A novel locus (DFNA23) for prelingual autosomal dominant nonsyndromic hearing loss maps to 14q21-q22 in a Swiss German kindred

Auricular fistula: a review of its clinical manifestations, genetics, and treatments

Auricular fistula is a common congenital auricular malformation, characterized as a small opening in the skin and a subcutaneous cyst. It can be classified in different ways according to positions of pits and directions of fistula tracts. The term preauricular fistula and variant type of preauricular fistula (postauricular fistula) are used. Auricular fistula prevalence varies in countries and populations, and its actual prevalence is presently unknown. The most accepted and widely cited theory of auricular fistula etiopathogenesis is an incorrect or incomplete fusion of six auricular hillocks that are mesenchymal proliferations. Auricular fistula can occur either sporadically or genetically. The pattern in inherited cases is thought to be incomplete autosomal dominant, with variable expressions, reduced penetrance, and inapparent gender differences. Auricular fistula has several forms and is reported as being a component of many syndromes. In the field of genetics, currently, there is no related review to comprehensively summarize the genetic basis of auricular fistula and related disorders. This article provides a comprehensive review of auricular fistula, especially congenital preauricular fistula, which accounts for the majority of auricular fistula, by summarizing the clinical manifestations, histological and embryological development, genetics, examinations, and treatments, as well as syndromes with auricular fistula.

Autosomal Dominant Non-Syndromic Hearing Loss (DFNA): A Comprehensive Narrative Review

Autosomal dominant non-syndromic hearing loss (HL) typically occurs when only one dominant allele within the disease gene is sufficient to express the phenotype. Therefore, most patients diagnosed with autosomal dominant non-syndromic HL have a hearing-impaired parent, although de novo mutations should be considered in all cases of negative family history. To date, more than 50 genes and 80 loci have been identified for autosomal dominant non-syndromic HL. DFNA22 (MYO6 gene), DFNA8/12 (TECTA gene), DFNA20/26 (ACTG1 gene), DFNA6/14/38 (WFS1 gene), DFNA15 (POU4F3 gene), DFNA2A (KCNQ4 gene), and DFNA10 (EYA4 gene) are some of the most common forms of autosomal dominant non-syndromic HL. The characteristics of autosomal dominant non-syndromic HL are heterogenous. However, in most cases, HL tends to be bilateral, post-lingual in onset (childhood to early adulthood), high-frequency (sloping audiometric configuration), progressive, and variable in severity (mild to profound degree). DFNA1 (DIAPH1 gene) and DFNA6/14/38 (WFS1 gene) are the most common forms of autosomal dominant non-syndromic HL affecting low frequencies, while DFNA16 (unknown gene) is characterized by fluctuating HL. A long audiological follow-up is of paramount importance to identify hearing threshold deteriorations early and ensure prompt treatment with hearing aids or cochlear implants.

A novel heterozygous SIX1 missense mutation resulted in non-syndromic unilateral hearing loss

Familial non-syndromic unilateral hearing loss (NS-UHL) is rare and its genetic etiology has not been clearly elucidated. This study aimed to identify the genetic cause of NS-UHL in a three-generation Chinese family. Detailed medical history consultation and clinical examination were conducted. Further, whole-exome sequencing (WES) was performed to identify the genetic etiology of the proband, and the variant was verified by Sanger sequencing. A novel missense mutation, c.533G>C (p.Arg178Thr), in the SIX homeobox 1 gene (SIX1) was identified in four patients and co-segregated with NS-UHL in a three-generation Chinese family as a dominant trait. Using bioinformatics analyses, we show that this novel mutation is pathogenic and affects the structure of SIX1 protein. These data suggest that mutations in SIX1 gene are associated with NS-UHL. Our study added the NS-UHL phenotype associated with SIX1, and thereby improving the genetic counseling provided to individuals with SIX1 mutations.

Update on CD164 and LMX1A genes to strengthen their causative role in autosomal dominant hearing loss

Novel hearing loss (HL) genes are constantly being discovered, and evidence from independent studies is essential to strengthen their position as causes of hereditary HL. To address this issue, we searched our genetic data of families with autosomal dominant HL (ADHL) who had been tested with high-throughput DNA sequencing methods. For CD164, only one pathogenic variant in one family has so far been reported. For LMX1A, just two previous studies have revealed its involvement in ADHL. In this study we found two families with the same pathogenic variant in CD164 and one family with a novel variant in LMX1A (c.686C>A; p.(Ala229Asp)) that impairs its transcriptional activity. Our data show recurrence of the same CD164 variant in two HL families of different geographic origin, which strongly suggests it is a mutational hotspot. We also provide further evidence for haploinsufficiency as the pathogenic mechanism underlying LMX1A-related ADHL.

