Clinical and molecular analysis of three Mexican families with Pendred's syndrome

Functional studies of deafness-associated pendrin and prestin variants

Pendrin and prestin are evolutionary conserved membrane proteins that are essential for normal hearing. Pendrin is an anion transporter required for normal development and maintenance of ion homeostasis in the inner ear, while prestin is a voltage-dependent motor responsible for cochlear amplification essential for high sensitivity and frequency selectivity of mammalian hearing. Dysfunction of these proteins result in hearing loss in humans, and numerous deafness-associated pendrin and prestin variants have been identified in patients. However, the pathogenic impacts of many of these variants are ambiguous. Here we report results from our ongoing efforts in experimentally characterizing pendrin and prestin variants using in vitro functional assays, providing invaluable information regarding their pathogenicity.

Genetic etiology of non-syndromic hearing loss in Latin America

Latin America comprises all countries from South and Central America, in addition to Mexico. It is characterized by a complex mosaic of regions with heterogeneous genetic profiles regarding the geographical origin of the ancestors and proportions of admixture between the Native American, European and African components. In the first years following the findings of the role of the GJB2/GJB6 genes in the etiology of hearing loss, most scientific investigations about the genetics of hearing loss in Latin America focused on assessing the frequencies of pathogenic variants in these genes. More recently, modern techniques allowed researchers in Latin America to make exciting contributions to the finding of new candidate genes, novel mechanisms of inheritance in previously known genes, and characterize a wide diversity of variants, many of them unique to Latin America. This review aimed to provide a general landscape of the genetic studies about non-syndromic hearing loss in Latin America and their main scientific contributions. It allows the conclusion that, although there are similar contributions of some genes, such as GJB2/GJB6, when compared to European and North American countries, Latin American populations revealed some peculiarities that indicate the need for tailored strategies of screening and diagnosis to specific geographic regions.

Study on the relationship between the pathogenic mutations of SLC26A4 and CT phenotypes of inner ear in patient with sensorineural hearing loss

To investigate the possible association of pathogenic mutations of SLC26A4 and computerized tomography (CT) phenotypes of inner ear, and explore the feasibility of using the method of gene sequence analysis. A total of 155 patients with bilateral hearing loss carrying SLC26A4 gene mutations were further subjected to high-resolution temporal bone CT and thyroid B ultrasound tests. The potential relationship between the pathogenic mutations of gene and the CT phenotypes were analyzed. As a result, 65 patients (41.9%, 65/155) carried SLC26A4 gene mutations, and 27 cases were detected with pathogenic mutations of SLC26A4 where IVS7-2A>G (55.6%, 15/27) was the most common pathogenic mutation. Amongst them, 19 patients carrying bi-allelic SLC26A4 mutations were all confirmed to have inner ear malformation by CT scan including four cases of enlarged vestibular aqueduct (EVA) and 15 cases of Mondini dysplasia (MD). However, there was only one in eight cases of single allele pathogenic mutation who was confirmed to have EVA by CT scan. Further, only one patient with EVA was confirmed to be slightly higher of total T3 than normal by thyroid ultrasound scan and thyroid hormone assays. These findings suggested that CT detection and SLC26A4 gene detection are efficient methods to diagnose EVA, which can complement each other. Also, the bi-allelic pathogenic mutations of SLC26A4 are more likely to induce inner ear malformation than single allele pathogenic mutation.

Novel pathogenic variants underlie SLC26A4-related hearing loss in a multiethnic cohort

OBJECTIVES

The genetics of sensorineural hearing loss is characterized by a high degree of heterogeneity. Despite this heterogeneity, DNA variants found within SLC26A4 have been reported to be the second most common contributor after those of GJB2 in many populations.

METHODS

Whole exome sequencing and/or Sanger sequencing of SLC26A4 in 117 individuals with sensorineural hearing loss with or without inner ear anomalies but not with goiter from Turkey, Iran, and Mexico were performed.

RESULTS

We identified 27 unique SLC26A4 variants in 31 probands. The variants c.1673A > G (p.N558S), c.1708-1G > A, c.1952C > T (p.P651L), and c.2090-1G > A have not been previously reported. The p.N558S variant was detected in two unrelated Mexican families.

CONCLUSION

A range of SLC26A4 variants without a common recurrent mutation underlies SLC26A4-related hearing loss in Turkey, Iran, and Mexico.

