Screening for monogenetic del(GJB6-D13S1830) and digenic del(GJB6-D13S1830)/GJB2 patterns of inheritance in deaf individuals from Eastern Austria

Analyses of del(GJB6-D13S1830) and del(GJB6-D13S1834) deletions in a large cohort with hearing loss: Caveats to interpretation of molecular test results in multiplex families

BACKGROUND

Mutations involving the closely linked GJB2 and GJB6 at the DFNB1 locus are a common genetic cause of profound congenital hearing loss in many populations. In some deaf GJB2 heterozygotes, a 309 kb deletion involving the GJB6 has been found to be the cause for hearing loss when inherited in trans to a GJB2 mutation.

METHODS

We screened 2,376 probands from a National DNA Repository of deaf individuals.

RESULTS

Fifty-two of 318 heterozygous probands with pathogenic GJB2 sequence variants had a GJB6 deletion. Additionally, eight probands had an isolated heterozygous GJB6 deletion that did not explain their hearing loss. In two deaf subjects, including one proband, a homozygous GJB6 deletion was the cause for their hearing loss, a rare occurrence not reported to date.

CONCLUSION

This study represents the largest US cohort of deaf individuals harboring GJB2 and GJB6 variants, including unique subsets of families with deaf parents. Testing additional members to clarify the phase of GJB2/GJB6 variants in multiplex families was crucial in interpreting clinical significance of the variants in the proband. It highlights the importance of determining the phase of GJB2/GJB6 variants when interpreting molecular test results especially in multiplex families with assortative mating.

Single Cell and Single Nucleus RNA-Seq Reveal Cellular Heterogeneity and Homeostatic Regulatory Networks in Adult Mouse Stria Vascularis

The stria vascularis (SV) generates the endocochlear potential (EP) in the inner ear and is necessary for proper hair cell mechanotransduction and hearing. While channels belonging to SV cell types are known to play crucial roles in EP generation, relatively little is known about gene regulatory networks that underlie the ability of the SV to generate and maintain the EP. Using single cell and single nucleus RNA-sequencing, we identify and validate known and rare cell populations in the SV. Furthermore, we establish a basis for understanding molecular mechanisms underlying SV function by identifying potential gene regulatory networks as well as druggable gene targets. Finally, we associate known deafness genes with adult SV cell types. This work establishes a basis for dissecting the genetic mechanisms underlying the role of the SV in hearing and will serve as a basis for designing therapeutic approaches to hearing loss related to SV dysfunction.

A Novel Mutation in SLC26A4 Causes Nonsyndromic Autosomal Recessive Hearing Impairment

BACKGROUND

Heterozygous mutations in GJB2 (MIM: 121011) encoding the gap junction protein connexin 26 are overrepresented in patient groups suffering from nonsyndromic sensorineural hearing impairment (HI) implying the involvement of additional genetic factors. Mutations in SLC26A4 (MIM: 605646), encoding the protein pendrin can cause both Pendred syndrome and autosomal recessive, nonsyndromic HI locus 4 type sensorineural HI (MIM: 600791).

OBJECTIVES

Aim of this study was to investigate the role of SLC26A4 coding mutations in a nonsyndromic hearing impairment (NSHI) patient group bearing heterozygous GJB2 35delG mutations.

DESIGN

We analyzed the 20 coding exons of SLC26A4 in a group of patients (n = 15) bearing heterozygous 35delG mutations and exclusively suffering from congenital HI.

RESULTS

In a case of bilateral congenital hearing loss we identified a rare, novel SLC26A4 exon 2 splice donor mutation (c.164+1delG) predicted to truncate pendrin in the first cytoplasmic domain, as a compound heterozygote with the pathogenic missense mutation c.1061T>C (p.354F>S; rs111033243).

CONCLUSIONS

Screening for SLC26A4 mutations may identify the genetic causes of hearing loss in patients bearing heterozygous mutations in GJB2.

HYPOTHESIS

SLC26A4 coding mutations are genetic causes for nonsyndromic HI in patients bearing heterozygous GJB2 35delG mutations.