SIX1 cooperates with RUNX1 and SMAD4 in cell fate commitment of Müllerian duct epithelium

During female mammal reproductive tract development, epithelial cells of the lower Müllerian duct are committed to become stratified squamous epithelium of the vagina and ectocervix, when the expression of ΔNp63 transcription factor is induced by mesenchymal cells. The absence of ΔNp63 expression leads to adenosis, the putative precursor of vaginal adenocarcinoma. Our previous studies with genetically engineered mouse models have established that fibroblast growth factor (FGF)/mitogen-activated protein kinase (MAPK), bone morphogenetic protein (BMP)/SMAD, and activin A/runt-related transcription factor 1 (RUNX1) signaling pathways are independently required for ΔNp63 expression in Müllerian duct epithelium (MDE). Here, we report that sine oculis homeobox homolog 1 (SIX1) plays a critical role in the activation of ΔNp63 locus in MDE as a downstream transcription factor of mesenchymal signals. In the developing mouse reproductive tract, SIX1 expression was restricted to MDE within the future cervix and vagina. SIX1 expression was totally absent in SMAD4 null MDE and was reduced in RUNX1 null and FGFR2 null MDE, indicating that SIX1 is under the control of vaginal mesenchymal factors: BMP4, activin A and FGF7/10. Furthermore, Six1, Runx1, and Smad4 gene-dose-dependently activated ΔNp63 expression in MDE within the vaginal fornix. Using a mouse model of diethylstilbestrol (DES)-associated vaginal adenosis, we found DES action through epithelial estrogen receptor α (ESR1) inhibits activation of ΔNp63 locus in MDE by transcriptionally repressing SIX1 and RUNX1 in the vaginal fornix.

Genetic Hearing Loss and Gene Therapy

Genetic hearing loss crosses almost all the categories of hearing loss which includes the following: conductive, sensory, and neural; syndromic and nonsyndromic; congenital, progressive, and adult onset; high-frequency, low-frequency, or mixed frequency; mild or profound; and recessive, dominant, or sex-linked. Genes play a role in almost half of all cases of hearing loss but effective treatment options are very limited. Genetic hearing loss is considered to be extremely genetically heterogeneous. The advancements in genomics have been instrumental to the identification of more than 6,000 causative variants in more than 150 genes causing hearing loss. Identification of genes for hearing impairment provides an increased insight into the normal development and function of cells in the auditory system. These defective genes will ultimately be important therapeutic targets. However, the auditory system is extremely complex which requires tremendous advances in gene therapy including gene vectors, routes of administration, and therapeutic approaches. This review summarizes and discusses recent advances in elucidating the genomics of genetic hearing loss and technologies aimed at developing a gene therapy that may become a treatment option for in the near future.

Human eye conditions: insights from the fly eye

The fruit fly Drosophila melanogaster has served as an excellent model to study and understand the genetics of many human diseases from cancer to neurodegeneration. Studying the regulation of growth, determination and differentiation of the compound eyes of this fly, in particular, have provided key insights into a wide range of diseases. Here we review the regulation of the development of fly eyes in light of shared aspects with human eye development. We also show how understanding conserved regulatory pathways in eye development together with the application of tools for genetic screening and functional analyses makes Drosophila a powerful model to diagnose and characterize the genetics underlying many human eye conditions, such as aniridia and retinitis pigmentosa. This further emphasizes the importance and vast potential of basic research to underpin applied research including identifying and treating the genetic basis of human diseases.

Discovery of MYH14 as an important and unique deafness gene causing prelingually severe autosomal dominant nonsyndromic hearing loss

BACKGROUND

Pathogenic variants of MYH14 are known to be associated (in either a syndromic or nonsyndromic manner) with hearing loss. Interestingly, all reported cases to date of MYH14-related nonsyndromic hearing loss with detailed phenotypes have demonstrated mild-to-moderate progressive hearing loss with postlingual onset.

METHODS

In the present study, targeted resequencing (TRS) of known deafness genes was performed to identify the causative variant in two multiplex families segregating autosomal dominant (AD) inherited hearing loss.

