Molecular analysis of the PDS gene in Pendred syndrome

Mutation screening of the thyroid peroxidase gene in a cohort of 55 Portuguese patients with congenital hypothyroidism

OBJECTIVE

Defects in the human thyroid peroxidase (TPO) gene are reported to be one of the causes of congenital hypothyroidism (CH) due to a total iodide organification defect. The aim of the present study was to determine the nature and frequency of TPO gene mutations in patients with CH, characterised by elevated TSH levels and orthotopic thyroid gland, identified in the Portuguese National Neonatal Screening Programme.

SUBJECTS AND METHODS

The sample comprised 55 patients, from 53 unrelated families, with follow-up in the endocrinology clinics of the treatment centres of Porto and Lisbon. Mutation screening in the TPO gene (exons 1-17) was performed by single-strand conformational analysis followed by sequencing of fragments with abnormal migration patterns.

RESULTS

Eight different mutations were detected in 13 patients (seven homozygotes and six compound heterozygotes). Novel mutations included three missense mutations, namely 391T > C (S131P), 1274A > G (N425S) and 2512T > A (C838S), as well as the predictable splice mutation 2748G > A (Q916Q/spl?). The undocumented polymorphism 180-47A > C was also detected.

CONCLUSION

The results are in accordance with previous observations confirming the genetic heterogeneity of TPO defects. The proportion of patients in which the aetiology was determined justifies the implementation of this molecular testing in our CH patients with dyshormonogenesis.

The immunohistochemical analysis of pendrin in the mouse inner ear

Pendred's syndrome (PS) is an autosomal recessive disorder characterized by deafness and goiter, which are caused by mutations in the Pendred's syndrome gene (PDS). PDS encodes a membrane protein named pendrin that is considered to act as an anion transporter. An expression pattern of the PDS ortholog (Pds) mRNA in the auditory and vestibular systems has been reported in mice, and the localization of pendrin has been reported recently. We generated antipeptide antibodies against human pendrin, and performed immunohistochemical analysis of mouse inner ears. We detected pendrin in the endolymphatic duct and sac, and the utricle, saccule, and external sulcus. In the endolymphatic duct and sac, the expression of pendrin was apparent at the apical membrane. In addition, we detected pendrin in the spiral ligament, Claudius cells, Deiter's cells, and the spiral ganglion of the cochlea. Our results are key to defining the role of pendrin in inner ear development and elucidating the pathogenic mechanisms underlying deafness in PS.

Mutational analysis of the human SLC26A8 gene: exclusion as a candidate for male infertility due to primary spermatogenic failure

SLC26A8 is an anion transporter that is solely expressed in the testes. It interacts with MgcRacGAP that shows strong structural similarity with the Drosophila protein RotundRacGAP, which is established to have an essential role for male fertility in the fruit fly. To explore whether the SLC26A8 gene has a role in human male infertility, we performed mutational analysis in the coding region of the SLC26A8 gene in 83 male infertility patients and two groups of controls using single-strand conformational polymorphism and direct sequencing methods. We found six novel coding sequence variations, of which five lead to amino acid substitutions. All variants were found with similar frequencies in both patients and controls, thus suggesting that none of them may be causally associated with infertility. We conclude that the SLC26A8 mutations are not a common cause of male infertility.

Intrafamilial variability of the deafness and goiter phenotype in Pendred syndrome caused by a T416P mutation in the SLC26A4 gene

Pendred syndrome (PS) is the most common cause of syndromic deafness, accounting for more than 5% of all autosomal-recessive hearing loss cases. It is characterized by bilateral sensorineural hearing loss and by goiter with or without hypothyroidism. Mutations in the SLC26A4 gene cause both classical PS and deafness associated with an enlarged vestibular aqueduct without goiter. To investigate a possible genotype-phenotype correlation in PS, we performed a detailed clinical and genetic study in three adult German sibs with typical PS caused by a common homozygous SLC26A4 mutation, T416P. An audiological long-term follow-up of 23 yr showed that the mutation T416P is associated with a distinct type of hearing loss in each of the three sibs: moderate-to-profound progressive deafness, profound nonprogressive deafness, and a milder but more rapidly progressing form. We show that these phenotypic differences are not caused by either different degrees of inner ear malformations or sequence variations in the GJB2/connexin 26 gene. Because the thyroid phenotype was also highly variable within the family, with thyroid sizes ranging from normal to large goiters requiring thyroidectomy, this study leads to the conclusion that other environmental and/or genetic factors have an impact on the PS phenotype.

Screening of SLC26A4 (PDS) gene in Pendred's syndrome: a large spectrum of mutations in France and phenotypic heterogeneity

Sensorineural hearing defect and goiter are common features of Pendred's syndrome. The clinical diagnosis of Pendred's syndrome remains difficult because of the lack of sensitivity and specificity of the thyroid signs. The identification of PDS as the causative gene allowed molecular screening and enabled a re-evaluation of the syndrome to identify potential diagnostic characteristics. This report presents the clinical and genotypic findings of 30 French families, for whom a diagnosis of Pendred's syndrome had been made. Twenty-seven families had at least one mutated allele. Twenty-eight different mutations were identified, 11 of which had never been previously reported. The main clinical characteristics were: early hearing loss, fluctuation in terms of during deafness evolution, and the presence of an enlarged vestibular aqueduct.

Distribution and frequencies of PDS (SLC26A4) mutations in Pendred syndrome and nonsyndromic hearing loss associated with enlarged vestibular aqueduct: a unique spectrum of mutations in Japanese

Molecular diagnosis makes a substantial contribution to precise diagnosis, subclassification, prognosis, and selection of therapy. Mutations in the PDS (SLC26A4) gene are known to be responsible for both Pendred syndrome and nonsyndromic hearing loss associated with enlarged vestibular aqueduct, and the molecular confirmation of the PDS gene has become important in the diagnosis of these conditions. In the present study, PDS mutation analysis confirmed that PDS mutations were present and significantly responsible in 90% of Pendred families, and in 78.1% of families with nonsyndromic hearing loss associated with enlarged vestibular aqueduct. Furthermore, variable phenotypic expression by the same combination of mutations indicated that these two conditions are part of a continuous category of disease. Interestingly, the PDS mutation spectrum in Japanese, including the seven novel mutations revealed by this study, is very different from that found in Caucasians. Of the novel mutations detected, 53% were the H723R mutation, suggesting a possible founder effect. Ethnic background is therefore presumably important and should be noted when genetic testing is being performed. The PDS gene mutation spectrum in Japanese may be representative of those in Eastern Asian populations and its elucidation is expected to facilitate the molecular diagnosis of a variety of diseases.