Whole-exome sequencing to identify the cause of congenital sensorineural hearing loss in carriers of a heterozygous GJB2 mutation

Bi-allelic variations in the gap junction protein beta-2 (GJB2) gene cause up to 50% of cases of newborn hearing loss. Heterozygous pathogenic GJB2 variations are also fivefold overrepresented in idiopathic patient groups compared to the normal-hearing population. Whether hearing loss in this group is due to unidentified additional variations within GJB2 or variations in other deafness genes is unknown in most cases. Whole-exome sequencing offers an effective approach in the search for causative variations in patients with Mendelian diseases. In this prospective genetic cohort study, we initially investigated a family of Turkish origin suffering from congenital autosomal recessive hearing loss. An index patient and his normal-hearing father, both bearing a single heterozygous pathogenic c.262G>T (p.Ala88Ser) GJB2 transversion as well as the normal-hearing mother were investigated by means of whole-exome sequencing. Subsequently the genetic screening was extended to a hearing-impaired cohort of 24 families of Turkish origin. A homozygous missense c.5492G>T transversion (p.Gly1831Val) in the Myosin 15a gene, previously linked to deafness, was identified as causative in the index family. This very rare variant is not listed in any population in the Genome Aggregation Database. Subsequent screening of index patients from additional families of Turkish origin with recessive hearing loss identified the c.5492G>T variation in an additional family. Whole-exome sequencing may effectively identify the causes of idiopathic hearing loss in patients bearing heterozygous GJB2 variations.

A deafness mechanism of digenic Cx26 (GJB2) and Cx30 (GJB6) mutations: Reduction of endocochlear potential by impairment of heterogeneous gap junctional function in the cochlear lateral wall

Digenic Connexin26 (Cx26, GJB2) and Cx30 (GJB6) heterozygous mutations are the second most frequent cause of recessive deafness in humans. However, the underlying deafness mechanism remains unclear. In this study, we created different double Cx26 and Cx30 heterozygous (Cx26^+/-/Cx30^+/-) mouse models to investigate the underlying pathological changes and deafness mechanism. We found that double Cx26^+/-/Cx30^+/- heterozygous mice had hearing loss. Endocochlear potential (EP), which is a driving force for hair cells producing auditory receptor current, was reduced. However, unlike Cx26 homozygous knockout (Cx26^-/-) mice, the cochlea in Cx26^+/-/Cx30^+/- mice displayed normal development and had no apparent hair cell degeneration. Gap junctions (GJs) in the cochlea form two independent networks: the epithelial cell GJ network in the organ of Corti and the connective tissue GJ network in the cochlear lateral wall. We further found that double heterozygous deletion of Cx26 and Cx30 in the epithelial cells did not reduce EP and had normal hearing, suggesting that Cx26^+/-/Cx30^+/- may mainly impair gap junctional functions in the cochlear lateral wall and lead to EP reduction and hearing loss. Most of Cx26 and Cx30 in the cochlear lateral wall co-expressed in the same gap junctional plaques. Moreover, sole Cx26^+/- or Cx30^+/- heterozygous mice had no hearing loss. These data further suggest that digenic Cx26 and Cx30 mutations may impair heterozygous coupling of Cx26 and Cx30 in the cochlear lateral wall to reduce EP, thereby leading to hearing loss.

Identification and genotype/phenotype correlation of mutations in a large German cohort with hearing loss

The prevalence of hearing impairment is estimated as approximately 1 on 1,000 newborn children. To assess a higher mutation detection rate in individuals with hearing loss a three-step mutation screening program consisting of GJB2 in first line, then GJB1, GJB3 and GJB6 (second step) and if tested negative or heterozygote, testing of GJA1, GJB4, SLC26A4 and PJVK (third) was performed. Audiograms were derived from all patients to characterize audiological features of GJB2 mutations especially. In 59 patients (31.3%) of the 188 probands, the hearing impairment was due to GJB2 mutations, 45 (23.9%) of these being homozygous for 35delG mutation and 14 (7.4%) compound heterozygous for GJB2 mutations in the coding region of exon 2 whereas no significant sequence variation was found in exon 1. In 22 (11.7%) additional patients a single recessive mutation in GJB2, GJB3, GJB6 and SLC26A4 without a second mutation on the other allele was identified, making genetic counseling difficult. Our study showed significant difference in hearing loss degree in the patients with GJB2-mutations. Forty-five (45.5%) GJB2-cases were identified in 99 individuals diagnosed with severe to profound hearing loss, 14 (17.7%) GJB2-cases were identified in 79 individuals with moderate deafness whereas no clear GJB2 mutation was found in 10 patients with mild hearing loss (p < 0.001). Revealing a high variability of hearing levels in identical genotypes (even intrafamilial), a significant genotype-phenotype correlation could not be established. Based on the identified mutations spectrum and frequencies, speaking mostly of GJB2, a step by step screening for mutations can be devised and in addition may lead to a better stratification of patients for specific therapeutical approaches.