RESULTS

TRS uncovered two novel variants of MYH14 (c.A572G: p.Asp191Gly in the myosin head domain and c.C73T:p.Gln25* in exon 2) from two multiplex deafness Korean families. Notably, both probands showed phenotypes of congenital or prelingual severe hearing loss. It is remarkably uncommon to encounter such a severe-to-profound, prelingual, AD hearing loss. Given that the first variant, p. Asp191Gly, was the first documented missense allele discovered in the myosin head domain of this gene related to either congenital or prelingual severe nonsyndromic hearing loss, and also that the second variant, p. Gln25*, lead to a null allele, more severe phenotypes from our probands may have been the result of either genotype-phenotype correlation or genetic backgrounds, or both.

CONCLUSIONS

In the present study, we report that MYH14 can manifest as nonsyndromic prelingual severe sensorineural hearing loss in an AD fashion in Koreans. The results of the present study suggest that further genetic studies of similar patients should consider MYH14 as a causative gene, and cochlear implantation during infant or early childhood should be indicated for those patients with certain MYH14 pathogenic variants.

c.G2114A MYH9 mutation (DFNA17) causes non-syndromic autosomal dominant hearing loss in a Brazilian family

We studied a family presenting 10 individuals affected by autosomal dominant deafness in all frequencies and three individuals affected by high frequency hearing loss. Genomic scanning using the 50k Affymetrix microarray technology yielded a Lod Score of 2.1 in chromosome 14 and a Lod Score of 1.9 in chromosome 22. Mapping refinement using microsatellites placed the chromosome 14 candidate region between markers D14S288 and D14S276 (8.85 cM) and the chromosome 22 near marker D22S283. Exome sequencing identified two candidate variants to explain hearing loss in chromosome 14 [PTGDR – c.G894A:p.R298R and PTGER2 – c.T247G:p.C83G], and one in chromosome 22 [MYH9, c.G2114A:p.R705H]. Pedigree segregation analysis allowed exclusion of the PTGDR and PTGER2 variants as the cause of deafness. However, the MYH9 variant segregated with the phenotype in all affected members, except the three individuals with different phenotype. This gene has been previously described as mutated in autosomal dominant hereditary hearing loss and corresponds to DFNA17. The mutation identified in our study is the same described in the prior report. Thus, although linkage studies suggested a candidate gene in chromosome 14, we concluded that the mutation in chromosome 22 better explains the hearing loss phenotype in the Brazilian family.

EYA1-related disorders: two clinical cases and a literature review

OBJECTIVES

To delineate the diagnostic and rehabilitative aspects of syndromes that have overlapping features, we present the cases of two unrelated Caucasian males affected by hearing impairment, preauricular pits and cervical fistulae. Specific findings that are helpful in the diagnosis and management of EYA1-related disorders are highlighted.

METHODS

Genetic, otologic, imaging, eye and renal evaluations were conducted to achieve a detailed and comprehensive assessment, leading to the most accurate diagnosis and appropriate treatment. A literature review was also carried out.

RESULTS

Diagnostic criteria indicated that the two patients were affected by BOS1 (Branchio-Otic Syndrome 1). We also identified a novel sporadic missense mutation in the EYA1 gene: p.G533R (c.1597G>A, NM_000503.4), a highly conserved, heterozygotic amino acid substitution. In the other case, we identified the p.X593QextX6 (c.1777T>A, NM_000503.4) substitution. Both variants lead to isoform 1 (EYA1B and EYA1C) which is composed of 592 amino acids. Clinical and in silico evidence suggests a pathogenic role for the new mutations. Imaging evaluation revealed a complex pathology, characterized by external, inner and middle ear malformations, without renal anomalies.

CONCLUSIONS

Our results demonstrate the importance of considering the imaging evaluation and the complete DNA sequencing of the EYA1 gene for the differential diagnosis of deafness and related branchio-oto-renal disorders.