Pendred syndrome and DFNB4-mutation screening of SLC26A4 by denaturing high-performance liquid chromatography and the identification of eleven novel mutations

Mutations in SLC26A4 cause Pendred syndrome, an autosomal-recessive disorder characterized by sensorineural deafness and goiter, and DFNB4, a type of autosomal recessive nonsyndromic deafness in which, by definition, affected persons do not have thyromegaly. The clinical diagnosis of these two conditions is difficult, making mutation screening of SLC26A4 a valuable test. Although screening can be accomplished in a variety of ways, all techniques are not equally accurate, timely or cost effective. We found single-strand conformational polymorphism analysis (SSCP) to be 63% effective in detecting mutations a panel of different SLC26A4 allele variants when compared to data from direct sequencing. Because direct sequencing can be time consuming and expensive, especially for a gene with 21 exons, we studied DHPLC as an alternative screening method. We found DHPLC as accurate and reliable as direct sequencing but to be more rapid and cost effective. In addition, we report 11 novel disease-causing allele variants of SLC26A4.

Mutations in the SLC26A4 (pendrin) gene in patients with sensorineural deafness and enlarged vestibular aqueduct

Pendred syndrome and the enlarged vestibular aqueduct (EVA) are considered phenotypic variations of the same entity due to mutations in the SLC26A4 (pendrin) gene. Pendred syndrome consists in sensorineural deafness, goiter and impaired thyroid hormone synthesis while in EVA thyroid function seems to be preserved. The aim of this study was to evaluate thyroid function and morphology and to look for mutations in the SLC26A4 gene in patients presented with EVA. Among 57 consecutive patients with sensorineural deafness 15 with EVA, as assessed by magnetic resonance imaging (MRI), were identified and studied. A complete evaluation of thyroid function including thyroid echography and perchlorate discharge test was carried out in all patients with EVA; all exons of the SLC26A4 gene were amplified from peripheral leukocytes and directly sequenced, using specific intronic primers. Out of 15 patients with EVA, goiter was present in 8 (53%), hypothyroidism in 7 (47%), increased serum thyroglobulin levels in 8 (53%) and a positive perchlorate discharge test in 10 (67%). Nine alleles of the SLC26A4 gene were mutated: 2 novel mutations (L465W and G497R) and 4 already known mutations (T410M, R409H, T505N and IVS1001+1G>A) were found. Four subjects were compound heterozygous and 1 heterozygous (G497R/wt). All patients harbouring mutations in the SLC26A4 gene had goiter and a positive perchlorate discharge test: 3 were slightly hypothyroid and 2 euthyroid. The remaining 10 patients had no mutations in the SLC26A4 gene: 4 of them were hypothyroid, 2 with goiter and positive perchlorate discharge test, 2 without goiter and with negative perchlorate discharge test. Two patients without mutations were euthyroid with positive perchlorate discharge test. Patients with mutations in the SLC26A4 gene had larger thyroid volume (p<0.002), higher serum thyroglobulin (Tg) levels (p<0.002) and greater radioiodine discharge after perchlorate (p=0.09) than patients without mutations. The results of the present study lend support to the concept that all patients with mutated SLC26A4 gene have abnormalities of thyroid function tests.

Molecular heterogeneity in two families with auditory pigmentary syndromes: the role of neuroimaging and genetic analysis in deafness

We report two cases in which the probands presented with deafness and a family history of a dominantly inherited auditory pigmentary syndrome, yet the cause of deafness in each proband was not associated with the pigmentary abnormalities but was a result of mutations in SLC26A4, the gene mutated in Pendred's syndrome. The first case is a young woman with congenital sensorineural hearing loss and a family history of piebaldism. Despite showing no pigmentary abnormalities, the proband was found to harbor the same KIT mutation as her relatives affected by piebaldism, as well as two mutations in the SLC26A4 gene. In the second case, 2-year-old identical twin boys born to deaf parents presented with congenital sensorineural deafness and an extensive maternal family history of Waardenburg's syndrome type I (WSI). Their father had recessively inherited deafness associated with dilated vestibular aqueducts and a clinical diagnosis of Pendred's syndrome was made in him, which was confirmed molecularly. As the twin boys did not have features of WSI, both the mother and children were tested for mutations in SLC26A4 which showed the mother to be a carrier of a single mutation and both boys to be compound heterozygotes, illustrating pseudodominant inheritance of the condition.

The SLC26 gene family of multifunctional anion exchangers

The ten-member SLC26 gene family encodes anion exchangers capable of transporting a wide variety of monovalent and divalent anions. The physiological role(s) of individual paralogs is evidently due to variation in both anion specificity and expression pattern. Three members of the gene family are involved in genetic disease; SLC26A2 in chondrodysplasias, SLC26A3 in chloride-losing diarrhea, and SLC26A4 in Pendred syndrome and hereditary deafness (DFNB4). The analysis of Slc26a4-null mice has significantly enhanced the understanding of the roles of this gene in both health and disease. Targeted deletion of Slc26a5 has in turn revealed that this paralog is essential for electromotor activity of cochlear outer hair cells and thus for cochlear amplification. Anions transported by the SLC26 family, with variable specificity, include the chloride, sulfate, bicarbonate, formate, oxalate and hydroxyl ions. The functional versatility of SLC26A6 identifies it as the primary candidate for the apical Cl(-)-formate/oxalate and Cl(-)-base exchanger of brush border membranes in the renal proximal tubule, with a central role in the reabsorption of Na(+)-Cl(-) from the glomerular ultrafiltrate. At least three of the SLC26 exchangers mediate electrogenic Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange; the stoichiometry of Cl(-)-HCO(3)(-) exchange appears to differ between SLC26 paralogs, such that SLC26A3 transports >/=2 Cl(-) ions per HCO(3)(-) ion, whereas SLC26A6 transports >/=2 HCO(3)(-) ions per Cl(-) ion. SLC26 Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange is activated by the cystic fibrosis transmembrane regulator (CFTR), implicating defective regulation of these exchangers in the reduced HCO(3)(-) transport seen in cystic fibrosis and related disorders; CFTR-independent activation of these exchangers is thus an important and novel goal for the future therapy of cystic fibrosis.