Connexin 26 and 30 mutations in paediatric patients with congenital, non-syndromic hearing loss treated with cochlear implantation in Mediterranean Turkey

Objective:

Mutations in the genes for connexin 26 (GJB2) and connexin 30 (GJB6) play an important role in autosomal recessive, non-syndromic hearing loss. This study aimed to detect the 35delG and 167delT mutations of theGJB2gene and the del(GJB6-D13S1830) mutation of theGJB6gene in paediatric patients diagnosed with congenital, non-syndromic hearing loss and treated with cochlear implantation in Mediterranean Turkey.

Materials and method:

We included 94 children diagnosed with congenital, non-syndromic hearing loss and treated with cochlear implantation. Blood samples were collected, DNA extracted and an enzyme-linked immunosorbent assay performed to enable molecular diagnosis of mutations.

Results:

Of the 94 children analysed, the 35delG mutation was detected in 12 (12.7 per cent): 10 (83.3 per cent) were homozygous and 2 (16.7 per cent) heterozygous mutant. The 167delT and del(GJB6-D13S1830) mutations were not detected.

Conclusion:

The GJB2-35delG mutation is a major cause of congenital, non-syndromic hearing loss in this study population.

Novel connexin 30 and connexin 26 mutational spectrum in patients with progressive sensorineural hearing loss

Objective:

Mutations in the gap junction protein beta-2 gene (‘GJB2’) are known to be responsible for mild to profound congenital and late-onset hearing loss. This study aimed to investigate the molecular basis of progressive hearing loss compared with non-progressive hearing loss.

Methods:

Following clinical otorhinolaryngological evaluation, a genetic analysis was performed in a cohort of 72 patients with progressive sensorineural hearing loss.

Results:

Pathological genotypes were established in 16 patients (22.2 per cent). Six different gap junction protein beta-2 gene mutations were detected in 15 patients, with the c.35delG mutation responsible for 56 per cent of the mutated alleles. A novel gap junction protein beta-6 gene (‘GJB6’) mutation (p.Met203Val) was observed in one patient with mild progressive hearing loss.

Conclusion:

Analyses of gap junction protein beta-2 and -6 genes revealed that similar pathological genotypes, occurring with similar frequencies, were responsible for progressive hearing loss, compared with reported genotypes for non-progressive hearing loss patients. Thus, genotype cannot be used to differentiate non-progressive from progressive hearing loss cases; in this study, patients both with and without an established pathological genotype had a similar clinical course.

Prevalence of GJB6 mutations in Chinese patients with non-syndromic hearing loss

OBJECTIVE

To investigate the distribution of GJB6 mutations in Central Chinese population with non-syndromic hearing loss.

METHOD

Totally 655 hearing impaired patients in Hubei province of China were screened for del(GJB6-D13S1830) deletions by using multiplex PCR and sequencing of GJB6 whole coding region.

RESULT

The del(GJB6-D13S1830) and other mutations in GJB6 gene were not observed in our study cohort.

CONCLUSION

The results suggest that GJB6 mutations is not a common cause among Central Chinese population and screening for the mutations of GJB6 can be ranked as unconventional deaf gene test for this population.

Prevalence of mutations located at the dfnb1 locus in a population of cochlear implanted children in eastern Romania

OBJECTIVE

Hearing loss is one of the major public health problems, with a genetic etiology in more than 60% of cases. Connexin 26 and connexin 30 mutations are the most prevalent causes of deafness. The aim of this study is to characterize and to establish the prevalence of the GJB2 and GJB6 gene mutations in a population of cochlear implanted recipients from Eastern Romania, this being the first report of this type in our country.