Pendred syndrome

Pendred syndrome is an autosomal recessive disorder that is classically defined by the combination of sensorineural deafness/hearing impairment, goiter, and an abnormal organification of iodide with or without hypothyroidism. The hallmark of the syndrome is the impaired hearing, which is associated with inner ear malformations such as an enlarged vestibular aqueduct (EVA). The thyroid phenotype is variable and may be modified by the nutritional iodine intake. Pendred syndrome is caused by biallelic mutations in the SLC26A4/PDS gene, which encodes the multifunctional anion exchanger pendrin. Pendrin has affinity for chloride, iodide, and bicarbonate, among other anions. In the inner ear, pendrin functions as a chloride/bicarbonate exchanger that is essential for maintaining the composition and the potential of the endolymph. In the thyroid, pendrin is expressed at the apical membrane of thyroid cells facing the follicular lumen. Functional studies have demonstrated that pendrin can mediate iodide efflux in heterologous cells. This, together with the thyroid phenotype observed in humans (goiter, impaired iodine organification) suggests that pendrin could be involved in iodide efflux into the lumen, one of the steps required for thyroid hormone synthesis. Iodide efflux can, however, also occur in the absence of pendrin suggesting that other exchangers or channels are involved. It has been suggested that Anoctamin 1 (ANO1/TMEM16A), a calcium-activated anion channel, which is also expressed at the apical membrane of thyrocytes, could participate in mediating apical efflux. In the kidney, pendrin is involved in bicarbonate secretion and chloride reabsorption. While there is no renal phenotype under basal conditions, severe metabolic alkalosis has been reported in Pendred syndrome patients exposed to an increased alkali load. This review provides an overview on the clinical spectrum of Pendred syndrome, the functional data on pendrin with a focus on its potential role in the thyroid, as well as the controversy surrounding the relative physiological roles of pendrin and anoctamin.

Mapping pathogenic mutations suggests an innovative structural model for the pendrin (SLC26A4) transmembrane domain

Human pendrin (SLC26A4) is an anion transporter mostly expressed in the inner ear, thyroid and kidney. SLC26A4 gene mutations are associated with a broad phenotypic spectrum, including Pendred Syndrome and non-syndromic hearing loss with enlarged vestibular aqueduct (ns-EVA). No experimental structure of pendrin is currently available, making phenotype-genotype correlations difficult as predictions of transmembrane (TM) segments vary in number. Here, we propose a novel three-dimensional (3D) pendrin transmembrane domain model based on the SLC26Dg transporter. The resulting 14 TM topology was found to include two non-canonical transmembrane segments crucial for pendrin activity. Mutation mapping of 147 clinically validated pathological mutations shows that most affect two previously undescribed TM regions.

Evaluation of genotype-phenotype relationships in patients referred for endocrine assessment in suspected Pendred syndrome

DESIGN

Patients with Pendred syndrome have genotypic and phenotypic variability, leading to challenges in definitive diagnosis. Deaf children with enlarged vestibular aqueducts are often subjected to repeated investigations when tests for mutations in SLC26A4 are abnormal. This study provides genotype and phenotype information from patients with suspected Pendred syndrome referred to a single clinical endocrinology unit.

METHODS

A retrospective analysis of 50 patients with suspected Pendred syndrome to investigate the correlation between genetic, perchlorate discharge test (PDT) and endocrine status.

RESULTS

Eight patients with monoallelic SLC26A4 mutations had normal PDT. Of the 33 patients with biallelic mutations, ten of 12 patients with >30% discharge developed hypothyroidism. In our cohort, c.626G>T and c.3-2A>G result in milder clinical presentations with lower median perchlorate discharge of 9.3% (interquartile range 4-15%) compared with 40% (interquartile range 21-60%) for the remaining mutations. Eight novel mutations were detected. All patients with PDT <30% remained euthyroid to date, although the majority are still under the age of 30. There was a significant correlation between PDT and goitre size (R=0.61, P=0.0009) and the age of onset of hypothyroidism (R=-0.62, P=0.0297). In our population, the hazard of becoming hypothyroid increased by 7% per percentage point increase in PDT (P<0.001).

CONCLUSION

There is a correlation between SLC26A4 genotype and thyroid phenotype. If results hold true for larger patient numbers and longer follow-up, then for patients with monoallelic mutations, PDT could be unnecessary. Patients with biallelic mutations and PDT discharge >30% have a high risk of developing goitre and hypothyroidism, and should have lifelong monitoring.

