Genotype phenotype correlations for hearing impairment: Approaches to management

Transcription

1 Clin Genet 2014: 85: Printed in Singapore. All rights reserved Review 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd CLINICAL GENETICS doi: /cge Genotype phenotype correlations for hearing impairment: Approaches to management Hoefsloot L. H., Feenstra I., Kunst H. P. M., Kremer H. Genotype phenotype correlations for hearing impairment: Approaches to management. Clin Genet 2014: 85: John Wiley & Sons A/S. Published by John Wiley & Sons Ltd, 2013 Hearing impairment is an extremely heterogeneous disorder, with both environmental as well as genetic causes. This review describes the known genes involved in non-syndromic hearing impairment and their genotype phenotype correlations where possible. Furthermore, some of the more frequent syndromic forms of hearing impairment are described, in particular where they overlap with the non-syndromic forms. Given the heterogeneity of the disorder, together with the indistinguishable phenotypes for many of the genes, it is suggested that testing for mutations is performed using massive parallel sequencing techniques, either by a large targeted set of genes or by an exome wide analysis. Conflict of interest The authors declare no conflict of interest. L. H. Hoefsloot a,b, I. Feenstra a,b,h.p.m.kunst c,d and H. Kremer a,c,d,e a Department of Human Genetics, b Institute for Genetic and Metabolic Disease, c Department of Otorhinolaryngology, Head and Neck Surgery, d Donders Institute for Brain, Cognition and Behaviour, and e Nijmegen Center for Molecular Life Sciences, Radboud University Medical Center, Nijmegen, the Netherlands Key words: diagnostic testing genetic genotype non-syndromic hearing impairment phenotype Corresponding author: Lies H. Hoefsloot, Department of Human Genetics, Radboud University Medical Center, 6500 HB Nijmegen, the Netherlands. Tel.: ; fax: ; lies.hoefsloot@radboudumc.nl [PO BOX 9101] Received 11 September 2013, revised and accepted for publication 29 December 2013 Hearing impairment with a genetic cause is an extremely heterogeneous group of disorders. Being the most common sensory disorder, hearing impairment affects one in every newborns (1). By adolescence, the prevalence increases to 3.5 per The incidence of hearing impairment steadily rises, with 50 80% of people having hearing impairment at the age of 80 (suppl 1 3). Hearing is a complex process. Airborne sound is received by the ears and is transmitted as a mechanical signal by the ossicles through the middle ear to the cochlea. In the cochlea, the signal is converted from a fluid-borne wave into an electrical signal. This signal is then transported through the auditory nerves to the central nervous system, which is needed for integration and comprehension. Genetic factors play a major role in hearing impairment. Half of the congenital/early-onset cases are thought to be due to an environmental cause, such as cytomegalovirus (CMV) infections during pregnancy or ototoxic medication and the remaining are considered to have a genetic origin (suppl 1, 4, 5). Within this last group, about 75 80% is inherited in an autosomal recessive mode, 15 20% autosomal dominant, 1 4% X-linked, and 1 2% due to a mitochondrial DNA mutation. In 30% of cases with a genetic basis, additional features lead to the diagnosis of syndromic hearing impairment, but in the remaining 70% hearing impairment is the only finding in otherwise healthy people. On the other hand, age-related hearing impairment is thought to be a complex disorder, with contributions both from genetic and from environmental factors. This review restricts itself to non-syndromic hearing impairment caused by mutations in specific nuclear genes and only touches upon syndromic forms when they can present as non-syndromic hearing impairment. 514

2 Genotype phenotype correlations for hearing impairment Mutations in the mitochondrial DNA in some cases can cause non-syndromic hearing loss, but this is not discussed here. Several reviews on similar subjects have been published. This review aims to be an update and focuses on the phenotype genotype correlations (2 7). An overview of all loci involved in hearing loss can also be found on the Hereditary Hearing loss Homepage ( Because of space constrictions, the tables and most of the references (indicated by suppl followed by a number) have been moved to the supplementary data, available online. The reader is strongly encouraged to view this data as an integral part of the article. Characteristics of the disorder Hearing impairment can be described in many ways, but one of the most informative characteristics is the type of hearing impairment. This can be divided into four subtypes: first of all is the conductive hearing impairment, which results from abnormalities in the structures of the outer ear, as well as from abnormalities of the ossicles. Second, sensorineural hearing impairment, which results from malfunction of inner ear structures (i.e., cochlea). Third, mixed hearing impairment, which is a combination of sensorineural and conductive hearing impairment. Fourth, central auditory dysfunction resulting from abnormalities or dysfunction of the structures downstream of the cochlea, such as the eighth cranial nerve, the auditory brain stem, or the cerebral cortex (7). The second characteristic is the age of onset. This can be either prelingual or postlingual. Prelingual hearing impairment can be congenital (present at birth), but not necessarily. Postlingual hearing impairment occurs after the start of normal speech differing widely in age of onset, varying from teenage years till the fourth or fifth decade of life. Onset in later decades is considered to be age-related hearing impairment. The third characteristic is the severity of hearing impairment. Hearing impairment can either be mild (hearing threshold db) moderate (41 55 db), moderate severe (56 70 db), severe (71 90 db) or profound (90 db). The fourth characteristic is whether the hearing impairment is progressive or not. This feature is assessed by evaluation of hearing levels during several years. In the majority of individuals with congenital hearing impairment, a non-progressive form is detected, and most