Aspectos genéticos do hipotireoidismo congênito

Hipotireoidismo congênito (HC) afeta cerca de 1:3000 a 1:4000 recém-nascidos (RN). Numerosos genes são essenciais, tanto para o desenvolvimento normal do eixo hipotálamo-hipófise-tireóide quanto para a produção hormonal, e estão associados ao HC. Cerca de 85% do hipotireoidismo primário é denominado disgenesia tireoidiana e evidências sugerem que mutações nos fatores de transcrição (TTF2, TTF1 e PAX-8) e no gene do receptor de TSH podem ser responsáveis pela doença. Os defeitos hereditários da síntese hormonal podem ser devidos a mutações nos genes NIS (natrium-iodide symporter), pendrina, tireoglobulina (TG), peroxidase (TPO). Mais recentemente, mutações no gene THOX-2 têm sido descritas para defeitos na organificacão. O hipotireoidismo central afeta cerca de 1:20.000 RN e tem sido associado com mutações nos fatores transcricionais hipofisários (POUIF1, PROP1, LHX3, HESX1). A síndrome de resistência periférica ao hormônio tireoidiano é uma doença rara que cursa com hipotireoidismo em alguns tecidos e, freqüentemente, está associada a mutações autossômicas dominantes no receptor beta (TRß).

Enlarged endolymphatic duct and sac syndrome: relationship between MR findings and genotype of mutation in pendred syndrome gene

Pendred syndrome (PDS) is characterized by profound deafness in childhood, positive perchlorate challenge, and goiter. PDS is often associated with enlarged endolymphatic duct and sac (EEDS), and recently, PDS gene mutations have been reported even in those patients with EEDS without classic Pendred syndrome. In a previous report, the number of mutant alleles was correlated with the degree of subclinical thyroid abnormality, but not with hearing loss, in patients with missense mutation H723R. It also has been reported that the hearing loss in EEDS was not correlated with the EEDS volume, cochlear modiolar area, or signal intensity of the endolymphatic sac. We evaluated the correlations between the number of mutant alleles and these parameters in patients with EEDS to investigate the mechanisms underlying this condition. The study group was comprised of 16 Japanese patients with EEDS diagnosed by MR imaging. The H723R mutation was homozygous in six patients and heterozygous in six patients, with no mutation found in four patients. The modiolar area, EEDS volume, and signal intensity ratio (sac signal/cerebrospinal fluid signal) were not significantly correlated with the number of mutant alleles. PDS gene mutations may not be the only cause of EEDS, and the mechanisms underlying EEDS remain unclear.

Thyroperoxidase gene mutations in congenital goitrous hypothyroidism with total and partial iodide organification defect

Mutations of the thyroperoxidase (TPO) gene have been reported as being the most severe and frequent abnormality in thyroid iodide organification defect (IOD) causing goitrous congenital hypothyroidism. The objective of this study was to screen and subsequently identify TPO gene mutations in patients with congenital hypothyroidism with evidence of total iodine organification defects (TIOD) or partial iodine organification defect (PIOD) as defined by the perchlorate discharge test. Seven goitrous patients with TIOD and seven patients with PIOD, from three and five unrelated families, respectively, were studied. We were able to detect different TPO genes mutations in patients with TIOD and PIOD. In TIOD families the results were as follows: (1) a homozygous GGCC insertion at exon 8, position 1277 (family 1); (2) compound heterozygosity with a GGCC insertion at exon 8 (1277) and a nucleotide substitution in exon 11 (2068G>C) (family 2); (3) compound heterozygosity with the mutation 2068G>C in exon 11 and a C insertion in exon 14 between positions 2505-2511 (family 3). In patients with PIOD we have detected: (1) only one heterozygous mutation in two families (4 and 5), in exons 11 and 10 (2084G>A and 1780C>A); (2) a compound heterozygous condition in one family (family 6), with mutations, respectively in exons 8 and 10 (1242G>T and 1780C>A); (3) only polymorphisms (family VII) and (4) a heterozygous mutation in the first base of the border exon/intron 9 +1G>T (family VIII). We did not detect inactivating mutations in exons 11, 16, and 21 of the THOX2 gene where mutations have been previously described. We concluded that homozygous and compound heterozygous mutations found in TIOD characterized the autosomal recessive mode of inheritance and will translate a nonfunctional protein or a protein that may not reach the apical membrane. As for PIOD, the majority of the studied kindreds had only heterozygous mutations and/or polymorphisms. It is conceivable that these TPO gene sequence alterations may partially affect the functional state of the translated protein or affect its transport to the apical membrane.

HUMANNONSYNDROMICSENSORINEURALDEAFNESS

Given the unique biological requirements of sound transduction and the selective advantage conferred upon a species capable of sensitive sound detection, it is not surprising that up to 1% of the approximately 30,000 or more human genes are necessary for hearing. There are hundreds of monogenic disorders for which hearing loss is one manifestation of a syndrome or the only disorder and therefore is nonsyndromic. Herein we review the supporting evidence for identifying over 30 genes for dominantly and recessively inherited, nonsyndromic, sensorineural deafness. The state of knowledge concerning their biological roles is discussed in the context of the controversies within an evolving understanding of the intricate molecular machinery of the inner ear.

Genetic screening for hearing loss

The recent discovery that mutations in GJB2, the gene that encodes connexin 26 (Cx26), are responsible for up to half the cases of autosomal recessive non-syndromic hearing loss and a significant proportion of sporadic hearing loss has had immense implications for medical evaluation and genetic screening. It is now possible to couple mutational analysis of GJB2 with universal screening and provide an unequivocal diagnosis of inherited hearing loss in up to 50% of babies with severe to profound non-syndromic hearing loss. Currently, other genetic tests should be performed on the basis of specific clinical features. Current potential candidates for screening include SLC26A4, in the presence of specific temporal bone anomalies, and WFS1, in the presence of a low-frequency hearing loss.

