An integrated genetic and functional analysis of the role of type II transmembrane serine proteases (TMPRSSs) in hearing loss

Building on our discovery that mutations in the transmembrane serine protease, TMPRSS3, cause nonsyndromic deafness, we have investigated the contribution of other TMPRSS family members to the auditory function. To identify which of the 16 known TMPRSS genes had a strong likelihood of involvement in hearing function, three types of biological evidence were examined: 1) expression in inner ear tissues; 2) location in a genomic interval that contains a yet unidentified gene for deafness; and 3) evaluation of hearing status of any available Tmprss knockout mouse strains. This analysis demonstrated that, besides TMPRSS3, another TMPRSS gene was essential for hearing and, indeed, mice deficient for Hepsin (Hpn) also known as Tmprss1 exhibited profound hearing loss. In addition, TMPRSS2, TMPRSS5, and CORIN, also named TMPRSS10, showed strong likelihood of involvement based on their inner ear expression and mapping position within deafness loci PKSR7, DFNB24, and DFNB25, respectively. These four TMPRSS genes were then screened for mutations in affected members of the DFNB24 and DFNB25 deafness families, and in a cohort of 362 sporadic deaf cases. This large mutation screen revealed numerous novel sequence variations including three potential pathogenic mutations in the TMPRSS5 gene. The mutant forms of TMPRSS5 showed reduced or absent proteolytic activity. Subsequently, TMPRSS genes with evidence of involvement in deafness were further characterized, and their sites of expression were determined. Tmprss1, 3, and 5 proteins were detected in spiral ganglion neurons. Tmprss3 was also present in the organ of Corti. TMPRSS1 and 3 proteins appeared stably anchored to the endoplasmic reticulum membranes, whereas TMPRSS5 was also detected at the plasma membrane. Collectively, these results provide evidence that TMPRSS1 and TMPRSS3 play and TMPRSS5 may play important and specific roles in hearing.     

Autosomal recessive postlingual hearing loss (DFNB8): compound heterozygosity for two novel TMPRSS3 mutations in German siblings

Mutations in the transmembrane protease, serine 3 (TMPRSS3) gene, encoding a transmembrane serine protease, cause autosomal recessive deafness childhood (DFNB8) or congenital onset (DFNB10). TMPRSS3 mutations have been mainly identified in patients from Asian and Mediterranean countries and seem to be a rare finding in the Northern European population so far. The identification of two novel pathogenic TMPRSS3 mutations (c.646C-->T - R216C; c.916G-->A - A306T) is described in four affected siblings of German origin with postlingual hearing loss, treated by bilateral cochlear implantation with good results. Although TMPRSS3 mutations are supposed to be a rare cause of autosomal recessive hearing loss, in families with postlingual disease onset TMPRSS3 is the most favourable candidate gene after exclusion of GJB2 mutations.     

Novel TMC1 structural and splice variants associated with congenital nonsyndromic deafness in a Sudanese pedigree

Mutations of the transmembrane channel-like gene 1 (TMC1) have been shown to cause autosomal dominant and recessive forms of congenital nonsyndromic deafness linked to the loci DFNA36 and DFNB7/B11, respectively. In a Sudanese pedigree affected by an apparently recessive form of nonsyndromic deafness, we performed a linkage analysis using markers covering the deafness loci DFNB1 - DFNB30. A two-point LOD score of 3.08 was obtained at marker position D9S1876, located within the first intron of the TMC1 gene at DFNB7/B11. By DNA sequencing of TMC1 exons 3-22, we identified the structural variant c.1165C>T in exon 13, leading to the stop codon p.Arg389X, and the splice-site variant c.19+5G>A, independently segregating with the deafness phenotype. The c.1165C>T [p.Arg389X] mutation was also observed in four out of 243 unrelated deaf Sudanese individuals, but none of the mutations was found among 292 normal hearing controls. The finding of TMC1 mutations contributing to deafness in Sudan confirms and extends previous reports on the role of TMC1 in recessive nonsyndromic deafness and shows that deafness-causing TMC1 mutations may occur in various ethnic groups.     