Identification of a founder mutation for Pendred syndrome in families from northwest Iran

OBJECTIVE

Mutations in the SLC26A4 gene cause both Pendred syndrome and autosomal recessive nonsyndromic hearing loss (ARNSHL) at the DFNB4 locus. The SLC26A4 mutations vary among different communities. Previous studies have shown that mutations in the SLC26A4 gene are responsible for the more common syndromic hereditary hearing loss in Iran. This study assesses the possibility of a founder mutation for Pendred syndrome in northwest Iran.

MATERIALS AND METHODS

In this study, we performed comprehensive clinical and genetic evaluations in two unrelated families from northwest Iran with nine members affected by hearing loss (HL). After testing short tandem repeat (STR) markers to confirm linkage to the SLC26A4 locus, we screened the SLC26A4 gene by Sanger sequencing of all 21 exons, exon-intron boundaries and the promoter region for any causative mutation. We identified the same causative mutation in these two families as we had detected earlier in two other Azeri families from northwest Iran. To investigate the possibility of a founder effect in these four families, we conducted haplotype analysis, and 14 single nucleotide polymorphisms (SNPs) throughout the SLC26A4 gene were genotyped.

RESULTS

Patients in the two families showed the phenotype of Pendred syndrome. A known frameshift mutation (c.965insA, p.N322Fs7X) in exon 8 was identified in the two families, which was the same mutation that we detected previously in two other Azeri families. The results of haplotype analysis showed that all 15 patients from four families shared the founder mutation. Common haplotypes were not observed in noncarrier members.

CONCLUSIONS

Based on the results of our two studies, the c.965insA mutation has only been described in Iranian families from northwest Iran, so there is evidence for a founder mutation originating in this part of Iran.

Atrophic thyroid follicles and inner ear defects reminiscent of cochlear hypothyroidism in Slc26a4-related deafness

Thyroid hormone is essential for inner ear development and is required for auditory system maturation. Human mutations in SLC26A4 lead to a syndromic form of deafness with enlargement of the thyroid gland (Pendred syndrome) and non-syndromic deafness (DFNB4). We describe mice with an Slc26a4 mutation, Slc26a4 (loop/loop) , which are profoundly deaf but show a normal sized thyroid gland, mimicking non-syndromic clinical signs. Histological analysis of the thyroid gland revealed defective morphology, with a majority of atrophic microfollicles, while measurable thyroid hormone in blood serum was within the normal range. Characterization of the inner ear showed a spectrum of morphological and molecular defects consistent with inner ear pathology, as seen in hypothyroidism or disrupted thyroid hormone action. The pathological inner ear hallmarks included thicker tectorial membrane with reduced β-tectorin protein expression, the absence of BK channel expression of inner hair cells, and reduced inner ear bone calcification. Our study demonstrates that deafness in Slc26a4 (loop/loop) mice correlates with thyroid pathology, postulating that sub-clinical thyroid morphological defects may be present in some DFNB4 individuals with a normal sized thyroid gland. We propose that insufficient availability of thyroid hormone during inner ear development plays an important role in the mechanism underlying deafness as a result of SLC26A4 mutations.

Analysis of the thyroid phenotype in 42 patients with Pendred syndrome and nonsyndromic enlargement of the vestibular aqueduct

BACKGROUND

Pendred syndrome (PS), a recessive disorder caused by mutations in the SLC26A4 (PDS) gene, is associated with deafness and goiter. SLC26A4 mutations have also been identified in patients exhibiting isolated sensorineural hearing loss without apparent thyroid abnormality (nonsyndromic enlargement of the vestibular aqueduct; nonsyndromic EVA). Our aim was to describe systematically the thyroidal phenotypes and the SLC26A4 genotypes of patients presenting with PS or nonsyndromic EVA.

METHODS

Nineteen patients with PS and 23 patients with nonsyndromic EVA, aged 5-53 years, were included. They underwent thyroid evaluation (physical examination, biological thyroid function tests, measurement of thyroglobulin level, thyroid ultrasonography, and thyroid (123)I scintigraphy with perchlorate discharge test), otological evaluation, and SLC26A4 mutation screening.

