Clinical and molecular characteristics of Pendred syndrome

SLC26A4 mutation in Pendred syndrome with hypokalemia: A case report

RATIONALE

Pendred syndrome is an autosomal recessive disorder characterized by sensorineural hearing loss, inner ear malformations, goiter, and abnormal organification of iodide. It is caused by mutations in SLC26A4 gene, which encodes pendrin (a transporter of chloride, bicarbonate, and iodide). Pendred syndrome is a common cause of syndromic deafness, but the metabolic abnormalities it causes are often overlooked. Here, we report the case of a patient diagnosed with Pendred syndrome with hypokalemia.

PATIENT CONCERNS

A 53-year-old deaf-mute woman was hospitalized due to severe limb asthenia. The emergency examination showed that her blood potassium level was 1.8 mmol/L.

DIAGNOSES

Through the genetic test, we found a mutation of SLC26A4 gene in NM_000441: c.2027T>A, p.L676Q, as well as the SLC26A4 exon 5-6 deletion. These genetic variations pointed to Pendred syndrome (an autosomal recessive disorder that mainly affects the inner ear, thyroid, and kidney) which is a common cause of syndromic deafness.

INTERVENTIONS

The patient was treated with potassium supplements and screened for the cause of hypokalemia.

OUTCOMES

The patient was discharged after her potassium levels rose to the normal range.

LESSONS

Patients with Pendred syndrome may also have certain metabolic abnormalities; thus, more attention should be paid to them during clinical diagnosis.

Outcomes of Cochlear Implantation in Patients with Pendred syndrome: A Systematic Review and Narrative Synthesis

Establish outcomes following cochlear implantation (CI) in patients with Pendred syndrome. Systematic review and narrative synthesis. Databases searched: Medline, Pubmed, Embase, Web of Science, Cochrane Collection and ClinicalTrials.gov. No limits placed on language or year of publication. Review conducted in accordance with the PRISMA statement. Searches identified 251 abstracts and 242 full texts. Of these, 22 studies met inclusion criteria reporting outcomes in 231 patients with at least 234 implants. Hearing outcomes were generally good with patients experiencing useful functional improvement. A total of 46 minor complications were reported in 78 cases. The methodological quality of included studies was modest, predominantly consisting of case reports and non-controlled case series with small numbers of patients. All studies were OCEBM grade III-IV. Hearing outcomes following CI in Pendred syndrome are generally good with useful functional improvement. However, outcomes reported in published studies lack long term follow up.

Association between Ménière's disease and thyroid diseases: a nested case-control study

The association of thyroid disease and Ménière’s disease would suggest that both are autoimmune diseases. This study aimed to investigate the relation of goiter, hypothyroidism, thyroiditis, hyperthyroidism, and autoimmune thyroiditis with Ménière’s disease. The Korean National Health Insurance Service-Health Screening Cohort data from 2002 through 2015 were used. The 8183 adult patients with Ménière’s disease were 1:4 matched with the 32,732 individuals of the control group for age, sex, income, and region of residence. The previous histories of thyroid disorders including goiter, hypothyroidism, thyroiditis, and hyperthyroidism were investigated using conditional logistic regression analyses. Subgroup analyses were conducted, including for age and sex. Smoking, alcohol consumption, obesity, Charlson Comorbidity Index, histories of benign paroxysmal vertigo, vestibular neuronitis, other peripheral vertigo, thyroid cancer, and levothyroxine medication were adjusted in the models. The histories of goiter (5.7% vs. 4.2%), hypothyroidism (4.7% vs. 3.6%), thyroiditis (2.1% vs. 1.6%), hyperthyroidism (3.6% vs. 2.5%), and autoimmune thyroiditis (0.99% vs. 0.67%) were higher in the Meniere’s disease group than in the control group (all P < 0.05). The histories of goiter, hypothyroidism, and hyperthyroidism were associated with Ménière’s disease (adjusted odds ratio (OR) = 1.19 [95% confidence interval (CI) = 1.04–1.36] for goiter, 1.21 [95% CI 1.02–1.44] for hypothyroidism, and 1.27 [95% CI 1.09–1.49] for hyperthyroidism, each of P < 0.05). In subgroup analyses, hypothyroidism was associated with Ménière’s disease in < 65-year-old women. Hyperthyroidism was related with Ménière’s disease in women overall. Thyroid diseases of goiter, hypothyroidism, and hyperthyroidism were associated with Ménière’s disease.