DFNB49 is an important cause of non-syndromic deafness in Czech Roma patients but not in the general Czech population

?afka Bro?ková D, La?t?vková J, ?těpánková H, Kr?tová M, Trková M, My?ka P, Seeman P. DFNB49 is an important cause of non-syndromic deafness in Czech Roma patients but not in the general Czech population. Due to endogamy, the Roma have a higher risk for autosomal recessive (AR) disorders. We used homozygosity mapping on single-nucleotide polymorphism chips in one Czech Roma consanguineous family with non-syndromic hearing loss (NSHL). The second largest homozygous region in a deaf patient was mapped to the previously reported DFNB49 region. The MARVELD2 gene was recently reported as a causal gene for NSHL DFNB49. Sequencing of the MARVELD2 gene revealed a previously reported homozygous mutation c.1331+2 T>C (IVS4 + 2 T>C) in the deaf child. Subsequently, the same mutation was found in two more Roma families from an additional 19 unrelated Czech Roma patients with deafness tested for the MARVELD2 gene. To explore the importance of MARVELD2 mutations and DFNB49 for the general Czech and Central European population with early hearing loss we also tested 40 unrelated Czech patients with AR NSHL. No pathogenic mutation in the MARVELD2 gene was found in a group of 40 Czech non-Roma patients. Mutations in the MARVELD2 gene seem to be a significant cause of early NSHL in Czech Roma and this gene should be tested in this group of patients after GJB2.     

SLC26A4 gene is frequently involved in nonsyndromic hearing impairment with enlarged vestibular aqueduct in Caucasian populations

Sensorineural hearing loss is the most frequent sensory deficit of childhood and is of genetic origin in up to 75% of cases. It has been shown that mutations of the SLC26A4 (PDS) gene were involved in syndromic deafness characterized by congenital sensorineural hearing impairment and goitre (Pendred's syndrome), as well as in congenital isolated deafness (DFNB4). While the prevalence of SLC26A4 mutations in Pendred's syndrome is clearly established, it remains to be studied in large cohorts of patients with nonsyndromic deafness and detailed clinical informations. In this report, 109 patients from 100 unrelated families, aged from 1 to 32 years (median age: 10 years), with nonsyndromic deafness and enlarged vestibular aqueduct, were genotyped for SLC26A4 using DHPLC molecular screening and sequencing. In all, 91 allelic variants were observed in 100 unrelated families, of which 19 have never been reported. The prevalence of SLC26A4 mutations was 40% (40/100), with biallelic mutation in 24% (24/100), while six families were homozygous. All patients included in this series had documented deafness, associated with EVA and without any evidence of syndromic disease. Among patients with SLC26A4 biallelic mutations, deafness was more severe, fluctuated more than in patients with no mutation. In conclusion, the incidence of SLC26A4 mutations is high in patients with isolated deafness and enlarged vestibular aqueduct and could represent up to 4% of nonsyndromic hearing impairment. SLC26A4 could be the second most frequent gene implicated in nonsyndromic deafness after GJB2, in this Caucasian population.     

Novel missense mutations of TMPRSS3 in two consanguineous Tunisian families with non‐syndromic autosomal recessive deafness

Recently the TMPRSS3 gene, which encodes a transmembrane serine protease, was found to be responsible for two non‐syndromic recessive deafness loci located on human chromosome 21q22.3, DFNB8 and DFNB10. We found evidence for linkage to the DFNB8/10 locus in two unrelated consanguineous Tunisian families segregating congenital autosomal recessive sensorineural deafness. The audiometric tests showed a loss of hearing greater than 70 dB, in all affected individuals of both families. Mutation screening of TMPRSS3 revealed two novel missense mutations, W251C and P404L, altering highly conserved amino acids of the serine protease domain. Both mutations were not found in 200 control Tunisian chromosomes. The detection of naturally‐occurring TMPRSS3 missense mutations in deafness families identifies functionally important amino acids. Comparative protein modeling of the TMPRSS3 protease domain predicted that W251C might lead to a structural rearrangement affecting the active site H257 and that P404L might alter the geometry of the active site loop and therefore affect the serine protease activity. Hum Mutat 18:101–108, 2001. © 2001 Wiley‐Liss, Inc.     