RESULTS

In 19 patients with PS, goiter was identified in 15 (79%) and hypothyroidism in 15 (79%); hypothyroidism was subclinical in four patients and congenital in six patients. The perchlorate discharge test (PDT) was positive in 10/16 (63%). Morphological evaluation of the inner ear using MRI and/or CT showed bilateral EVA in 15/15 PS patients. Mutation screening revealed two SLC26A4 mutant alleles in all 19 PS patients that were homozygous in two families and compound heterozygous in 12 families. In the 23 patients with nonsyndromic EVA, systematic thyroid evaluation found no abnormalities except for slightly increased thyroglobulin levels in two patients. SLC26A4 mutations were identified in 9/23 (39%). Mutations were biallelic in two (compound heterozygous) and monoallelic in seven patients.

CONCLUSION

The thyroid phenotype is widely variable in PS. SLC26A4 mutation screening is needed in patients exhibiting PS or nonsyndromic EVA. PS is associated with biallelic SLC26A4 mutations and nonsyndromic EVA with no, monoallelic, or biallelic SLC26A4 mutations. Systematic thyroid evaluation is recommended in patients with nonsyndromic EVA associated with one or two SLC26A4 mutations. We propose using a combination of three parameters to define and diagnose PS: (i) sensorineural deafness with bilateral EVA; (ii) thyroid abnormality comprising goiter and/or hypothyroidism and/or a positive PDT; (iii) biallelic SLC26A4 mutations.

Identification of two heterozygous deafness mutations in SLC26A4 (PDS) in a Chinese family with two siblings

OBJECTIVE

To detect genetic cause of two Chinese siblings (patient 1 and 2) with Pendred syndrome.

DESIGN

Patients and their parents underwent clinical and genetic evaluations. To identify genetic mutations, sequencing of SLC26A4 was carried out.

STUDY SAMPLE

Two siblings and their parents.

RESULTS

Clinical evaluations showed that patient 1 suffered from bilateral postlingual progressive sensorineural hearing loss with enlarged vestibular aqueduct and slight diffuse multinodular goiter with euthyroid, and patient 2 suffered from bilateral prelingual progressive sensorineural hearing loss with enlarged vestibular aqueduct and no goiter with euthyroid. Furthermore, the sequence analysis of SLC26A4 indicated that either of the two siblings presented a compound heterozygote for the c.919A>G mutation in the splice site of intron 7 and for the c.1548insC mutation in exon 14. Their mother was a heterozygous carrier of the splice site mutation in intron 7, and their father was a heterozygous carrier of the insertion mutation in exon 14.

CONCLUSIONS

Mutation analysis identified a compound heterozygous mutation (c.919A>G/c.1548insC) in SLC26A4 in two Chinese siblings with Pendred syndrome. Also, c.1548insC was first reported in the Chinese population. Although the two siblings from the same family carried the same genotype, they presented different phenotypes.

Absence of primary hypothyroidism and goiter in Slc26a4 (-/-) mice fed on a low iodine diet

BACKGROUND

Mutations in the SLC26A4 gene, coding for the anion transporter pendrin, are responsible for Pendred syndrome, characterized by congenital sensorineural deafness and dyshormonogenic goiter. The physiological role of pendrin in the thyroid is still unclear and the lack of a thyroid phenotype in some patients with SLC26A4 mutations and in Slc26a4 (-/-) mice indicate the existence of environmental or individual modifiers able to compensate for pendrin inactivation in the thyroid. Since pendrin can transport iodide in vitro, variations in iodide supply have been claimed to account for the thyroid phenotype associated with pendrin defects.

AIM

The Slc26a4 (-/-) mouse model was used to test the hypothesis that iodide supply may influence the penetrance and expressivity of SLC26A4 mutations.

MATERIALS AND METHODS

Slc26a4 (-/-) and (+/+) mice were fed up to 6 months on a standard or low iodine diet and were evaluated for thyroid structural abnormalities or biochemical hypothyroidism.

RESULTS

A 27-fold iodide restriction induced similar modifications in thyroid histology, but no differences in thyroid size, T4 or TSH levels were observed between between Slc26a4 (-/-) and (+/+) mice, either in standard conditions and during iodine restriction.

CONCLUSIONS

Iodide restriction is not able to induce a thyroid phenotype in Slc26a4 (-/-) mice. These experimental data, together with those coming from a review of familial Pendred cases leaving in regions either with low or sufficient iodide supply, support the idea that the expression of thyroid phenotype in Pendred syndrome is more powerfully influenced by individual factors than by dietary iodide.

