The development of the Human Genome Project has promoted the combination of genetics and clinical medicine for diagnostics. The emergence of genetic screening and diagnosis for deafness is of great significance for the clarification of the cause of deafness and to prevent its occurrence [9; 10; 11]. Since the 1980s, with the development of technology, great progress has been made in the study of the genetic etiology of deafness. To date, more than 250 deafness genes have been cloned or identified (https://hereditaryhearingloss.org/) and it has been reported that more than 500 genes may contribute to the deafness phenotype. However, the diagnosis of hereditary deafness is extremely challenging for the following reasons: Firstly, hereditary deafness is a disease with very high genetic, clinical and ethnic heterogeneity. Secondly, hereditary deafness genes may have different categories of genetic variants, such as single nucleotide variations (SNVs), insertions and deletions (INDELs) and CNVs. Thirdly, most deafness gene variants do not occur repeatedly.
The introduction of NGS into the field of medical genetics has opened up new horizons for clinical diagnosis and scientific research and it has become a powerful auxiliary technology. Over the past decade, studies have attempted to enrich the known HL genes using capture-based or PCR-based targeted enrichment methods, followed by NGS, to screen for variants [12]. It can be concluded that the sensitivity and specificity of NGS for HL genetic detection is comparable to conventional Sanger sequencing, based on previous studies. Recently, Gema García-García et al [13] detected the variants responsible for the disease in 47 out of 118 families (40%), with a custom panel that included 59 genes associated with non-syndromic or syndromic HL. Seco et al [14] published a study that evaluated the diagnostic utility of NGS targeting using a gene panel of 120 HL-related genes. Using this strategy, they were able to identify a causative variant of HL in 67 out of 200 patients (33.5%). In this study, 518 deafness-related genes were identified in 879 patients. Deafness gene variants were detected in 429 children and the detection rate was 48.8%, which was slightly higher than the detection rates observed when using the smaller gene panels that contain the common deafnessrelated variants. Our study showed that the more deafness-related genes that are included, the higher the detection rate, which indicates that some populations have rare variants. These results are significant for the molecular diagnosis of deafness and further research.
Deafness is a disease with high genetic heterogeneity and one of the manifestations is that the high-frequency mutation genes are different in different populations. The GJB2 and MYO6 genes are the most commonly mutated HL genes in the global population. In Czech patients, the common HL genes are GJB2 and STRC [15; 16]. However, in the Chinese population, the common HL genes are GJB2 and SLC26A4 [9]. These studies show that molecular diagnostic analyses are highly population dependent, as the mutation spectrum for HL can differ greatly between populations. In this study, variants were detected in ten genes (GJB2, SLC26A4, MYO15A, MYO7A, TMC1, CDH23, MITF, USH2A, WFS1 and SOX10), which explained 55.7% of patients with deafness variants. With the exception of mutations in two relatively common genes, GJB2 (16.6%) and SLC26A4 (11.9%), most reported mutations were present in only a single or a small number of families. After excluding GJB2 and SLC26A4 mutations, we detected P, LP or VUS mutations in the known HL genes in 34.9% of the probands.
Our results demonstrated that NGS screening is a powerful technology for the identification of mutations in HL genes. From an epidemiological perspective, GJB2 and SLC26A4 mutations were involved in 28.44% of cases of deafness with a gene variant, while the remaining cases arose from rare gene mutations. For instance, CDH23, PCDH15 and MYO7A are large genes and it appears that these genes are more prone to mutations [17; 18]. The mutations in some genes did not appear repeatedly. For example, none of the mutations in the SOX10 and MITF genes were recurrent in our cohort. This may indicate that SOX10 and MITF are relatively more prone to de novo mutations or that they are highly conserved genes in which variation is rarely tolerated. Some genes, such as TNIE and TPRN, had less than 20 loci reported in the HGMD database. In these two genes, only one case of a rare gene mutation was detected in our cohort. In contrast, five patients in our study had BDP1 gene mutations, despite this gene having less than 20 mutation sites in the HGMG database. It is worth noting that, among these five individuals, four had the same mutation, which has not been reported in the literature. This suggested that this locus may be a hotspot in this population and patients with negative test results are likely to carry undetected mutations.
Mutations in introns may be relatively common in deafness patients. Among the 98 deafness-related genes detected in this study, SLC26A4 (62.7%, 32/51), MYO15A (63.6%, 14/22), OTOF (40.0%, 4/10) and PTPRQ (75%.0, 3/4) contained intronic variants. In previous studies, it has been confirmed that the intron of SLC26A4 is prone to mutations, especially at the c.919-2A > G site, which is one of the mutation hotspots in the Asian population [19; 20]. Our research also confirmed this. The variants detected in this study included common intronic mutations and also some rare or novel variants. This suggested that there are some unknown variant sites that require comprehensive testing, in order to find more genetic variants.
Copy number variants are a major contributor to hereditary HL, which shows the importance of the inclusion of CNV detection in a molecular diagnostic analysis for HL [21; 22]. Although there have been studies that show the importance of CNV analysis of NGS data, in the etiology of deafness, this analysis method is still being optimized for clinical use. The study by Shearer et al [23] used targeted enrichment and NGS with integrated CNV detection for HL in 686 patients. At least one CNV within a known deafness gene was observed in 15.2% (104) of patients. In our cohort, we used NGS to identify CNVs in four different genes (SLC26A4, MITF, EYA1 and CDH23), in nine patients (2.1%, 9/429). Several studies have shown that STRC is one of the deafness genes that is more prone to CNVs [23; 24; 25]. These results demonstrated that NGS can detect CNVs but more medical records and studies are needed to confirm whether all CNVs can be detected [26].
This study was not able to fully identify and explain the molecular etiology of all patients. Deafness-related gene variants were not detected in 20.8% of the 879 patients. These patients were likely to have undetected variants but more research is needed to confirm this speculation. It was noted that 43.2% of the unsolved patients had a pathogenic variant in a known HL gene. These patients may have had another mutation that had not been detected, or they may have been carriers who had developed deafness [27; 28]. Deafness can also occur through an oligogenic pattern of inheritance, were two or more pathogenic variants in different genes are detected in individuals with unsolved HL. Combinations of some variants have been reported to be the cause of oligogenesis, such as variants in SLC26A4 and KCNJ10, or SLC26A4 and FOXI1 [29]. However, we did not find any patients with the previously reported combinations of variants.
In this study, we applied NGS technology to (1) identify the molecular epidemiological etiology of HL in Chinese children and (2) discover novel mutations in causative genes. The results of the study provide a reference for the development of genetic screening or diagnosis of deafness, which is suitable for this region. In our analysis, we only considered coding regions, which is a limitation of this study. Pathogenic variants in noncoding regions or transcripts may also cause HL. In addition, due to the lack of family history, it was not clear whether the deafness gene variants carried by some patients contributed to the development of the disease.
Our data indicated that many rare variants are responsible for HL in this cohort and these mutations can be detected using NGS, which is approximately comparable in accuracy to Sanger sequencing. It is likely that there are undetected, rare variants that are specific to the Chinese population. Therefore, the main challenge for the future will be the establishment of population specific mutation-spectra, to achieve accurate, personalized, comprehensive molecular testing for HL.