Profound, prelingual nonsyndromic deafness maps to chromosome 10q21 and is caused by a novel missense mutation in the Usher syndrome type IF gene PCDH15

We studied a consanguineous family (Family A) from the island of Newfoundland with an autosomal recessive form of prelingual, profound, nonsyndromic sensorineural hearing loss. A genome-wide scan mapped the deafness trait to 10q21-22 (max LOD score of 4.0; D10S196) and fine mapping revealed a 16 Mb ancestral haplotype in deaf relatives. The PCDH15 gene was mapped within the critical region and was an interesting candidate because truncating mutations cause Usher syndrome type IF (USH1F) and two missense mutations have been previously associated with isolated deafness (DFNB23). Sequencing of the PCDH15 gene revealed 33 sequencing variants. Three of these variants were homozygous exclusively in deaf siblings but only one of them was not seen in ethnically matched controls. This novel c.1583 T>A transversion predicts an amino-acid substitution of a valine with an aspartic acid at codon 528 (V528D). Like the two DFNB23 mutations, the V528D mutation in Family A occurs in a highly conserved extracellular cadherin (EC) domain of PCDH15 and is predicted to be more deleterious than the previously identified DFNB23 missense mutations (R134G and G262D). Physical assessment, vestibular and visual function testing in deaf adults ruled out syndromic deafness because of Usher syndrome. This study validates the DFNB23 designation and supports the hypothesis that missense mutations in conserved motifs of PCDH15 cause nonsyndromic hearing loss. This emerging genotype–phenotype correlation in USH1F is similar to that in several other USH1 genes and cautions against a prognosis of a dual sensory loss in deaf children found to be homozygous for hypomorphic mutations at the USH1F locus.     

Vestibular dysfunction in DFNB1 deafness

Mutations of GJB2 and GJB6 (connexin‐26 and 30) at the DFNB1 locus are the most common cause of autosomal recessive, nonsyndromic deafness. Despite their widespread expression throughout the vestibular system, vestibular dysfunction has not been widely recognized as a commonly associated clinical feature. The observations of vertigo accompanying DFNB1 deafness in several large families prompted our hypothesis that vestibular dysfunction may be an integral, but often overlooked, component of DFNB1 deafness. Our aim was to define the prevalence of vestibular dysfunction in Cases of DFNB1 deafness and Controls with other forms of deafness. We developed and used a survey to assess symptoms of vestibular dysfunction, medical, and family history was distributed to Cases with deafness due to pathogenic GJB2 and/or GJB6 mutations and deaf Controls without DFNB1 deafness. Our results showed: Surveys were returned by 235/515 Cases (46%) with DFNB1 mutations and 121/321 Controls (38%) without these mutations. The mean age of Cases (41) was younger than Controls (51; P < 0.001). Vestibular dysfunction was reported by 127 (54%) of Cases and was present at significantly higher rates in Cases than in deaf Controls without DFNB1 deafness (P < 0.03). Most (63%) had to lie down in order for vertigo to subside, and 48% reported that vertigo interfered with activities of daily living. Vertigo was reported by significantly more Cases with truncating than non‐truncating mutations and was also associated with a family history of dizziness. We conclude that vestibular dysfunction appears to be more common in DFNB1 deafness than previously recognized and affects activities of daily living in many patients. © 2011 Wiley‐Liss, Inc.     

Involvement of DFNB59 mutations in autosomal recessive nonsyndromic hearing impairment