METHODS

We present a retrospective study that enrolled 45 Caucasian cochlear implanted patients with non-syndromic sensorineural severe to profound, congenital or progressive with early-onset idiopathic hearing loss. We performed sequential analysis of exon 1 and the coding exon 2 of the GJB2 gene including also the splice sites and analysis of the deletions del(GJB6-D13S1830), del(GJB6-D13S1854) and del(chr13:19,837,343-19,968,698).

RESULTS

The genetic analysis of the GJB2 gene identified connexin 26 mutations in 22 patients out of 45 (12 homozygous for c.35delG, 6 compound heterozygous and 4 with mutations only on one allele). We found 6 different mutations, the most prevalent being c.35delG - found on 32 alleles, followed by p.W24* - found on 2 alleles. We did not identify the deletions del(GJB6-D13S1830), del(GJB6-D13S1854) and del(chr13:19,837,343-19,968,698).

CONCLUSIONS

Although the most prevalent mutation was c.35delG (80% from all types of mutations), unexpectedly we identified 5 more different mutations. The presence of 6 different mutations on the GJB2 gene has implications in hearing screening programs development in our region and in genetic counseling.

Prevalence of the GJB2 IVS1+1G >A mutation in Chinese hearing loss patients with monoallelic pathogenic mutation in the coding region of GJB2

Background

Mutations in the GJB2 gene are the most common cause of nonsyndromic recessive hearing loss in China. In about 6% of Chinese patients with severe to profound sensorineural hearing impairment, only monoallelic GJB2 mutations known to be either recessive or of unclear pathogenicity have been identified. This paper reports the prevalence of the GJB2 IVS1+1G>A mutation in a population of Chinese hearing loss patients with monoallelic pathogenic mutation in the coding region of GJB2.

Methods

Two hundred and twelve patients, screened from 7133 cases of nonsyndromic hearing loss in China, with monoallelic mutation (mainly frameshift and nonsense mutation) in the coding region of GJB2 were examined for the GJB2 IVS1+1G>A mutation and mutations in the promoter region of this gene. Two hundred and sixty-two nonsyndromic hearing loss patients without GJB2 mutation and 105 controls with normal hearing were also tested for the GJB2 IVS1+1G>A mutation by sequencing.

Results

Four patients with monoallelic mutation in the coding region of GJB2 were found carrying the GJB2 IVS1+1G>A mutation on the opposite allele. One patient with the GJB2 c.235delC mutation carried one variant, -3175 C>T, in exon 1 of GJB2. Neither GJB2 IVS1+1G>A mutation nor any variant in exon 1 of GJB2 was found in the 262 nonsyndromic hearing loss patients without GJB2 mutation or in the 105 normal hearing controls.

Conclusion

Testing for the GJB2 IVS 1+1 G to A mutation explained deafness in 1.89% of Chinese GJB2 monoallelic patients, and it should be included in routine testing of patients with GJB2 monoallelic pathogenic mutation.

Low prevalence of GJB2 mutations in non-syndromic hearing loss in Western India

OBJECTIVES

To identify the prevalence of GJB2 (Cx 26)and GJB6 (Cx 30) mutations in hearing impaired individuals from Western and South India.

STUDY DESIGN

Cross-sectional study.

METHODS

Families with hearing impaired individuals (prelingual, non-syndromic, sensori-neural hearing loss) were enrolled and genomic DNA was extracted. Primers were designed for amplifying the coding and non-coding exons including flanking splice sites of the Cx 26 gene. Probands heterozygous or negative for Cx 26 mutations were further analyzed for the 342Kb deletion encompassing D13S1830 microsatellite marker on Cx 30.

