Shannon P. Pryor

SLC26A4/PDS genotype-phenotype correlation in hearing loss with enlargement of the vestibular aqueduct (EVA): evidence that Pendred syndrome and non-syndromic EVA are distinct clinical and genetic entities

Enlargement of the vestibular aqueduct (EVA) and its contents, the endolymphatic sac and duct, is the most common radiologic malformation of the inner ear associated with sensorineural hearing loss.1 It may occur alone or in combination with an incomplete partition of the apical turn of the cochlea as part of a complex of malformations known as a Mondini deformity.2 Hearing loss in ears with EVA is typically pre- or perilingual in onset, sensorineural or mixed, and fluctuating or progressive. EVA may be unilateral or bilateral; asymmetry of the hearing loss and the anatomic defect is common in bilateral cases.3–5 EVA has been observed in Pendred syndrome (PS; MIM 274600),6 branchio-oto-renal syndrome (MIM 113650),7 CHARGE (MIM 214800),8 Waardenburg syndrome (MIM 193500, 193510, 600193, 606662),9 and distal renal tubular acidosis with deafness (MIM 267300).10 Familial non-syndromic hearing loss with EVA was described in 199611 and numerous subsequent reports (DFNB4 (MIM 600791), enlarged vestibular aqueduct syndrome (MIM 603545)). EVA is always detected when the ears of individuals with PS are evaluated by both computed tomography (CT) and magnetic resonance imaging (MRI),6 and it has been estimated that PS may comprise up to 10% of prelingual deafness worldwide.3,12,13 PS is inherited in an autosomal recessive manner and is comprised of bilateral hearing loss, EVA, and an iodine organification defect in the thyroid gland, which may lead to goitre. PS is clinically differentiated from non-syndromic EVA by the presence of the thyroid iodine organification defect because goitre is an incompletely penetrant feature of PS.3 When goitre does occur in PS, it is most often euthyroidal and not evident until the second decade of life.3,12,14,15 There can be intrafamilial variability of the goitre, and PS phenocopies with …

Hypo-functional SLC26A4 variants associated with nonsyndromic hearing loss and enlargement of the vestibular aqueduct: genotype-phenotype correlation or coincidental polymorphisms?

Hearing loss with enlargement of the vestibular aqueduct (EVA) can be associated with mutations of the SLC26A4 gene encoding pendrin, a transmembrane Cl−/I−/HCO  3− exchanger. Pendrins critical transport substrates are thought to be I− in the thyroid gland and HCO  3− in the inner ear. We previously reported that bi‐allelic SLC26A4 mutations are associated with Pendred syndromic EVA whereas one or zero mutant alleles are associated with nonsyndromic EVA. One study proposed a correlation of nonsyndromic EVA with SLC26A4 alleles encoding pendrin with residual transport activity. Here we describe the phenotypes and SLC26A4 genotypes of 47 EVA patients ascertained since our first report of 39 patients. We sought to determine the pathogenic potential of each variant in our full cohort of 86 patients. We evaluated the trafficking of 11 missense pendrin products expressed in COS‐7 cells. Products that targeted to the plasma membrane were expressed in Xenopus oocytes for measurement of anion exchange activity. p.F335L, p.C565Y, p.L597S, p.M775T, and p.R776C had Cl−/I− and Cl−/HCO  3− exchange rate constants that ranged from 13 to 93% of wild type values. p.F335L, p.L597S, p.M775T and p.R776C are typically found as mono‐allelic variants in nonsyndromic EVA. The high normal control carrier rate for p.L597S indicates it is a coincidentally detected nonpathogenic variant in this context. We observed moderate differential effects of hypo‐functional variants upon exchange of HCO  3− versus I− but their magnitude does not support a causal association with nonsyndromic EVA. However, these alleles could be pathogenic in trans configuration with a mutant allele in Pendred syndrome. Hum Mutat 0, 1–10, 2009.