Origins and frequencies of SLC26A4 (PDS) mutations in east and south Asians: global implications for the epidemiology of deafness

Recessive mutations of SLC26A4 (PDS) are a common cause of Pendred syndrome and non-syndromic deafness in western populations. Although south and east Asia contain nearly one half of the global population, the origins and frequencies of SLC26A4 mutations in these regions are unknown. We PCR amplified and sequenced seven exons of SLC26A4 to detect selected mutations in 274 deaf probands from Korea, China, and Mongolia. A total of nine different mutations of SLC26A4 were detected among 15 (5.5%) of the 274 probands. Five mutations were novel and the other four had seldom, if ever, been identified outside east Asia. To identify mutations in south Asians, 212 Pakistani and 106 Indian families with three or more affected offspring of consanguineous matings were analysed for cosegregation of recessive deafness with short tandem repeat markers linked to SLC26A4. All 21 SLC26A4 exons were PCR amplified and sequenced in families segregating SLC26A4 linked deafness. Eleven mutant alleles of SLC26A4 were identified among 17 (5.4%) of the 318 families, and all 11 alleles were novel. SLC26A4 linked haplotypes on chromosomes with recurrent mutations were consistent with founder effects. Our observation of a diverse allelic series unique to each ethnic group indicates that mutational events at SLC26A4 are common and account for approximately 5% of recessive deafness in south Asians and other populations.

Neuro-otological findings in Pendred syndrome

Pendred syndrome is an autosomal recessive inherited disorder characterized by profound hearing impairment and inappropriate iodine release by the thyroid on perchlorate challenge. Thirty-three cases comprising members of 13 families and eight isolated cases were studied, with detailed audiological and vestibular investigation and computerized tomography. A uniform, profound, symmetrical sensorineural hearing loss was identified in all cases. Approximately one-third of the group reported progressive hearing impairment, in childhood or adolescence, associated with head injury, infection, or delayed secondary hydrops. Ninety per cent of the cases scanned showed dilated vestibular aqueducts, and all cases with progression of the hearing impairment demonstrated this structural abnormality. Approximately one-third of the cases had normal vestibular function, but a further third demonstrated a unilateral peripheral deficit, while the remaining third showed bilateral vestibular hypofunction. There was no intra-familial concordance of vestibular findings, and no correlation between vestibular abnormality and presence or absence of a dilated vestibular aqueduct, with or without a Mondini malformation. In older children and adults, Pendred syndrome was associated with a profound, symmetrical, sensorineural auditory impairment, and a variety of vestibular abnormalities, which are not uniform within families, or correlated with structural labyrinthine deformities.

Long-term consequences of congenital hypothyroidism in the era of screening programmes

Newborn screening for congenital hypothyroidism (CH), is one of the major achievements of medicine because early diagnosis and treatment has resulted in normal development in the vast majority of cases. However, all studies on outcome report up to 10% of patients with residual problems regarding mental development and neurological symptoms despite early diagnosis. Factors clearly associated with a less favourable outcome are late onset and an inadequate dosage of thyroid hormone substitution, a poor social-economic environment and compliance problems, while the impact of severity of CH at diagnosis on outcome is not completely settled, although most studies demonstrate a correlation between severity of hypothyroidism and poorer outcome. More recently in a few cases the molecular basis of CH has been clarified. It has become evident that, in some patients with persistent mental retardation and neurological symptoms, defects in transcription factors which are expressed in the thyroid gland as well as in the central nervous system (CNS) during embryonic development cause both defective thyroid and CNS development. The clarification of further molecular defects which affect the thyroid gland and brain development will help us to understand the poor outcome of patients with CH in the era of newborn screening and these diagnostic advances will ensure adequate counselling and care for these patients.

Differential diagnosis between Pendred and pseudo-Pendred syndromes: clinical, radiologic, and molecular studies

The disease gene for Pendred syndrome has been recently characterized and named PDS. It codes for a transmembrane protein called pendrin, which is highly expressed at the apical surface of the thyroid cell and functions as a transporter of chloride and iodide. Pendrin is also expressed at the inner ear level, where it appears to be involved in the maintenance of the endolymph homeostasis in the membranous labyrinth, and in the kidney, where it mediates chloride-formate exchange and bicarbonate secretion. Mutations in the PDS gene and the consequent impaired function of pendrin leads to the classic phenotype of Pendred syndrome, i.e. dyshormonogenic goiter and congenital sensorineural hearing loss. In the present study, we performed a detailed clinical, radiologic, and molecular analysis of six families presenting with clinical diagnosis of Pendred syndrome. In two families a homozygous pattern for PDS mutations was found, whereas the affected members of the other four families were compound heterozygotes. One family did not harbor PDS mutations. Among the four novel mutations described, one is a transversion in exon 2 (84C>A), leading to the substitution S28R. Two other novel mutations lie in exon 4 (398T>A) and in exon 16 (1790T>C), leading to the substitutions S133T and L597S, respectively. The fourth novel mutation (1614+1G>A) is located in the first base pair of intron 14, probably affecting the splicing of the PDS gene. Clinically, all patients had goiter with positive perchlorate test, hypothyroidism, and severe or profound sensorineural hearing loss. In all the individuals harboring PDS mutations, but not in the family without PDS mutations, inner ear malformations, such as enlargement of the vestibular aqueduct and of the endolymphatic duct and sac, were documented. The pseudo-Pendred phenotype exhibited by the family without PDS mutations is likely caused by an autoimmune thyroid disease associated with a sensorineural hearing loss of different origin.

Mutations of the PDS gene, encoding pendrin, are associated with protein mislocalization and loss of iodide efflux: implications for thyroid dysfunction in Pendred syndrome

Pendred syndrome (PDS) is an autosomal recessive disorder characterized by deafness and goiter. Phenotypic heterogeneity is observed in affected individuals, and thyroid dysfunction is particularly variable. The syndrome is caused by mutations in the PDS (SLC26A4) gene, encoding an anion transporter pendrin, which localizes to the apical membrane of thyroid follicular cells. PDS is thought to enable efflux iodide into the follicle lumen. More than 50 diseases causing mutations of PDS have been reported. Here we have investigated the effect of nine PDS missense mutations on pendrin localization and iodide transport with the view to understanding their functional impact. As demonstrated by transient expression of green fluorescent protein-tagged pendrin mutant constructs in mammalian cell lines, appropriate trafficking to the plasma membrane was observed for only two mutants. The remaining PDS mutants appear to be retained within the endoplasmic reticulum following transfection. Iodide efflux assays were performed using human embryonic kidney 293 cells transfected with mutant pendrin and cotransfected with sodium iodide transporter to provide a mechanism of iodide uptake. The results indicated loss of pendrin iodide transport for all mislocalizing mutations. However, PDS mutants are associated with variable thyroid dysfunction in affected subjects. We concluded that additional genetic and/or environmental factors influence the thyroid activity in Pendred syndrome.