late-onset hearing impairment is progressive, although this is not always the case. The fifth characteristic is the tone frequency affected by hearing impairment, which can be low (<1000 Hz), middle ( Hz), or high (>2000 Hz). The sixth characteristic is whether the hearing impairment is syndromic or not. People with nonsyndromic hearing impairment have no associated abnormalities. Syndromic hearing impairment patients have associated abnormalities, usually involving one or more other organ systems, intellectual disability, with or without dysmorphic features. The seventh characteristic is the mode of inheritance. Mostly patients present as a sporadic case, but when there is a family history, all kinds of inheritance can be found: autosomal dominant, autosomal recessive, X-linked, Y-linked, and mitochondrial (maternal inheritance) [CAVE: sporadic does NOT exclude a genetic cause; this could be autosomal recessive or autosomal dominant with a de novo mutation, or X-linked in male patients]. The eighth characteristic is whether the hearing impairment is bilateral or unilateral. Bilateral hearing impairment is more likely to have a genetic cause, but unilateral hearing impairment is described in, for instance, Waardenburg syndrome. The ninth and final characteristic is whether the hearing impairment is associated with vestibular dysfunction. How to diagnose hearing impairment Different protocols exist in different countries, and even vary between different hospitals, but the common practice in the Otogenetics Clinic Nijmegen involves the following: A detailed intake includes the onset of hearing impairment, progression, impact on functioning in daily life, and family history. All possible causes of hearing loss should be investigated. These include ototoxic medication, possible head trauma, noise-induced trauma, meningitis, and ear disease or ear operations in the past. In cases where the onset is congenital or in early childhood, perinatal infections, icterus, and asphyxia should be asked for. In all cases, vestibular complaints such as instability, vertigo, difficulty walking straight, delayed motor development, difficulty walking in the dark, falling to one side (left or right), and the dandy phenomenon (patient sees the horizon moving vertically when walking) should be taken into account. Otorhinolaryngological examination, i.e., otoscopy of the ear canal and eardrum, to exclude common causes of an air bone gap such as ear wax, eardrum perforations, or cholesteatoma, should be performed. Vestibular problems can be assessed in a child by the history of motor milestones, and observation of walking, or in an adult, using Romberg test or head thrust test. From the age of 4 5 years, hearing impairment can be assessed by performing pure-tone audiometry. Frequencies between 250 and 8000 Hz are tested at different sound intensities to determine the threshold. Both the air and the bone conduction are measured to rule out an air bone gap. An air bone gap is the result of middle ear and/or external ear pathology. In sensorineural hearing impairment, an air bone gap is usually absent. In addition to pure-tone audiometry, speech audiometry can be performed. In speech audiometry test, words are presented to the patient. The hearing level at which the patient can correctly repeat 50% of the words presented is called the speech reception threshold. The objective tests performed in young children are otoacoustic emissions (OAEs), brainstem-evoked response 515

3 Hoefsloot et al. audiometry (BERA), and auditory steady state response testing (ASSR). OAEs are sounds that are generated by the outer hair cells in response to a sound stimulus. These are evoked by administering a sound to the outer ear canal with a probe that can also record the responses of the hair cells. Normal transient OAEs suggest a hearing threshold better than 40 db. In both BERA and ASSR, a click is administered to the ears, which evokes responses at the level of the cochlea, eight nerve, and auditory brainstem. These responses are recorded with surface electrodes placed on the head. The threshold measured with the BERA corresponds with hearing in the Hz region. The more recently developed ASSR is frequency specific and hearing thresholds can be measured for various frequencies, most often 500, 1000, 2000, and 4000 Hz. The electronystagmogram (ENG) assesses the horizontal movements of each eye separately by placing electrodes to the left and right side of each eye. Subsequently, rotary chair testing is performed. A manner of rotary chair testing is the velocity step (VS) test. In this test, the chair accelerates slowly to a maximum velocity of 90 /s. When there is no nystagmus anymore, the chair stops suddenly within 1 s. This abrupt stop produces a post-rotary nystagmus, which is being recorded. Another part of the vestibular examination is the caloric test, which is performed to assess the total responsiveness of the horizontal semicircular canals. It is a method to investigate each labyrinth separately. For this test, the subject is placed on a lying posture and both ear canals are consecutively irrigated with warm and cold water. Irrigation of the ear with cold and warm water ends in a horizontal nystagmus to the contralateral and ipsilateral side, respectively. In affected ears, a hypofunction or areflexia can be demonstrated with both the VS and the caloric tests. Nomenclature Different types of non-syndromic hearing impairment are abbreviated as DFN (DeaFNess), followed by the letter A for autosomal dominant loci, and a number. DFNB is used for autosomal recessive loci and DFNX is used for X-linked loci. Until now, one Y-linked locus has been described [DFNY (8, 9)]. Gene names are approved according to the HUGO nomenclature; therefore, they might differ from the previous literature [ (10)]. Only the genes and loci that could be found in the OMIM database have been included in Tables S1 S6 in Appendix S1 ( The genes involved: non-syndromic autosomal recessive hearing impairment An overview of all known loci and genes for autosomal recessive non-syndromic hearing impairment can be found in Table S1 in Appendix S1. The loci for which the gene has not yet been identified are listed in Table S2 in Appendix S1. The identification