A case of palmoplantar lichen planus in a patient with congenital sensorineural deafness

Digenic inheritance of mutations in EPHA2 and SLC26A4 in Pendred syndrome

Enlarged vestibular aqueduct (EVA) is one of the most commonly identified inner ear malformations in hearing loss patients including Pendred syndrome. While biallelic mutations of the SLC26A4 gene, encoding pendrin, causes non-syndromic hearing loss with EVA or Pendred syndrome, a considerable number of patients appear to carry mono-allelic mutation. This suggests faulty pendrin regulatory machinery results in hearing loss. Here we identify EPHA2 as another causative gene of Pendred syndrome with SLC26A4. EphA2 forms a protein complex with pendrin controlling pendrin localization, which is disrupted in some pathogenic forms of pendrin. Moreover, point mutations leading to amino acid substitution in the EPHA2 gene are identified from patients bearing mono-allelic mutation of SLC26A4. Ephrin-B2 binds to EphA2 triggering internalization with pendrin inducing EphA2 autophosphorylation weakly. The identified EphA2 mutants attenuate ephrin-B2- but not ephrin-A1-induced EphA2 internalization with pendrin. Our results uncover an unexpected role of the Eph/ephrin system in epithelial function.

While biallelic mutations of the SLC26A4 gene cause non-syndromic hearing loss with enlarged vestibular aqueducts or Pendred syndrome, a considerable number of patients carry mono-allelic mutations. Here the authors identify EPHA2 as another causative gene of Pendred syndrome with SLC26A4.

Carrier re-sequencing reveals rare but benign variants in recessive deafness genes

For recessive Mendelian disorders, determining the pathogenicity of rare, non-synonymous variants in known causative genes can be challenging without expanded pedigrees and/or functional analysis. In this study, we proposed to establish a database of rare but benign variants in recessive deafness genes by systematic carrier re-sequencing. As a pilot study, 30 heterozygous carriers of pathogenic variants for deafness were identified from unaffected family members of 18 deaf probands. The entire coding regions of the corresponding genes were re-sequenced in those carriers by targeted next-generation sequencing or Sanger sequencing. A total of 32 non-synonymous variants were identified in the normal-hearing carriers in trans with the pathogenic variant and therefore were classified as benign. Among them were five rare (minor allele frequencies less than 0.005) variants that had previously undefined, disputable or even misclassified function: p.A434T (c.1300 G > A) in SLC26A4, p.R266Q (c.797 G > A) in LOXHD1, p.K96Q (c.286 A > C) in MYO15A, p.T123N (c.368 C > A) in GJB2 and p.V1299I (c.797 G > A) in CDH23. Our results suggested that large scale carrier re-sequencing may be warranted to establish a database of rare but benign variants in causative genes in order to reduce false positive genetic diagnosis of recessive Mendelian disorders.

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.