Clinical and molecular characteristics of Pendred syndrome

Pendred syndrome is an autosomal recessive disorder defined by sensorineural deafness, goiter and a partial defect in the organification of iodide. It is caused by biallelic mutations in the SLC26A4 gene, which encodes pendrin, a multifunctional anion exchanger. At the level of the inner ear, pendrin is important for the creation of a normal endolymph composition and the maintenance of the endocochlear potential. In the thyroid, pendrin is expressed at the apical membrane of thyroid follicular cells and it appears to be involved in mediating iodide efflux into the lumen and/or maintenance of the follicular pH. Goiter development and hypothyroidism vary among affected individuals and seem to be partially dependent on nutritional iodide intake. In the kidney, pendrin functions as a chloride/bicarbonate exchanger. Elucidation of the molecular basis of Pendred syndrome and the function of pendrin has provided unexpected novel insights into the pathophysiology of the inner ear, thyroid hormone synthesis, and chloride/bicarbonate exchange in the kidney.

A coherent organization of differentiation proteins is required to maintain an appropriate thyroid function in the Pendred thyroid

CONTEXT

Pendred syndrome is caused by mutations in the gene coding for pendrin, an apical Cl-/I- exchanger.

OBJECTIVE

To analyze intrathyroidal compensatory mechanisms when pendrin is lacking, we investigated the thyroid of a patient with Pendred syndrome. The expression of proteins involved in thyroid hormone synthesis, markers of oxidative stress (OS), cell proliferation, apoptosis, and antioxidant enzymes were analyzed.

RESULTS

Three morphological zones were identified: nearly normal follicles with iodine-rich thyroglobulin in the colloid (zone 1.a), small follicles without iodine-rich thyroglobulin in lumina (zone 1.b), and destroyed follicles (zone 2). In zones 1.a, dual oxidase (Duox) and thyroid peroxidase (TPO) were localized at the apical pole, OS and cell apoptosis were absent, but ClC-5 expression was strongly increased. In zones 1.b, Duox and TPO were aberrantly present and increased in the cytosol and associated with high OS, apoptosis, cell proliferation, and increased expression of peroxiredoxin-5, catalase, and dehalogenase-1 but moderate ClC-5 expression.

CONCLUSION

In conclusion, the absence of pendrin is accompanied by increased ClC-5 expression that may transiently compensate for apical iodide efflux. In more affected follicles, Duox and TPO are relocated in the cytosol, leading to abnormal intracellular thyroid hormone synthesis, which results in cell destruction presumably because intracellular OS cannot be buffered by antioxidant defenses.

[Pendred's syndrome: a cause of goiter associated with deafness]

Pendred's syndrome is an autosomal recessive disorder leading to congenital sensorineural hearing loss and a variable degree of goiter due to reduced iodine organification. The cause of this disease is dysfunction of an anion transporter protein located on the apical membrane of thyrocytes, called pendrin, which is also found in the kidney and cochlea. Molecular analysis of the gene is useful to identify other affected family members and provide proper genetic advice and early diagnosis in descendants. We present the cases of two siblings with sensorineural deafness who were diagnosed with Pendred's syndrome as adults because one of them consulted for goiter.

Distinct and novel SLC26A4/Pendrin mutations in Chinese and U.S. patients with nonsyndromic hearing loss

Mutations of the human SLC26A4/PDS gene constitute the most common cause of syndromic and nonsyndromic hearing loss. Definition of the SLC26A4 mutation spectrum among different populations with sensorineural hearing loss is important for development of optimal genetic screening services for congenital hearing impairment. We screened for SLC26A4 mutations among Chinese and U.S. subjects with hearing loss, using denaturing HPLC (DHPLC) and direct DNA sequencing. Fifty-two of 55 Chinese subjects with deafness accompanied by enlargement of the vestibular aqueduct (EVA) exhibited at least one mutant SLC26A4 allele, whereas SLC26A4 mutations were found in only 2 of 116 deaf Chinese patients without EVA. The spectrum of SLC26A4 mutations differed among Chinese and U.S. subjects and included 10 previously unreported SLC26A4 variants: 4 in the Chinese population (p.E303Q, p.X329, p.X467, p.X573) and 6 in the U.S. population (p.V250A, p.D266N, p.F354S, p.D697A, p.K715N, p.E737D). Among the seven novel in-frame missense mutations, five encoded SLC26A4 proteins with substantially reduced Cl(-)/anion exchange activity as expressed and measured in Xenopus oocytes, but four of these were sufficiently active to allow study of anion selectivity. The only mutant polypeptide exhibiting complete loss of anion exchange function, p.E303Q, was expressed at or near the oocyte surface at near-wild-type levels. Two variants, p.F354S and p.E737D, displayed selective reduction in relative rate of Cl(-)/HCO(3)(-) exchange compared with similarly measured rates of Cl(-)/Cl(-) and Cl(-)/I(-) exchange. Our data show that mutation analysis of the SLC26A4 gene is of high diagnostic yield among subjects with deafness and bilateral EVA in both China and the U.S. However, the pathogenicity of monoallelic SLC26A4 gene variants in patients with hearing loss remains unclear in many instances.

