Molecular and Functional Characterization of Human Pendrin and its Allelic Variants

Abstract

Pendrin (SLC26A4, PDS) is an electroneutral anion exchanger transporting I ^-, Cl ^-, HCO₃^-, 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 ^-/HCO₃^- exchange activities should be carefully evaluated.

Reduction of Cellular Expression Levels Is a Common Feature of Functionally Affected Pendrin (SLC26A4) Protein Variants

“…Despite that iodide transport may be of no physiological relevance in the inner ear, the chloride/iodide exchange activity generally reflects the chloride/ bicarbonate exchange activity (10) and can therefore be used as a tool to discrim inate between variants with and without pathogenic potential. Accordingly, when differences between the chloride/iodide Expression levels of the different pen drin variants in the PM region mirrored total expression levels ( Figures 3, 4).…”

“…These variants, together with a variant (p.R776C) for which previous functional studies led to contradictory 500 different sequence alterations of the pendrin gene have been reported to date, but most of the corresponding protein variants miss a precise functional and molecular characterization (10). In the absence of careful genetic and functional assessments, two factors, i.e., the clinical condition of pseudoPendred syndromes (the association of deafness and goiter with no pendrin mutations [11][12][13][14][15]) and the existence of pendrin variants with no functional impairment (10), could lead to an incorrect assignment of the genetic cause of the disease.…”

Synopsis of the 48thAnnual Meeting of the Lake Cumberland Biological Transport Group and the Second Biannual Meeting of the Pendrin Consortium

“…Later, functional studies established that pendrin is an electroneutral anion exchanger capable of transporting a broad range of monovalent anions, including iodide, chloride, bicarbonate, thiocyanate, hydroxide and formate, but with no affinity for divalent ions such as sulfate [22]. Pendrin is expressed on the apical aspect of a variety of epithelial cells, being particularly abundant in the thyroid.…”

“…This disorder is disclosed by a positive perchlorate discharge test, and may lead to subclinical or overt hypothyroidism with or without goiter. Pendred syndrome is usually associated with homozygous or compound heterozygous biallelic mutations in the pendrin gene, while non-syndromic EVA has been found in association with one, two or no pendrin mutations [22]. Common radiological findings at the level of the inner ear associated with pendrin mutations include EVA and Mondini's dysplasia.…”

Estudo do Papel do Gene SLC26A4 na Surdez Neurossensorial Não-Sindrômica Pré-Lingual em uma Série de Casos no Sudeste Brasileiro

“…A mutação encontrada no aminoácido 382 codificado pelo éxon 9 promoveu a troca de uma asparagina por uma lisina e foi observada em heterozigose em apenas um indivíduo (Paciente 2292) da população de estudo (Figura 11 e Figura 12 PERA et al, 2008;KAHRIZI et al, 2009;CHEN et al, 2011;DOSSENA et al, 2011b;RENDTORFF et al, 2013 heterozigose composta e/ou homozigose em muitos indivíduos não afetados, inclusive na população brasileira (KENNA et al, 2001;GUO et al, 2008;CASTRO et al, 2013;CHEN et al, 2014 "N" refere-se à ausência do fator estudado e "S" à presença do fator estudado. "NR" corresponde a um fato na relatado.…”