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Mutations in the OTOF gene encoding otoferlin result in a disrupted function of the ribbon synapses with impairment of the multivesicular glutamate release. Most affected subjects present with congenital hearing loss and abnormal auditory brainstem potentials associated with preserved cochlear hair cell activities (otoacoustic emissions, cochlear microphonics [CMs]). Transtympanic electrocochleography (ECochG) has recently been proposed for defining the details of potentials arising in both the cochlea and auditory nerve in this disorder, and with a view to shedding light on the pathophysiological mechanisms underlying auditory dysfunction. We review the audiological and electrophysiological findings in children with congenital profound deafness carrying two mutant alleles of the OTOF gene. We show that cochlear microphonic (CM) amplitude and summating potential (SP) amplitude and latency are normal, consistently with a preserved outer and inner hair cell function. In the majority of OTOF children, the SP component is followed by a markedly prolonged low-amplitude negative potential replacing the compound action potential (CAP) recorded in normally-hearing children. This potential is identified at intensities as low as 90 dB below the behavioral threshold. In some ears, a synchronized CAP is superimposed on the prolonged responses at high intensity. Stimulation at high rates reduces the amplitude and duration of the prolonged potentials, consistently with their neural generation. In some children, however, the ECochG response only consists of the SP, with no prolonged potential. Cochlear implants restore hearing sensitivity, speech perception and neural CAP by electrically stimulating the auditory nerve fibers. These findings indicate that an impaired multivesicular glutamate release in OTOF-related disorders leads to abnormal auditory nerve fiber activation and a consequent impairment of spike generation. The magnitude of these effects seems to vary, ranging from no auditory nerve fiber activation to an abnormal generation of EPSPs that occasionally trigger a synchronized electrical activity, resulting in high-threshold CAPs.

Objective: Our objective was to compare acoustically-and electrically-evoked potentials of the auditory nerve in patients with postsynaptic or presynaptic auditory neuropathy with underlying mutations in the OPA1 or OTOF gene. Study design: Transtympanic electrocochleography (ECochG) was recorded from two adult patients carrying the R445H OPA1 mutation, and from five children with mutations in the OTOF gene. Cochlear potentials to clicks or tone-bursts were compared to recordings obtained from 16 normally hearing subjects. Electrically-evoked neural responses recorded through the cochlear implant were also obtained. Results: The cochlear microphonic (CM) was recorded from all subjects, with normal amplitudes. After cancelling the CM, cochlear potentials were of negative polarity with reduced amplitude and prolonged duration compared to controls in both groups of patients. Prolonged negative responses were recorded as low as 50-90dB below behavioural threshold in subjects with OTOF mutations whereas in the OPA1 disorder the prolonged potentials were correlated with hearing threshold. A compound action potential (CAP) was superimposed on the prolonged activity at high stimulation intensity in two children with mutations in the OTOF gene while CAPs were absent in the OPA1 disorder. Electrically-evoked compound action potentials (e-CAPs) were only recorded from subjects with OTOF mutations following cochlear implantation. Conclusions: The findings are consistent with abnormal function of distal portions of auditory nerve fibres in patients carrying the OPA1 mutation whereas the low-threshold prolonged potentials recorded from children with mutations in the OTOF gene are consistent with abnormal neurotransmitter release resulting in reduced dendritic activation and impairment of spike initiation. © 2011 Informa Healthcare.

Otoferlin is involved in neurotransmitter release at the synapse between inner hair cells (IHCs) and auditory nerve fibres, and mutations in the OTOF gene result in severe to profound hearing loss. Abnormal sound-evoked cochlear potentials were recorded with transtympanic electrocochleography from four children with otoferlin (OTOF) mutations to evaluate physiological effects in humans of abnormal neurotransmitter release from IHCs. The subjects were profoundly deaf with absent auditory brainstem responses and preserved otoacoustic emissions consistent with auditory neuropathy. Two children were compound heterozygotes for mutations c.2732_2735dupAGCT and p.Ala964Glu; one subject was homozygous for mutation p.Phe1795Cys, and one was compound heterozygote for two novel mutations c.1609delG in exon 16 and c.1966delC in exon 18. Cochlear potentials evoked by clicks from 60 to 120 dB peak equivalent sound pressure level were compared to recordings obtained from 16 normally hearing children. Cochlear microphonic (CM) was recorded with normal amplitudes from all but one ear. After cancelling CM, cochlear potentials were of negative polarity with reduced amplitude and prolonged duration compared to controls. These cochlear potentials were recorded as low as 50-90 dB below behavioural thresholds in contrast to the close correlation in controls between cochlear potentials and behavioural threshold. Summating potential was identified in five out of eight ears with normal latency whilst auditory nerve compound action potentials were either absent or of low amplitude. Stimulation at high rates reduced amplitude and duration of the prolonged potentials, consistent with neural generation. This study suggests that mechano-electrical transduction and cochlear amplification are normal in patients with OTOF mutations. The low-amplitude prolonged negative potentials are consistent with decreased neurotransmitter release resulting in abnormal dendritic activation and impairment of auditory nerve firing.