Sex-linked deafness

Genomic and phenotypic landscapes of X-linked hereditary hearing loss in the Chinese population

BACKGROUND

Hearing loss (HL) is the most common sensory birth deficit worldwide, with causative variants in more than 150 genes. However, the etiological contribution and clinical manifestations of X-linked inheritance in HL remain unclear within the Chinese HL population. In this study, we focused on X-linked hereditary HL and aimed to assess its contribution to hereditary HL and identify the genotype-phenotype relationship.

METHODS

We performed a molecular epidemiological investigation of X-linked hereditary HL based on next-generation sequencing and third-generation sequencing in 3646 unrelated patients with HL. We also discussed the clinical features associated with X-linked non-syndromic HL-related genes based on a review of the literature.

RESULTS

We obtained a diagnostic rate of 52.72% (1922/3646) among our patients; the aggregate contribution of HL caused by genes on the X chromosome in this cohort was ~ 1.14% (22/1922), and POU3F4 variants caused ~ 59% (13/22) of these cases. We found that X-linked HL was congenital or began during childhood in all cases, with representative audiological profiles or typical cochlear malformations in certain genes. Genotypic and phenotypic analyses showed that causative variants in PRPS1 and AIFM1 were mainly of the missense type, suggesting that phenotypic variability was correlated with the different effects that the replaced residues exert on structure and function. Variations in SMPX causing truncation of the protein product were associated with DFNX4, which resulted in typical audiological profiles before and after the age of 10 years, whereas nontruncated proteins typically led to distal myopathy. No phenotypic differences were identified in patients carrying POU3F4 or COL4A6 variants.

CONCLUSIONS

Our work constitutes a preliminary evaluation of the molecular contribution of X-linked genes in heritable HL (~ 1.14%). The 15 novel variants reported here expand the mutational spectrum of these genes. Analysis of the genotype-phenotype relationship is valuable for X-linked HL precise diagnostics and genetic counseling. Elucidation of the pathogenic mechanisms and audiological profiles of HL can also guide choices regarding treatment modalities.

Progressive Loss of Sensitivity to Electrical Stimulation After Cochlear Implantation in X-Linked Incomplete Partition Type III Deafness

OBJECTIVES

Patients with X-linked incomplete partition type III (IP3) deafness treated with cochlear implants exhibit higher "Most Comfortable Loudness" (MCL) levels of stimulation and more electrode deactivation than patients with normal morphology. We endeavored to analyze the progression of the MCL levels and electrode deactivation over time and assess those factors that could have led to deactivation. Furthermore, we aimed to assess whether speech perception was affected by a progressive loss of neural contact.

DESIGN

All 13 patients with the IP3 malformation in our clinical database were analyzed retrospectively with regard to impedance, stimulation levels, deactivated electrodes, and speech perception. A control group of patients with normal anatomy was included.

RESULTS

MCL levels increased over time by 2.5 charge units (qu) per year, which was not seen in the control group. Electrode deactivation was more common in IP3 malformation, and it was estimated that 25% of electrodes would be deactivated by 15 years of age. Impedance was stable but higher in the study population. Speech perception was lower in IP3 malformation generally and was correlated to the number of deactivated electrodes.

CONCLUSIONS

Patients diagnosed with IP3 malformation deafness may suffer a greater risk of cochlear implant discontinuation compared with those with normal anatomy. A progressive loss of sensitivity to electrical stimulation may indicate a form of neural degradation in the abnormal cochlea. With time, patients in this group, even with cochlear implant technology, may experience gradual deterioration of speech perception. This has clinical implications for the counseling of parents.

A novel cell-free therapy using exosomes in the inner ear regeneration

Cellular and molecular alterations associated with hearing loss are now better understood with advances in molecular biology. These changes indicate the participation of distinct damage and stress pathways that are unlikely to be fully addressed by conventional pharmaceutical treatment. Sensorineural hearing loss is a common and debilitating condition for which comprehensive pharmacologic intervention is not available. The complex and diverse molecular pathology that underlies hearing loss currently limits our ability to intervene with small molecules. The present review focuses on the potential for the use of extracellular vesicles in otology. It examines a variety of inner ear diseases and hearing loss that may be treatable using exosomes (EXOs). The role of EXOs as carriers for the treatment of diseases related to the inner ear as well as EXOs as biomarkers for the recognition of diseases related to the ear is discussed.

X-Linked Gusher Disease DFNX2 in Children, a Rare Inner Ear Dysplasia with Mixed Hearing and Vestibular Loss

Conductive hearing losses are typically present in disorders of the external/middle ear. However, there is a rare group of inner ear conditions called third windows that can also generate a conductive hearing loss. This is due to an abnormal connection between the middle and the inner ear or between the inner ear and the cranial cavity. X-linked gusher disorder is an extremely rare congenital inner ear dysplastic syndrome with an abnormal connection due to a characteristic incomplete cochlear partition type III and an incomplete internal auditory meatus fundus. The disorder is inherited in an X-linked fashion due to the mutation of the POU3F4 gene. We present two siblings diagnosed with the condition and their long-term follow-ups. They both presented audiovestibular symptoms and showed progressive mixed losses and bilateral vestibular weakness. They were treated with cochlear implant, digital amplification and with vestibular rehabilitation. Significant others around them were involved in their journey with the medical team, and in both, a very favourable outcome was achieved. This is the first time that we have reported evolving audiovestibular function with vestibular quantification in X-linked gusher disorder and emphasize on the multidisciplinary holistic approach to manage these children effectively.

Considering gene therapy to protect from X-linked deafness DFNX2 and associated neurodevelopmental disorders

Mutations and deletions in the gene or upstream of the gene encoding the POU3F4 transcription factor cause X-linked progressive deafness DFNX2 and additional neurodevelopmental disorders in humans. Hearing loss can be purely sensorineural or mixed, that is, with both conductive and sensorineural components. Affected males show anatomical abnormalities of the inner ear, which are jointly defined as incomplete partition type III. Current approaches to improve hearing and speech skills of DFNX2 patients do not seem to be fully effective. Owing to inner ear malformations, cochlear implantation is surgically difficult and may predispose towards severe complications. Even in cases where implantation is safely performed, hearing and speech outcomes remain highly variable among patients. Mouse models for DFNX2 deafness revealed that sensorineural loss could arise from a dysfunction of spiral ligament fibrocytes in the lateral wall of the cochlea, which leads to reduced endocochlear potential. Highly positive endocochlear potential is critical for sensory hair cell mechanotransduction and hearing. In this context, here, we propose to develop a therapeutic approach in male Pou3f4 -/y mice based on an adeno-associated viral (AAV) vector-mediated gene transfer in cochlear spiral ligament fibrocytes. Among a broad range of AAV vectors, AAV7 was found to show a strong tropism for the spiral ligament. Thus, we suggest that an AAV7-mediated delivery of Pou3f4 complementary DNA in the spiral ligament of Pou3f4 -/y mice could represent an attractive strategy to prevent fibrocyte degeneration and to restore normal cochlear functions and properties, including a positive endocochlear potential, before hearing loss progresses to profound deafness.

