Pendred Syndrome, or Not Pendred Syndrome? That Is the Question

Abstract

Pendred syndrome (PDS) is the most common form of syndromic Hearing Loss (HL), characterized by sensorineural HL, inner ear malformations, and goiter, with or without hypothyroidism. SLC26A4 is the major gene involved, even though ~50% of the patients carry only one pathogenic mutation. This study aims to define the molecular diagnosis for a cohort of 24 suspected-PDS patients characterized by a deep radiological and audiological evaluation. Whole-Exome Sequencing (WES), the analysis of twelve variants upstream of SLC26A4, constituting the "CEVA haplotype" and Multiplex Ligation Probe Amplification (MLPA) searching for deletions/duplications in SLC26A4 gene have been carried out. In five patients (20.8%) homozygous/compound heterozygous SLC26A4 mutations, or pathogenic mutation in trans with the CEVA haplotype have been identified, while five subjects (20.8%) resulted heterozygous for a single variant. In silico protein modeling supported the pathogenicity of the detected variants, suggesting an effect on the protein stabilization/function. Interestingly, we identified a genotype-phenotype correlation among those patients carrying SLC26A4 mutations, whose audiograms presented a characteristic slope at the medium and high frequencies, providing new insights into PDS. Finally, an interesting homozygous variant in MYO5C has been identified in one patient negative to SLC26A4 gene, suggesting the identification of a new HL candidate gene.

Keywords: Pendred syndrome; Whole-Exome Sequencing; genotype-phenotype correlation.

Figure 1

Membrane topology prediction for the protein expressed by SLC26A4 (pendrin) according to TOPCONS [27]. Boxes refer to the transmembrane (TM) region (in vs. out, grey; out vs. in, white); red lines refer to intracellular regions; blue lines refer to extracellular regions. X-axis refers to the position within protein sequence. Reliability estimate of the predictions is reported in the Y-axis.

Figure 2

Audiograms of the carriers of SLC26A4 mutations. Audiometric features, displayed as audiograms, of the ten patients resulted in being carriers of SLC26A4 homozygous or heterozygous mutations. The hearing threshold levels are reported as the mean value between the right and the left ear.

Figure 3

Structural model of the human pendrin monomer as predicted by Alphafold [28]. Protein backbone is represented by cyan cartoons and residues target of missense mutations identified in this study are represented by red sticks and labeled. The red arrows indicate the variants that were detected in homozygosis in the cohort. (A) View in a direction crossing the cell membrane. The membrane bilayer is represented by shaded thick gray lines. The cytoplasmic side of the protein is in the upper part while the extracellular portion is in the lower part. (B) View of the transmembrane region as seen from the top of the view in (A). Protein domains not belonging to the transmembrane region have been omitted for clarity.

Figure 4

Pedigree and audiograms of patient ID24, DNA chromatograms, and schematic representation of the protein domains (A) Pedigree of the proband ID24, which is indicated with a black arrow. Both his parents (I:1 and I:2) and his brother (II:1) carry the variant on MYO5C (NM_018728.4, c.3592C>T p.(R1198C)) at the heterozygous state. The genotypes of the four individuals are indicated. The only affected subject is ID24, who therefore is represented with a filled symbol. square: males; circle: females. (B) Audiometric features of ID24, displayed as audiograms. The hearing threshold levels of the right ear are reported in blue while those of the left ear are reported in red. (C) Schematic representation of myosin-Vc protein domains and the localization of the variant identified in ID24. The MYSc (myosin large ATPase) domain is displayed in orange, in green the five IQ domains, which are formed by short calmodulin-binding motif and finally the coiled coil domains are reported in yellow. A pink star points out the position of the interesting MYO5C variant identified in ID24. (D) Chromatograms displaying part of MYO5C sequence. The sequence of ID24 is reported on top, followed by the one of his mother, father, and brother, respectively. The red box indicates the position of the variant.