Hereditary deafness and phenotyping in humans

Hereditary deafness has proved to be extremely heterogeneous genetically with more than 40 genes mapped or cloned for non-syndromic dominant deafness and 30 for autosomal recessive non-syndromic deafness. In spite of significant advances in the understanding of the molecular basis of hearing loss, identifying the precise genetic cause in an individual remains difficult. Consequently, it is important to exclude syndromic causes of deafness by clinical and special investigation and to use all available phenotypic clues for diagnosis. A clinical approach to the aetiological investigation of individuals with hearing loss is suggested, which includes ophthalmology review, renal ultrasound scan and neuro-imaging of petrous temporal bone. Molecular screening of the GJB2 (Connexin 26) gene should be undertaken in all cases of non-syndromic deafness where the cause cannot be identified, since it is a common cause of recessive hearing impairment, the screening is straightforward, and the phenotype unremarkable. By the same token, mitochondrial inheritance of hearing loss should be considered in all multigeneration families, particularly if there is a history of exposure to aminoglycoside antibiotics, since genetic testing of specific mitochondrial genes is technically feasible. Most forms of non-syndromic autosomal recessive hearing impairment cause a prelingual hearing loss, which is generally severe to profound and not associated with abnormal radiology. Exceptions to this include DFNB2 (MYO7A), DFNB8/10 (TMPRSS3) and DFNB16 (STRC) where age of onset may sometimes be later on in childhood, DFNB4 (SLC26A4) where there may be dilated vestibular aqueducts and endolymphatic sacs, and DFNB9 (OTOF) where there may also be an associated auditory neuropathy. Unusual phenotypes in autosomal dominant forms of deafness, include low frequency hearing loss in DFNA1 (HDIA1) and DFNA6/14/38 (WFS1), mid-frequency hearing loss in DFNA8/12 (TECTA), DFNA13 (COL11A2) and vestibular symptoms and signs in DFNA9 (COCH) and sometimes in DFNA11 (MYO7A). Continued clinical evaluation of types and course of hearing loss and correlation with genotype is important for the intelligent application of molecular testing in the next few years.

Genetics of hearing impairment

Our understanding of the genetics of hearing impairment (HI) has advanced rapidly during the last decade. In this review, we focus on HI due to single gene abnormalities, highlighting some of the more common causes of syndromic HI. We also outline the current state of knowledge of the genetics of non-syndromic HI. The most significant clinical advance has been the finding that mutations in GJB2 cause half of moderate-to-profound congenital hereditary deafness in many world populations. The implications of this finding for screening and genetic counseling are discussed.

Fluctuant, progressive hearing loss associated with Menière like vertigo in three patients with the Pendred syndrome

OBJECTIVE

To evaluate vestibular and long-term audiometric findings in patients with Pendred syndrome.

STUDY DESIGN

Retrospective analysis of long-term clinical data.

SETTING

University hospital department.

PATIENTS

Three patients with Pendred syndrome caused by a mutation in the SLC26A4 gene.

METHODS

Perchlorate discharge test, mutation analysis of the SLC26A4 gene, MR imaging of temporal bones, vestibular function test (in two cases) and serial audiometry. A saturation hyperbola with onset age was fitted to the audiometric threshold-on-age data using a nonlinear regression method. The residues remaining after regression were analyzed in a correlation analysis to detect significant ipsilateral or contralateral cofluctuation.

RESULTS

All three patients had a mutation in the SLC26A4 gene and bilateral enlarged vestibular aqueduct; two of them had a positive perchlorate discharge test but in one of two siblings this test was negative. Hearing loss was significantly progressive with significant ipsilateral and contralateral cofluctuation in all evaluable cases, combined with episodes of Menière like vertigo in two cases. The episodes of vertigo are as seen in Menière disease. One case had unilateral caloric areflexia and one had bilateral vestibular hyporeflexia, proven to be progressive in a repeat examination.

CONCLUSIONS

Patients with Pendred syndrome may exhibit progressive and fluctuant hearing loss with episodes of vertigo.

Aggressive metastatic follicular thyroid carcinoma with anaplastic transformation arising from a long-standing goiter in a patient with Pendred's syndrome

In this article we describe detailed pathological and molecular genetics studies in a consanguineous kindred with Pendred's syndrome. The index patient was a 53-year-old female patient with congenital deafness and goiter. Her parents were first-degree cousins. She had a large goiter (150 g) that had been present since childhood. One of her sisters and a niece are also deaf and have goiter as well. The presence of Pendred's syndrome was confirmed by a positive perchlorate test and the demonstration of a Mondini malformation. Thyroid function tests (under levothyroxine [LT4] therapy) were in the euthyroid range with a thyrotropin [TSH] level of 2.8 microU/mL (0.2-3.2), a serum total thyroxine (T4) of 90 nmol/L (54-142), and a serum total triiodothyronine (T3) of 2.7 nmol/L (0.8-2.4). Total thyroidectomy was performed, and the mass in the right lobe was found to have invaded adjacent tissues. The histopathological findings were consistent with a follicular carcinoma with areas of anaplastic transformation and lung metastasis. The patient was treated twice with 100 mCi 131iodine (3,700 MBq) and received suppressive doses of LT4. Postoperatively, the serum thyroglobulin (Tg) levels remained markedly elevated (2,352 to 41,336 ng/mL). The patient died of a sudden severe episode of hemoptysis. Sequence analysis of the PDS gene performed with DNA from the two relatives with Pendred's syndrome revealed the presence of a deletion of thymidine 279 in exon 3, a point mutation that results in a frameshift and a premature stop codon at codon 96 in the pendrin molecule. We concluded that prolonged TSH stimulation because of iodine deficiency or dyshormonogenesis in combination with mutations of oncogenes and/or tumor suppressor genes, may result in the development of follicular thyroid carcinomas that undergo transformation into anaplastic cancers. It is likely that these pathogenetic mechanisms have been involved in the development of aggressive metastatic thyroid cancer in this unusual patient with Pendred's syndrome.

