the audiometric data associated with the molecular diagnose of hearing loss helped us to outline a genotype-phenotype correlation in ten patients with 35delG mutation. However, it is still necessary to run multicentric studies to verify the real phenotypic expression in the Brazilian population, as far as the 35delG mutation is concerned.

Analysis of differential gene expression in the cochlea and kidney of mouse by cDNA microarrays

Microarray hybridization analysis of gene expression in the cochlea and kidney suggest a relationship between these tissues at the genomic level, indicating the common gene expression, likely serving a common function in both the organs primarily maintaining ion transport, and implied previously from morphological, pharmaco-kinetic and teratogenic studies. The cDNAs of more than 100 genes listed on the hereditary hearing loss homepage were amplified as targets by RT-PCR and were hybridized with probes prepared from total RNA of the cochlea and the kidney. Thirteen of the genes analyzed showed altered fluorescence ratios of more than two logs. Of these, the expressions of 11 genes were over expressed and two were under expressed in the cochlea than in the kidney. Our data is the first report to corroborate the genomic similarities between these two important organs and may help to explain the somewhat similar response of these organs to certain therapeutic drugs.

The potential use of stem cells for cochlear repair

In light of the currently defined characteristics of stem cells, a re-evaluation of hair cell regeneration in birds suggests that there may be a stem cell population located in the inner ear. It is yet to be determined if the mammalian cochlea contains stem cells, but the presence of a mammalian vestibular stem cell population would not appear to be out of the realm of possibility. This paper reviews the latest advances in stem cell biology and suggests that stem cells may be an appropriate biological tool to be used for cochlear repair. The potential use of several types of stem cells, including embryonic, neural and hematopoietic stem cells, as agents for cochlear repair is examined.

A PCR-RFLP test for simultaneous detection of two single-nucleotide insertions in the Connexin-26 gene promoter

Comparisons of Connexin-26 (GJB2) gene sequences available in the GenBank data base indicate the presence of a polymorphism in the promoter, but no easy method is available for the detection of this polymorphism. We have developed a PCR-RFLP test for simultaneous detection of two single nucleotide insertions (G and A) in the GJB2 promoter. The test is based on amplification of a 146-bp DNA fragment, which was digested with Mae I to detect the G insertion in the promoter. A similar digestion with Hinf I detects the A insertion. The test was validated using direct DNA sequencing of amplified DNA from 33 samples. After validation, we have used it to investigate DNA samples from 160 control subjects and 51 unrelated patients with nonsyndromic autosomal recessive deafness. All of the samples analyzed using the PCR test and DNA sequencing were found to contain both the G and A insertions in the GJB2 gene promoter. This PCR test will be useful in studying the prevalence of these two insertions in other populations.

Auditory and vestibular defects in the circling (ci2) rat mutant

The circling rat is an autosomal recessive mutant (homozygous ci2/ci2) that displays lateralized circling behaviour, locomotor hyperactivity, ataxia and stereotypic head-movement. These abnormal behaviours occur in phases or bursts either spontaneously or in response to stress. Heterozygous (ci2/+) littermates display normal spontaneous behaviours. We have previously found that ci2/ci2 rats of both genders have a lower tissue content of dopamine in the striatum ipsilateral to the preferred direction of rotation, indicating that the rats turn away from the brain hemisphere with higher striatal dopaminergic activity. In view of the similarities of the motor syndrome of the ci2/ci2 mutant rat to that of mouse deafness mutants, the present study evaluated the hearing ability of the circling rat mutant by recording brainstem auditory-evoked potentials. To test for vestibular dysfunction, a swimming test was conducted. Histological methods were used to examine the cochlear and vestibular parts of the inner ear and the cochlear and vestibular brainstem nuclei for defects. The absence of auditory-evoked potentials demonstrated a complete hearing loss in the adult ci2/ci2 mutant rat, whereas heterozygous littermates exhibited auditory-evoked potentials with thresholds resembling those of other laboratory strains. Furthermore, the mutant rats were unable to swim. Histological analysis of the inner ear of adult mutants revealed virtually complete loss of the cochlear neuroepithelium, while no such hair cell degeneration was seen in the vestibular parts of the inner ear. However, part of the vestibular hair cells showed protrusions into the endolymphatic space, suggesting alterations in the cytoskeletal architecture. The histological findings in mutant circling rats strongly indicate that the hearing loss of the mutants is of the sensory neural type, the most prevalent type of hearing loss. In the cochlear nuclei of the brain stem of mutant rats, neurons exhibited an abnormal shape, reduced size and increased density compared to controls. In contrast, no abnormal neuronal morphology was seen in the vestibular nuclei, but a significantly reduced neuronal density was found in the medial vestibular nucleus. Abnormal vestibular function would be a likely explanation for the disturbed balance of mutant rats as exemplified by the ataxia and the inability to swim, whereas the previous data on these rats strongly indicate an involvement of the basal ganglia in the abnormal circling behaviour. The genetic defect in the mutant rats, thus, results in a clinical syndrome with features also seen in human genetic disorders with deafness and hyperkinesia, making the ci2/ci2 rat an excellent model for investigating both cochlear/vestibular dysfunction and hyperkinetic movement disorders.