Minireview: The sodium-iodide symporter NIS and pendrin in iodide homeostasis of the thyroid

Thyroid hormones are essential for normal development and metabolism. Thyroid hormone biosynthesis requires iodide uptake into the thyrocytes and efflux into the follicular lumen, where it is organified on selected tyrosyls of thyroglobulin. Uptake of iodide into the thyrocytes is mediated by an intrinsic membrane glycoprotein, the sodium-iodide symporter (NIS), which actively cotransports two sodium cations per each iodide anion. NIS-mediated transport of iodide is driven by the electrochemical sodium gradient generated by the Na(+)/K(+)-ATPase. NIS is expressed in the thyroid, the salivary glands, gastric mucosa, and the lactating mammary gland. TSH and iodide regulate iodide accumulation by modulating NIS activity via transcriptional and posttranscriptional mechanisms. Biallelic mutations in the NIS gene lead to a congenital iodide transport defect, an autosomal recessive condition characterized by hypothyroidism, goiter, low thyroid iodide uptake, and a low saliva/plasma iodide ratio. Pendrin is an anion transporter that is predominantly expressed in the inner ear, the thyroid, and the kidney. Biallelic mutations in the SLC26A4 gene lead to Pendred syndrome, an autosomal recessive disorder characterized by sensorineural deafness, goiter, and impaired iodide organification. In thyroid follicular cells, pendrin is expressed at the apical membrane. Functional in vitro data and the impaired iodide organification observed in patients with Pendred syndrome support a role of pendrin as an apical iodide transporter.

The genetic bases for non-syndromic hearing loss among Chinese

Deafness is an etiologically heterogeneous trait with many known genetic, environmental causes or a combination thereof. The identification of more than 120 independent genes for deafness has provided profound new insights into the pathophysiology of hearing. However, recent findings indicate that a large proportion of both syndromic and non-syndromic forms of deafness in the Chinese population are caused by defects in a small number of genes. Studies of the genetic epidemiology and molecular genetic features revealed that there is a clear relevance of genes causing deafness in Chinese deaf patients as well as a unique spectrum of common and rare deafness gene mutations in the Chinese population. This review is focused on the genetic aspects of non-syndromic and mitochondrial deafness, in which unique molecular genetic features of hearing impairment have been identified in the Chinese population. The current China population is approximately 1.3 billion. It is estimated that 30,000 infants are born with congenital sensorineural hearing loss each year. Better understanding of the genetic causes of deafness in the Chinese population is important for accurate genetics counseling and early diagnosis for timely intervention and treatment options.

SLC26A4 c.919-2A>G varies among Chinese ethnic groups as a cause of hearing loss

PURPOSE

Mutations in the SLC26A4 gene are second only to GJB2 mutations as a currently identifiable genetic cause of sensorineural hearing loss. In most areas of China, genetic testing for sensorineural hearing loss is unavailable because of limited knowledge of the mutation spectrum. Although SLC26A4 c.919-2A>G (IVS7-2A>G) is a common mutation among some Asian populations, the mutation prevalence among various ethnic groups within China has not been studied.

METHODS

DNA specimens from 3271 subjects with moderate to profound sensorineural hearing loss from 27 regions of China were genotyped for the c.919-2A>G mutation by polymerase chain reaction/restriction-fragment-length polymorphism. Normal hearing controls from Han (n = 185) and Uigur (n = 152) populations were also tested.

