Functional assessment of allelic variants in the SLC26A4 gene involved in Pendred syndrome and nonsyndromic EVA

Abstract

Pendred syndrome is an autosomal recessive disorder characterized by sensorineural hearing loss, with malformations of the inner ear, ranging from enlarged vestibular aqueduct (EVA) to Mondini malformation, and deficient iodide organification in the thyroid gland. Nonsyndromic EVA (ns-EVA) is a separate type of sensorineural hearing loss showing normal thyroid function. Both Pendred syndrome and ns-EVA seem to be linked to the malfunction of pendrin (SLC26A4), a membrane transporter able to exchange anions between the cytosol and extracellular fluid. In the past, the pathogenicity of SLC26A4 missense mutations were assumed if the mutations fulfilled two criteria: low incidence of the mutation in the control population and substitution of evolutionary conserved amino acids. Here we show that these criteria are insufficient to make meaningful predictions about the effect of these SLC26A4 variants on the pendrin-induced ion transport. Furthermore, we functionally characterized 10 missense mutations within the SLC26A4 ORF, and consistently found that on the protein level, an addition or omission of a proline or a charged amino acid in the SLC26A4 sequence is detrimental to its function. These types of changes may be adequate for predicting SLC26A4 functionality in the absence of direct functional tests.ion transport physiology ͉ genotype-phenotype correlation P endred syndrome (PS) (OMIM#274600) (1) is an autosomal recessive disorder characterized by sensorineural hearing loss (SNHL) and malformations of the inner ear, ranging from enlarged vestibular aqueduct (EVA) (2) to Mondini malformation (3), combined with deficient iodide organification in the thyroid gland, as demonstrated by the positive perchlorate discharge test in affected individuals (4-7). Another form of SNHL associated with EVA, however, showing normal thyroid function, is called nonsyndromic EVA (ns-EVA) (OMIM#600791). The clinical features of PS are the consequence of impaired pendrin function (1), a protein encoded by the SLC26A4 gene (NM 000441). It is a member of the multifunctional anion transporter family SLC26, which mediates the exchange of anions including Cl Ϫ , HCO 3 Ϫ , OH Ϫ , I Ϫ , or formate (8). Pendrin seems to be responsible for the efflux of iodide in thyrocytes (9-11), and for mediating Cl Ϫ /HCO 3 Ϫ exchange in the kidney cortex (12) and inner ear. In the latter, pendrin is involved in the conditioning of endolymphatic fluid, presumably because of HCO 3 Ϫ secretion (13), thereby modifying inner ear acid-base homeostasis. A variable feature of PS is the development of goitre (apparent in only about 50% of the affected individuals). At the thyroid level, the role of pendrin is not conclusive. The transporter could act as an iodide transporter at the apical membrane of thyroid cells and impaired function could therefore lead to the iodide organification defect observed in PS patients (10,11,(14)(15)(16). PS seems to be linked to bi-allelic mutations of the SLC26A4 genes. ns-EVA is genetically more heterogeneous relative to PS, and a...

Spectrum and Frequency of SLC26A4 Mutations Among Czech Patients with Early Hearing Loss with and without Enlarged Vestibular Aqueduct (EVA)

“…This frequency is significantly higher than the expected heterozygote frequency in the general population which is estimated to be 1% (Pera et al, 2008a). Two explanations could account for this finding: 1) the heterozygote frequency in the general population is higher than expected or 2) a single mutation in the SLC26A4 gene can contribute to deafness by a mechanism other than by causing EVA.…”

Mutation analysis of the SLC26A4, FOXI1 and KCNJ10 genes in individuals with congenital hearing loss

“…Assuming Hardy-Weinberg equilibrium, the carrier frequency would be about 1:50 (2%). This is congruent with a study in which pathogenic or possibly pathogenic mutations were identified in 1.9% of normal-hearing controls (8/428 controls; p.E29Q, p.F354S, p.F667C, p.D724G, p.G740S) (Pera et al, 2008b). An "excess" of heterozygous mutations in individuals with SNHL compared to controls has also been observed for GJB2 (Putcha et al, 2007), a gene for which neighboring deletions can affect gene expression Given that SLC26A4 related SNHL is autosomal recessive, we postulated that these 'missing' SLC26A4 mutations may be the result of intragenic deletions or duplications of one or more exons for which patients are not routinely tested.…”

Identification of single nucleotide polymorphisms in the bovine solute carrier family 11 member 1 (SLC11A1) gene and their association with infection by Mycobacterium avium subspecies paratuberculosis

“…Moreover, Pera et al (2008) observed that the addition or omission of a proline or a charged AA in the solute carrier family 26 member 4 (SLC26A4) protein is detrimental to its function. Referring back to the SLC11A1 gene, the nonconservative G to C change at c.1067C > G is predicted to generate a change in the secondary structure of the protein that would give rise to a helix instead of a strand structure in TM8 (Martinez et al, 2008).…”