Tat1, a novel sulfate transporter specifically expressed in human male germ cells and potentially linked to rhogtpase signaling

RhoGTPases (Rho, Rac, and Cdc42) are known to regulate multiple functions, including cell motility, adhesion, and proliferation; however, the signaling pathways underlying these pleiotropic effects are far from fully understood. We have recently described a new RhoGAP (GTPase activating protein for RhoGTPases) gene, MgcRacGAP, primarily expressed in male germ cells, at the spermatocyte stage. We report here the isolation, through two-hybrid cloning, of a new partner of MgcRacGAP, very specifically expressed in the male germ line and showing structural features of anion transporters. This large protein (970 amino acids and a predicted size of 109 kDa), we provisionally designated Tat1 (for testis anion transporter 1), is closely related to a sulfate permease family comprising three proteins in human (DRA, Pendrin, and DTD); it is predicted to be an integral membrane protein with 14 transmembrane helices and intracytoplasmic NH(2) and COOH termini. In situ hybridization studies demonstrate that Tat1 and MgcRacGAP genes are coexpressed in male germ cells at the spermatocyte stage. On testis sections, Tat1 protein can be immunodetected in spermatocytes and spermatids associated with plasma membrane. Two-hybrid and in vitro binding assays demonstrate that MgcRacGAP stably interacts through its NH(2)-terminal domain with the Tat1 COOH-terminal region. Expression of Tat1 protein in COS7 cells generates a 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene and chloride-sensitive sulfate transport. Therefore we conclude that Tat1 is a novel sulfate transporter specifically expressed in spermatocytes and spermatids and interacts with MgcRacGAP in these cells. These observations raise the possibility of a new regulatory pathway linking sulfate transport to Rho signaling in male germ cells.

Pendred syndrome, DFNB4, and PDS/SLC26A4 identification of eight novel mutations and possible genotype-phenotype correlations

Mutations in PDS (SLC26A4) cause both Pendred syndrome and DFNB4, two autosomal recessive disorders that share hearing loss as a common feature. The hearing loss is associated with temporal bone abnormalities, ranging from isolated enlargement of the vestibular aqueduct (dilated vestibular aqueduct, DVA) to Mondini dysplasia, a complex malformation in which the normal cochlear spiral of 2(1/2) turns is replaced by a hypoplastic coil of 1(1/2) turns. In Pendred syndrome, thyromegaly also develops, although affected persons usually remain euthyroid. We identified PDS mutations in the proband of 14 of 47 simplex families (30%) and nine of 11 multiplex families (82%) (P=0.0023). In all cases, mutations segregated with the disease state in multiplex families. Included in the 15 different PDS allele variants we found were eight novel mutations. The two most common mutations, T416P and IVS8+1G>A, were present in 22% and 30% of families, respectively. The finding of PDS mutations in five of six multiplex families with DVA (83%) and four of five multiplex families with Mondini dysplasia (80%) implies that mutations in this gene are the major genetic cause of these temporal anomalies. Comparative analysis of phenotypic and genotypic data supports the hypothesis that the type of temporal bone anomaly may depend on the specific PDS allele variant present.

Pendrin, encoded by the Pendred syndrome gene, resides in the apical region of renal intercalated cells and mediates bicarbonate secretion

Pendrin is an anion transporter encoded by the PDS/Pds gene. In humans, mutations in PDS cause the genetic disorder Pendred syndrome, which is associated with deafness and goiter. Previous studies have shown that this gene has a relatively restricted pattern of expression, with PDS/Pds mRNA detected only in the thyroid, inner ear, and kidney. The present study examined the distribution and function of pendrin in the mammalian kidney. Immunolocalization studies were performed using anti-pendrin polyclonal and monoclonal antibodies. Labeling was detected on the apical surface of a subpopulation of cells within the cortical collecting ducts (CCDs) that also express the H(+)-ATPase but not aquaporin-2, indicating that pendrin is present in intercalated cells of the CCD. Furthermore, pendrin was detected exclusively within the subpopulation of intercalated cells that express the H(+)-ATPase but not the anion exchanger 1 (AE1) and that are thought to mediate bicarbonate secretion. The same distribution of pendrin was observed in mouse, rat, and human kidney. However, pendrin was not detected in kidneys from a Pds-knockout mouse. Perfused CCD tubules isolated from alkali-loaded wild-type mice secreted bicarbonate, whereas tubules from alkali-loaded Pds-knockout mice failed to secrete bicarbonate. Together, these studies indicate that pendrin is an apical anion transporter in intercalated cells of CCDs and has an essential role in renal bicarbonate secretion.

Pendrin: an apical Cl-/OH-/HCO3- exchanger in the kidney cortex

The identities of the apical Cl-/base exchangers in kidney proximal tubule and cortical collecting duct (CCD) cells remain unknown. Pendrin (PDS), which is expressed at high levels in the thyroid and its mutation causes Pendred's syndrome, is shown to be an anion exchanger. We investigated the renal distribution of PDS and its function. Our results demonstrate that pendrin mRNA expression in the rat kidney is abundant and limited to the cortex. Proximal tubule suspensions isolated from kidney cortex were highly enriched in pendrin mRNA. Immunoblot analysis studies localized pendrin to cortical brush-border membranes. Nephron segment RT-PCR localized pendrin mRNA to proximal tubule and CCD. Expression studies in HEK-293 cells demonstrated that pendrin functions in the Cl-/OH-, Cl-/HCO3-, and Cl-/formate exchange modes. The conclusion is that pendrin is an apical Cl-/base exchanger in the kidney proximal tubule and CCD and mediates Cl-/OH-, Cl-/HCO3-, and Cl-/formate exchange.