A novel mutation of X-linked recessive deafness gene POU3F4 in a boy with congenital deafness

Purpose

To report an interstitial deletion of Xq21.1 in chromosome X in a boy with congenital deafness.

Methods

The proband underwent a thorough physical examination and a detailed audiological and temporal bone computed tomography (CT) scan. Cochlear implantation was performed on the proband, and follow-up was conducted. High throughput sequencing and copy number analysis was made of peripheral blood samples from the proband, family members, and control subjects.

Results

Sensorineural hearing loss was present in the boy and temporal bone CT scan showed a bilateral incomplete partition type III anomaly (IP-III). Q21.1 (79.40-83.32 Mb) of chromosome X in the proband had a copy number deletion with a fragment size of about 3.92 Mb. Categories of auditory performance scores and SIR scores of the cochlea in this child improved after surgery.

Conclusion

Through the analysis of POU3F4, a novel mutation site with potentially pathogenic significance was found.Level of Evidence: 5.

Genetic etiology of non-syndromic hearing loss in Europe

Hearing impairment not etiologically associated with clinical signs in other organs (non-syndromic) is genetically heterogeneous, so that over 120 genes are currently known to be involved. The frequency of mutations in each gene and the most frequent mutations vary throughout populations. Here we review the genetic etiology of non-syndromic hearing impairment (NSHI) in Europe. Over the years, epidemiological data were scarce because of the large number of involved genes, whose screening was not cost-effective until implementation of massively parallel DNA sequencing. In Europe, the most common form of autosomal recessive NSHI is DFNB1, which accounts for 11-57% of the cases. Mutations in STRC account for 16% of the recessive cases, and only a few more (MYO15A, MYO7A, LOXHD1, USH2A, TMPRSS3, CDH23, TMC1, OTOF, OTOA, SLC26A4, ADGRV1 and TECTA) have contributions higher than 2%. As regards autosomal-dominant NSHI, DFNA22 (MYO6) and DFNA8/12 (TECTA) represent the most common forms, accounting for 21% and 18% of elucidated cases, respectively. The contribution of ACTG1 and WFS1 drops to 9% in both cases, followed by POU4F3 (6.5%), MYO7A (5%), MYH14 and COL11A2 (4% each). Four additional genes contribute 2.5% each one (MITF, KCNQ4, EYA4, SOX10) and the remaining are residually represented. X-linked hearing loss and maternally-inherited NSHI have minor contributions in most countries. Further knowledge on the genetic epidemiology of NSHI in Europe needs a standardization of the experimental approaches and a stratification of the results according to clinical features, familial history and patterns of inheritance, to facilitate comparison between studies.

X-linked Malformation Deafness: Neurodevelopmental Symptoms Are Common in Children With IP3 Malformation and Mutation in POU3F4

Supplemental Digital Content is available in the text.

Incomplete partition type 3 (IP3) malformation deafness is a rare hereditary cause of congenital or rapid progressive hearing loss. The children present with a severe to profound mixed hearing loss and temporal bone imaging show a typical inner ear malformation classified as IP3. Cochlear implantation is one option of hearing restoration in severe cases. Little is known about other specific difficulties these children might exhibit, for instance possible neurodevelopmental symptoms.

A novel missense mutation in SMPX causes a rare form of X-linked postlingual sensorineural hearing loss in a Chinese family

BACKGROUND

X-linked deafness-4 (DFNX4) caused by the functional loss of the SMPX gene is one form of nonsyndromic hearing loss with postlingual onset. This study aimed to investigate the cause of X-linked inherited sensorineural nonsyndromic hearing loss in a four-generation Chinese family and to explain the reason for extremely different hearing phenotypes between the proband and other family members.

METHODS

Whole-exome sequencing (WES) and co-segregation analysis were used to identify the pathogenic variants. Furthermore, methylation differences among the androgen receptor genes were utilized to investigate whether the severe phenotype of the proband is related to X-chromosome inactivation (Xi).

RESULTS

We described in detail the clinical characteristics of the family and identified a novel missense mutation (c.262C>G: p.Gln88Glu) in SMPX by WES. This variant was co-segregated with the postlingual hearing loss phenotype and was absent in 300 normal controls. Also, we found that the proband, a 4-year-old female, carries two new compound heterozygous mutations (c.9259G>A: p.Val3087Ile and c.8576G>A: p.Arg2859His) in the USH2A gene, but to date without any other symptoms except profound sensorineural hearing loss. Additionally, analysis of X-chromosome inactivation indicated moderate skewing in the proband, which is probably related to the heterogeneity of clinical characteristics.

CONCLUSIONS

This is the first study to report a missense mutation of SMPX in a Chinese family. Our findings have enriched the mutation and phenotypic spectrum of the SMPX gene.

[Presentation of a rare case of hereditary hearing loss with X-linked recessive inheritance associated with the POU3F4 gene]

Congenital hearing loss is one of the most frequent inherited human pathologies, occurring in 1-2 out of 1000 newborns. X-linked hearing loss occurs in 1-5% of all congenital hearing impairments. The proband (a man) and his affected brother have profound prelingual non-syndromic neurosensory hearing loss. Their parents are healthy. The aim of the study was to determine the cause of hearing loss in a given family and to assess the population frequency of the revealed pathogenic genetic variant. NGS analysis identified a pathogenic variant c.907C>T (p.Pro303Ser) in the POU3F4 gene mapped to the Xq21.1 locus. This is the second case of X-linked hearing loss (DFNX2, OMIM 304400) in Europe, caused by the c.907C>T variant in the POU3F4 gene. DFNX2-hearing loss is manifested with abnormalities of the inner ear, predisposing to the "gusher effect" - otoliquorrhea during stapedoplasty. The brother was diagnosed with a c.907C>T variant in the POU3F4 gene in the hemizygous state while in their mother - in the heterozygous state. Their father had no variant c.907C>T. Molecular genetic analysis showed that the genetic variant c.907C>T was not detected in the control sample of healthy female from the Nogai population, which suggests its low frequency in the population.

Cochlear Implantation in a Patient with a Novel POU3F4 Mutation and Incomplete Partition Type-III Malformation

Aims

This study is aimed at (1) analyzing the clinical manifestations and genetic features of a novel POU3F4 mutation in a nonsyndromic X-linked recessive hearing loss family and (2) reporting the outcomes of cochlear implantation in a patient with this mutation.

Methods

A patient who was diagnosed as the IP-III malformation underwent cochlear implantation in our hospital. The genetic analysis was conducted in his family, including the whole-exome sequencing combined with Sanger sequencing and bioinformatic analysis. Clinical features, preoperative auditory and speech performances, and postoperative outcomes of cochlear implant (CI) were assessed on the proband and his family.