of the genes for autosomal recessive hearing impairment has greatly benefited from the work that has been carried out in populations with a high proportion of consanguineous families, where the assumption that a homozygous familial mutation is the cause of the disorder has been proven correct in over 50% of all cases [see references for the loci in Table S1 in Appendix S1, and reviewed in 2012 (6)]. Here, we describe in more detail the genes that are more commonly involved and those that are associated with hearing impairment and characteristic features, with emphasis on the genotype phenotype relation. In general, autosomal recessive hearing impairment is non-progressive and prelingual/congenital, although certain genes are known to cause late-onset hearing impairment, and the hearing impairment can be progressive in some cases. GJB2 and GJB6: DFNB1 By far the biggest contribution to congenital/prelingual hearing impairment is made by the DFNB1 locus. In the Caucasian population, the most common mutation has a carrier frequency of 1:45 (suppl 6). The frequency of carriership at least for some populations studied is between 1% and 3% (suppl 6 11), with exceptions for certain other populations (suppl 12 14), such as the African-American population (suppl 15). The locus was identified in 1994 (suppl 16), and the GJB2 gene in that locus has been one of the first causative genes to be found (suppl 17, 18). Later on, it was found that next to mutations in the GJB2 gene, a specific deletion involving part of the GJB6 gene is one of the more frequent alleles (suppl 19 22). This deletion has an uneven distribution in different populations; for instance, in China it was found to be present in one population and almost absent in another (suppl 23, 24). Although there has been some discussion whether this would present a digenic model, it was proven that the deletion contains a regulatory element for the GJB2 gene; therefore, DFNB1 is a singlegene disorder, with compound heterozygosity instead of double heterozygosity (suppl 25, 26). The onset is in the majority of cases congenital, or at least prelingual, and the hearing impairment is mainly in the high frequencies. The severity can vary, depending on the specific mutations. Overall, missense mutations are associated with a milder phenotype, nonsense, and frameshift mutations with a severe phenotype (11). This means that for patients with congenital and/or prelingual hearing impairment, in most populations, the DFNB1 locus is the first gene to be investigated (12 14). However, late onset (but still in the first decade) with mutations in the DFNB1 locus have been described (15). Some families have been described with nonsyndromic autosomal dominant hearing impairment and a heterozygous mutation in GJB2 or GJB6, but this seems to be a rare finding (suppl 27 29). Mutations in both genes have been associated with autosomal dominant skin disease (see notes to Table S3 in Appendix S1). 516

4 MYO15A: DFNB3 Mutations in the MYO15A gene are associated with severe to profound hearing impairment (suppl 30 33). The analysis of this gene has not been systematically carried out in routine diagnostics, because of the prohibitive size of the gene (66 exons). However, it seems that the frequency of mutations in this gene has been underestimated, and this suggests that MYO15A is one of the genes that should be analyzed routinely in sporadic patients with non-syndromic early-onset hearing impairment. The suggestion that mutations in the large exon 2 are sometimes associated with a less severe hearing impairment has been raised (suppl 34). Genotype phenotype correlations for hearing impairment TMPRSS3: DFNB8 and DFNB10 DFNB8 was originally described with postlingual onset whereas DFNB10 was described with prelingual onset (suppl 35, 36). Mutations in TMPRSS3 have been described in the Asian and Mediterranean families (suppl 37 42), but it is also one of the more frequently mutated genes in the Caucasian population, although earlier reports found a rare occurrence (suppl 43). The phenotype can differ according to the mutation identified, with the severe mutations leading to prelingual onset, whereas milder mutations are associated with a later, more variable postlingual onset (suppl 44). In children with an onset of hearing impairment within the first decade, that is rapidly progressive, and showing a ski slope audiogram, testing of TMPRSS3 should be at the top of the list. An example of age-related typical audiograms (ARTA) is shown in Fig. 1. OTOF: DFNB9 and PJVK: DFNB59; auditory neuropathy DFNB9 autosomal recessive hearing impairment is better characterized as an auditory neuropathy, because affected individuals have distinctive test results. They have hearing impairment based on pure-tone audiometry and auditory brainstem response measurements. But, using the OAE test, the results are initially normal, indicating a normal response of the outer hair cells to environmental sound. Furthermore, their speech perception is poor when compared to the pure-tone threshold. Consequently, patients with mutations in the OTOF gene are not helped by hearing aids but instead may benefit from cochlear implants (suppl 45). Also unique maybe for mutations in this gene, patients have been described with a temperature-sensitive hearing impairment: when feverish, their hearing deteriorated and improved again when the temperature returned to normal (suppl 46, 47). Mutations in the PJVK gene, DFNB59, have also been found in families with auditory neuropathy. In these families, vestibular dysfunction is described, distinguishing these families from the patients with OTOF mutations (suppl 48 52). Finally, mutations in the DIAPH3 gene (OMIM ) have been described in AUNA1 (OMIM Fig. 1. Age-related typical audiograms (ARTA) from an individual with DFNB8, caused by two mutations in the TMPRSS3 gene (modified from suppl 44). Age is represented in italics ), an autosomal dominant form of auditory neuropathy (16). GIPC3: DFNB15/72/95 GIPC3 gene mutations have been described in humans, after the establishment of the involvement with hearing impairment in mice. The hearing impairment is early/congenital, moderate to profound. In the audiograms of patients, sometimes a specific pattern