RESULTS

Overall, 408 subjects with sensorineural hearing loss (12.5%) carried at least one c.919-2A>G allele, with 158 (4.8%) homozygotes and 250 (7.6%) heterozygotes. Within the subpopulations examined, the rate varies from 0% to 12.2% for c.919-2A>G homozygotes and from 0% to 17.6% for heterozygotes. Based on this cohort, Chinese subjects with sensorineural hearing loss seem to have a relatively higher c.919-2A>G frequency than that of other Asian populations.

CONCLUSION

These results demonstrate that a simple and efficient genetic test for the c.919-2A>G mutation alone would identify the molecular cause in up to 8-12% of individuals with sensorineural hearing loss in a few eastern and central regions of China. Those who are negative for the c.919-2A>G mutation would be candidates for further mutational analysis of SLC26A4 or other deafness-related genes. This would greatly improve genetic diagnosis and counseling for a huge number of Chinese individuals and family members with sensorineural hearing loss in China, and many more ethnic Chinese in other countries, which might be up to one million.

Pendred syndrome and iodide transport in the thyroid

Pendred syndrome is an autosomal recessive disorder characterized by sensorineural hearing impairment, presence of goiter, and a partial defect in iodide organification, which may be associated with insufficient thyroid hormone synthesis. Goiter development and development of hypothyroidism are variable and depend on nutritional iodide intake. Pendred syndrome is caused by biallelic mutations in the SLC26A4 gene, which encodes pendrin, a transporter of chloride, bicarbonate and iodide. This review discusses the controversies surrounding the potential role of pendrin in mediating apical iodide efflux into the lumen of thyroid follicles, and discusses its functional role in the kidney and the inner ear.

Pendred syndrome

Pendred syndrome (PDS) is an autosomal recessive disorder clinically characterized by sensorineural hearing loss and goiter. PDS is mainly caused by mutations in the SLC26A4 gene, although a few cases are due to mutations in the FOXI1 gene. SLC26A4 encodes pendrin, a sodium-independent transporter of iodide/chloride, chloride/formate and bicarbonate, that is expressed in the inner ear, thyroid gland, syncytiotrophoblast cells, endometrium and kidney. FOXI1 encodes a transcription factor necessary for pendrin expression. Patients with PDS show a bilateral and severe-to-profound hearing loss, although some cases present with a slowly progressive and fluctuating course. Temporal bone abnormalities with enlargement of the vestibular aqueduct, alone or with Mondini dysplasia, are common. Goiter appears most frequently in the second decade of life with a range of variations in size, depending on the amount of iodide intake and the effect that the mutation causes in pendrin function in any individual patient. A standard thyroid hormone-replacement regimen should be given to PDS patients with hypothyroidism to re-establish euthyroidism and prevent or decrease goiter growth. Total or partial thyroidectomy is occasionally the treatment of choice. Hearing aids and proper educational programs should also be offered to patients.

Pendred syndrome in two Galician families: insights into clinical phenotypes through cellular, genetic, and molecular studies

CONTEXT

We studied two families from Galicia (northwest Spain) with Pendred syndrome (PS) and unusual thyroid phenotypes. In family A, the proposita had a large goiter and hypothyroxinemia but normal TSH and free T3 (FT3). In family B, some affected members showed deafness but not goiter.

OBJECTIVE

Our objective was to identify the mutations causing PS and molecular mechanisms underlying the thyroid phenotypes.

INTERVENTIONS

Interventions included extraction of DNA and of thyroid tissue.

PATIENTS

Propositi and 10 members of the two families participated in the study.

MAIN OUTCOME MEASURES

Main outcome measures included SLC26A4 gene analysis, deiodinase activities in thyroid tissue, and c.416-1G-->A effects on SLC26A4 splicing. In addition, a primary PS thyrocyte culture, T-PS2, was obtained from propositus B and compared with another culture of normal human thyrocytes, NT, by Western blotting, confocal microscopy, and iodine uptake kinetics.

RESULTS

Proposita A was heterozygous for c.578C-->T and c.279delT, presented with goiter, and had normal TSH and FT3 but low FT4 attributable to high type 1 and type 2 iodothyronine deiodinase activities in the goiter. Propositus B bore c.279delT and a novel mutation c.416-1G-->A; some deaf relatives were homozygous for c.416-1G-->A but did not present goiter. The c.279delT mutation was associated with identical haplotype in the two families. T-PS2 showed truncated pendrin retained intracellularly and high iodine uptake with low efflux leading to iodine retention.