Hedgehog Signaling Regulates Sensory Cell Formation and Auditory Function in Mice and Humans

Auditory perception is mediated through a finite number of mechanosensory hair cells located in a specialized sensory epithelium within the inner ear. The formation of the appropriate number of hair cells and the location of those cells is crucial for normal auditory function. However, the factors that regulate the formation of this epithelium remain poorly understood. Truncating mutations in the transcription factor GLI3, a downstream effector of the Hedgehog (HH) pathway, lead to a partial loss of HH signaling and cause Pallister-Hall syndrome (PHS). Here, we report that cochleae from a mouse model of PHS (Gli3Δ699), which produces only the truncated, repressor form of GLI3, have a variably penetrant phenotype that includes an increase in the size of the sensory epithelium and the development of large ectopic sensory patches in Köllikers organ (KO). Consistent with the mouse model, some PHS individuals exhibit hearing loss across a broad range of frequencies. Moreover, inhibition of HH signaling in vitro results in an increase in the size of the prosensory domain, a precursor population that gives rise to the sensory epithelium, whereas treatment with Sonic hedgehog (SHH) inhibits prosensory formation. Finally, we demonstrate that HH signaling within the cochlea regulates expression of prosensory markers and that the effects of HH in KO are dependent on activation of Notch, an inducer of prosensory fate. These results suggest that HH signaling plays a key role in the specification, size, and location of the prosensory domain, and therefore of hair cells, within the cochlea.

SLC26A4 genotype, but not cochlear radiologic structure, is correlated with hearing loss in ears with an enlarged vestibular aqueduct†

Identify correlations among SLC26A4 genotype, cochlear structural anomalies, and hearing loss associated with enlargement of the vestibular aqueduct (EVA).

Cochleosaccular Dysplasia Associated With a Connexin 26 Mutation in Keratitis–Ichthyosis–Deafness Syndrome

Objective: The objective of this study was to characterize the temporal bone phenotype associated with a mutation of GJB2 (encoding connexin 26).

Recent Advances in the Understanding of Syndromic Forms of Hearing Loss

There are hundreds of different syndromes that include an auditory phenotype as a prominent feature and nearly as many reviews of this topic (Ahmed, Riazuddin, Riazuddin, & Wilcox, 2003; Griffith & Friedman, 2002; Gurtler & Lalwani, 2002; Lalwani, 2002; Morton, 2002; Petit, 2001; Steel, Erven, & Kiernan, 2002). Approximately 30% of individuals with hereditary hearing loss also have abnormalities of other organ systems and are considered to have a syndromic form of deafness (Gorlin, Toriello, & Cohen, 1995). The accompanying abnormalities range from subtle to obvious and may be congenital or delayed in appearance. The majority of these syndromes are inherited as monogenic disorders. Some of these genes have been mapped to a chromosomal map position and a subset of these mapped genes have been identified (cloned). In Table 1 we list many of the syndromic forms of hearing loss and some of the distinguishing clinical features. In many cases where hearing loss is listed as a part of a syndrome, the loss develops late and may simply be secondary to the general neurological decay. For virtually all inherited syndromic forms of hearing loss the Online Mendelian Inheritance in Man (www.ncbi.nlm.nih.gov/Omim/) has comprehensive descriptions of the clinical features and molecular genetics as well as an all-inclusive list of references. In this review we discuss only advances in our understanding of syndromic forms of deafness that have been made in the past few years. Several deafness syndromes are named after the clinician(s) who first or more fully described the disorder such as Jervell and Lange-Nielsen syndrome, Marshall syndrome, Stickler syndrome, Usher syndrome and Waardenburg syndrome (Table 1). Other hereditary deafness syndromes have been given labels that encompass part or all of the clinical presentation (phenotype) such as Branchio-Oto-Renal syndrome (abbreviated BOR, Table 1). As might be expected, there is a gene for each of these clinically distinct inherited syndromes that include hearing loss as one feature. However, there are exceptions to this generalization. Two phenotypically distinct syndromes may be due to different mutations of the same gene. Geneticists refer to different mutations of the same gene as allelic mutations or, more simply, as alleles. There are many examples of phenotypically distinct syndromes that are caused by allelic mutations such as Marshall syndrome (OMIM 154780) and Stickler syndrome type 2 (OMIM 604841), both of which can be caused by mutations of COL11A1 on chromosome 1p21 (Griffith, Sprunger, Sirko-Osadsa, Tiller, Meisler, & Warman, 1998) (Table 1). Another example is Waardenburg syndrome type 1 (OMIM 193500), Waardenburg syndrome type 3 (OMIM 148820) and Craniofacial-Deafness-Hand syndrome (OMIM 122880), which are clinically distinct but, in fact, are caused by allelic mutations of PAX3 on chromosome 2q35 (Asher, Sommer, Morell, & Friedman, 1996). Moreover, the converse is also true. Mutations of more than one gene may cause the identical clinical phenotype. This is referred to as genetic heterogeneity. For example, Usher syndrome type 1 is characterized by congenital, severe to profound hearing loss, retinitis pigmentosa (RP) with prepubertal onset and vestibular areflexia. Surprisingly, there are at least seven different genes that can cause clinically indistinguishable Usher syndrome type 1 (Table 1). Sometimes a patient has hearing loss that is obvious while their other associated abnormalities escape notice. There are many different reasons for incomplete diagnoses. Two examples illustrate this point. The hearing loss in Usher syndrome types 1 and 2 is congenital, while the onset of RP may be delayed and not noticed until adolescence. Patients who have Jervell and Lange-Nielsen syndrome are hearing impaired, but their heart conduction problems are easily overlooked (Table 1). Therefore, Section on Human Genetics (T.B.F., J.M.S., T.B-Y., A.L., E.R.W., S.R., Z.A., I.A.B.), Section on Gene Structure and Function (S.P.P, A.J.G.), and Hearing Section (S.P.P., A.J.G.), National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Rockville, Maryland; and Department of Pediatrics and Human Development (R.A.F.), Michigan State University, East Lansing, Michigan.