RESULTS

Two hundred and eighty-eight patients were enrolled in the study and 116 (40.3%) were diagnosed to have mutations in the coding exon 2 of Cx 26 gene. Fifty-four (18.8%) probands were found to have mutations in both the alleles while the remaining 62 (21.5%) were heterozygous for Cx 26 mutations. W24X, and W77X were the common mutations identified. The prevalence of familial deafness was similar in both consanguineous and non-consanguineous families (33% and 34.9% respectively). Mutations in the non-coding exon 1 and intron 1 as well as the 342 kb deletion involving D13S1830 marker on Cx 30 were ruled out in two hundred and thirty-four deaf individuals carrying none or only one mutation in the exon 2 of Cx 26 gene.

CONCLUSION

Cx30 mutations do not contribute to the autosomal recessive non-syndromic hearing loss (NSHL) in the Indian population. Homozygous Cx26 mutations account only for about 1/5th (18.8%) of autosomal recessive non-syndromic hearing implying the need to explore other contributory loci.

Mutation in gap and tight junctions in patients with non-syndromic hearing loss

Biallelic mutations in the GJB2, GJB3, GJB6 and CLDN14 genes have been implicated in autosomal recessive non-syndromic hearing impairment (ARNSHI). Moreover, a large number of GJB2 heterozygous patients was reported. The phenotype was in partly justified by the occurrence of two deletions including GJB6. We analysed GJB2, GJB6, GJB3 and CLDN14 in 102 Tunisian patients with ARNSHI. The deletions del(GJB6-D13S1830) and del(GJB6-D13S1854) were also screened. The c.35delG in GJB2 was the most frequent mutation (21.57%). It was detected at heterozygous state in 2 patients. The del(GJB6-D13S1830) was identified in one case at heterozygous state. No other mutation in studied gap junction genes was detected in heterozygous patients. Several polymorphisms were identified in GJB3, GJB6 and CLDN14. Our study confirms the importance of GJB2 screening in ARNSHI and suggests that in consanguineous populations, a single DFNB1 mutant allele in individuals with HI is likely due to a coincidental carrier state.

Relevance of the A1555G Mutation in the 12S rRNA Gene for Hearing Impairment in Austria

OBJECTIVE:: To analyze the prevalence and importance of the maternally inherited A1555G mutation in the 12S rRNA gene in the Austrian population. STUDY DESIGN:: Investigation for mutations of genetically affected familial and sporadic cases of hearing impairment (HI), including analyses of audiometric data. SETTING:: Teaching hospital, tertiary referral center. PATIENTS:: Forty-five familial and 77 sporadic cases of nonsyndromic HI in an Austrian Caucasian ethnic group. MAIN OUTCOME MEASURE(S):: Pure-tone audiometric data and screening by restriction fragment length polymorphism analysis after exclusion of GJB2 (Connexin 26) caused hearing loss. RESULTS:: In the investigated hearing-impaired population, the mutation A1555G in the mitochondrial 12S rRNA gene was not detected. CONCLUSION:: The A1555G mutation in the mitochondrial DNA 12S rRNA is not a major cause of HI in the Austrian Caucasian population.

Connexin 26 mutations in autosomal recessive deafness disorders: a review

This review explores the association between GJB2 gene mutations, encoding connexin 26 (Cx26), and nonsyndromic hearing loss. Connexins are proteins that form intracellular membrane channels and regulate ion movement between contiguous fluid spaces. A family of autosomal gene mutations has been identified that lead to abnormal connexin expression within the inner ear that are associated with hearing loss. The exact mechanism by which this link is elicited remains unclear. We aim to highlight the clinically underestimated prevalence of GJB2 gene mutations, to explore the influential role of ethnic diversity in mutation frequency, and to provide a framework for hearing specialists in considering the differential diagnosis of nonsyndromic hearing loss. By linking an observed phenotype associated with abnormal Cx26 expression to the current understanding of the biological and genetic basis underlying it will allow a more accurate clinical description of associated hearing loss, and therefore enable more effective patient management and genetic counselling.

High Incidence of GJB2 Mutations During Screening of Newborns for Hearing Loss in Austria

OBJECTIVES

The aim of the present study was to evaluate gap junction protein beta2 (GJB2) genetic testing within a national neonate screening program for hearing loss (HL) in a European population.

DESIGN

Neonatal cases of nonsyndromic HL (N = 21) were identified by postpartal otoacoustic emissions (OAE) and brain stem electric response audiometry (BERA) analysis. GJB2 testing was performed by direct sequencing.

