A mutation in the gamma actin 1 (ACTG1) gene causes autosomal dominant hearing loss (DFNA20/26).

Linkage analysis in a multigenerational family with autosomal dominant hearing loss yielded a chromosomal localisation of the underlying genetic defect in the DFNA20/26 locus at 17q25-qter. The 6-cM critical region harboured the gamma-1-actin (ACTG1) gene, which was considered an attractive candidate gene because actins are important structural elements of the inner ear hair cells. In this study, a Thr278Ile mutation was identified in helix 9 of the modelled protein structure. The alteration of residue Thr278 is predicted to have a small but significant effect on the gamma 1 actin structure owing to its close proximity to a methionine residue at position 313 in helix 11. Met313 has no space in the structure to move away. Moreover, the Thr278 residue is highly conserved throughout eukaryotic evolution. Using a known actin structure the mutation could be predicted to impair actin polymerisation. These findings strongly suggest that the Thr278Ile mutation in ACTG1 represents the first disease causing germline mutation in a cytoplasmic actin isoform.

[From gene to disease; a progressive cochlear-vestibular dysfunction with onset in middle-age (DFNA9)]. / Van gen naar ziekte; een op middelbare leeftijd optredende progressieve cochleavestibulaire disfunctie (DFNA9).

DFNA9 is an autosomal dominant genetic inner-ear hearing impairment that starts to show itself in the 3rd and 4th decades of life. This hearing impairment may be of a different degree of severity in each ear. Progression of hearing loss is about 3 dB/year. In about one in three patients severe vestibular symptoms similar to those in Ménière's disease are present as a result of a progressive impairment of the vestibular system. Several mutations were found in the COCH-gene on chromosome 14. There are indications that some of the mutations disrupt the folding of the cochlin protein, an important component of the extracellular matrix in the inner ear. DNA-diagnostics confirming the diagnosis ofDFNA9 are possible.

Progressive cochleovestibular impairment caused by a point mutation in the COCH gene at DFNA9.

OBJECTIVES: Analysis of phenotype-genotype correlation. STUDY DESIGN: Family study. METHODS: Auditory and vestibulo-ocular functions were examined in a Dutch family with autosomal dominantly inherited sensorineural hearing impairment caused by a 208C > T mutation in the COCH gene, located in chromosome 14q12-q13 (DFNA9). Linear regression analysis of individual longitudinal hearing threshold data (n = 11) on age was performed. RESULTS: Fifteen of the 16 genetically affected persons could be evaluated. They all developed hearing and vestibular impairment symptoms--and in many cases also cardiovascular disease--in the fourth to fifth decade. At the low frequencies (0.25-2 kHz), hearing loss started at the age of about 40 years and showed an average annual progression of approximately 3 dB, finally resulting in profound hearing losses. In two exceptional cases, annual progression attained levels of up to 24 dB. At the high frequencies (4-8 kHz), the average threshold increased from about 50 dB at the age of 35 years to about 120 dB at the age of 75 years (which amounts to 1.8 dB annual threshold increase). All affected individuals tested showed normal ocular motor functions. The patients older than 46 years generally showed absence of the vestibulo-ocular reflex, but their cervico-ocular reflex was enhanced compared with normal subjects, whereas those aged 40 to 46 years showed either severe vestibular hyporeflexia or unilateral caloric areflexia. CONCLUSION: These findings suggest a gradual development of cochleovestibular impairment caused by the new mutation found.

Speech recognition scores related to age and degree of hearing impairment in DFNA2/KCNQ4 and DFNA9/COCH.

OBJECTIVE: To analyze the relationship between pure-tone hearing threshold and speech recognition performance in DFNA2/KCNQ4 and DFNA9/COCH, 2 types of high-frequency nonsyndromic hearing impairment. DESIGN: Case series with cross-sectional analysis of phoneme recognition scores related to age and hearing level. SETTING: University hospital. PATIENTS: Forty-five members of 4 separate families, all carrying 1 of 3 different mutations in the KCNQ4 gene at the DFNA2 locus (1p34); 42 members of 7 separate families, all carrying the same Pro51Ser mutation in the COCH gene at the DFNA9 locus (14q12-q13). RESULTS: The deterioration of speech recognition dropped to a 90% score at a higher level of hearing impairment (pure-tone-average at 1, 2, and 4 kHz) in DFNA2-affected patients (65 dB) than in DFNA9-affected patients (46 dB). CONCLUSION: At similar levels of hearing impairment, DFNA2/KCNQ4-affected patients showed better speech recognition performance than DFNA9/COCH-affected patients.

Progressive fluctuant hearing loss, enlarged vestibular aqueduct, and cochlear hypoplasia in branchio-oto-renal syndrome.

OBJECTIVE: To study the results of petrosal bone imaging and audiometric long-term follow-up of two patients with branchio-oto-renal (BOR) syndrome and relate them to the clinical features, including caloric responses. STUDY DESIGN: Longitudinal case study. SETTING: Tertiary referral center. PATIENTS: A father and son with the BOR syndrome. MAIN OUTCOME MEASURES: Both patients underwent imaging studies to detect and evaluate inner ear anomalies. Longitudinal audiometric analysis of the hearing threshold data over the previous 23 years was performed. Caloric tests were performed at various ages. RESULTS: The son had a short, wide internal acoustic canal, a hypoplastic cochlea, a plump vestibule, and a wide vestibular aqueduct on both sides; the semicircular canals and endolymphatic sac were of normal size. He showed progressive fluctuant sensorineural hearing loss. Caloric tests disclosed hyporeflexia on the left side. The father had a plump internal acoustic canal and hypoplastic cochlea on both sides. The left vestibule was hypoplastic, and the left vestibular aqueduct was marginally enlarged. He showed severe hearing impairment, without substantial progression or fluctuation, and caloric areflexia on the left side. CONCLUSION: These findings suggest a correlation between progressive fluctuant sensorineural hearing loss with caloric hypofunction and the presence of an enlarged vestibular aqueduct in the BOR syndrome. Additional longitudinal case studies are needed to further evaluate such a correlation.

A Pro51Ser mutation in the COCH gene is associated with late onset autosomal dominant progressive sensorineural hearing loss with vestibular defects.

We analysed a Dutch family with autosomal dominant non-syndromic progressive sensorineural hearing loss and mapped the underlying gene defect by genetic linkage analysis to a 11.0 cM region overlapping the DFNA9 interval on chromosome 14q12-q13. Clinically, the Dutch family differs from the original DFNA9 family by a later age at onset and a more clearly established vestibular impairment. A gene that is highly and specifically expressed in the human fetal cochlea and vestibule, COCH (previously described as Coch5B2 ), was mapped to the DFNA9 critical region. Sequence analysis revealed a 208C-->T mutation in the COCH gene, resulting in a Pro51Ser substitution in the predicted protein in all affected individuals of the family but not in unaffected family members and 200 control individuals. The same mutation was also identified in three apparently unrelated families with a similar phenotype, suggesting the presence of a Dutch founder mutation. The function of COCH is unknown but several characteristics of the protein point to a structural role in the extracellular matrix. The mutant serine at position 51 is situated between cysteines and possibly interferes with proper COCH protein folding or its interaction with extracellular matrix proteins.