Segregation of enlarged vestibular aqueducts in families with non-diagnostic SLC26A4 genotypes

Background: Hearing loss with enlarged vestibular aqueduct (EVA) can be inherited as an autosomal recessive trait caused by bi-allelic mutations of SLC26A4. However, many EVA patients have non-diagnostic SLC26A4 genotypes with only one or no detectable mutant alleles. Methods and results: In this study, the authors were unable to detect occult SLC26A4 mutations in EVA patients with non-diagnostic genotypes by custom comparative genomic hybridisation (CGH) microarray analysis or by sequence analysis of conserved non-coding regions. The authors sought to compare the segregation of EVA among 71 families with two (M2), one (M1) or no (M0) detectable mutant alleles of SLC26A4. The segregation ratios of EVA in the M1 and M2 groups were similar, but the segregation ratio for M1 was significantly higher than in the M0 group. Haplotype analyses of SLC26A4-linked STR markers in M0 and M1 families revealed discordant segregation of EVA with these markers in eight of 24 M0 families. Conclusion: The results support the hypothesis of a second, undetected SLC26A4 mutation that accounts for EVA in the M1 patients, in contrast to non-genetic factors, complex inheritance, or aetiologic heterogeneity in the M0 group of patients. These results will be helpful for counselling EVA families with non-diagnostic SLC26A4 genotypes.

Evaluation of the Thyroid in Patients With Hearing Loss and Enlarged Vestibular Aqueducts