RESULTS

Mutations in GJB2 were found in 15 of 21 children (71.4%) identified by neonatal audiological screening. The 35delG mutation in GJB2 was found homozygous in 10 cases (47.6%) and also as a clear cause of HL as the heterozygous alterations 35delG/del311-324 and 35delG/L90P. In a single case, L90P/R143Q was also identified as a cause of HL. In 3 HL cases that were not identifiable during initial OAE testing, homozygous 35delG and 35delG/R184P defined the genetic basis for HL in 2 cases, whereas one case had wild-type GJB2.

CONCLUSIONS

Our findings of the high mutation rate in the Austrian population, especially in neonates identified during the newborn screening program, confirm the importance of screening for mutations in GJB2.

High prevalence of the IVS 1 + 1 G to A/GJB2 mutation among Czech hearing impaired patients with monoallelic mutation in the coding region of GJB2

Biallelic pathogenic GJB2 gene mutations cause pre-lingual genetic hearing loss in up to 50% of individuals with bilateral sensorineural hearing loss worldwide. Sequencing of the entire GJB2 gene-coding region in Czech patients with pre-lingual bilateral hearing loss revealed that 10.3% of Czech patients carry only one monoallelic pathogenic mutation in the coding region of the GJB2 gene, which is significantly more than the population frequency of 3.4%. The 309-kb GJB6 deletion, frequent in Spain and France, is very rare in the Czech population. In order to evaluate the impact of the IVS1 + 1 G to A splice site mutation in the non-coding part of the GJB2 gene among Czech patients, we tested all available patients with pre-lingual hearing loss with only one monoallelic mutation in the coding part of GJB2. By sequencing of the exon 1 region of the GJB2 gene and HphI restriction analysis in 20 Czech patients we identified nine patients carrying IVS1 + 1 G to A. Testing for this mutation explained deafness in 45% of Czech GJB2 monoallelic patients. This mutation represents now 4% of GJB2 pathogenic mutations in Czech patients and is the third most common GJB2 mutation found in our cohort of 242 unrelated Czech patients with prelingual hearing loss. A similar frequency may also be expected in other Central European or Slavic populations.

GJB2 mutations in hearing impairment: identification of a broad clinical spectrum for improved genetic counseling

OBJECTIVES/HYPOTHESIS

Hearing impairment has a high prevalence affecting approximately 1 in 1000 newborn children. Alterations in the gap junction protein beta 2 (GJB2) and gap junction protein beta 6 (GJB6) are associated with nonsyndromic hearing impairment and should have a significant impact on genetic counseling.

STUDY DESIGN

Various cases of nonsyndromic hearing impairment were screened for alterations in GJB2 and GJB6 in this clinical study.

METHODS

The prevalence of mutations in GJB2 encoding for connexin 26 in a patient group with nonsyndromic hearing impairment comprising 45 families and 57 sporadic cases was initially determined by sequencing. The role of GJB2 was then assessed in individuals with hearing impairment (3 families and 20 sporadic cases) who are usually excluded from analysis because of the presence of additional symptoms or in cases in which a role for nongenetic factors cannot be eliminated. In hearing-impaired individuals with heterozygous GJB2 mutations the recently identified 342-kb deletion truncating GJB6 called del(GJB6-D13S1830) as a digenetic component in hearing impairment was excluded by polymerase chain reaction.

RESULTS

Autosomal recessively inherited GJB2 mutations induced hearing impairment in 25.5% of individuals in the nonsyndromic hearing impairment group. GJB2 alterations were also seen in 17.4% of individuals in whom additional symptoms or a role for nongenetic involvement could not be excluded. In all, 15 different alterations in GJB2 were detected, including the previously unknown 154G>C, 557C>T, and 682C>T mutations, and these were correlated to clinical parameters.

CONCLUSION

Improved genetic counseling can be performed by screening for GJB2 alterations in patients with nonsyndromic hearing impairment including patients within groups for which a role for exogenetic factors cannot be excluded. Specific genetic counseling for GJB2-linked hearing impairment in heterozygotes will depend on future research.