Molecular analysis of the Pendred's syndrome gene and magnetic resonance imaging studies of the inner ear are essential for the diagnosis of true Pendred's syndrome

Pendred's syndrome is a combination of congenital sensorineural hearing loss and iodine organification defect leading to a positive perchlorate test and goiter. Although it is the commonest form of syndromic hearing loss, the variable clinical presentation contributes to the difficulty in securing a diagnosis. The identification of the disease gene (PDS) prompts the need to reevaluate the syndrome to identify possible clues for the diagnosis. To this purpose, in three Italian families presenting with the clinical features of Pendred's syndrome, the molecular analysis was accompanied by full clinical, biochemical, and radiological examination. A correlation between genotype and phenotype was found in the only patient with enlargement of vestibular aqueduct and endolymphatic duct and sac at magnetic resonance imaging. This subject was a compound heterozygote for a deletion in PDS exon 10 (1197delT, FS400) and a novel insertion in exon 19 (2182-2183insG, Y728X). The present study demonstrates for the first time the value of the combination of clinical/radiological and genetic studies in the diagnosis of Pendred's syndrome. The positivity of a perchlorate discharge test and the malformations of membranous labyrinth fit well with the recent achievements on the role of pendrin in thyroid hormonogenesis and the maintenance of endolymph homeostasis.

Expression of pendrin and the Pendred syndrome (PDS) gene in human thyroid tissues

The gene recently cloned that is responsible for the Pendred syndrome (PDS), an autosomal recessive disease characterized by goiter and congenital sensorineural deafness, is mainly expressed in the thyroid gland. Its product, designated pendrin, was shown to transport chloride and iodide. To investigate whether the PDS gene is altered during thyroid tumorigenesis, PDS gene expression and pendrin expression were studied using real-time kinetic quantitative PCR and antipeptide antibodies, respectively, in normal, benign, and malignant human thyroid tissues. The results were then compared to those observed for sodium/iodide symporter (NIS) expression. In normal tissue, pendrin is localized at the apical pole of thyrocytes, and this in contrast to the basolateral location of NIS. Immunostaining for pendrin was heterogeneous both inside and among follicles. In hyperfunctioning adenomas, the PDS messenger ribonucleic acid level was in the normal range, although immunohistochemical analysis showed strong staining in the majority of follicular cells. In hypofunctioning adenomas, mean PDS gene expression was similar to that detected in normal thyroid tissues, but pendrin immunostaining was highly variable. In thyroid carcinomas, PDS gene expression was dramatically decreased, and pendrin immunostaining was low and was positive only in rare tumor cells. This expression profile was similar to that observed for the NIS gene and its protein product. In conclusion, our study demonstrates that pendrin is located at the apical membrane of thyrocytes and that PDS gene expression is decreased in thyroid carcinomas.

A novel mutation in the pendrin gene associated with Pendred's syndrome

OBJECTIVE

Pendred's syndrome is an autosomal recessive disorder characterized by goitre, sensorineural deafness and iodide organification defect. It is one of the most frequent causes of congenital deafness, accounting for about 10% of hereditary hearing loss. It is caused by mutations in the pendrin (PDS) gene, a 21 exon gene located on chromosome 7. The aim of this study was to examine an Italian family affected with Pendred's syndrome at the molecular level.

PATIENTS

Thirteen subjects belonging to a family from Southern Italy were evaluated for the clinical and genetic features of Pendred's syndrome.

MEASUREMENTS

Exons 2-21 of the PDS gene were amplified from peripheral leucocytes by the polymerase chain reaction; mutation analysis was performed by single strand conformation polymorphism, direct sequencing and restriction analysis.

RESULTS

The index patient had the classical triad of the syndrome and harboured two mutations in the PDS gene in the form of compound heterozygosity. He was found to be heterozygous for a cytosine to adenosine mutation at nucleotide 1523 in exon 13 and for a IVS 1001 + 1G --> A mutation. The former is a novel mutation which results in a change of 508 threonine to asparagine in the putative eleventh transmembrane domain. The latter mutation in the donor splice site has already been described in other patients and is thought to lead to aberrant splicing and premature protein truncation. Three subjects who were heterozygous for one mutation had normal phenotypes. Two subjects had sensorineural deafness and were heterozygous for a single mutation. Goitre was found only in patients with Pendred's syndrome and was absent in all other individuals, albeit residing in an iodine-deficient area.

CONCLUSIONS

We have identified a novel mutation in the pendrin gene causing Pendred's syndrome, and confirm that molecular analysis is a useful tool for a definitive diagnosis. This is particularly relevant in cases such as in the subjects of our family in which the clinical features might be misleading and other genetics factors might be responsible for deafness.

Pendrin does not increase sulfate uptake in mammalian COS-7 cells

Pendred's syndrome is characterized by goiter, sensorineural deafness and impaired iodide organification. It is one of the most frequent causes of congenital deafness accounting for about 10% of hereditary hearing loss. It is caused by mutations in the pendrin (PDS) gene, which was postulated to be a sulfate transporter, because of its homology with other genes. We tested sulfate transport in mammalian COS-7 cells that were transiently transfected with PDS cDNA. 35SO4 uptake increased in a time-dependent manner, but this phenomenon was similar in cells transfected with PDS and in mock-transfected cells (450 and 360 cpm/beta-gal units at 10 min, respectively; 38,250 and 31,000 cpm/beta-gal units, at 12 h, respectively). There was no significant increase in 35SO4 uptake using increasing amounts of PDS-containing plasmid (up to 12 microg per dish). These data indicate that pendrin is not a sulfate transporter. Additional functional studies on this protein are warranted to clarify its role in thyroid pathophysiology and inner ear development.

Mutations of the Pendred syndrome gene (PDS) in patients with large vestibular aqueduct

A recent report demonstrated the presence of a mutation in the Pendred syndrome gene (PDS) of patients with large vestibular aqueducts but without goitre. We studied PDS mutations in members of four Japanese families, among which five affected members showed bilateral enlarged vestibular aqueducts. All affected members exhibited moderate to severe bilateral fluctuating sensorineural hearing loss and the absence of goitre. Three members also suffered from recurrent episodic vertiginous spells. Analysis of PDS mutation revealed two single base changes (mis-sense mutations) in exons 19 and 10. The first was an A-->G transition at nucleotide position 2168, resulting in a predicted His-->Arg substitution at position 723 (H723R), whereas the second was a C-->T transition at nucleotide position 1229, resulting in a predicted Thr-->Met substitution at position 410 (T410M). Both mutations are situated in the extracellular domain close to the C terminal. It thus appears that PDS mutations can lead not only to classic Pendred syndrome, but also to large vestibular aqueduct syndrome.

