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Mutations in the SLC26A4 gene are a common cause of human hereditary hearing impairment worldwide. Previous studies have demonstrated that different SLC26A4 mutations have different pathogenetic mechanisms. By using a genotype-driven approach, we established a knock-in mouse model (i.e., Slc26a4tm2Dontuh/tm2Dontuh mice) homozygous for the common p.H723R mutation in the East Asian population. To verify the pathogenicity of the p.H723R allele in mice, we further generated mice with compound heterozygous mutations (i.e., Slc26a4tm1Dontuh/tm2Dontuh) by intercrossing Slc26a4+/tm2Dontuh mice with Slc26a4tm1Dontuh/tm1Dontuh mice, which segregated the c.919-2A>G mutation with an abolished Slc26a4 function. Mice were then subjected to audiologic assessments, a battery of vestibular evaluations, inner ear morphological studies, and noise exposure experiments. The results were unexpected; both Slc26a4tm2Dontuh/tm2Dontuh and Slc26a4tm1Dontuh/tm2Dontuh mice showed normal audiovestibular phenotypes and inner ear morphology, and they did not show significantly higher shifts in hearing thresholds after noise exposure than the wild-type mice. The results indicated not only the p.H723R allele was non-pathogenic in mice, but also a single p.H723R allele was sufficient to maintain normal inner ear physiology in heterozygous compound mice. There might be discrepancies in the pathogenicity of specific SLC26A4 mutations in humans and mice; therefore, precautions should be taken when extrapolating the results of animal studies to humans. [ABSTRACT FROM AUTHOR]

Accurate epidemiological data on common deafness genes are essential to improve the efficiency and to reduce the cost of molecular diagnosis. They may depend on several factors, including a clear delineation of the source of patients being studied. In the present study, we hypothesize that patients with idiopathic sensorineural hearing loss recruited from different sources might reveal discrepancies in the epidemiological results of genetic screening, because patients from different sources might demonstrate distinct clinical or audiologic features and thus result in biased selection of subjects. To elucidate the relative importance of common deafness genes in Taiwanese and to verify our hypothesis, we conducted a prospective project screening mutations in GJB2, SLC26A4 and mitochondrial 12S rRNA gene in a total of 420 Taiwanese families with idiopathic bilateral sensorineural hearing loss, of which 325 families were recruited from hospitals and 95 from hearing rehabilitation facilities. Allele frequencies of common mutations in these three genes and distributions of the corresponding genotypes were then compared between the two groups. The allele frequencies of mutations in SLC26A4, GJB2 and mitochondrial 12S rRNA in the probands of the 420 families were 14.4, 21.7 and 3.8%, respectively. The allele frequency of SLC26A4 mutations in the hospital group was significantly higher than that in the rehabilitation facility group (16.2 vs. 8.4%, chi(2)-test, p < 0.05), whereas no difference in the frequencies of GJB2 mutations and mitochondrial 12S rRNA mutations was found between the two groups. Distributions of probands classified by SLC26A4 genotypes were also different between the two groups (chi(2)-test, p < 0.05). Accordingly, a discrepancy in the genetic screening results might exist between different sources of idiopathic hearing-impaired patients. Further analysis of audiological results and construction of a logistic regression model showed that different audiological features, namely hearing levels and hearing loss patterns, might be responsible for the unequal distributions of mutations and probands between the hospital and rehabilitation facility groups. [ABSTRACT FROM AUTHOR]

Recessive mutations in the SLC26A4 gene are responsible for nonsyndromic enlarged vestibular aqueduct (EVA) and Pendred syndrome. However, in some affected families, only 1 or 0 mutated allele can be identified, as well as no clear correlation between SLC26A4 genotypes and clinical phenotypes, hampering the accuracy of genetic counseling. To elucidate the genetic composition of nonsyndromic EVA and Pendred syndrome, we screened related genomic fragments, including the SLC26A4 coding regions, the SLC26A4 promoter and the FOXI1 transcription factor gene, in 101 Taiwanese families, and analyzed their phenotypic and genotypic results. Mutation screening in the SLC26A4 coding regions by direct sequencing and quantitative polymerase chain reaction detected 2 mutations in 63 (62%) families, 1 mutation in 24 (24%) families and no mutation in 14 (14%) families. The radiological findings, the presence of goiters and the audiological results were not different among probands (i.e. index cases of the families) with different SLC26A4 genotypes. Specifically, probands heterozygous for SLC26A4 mutations demonstrated clinical features indistinguishable from those of probands with 2 mutated alleles, implicating that there might be undetected mutations. However, except for a variant (c.-2554G>A of SLC26A4) with possible pathological consequences, no definite mutation was detected after extensive screening in the SLC26A4 promoter and FOXI1. In other words, in most Taiwanese families nonsyndromic EVA or Pendred syndrome might not result from aberrance in the transcriptional control of SLC26A4 by FOXI1. Meanwhile, exploration of undetected mutations in the SLC26A4 noncoding regions revealed 9 divergent haplotypes among the 21 no-mutation-detected SLC26A4 alleles of the c.919-2A>G heterozygotes, indicating that there might be no common and predominant mutations in the SLC26A4 introns.