We reported the case of a 23-year-old patient who presented a series of typical clinical features with a novel homozygous mutation, p.Ser2828* (rs1199684717), in USH2A, a gene responsible for USH2 (OMIM:276901). The frequency of the mutation is 0.000004 in the Genome Aggregation Database, and it was found in a heterozygous state in one European non-Finnish individual. Mutations in USH2A are associated with USH2, which is responsible for almost 50% of USH cases.
USH2A codes two alternatively spliced isoforms of usherin. The short ~170 kDa isoform a, consisting of 21 exons, is regarded as an extracellular protein. The full-length ~580 kDa isoform b is a complex transmembrane protein composed of three regions: a large extracellular region consisting of an N-terminal signal peptide, laminin G-like domain (LamGL), laminin domain N-terminal (LamNT), laminin-type EGF-like modules (EGF-Lam), fibronectin type III (FN3) repeats, laminin G domains (LamG); a transmembrane region (TM); and a cytoplasmic C-terminal domain containing a PDZ-binding motif[8, 9]. Usherin is distributed in the periciliary membrane complex and synapse in photoreceptors. All USH1 and USH2 proteins are organized as protein networks by the scaffold proteins harmonin (USH1C), whirlin (USH2D) and SANS (USH1G). Usherin (USH2A) and VLGR1b (USH2C) are part of the links that are intracellularly attached to the scaffold proteins. However, during the differentiation of the hair bundle, both USH1 and USH2 proteins contribute to the formation of side links located at the tip and the base of the stereocilia, respectively. They exist in multiprotein complexes that work together as molecular networks to anchor them to the stereocilia actin filaments[10-14].
The homozygous mutation (p.Ser2828*) in USH2A caused premature termination of translation, and as a result, 19 FN3, TM and PDZ-binding motif domains were deleted. FN3 plays a key role in cell adhesion, cell morphology, thrombosis, cell migration, and embryonic differentiation as well as pathophysiologic processes such as angiogenesis and vascular remodelling. A TM domain is present at the base of differentiating stereocilia and causes the mechanosensitive hair bundles to be receptive to sound. PDZ-binding motif domains provide the anchoring of interstereocilia lateral links to the F-actin core of stereocilia. In this regard, we suppose that the absence of these domains corresponding to the incompleteness of usherin might have affected the process of differentiation and maturation of the stereocilia, resulting in a milder dysmorphic phenotype of the stereocilia. Several missense hotspots have been associated with the pathogenesis of FN3 in usherin, which supports our hypothesis. However, this pathway needs to be confirmed by molecular experiments in the future.
Whole-genome sequencing (WGS), whole-exome sequencing (WES) and TES are three major methodologies for molecular diagnosis of IRDs. WGS is useful for detecting copy number and structural variations. WES is especially useful for identifying novel IRD-related genes. TES is an accurate, rapid and cost-effective approach for screening of multiple genes, but it still has some major limitations, such as detecting variants in low-depth regions and copy number variations[18, 20]. Because of their high cost, both of WGS and WES are less widely used than TES. TES is suitable for molecular diagnosis of USH. Because of the great diversity of various types of pathogenic genes and the frequent occurrence of new mutations, array-based diagnosis often can not accurately reflect pathogenicity. Pathogenic USH genes have many subtypes and numerous exons. At present, more than 400 coding exons have been identified. Therefore, a higher diagnosis rate can be obtained using a sequence-based diagnosis method.