can be seen, impairment at all frequencies, downsloping, with a slight elevation at 4000 Hz (suppl 53, 54). Seeing this pattern in a child with hearing impairment, the testing of GIPC3 should be considered. STRC: DFNB16 Mutations in the STRC gene have been described as early as The STRC gene product has a function in the outer hair cell stereocilia (suppl 55, 56). Further elucidation of the importance of the locus came in 2009 with the finding that a copy number variation (CNV), containing the STRC gene, is present at relatively high frequencies in the population and can be involved in hearing impairment. The analysis of the gene is hampered by the presence of a pseudogene with >99% homology that lies in tandem with the normal gene, but STRC has been shown to be one of the more frequent genes to be involved in non-syndromic hearing impairment, especially in the mild-to-moderate group(suppl 57 59). Furthermore, a contiguous gene syndrome has been described, the 517

5 Hoefsloot et al. deletion also involving the CATSPER gene involved in male fertility (suppl 60, 61). So, in patients with prelingual, mild-to-moderate hearing loss, the STRC gene could be the first gene to test after exclusion of mutations in the DFNB1 locus. OTOG: DFNB18B and OTOGL: DFNB84B Both genes have been reported recently, and both genes are located at the same locus as another hearing impairment gene: the USH1C for OTOG, and the PTPRQ gene for OTOGL. Both genes code for proteins that are present in the tectorial membrane. Patients with mutations in the OTOG gene have a mild-tomoderate hearing impairment, stable, and prelingual. Vestibular hyporeflexia has been noted (17), and this may differentiate these patients from patients with mutations in the STRC gene. Mutations in the OTOGL gene have also been described in mild-to-moderate hearing impairment (18, 19). The genes involved: autosomal dominant hearing impairment There are several reasons why this type of hearing impairment has received much attention from researchers, one of them being that autosomal dominant hearing impairment is easily recognized in families to have a genetic component, as opposed to sporadic cases with an autosomal recessive background. The other feature that differentiates autosomal dominant hearing impairment from the autosomal recessive types is that it is almost always postlingual hearing impairment. Some types even have an onset after the third decade of life. As a consequence, these types of hearing impairment are progressive, as opposed to the more or less stable hearing impairment seen for the autosomal recessive forms. But there are always exceptions. In Tables S3 and S4 in Appendix S1, the loci are listed with already known genes (Table S3 in Appendix S1) or with unidentified genes (Table S4 in Appendix S1). The progressive nature of the hearing impairment can be assessed in several ways, but one of the most common is the assembly of an audioprofile. This is a figure in which several audiograms are plotted together. This can be successive audiograms taken from either the same individual over the years or from members of the same family with varying ages. Audioprofiles for most of the autosomal dominant hearing impairment loci have been assembled and can be found at (20, 21). It has become apparent that audioprofiles can be very typical for certain loci (mutated genes), and using the audiogene tool with as input an audiogram, it is possible to predict which gene is most likely to be involved, sometimes with considerable success (22 24). KCNQ4 and GJB3: DFNB2 The DFNA2 locus has turned out to be complicated: since the determination of the involvement of the KCNQ4 gene in a percentage of families (suppl 62, 63), the second gene in this locus is GJB3 and it seems there is a third gene involved in this locus (suppl 64, 65). The hearing impairment starts in the first decade, but late onset has been described, with mainly the higher frequencies affected. The hearing impairment is progressive, with parallel lines in the audioprofile. Audioprofiles for all three genes can be found at eng.uiowa.edu/audioprofiles. DFNA5 and DFNA5 DFNA5 was first described in a five-generation family in 1966 (suppl 66, 67), but it took until 1998 to identify the causative mutation in the DFNA5 gene (suppl 68). The remarkable finding is that all mutations described to date are leading to a loss of exon 8 in the mrna. A truncating mutation initially thought to be causative did not segregate with the phenotype, further strengthening the hypothesis that only mutations leading to exon 8 skipping lead to hearing impairment (suppl 69) The DFNA5 gene is implicated in apoptosis and has been linked to several forms of cancer, indicating its role as a tumor suppressor gene (suppl 70, 71). The hearing impairment is progressive, with the higher frequencies more affected in later years. COCH and DFNA9 Mutations in the COCH gene have been identified in families with late-onset, high-frequency hearing impairment that rapidly progresses (suppl 72 74). In almost all affected persons, vestibular dysfunction is noted. The vestibular dysfunction has been likened to Meniere s disease, but later publications showed that in Meniere s patients, no mutations in the COCH gene could be identified (suppl 75). Remarkably, the onset of vestibular symptoms seems to precede the actual hearing impairment (suppl 76). In the Dutch and Belgian population, a founder mutation has been identified, which is the most prevalent mutation found in diagnostics for this gene (suppl 77, 78). Given the characteristic phenotype (late onset, vestibular dysfunction, and an autosomal dominant family history of similar complaints), this is one of the few genes associated with hearing impairment that has a high clinical sensitivity in diagnostic service (L. H. H., pers. comm.). EYA4 and DFNA10 The locus was mapped in 1996 in a multigenerational family where hearing impairment started in the second to fifth decade, leading progressively to severeprofound loss involving all frequencies (suppl 79 82). In a family with a large deletion in the gene, an associated phenotype of dilated cardiomyopathy was found, a phenotype that develops after the hearing impairment (suppl 83, 84). However, in most families described, no cardiac phenotype could be identified, so in most cases, mutations in EYA4 lead to autosomal non-syndromic hearing impairment (suppl 85). 518

