Mutations were revealed during two independent screens of NSRHL families. In the Nebraska group, STRP genotyping of the OTOF locus was performed on 47 families (38 NSRHL families and nine families with non‐syndromic recessive auditory neuropathy (NSRAN) families), 17 (14 NSRHL and three NSRAN) of whom were consistent with linkage to the OTOF locus by haplotyping and were therefore included in the genetic screening. Sixteen families were uninformative for linkage (12 NSRHL and four NSRAN). Fourteen families (12 NSRHL and two NSRAN) were informative and not consistent with linkage. The 17 families consistent for linkage and the 16 uninformative for linkage were included in the mutation screen. Seven mutations believed to be pathogenic were found in five families in this group (table 1, fig 1).
Table 1OTOF mutations including mutations from genetic screening of 65 NSRAN and NSRHL families (in bold)
Figure 1Pedigrees of families with OTOF mutations. Proband is identified by arrow.
In the Iowa group, 18 NSRHL families were screened with markers linked to OTOF. None of the Iowa families had been previously diagnosed with auditory neuropathy. Of these 18 families, all were informative for linkage but only three were consistent for linkage to OTOF. These families were screened by SSCP. A single missense mutation deemed pathogenic was identified in family 53510 in this group (table 1, fig 1). An audiological summary is given for each family (table 2).
Table 2Summary of audiological data
We found a high degree of neutral variation within the subject population during the mutation screen; 23 coding polymorphic changes were noted (*denotes alleles found in the Spanish population).20
Eleven of these were missense alleles (158C→T, A53V; 244C→T*, R82C*; 1723G→A, V575M; 2317C→T*, R773S*; 2464C→T, R822W; 3247G→C, A1083P; 3470G→A, R1157Q; 3966C→G, D1322E; 4874G→A, V1625M; 4936C→T, P1646S; and 5663G→A, G1888D), and 12 were silent alleles (372A→G*, T124T*; 945G→A*, K315K*; 1926C→T, N642N; 2022CvT, D674D; 2025G→A, E675E; 2580C→G*, V860V*; 2736G→C*, L919L*; 3189G→A, A1063A; 4677G→A, V1559V; 4767C→T, R1589R; 5391C→T*, F1797F*; and 5655C→T*, R1885R*).
Probably the most unusual family of the group is 3467, a family with temperature sensitive AN/AD, with two affected siblings. The variant 1544T→C, I515T was found heterozygously in the father and the two affected children of this temperature sensitive NSRAN family. The maternal mutation is still unknown.
The audiogram for individual 3467‐1, when she was afebrile, showed a mild low frequency hearing loss and speech comprehension was below the 10th percentile for both quiet and noise. Tympanometry was normal and AR were absent. ABR was abnormal, but CM were present. On two occasions, testing was performed during febrile illness. Her core temperature was defined approximately 2 hours before testing. At a temperature of 38.1°C, her pure tone thresholds decreased to profound deafness in the low frequencies, rising to severe hearing loss in the high frequencies. SAT was 80 dB HL, but she was unable to repeat any of the test spondee words. Tympanometry and OAE were normal, but AR and ABR were absent. With a temperature of 37.8°C, she was tested again and showed a mild to moderate hearing loss and zero speech comprehension. ABR and OAE were not tested. The following day her auditory functions returned to normal after her fever abated. She has reported to her parents that her hearing becomes affected “suddenly” when she is febrile.
Individual 3467‐2 was examined twice when afebrile. He has a mild low frequency hearing loss, normal tympanograms, absent AR, and abnormal ABR. CM and OAE were present bilaterally. We were unable to test 3467‐2 when febrile, but his parents report that under those conditions he experienced hearing loss similar to his sister. OAE suppression was tested to determine medial olivocochlear (MOC) neurone integrity. Activation of these neurones by presenting sound to the contralateral cochlea will induce a suppression of ipsilateral OAEs.30
Patients with AN/AD cannot suppress their OAE during this test.31
As expected, the OAE of 3467‐2 were not suppressed, which indicates that the sound signal reached the OHC, but not the efferent MOC neurones that feed back to suppress the OHC. We have tested the parent carrying the I515T mutation and no abnormality of pure tone threshold, speech comprehension in quiet and noise, ABR, AR, or OAE was identified. The clinical details of this family have been published previously.32
pathological alleles have been found in family 3456. The maternally inherited mutation is a novel splice site mutation, IVS2 –2A→C. The mutation inherited through their father is a previously published nonsense mutation 2485C→T, Q829X, found in a group of Spanish families and one Cuban family.3,20
Family 3456 is a white family from England with no known Hispanic ancestry, with two sons both having AN/AD. Pregnancy and birth history were unremarkable for the two affected boys except for slight jaundice in the older boy (individual 3467‐1). Motor milestones were met at an appropriate age. As is consistent with AN/AD, ABR was absent and OAE were present. Both boys have profound hearing loss and corner audiograms. OAE suppression was tested in individual 3467‐1 and, as expected, his OAE were not suppressed, but were instead increased in amplitude. Vestibular function testing in the older boy indicated that there may be a slight hypofunction on the left side. The younger was not tested for vestibular function. Computed tomography scans were normal in both boys, and magnetic resonance imaging in the older boy. Both boys have had positive experiences with their cochlear implants and consider them to be beneficial.
Both mutations in family 3466 result in a frameshift. The maternally inherited mutation in exon 17 contains an insertion 1886_1887insA (K629fs). The paternally inherited mutation is a deletion in exon 21, 2348delG (G783fs). This family has two affected children, a boy (3466‐1) and a girl (3466‐2) with normal OAE and absent ABR. Birth histories were uneventful and developmental milestones, except speech, were attained at a normal age. The children underwent neurological examinations at the age of 5 years (3466‐1) and 7 years (3466‐2), which were normal. However, 3466‐1 may also have a vestibular neuropathy or hypoactive vestibular function, as no nystagmus developed after spinning in a chair and his father reported that the child can spin without falling down.
Family 3539 has two affected children with normal tympanometry, OAE and CM, but absent AR and ABR. The daughter was born prematurely at 32 weeks gestation; otherwise, the birth histories were unremarkable. A splice site mutation, IVS18 +1G→T, was found in the maternal allele.
The Hispanic nonsense mutation 2485C→T, Q829X has been found heterozygously in family 3450. This family has Mexican ancestry. Both affected children are “typical” of NSRAN, in that OAE and CM are present, but ABR and AR are absent.
In the Iowa group of families, an OTOF mutation was found in one family. There were three affected children in family 53510, all heterozygous for 2381G→A, R794H. Two of these individuals had a profound hearing loss while one had a severe to profound hearing loss. All three individuals had a corner audiogram.