Results

A novel variant c.400_401insACTC (p.Q136LfsX58) in the POU3F4 gene was detected in the family, which was cosegregated with the hearing loss. This variant was absent in 200 normal-hearing persons. The phylogenetic analysis and structure modeling of Pou3f4 protein further confirmed that the novel mutation was pathogenic. The proband underwent cochlear implantation on the right ear at four years old and gained greatly auditory and speech improvement. However, the benefits of the CI declined about three and a half years postoperation. Though the right ear had been reimplanted, the outcomes were still worse than before.

Conclusion

A novel frame shift variant c.400_401insACTC (p.Q136LfsX58) in the POU3F4 gene was identified in a Chinese family with X-linked inheritance hearing loss. A patient with this mutation and IP-III malformation could get good benefits from CI. However, the outcomes of the cochlear implantation might decline as the patient grows old.

New Imaging Findings of Incomplete Partition Type III Inner Ear Malformation and Literature Review

Incomplete partition type III, also referred to as X-linked deafness, is a rare genetic inner ear malformation. Its characteristic CT findings, including bulbous dilation of the internal auditory canal and absence of the modiolus with the interscalar septa present, have been well-recognized. In this series of 19 cases, we report the abnormalities of the vestibule and semicircular canals and provide a comprehensive description of their CT and MR imaging findings. The inner ear malformations in incomplete partition type III were bilateral and basically symmetric, with involvement of the internal auditory canal, nerve canals in the fundus, cochlea, vestibule, semicircular canals, vestibular aqueduct, otic capsule, round window, oval window, and stapes. An irregular vestibule with a cystic appearance is also a distinctive imaging feature, which could be seen in about 90% of our patients, with a cystic appearance of the semicircular canals present in nearly half of the cases.

In silico analysis of nonsynonymous single-nucleotide polymorphisms (nsSNPs) of the SMPX gene

Mutations in the SMPX gene can disrupt the regular activity of the SMPX protein, which is involved in the hearing process. Recent reports showing a link between nonsynonymous single-nucleotide polymorphisms (nsSNPs) in SMPX and hearing loss, thus classifying deleterious SNPs in SMPX will be an uphill task before designing a more extensive population study. In this study, damaging nsSNPs of SMPX from the dbSNP database were identified by using 13 bioinformatics tools. Initially, the impact of nsSNPs in the SMPX gene were evaluated through different in silico predictors; and the deleterious convergent changes were analyzed by energy-minimization-guided residual network analysis. In addition, the pathogenic effects of mutations in SMPX-mediated protein-protein interactions were also characterized by structural modeling and binding energy calculations. A total of four mutations (N19D, A29T, K54N, and S71L) were found to be highly deleterious by all the tools, which are located at highly conserved regions. Furthermore, all four mutants showed structural alterations, and the communities of amino acids for mutant proteins were readily changed, compared to the wild-type. Among them, A29T (rs772775896) was revealed as the most damaging nsSNP, which caused significant structural deviation of the SMPX protein, as a result reducing the binding affinity to other functional partners. These findings reflect the computational insights into the deleterious role of nsSNPs in SMPX, which might be helpful for subjecting wet-lab confirmatory analysis.

Whole-exome sequencing identifies a donor splice-site variant in SMPX that causes rare X-linked congenital deafness

Background

X‐linked deafness‐4 (DFNX4) caused by functional loss of SMPX is a nonsyndromic form of progressive hearing loss with post‐lingual onset. Herein, we describe a male neonate from an ethnic Han Chinese family who presented with congenital deafness.

Methods

The proband and the family members were subjected to comprehensively hearing screen. Genetic testing was carried out using whole‐exome sequencing (WES). The result was verified by Sanger sequencing. Functional characterization of the identified variant was completed by reverse transcription PCR (RT‐PCR), Sanger sequencing, and fluorogenic quantitative PCR (qPCR).

Results

The proband was diagnosed with progressive sensorineural hearing loss. The proband's mother showed normal hearing at present. The proband's maternal grandmother exhibited mild HL since the age of 50. Using whole‐exome sequencing (WES), we identified a donor splice‐site variant (NM_014332.2: c.132 + 1G>A) in the SMPX gene in the proband. The mother and maternal grandmother were both carriers, which suggested a X‐linked inheritance of the condition in the family. RT‐PCR and Sanger sequencing revealed that four alternative splice pairs within intron 3 have led to four aberrant RNAs transcripts, including two non‐canonical splice‐pairs (GC‐AG and CT‐AG). The variant generated a novel frameshift variant, creating a premature termination codon (PTC) upstream of a newly formed splice site (p.Met45Glyfs*16). SMPX mRNA expression assay showed that the PTC has caused degradation of mRNA via nonsense‐mediated mRNA decay (NMD).

Conclusion

This is the first study to report a SMPX (DFNX4) splicing variant in a Chinese family. These findings, especially congenital deafness, contributed to existing knowledge regarding the genotypic and phenotypic spectrum of SMPX‐associated hearing loss.

Zebrafish Model for Nonsyndromic X-Linked Sensorineural Deafness, DFNX1

Hereditary deafness is often a neurosensory disorder and affects the quality of life of humans. Only three X-linked genes (POU class 3 homeobox 4 (POU3F4), phosphoribosyl pyrophosphate synthetase 1 (PRPS1), and small muscle protein X-linked (SMPX)) are known to be involved in nonsyndromic hearing loss. Four PRPS1 missense mutations have been found to associate with X-linked nonsyndromic sensorineural deafness (DFNX1/DFN2) in humans. However, a causative relationship between PRPS1 mutations and hearing loss in humans has not been well studied in any animal model. Phosphoribosyl pyrophosphate synthetase 1 (PRS-I) is highly conserved in vertebrate taxa. In this study, we used the zebrafish as a model to investigate the auditory role of zebrafish orthologs (prps1a and prps1b) of the human PRPS1 gene with whole mount in situ hybridization, reverse transcription polymerase chain reaction, phenotypic screening, confocal imaging, and electrophysiological methods. We found that both prps1a and prps1b genes were expressed in the inner ear of zebrafish. Splice-blocking antisense morpholino oligonucleotides (MO1 and MO2) caused exon-2 skip and intron-2 retention of prps1a and exon-2 skip and intron-1 retention of prps1b to knock down functions of the genes, respectively. MO1 and MO2 morphants had smaller otic vesicles and otoliths, fewer inner ear hair cells, and lower microphonic response amplitude and sensitivity than control zebrafish. Therefore, knockdown of either prps1a or prps1b resulted in significant sensorineural hearing loss in zebrafish. We conclude that the prps1 genes are essential for hearing in zebrafish, which has the potential to help us understand the biology of human deafness DFNX1/DFN2. Anat Rec, 303:544-555, 2020. © 2019 American Association for Anatomy.

Skewed X-chromosome inactivation and next-generation sequencing to identify a novel SMPX variants associated with X-linked hearing loss in a Chinese family

OBJECTIVE

Hereditary nonsyndromic hearing loss is extremely heterogeneous and an X-linked form accounts for 1-5% of all cases. The aim of this study was to identify the pathogenic variants in a nonsyndromic X-linked dominant hearing loss family, and explain the reason of different hearing phenotype in hearing between the two sisters with the same variant.