6 Genotype phenotype correlations for hearing impairment The genes involved: X-linked non-syndromic hearing impairment POU4F3: DFNX2 DFNX2 (DFN3) was described already in 1988 (suppl 86) and was further refined in later years (suppl 87, 88). In 1995, it was shown that mutations in the POU3F4 gene, as well as deletions encompassing a putative regulatory element about 400 kb proximal to the gene, are the cause of the hearing impairment in DFNX2 (suppl 89, 90). The hearing impairment is mixed, because apart from the cochlear defect, stapes fixation can be found in patients. The increased perilymphatic pressure is thought to lead to the so-called gusher phenotype that can be seen in these patients during the surgical correction of the ossicular chain. However, in other DFNX2 patients, the hearing impairment seems to be mixed, because of the presence of an inner ear malformation, which causes a pseudo conductive hearing impairment. In these patients, a normal mobility of the stapes is found and normal stapedial reflexes can be present. Computerized tomography shows abnormal dilatation of the internal acoustic canal, as well as an abnormally wide communication between the internal acoustic canal and the inner ear compartment (suppl 91). The mixed component can be missed, because the sensorineural hearing impairment can be profound (suppl 92). Female carriers can be affected as well, but the phenotype is much more varied (Tables S5 and S6 in Appendix S1). PRPS1: DFNX1 DFNX1 has been recognized for a long time (OMIM and references therein), but the gene was identified in 2010 (suppl 93). Mutations in the PRPS1 gene have also been described in X-linked Charcot-Marie-Tooth disease-5 (CMTX5) and Arts syndrome, both syndromes including sensorineural hearing loss, but also peripheral neuropathy and optic atrophy (CMTX5) and additionally for Arts syndrome also central neuropathy and an impaired immune system [see for review de Brouwer (25)]. As an isolated hearing impairment syndrome, there is a marked difference between males and females. In males, the onset is between 5 and 15 years, and the hearing impairment is progressive to profound, with flat audioprofiles. The onset in female carriers is not until their fifties, and the hearing impairment is generally mild to moderate. SMPX: DFNX4 The gene in the DFNX4 locus, SMPX, was identified recently (suppl 94, 95). The gene has been known for a long time, but it was thought that the protein was only expressed in muscle. The clinical features of the X-linked inheritance have been described by del Castillo (suppl 96). It shows the typical pattern for an X-linked inherited disorder, where males are more severely affected than females, and the phenotype in females is very variable, presumably because of (non-random) X-inactivation. The genes involved: autosomal recessive and autosomal dominant hearing impairment TECTA: DFNB21 and DFNA8/12 Mutations in the TECTA gene are associated both with autosomal dominant and with autosomal recessive hearing impairment (suppl 97). In both cases, the audiogram is flat, or cookie bite shaped, but depending on the type of mutation, there can also be an impairment of predominantly the higher frequencies (suppl ). The autosomal recessive form has an early onset, and the hearing impairment is severe to profound. In contrast, the hearing impairment of the autosomal dominant form is more varied in the presentation, and onset of clinical symptoms can be postlingual (suppl 101). In an autosomal dominant family, the first mutation in an exonic splice enhancer (ESE) element involved in hearing impairment was described (suppl 102). The representative audiograms of both DFNB21 and DFNA8/12 are shown in Fig. 2. The other loci presenting with mid-frequency hearing impairment are DFNA13, DFNA21, DFNA31, DFNA44, and DFNA49 (Tables S3 and S4 in Appendix S1, OMIM entries and references therein); only for DFNA13 and DFNA44 have the genes been described, COL11A2 and CCDC50, respectively. TMC1: DFNB7/11 and DFNA36 The mutations in this gene not only cause autosomal recessive hearing impairment (DFNB7/11) but also cause autosomal dominant hearing impairment (DFNA36). The autosomal recessive form is characterized by early-onset, stable, severe to profound hearing impairment (suppl 103, 104), whereas the autosomal dominant form is characterized by late-onset, progressive hearing impairment (suppl 105, 106). But this distinction is not always true: an autosomal recessive family with late-onset, progressive hearing impairment and mutations in the TMC1 gene has also been described (suppl 107, Fig. 3). This might be explained by a reduced amount of normal splicing in addition to abnormal splicing of the mrna mimicking a dominant mutation. The hearing impairment involves mainly the high frequencies. MYO6: DFNB37 and DFNA22 Mutations in the MYO6 gene have been described both in autosomal dominant and in autosomal recessive families with non-syndromic hearing loss (suppl ). The family with the recessive hearing impairment exhibited prelingual, profound hearing impairment, whereas the autosomal dominant form is characterized by postlingual, progressive hearing impairment. Additionally, a syndromic form of hearing 519