Hypokalemic salt-losing tubulopathy with chronic renal failure and sensorineural deafness

OBJECTIVE

To characterize a rare inherited hypokalemic salt-losing tubulopathy with linkage to chromosome 1p31.

METHODS

We conducted a retrospective analysis of the clinical data for 7 patients in whom cosegregation of the disease with chromosome 1p31 had been demonstrated. In addition, in 1 kindred, prenatal diagnosis in the second child was established, allowing a prospective clinical evaluation.

RESULTS

Clinical presentation of the patients was homogeneous and included premature birth attributable to polyhydramnios, severe renal salt loss, normotensive hyperreninemia, hypokalemic alkalosis, and excessive hyperprostaglandin E-uria, which suggested the diagnosis of hyperprostaglandin E syndrome/antenatal Bartter syndrome. However, the response to indomethacin was only poor, accounting for a more severe variant of the disease. The patients invariably developed chronic renal failure. The majority had extreme growth retardation, and motor development was markedly delayed. In addition, all patients turned out to be deaf.

CONCLUSION

The hypokalemic salt-losing tubulopathy with chronic renal failure and sensorineural deafness represents not only genetically but also clinically a disease entity distinct from hyperprostaglandin E syndrome/antenatal Bartter syndrome. A pleiotropic effect of a single gene defect is most likely causative for syndromic hearing loss.

Prenatal diagnosis for inherited deafness--what is the potential demand?

Genetic testing for inherited deafness is now available within some genetics centres. This study used a structured questionnaire to assess the potential uptake of prenatal diagnosis (PND) for inherited deafness, and document the opinions of deaf and hearing individuals toward PND and termination of pregnancy (TOP) for hearing status. Participants were self-selected from the whole of the UK, of whom 644 were deaf, 143 were hard of hearing or deafened, and 527 were hearing individuals who had either a deaf parent or child. The results showed that 21% of deaf, 39% of hard of hearing and deafened, and 49% of hearing participants said they would consider PND for deafness. Six percent of deaf, 11% of hard of hearing and deafened, and 16% of hearing participants said they would consider a TOP if the fetus was found to be deaf. Two percent of deaf participants said they would prefer to have deaf children and would consider a TOP if the fetus was found to be hearing.

A review of evidence in support of a role for 5-HT in the perception of tinnitus

Tinnitus is a debilitating condition from which some 0.5-1% of the population of the Western world suffer sufficiently badly as to interfere with their normal working and leisure life. There is no satisfactory treatment at the present time and the uncertainty surrounding the mechanism of its generation makes it difficult to devise an effective cure. After much debate, the consensus of opinion amongst researchers regarding its site of origin is that it is primarily a central nervous system pathology although there certainly exists a class of patients whose tinnitus is peripherally based. In this paper, we provide some speculative ideas on how an initial auditory insult can be translated into central neurological substrates that represent tinnitus. Plastic changes arising from sensory deprivation trigger a change in synaptology and neurotransmission with a consequent change in receptor configuration. From neuroanatomical considerations and analogies with other clinical conditions, we postulate the involvement of serotonin (5-HT) in these plastic changes and consider the evidence available from the use of serotonergic drugs in different conditions. A possible relationship between 5-HT and lidocaine is also discussed.