In a consanguineous Turkish family, a locus for autosomal recessive nonsyndromic hearing impairment (ARNSHI) was mapped to chromosome 2q31.1-2q33.1. Microsatellite marker analysis in the complete family determined the critical linkage interval that overlapped with DFNB27, for which the causative gene has not yet been identified, and DFNB59, a recently described auditory neuropathy caused by missense mutations in the DFNB59 gene. The 352-amino acid (aa) DFNB59 gene product pejvakin is present in hair cells, supporting cells, spiral ganglion cells, and the first three relays of the afferent auditory pathway. A novel homozygous nonsense mutation (c.499C>T; p.R167X) was detected in the DFNB59 gene, segregating with the deafness in the family. The mRNA derived from the mutant allele was found not to be degraded in lymphocytes, indicating that a truncated pejvakin protein of 166 aa may be present in the affected individuals. Screening of 67 index patients from additional consanguineous Turkish families with autosomal recessive hearing impairment revealed a homozygous missense mutation (c.547C>T; p.R183W) that segregates with the hearing impairment in one family. Furthermore, in a panel of 83 Dutch patients, two additional novel mutations (c.509_512delCACT; p.S170CfsX35 and c.731T>G; p.L244R), which were not present in ethnically matched controls, were found heterozygously. Together, our data indicate that also nonsense mutations in DFNB59 cause nonsyndromic hearing loss, but that mutations in DFNB59 are not a major cause of nonsyndromic hearing impairment in the Turkish and Dutch population.     

Prelingual deafness: high prevalence of a 30delG mutation in the connexin 26 gene

Prelingual non-syndromic (isolated) deafness is the most frequent hereditary sensory defect. In >80% of the cases, the mode of transmission is autosomal recessive. To date, 14 loci have been identified for the recessive forms (DFNB loci). For two of them, DFNB1 and DFNB2, the genes responsible have been characterized; they encode connexin 26 and myosin VIIA, respectively. In order to evaluate the extent to which the connexin 26 gene (Cx26) contributes to prelingual deafness, we searched for mutations in this gene in 65 affected Caucasian families originating from various countries, mainly tunisia, France, New Zealand and the UK. Six of these families are consanguineous, and deafness was shown to be linked to the DFNB1 locus, 10 are small non consanguineous families in which the segregation of the trait has been found to be compatible with the involvement of DFNB1, and in the remaining 49 families no linkage analysis has been performed. A total of 62 mutant alleles in 39 families were identified. Therefore, mutations in Cx26 represent a major cause of recessively inherited prelingual deafness since according to the present results they would underlie approximately half of the cases. In addition, one specific mutation, 30delG, accounts for the majority (approximately 70%) of the Cx26 mutant alleles. It is therefore one of the most frequent disease mutations so far identified. Several lines of evidence indicate that the high prevalence of the 30delG mutation arises from a mutation hot spot rather than from a founder effect. Genetic counseling for prelingual deafness has been so far considerably impaired by the difficulty in distinguishing genetic and non genetic deafness in families presenting with a single deaf child. Based on the results presented here, the development of a simple molecular test could be designed which should be of considerable help.      

Unconventional myosins and the genetics of hearing loss

Mutations of the unconventional myosins genes encoding myosin VI, myosin VIIA and myosin XV cause hearing loss and thus these motor proteins perform fundamental functions in the auditory system. A null mutation in myosin VI in the congenitally deaf Snell's waltzer mice (Myo6(sv)) results in fusion of stereocilia and subsequent progressive loss of hair cells, beginning soon after birth, thus reinforcing the vital role of cytoskeletal proteins in inner ear hair cells. To date, there are no human families segregating hereditary hearing loss that show linkage to MYO6 on chromosome 6q13. The discovery that the mouse shaker1 (Myo7(ash1)) locus encodes myosin VIIA led immediately to the identification of mutations in this gene in Usher syndrome type 1B; subsequently, mutations in this gene were also found associated with recessive and dominant nonsyndromic hearing loss (DFNB2 and DFNA11). Stereocilla of sh1 mice are severely disorganized, and eventually degenerate as well. Myosin VIIA has been implicated in membrane trafficking and/or endocytosis in the inner ear. Mutant alleles of a third unconventional myosin, myosin XV, are associated with nonsyndromic, recessive, congenital deafness DFNB3 on human chromosome 17p11.2 and deafness in shaker2 (Myo15(sh2)) mice. In outer and inner hair cells, myosin XV protein is detectable in the cell body and stereocilia. Hair cells are present in homozygous sh2 mutant mice, but the stereocilia are approximately 1/10 of the normal length. This review focuses on what we know about the molecular genetics and biochemistry of myosins VI, VIIA and XV as relates to hereditary hearing loss. Am. J. Med. Genet. (Semin. Med. Genet.) 89:147-157, 1999. Published 2000 Wiley-Liss, Inc.     