7 Hoefsloot et al. Fig. 2. Age-related typical audiograms (ARTA for DFNB21 (a) and DFNA8/12 (b), both caused by mutations in the TECTA gene. (a) Modified from (suppl 180) (b) modified from (suppl 101). The mid-frequency phenotype is indicated by mf. of a syndrome. In total, approximately 400 syndromes are known that include hearing impairment among the anomalies. We here describe the main characteristics of a number of common syndromes, because they can be masked by the hearing impairment, which is then the most obvious symptom. Fig. 3. Age-related typical audiograms (ARTA for DFNB7/11, with two mutations in TMC1, modified from (suppl 107). Italic numbers indicate age in years. impairment has been described, where there is an association with hypertrophic cardiomyopathy (suppl 114). Notably, the mutation in the last family was a missense mutation, as opposed to nonsense and splice site mutations in the non-syndromic families. This suggests a gain-of-function or a dominant-negative effect for the missense mutation. The genes involved: non-syndromic and syndromic hearing impairment Hearing impairment can be an isolated feature, although one should be aware of the fact that it may also be part SLC26A4: DFNB4 and Pendred syndrome Pendred syndrome (PDS) and DFNB4 comprise a phenotypic spectrum of hearing impairment with or without other findings. PDS is characterized by prelingual, severe-to-profound bilateral sensorineural hearing impairment, variable vestibular dysfunction, temporal bone abnormalities, and development of (most often euthyroid) goiter in late childhood to early adulthood. Affected individuals display a considerable variability of findings, even within the same family (suppl ). DFNB4 is characterized by sensorineural hearing impairment, variable vestibular dysfunction, and enlarged vestibular aqueduct (EVA). Thyroid defects are not seen in DFNB4. Mutations in the SLC26A4 can be found in approximately 50% of affected individuals from either simplex or multiplex families with PDS or DFNB4. These persons are often compound heterozygotes for disease-causing variants in SLC26A4, although not infrequently only a single variant is detected. Digenic inheritance with the FOXI1 gene (<1% of affected individuals) and the KCNJ10 (<1% of affected individuals) has been described, suggesting further genetic heterogeneity, although this has been disputed recently (suppl 117, ). A specific feature of the EVA is apparently that upon trauma, the hearing impairment can be exacerbated (suppl 122, 123). 520

8 Genotype phenotype correlations for hearing impairment CLPP: DFNB81 and Perrault syndrome DFNB81 with mutations in the CLPP gene is now known to be Perrault Syndrome (PRLTS3) where females have associated genital abnormalities and/or suffer from premature ovarian failure (POF) (suppl 114). Although probably a rare condition, this could be missed in clinic because POF is usually not evident at the time when children are assessed for their hearing impairment. Perrault syndrome is genetically heterogeneous, and mutations in the LARS2 gene (PRLTS4), the HSD17B4 gene (PRLTS1), and the HARS2 gene (PRLTS2) have also been described, all inherited as an autosomal recessive condition. WFS1: DFNA6/14/38 and Wolfram syndrome Mutations in the WFS1 gene are associated with autosomal dominant low-frequency hearing impairment, and this gene is therefore the first gene to test once lowfrequency hearing impairment has been established, followed by the analysis maybe of DIAPH1, although for this gene the Puerto Rican family seems to be the only family identified as far as the literature describes (suppl 124). Specific mutations may be associated with additional optic atrophy (suppl 125). Mutations in the WFS1 gene are also associated with Wolfram syndrome, an autosomal recessive neurodegenerative disease characterized by diabetes mellitus, optic atrophy, diabetes insipidus, and deafness (DIDMOAD) (OMIM ), and Wolfram-like syndrome, characterized by a prelingual onset, progressive hearing impairment, diabetes mellitus, and optic atrophy (OMIM ). The hearing impairment is prelingual, with the low frequencies first affected, and therefore might be missed in less affected subjects, because of the relatively good speech perception. The hearing impairment progresses to include the higher frequencies, and in later age groups the audiogram is more or less flat (suppl 126). This means that in older patients with autosomal dominant non-syndromic hearing impairment, a mutation in the WFS1 gene may be looked for, also if the hearing impairment is not only in the lower frequencies. ACTG1: DFNA20/26 and Baraitser Winter syndrome Mutations in the ACTG1 gene have been described not only in families with non-syndromic autosomal dominant hearing impairment (suppl ) but also in families with Baraitser Winter syndrome (BRWS), which is characterized by ptosis, colobomata, neuronal migration disorder, distinct facial anomalies, postnatal growth retardation, and intellectual disability, but also sensorineural hearing impairment is part of the spectrum. In most of these patients, the mutation could be shown to have arisen de novo, as a clear proof of pathogenicity. It was suggested that BRWS represents the severe end of a spectrum of cytoplasmic actin-associated phenotypes that begins with BRWS and extends to non-syndromic hearing impairment (suppl 134). MYH9 and DFNA17: MYH9-related disorders Mutations in MYH9 are found in patients with autosomal dominant giant-platelet disorders May Hegglin anomaly (155100), Fechtner syndrome (153640), and Sebastian syndrome (605249). Further studies have shown that these syndromes are overlapping, both in phenotype and genotype. Therefore, the term MYH9- related disease was coined for a disorder with thrombocytopenia, giant platelets, and inclusion bodies in neutrophils. Only a limited number of mutations have been reported, indicating a dominant-negative mechanism for this autosomal dominant syndrome (suppl ). However, two families have been described segregating autosomal dominant non-syndromic hearing impairment due to a mutation in MYH9.Notably, it is the same mutation in both families, indicating a specific mechanism in these families for the isolated non-syndromic hearing impairment without the extra clinical features that have been described for mutations in MYH9 -related disease (suppl 151, 152). MYO7A, USH1C, CDH23, PCDH15, USH1G, CIB2, DFNB31: Usher syndrome Usher syndrome is characterized by hearing impairment and retinitis pigmentosa. Three types of Usher syndrome can be distinguished, based on their clinical features and genes involved [see also a recent review by Bonnet (26)]. The hearing impairment in Usher syndrome type I is congenital, bilateral, and profound. Vestibular areflexia is associated with the hearing impairment and is a defining feature of this disorder. Therefore, children with Usher syndrome type I typically walk later than usual. A child with Usher syndrome type I is often misdiagnosed as having nonsyndromic deafness until night blindness and tunnel vision, early signs of retinitis pigmentosa (RP), become severe enough to be noticeable. Late