A novel dominant mutation in SIX1, affecting a highly conserved residue, result in only auditory defects in humans

Branchio-oto-renal (BOR) and Branchio-otic (BO) syndromes are dominant disorders characterized by variable hearing impairment (HI) and branchial defects. BOR includes additional kidney malformations. BO/BOR syndromes are genetically heterogeneous and caused by mutations in EYA1 and SIX1 genes. Mutation in SIX1 is responsible also for DFNA23, a locus for non-syndromic HI. Strikingly, the severity of the phenotype did not seem to correlate with the type of SIX1 mutation. Herein, we identified a novel mutation in SIX1 (p.E125K) in a Tunisian family with variable HI and preauricular pits. This mutation is located at the same position as the mutation identified in the Catwhesel (Cwe) mouse. No renal and branchial defects were observed in our family nor in Cwe/+ mice. A homology model revealed that the replacement of the Glutamate by a Lysine alters the electrostatic potential surface propriety which may affect the DNA-binding activity.

Audiovestibular findings in a branchio-oto syndrome patient with a SIX1 mutation

CONCLUSION

A reported mutation in SIX1 was identified in a patient with familial hearing loss (HL), a left preauricular pit, and bilateral enlarged vestibular aqueducts (EVA). Although the characteristic symptoms of EVA including fluctuating HL and repetitive vertigo were not seen in the patient, further studies are needed to clarify the association between EVA and such symptoms.

OBJECTIVES

To study the audiovestibular functions, and to identify the causative gene in a patient with branchio-oto syndrome.

METHODS

We enrolled a 30-year-old female in whom HL was pointed out at the age of 6 years. She visited our department at the age of 21 years, and had not experienced any progression of her HL, tinnitus, or vertigo. Pure-tone audiograms showed bilateral moderate mixed HL with no apparent progression during a 9-year follow-up period. Audiovestibular examinations included distortion product otoacoustic emissions (DPOAEs), electrocochleography (ECochG), and electronystagmography (ENG). Direct sequencing was utilized to screen for SIX1, EYA1, SLC26A4, GJB2, and mitochondrial DNA MTRNR1 including 1555 position.

RESULTS

The findings of DPOAEs, ECochG, and ENG indicated cochlear HL with no vestibular dysfunction. A previously reported mutation of a heterozygous c.386A > G (p.Y129C) in SIX1 was detected. No mutation was identified in EYA1, SLC26A4, GJB2, or MTRNR1.

Modifiers of hearing impairment in humans and mice

Lack of penetrance and variability of expression are common findings in nonsyndromic hearing loss with autosomal dominant mode of inheritance, but are also seen with recessive inheritance. Now we know that genotype cannot necessarily predict phenotype due to the complexity of the genome, the proteome interacting with the transcriptome, and the dynamically coupled systems that are involved. The contribution of genetic background to phenotypic diversity reflects the additive and interactive (epistasis) effects of multiple genes. Because, individual genes do not act alone but rather in concert with many other genes, it is not surprising that, modifier genes are common source of phenotypic variation in human populations. They can affect the phenotypic outcome of a given genotype by interacting in the same or in a parallel biological pathway as the disease gene. These modifier genes modulate penetrance, dominance, pleiotropy or expressivity in individuals with Mendelian traits and can also be exerted by influencing the severity, the penetrance, the age of onset and the progression of a disease. In this review, we focus on modifier genes that specifically affect hearing loss phenotypes in humans as well as those described in mice. We also include examples of digenic inheritance of deafness, because additive or interactive effects can also result from interaction between two mutant genes.

Branchio-oto-renal syndrome: detection of EYA1 and SIX1 mutations in five out of six Danish families by combining linkage, MLPA and sequencing analyses

The branchio-oto-renal (BOR) syndrome is an autosomal-dominant disorder characterized by hearing loss, branchial and renal anomalies. BOR is genetically heterogeneous and caused by mutations in EYA1 (8q13.3), SIX1 (14q23.1), SIX5 (19q13.3) and in an unidentified gene on 1q31. We examined six Danish families with BOR syndrome by assessing linkage to BOR loci, by performing EYA1 multiplex ligation-dependent probe amplification (MLPA) analysis for deletions and duplications and by sequencing of EYA1, SIX1 and SIX5. We identified four EYA1 mutations (c.920delG, IVS10-1G>A, IVS12+4A>G and p.Y591X) and one SIX1 mutation (p.W122R), providing a molecular diagnosis in five out of the six families (83%). The present, yet preliminary, observation that renal and temporal bone malformations are less frequent in SIX1-related disease suggests a slightly different clinical profile compared to EYA1-related disease. Unidentified mutations impairing mRNA expression or further genetic heterogeneity may explain the lack of mutation finding in one family despite LOD score indications of EYA1 involvement.