Mutation spectrum of MYO7A and evaluation of a novel nonsyndromic deafness DFNB2 allele with residual function

Recessive mutations of MYO7A, encoding unconventional myosin VIIA, can cause either a deaf-blindness syndrome (type 1 Usher syndrome; USH1B) or nonsyndromic deafness (DFNB2). In our study, deafness segregating as a recessive trait in 24 consanguineous families showed linkage to markers for the DFNB2/USH1B locus on chromosome 11q13.5. A total of 23 of these families segregate USH1 due to 17 homozygous mutant MYO7A alleles, of which 14 are novel. One family segregated nonsyndromic hearing loss DFNB2 due to a novel three-nucleotide deletion in an exon of MYO7A (p.E1716del) encoding a region of the tail domain. We hypothesized that DFNB2 alleles of MYO7A have residual myosin VIIA. To address this question we investigated the effects of several mutant alleles by making green fluorescent protein (GFP) tagged cDNA expression constructs containing engineered mutations of mouse Myo7a at codons equivalent to pathogenic USH1B and DFNB2 alleles of human MYO7A. We show that in transfected mouse hair cells an USH1B mutant GFP-myosin VIIa does not localize properly to inner ear hair cell stereocilia. However, a GFP-myosin VIIa protein engineered to have an equivalent DFNB2 mutation to p.E1716del localizes correctly in transfected mouse hair cells. This finding is consistent with the hypothesis that p.E1716del causes a less severe phenotype (DFNB2) than the USH1B-associated alleles because the resulting protein retains some degree of normal function.     

Screening for MYO15A gene mutations in autosomal recessive nonsyndromic, GJB2 negative Iranian deaf population

MYO15A is located at the DFNB3 locus on chromosome 17p11.2, and encodes myosin-XV, an unconventional myosin critical for the formation of stereocilia in hair cells of cochlea. Recessive mutations in this gene lead to profound autosomal recessive nonsyndromic hearing loss (ARNSHL) in humans and the shaker2 (sh2) phenotype in mice. Here, we performed a study on 140 Iranian families in order to determine mutations causing ARNSHL. The families, who were negative for mutations in GJB2, were subjected to linkage analysis. Eight of these families showed linkage to the DFNB3 locus, suggesting a MYO15A mutation frequency of 5.71% in our cohort of Iranian population. Subsequent sequencing of the MYO15A gene led to identification of 7 previously unreported mutations, including 4 missense mutations, 1 nonsense mutation, and 2 deletions in different regions of the myosin-XV protein.     

Use of a multiplex PCR/sequencing strategy to detect both connexin 30 (GJB6) 342 kb deletion and connexin 26 (GJB2) mutations in cases of childhood deafness

Hearing loss is a common congenital disorder that is frequently associated with mutations in the Cx26 gene (GJB2). Three recent reports that found a large deletion in another DFNB1 gene, Cx30 (GJB6), suggest that this defect may cause nonsyndromic recessive hearing loss through either a homozygous deletion of Cx30, or digenic inheritance of a Cx30 deletion and a Cx26 mutation in trans. We designed a simple diagnostic strategy with multiplex PCR followed by direct sequencing to allow for the simultaneous detection of Cx26 mutations and Cx30 deletions, and evaluated its effectiveness as a clinical genetic test by examining 200 DNA samples. In the 108 samples from deaf subjects, two digenic mutations were identified in Cx26 and Cx30 (E47X/342 kb deletion and 167delT/342 kb deletion); 69 had only Cx26 mutations (29 biallelic, 40 singleton), including two novel frameshift mutations 511-512insAACG and 358-360delAG; and 37 had no detectable mutation in either Cx26 or Cx30. Our deletion mapping suggested that the proximal breakpoint of all reported Cx30 large deletions are between the nucleotide 444 and 627 at the Cx30 coding region within a maximal interval of 78 or 184 bp. This simultaneous examination of Cx26 and Cx30 is a practical and efficient diagnostic approach for patients with nonsyndromic congenital deafness.     