METHODS

Targeted gene capture and next-generation sequencing were used to study the genetic cause. What's more, methylation differences among the androgen receptor genes were used to investigate whether the different hearing levels of the two sisters is related to X-chromosome inactivation (Xi).

RESULTS

We identified SMPX c.29insA (p.Asn10Lysfs*3) as the novel variant causing deafness. The skewed X-chromosome inactivation was relevant to the hearing difference between the two sisters.

CONCLUSION

Targeted gene capture and NGS is an efficient way to identify pathogenic variants in genes. Analysis of X-chromosome inactivation is beneficial to the diagnosis and genetic counseling of X-linked dominant hearing loss families.

X-Linked Sensorineural Hearing Loss: A Literature Review

Sensorineural hearing loss is a very diffuse pathology (about 1/1000 born) with several types of transmission. X-linked hearing loss accounts for approximately 1% - 2% of cases of non-syndromic forms, as well as for many syndromic forms. To date, six loci (DFNX1-6) and five genes (PRPS1 for DFNX1, POU3F4 for DFNX2, SMPX for DFNX4, AIFM1 for DFNX5 and COL4A6 for DFNX6) have been identified for X-linked non-syndromic hearing loss. For the syndromic forms, at least 15 genes have been identified, some of which are also implicated in non-syndromic forms. Moreover, some syndromic forms, presenting large chromosomal deletions, are associated with mental retardation too. This review presents an overview of the currently known genes related to X-linked hearing loss with the support of the most recent literature. It summarizes the genetics and clinical features of X-linked hearing loss to give information useful to realize a clear genetic counseling and an early diagnosis. It is important to get an early diagnosis of these diseases to decide the investigations to predict the evolution of the disease and the onset of any other future symptoms. This information will be clearly useful for choosing the best therapeutic strategy. In particular, regarding audiological aspects, this review highlights risks and benefits currently known in some cases for specific therapeutic intervention.

A novel mutation in the SMPX gene associated with X-linked nonsyndromic sensorineural hearing loss in a Chinese family

X-linked inheritance is very rare and is estimated to account for only 1-5% of all nonsyndromic hearing loss cases. We found a multiplex family from China segregating with X-linked nonsyndromic hearing loss. After exclusive analysis of 10 common variations of three hearing loss-related genes, GJB2, mtDNA12srRNA and SLC26A4, a novel truncated variant of SMPX, c.87dupA (p.Gly30Argfs*12) (NCBI ClinVar Submission ID: SUB3136126), was identified by whole-exome sequencing. This variant was co-segregated with hearing loss in the entire family and was absent in 576 unrelated ethnically and geographically matched controls. We also detected a single nucleotide variation in two male controls with normal hearing, SMPX c.55A>G (p.Asn19Asp), which has been annotated as a rare variant in the Single Nucleotide Polymorphism (dbSNP) (rs759552778) and Exome Aggregation Consortium (ExAC) databases. This study has enriched the mutation spectrum of the SMPX gene.

Fluvastatin protects cochleae from damage by high-level noise

Exposure to noise and ototoxic drugs are responsible for much of the debilitating hearing loss experienced by about 350 million people worldwide. Beyond hearing aids and cochlear implants, there have been no other FDA approved drug interventions established in the clinic that would either protect or reverse the effects of hearing loss. Using Auditory Brainstem Responses (ABR) in a guinea pig model, we demonstrate that fluvastatin, an inhibitor of HMG-CoA reductase, the rate-limiting enzyme of the mevalonate pathway, protects against loss of cochlear function initiated by high intensity noise. A novel synchrotron radiation based X-ray tomographic method that imaged soft tissues at micrometer resolution in unsectioned cochleae, allowed an efficient, qualitative evaluation of the three-dimensional internal structure of the intact organ. For quantitative measures, plastic embedded cochleae were sectioned followed by hair cell counting. Protection in noise-exposed cochleae is associated with retention of inner and outer hair cells. This study demonstrates the potential of HMG-CoA reductase inhibitors, already vetted in human medicine for other purposes, to protect against noise induced hearing loss.

A novel splicing mutation in SMPX is linked to nonsyndromic progressive hearing loss

OBJECTIVE

X-linked nonsyndromic hearing impairment is the rarest form of genetic hearing loss and represents only a minor fraction of all cases. The aim of this study was to investigate the cause of X-linked nonsyndromic sensorineural hearing loss in a three-generation American family.

METHODS

Whole-exome sequencing and co-segregation analysis were used to identify disease-causing genes.

RESULTS

In this study, we described in detail the clinical characteristics of the family and identified a novel frameshift mutation creating a premature stop codon (c.133-1 G > A, p.(Gly45fs*36)) of SMPX. The loss-of-function mutation was co-segregated with the progressive hearing loss phenotype and was absent in 200 normal controls.

CONCLUSIONS

We report the first SMPX (DFNX4) mutation in a North American family. Our findings contribute to the existing genotypic and phenotypic spectrum of SMPX associated hearing loss. Furthermore, our data suggest that exome sequencing is promising in the genetic diagnosis of hearing loss.

X-linked Malformation and Cochlear Implantation

OBJECTIVE

To evaluate if cochlear implantation is safe and constitutes an option for hearing rehabilitation of children with x-linked inner ear malformation.

STUDY DESIGN

Retrospective patient review in combination with a multidisciplinary follow-up.

SETTING

Tertiary referral hospital and cochlear implant program.

PATIENTS

Ten children with severe-profound mixed hearing loss and radiological findings consistent with Incomplete Partition type 3 cochlear malformation received cochlear implants during the years 2007 to 2015. Nine of the children had a mutation affecting the gene POU3F4 on Xq21.

INTERVENTION

Cochlear implantation.

MAIN OUTCOME MEASURES

Surgical events, intraoperative measures and electrical stimulation levels, hearing and spoken language abilities.

RESULTS

In all, 15 cochlear implantations were performed. In three cases the electrode was found to be in the internal auditory canal on intraoperative x-ray and repositioned successfully. One child had a postoperative rhinorrhea confirmed to be cerebrospinal fluid but this resolved on conservative treatment. No severe complications occurred. Postoperative electrical stimulation levels were higher in 9 of 10 children, as compared with typically reported average levels in patients with a normal cochlea. Eight patients developed spoken language to various degrees while two were still at precommunication level. However, speech recognition scores were lower than average pediatric cases.

CONCLUSION

Cochlear implantation is a safe procedure for children with severe-profound mixed hearing loss related to POU3F4 mutation inner ear malformation. The children develop hearing and spoken language but outcome is below average for pediatric CI recipients.