The role of bone morphogenetic protein 4 in inner ear development and function

Bone Morphogenetic Protein 4 (BMP4) is a member of the TGF-beta superfamily and is known to be important for the normal development of many tissues and organs, including the inner ear. Bmp4 homozygous null mice die as embryos, but Bmp4 heterozygous null (Bmp4(+/-)) mice are viable and some adults exhibit a circling phenotype, suggestive of an inner ear defect. To understand the role of BMP4 in inner ear development and function, we have begun to study C57BL/6 Bmp4(+/-) mice. Quantitative testing of the vestibulo-collic reflex, which helps maintain head stability, demonstrated that Bmp4(+/-) mice that exhibit circling behavior have a poor response in the yaw axis, consistent with semicircular canal dysfunction. Although the hair cells of the ampullae were grossly normal, the stereocilia were greatly reduced in number. Auditory brainstem responses showed that Bmp4(+/-) mice have elevated hearing thresholds and immunohistochemical staining demonstrated decreased numbers of neuronal processes in the organ of Corti. Thus Bmp4(+/-) mice have structural and functional deficits in the inner ear.

A novel locus for autosomal dominant non-syndromic deafness, DFNA53, maps to chromosome 14q11.2-q12

BACKGROUND

Non-syndromic hearing loss is among the most genetically heterogeneous traits known in humans. To date, at least 50 loci for autosomal dominant non-syndromic sensorineural hearing loss (ADNSSHL) have been identified by linkage analysis.

OBJECTIVE

To report the mapping of a novel autosomal dominant deafness locus on the long arm of chromosome 14 at 14q11.2-q12, DFNA53, in a large multigenerational Chinese family with post-lingual, high frequency hearing loss that progresses to involve all frequencies.

RESULTS

A maximum multipoint LOD score of 5.4 was obtained for marker D14S1280. The analysis of recombinant haplotypes mapped DFNA53 to a 9.6 cM region interval between markers D14S581 and D14S1021. Four deafness loci (DFNA9, DFNA23, DFNB5, and DFNB35) have previously been mapped to the long arm of chromosome 14. The critical region for DFNA53 contains the gene for DFNA9 but does not overlap with the regions for DFNB5, DFNA23, or DFNB35. Screening of the COCH gene (DFNA9), BOCT, EFS, and HSPC156 within the DFNA53 interval did not identify the cause for deafness in this family.

CONCLUSIONS

Identifying the DFNA53 locus is the first step in isolating the gene responsible for hearing loss in this large multigeneration Chinese family.

Nuclear and mitochondrial genes mutated in nonsyndromic impaired hearing

Half of the cases with congenital impaired hearing are hereditary (HIH). HIH may occur as part of a multisystem disease (syndromic HIH) or as disorder restricted to the ear and vestibular system (nonsyndromic HIH). Since nonsyndromic HIH is almost exclusively caused by cochlear defects, affected patients suffer from sensorineural hearing loss. One percent of the total human genes, i.e. 300-500, are estimated to cause syndromic and nonsyndromic HIH. Of these, approximately 120 genes have been cloned thus far, approximately 80 for syndromic HIH and 42 for nonsyndromic HIH. In the majority of the cases, HIH manifests before (prelingual), and rarely after (postlingual) development of speech. Prelingual, nonsyndromic HIH follows an autosomal recessive trait (75-80%), an autosomal dominant trait (10-20%), an X-chromosomal, recessive trait (1-5%), or is maternally inherited (0-20%). Postlingual nonsyndromic HIH usually follows an autosomal dominant trait. Of the 41 mutated genes that cause nonsyndromic HIH, 15 cause autosomal dominant HIH, 15 autosomal recessive HIH, 6 both autosomal dominant and recessive HIH, 2 X-linked HIH, and 3 maternally inherited HIH. Mutations in a single gene may not only cause autosomal dominant, nonsyndromic HIH, but also autosomal recessive, nonsyndromic HIH (GJB2, GJB6, MYO6, MYO7A, TECTA, TMC1), and even syndromic HIH (CDH23, COL11A2, DPP1, DSPP, GJB2, GJB3, GJB6, MYO7A, MYH9, PCDH15, POU3F4, SLC26A4, USH1C, WFS1). Different mutations in the same gene may cause variable phenotypes within a family and between families. Most cases of recessive HIH result from mutations in a single locus, but an increasing number of disorders is recognized, in which mutations in two different genes (GJB2/GJB6, TECTA/KCNQ4), or two different mutations in a single allele (GJB2) are involved. This overview focuses on recent advances in the genetic background of nonsyndromic HIH.