长治地区耳聋患者常见耳聋基因GJB2、GJB3和线粒体DNA 12S rRNA 1555A〉G相关性分析

目的通过对长治地区115例耳聋患者常见基因GJB2、GJB3和线粒体DIqA 12S rRNA 1555A〉G测序分析,研究该地区耳聋基因的突变情况及热点突变位点。方法收集长治地区特殊教育学校的115q,J耳聋患者的外周血样本,提取其DblA后对它的目的基因进行扩增并测序。结果115例耳聋患者中GJB2基因检测到81例发生突变,并发现了10个突变位点,其中C．7657T〉c是主要的多态位点,其携带率为22．6％（52／115）。另外,长治地区发现2例线粒体DNAl555A〉G位点突变,未检测出GJB3基因突变位点。结论通过对长治地区常见耳聋基因突变位点的研究,了解该地区的耳聋基因突变谱,为后续国内耳聋基因型分布提供数据支持,同时也为耳聋的早期诊断,治疗提供理论依据。     

Identification of three novel TECTA mutations in Iranian families with autosomal recessive nonsyndromic hearing impairment at the DFNB21 locus

Forty-five consanguineous Iranian families segregating autosomal recessive nonsyndromic hearing loss (ARNSHL) and negative for mutations at the DFNB1 locus were screened for allele segregation consistent with homozygosity by descent (HBD) at the DFNB21 locus. In three families demonstrating HBD at this locus, mutation screening of TECTA led to the identification of three novel homozygous mutations: one frameshift mutation (266delT), a transversion of a cytosine to an adenine (5,211C > A) leading to a stop codon, and a 9.6 kb deletion removing exon 10. In total, six mutations in TECTA have now been described in families segregating ARNSHL. All of these mutations are inactivating and produce a similar phenotype that is characterized by moderate-to-severe hearing loss across frequencies with a mid frequency dip. The truncating nature of these mutations is consistent with loss-of-function, and therefore the existing TECTA knockout mouse mutant represents a good model in which to study DFNB21-related deafness.     

Searching for evidence of DFNB2

Deafness is the most common form of sensory impairment in humans, affecting about 1 in 1,000 births in the United States. Of those cases with genetic etiology, approximately 80% are nonsyndromic and recessively inherited. Mutations in several unconventional myosins, members of a large superfamily of actin-associated molecular motors, have been found to cause hearing loss in both humans and mice. Mutations in the human unconventional Myosin VIIa (MYO7A), located at 11q13.5, are reported to be responsible for both syndromic and nonsyndromic deafness. MYO7A mutations are responsible for Usher syndrome type Ib, the most common genetic subtype of Usher I. Usher I is clinically characterized by congenital profound deafness, progressive retinal degeneration called retinitis pigmentosa (RP), and vestibular areflexia. Although a wide spectrum of MYO7A mutations have been identified in Usher Ib patients, four mutations have been reported to cause DFNB2, a recessive deafness without retinal degeneration, and one mutation has been implicated in a single case of dominant nonsyndromic hearing loss (DFNA11). Our study attempts to ascertain additional DFNB2 families to investigate the disparate nonsyndromic phenotype and alleged causative mutations. Data from both linkage and heterogeneity analyses on 36 selected autosomal recessive nonsyndromic deafness (RNSD) families, all previously excluded by mutational analysis from GJB2 (Cx26), the leading cause of nonsyndromic deafness, showed no evidence of DFNB2 within the sample. These negative results and the isolated reports of DFNB2 bring into question whether certain MYO7A mutations produce nonsyndromic recessive hearing loss.     

