Abstract
Purpose
The aim of this study was to characterize the genetic, audiologic, and epidemiologic characteristics of unilateral hearing loss (HL) in a national hereditary deafness repository.
Materials and Methods
This is a prospective clinical study involving 34 subjects identified in a national hereditary deafness repository. Clinical data and family history of HL were obtained on enrollment. Candidate deafness genes were screened by single-stranded conformation polymorphism, and mutations were confirmed with sequencing.
Results
Thirty-four subjects (19 males, 15 females) with unilateral HL were identified, ranging in age from 2 months to 36 years. The mean age at diagnosis was 7 years, and the left ear was affected in 62% of the cases. The racial distribution of our sample was 62% white, 23% African American, and 15% Hispanic. Imaging results were available in 47%, and most (69%) were considered normal. Nineteen percent had enlarged vestibular aqueducts, 2 had ipsilateral Mondini dysplasia, and 1 had a common cavity deformity. Twenty subjects (59%) had a family history of HL, with 26% specifically reporting familial unilateral HL. Mutational screening revealed sequence variants in the GJB2 (connexin 26), GJB3 (connexin 31), TECTA , and COCH genes. Two novel mutations were detected in COCH and TECTA .
Conclusions
Sequence variants in known deafness genes were detected in more than one-third of our study population, suggesting that gene/gene or gene/environmental interactions may indeed play a role in the etiology of some cases of unilateral deafness. Further prospective studies including congenital cytomegalovirus screening at birth and molecular screening of deafness genes in children with congenital unilateral HL will be required to establish the etiology of unilateral deafness with certainty.
1
Introduction
Hearing loss (HL) affects 3 per 1000 infants at birth, and the prevalence roughly doubles by age 4 years . Genetic factors account for at least 50% to 60% of cases of bilateral severe to profound congenital HL . Environmental or infectious etiologies, most commonly congenital cytomegalovirus (CMV) infections, are implicated in the remaining 40% to 50% of children with “non-genetic” bilateral HL . However, the factors contributing to unilateral HL are much less well defined. In data from newborn hearing screening programs, infants with unilateral HL are reported to range from less than half a percent to as many as 3.4% of all screened infants, although poor follow-up may contribute to an underestimation of the actual prevalence in newborns . The exact prevalence of unilateral HL in older children is unknown but has been estimated to range from 0.5% to 5% in school-aged children and varies based on the mode of ascertainment of the HL . It is likely that when cases of delayed onset and progressive forms of unilateral HL are added to the known figures from universal newborn hearing screening programs, the true prevalence may approach or exceed 5%. Data from the Virginia Early Hearing Detection and Intervention program reveals that more than one-third of children identified with HL at birth by universal newborn hearing screening are unilateral cases . Similarly, 31% of these children also have coexisting birth defects, similar to the rate of comorbid birth defects in children with congenital bilateral HL .
Significant progress has been made in identifying the genetic causes of profound congenital deafness, as well as progressive late-onset deafness, but less is known about the potential causes of milder forms of HL, especially unilateral losses, despite its higher prevalence in the population. Until the molecular basis of these forms of deafness is fully understood, etiologically specific forms of treatment will not be possible. Hereditary transmission of unilateral HL has been reported and is known to be associated with genetic syndromes such as Waardenburg syndrome, Pendred syndrome, and delayed endolymphatic hydrops . Delayed-onset unilateral losses are also common in patients with recognized or unrecognized congenital CMV infections, and early data suggest that children who are heterozygous carriers of GJB2/6 mutations may be at higher risk for developing sensorineural HL in association with congenital CMV infection . The molecular basis for other, perhaps, more common causes of unilateral HL still remains an important gap in existing knowledge. For the current study, we selected 10 candidate deafness genes ( Table 1 ) including GJB2/6 for mutational analysis due to their frequency in bilateral hearing impairment, their potential for gene/environmental interactions in the heterozygous state, or their ability to cause asymmetric or progressive forms of HL.
| Gene | Locus | Rationale |
|---|---|---|
| GJB2/6 (connexins 26 and 30) | DFNB1 | Most common cause of nonsyndromic recessive HL, potential for gene/environmental interactions with CMV |
| SLC26A4 | DFNB4 | Association with EVA, progressive, asymmetric, and varying HL even in heterozygous state |
| GJB3 (connexin 31) | DFNA2 | Connexin protein, potential digenic or gene/environmental interactions, varying audiologic phenotypes |
| KCNQ4 | DFNA2 | Potential gene/environmental interactions, mutations known to produce varying audiologic phenotypes |
| DFNA5 | DFNA5 | Potential gene/environmental interactions, mutations known to produce varying audiologic phenotypes |
| COCH | DFNA9 | Potential gene/environmental interactions, mutations known to produce varying audiologic phenotypes |
| TECTA | DFNA12 | Potential gene/environmental interactions, mutations known to produce varying audiologic phenotypes |
| MYH9 | DFNA 17 | Potential gene/environmental interactions, mutations known to produce varying audiologic phenotypes |
| ACTG1 | DFNA20/26 | Potential gene/environmental interactions, mutations known to produce varying audiologic phenotypes |
| A1555G MTRNR1 | mt12srRNA | Potential gene/environmental interactions, mutations known to produce varying audiologic phenotypes |
2
Materials and methods
Subjects meeting the audiologic criteria of unilateral HL were ascertained through the Hereditary Deafness Repository at Virginia Commonwealth University (VCU) Medical Center after institutional review board approval was obtained from VCU. The Hereditary Deafness Repository at VCU currently consists of more than 4500 DNA samples from 2434 probands and other family members. These patients were ascertained through surveys of Gallaudet University alumni, the Annual Survey of Deaf and Hard of Hearing Children and Youth, the Gallaudet University student body, the Virginia Newborn Hearing Screening program, and clinics at VCU. Blood samples were collected after institutional review board–approved informed consent was obtained. DNA was purified according to standard methods and stored in −20°C freezers. Subjects were assigned unique identifiers in the Repository, thus ensuring strict confidentiality. For the current study, we identified 34 patients in our database with audiometric characteristics of unilateral deafness. Clinical data including the age of onset of HL, results of newborn screening, and medical, perinatal, and family history were systematically collected on each subject. Data on the results of radiologic studies of the inner ear were collected when available.