Early onset and rapid progression of dominant nonsyndromic DFNA36 hearing loss

OBJECTIVE

To characterize the auditory and vestibular phenotype of autosomal dominant nonsyndromic DFNA36 hearing loss.

STUDY DESIGN

Clinical evaluation of individuals with DFNA36 hearing loss linked to the D572N mutation of transmembrane channel-like gene 1 (TMC1). Medical history interviews, physical examinations, and pure-tone air conduction audiometry were performed in the field. Audiology and radiology reports were available and retrospectively reviewed for a subset of subjects.

SETTING

Primary, secondary, and tertiary referral centers (retrospectively reviewed studies); subjects' homes (prospective clinical evaluations).

PATIENTS

Thirteen affected members of a North American Caucasian family segregating DFNA36 hearing loss.

MAIN OUTCOME MEASURES

Pure-tone audiometric thresholds and their rates of progression.

RESULTS

Subjects had bilateral, symmetric, sensorineural hearing loss with a postlingual onset in the first decade of life. High frequencies were initially affected, followed by rapid progression (5.9 dB/yr for the 0.5/1/2/4-kHz pure-tone average) to profound deafness across all frequencies by the second decade of life. Two individuals had excellent auditory-verbal communication after rehabilitation with cochlear implants placed over two decades after total deafening.

CONCLUSIONS

DFNA36 has one of the earliest onsets and most rapid rates of progression among the autosomal dominant non-syndromic hearing loss phenotypes. These distinctive features should facilitate its clinical detection and the development of clinical-molecular genetic diagnostic algorithms for dominant nonsyndromic hearing loss.

Identification and characterization of two novel brain-derived immunoglobulin superfamily members with a unique structural organization

We recently used a differential display PCR screen to identify secreted and transmembrane proteins that are highly expressed in the developing rat basilar pons, a prominent ventral hindbrain nucleus used as a model for studies of neuronal migration, axon outgrowth, and axon-target recognition. Here we describe cloning and characterization of one of these molecules, now called MDGA1, and a closely related homologue, MDGA2. Analyses of the full-length coding region of MDGA1 and MDGA2 indicate that they encode proteins that comprise a novel subgroup of the Ig superfamily and have a unique structural organization consisting of six immunoglobulin (Ig)-like domains followed by a single MAM domain. Biochemical characterization demonstrates that MDGA1 and MDGA2 proteins are highly glycosylated, and that MDGA1 is tethered to the cell membrane by a GPI anchor. The MDGAs are differentially expressed by subpopulations of neurons in both the central and peripheral nervous systems, including neurons of the basilar pons, inferior olive, cerebellum, cerebral cortex, olfactory bulb, spinal cord, and dorsal root and trigeminal ganglia. Little or no MDGA expression is detected outside of the nervous system of developing rats. The similarity of MDGAs to other Ig-containing molecules and their temporal-spatial patterns of expression within restricted neuronal populations, for example migrating pontine neurons and D1 spinal interneurons, suggest a role for these novel proteins in regulating neuronal migration, as well as other aspects of neural development, including axon guidance.

SIX1 mutations cause branchio-oto-renal syndrome by disruption of EYA1-SIX1-DNA complexes

Urinary tract malformations constitute the most frequent cause of chronic renal failure in the first two decades of life. Branchio-otic (BO) syndrome is an autosomal dominant developmental disorder characterized by hearing loss. In branchio-oto-renal (BOR) syndrome, malformations of the kidney or urinary tract are associated. Haploinsufficiency for the human gene EYA1, a homologue of the Drosophila gene eyes absent (eya), causes BOR and BO syndromes. We recently mapped a locus for BOR/BO syndrome (BOS3) to human chromosome 14q23.1. Within the 33-megabase critical genetic interval, we located the SIX1, SIX4, and SIX6 genes, which act within a genetic network of EYA and PAX genes to regulate organogenesis. These genes, therefore, represented excellent candidate genes for BOS3. By direct sequencing of exons, we identified three different SIX1 mutations in four BOR/BO kindreds, thus identifying SIX1 as a gene causing BOR and BO syndromes. To elucidate how these mutations cause disease, we analyzed the functional role of these SIX1 mutations with respect to protein-protein and protein-DNA interactions. We demonstrate that all three mutations are crucial for Eya1-Six1 interaction, and the two mutations within the homeodomain region are essential for specific Six1-DNA binding. Identification of SIX1 mutations as causing BOR/BO offers insights into the molecular basis of otic and renal developmental diseases in humans.