A novel frameshift mutation of SMPX causes a rare form of X-linked nonsyndromic hearing loss in a Chinese family

X-linked hearing impairment is the rarest form of genetic hearing loss (HL) and represents only a minor fraction of all cases. The aim of this study was to investigate the cause of X-linked inherited sensorineural HL in a four-generation Chinese family. A novel duplication variant (c.217dupA, p.Ile73Asnfs*5) in SMPX was identified by whole-exome sequencing. The frameshift mutation predicted to result in the premature truncation of the SMPX protein was co-segregated with the HL phenotype and was absent in 295 normal controls. Subpopulation screening of the coding exons and flanking introns of SMPX was further performed for 338 Chinese patients with nonsydromic HL by Sanger sequencing, and another two potential causative substitutions (c.238C>A and c.55A>G) in SMPX were identified in additional sporadic cases of congenital deafness. Collectively, this study is the first to report the role of SMPX in Chinese population and identify a novel frameshift mutation in SMPX that causes not only nonsyndromic late-onset progressive HL, but also congenital hearing impairment. Our findings extend the mutation and phenotypic spectrum of the SMPX gene.

A quantitative cSMART assay for noninvasive prenatal screening of autosomal recessive nonsyndromic hearing loss caused by GJB2 and SLC26A4 mutations

PurposeThe aim of this study was to assess the performance of a noninvasive prenatal screening (NIPS) assay for accurate fetal genotyping of pregnancies at genetic risk for autosomal recessive nonsyndromic hearing loss (ARNSHL).MethodsA total of 80 pregnant couples carrying known mutations in either the GJB2 or SLC26A4 genes associated with a risk for ARNSHL were recruited to the study. Fetal amniocyte samples were genotyped by invasive prenatal screening (IPS), whereas the cell-free fetal DNA present in maternal plasma samples was genotyped using a novel NIPS method based on circulating single-molecule amplification and resequencing technology (cSMART).ResultsIPS of the 80 at-risk pregnancies identified 20 normal homozygote, 42 heterozygote, 5 affected homozygote, and 13 affected compound heterozygote fetuses. Benchmarking against IPS, 73 of 80 fetuses (91.3%) were correctly genotyped by the cSMART NIPS assay. A low fetal DNA fraction (<6%) was identified as the main contributing factor in five of seven discordant NIPS results. At fetal DNA fractions >6%, the sensitivity and specificity of the cSMART assay for correctly diagnosing ARNSHL were 100 and 96.5%, respectively.ConclusionBased on key performance indicators, the cSMART NIPS assay has clinical potential as an alternative to traditional IPS of ARNSHL.

GPRASP2, a novel causative gene mutated in an X-linked recessive syndromic hearing loss

Background

A substantial amount of nuclear genes have been identified to be implicated in genetic hearing loss, while X-linked hearing loss is genetically heterogeneous and relatively infrequent.

Objective

To identify the causative gene mutation in a five-generation Chinese family with an X-linked recessive syndromic hearing loss (SHL).

Methods

Targeted X-chromosome exome sequencing was conducted, and cosegregation analysis was performed in the members of the affected family. The in silico and expression studies were also performed.

Results

A 2-bp missense mutation (c.1717_1718GC>AA, p.A573N) in the G protein-coupled receptor associated sorting protein 2 (GPRASP2) gene was identified in four hemizygous male patients and two heterozygous female carriers, which was cosegregated with the clinical phenotypes in this family. In silico analysis supported that this gene mutation is functionally deleterious, and it was detected that homologous Gprasp2 was highly expressed in multiple structures of the mouse cochlea, which suggested that GPRASP2 might be the genetic cause for the described disease phenotypes.

Conclusions

This study presented a novel X-linked SHL combined with unique and unrecognised clinical features, and a missense variation of GPRASP2 was first identified to be implicated in X-linked SHL.

Answer to quiz case: Temporal bone imaging

Computed tomographic scanning of the petrous bone and magnetic resonance imaging sequences of the inner ear and cerebellopontine angle of a deaf patient were performed to find an explanation for his deafness, and to establish whether he would be a good candidate for cochlear implantation. The imaging features were considered pathognomonic for incomplete partition type III (IP type III). Further management and discussion of this deafness subtype are detailed.

Prevalence of various etiologies of hearing loss among cochlear implant recipients: Systematic review and meta-analysis

OBJECTIVE

To investigate the etiology of deafness in cochlear implanted children and to address the question whether there is a need for more thorough diagnostics, especially concerning genetics.

DESIGN

Systematic review. Four databases were searched for studies (year 2000-2014) on cochlear implanted children (n > 100). Studies were excluded if etiology had influenced their inclusion criteria. Eligibility and methodological quality were assessed independently by three authors. The studies' description of diagnostic evaluation was categorized in three groups.

STUDY SAMPLE

Sixteen studies were included (5069 children).

RESULTS

The most common etiological categories were 'Unknown' 40.3% (95% CI 32.8 to 48.0), 'Non-syndromic' 22.4% (95% CI 17.1 to 28.2), and 'Postnatal' 11.3% (95% CI 7.2 to 16.2). Studies published after 2006 had a lower proportion of 'Unknown' etiology 35.3% (95% CI 28.0 to 42.8) than older 45.5% (95% CI 31.0 to 60.4). Important information was missing from several studies: 11 (69%) studies did not provide detailed description on diagnostic evaluation of the etiology of deafness and had a higher proportion of 'Unknown' etiology.

CONCLUSIONS

In order to ensure a higher level of comparability in future studies, we recommend agreement upon an international standard of diagnostics and the introduction of an international standard for reporting etiology.

A novel mutation in POU3F4 in a Chinese family with X-linked non-syndromic hearing loss

Objective

Based on the clinical manifestations of a hearing loss patient, the POU3F4 gene was tested for diagnosis of etiology.

Methods

A comprehensive physical examination was performed on the proband to exclude abnormalities of other organs, and detailed audiological testing and temporal bone CT scan were also performed. Genomic DNA was extracted using the proband's peripheral blood leukocytes. Polymerase chain reactions (PCR) were performed in the coding sequence of the POU3F4 gene. Direct DNA sequencing was subsequently applied to screen the entire coding region of the POU3F4 gene.

Results

The proband had severe sensorineural hearing loss. Temporal CT showed bilateral cochlear incomplete partition, vestibule dysplasia, internal auditory canal fundus expansion, and cochlear interlink with the internal auditory canal fundus. A novel mutation (c.530C > A (p.S177X)) in the POU3F4 gene was found in this patient, creating an new stop codon and was predicted to result in a truncated protein lacking normal POU3F4 transcription factor function.

Conclusion

Through analysis of the POU3F4 gene and clinical manifestations in the patient, we conclude that a novel mutation may have resulted in a premature stop codon, contributing to the mutation of POU3F4 gene.