Molecular screening of deafness in Algeria: High genetic heterogeneity involving DFNB1 and the Usher loci,DFNB2/USH1B,DFNB12/USH1D and DFNB23/USH1F

A systematic approach, involving haplotyping and genotyping, to the molecular diagnosis of non-syndromic deafness within 50 families and 9 sporadic cases from Algeria is described.Mutations at the DFNB1 locus (encompassing the GJB2 and GJB6 genes) are responsible for more than half of autosomal recessive prelingual non-syndromic deafness in various populations. A c.35delG mutation can account for up to 85% of GJB2 mutations and two large deletions del(GJB6-D13S1830) and del(GJB6-D13S1854) have also been reported in several population groups.In view of the genetic heterogeneity a strategy was developed which involved direct analysis of DFNB1. In negative familial cases, haplotype analysis was carried out, where possible, to exclude DFNB1 mutations. Following this, haplotype analysis of five Usher syndrome loci, sometimes involved in autosomal non-syndromic hearing loss, was carried out to identify cases in which Usher gene sequencing was indicated. When homozygosity was observed at a locus in a consanguineous family, the corresponding gene was exhaustively sequenced.Pathogenic DFNB1 genotypes were identified in 40% of the cases. Of the 21 cases identified with 2 pathogenic mutations, c.35delG represented 76% of the mutated alleles. The additional mutations were one nonsense, two missense and one splicing mutation. Four additional patients were identified with a single DFNB1 mutation. None carried the large deletions.Three families with non-syndromic deafness carried novel unclassified variants (UVs) in MYO7A (1 family) and CDH23 (2 families) of unknown pathogenic effect.Additionally, molecular diagnosis was carried out on two Usher type I families and pathogenic mutations in MYO7A and PCDH15 were found.     

Identification of a novel frameshift mutation in the DFNB31/WHRN gene in a Tunisian consanguineous family with hereditary non-syndromic recessive hearing loss

Approximately 80% of hereditary hearing loss is non-syndromic. Non-syndromic deafness is the most genetically heterogeneous trait. The most common and severe form of hereditary hearing impairment is autosomal recessive non-syndromic hearing loss (ARNSHL), accounting for approximately 80% of cases of genetic deafness. To date, 22 genes implicated in ARNSHL have been identified. Recently a gene, DFNB31/WHRN, which encodes a putative PDZ scaffold protein called whirlin, was found to be responsible for the ARNSHL DFNB31. We found evidence for linkage to the DFNB31locus in a consanguineous Tunisian family segregating congenital profound ARNSHL. Mutation screening of DFNB31/WHRNrevealed four nonpathogenic sequence variants and a novel frameshift mutation [c.2423delG] + [c.2423delG] that changed the reading frame and induced a novel stop codon at amino acid 818 ([p.Gly808AspfsX11] + [p.Gly808AspfsX11]). To determine the contribution of the DFNB31locus in the childhood deafness, we performed linkage analysis in 62 unrelated informative families affected with ARNSHL. No linkage was found to this locus. From this study, we concluded that DFNB31/WHRN is most likely to be a rare cause of ARNSHL in the Tunisian population.     

Mapping of a new autosomal recessive nonsyndromic hearing loss locus (DFNB32) to chromosome 1p13.3-22.1

Approximately 80% of the hereditary hearing loss is nonsyndromic. Isolated deafness is the most genetically heterogeneous trait. We have ascertained 10 individuals from a large consanguineous Tunisian family with congenital profound autosomal recessive deafness. All affected individuals are otherwise healthy. Genotype analysis excluded linkage to known recessive deafness loci in this family. Following a genome wide screening, a linkage was detected only with locus D1S206 on chromosome 1, thereby defining a novel deafness locus, DFNB32. In order to confirm linkage and for fine mapping the genetic interval, 12 individuals belonging to this family were added and 19 microsatellite markers were tested. A maximum two-point lodscore of 4.96 was obtained at a new polymorphic marker D1S21401. Haplotype analysis defined a 16 Mb critical region between D1S2868 and afmb014zb9. The interval of DFNB32 locus overlap with DFNA37 locus and the Marshall and Stickler syndromes locus. The entire coding region of COL11A1, responsible of the later syndromes, was screened and no mutation was observed. Towards the identification of the DFNB32 gene, a search on the Human Cochlear cDNA Library and EST Database was done. The genes corresponding to the ESTs found in the DFNB32 interval are being screened for deafness-causing mutations.