diagnosis can be avoided if the clinician is alert to the combination of profound congenital deafness and delayed motor milestones as being highly suggestive of type 1 Usher syndrome. Six genes in which mutations are known to cause Usher syndrome type I have been identified: USH1B (MYO7A, 50 60% of the cases), USH1C (USH1C ), USH1D (CDH23 ), USH1F (PCDH15 ), USH1G (USH1G), and USH1J (CIB2 ). Mutations in the CDH23 gene seem to be the second most frequent cause of type 1 Usher syndrome (suppl 153, 154). Usher syndrome type II is characterized by congenital, bilateral sensorineural hearing impairment predominantly in the higher frequencies that ranges from mild to severe, normal vestibular function, and adolescent-toadult onset of retinitis pigmentosa (RP). One of the most important clinical distinctions between Usher syndrome type I and Usher syndrome type II is that children with Usher syndrome type II nearly always have normal motor development. The genes involved in Usher syndrome type II are USH2A (approximately 80% of the cases), GPR98 (VLGR1 ) (approximately 15% of cases), and DFNB31 (<5% of cases) (suppl ). 521

9 Hoefsloot et al. Usher syndrome type III is characterized by postlingual progressive sensorineural hearing impairment, late-onset RP, and variable impairment of vestibular function (suppl 157). Mutations in CLRN1 or HARS are causative (suppl ). Some individuals with Usher syndrome type III may have profound hearing impairment and vestibular disturbance and thus be clinically misdiagnosed as having Usher syndrome type I (suppl 161). Although Usher syndrome is thought to be rare, a recent study of children with hearing impairment found that 11% of them had one or two mutations in genes associated with Usher syndrome. It is however unclear from this study how many of those children actually have the full blown clinical symptoms associated with Usher syndrome (suppl 162). Apart from the fact that Usher syndrome initially presents with hearing impairment, some Usher syndrome-related genes have been described to be involved in non-syndromic forms. Leaky mutations in USH1C have been described to cause only non-syndromic deafness (DFNB18A, suppl 163, 164). The hearing impairment is severe to profound and prelingual, but no vestibular problems have been reported. Also mutations in the CDH23 gene can cause non-syndromic hearing impairment (DFNB12). Mostly missense mutations are detected, inferring that some residual activity can be enough to prevent an eye phenotype. The hearing impairment is moderate to profound, affects all frequencies with the higher frequencies more severely affected, and the onset is prelingual. Also the PCDH15 (DFNB23 ), CIB2 (DFNB48), and DFNB31 (DFNB31) genes have been described in non-syndromic autosomal recessive hearing impairment. MYO7A mutations have been associated with autosomal recessive (DFNB21) and autosomal dominant (DFNA11) non-syndromic hearing impairment (see Tables S1 and S3 in Appendix S1 for specific references). COL11A2: DFNA13andDFNB53 Mutations in the COL11A2 gene have been described not only in DFNA13 (OMIM ) but also in DFNB53 (OMIM ), fibrochondrogenesis 2 (OMIM ), otospondylomegaepiphyseal dysplasia (OSMED) (OMIM ), Stickler syndrome type III (OMIM ), and Weissenbacher Zweymuller syndrome (OMIM ). All these syndromes have other manifestations apart from the hearing impairment Depending on the type of mutations, the phenotype varies, but a clear genotype phenotype relation has not emerged as of yet. The audiogram is quite often cookie bite or bowl shaped, indicating a defect in the tectorial membrane (suppl ). TIMM8A: Mohr Tranebjaerg syndrome Mohr Tranebjaerg syndrome is also called deafness dystonia optic neuronopathy (DDON) (OMIM # ) syndrome and was originally reported as nonsyndromic deafness in males (suppl 169). However, at reinvestigation, Tranebjaerg and colleagues found several ocular features in the affected individuals, including myopia, decreased visual acuity, constricted visual fields, and abnormal electroretinogram (suppl 170, 171). The deafness was shown to be part of a progressive syndrome. Males with DDON syndrome have prelingual or postlingual sensorineural hearing impairment in early childhood, slowly progressive dystonia or ataxia in the teens, slowly progressive decreased visual acuity from optic atrophy beginning approximately age 20 years, and dementia beginning at approximately age 40 years. Psychiatric symptoms such as personality change and paranoia may appear in childhood and progress. The hearing impairment appears to be consistent in age of onset and progression, whereas the neurologic, visual, and neuropsychiatric signs vary in degree of severity and rate of progression. Female carriers may have mild hearing impairment and focal dystonia. DDON is caused by a heterozygous mutation or deletion of TIMM8A (suppl 172). KCNQ1, KCNE1: Jervell and Lange Nielsen syndrome The Jervell and Lange Nielsen syndrome (JLNS, OMIM # , ) is an autosomal recessive syndrome of abnormal cardiac ventricular repolarization with prolonged QT interval and bilateral, profound congenital sensorineural deafness. Due to ventricular arrhythmias, there is a relatively high risk for sudden death if no treatment is started (suppl 173). JLNS is an autosomal recessive syndrome including two subtypes; the majority of patients have JLNS type 1, caused by mutations in KCNQ1 and the remaining 10% of patients who usually display a milder phenotype have JLNS type 2, due to mutations in KCNE1 (suppl 174). Although heterozygous mutations in both genes are associated with long QT-syndrome, the at-risk families are not always recognized, because of the different nature of the mutations (dominant negative in the long QTsyndrome and loss of function in the JLNS syndrome (suppl 175, 176). However, physicians should ask for a family history of sudden death in patients with profound congenital sensorineural deafness. How to diagnose the genetic cause after hearing impairment has been established After the diagnosis of hearing impairment, nowadays usually in the neonatal period, rehabilitation with hearing aids is started as quickly as possible, followed sometimes by early cochlear implantation in case of a severe to profound hearing impairment. For many reasons, it is imperative for families to establish the cause of the hearing impairment, whether or not a genetic factor is involved. A correct genetic diagnosis might influence treatment options, such as when mutations in KCNQ1 are found in a child with profound congenital hearing loss, the concurrent long QT-syndrome can be treated early on. But also lifedetermining events such as career choices and family 522