OBJECTIVE To evaluate thyroid structure and function in patients with enlargement of the vestibular aqueduct (EVA) and sensorineural hearing loss. DESIGN Prospective cohort survey. SETTING National Institutes of Health Clinical Center, a federal biomedical research facility. PATIENTS The study population comprised 80 individuals, aged 1.5 to 59 years, ascertained on the basis of EVA and sensorineural hearing loss. MAIN OUTCOME MEASURES Associations among the number of mutant alleles of SLC26A4; volume and texture of the thyroid; percentage of iodine 123 ((123)I) discharged at 120 minutes after administration of perchlorate in the perchlorate discharge test; and peripheral venous blood levels of thyrotropin, thyroxine, free thyroxine, triiodothyronine, thyroglobulin, antithyroid peroxidase and antithyroglobulin antibodies, and thyroid-binding globulin. RESULTS Thyroid volume is primarily genotype dependent in pediatric patients but age dependent in older patients. Individuals with 2 mutant SLC26A4 alleles discharged a significantly (P < or = .001) greater percentage of (123)I compared with those with no mutant alleles or 1 mutant allele. Thyroid function, as measured by serologic testing, is not associated with the number of mutant alleles. CONCLUSIONS Ultrasonography with measurement of gland volume is recommended for initial assessment and follow-up surveillance of the thyroid in patients with EVA. Perchlorate discharge testing is recommended for the diagnostic evaluation of patients with EVA along with goiter, nondiagnostic SLC26A4 genotypes (zero or 1 mutant allele), or both.

Do mutations of the Pendred syndrome gene, SLC26A4, confer resistance to asthma and hypertension?

Background and aims: Mutations of SLC26A4 cause Pendred syndrome, an autosomal recessive disorder comprising goitre and deafness with enlarged vestibular aqueducts (EVA). Recent studies in mouse models implicate Slc26a4 in the pathogenesis of asthma and hypertension. We hypothesise that asthma and hypertension are less prevalent among humans with SLC26A4 mutations. Methods: We reviewed medical histories and SLC26A4 genotypes for 80 individuals with EVA and 130 of their unaffected family members enrolled in a study of EVA. We used Fisher’s exact test to compare the prevalence of asthma and hypertension among groups of subjects with zero, one, or two mutant alleles of SLC26A4. Results: Although none of the 21 subjects with two mutant alleles of SLC26A4 had asthma or hypertension, there were no statistically significant differences in the prevalence of asthma or hypertension among subjects with zero, one, or two mutant alleles. Conclusion: There might be a protective effect of SLC26A4 mutations for asthma and hypertension but our study is statistically underpowered to detect this effect. Study sizes of at least 1125 and 504 individuals will be needed for 80% power to detect an effect at α = 0.05 for asthma and hypertension, respectively. Our hypothesis merits a larger study since it has implications for potential strategies to treat hearing loss by manipulating SLC26A4 expression or function.

Use of SLC26A4 Mutation Testing for Unilateral Enlargement of the Vestibular Aqueduct

IMPORTANCE Approximately one-half of all subjects with unilateral or bilateral hearing loss with enlargement of the vestibular aqueduct (EVA) will have SLC26A4 gene mutations. The number (0, 1, or 2) of mutant alleles of SLC26A4 detected in an individual subject with EVA is each associated with a distinct combination of diagnostic and prognostic information as well as probability of recurrence of EVA in siblings. OBJECTIVE To evaluate the results of SLC26A4 mutation testing in subjects with unilateral EVA. (The study objective was formulated before data were collected.) DESIGN Prospective cross-sectional study of cohort ascertained between 1998 and 2012. SETTING National Institutes of Health Clinical Center, a federal biomedical research facility. PARTICIPANTS Twenty-four subjects (10 males, 14 females) with unilateral EVA, defined as a midpoint diameter greater than 1.5 mm, who were referred or self-referred to participate in a study about the clinical and molecular analysis of EVA. Twenty-one (87.5%) of 24 subjects were white. Mean age was 10.3 years (age range, 5-39 years). INTERVENTION SLC26A4 mutation analysis. MAIN OUTCOMES AND MEASURES Audiometric results, the presence or absence of EVA, and the number of mutant alleles of SLC26A4. RESULTS Approximately 8.3% of the subjects with unilateral EVA had 2 mutant SLC26A4 alleles, 16.7% had 1 mutant allele, and 75.0% had 0 mutant alleles. CONCLUSIONS AND RELEVANCE Unilateral EVA can be associated with all possible SLC26A4 genotype results. The distinct combination of prognoses and recurrence probability associated with each genotype supports the clinical use of testing for SLC26A4 mutations in subjects with unilateral EVA.