The Genetics of Deafness in Domestic Animals

Although deafness can be acquired throughout an animal's life from a variety of causes, hereditary deafness, especially congenital hereditary deafness, is a significant problem in several species. Extensive reviews exist of the genetics of deafness in humans and mice, but not for deafness in domestic animals. Hereditary deafness in many species and breeds is associated with loci for white pigmentation, where the cochlear pathology is cochleo-saccular. In other cases, there is no pigmentation association and the cochlear pathology is neuroepithelial. Late onset hereditary deafness has recently been identified in dogs and may be present but not yet recognized in other species. Few genes responsible for deafness have been identified in animals, but progress has been made for identifying genes responsible for the associated pigmentation phenotypes. Across species, the genes identified with deafness or white pigmentation patterns include MITF, PMEL, KIT, EDNRB, CDH23, TYR, and TRPM1 in dog, cat, horse, cow, pig, sheep, ferret, mink, camelid, and rabbit. Multiple causative genes are present in some species. Significant work remains in many cases to identify specific chromosomal deafness genes so that DNA testing can be used to identify carriers of the mutated genes and thereby reduce deafness prevalence.

Positive Outcomes and Surgical Strategies for Bilateral Cochlear Implantation in a Child With X-Linked Deafness

OBJECTIVES

To recognize that bilateral cochlear implantation (CI) in X-linked deafness is safe, describe techniques to maximize successful electrode placement and minimize surgical risks, and recognize that normalization of hearing as well as language acquisition is achievable.

METHODS

A 6-month-old male patient presented with bilateral profound sensorineural hearing loss and was confirmed to have X-linked deafness secondary to POU3F4 gene mutation. Due to lack of benefit from amplification, he underwent bilateral CI in a staged fashion at 12 (right) and 15 months (left) of age. A transmastoid-facial recess approach was used bilaterally utilizing perimodiolar electrodes. High-flow gushers were controlled with muscle plugs. C-arm fluoroscopy was used during insertion of electrodes.

RESULTS

Follow-up data are available for 12 months. No complications were encountered, including no postoperative cerebrospinal fluid (CSF) leakage or facial stimulation. Postoperative audiograms in aided conditions showed hearing thresholds <40 dB. At 11 months following activation of his second CI, he scored in the normal range for his chronological age on standardized language measures.

CONCLUSION

With careful preparation and the assistance of intraoperative fluoroscopy, CI in patients with congenital X-linked deafness can be done safely. Performing bilateral CI followed by dedicated auditory-verbal rehabilitation may allow patients to achieve normal language development.

X-linked deafness-2 (DFNX2) phenotype associated with a paracentric inversion upstream of POU3F4

PURPOSE

The authors report on a 7-year-old male, designated FR, who has severe sensorineural hearing loss. Features include a round face, hypertelorism, epicanthal folds, and flat nasal root. Although there were early developmental concerns regarding FR, all but his speech delay resolved when he was placed in an educational program that accommodated his hearing loss. Genetic studies were performed to investigate genetic causes for his hearing loss.

METHOD

History, physical examination, audiologic assessment, and imaging were performed according to usual practice. FMR1,GJB2,GJB6, and POU3F4 genes were sequenced. Chromosomal studies consisted of karyotyping and breakpoint analysis by fluorescence in situ hybridization (FISH).

RESULTS

Results from FMR1,GJB2,GJB6, and POU3F4 sequencing and echocardiography, electrocardiogram, and abdominal ultrasound were normal. A computed tomography (CT) scan revealed a large fundus of the internal auditory canals and absence of the bony partition between the fundus and the adjacent cochlear turns, with a widened modiolus bilaterally. FR's CT findings were consistent with those described in persons with X-linked deafness-2 (DFNX2) hereditary deafness. FR's karyotype was 46,inv(X)(q13q24),Y.ish inv(X)(XIST+)mat. FISH refined the breakpoints to inv(X)(q21.1q22.3). The Xq21.1 breakpoint was narrowed to a 25-kb region 450 kb centromeric to the DFNX2 gene, POU3F4. There are rare case reports of DFNX2 patients with chromosomal rearrangements positioned centromeric to POU3F4 and no mutations within the gene.

CONCLUSION

Authors hypothesized that FR's hearing loss was caused by dysregulation of POU3F4 due to separation from regulatory elements affected by the inversion.

De novo mutation in X-linked hearing loss-associated POU3F4 in a sporadic case of congenital hearing loss

OBJECTIVES

In this report, we present a male patient with no family history of hearing loss, in whom we identified a novel de novo mutation in the POU3F4 gene.

METHODS

One hundred ninety-four (194) Japanese subjects from unrelated and nonconsanguineous families were enrolled in this study. We used targeted genomic enrichment and massively parallel sequencing of all known nonsyndromic hearing loss genes for identifying the genetic causes of hearing loss.

RESULTS

A novel de novo frameshift mutation of POU3F4 to c.727_728insA (p.N244KfsX26) was identified. The patient was a 7-year-old male with congenital progressive hearing loss and inner ear deformity. Although the patient had received a cochlear implant, auditory skills were still limited. The patient also exhibited developmental delays similar to those previously associated with POU3F4 mutation.

CONCLUSION

This is the first report of a mutation in POU3F4 causing hearing loss in a Japanese patient without a family history of hearing loss. This study underscores the importance of comprehensive genetic testing of patients with hearing loss for providing accurate prognostic information and guiding the optimal management of patient rehabilitation.

X-linked hearing loss: two gene mutation examples provide generalizable implications for clinical care

PURPOSE

To describe the inheritance patterns and auditory phenotype features of 3 Canadian families with mutations in 2 X-linked "deafness" genes (DFNX).

METHOD

Audiological, medical, and family histories were collected and family members interviewed to compare hearing thresholds and case histories between cases with mutations in SMPX versus POU3F4.

RESULTS

The family pedigrees reveal characteristic X-linked inheritance patterns. Phenotypic features associated with the SMPX (DFNX4) mutation include early onset in males with rapid progression from mild and flat to sloping sensorineural loss, with highly variable onset and hearing loss severity in females. In contrast, phenotypic features associated with the POU3F4 (DFNX2) mutation are characterized by an early onset, mixed hearing loss with fluctuation in males, and a normal hearing phenotype reported for females.

CONCLUSIONS

The study shows how this unique inheritance pattern and both gender and mutation-specific phenotype variations can alert audiologists to the presence of X-linked genetic etiologies in their clinical practice. By incorporating this knowledge into clinical decision making, audiologists can facilitate the early identification of X-linked hearing loss and contribute to the effective team management of affected families.

[Hereditary hearing loss: Part 1: diagnostic overview and practical advice]

Hearing loss is the most frequently occurring congenital sensory defect in humans. It is believed that between one and five of every 1000 children born suffers from hearing loss of at least 40 dB. The economic consequences of deafness are staggering and affect not only the individual patient but also society as a whole. A genetic cause is suspected in 50 %-70 % of cases of congenital hearing loss. To date over 130 loci have been associated with genetic hearing loss, with some loci containing more than one gene and others containing as yet unidentified genes. The present article is intended to provide some insight into the complex background issues involved and offer guidance on appropriate decision-making with regard to genetic testing in affected patients. Part 1 is concerned with non-syndromic hearing loss, while part 2 deals with syndromic hearing loss.