10 Genotype phenotype correlations for hearing impairment planning might benefit from a correct genetic diagnosis. For instance, when a child is found to be hearing impaired due to mutations in the USH2A gene, the onset of the RP might be anticipated and acted upon. When an X-linked gene is found to be the cause of hearing impairment, the risk for future children and grandchildren is quite different than when an autosomal recessive gene is found to be causative. Because in only a limited number of cases the characteristics of the hearing impairment (shape audiogram, onset, progression, and family history) point to the gene to test, it is advisable in the other cases to use a technique that assays for most or all the known genes at the same time. With the arrival of the massive parallel sequencing techniques, it has become feasible to do so. Several of these tests have been developed, either targeted or exome wide, either commercially or by diagnostic laboratories (suppl ). Both approaches have their value. We strongly encourage further development of these tests, as it will also increase our knowledge, like the assignment of new phenotypes to genotypes and vice versa. We firmly believe that in the end it should be possible to identify the causative mutations in >90% of individuals with non-syndromic hearing impairment where non-genetic causes have been excluded. Supporting Information The following Supporting information is available for this article: Appendix S1. Supplementary references; nonsyndromic hearing impairment, genes and loci: Tables 1 6. Additional Supporting information may be found in the online version of this article. Acknowledgements The authors wish to thank Dr Patrick Huygen for his help, especially for providing the figures for this article. References 1. Fortnum HM, Summerfield AQ, Marshall DH et al. Prevalence of permanent childhood hearing impairment in the United Kingdom and implications for universal neonatal hearing screening: questionnaire based ascertainment study. BMJ 2001: 323: Willems PJ. Genetic causes of hearing loss. N Engl J Med 2000: 342: Bitner-Glindzicz M. Hereditary deafness and phenotyping in humans. Br Med Bull 2002: 63: Petersen MB, Willems PJ. Non-syndromic, autosomal-recessive deafness. Clin Genet 2006: 69: Hilgert N, Smith RJ, Van Camp G. Forty-six genes causing nonsyndromic hearing impairment: which ones should be analyzed in DNA diagnostics? Mutat Res 2009: 681: Duman D, Tekin M. Autosomal recessive nonsyndromic deafness genes: a review. Front Biosci 2012: 17: Smith RJH, Shearer AE, Hildebrand MS et al. Deafness and hereditary hearing loss overview In: Pagon RA, Adam MP, Bird TD, et al., eds. GeneReviews [Internet]. Seattle, WA: University of Washington, Seattle; Available from: gov/books/nbk1434/. 8. Wang Q, Xue Y, Zhang Y et al. Genetic basis of Y-linked hearing impairment. Am J Hum Genet 2013: 92: Wang QJ, Lu CY, Li N et al. Y-linked inheritance of non-syndromic hearing impairment in a large Chinese family. J Med Genet 2004: 41: e Gray KA, Daugherty LC, Gordon SM et al. Genenames.org: the HGNC resources in Nucleic Acids Res 2013: 41: D545 D Snoeckx RL, Huygen PL, Feldmann D et al. GJB2 mutations and degree of hearing loss: a multicenter study. Am J Hum Genet 2005: 77: Lim LH, Bradshaw JK, Guo Y et al. Genotypic and phenotypic correlations of DFNB1-related hearing impairment in the Midwestern United States. Arch Otolaryngol Head Neck Surg 2003: 129: MacArdle B, Bitner-Glindzicz M. Investigation of the child with permanent hearing impairment. Arch Dis Child Educ Pract Ed 2010: 95: Hoefsloot LH, Roux AF, Bitner-Glindzicz M. EMQN Best Practice guidelines for diagnostic testing of mutations causing non-syndromic hearing impairment at the DFNB1 locus. Eur J Hum Genet 2013: 21: Pagarkar W, Bitner-Glindzicz M, Knight J et al. Late postnatal onset of hearing loss due to GJB2 mutations. Int J Pediatr Otorhinolaryngol 2006: 70: Schoen CJ, Emery SB, Thorne MC et al. Increased activity of Diaphanous homolog 3 (DIAPH3)/diaphanous causes hearing defects in humans with auditory neuropathy and in Drosophila. Proc Natl Acad Sci U S A 2010: 107: Schraders M, Ruiz-Palmero L, Kalay E et al. Mutations of the gene encoding otogelin are a cause of autosomal-recessive nonsyndromic moderate hearing impairment. Am J Hum Genet 2012: 91: Bonnet C, Louha M, Loundon N et al. Biallelic nonsense mutations in the otogelin-like gene (OTOGL) in a child affected by mild to moderate hearing impairment. Gene 2013: 11: Yariz KO, Duman D, Seco CZ et al. Mutations in OTOGL, encoding the inner ear protein otogelin-like, cause moderate sensorineural hearing loss. Am J Hum Genet 2012: 91: Hildebrand MS, DeLuca AP, Taylor KR et al. A contemporary review of AudioGene audioprofiling: a machine-based candidate gene prediction tool for autosomal dominant nonsyndromic hearing loss. Laryngoscope 2009: 119: Taylor KR, Deluca AP, Shearer AE et al. AudioGene: predicting hearing loss genotypes from phenotypes to guide genetic screening. Hum Mutat 2013: 34: Bischoff AM, Luijendijk MW, Huygen PL et al. A novel mutation identified in the DFNA5 gene in a Dutch family: a clinical and genetic evaluation. Audiol Neurootol 2004: 9: Hildebrand MS, Tack D, McMordie SJ et al. Audioprofile-directed screening identifies novel mutations in KCNQ4 causing hearing loss at the DFNA2 locus. Genet Med 2008: 10: de Heer AM, Schraders M, Oostrik J et al. Audioprofile-directed successful mutation analysis in a DFNA2/KCNQ4 (p.leu274his) family. Ann Otol Rhinol Laryngol 2011: 120: de Brouwer AP, van Bokhoven H, Nabuurs SB et al. PRPS1 mutations: four distinct syndromes and potential treatment. Am J Hum Genet 2010: 86: Bonnet C, El-Amraoui A. Usher syndrome (sensorineural deafness and retinitis pigmentosa): pathogenesis, molecular diagnosis and therapeutic approaches. Curr Opin Neurol 2012: 25: