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’s found at heterozygous state in one European non Finnish individual. Mutations in USH2A are associated with USH2, it is responsible for almost 50% of USH cases[7].
USH2A codes two alternatively spliced isoforms of usherin. Short ~170 kDa isoform a, consisting of 21 exons, is regarded as an extracellular protein. 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. On the other hand, 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 made the premature termination translation, 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 and pathophysiologic processes such as angiogenesis and vascular remodeling[15]. TM exists at the base of the differentiating stereocilia, and it makes up the mechanosensitive hair bundles receptive to sound. PDZ-binding motif domains provide the anchoring of interstereocilia lateral links to the F-acin core of stereocilia[16]. In this regard, we suppose that the absence these domains corresponding to the incompleteness of usherin, which might probably have in turn affected the process of stereocilia differentiation and maturation, resulting in a milder stereocilia dysmorphic phenotype. Several positions are found associated exclusively with pathogenic of FN3 in usherin[17], which support our hypothesis. However, the pathway needs to be confirmed by molecular experiments in the future.
Whole-genome sequencing (WGS), whole-exome sequencing (WES) and TES is three major methodologies for molecular diagnosis of IRD. WGS is useful for detecting copy number variations and structural variations[18]. WES is especially useful for identifying novel IRD related genes. TES is an accurate, rapid and cost-effective approach for screening of multiple genes[19], but still have some major limitations, such as detecting variants in low-depth regions and copy number variations[18, 20]. Because of the higher cost, both of the WGS and WES is less widely used than TES. TES is suitable for molecular diagnosis of USH. As the great diversity of various types of pathogenic genes and the frequent occurrence of new mutations, array-based diagnosis often could not accurately reflect the pathogenicity. USH pathogenic genes have many subtypes and numerous exons. At present, more than 400 coding exons have been commented.[21] Therefore, a higher diagnosis rate can be obtained by using sequence-based diagnosis method.