A comparison of thyroidal protection by stable iodine or perchlorate in the case of acute or prolonged radioiodine exposure

In the case of a nuclear power plant accident, repetitive/prolonged radioiodine release may occur. Radioiodine accumulates in the thyroid and by irradiation enhances the risk of cancer. Large doses of non-radioactive iodine may protect the thyroid by inhibiting radioiodine uptake into the gland (iodine blockade). Protection is based on a competition at the active carrier site in the cellular membrane and the Wolff-Chaikoff effect, the latter being, however, only transient (24-48 h). Perchlorate may alternatively provide protection by a carrier competition mechanism only. Perchlorate has, however, a stronger affinity to the carrier than iodide. Based on an established biokinetic-dosimetric model developed to study iodine blockade, and after its extension to describe perchlorate pharmacokinetics and the inhibition of iodine transport through the carrier, we computed the protective efficacies that can be achieved by stable iodine or perchlorate in the case of an acute or prolonged radioiodine exposure. In the case of acute radioiodine exposure, perchlorate is less potent than stable iodine considering its ED50. A dose of 100 mg stable iodine has roughly the same protective efficacy as 1000 mg perchlorate. For prolonged exposures, single doses of protective agents, whether stable iodine or perchlorate, offer substantially lower protection than after acute radioiodine exposure, and thus repetitive administrations seem necessary. In case of prolonged exposure, the higher affinity of perchlorate for the carrier in combination with the fading Wolff-Chaikoff effect of iodine confers perchlorate a higher protective efficacy compared to stable iodine. Taking into account the frequency and seriousness of adverse effects, iodine and perchlorate at equieffective dosages seem to be alternatives in case of short-term acute radioiodine exposure, whereas preference should be given to perchlorate in view of its higher protective efficacy in the case of longer lasting radioiodine exposures.

Recombinant Human Thyrotropin Worsens Renal Cortical Perfusion and Renal Function in Patients After Total Thyroidectomy Due to Differentiated Thyroid Cancer

Background: Although thyrotropin (TSH) receptors are found in many nonthyroid tissues, we know little about the direct action of TSH on these receptors. Patients after total thyroidectomy for differentiated thyroid cancer (DTC) provide an interesting model for studying this issue. The administration of exogenous TSH in patients with an established thyroid state on levothyroxine (LT4) treatment allows us to study the effect of elevated TSH concentrations independent of thyroid status on the function of various organs, including the kidneys. The aim of this study was to assess the effects of the administration of recombinant human TSH (rhTSH) on renal perfusion and glomerular filtration in this group of patients. Methods: The study included 24 patients after total thyroidectomy due to DTC, without concomitant diseases, receiving only LT4 who qualified for radioiodine treatment (RIT). For two consecutive days, the patients received rhTSH and subsequently the RIT. Clinical and biochemical evaluation of thyroid and renal function was carried out before and 24 hours after the second dose of rhTSH and before the RIT. On the sixth day of hospitalization, the patients' glomerular filtration rate was re-evaluated. Kidney perfusion was assessed using color Doppler ultrasound imaging before and 24 hours after the second dose of rhTSH and before the RIT. Results: The administration of rhTSH to patients after total thyroidectomy due to DTC caused significant deterioration of renal perfusion after the second dose of rhTSH before the RIT, which was followed by a significant reduction in glomerular filtration. Furthermore, rhTSH did not significantly affect the hemodynamic parameters that could worsen renal function. Conclusions: This study indicates that TSH alone, independent of thyroid hormone concentrations, can influence renal perfusion and renal function.

Novel Roles for Chloride Channels, Exchangers, and Regulators in Chronic Inflammatory Airway Diseases

Chloride transport proteins play critical roles in inflammatory airway diseases, contributing to the detrimental aspects of mucus overproduction, mucus secretion, and airway constriction. However, they also play crucial roles in contributing to the innate immune properties of mucus and mucociliary clearance. In this review, we focus on the emerging novel roles for a chloride channel regulator (CLCA1), a calcium-activated chloride channel (TMEM16A), and two chloride exchangers (SLC26A4/pendrin and SLC26A9) in chronic inflammatory airway diseases.