CONCLUSIONS

c.279delT is a founder mutation in Galicia. Proposita A adapted to poor organification by increasing deiodinase activities in the goiter, avoiding hypothyroidism. Lack of goiter in subjects homozygous for c.416-1G-->A was due to incomplete penetrance allowing synthesis of some wild-type pendrin. Intracellular iodine retention, as seen in T-PS2, could play a role in thyroid alterations in PS.

Molecular analysis of the PDS gene in a nonconsanguineous Sicilian family with Pendred's syndrome

OBJECTIVE

The autosomal recessive Pendred's syndrome is defined by congenital sensorineural deafness, goiter, and impaired iodide organification. It is caused by mutations in the Pendred's syndrome (PDS) gene that encodes pendrin, a chloride/iodide transporter expressed in the thyroid, the inner ear, and the kidney. In this study we performed clinical and molecular analyses in three siblings from a nonconsanguineous Sicilian family who presented with the clinical features of Pendred's syndrome. PATIENTS AND MOLECULAR ANALYSES: In two sisters and one brother, the clinical diagnosis of Pendred's syndrome was established based on the findings of sensorineural hearing loss and large goiters. Thyroid function tests, perchlorate discharge tests, thyroid ultrasound, and scintigraphy were performed in all affected individuals. Exons 2 to 21 of the PDS gene were amplified by polymerase chain reaction (PCR) and both strands were submitted to direct sequence analysis.

RESULTS

The clinical diagnosis of Pendred's syndrome was supported by a positive perchlorate discharge test in the three afflicted siblings. Direct sequence analysis of the PDS gene revealed that all three harbored one allele with a novel mutation 890delC leading to a frameshift mutation and premature stop codon at position 302 (FS297 > 302X). On the other allele, two of the siblings had a previously described transition 1226G > A, which results in the substitution of arginine by histidine at position 409 (R409H). In the index patient, no mutation could be identified on the other allele. In functional studies, these mutants lose the ability of pendrin to mediate iodide efflux.

CONCLUSIONS

All three patients included in this study presented with the classic Pendred syndrome triad. Two siblings were compound heterozygous for mutations in the coding region of the PDS gene. The third individual could have an unidentified mutation in a regulatory or intronic region of the PDS gene, or an identical phenotype caused by distinct pathogenic mechanisms.

Specificity of SLC26A4 mutations in the pathogenesis of inner ear malformations

The traditional hypothesis concerning the pathogenesis of inner ear malformations holds that various types of malformations represent different stages of developmental arrest during embryogenesis. In order to verify this hypothesis, we surveyed mutations in the SLC26A4(PDS) gene, which were documented to cause enlarged vestibular aqueduct (EVA) and Mondini's dysplasia (incomplete partition of the cochlea), in 35 families with various types of inner ear malformations. In 25 families, the probands showed EVA or Mondini's dysplasia as the main temporal bone abnormalities, whereas the probands in the remaining 10 families revealed other types of malformations. In total, 7 mutated SLC26A4 alleles, including 6 missense mutations (A372V, A387V, T410M, S448L, T721M, and H723R) and 1 splice site mutation (IVS7-2A-->G), were detected. All mutated alleles segregated the malformations of EVA and Mondini's dysplasia, whereas no mutated alleles were found in the 10 probands with other types of malformations. SLC26A4 mutations were found in 22 of the 25 probands with EVA or Mondini's dysplasia, indicating that these might be specific to the development of Mondini's dysplasia and EVA. It is inferred that the pathogenetic mechanisms of the various malformations essentially differ, although their radiological findings appear to follow a continuum of morphological changes.

Analysis of differential gene expression in the cochlea and kidney of mouse by cDNA microarrays

Microarray hybridization analysis of gene expression in the cochlea and kidney suggest a relationship between these tissues at the genomic level, indicating the common gene expression, likely serving a common function in both the organs primarily maintaining ion transport, and implied previously from morphological, pharmaco-kinetic and teratogenic studies. The cDNAs of more than 100 genes listed on the hereditary hearing loss homepage were amplified as targets by RT-PCR and were hybridized with probes prepared from total RNA of the cochlea and the kidney. Thirteen of the genes analyzed showed altered fluorescence ratios of more than two logs. Of these, the expressions of 11 genes were over expressed and two were under expressed in the cochlea than in the kidney. Our data is the first report to corroborate the genomic similarities between these two important organs and may help to explain the somewhat similar response of these organs to certain therapeutic drugs.

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.

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.

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.

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.