Usher syndrome is a severe disease resulting in significant vision and hearing impairments. Based on the phenotypic characterization, the disease has been classified into three subtypes [7]. USH2 patients show moderate-to-severe hearing loss, intact vestibular function, and onset of Retinitis pigmentosa (RP) within the second decade of life [8]. The patient in our study was five years old and exhibited bilateral moderate deafness, without absence of vestibular dysfunction, vision, or visual field involvement. Because RP in USH2 patients usually occurs after puberty, this proband cannot be excluded as USH2. However, there is considerable variability within the subtypes resulting in overlapping phenotypes between USH1, USH2, and USH3 [3–5]. Therefore, the diagnosis of USH in childhood may be difficult because although some features exist at birth, others appear as the child matures. Genetic testing should be undertaken when the clinical phenotype does not enable clear clinical diagnosis.
To date, sixteen genes have been reported to be associated with USH: nine are involved in USH1, three in USH2, two in USH3, and two are not specified. The USH2A, USH2C and USH2D genes are responsible for USH2 [9]. Due to the large number of coding exons existing in these genes, Sanger sequencing is not feasible for clinical application. In this study, we therefore attempted to target a group of genes responsible for hearing loss, including the USH2A, USH2C, and USH2D genes. As a result, we have proven that targeted deep exome sequencing of 162 known causative genes of hearing loss can serve as a fast and efficient way to diagnose USH2. Compared with whole exome sequencing and whole genome sequencing, the cost of the disease-targeted gene panel was much lower and the workload was lesser [10].
The USH2A gene, located on chromosome 1q41, consists of 72 exons. In mammalian photoreceptors, usherin is localized to a spatially restricted membrane microdomain at the apical inner segment recess that wraps around the connecting cilia, corresponding to the periciliary ridge complex described in amphibian photoreceptors [9]. USH2A gene mutations have been implicated in the disease etiology of several inherited diseases, including USH2, nonsyndromic RP, and nonsyndromic deafness [11]. To date, 1348 variants in the USH2A gene have been reported in patients with USH2 or RP in the HGMD. Here, we reported two variants of USH2A, including one splicing variant (c.8559-2A > G) and one frameshift variant (c.4749delT, p.F1583fs), in the patient with hearing loss; the former variant has been previously reported [12], while the latter was novel. According to the criteria of ACMG, these mutations were pathogenic and the evidence include: (1) very strong evidence of pathogenicity (PVS1): Splicing, frameshift; (2) Moderate evidence of pathogenicity (PM2): absent from controls in the general population databases (or at extremely low frequency if recessive); (3) Supporting evidence of pathogenicity (PP4): Patient's phenotype is highly specific for a disease with a single genetic etiology [13].
The mutation c.8559-2A > G has been previously reported as a DM variant in HGMD. Bioinformatic analysis predicted that the mutation c.8559-2A > G would cause shearing abnormality, resulting in the skipping of exon 43 during transcription [2, 12, 14]. The mutation c.8559-2A > G in USH2A accounts for 19.1% of mutations in a Chinese USH2 cohort [15] and 26% in all Western Japanese USH patients, but was never reported in Europeans[15–16]. These results suggested that c.8559-2A > G may be one of the hot spot mutations of the USH2A gene in Asian populations. Hence, mutation screening for c.8559-2A > G in USH2A may prove very effective for the early diagnosis of USH2.
At present, the main abnormal phenotypes of USH are RP and sensorineural deafness. Despite the lack of cure for USH, cochlear implants can help improve the hearing functions of USH2 patients. Moreover, cochlear implants and sensory prosthesis implantation can improve the symptoms of hearing loss or retinal degeneration [2, 9]. At present, although there is no effective cure for human USH patients, virus vector and antisense oligonucleotide targeting therapies have been successfully used to treat USH in animal experiments [11].
In this report, we successfully performed genetic diagnosis of Usher syndrome by disease-targeted gene panel and have thus proven that this method can serve as a rapid, high-throughput, and efficient screening strategy. We describe a Chinese patient presenting clinical features compatible with USH2. Using a disease-targeted gene panel, we identified novel compound heterozygous mutations in exons 22 and 43. The novel mutation expands the spectrum of USH2A variants in USH. According to the criteria of ACMG, both mutations were pathogenic. Specific DNA sequencing of the two mutation sites was carried out in the fetus of the proband's mother. Subsequently, the mother of the proband gave birth to a healthy baby without any abnormal phenotype.