Cytoplasmic mislocalization of POU3F4 due to novel mutations leads to deafness in humans and mice

POU3F4 is a POU domain transcription factor that is required for hearing. In the ear, POU3F4 is essential for mesenchymal remodeling of the bony labyrinth and is the causative gene for DFNX2 human nonsyndromic deafness. Ear abnormalities underlie this form of deafness, characterized previously in multiple spontaneous, radiation-induced and transgenic mouse mutants. Here, we report three novel mutations in the POU3F4 gene that result in profound hearing loss in both humans and mice. A p.Gln79* mutation was identified in a child from an Israeli family, revealed by massively parallel sequencing (MPS). This strategy demonstrates the strength of MPS for diagnosis with only one affected individual. A second mutation, p.Ile285Argfs*43, was identified by Sanger sequencing. A p.Cys300* mutation was found in an ENU-induced mutant mouse, schwindel (sdl), by positional cloning. The mutation leads to a predicted truncated protein, similar to the human mutations, providing a relevant mouse model. The p.Ile285Argfs*43 and p.Cys300* mutations lead to a shift of Pou3f4 nuclear localization to the cytoplasm, demonstrated in cellular localization studies and in the inner ears of the mutant mice. The discovery of these mutations facilitates a deeper comprehension of the molecular basis of inner ear defects due to mutations in the POU3F4 transcription factor.

Hearing loss and PRPS1 mutations: Wide spectrum of phenotypes and potential therapy

OBJECTIVE

The purpose of this review was to evaluate the current literature on phosphoribosylpyrophosphate synthetase 1 (PRPS1)-related diseases and their consequences on hearing function.

DESIGN

A literature search of peer-reviewed, published journal articles was conducted in online bibliographic databases.

STUDY SAMPLE

Three databases for medical research were included in this review.

RESULTS

Mutations in PRPS1 are associated with a spectrum of non-syndromic to syndromic hearing loss. Hearing loss in male patients with PRPS1 mutations is bilateral, moderate to profound, and can be prelingual or postlingual, progressive or non-progressive. Audiogram shapes associated with PRPS1 deafness are usually residual and flat. Female carriers can have unilateral or bilateral hearing impairment. Gain of function mutations in PRPS1 cause a superactivity of the PRS-I protein whereas the loss-of-function mutations result in X-linked nonsyndromic sensorineural deafness type 2 (DFN2), or in syndromic deafness including Arts syndrome and X-linked Charcot-Marie-Tooth disease-5 (CMTX5).

CONCLUSIONS

Lower residual activity in PRS-I leads to a more severe clinical manifestation. Clinical and molecular findings suggest that the four PRPS1 disorders discovered to date belong to the same disease spectrum. Dietary supplementation with S-adenosylmethionine (SAM) appeared to alleviate the symptoms of Arts syndrome patients, suggesting that SAM could compensate for PRS-I deficiency.

A novel deletion in SMPX causes a rare form of X-linked progressive hearing loss in two families due to a founder effect

X-linked hearing loss is the rarest form of genetic hearing loss contributing to <1% of cases. We identified a multiplex family from Newfoundland (Family 2024) segregating X-linked hearing loss. Haplotyping of the X chromosome and sequencing of positional candidate genes revealed a novel point deletion (c.99delC) in SMPX which encodes a small muscle protein responsible for reducing mechanical stress during muscle contraction. This novel deletion causes a frameshift and a premature stop codon (p.Arg34GlufsX47). We successfully sequenced both SMPX wild-type and mutant alleles from cDNA of a lymphoblastoid cell line, suggesting that the mutant allele may not be degraded via nonsense-mediated mRNA decay. To investigate the role of SMPX in other subpopulations, we fully sequenced SMPX in 229 Canadian probands with hearing loss and identified a second Newfoundland Family (2196) with the same mutation, and a shared haplotype on the X chromosome, suggesting a common ancestor.

Congenital X-linked Stapes Gusher Syndrome in an Infant

Congenital X-linked deafness with stapes gusher is a rare, but well-known clinical entity with characteristic radiological features. Recognition of these findings is important as it changes the treatment course and precludes stapedectomy, which if done can lead to stapes gusher in these patients. We present the case of a nine-month-old male infant with hearing loss. CT scan showed typical radiological features of X-linked stapes gusher syndrome. In addition, the superior semicircular canals appeared dehiscent which could be due to immature ossification or less likely due to defective development as part of the disease. To the best of our knowledge this is the youngest patient prospectively identified by imaging in the English medical literature.

Genetics of hearing loss: where are we standing now?

Hearing loss (HL) is the most common sensory impairment and is caused by a broad range of inherited to environmental causes. Inherited HL consists 50-60% of all HL cases. The inherited form of HL is further classified to different categories. More than 300 syndromes and 40 genes have been identified to result in different levels of HL. Although several diagnostic or screening tests have been developed, yet there are controversies around their use.

Variable degrees of hearing impairment in a Dutch DFNX4 (DFN6) family

OBJECTIVE

Investigation of the audiometric characteristics of a large Dutch DFNX4 family with a p.Glu72X mutation in the SMPX gene.

PATIENTS AND METHODS

Sixty family members participated in this study and examination consisted of medical history, otoscopy, pure tone and speech audiometry. Linkage and mutation analysis revealed a pathogenic mutation in the SMPX gene.

RESULTS

All 25 mutation carriers exhibited hearing impairment, except one woman aged 25 years. The men (n = 10) showed more severe hearing impairment than the women (n = 14) and already at a younger age. The age of onset according to history was 2-10 years (mean: 3.3 years) in men and 3-48 years (mean: 26.4 years) in women. In the men, severe threshold deterioration mainly occurred during the first two decades of life, especially at the higher frequencies. The women showed milder threshold deterioration and more pronounced across-subjects and individual inter-aural variation, especially at 2-8 kHz. Longitudinal linear regression analysis demonstrated significant progression of at least two frequencies in five individuals (3 men and 2 women). The speech recognition scores of the mutation carriers with hearing impairment were decreased at relatively young ages compared to a reference group of patients with only presbycusis, especially in men. However, all these patients tended to have better speech recognition scores than the presbycusis patients at matching PTA(1,2,4 kHz) levels.

CONCLUSION

This study demonstrates the phenotypic heterogeneity in this large family with an X-linked pattern of inherited sensorineural hearing impairment. The men showed more severe hearing impairment at a younger age with more pronounced progression during the first two decades of life, while women demonstrated less severe hearing impairment with more gradual progression and a wider variation in age of onset, degree of hearing impairment and inter-aural asymmetry in thresholds.

Molecular and clinical studies of X-linked deafness among Pakistani families

There are 68 sex-linked syndromes that include hearing loss as one feature and five sex-linked nonsyndromic deafness loci listed in the OMIM database. The possibility of additional such sex-linked loci was explored by ascertaining three unrelated Pakistani families (PKDF536, PKDF1132, PKDF740) segregating X-linked recessive deafness. Sequence analysis of POU3F4 (DFN3) in affected members of families PKDF536 and PKDF1132 revealed two novel nonsense mutations, p.Q136X and p.W114X, respectively. Family PKDF740 is segregating congenital blindness, mild to profound progressive hearing loss that is characteristic of Norrie disease (MIM#310600). Sequence analysis of NDP among affected members of this family revealed a novel single nucleotide deletion c.49delG causing a frameshift and premature truncation (p.V17fsX1) of the encoded protein. These mutations were not found in 150 normal DNA samples. Identification of pathogenic alleles causing X-linked recessive deafness will improve molecular diagnosis, genetic counseling, and molecular epidemiology of hearing loss among Pakistanis.