A novel autosomal recessive non-syndromic deafness locus (DFNB35) maps to 14q24.1-14q24.3 in large consanguineous kindred from Pakistan

Autosomal recessive nonsyndromic deafness is one of the most frequent forms of inherited hearing impairment. Over 30 autosomal recessive nonsyndromic hearing loss loci have been mapped, and 15 genes have been isolated. Of the over 30 reported autosomal recessive nonsyndromic hearing loss (NSHL) loci, the typical phenotype is prelingual non-progressive severe to profound hearing loss with the exception of DFNB8, which displays postlingual onset and DFNB13, which is progressive. In this report we describe a large inbred kindred from a remote area of Pakistan, comprising six generations and segregating autosomal recessive nonsyndromic prelingual deafness. DNA samples from 24 individuals were used for genome wide screen and fine mapping. Linkage analysis indicates that in this family the NSHL locus, (DFNB35) maps to a 17.54 cM region on chromosome 14 flanked by markers D14S57 and D14S59. Examination of haplotypes reveals a region that is homozygous for 11.75 cM spanning between markers D14S588 and D14S59. A maximum two-point LOD score of 5.3 and multipoint LOD score of 7.6 was obtained at marker D14S53. The interval for DFNB35 does not overlap with the regions for DFNA9, DFNA23 or DFNB5.

Non-syndromic autosomal-dominant deafness

Non-syndromic deafness is a paradigm of genetic heterogeneity. More than 70 loci have been mapped, and 25 of the nuclear genes responsible for non-syndromic deafness have been identified. Autosomal-dominant genes are responsible for about 20% of the cases of hereditary non-syndromic deafness, with 16 different genes identified to date. In the present article we review these 16 genes, their function and their contribution to deafness in different populations. The complexity is underlined by the fact that several of the genes are involved in both dominant and recessive non-syndromic deafness or in both non-syndromic and syndromic deafness. Mutations in eight of the genes have so far been detected in only single dominant deafness families, and their contribution to deafness on a population base might therefore be limited, or is currently unknown. Identification of all genes involved in hereditary hearing loss will help in the understanding of the basic mechanisms underlying normal hearing, will facilitate early diagnosis and intervention and might offer opportunities for rational therapy.

Genetics and molecular biology of deafness. Update

This article discusses the latest research in the molecular biology and genetics of hearing impairment and its importance to otolaryngologists. Recent research has led to the discovery of many of the genes and gene products that are responsible for hereditary hearing impairment. State mandated screening of newborn infants for hearing loss ensures that a large number of hearing-impaired children will be detected at a very early age. Additionally, these children often will be referred to the otolaryngologist for evaluation of the hearing impairment. It is the otolaryngologist who must gather a detailed family history and perform a thorough physical examination to fully assess the cause of the hearing impairment. In taking the family history, it is important to note that the diagnosis of a hereditary hearing impairment often involves the evaluation of a large-sized family that has a history of hearing disorders. A history of an affected individual in a small family does not necessarily support a diagnosis of hearing impairment in later affected offspring because of the small sample size. Often, a hearing impairment that is part of a syndrome may not be detected because the physical findings associated with a syndrome are subtle in a young infant. For example, the white forelock seen in patients with Waardenburg's syndrome type I cannot be visualized in the infant who lacks hair. Additionally, some patients with syndromic hearing impairment do not present with physical findings, but rather they exhibit abnormal laboratory studies. Additional points to remember include the following: As infectious iatrogenic causes of hearing impairment decrease, the relative incidence of hereditary hearing impairment will increase. Hereditary hearing impairment can present as an isolated finding, or in association with a number of anomalies recognizable as a syndrome. The study of genetics and molecular biology has led to the identification of genes associated with hearing impairment and will allow for future screening and possible therapy for the hearing-impaired. The screening of newborns for hearing impairment using the techniques of molecular biologists and geneticists will result in early identification and appropriate intervention for those at risk for hereditary hearing impairment. An understanding of the syndromic and nonsyndromic causes of hereditary hearing impairment can help the otolaryngologist make a diagnosis and provide appropriate audiologic and educational management to the patient.