The Repository DNA samples on the 34 subjects were then screened for mutations involving known genes for deafness ( Table 1 ) using direct sequencing for GJB2 and GJB6 and the single-stranded conformation polymorphism technique for the remaining genes. We selected genes based on their potential for gene/environmental interactions in the heterozygous state or the ability to produce varying audiologic phenotypes, particularly asymmetric forms of HL, fluctuating, or progressing losses. Any changes found on single-stranded conformation polymorphism were confirmed using direct DNA sequencing with an ABI (Carlsbad, CA, USA) sequencer.
2
Materials and methods
Subjects meeting the audiologic criteria of unilateral HL were ascertained through the Hereditary Deafness Repository at Virginia Commonwealth University (VCU) Medical Center after institutional review board approval was obtained from VCU. The Hereditary Deafness Repository at VCU currently consists of more than 4500 DNA samples from 2434 probands and other family members. These patients were ascertained through surveys of Gallaudet University alumni, the Annual Survey of Deaf and Hard of Hearing Children and Youth, the Gallaudet University student body, the Virginia Newborn Hearing Screening program, and clinics at VCU. Blood samples were collected after institutional review board–approved informed consent was obtained. DNA was purified according to standard methods and stored in −20°C freezers. Subjects were assigned unique identifiers in the Repository, thus ensuring strict confidentiality. For the current study, we identified 34 patients in our database with audiometric characteristics of unilateral deafness. Clinical data including the age of onset of HL, results of newborn screening, and medical, perinatal, and family history were systematically collected on each subject. Data on the results of radiologic studies of the inner ear were collected when available.
The Repository DNA samples on the 34 subjects were then screened for mutations involving known genes for deafness ( Table 1 ) using direct sequencing for GJB2 and GJB6 and the single-stranded conformation polymorphism technique for the remaining genes. We selected genes based on their potential for gene/environmental interactions in the heterozygous state or the ability to produce varying audiologic phenotypes, particularly asymmetric forms of HL, fluctuating, or progressing losses. Any changes found on single-stranded conformation polymorphism were confirmed using direct DNA sequencing with an ABI (Carlsbad, CA, USA) sequencer.
3
Results
3.1
Clinical results
Our 34 subjects included 19 males and 15 females, ranging in age at enrollment from 2 months to 36 years. The mean age at diagnosis of HL was 7 years, with the left ear affected in 62%. Sixty-two percent of the study population was white, 23% were African American, and 15% reported a Hispanic or Latino heritage. Imaging reports were available in 16 subjects (47%), with most (11/16, or 69%) of these read as normal. Nineteen percent of those who had imaging results available (n = 3) had enlarged vestibular aqueduct (2 bilateral, 1 unilateral); there was also 1 case each of ipsilateral Mondini dysplasia and an ipsilateral common cavity deformity. All the unilateral findings coincided with the side of the HL. Twenty (59%) of the 34 subjects had a family history of unilateral, bilateral or unspecified HL. Nine subjects (26% of the total population, 45% of the population with a family history of HL) specifically related a history of familial unilateral HL. One of these subjects was diagnosed clinically with Waardenburg syndrome type I and had a family history of the syndrome.
3.2
Results of molecular screening
3.2.1
Summary
Mutational screening of deafness genes ( Table 1 ) was performed as previously described . Sequence variants ( Table 2 ) in 1 or more of the candidate deafness genes were identified in 12 (35%) of the 34 subjects, with 4 (33%) of the 12 subjects possessing changes in 2 deafness genes. However, most of these changes alone would not be considered pathogenic, for example, those carrying single recessive GJB2 (connexin 26) or GJB3 (connexin 30) mutations. Of the 10 genes tested, sequence variants were seen in GJB2 (connexin 26), GJB3 (connexin 31), COCH , and TECTA ( Table 2 ). To summarize the findings, 3 subjects were heterozygous for mutations of GJB2 (connexin 26), 3 subjects were heterozygous for mutations of GJB3 , 1 subject was heterozygous for a novel mutation in COCH , and 1 subject was homozygous for a novel TECTA mutation ( Table 2 ).