Recent advances in understanding the molecular basis of primary congenital hypothyroidism

Primary congenital hypothyroidism is characterized by low levels of circulating thyroid hormones and raised levels of thyrotropin at birth. It can be either permanent or transitory. Most permanent cases (80-85%) result from alterations in the formation of the thyroid gland during embryogenesis (thyroid dysgenesis), and several were shown recently to be produced by mutations in genes responsible for the development of thyroid follicular cells (TITF1, TITF2, PAX8 and TSHR). Less frequently, congenital hypothyroidism is determined by defects in thyroid hormone synthesis (hormonogenesis defects). The latter are usually associated with goiter. Recently, the molecular mechanisms of two forms of hormonogenesis defects (iodine transport defects and Pendred syndrome) were elucidated.

Histopathological findings suggest the diagnosis in an atypical case of Pendred syndrome

Radiological malformation of the labyrinth, specifically dilatation of the vestibular aqueduct, has been clearly established as a feature in the majority of patients with Pendred syndrome. Mutations of the Pendred syndrome (PDN) gene have been identified in this autosomal recessive form of deafness. There is no direct correlation between the nature of the underlying mutation and the clinical features of deafness, thyroid dysfunction and cochlear malformation. We report a family, the proband of which was thought to deafness secondary to congenital hypothyroidism. At autopsy, histopathological examination of this patient revealed a dilated vestibular aqueduct. Subsequent work on the family has confirmed the diagnosis of Pendred syndrome in the proband and her affected sister.

Splice-site mutation in the PDS gene may result in intrafamilial variability for deafness in Pendred syndrome

Pendred syndrome is a recessive inherited disorder that consists of developmental abnormalities of the cochlea, sensorineural hearing loss, and diffuse thyroid enlargement (goiter). This disorder may account for up to 10% of cases of hereditary deafness. The disease gene (PDS) has been mapped to chromosome 7q22-q31, and encodes a chloride-iodide transport protein. We performed mutation analysis of individual exons of the PDS gene in one Spanish family that shows intrafamilial variability of the deafness phenotype (two patients with profound and one with moderate-severe deafness). We identified a new splice-site mutation affecting intron 4 of the PDS gene, at nucleotide position 639+7. RNA analysis from lymphocytes of the affected patients showed that mutation 639+7A-->G generates a new donor splice site, leading to an mRNA with an insertion of six nucleotides from intron 4 of PDS. Since the newly created donor splice site is likely to compete with the normal one, variations of the levels of normal and aberrant transcripts of the PDS gene in the cochlea may explain the variability in the deafness presentation.

Expression pattern of the mouse ortholog of the Pendred's syndrome gene (Pds) suggests a key role for pendrin in the inner ear

Pendred's syndrome is an autosomal-recessive disorder characterized by deafness and goiter. After our recent identification of the human gene mutated in Pendred's syndrome (PDS), we sought to investigate in greater detail the expression of the gene and the function of its encoded protein (pendrin). Toward that end, we isolated the corresponding mouse ortholog (Pds) and performed RNA in situ hybridization on mouse inner ears (from 8 days postcoitum to postnatal day 5) to establish the expression pattern of Pds in the developing auditory and vestibular systems. Pds expression was detected throughout the endolymphatic duct and sac, in distinct areas of the utricle and saccule, and in the external sulcus region within the cochlea. This highly discrete expression pattern is unlike that of any other known gene and involves several regions thought to be important for endolymphatic fluid resorption in the inner ear, consistent with the putative functioning of pendrin as an anion transporter. These studies provide key first steps toward defining the precise role of pendrin in inner ear development and elucidating the pathogenic mechanism for the deafness seen in Pendred's syndrome.

Concurrence of Pendred syndrome, autoimmune thyroiditis, and simple goiter in one family

Pendred syndrome is the autosomal recessively transmitted association of familial goiter and congenital deafness. There is no specific biochemical marker of this disease, and the diagnosis depends upon the demonstration of the triad of congenital sensorineural hearing loss, goiter, and abnormal perchlorate discharge test. Pendred syndrome is caused by mutations within the putative ion transporter gene (PDS gene), located on chromosome 7q. A wide variation in the clinical presentation of this condition, and its well documented phenotypic overlap with other thyroid disorders (such as Hashimoto's thyroiditis), can lead to diagnostic difficulties. The potential for misdiagnosis increases when these disorders occur coincidentally in the same family. We describe a kindred in which Pendred syndrome, autoimmune thyroiditis, and simple goiter coexisted, to highlight these diagnostic pitfalls and to illustrate the use of mutational analysis in resolving diagnostic confusion.

Molecular genetics of congenital hypothyroidism

Congenital thyroid gland defects - resulting in reduced production of the hormones triiodothyronine (T3) and thyroxine (T4) - can be a consequence of either reduced or absent thyroid tissue (thyroid dysgenesis) or, less frequently, of impairment in the biochemical mechanisms responsible for hormone biosynthesis (thyroid dyshormonogenesis). Recent studies have revealed how mutations in the genes encoding either transcription factors or the thyroid stimulating hormone receptor cause, in humans or in mouse models, thyroid dysgenesis. This demonstrates, for the first time, the heritability of this condition. New genes responsible for thyroid dyshormonogenesis have also been discovered.

Pendred's syndrome: identification of the genetic defect a century after its recognition

Pendred's syndrome is an autosomal recessive disease characterized by goiter and congenital sensorineural deafness. Most patients with Pendred's syndrome are euthyroid, but the perchlorate test is positive indicating an impaired iodide organification. The sensorineural deafness is typically associated with a malformation of the inner ear, referred to as Mondini cochlea. The incidence of Pendred's syndrome is thought to be as high as 7.5 to 10 in 100,000 individuals, and it has been estimated to account for about 10% of the cases with hereditary deafness. Linkage of Pendred's syndrome to chromosome 7q22-31.1 was first established in 1996, and the Pendred's syndrome gene (PDS gene) was cloned in 1997. The PDS gene encodes pendrin, a highly hydrophobic 780 aminoacid protein with 11 transmembrane domains. Its function is unknown. Sequence comparison reveals a very high homology to several sulfate transporters suggesting that it could be a sulfate or anion transporter. A wide spectrum of mutations in the PDS gene has now been associated with Pendred's syndrome. Molecular analysis of the PDS gene is useful to make a definite diagnosis in familial and sporadic cases with Pendred's syndrome, and will be helpful for determining the true prevalence of this disorder.