Iodide transport: implications for health and disease

Disorders of the thyroid gland are among the most common conditions diagnosed and managed by pediatric endocrinologists. Thyroid hormone synthesis depends on normal iodide transport and knowledge of its regulation is fundamental to understand the etiology and management of congenital and acquired thyroid conditions such as hypothyroidism and hyperthyroidism. The ability of the thyroid to concentrate iodine is also widely used as a tool for the diagnosis of thyroid diseases and in the management and follow up of the most common type of endocrine cancers: papillary and follicular thyroid cancer. More recently, the regulation of iodide transport has also been the center of attention to improve the management of poorly differentiated thyroid cancer. Iodine deficiency disorders (goiter, impaired mental development) due to insufficient nutritional intake remain a universal public health problem. Thyroid function can also be influenced by medications that contain iodide or interfere with iodide metabolism such as iodinated contrast agents, povidone, lithium and amiodarone. In addition, some environmental pollutants such as perchlorate, thiocyanate and nitrates may affect iodide transport. Furthermore, nuclear accidents increase the risk of developing thyroid cancer and the therapy used to prevent exposure to these isotopes relies on the ability of the thyroid to concentrate iodine. The array of disorders involving iodide transport affect individuals during the whole life span and, if undiagnosed or improperly managed, they can have a profound impact on growth, metabolism, cognitive development and quality of life.

Congenital hypothyroidism and the importance of universal newborn screening

Congenital hypothyroidism (CH) is one of the most common preventable causes of mental retardation in children. Early diagnosis and treatment prevent the devastating outcome of mental retardation. Clinical features of CH are subtle and are not evident early in the neonatal period. Therefore, universal newborn screening (NS) is effective in detecting CH and implementing early treatment. This article reviews the current literature regarding the epidemiology, etiology, classification, clinical features, diagnosis of primary CH, and the management of abnormal newborn screen as well as treatment and prognosis of primary CH and outlines the importance of universal newborn screening in preventing mental retardation.

Recent insights into the cell biology of thyroid angiofollicular units

In thyrocytes, cell polarity is of crucial importance for proper thyroid function. Many intrinsic mechanisms of self-regulation control how the key players involved in thyroid hormone (TH) biosynthesis interact in apical microvilli, so that hazardous biochemical processes may occur without detriment to the cell. In some pathological conditions, this enzymatic complex is disrupted, with some components abnormally activated into the cytoplasm, which can lead to further morphological and functional breakdown. When iodine intake is altered, autoregulatory mechanisms outside the thyrocytes are activated. They involve adjacent capillaries that, together with thyrocytes, form the angiofollicular units (AFUs) that can be considered as the functional and morphological units of the thyroid. In response to iodine shortage, a rapid expansion of the microvasculature occurs, which, in addition to nutrients and oxygen, optimizes iodide supply. These changes are triggered by angiogenic signals released from thyrocytes via a reactive oxygen species/hypoxia-inducible factor/vascular endothelial growth factor pathway. When intra- and extrathyrocyte autoregulation fails, other forms of adaptation arise, such as euthyroid goiters. From onset, goiters are morphologically and functionally heterogeneous due to the polyclonal nature of the cells, with nodules distributed around areas of quiescent AFUs containing globules of compact thyroglobulin (Tg) and surrounded by a hypotrophic microvasculature. Upon TSH stimulation, quiescent AFUs are activated with Tg globules undergoing fragmentation into soluble Tg, proteins involved in TH biosynthesis being expressed and the local microvascular network extending. Over time and depending on physiological needs, AFUs may undergo repetitive phases of high, moderate, or low cell and tissue activity, which may ultimately culminate in multinodular goiters.

Papillary thyroid carcinoma in one of identical twin patients with Pendred syndrome

Pendred syndrome is an autosomal recessive disorder characterized by sensorineural deafness, a partial defect in iodide organification, and dyshormonogenetic goiter. Several cases of Pendred syndrome with follicular thyroid carcinomas were reported previously. Here we report identical twin patients with Pendred syndrome, who had thyroid tumors with distinct histopathological findings. 34-year-old identical twins with congenital deafness and goiter were referred to our hospital with complaint of neck discomfort. The genetic testing showed that these twin patients were compound heterozygotes carrying the same two mutations in the Pendred's syndrome (PDS / SLC26A4) gene (c2168A > G and ins2110GCTGG), which confirmed the diagnoses of Pendred syndrome. They underwent thyroidectomy. Histological examination of the thyroid tumors resected from these twin patients revealed follicular variant of papillary thyroid carcinoma, and diffuse and nodular goiter without any evidence of malignancy, respectively. To our knowledge, the former is the first case of follicular variant of papillary thyroid carcinoma in Pendred Syndrome.