Pediatric sensorineural hearing loss, part 2: syndromic and acquired causes

This article is the second in a 2-part series reviewing neuroimaging in childhood SNHL. Previously, we discussed the clinical work-up of children with hearing impairment, the classification of inner ear malformations, and congenital nonsyndromic causes of hearing loss. Here, we review and illustrate the most common syndromic hereditary and acquired causes of childhood SNHL, with an emphasis on entities that demonstrate inner ear abnormalities on cross-sectional imaging. Syndromes discussed include BOR syndrome, CHARGE syndrome, Pendred syndrome, Waardenburg syndrome, and X-linked hearing loss with stapes gusher. We conclude the article with a review of acquired causes of childhood SNHL, including infections, trauma, and neoplasms.

Nonsense mutations in SMPX, encoding a protein responsive to physical force, result in X-chromosomal hearing loss

The fact that hereditary hearing loss is the most common sensory disorder in humans is reflected by, among other things, an extraordinary allelic and nonallelic genetic heterogeneity. X-chromosomal hearing impairment represents only a minor fraction of all cases. In a study of a Spanish family the locus for one of the X-chromosomal forms was assigned to Xp22 (DFNX4). We mapped the disease locus in the same chromosomal region in a large German pedigree with X-chromosomal nonsyndromic hearing impairment by using genome-wide linkage analysis. Males presented with postlingual hearing loss and onset at ages 3-7, whereas onset in female carriers was in the second to third decades. Targeted DNA capture with high-throughput sequencing detected a nonsense mutation in the small muscle protein, X-linked (SMPX) of affected individuals. We identified another nonsense mutation in SMPX in patients from the Spanish family who were previously analyzed to map DFNX4. SMPX encodes an 88 amino acid, cytoskeleton-associated protein that is responsive to mechanical stress. The presence of Smpx in hair cells and supporting cells of the murine cochlea indicates its role in the inner ear. The nonsense mutations detected in the two families suggest a loss-of-function mechanism underlying this form of hearing impairment. Results obtained after heterologous overexpression of SMPX proteins were compatible with this assumption. Because responsivity to physical force is a characteristic feature of the protein, we propose that long-term maintenance of mechanically stressed inner-ear cells critically depends on SMPX function.

Identification of a novel mutation in POU3F4 for prenatal diagnosis in a Chinese family with X-linked nonsyndromic hearing loss

We present the clinical and genetic findings for a Chinese family with X-linked non-syndromic hearing loss in which the affected males showed congenital profound sensorineural hearing impairment. In two affected brothers, the computer tomography of temporal bone showed bilateral dilation of the internal auditory canal with fistulous communication between the lateral canal and the basal cochlear turn, which is consistent with the typical DFNX2 phenotype. A missense mutation (c.647G→A) in the POU3F4 gene caused a substitution from glycine to glutamic acid at position 216 (p.G216E), and this mutation was found to consistently cosegregate with the deafness phenotype in the family. The mutation resulted in the loss of function of the POU3F4 by decreasing the affinity between the protein and DNA, as shown in silico by the structural analysis. Prenatal diagnosis of pregnant proband of this family revealed the c.647G→A mutation in DNA extracted from the amniotic fluid surrounding the fetus. The appropriate use of genetic testing and prenatal diagnosis plays a key role in reducing the recurrence of genetic defects in high-risk families.

Clinical evaluation of DFN3 patients with deletions in the POU3F4 locus and detection of carrier female using MLPA

X-linked deafness type 3 (DFN3), the most prevalent X-linked form of hereditary deafness, is caused by mutations of the POU3F4 locus in the Xq21 region. We evaluated two Korean families showing typical characteristics of DFN3, such as congenital hearing loss and pathognomonic inner ear anomalies. Genetic analysis of these families did not reveal any mutations in the POU3F4 coding sequence. Instead, one family carried a genomic deletion upstream of POU3F4 gene, where the regulatory element is predicted to reside, and the other family possessed a deletion of almost the entire Xq21 region. The lack of mutation in the POU3F4 coding sequence makes the detection of carrier females using conventional sequencing methods difficult. By applying the multiplex ligation-dependent probe amplification (MLPA) method, we successfully determined the carrier status of female members in these families, demonstrating that MLPA is a rapid and accurate way to detect POU3F4 deletions in sporadic undiagnosed carriers of DNF3.

Hearing loss: a common disorder caused by many rare alleles

Perception of sound is a fundamental role of the auditory system. Traveling with the force of their mechanical energy, sound waves are captured by the ear and activate the sensory pathway of this complex organ. The hair cells, specialized sensory cells within the inner ear, transmit the mechanical energy into electrical nerve stimuli that reach the brain. A large number of proteins are responsible for the overarching tasks required to maintain the complex mechanism of sound sensation. Many hearing disorders are due to single gene defects inherited in a Mendelian fashion, thus enabling clinical diagnostics. However, at the same time, hearing impairment is genetically heterogeneous, with both common and rare forms occurring due to mutations in over 100 genes. The crosstalk between human and mouse genetics has enabled comprehensive studies on gene identification and protein function, taking advantage of the tools animal models have to offer. The aim of the following review is to provide background and examples of human deafness genes and the discovery of their function in the auditory system.

Pou3f4 deficiency causes defects in otic fibrocytes and stria vascularis by different mechanisms

DFN3, the most prevalent X-linked hearing loss, is caused by mutations in the POU3F4 gene. Previous studies in Pou3f4 knockout mice suggest that defective otic fibrocytes in the spiral ligament of the cochlear lateral wall may underlie the hearing loss in DFN3. To better understand the pathological mechanisms of the DFN3 hearing loss, we analyzed inner ears of Pou3f4-deficient mice during development. Our results indicate that compartmentalization of the spiral ligament mesenchyme setting up boundaries for specific otic fibrocytes occurs normally in Pou3f4-deficient cochlea. However, differentiation of the compartmentalized mesenchyme into specific otic fibrocytes was blocked in the absence of Pou3f4 function. In addition, we found that stria vascularis in the cochlear lateral wall was also affected in Pou3f4-deficient cochlea. Unlike the otic fibrocytes, differentiation of stria vascularis was completed in the absence of Pou3f4 function, yet expression of Kir4.1 channels in the strial intermediate cells, essential for the sound transduction, was lost afterwards. These results suggest that Pou3f4 deficiency causes defects in both otic fibrocytes and stria vascularis at different developmental stages and by different pathological mechanisms, which may account for the progressive nature of DFN3 hearing loss.