Molecular and functional characterization of human pendrin and its allelic variants

Pendrin (SLC26A4, PDS) is an electroneutral anion exchanger transporting I(-), Cl(-), HCO(3)(-), OH(-), SCN(-) and formate. In the thyroid, pendrin is expressed at the apical membrane of the follicular epithelium and may be involved in mediating apical iodide efflux into the follicle; in the inner ear, it plays a crucial role in the conditioning of the pH and ion composition of the endolymph; in the kidney, it may exert a role in pH homeostasis and regulation of blood pressure. Mutations of the pendrin gene can lead to syndromic and non-syndromic hearing loss with EVA (enlarged vestibular aqueduct). Functional tests of mutated pendrin allelic variants found in patients with Pendred syndrome or non-syndromic EVA (ns-EVA) revealed that the pathological phenotype is due to the reduction or loss of function of the ion transport activity. The diagnosis of Pendred syndrome and ns-EVA can be difficult because of the presence of phenocopies of Pendred syndrome and benign polymorphisms occurring in the general population. As a consequence, defining whether or not an allelic variant is pathogenic is crucial. Recently, we found that the two parameters used so far to assess the pathogenic potential of a mutation, i.e. low incidence in the control population, and substitution of evolutionary conserved amino acids, are not always reliable for predicting the functionality of pendrin allelic variants; actually, we identified mutations occurring with the same frequency in the cohort of hearing impaired patients and in the control group of normal hearing individuals. Moreover, we identified functional polymorphisms affecting highly conserved amino acids. As a general rule however, we observed a complete loss of function for all truncations and amino acid substitutions involving a proline. In this view, clinical and radiological studies should be combined with genetic and molecular studies for a definitive diagnosis. In performing genetic studies, the possibility that the mutation could affect regions other than the pendrin coding region, such as its promoter region and/or the coding regions of functionally related genes (FOXI1, KCNJ10), should be taken into account. The presence of benign polymorphisms in the population suggests that genetic studies should be corroborated by functional studies; in this context, the existence of hypo-functional variants and possible differences between the I(-)/Cl(-) and Cl(-)/HCO(3)(-) exchange activities should be carefully evaluated.

Identification of allelic variants of pendrin (SLC26A4) with loss and gain of function

BACKGROUND

Pendrin is a multifunctional anion transporter that exchanges chloride and iodide in the thyroid, as well as chloride and bicarbonate in the inner ear, kidney and airways. Loss or reduction in the function of pendrin results in both syndromic (Pendred syndrome) and non-syndromic (non-syndromic enlarged vestibular aqueduct (ns-EVA)) hearing loss. Factors inducing an up-regulation of pendrin in the kidney and the lung may have an impact on the pathogenesis of hypertension, chronic obstructive pulmonary disease (COPD) and asthma. Here we characterize the ion transport activity of wild-type (WT) pendrin and seven of its allelic variants selected among those reported in the single nucleotide polymorphisms data base (dbSNPs), some of which were previously identified in a cohort of individuals with normal hearing or deaf patients belonging to the Spanish population.

METHODS

WT and mutated pendrin allelic variants were functionally characterized in a heterologous over-expression system by means of fluorometric methods evaluating the I(-)/Cl(-) and Cl(-)/OH(-) exchange and an assay evaluating the efflux of radiolabeled iodide.

RESULTS

The transport activity of pendrin P70L, P301L and F667C is completely abolished; pendrin V609G and D687Y allelic variants are functionally impaired but retain significant transport. Pendrin F354S activity is indistinguishable from WT, while pendrin V88I and G740S exhibit a gain of function.

CONCLUSION

Amino acid substitutions involving a proline always result in a severe loss of function of pendrin. Two hyperfunctional allelic variants (V88I, G740S) have been identified, and they may have a contributing role in the pathogenesis of hypertension, COPD and asthma.