Clinical features associated with enlarged vestibular aqueduct
Enlarged vestibular aqueduct (EVA) is a radiologic malformation of the inner ear most commonly seen in children with sensorineural hearing loss. First described by Valvassori and Clemis in 1978, the radiological criteria of EVA was defined as the bony vestibular aqueduct exceeding 1.5 mm at the midpoint. Hearing loss with EVA may be associated with other abnormalities as part of a syndrome, or it could be an isolated clinical entity (deafness, autosomal recessive 4 [DFNB4], MIM 600791, or non-syndromic EVA).[3,4] Pendred syndrome (MIM 274600) is the most common syndrome associated with EVA. It is typically characterized by the presence of EVA, sensorineural hearing loss, and defect in thyroid iodine organification, which in some cases may result in goiter in late childhood to early adulthood. EVA has also been observed in branchio-oto-renal syndrome (MIM 113650), Waardenburg syndrome (MIM 606662,193500, 608890, 600193,193510), distal renal tubular acidosis with sensorineural deafness (MIM 267300), and CHARGE syndrome (MIM 214800). EVA may occur either unilaterally or bilaterally and may be accompanied by Mondini dysplasia, an incomplete partition of cochlea. Hearing loss associated with EVA in children is typically congenital or prelingual, usually progressive or fluctuating and variable in severity.[7,8] Some patients may suffer a sudden decrease in hearing after mild barotrauma or head trauma. It seems that the severity of the enlargement of vestibular aqueduct does not correlate with the severity of hearing loss, and the presence of a cochlear malformation is not associated with the degree of hearing loss. Therefore, the dilation of the osseous vestibular aqueduct itself is unlikely the direct cause of hearing impairment in EVA patients.
Pathophysiological mechanisms of EVA with SLC26A4 mutations
Mutations in the gene SLC26A4 (solute carrier family 26 members 4, SLC26A4) are the most prevalent cause of hearing loss associated with EVA and the second most frequent cause of autosomal recessive non-syndromic sensorineural hearing loss in many populations throughout the world.[4,10,11]SLC26A4 encodes a multipass transmembrane protein SLC26A4, also known as pendrin, which is mainly expressed in the thyroid gland, kidney, and inner ear.[12,13] Pendrin functions as a Cl−/I− transporter at the apical membrane of follicular epithelial cells in the thyroid, where SLC26A4 is expressed at extremely high levels. It mediates the transport of intracellular inorganic iodine into the thyroid follicular lumen (a process called “iodide efflux”) for the thyroid hormone biosynthesis.[14–16] The role of pendrin in iodide efflux appears to correlate with thyroid pathology in individuals affected by Pendred syndrome. In the inner ear, expression of pendrin occurs in non-sensory epithelial cells of the external sulcus region in the cochlea, transitional cells in the saccule and utricle, and in the endolymphatic sac and duct.[17,18] The significant insights into the role of pendrin in hearing and the mechanism of hearing impairment with EVA have been gained using the SLC26A4 knockout (SLC26A4−/−) mouse. The SLC26A4−/− mouse has profound hearing loss and displays signs of vestibular dysfunction. The endolymphatic sac and duct, as well as the entire inner ear of these mice, are dilated, similar to that found radiologically in patients with EVA. Subsequent studies of SLC26A4−/− mice indicate that pendrin functions as a Cl−/HCO3− exchanger in the inner ear. At embryonic day (E) 14.5 the cochlea and endolymphatic sac of SLC26A4−/− mice begin to dilate, as the result of the failed onset of pendrin expression in the inner ear at E11.5. Loss of pendrin leads to endolymphatic acidification at postnatal day (P)10, which may inhibit the reabsorption of Ca2+ via acid-sensitive Ca2+ channels. Ca2+ overloading may eventually lead to inhibition of the mechanosensory transduction essential for auditory function and promote the degeneration of hair cells. Moreover, lack of pendrin in SLC26A4−/− mice results in bicarbonate accumulation and alkalization of the intrastrial spaces. The extracellular alkalization in stria vascularis may enhance oxidative stress leading to a loss of KCNJ10 (adenosine triphosphate-sensitive inward rectifier potassium channel 10) protein expression at P15 and failure to establish endocochlear potential, which was thought to play a direct causative role in hearing impairment associated with EVA. Pendrin insufficiency also leads to strial dysfunction and degeneration. More recently, a study demonstrated in vitro that during morphogenesis of the inner ear the endolymphatic sac absorbs fluid in a SLC26A4-dependent manner. The study shows that the mitochondria-rich cells in endolymphatic sacs express various transporters, channels, pumps, and exchangers, among which the chloride-bicarbonate exchanger SLC26A4 plays a critical role in the transport of chloride from endolymph across the apical membrane into the cytosol of the mitochondria-rich cells. Disruption of this mechanism of transepithelial endolymph absorption by endolymphatic sac caused by SLC26A4 mutations might be the root cause of hearing loss associated with EVA.
Detection rates of SLC26A4 mutations in different populations
To date, there have been over 300 SLC26A4 variants identified as causative mutations in individuals affected by non-syndromic EVA or Pendred syndrome. Although non-syndromic EVA and Pendred syndrome are recessively inherited, for many patients molecular testing for the SLC26A4 coding regions and splice sites mutations has failed to identify causative mutations in both alleles.[24–26] The reported detection rates of biallelic and monoallelic SLC26A4 mutations in affected individuals varies among different ethnic groups, ranging from as low as 16% in North American Caucasians to as high as 62% or more in Chinese. A number of studies analyzing SLC26A4 mutations in EVA patients from different districts of China have been reported. In one study from northern China screening SLC26A4 mutations in 107 probands with EVA, two mutant alleles were found in 88.4% of the affected patients, and 1 mutant allele was detected in 9.5%. Another study in Taiwan, China tested SLC26A4 mutations in 101 families with non-syndromic EVA or Pendred syndrome. They detected two mutant alleles in 62% of the families, 1 mutant allele in 24% and no mutation in 14%. Our research group performed mutation analysis of SLC26A4 gene in a clinically well-phenotyped cohort of 140 probands from eastern China. We identified biallelic mutations in 76.4% of patients and monoallelic mutations in 8.6%. In other populations such as North American Caucasians, the detection rate of SLC26A4 is quite low, with nearly 50% of the affected patients having no detectable mutations.
Other genetic contributors to hearing loss with EVA
The inability to identify two mutant alleles of SLC26A4 in individuals with hearing loss associated with EVA suggests the possible involvement of (1) multiexon deletions or duplications in SLC26A4; (2) mutations in the SLC26A4 gene noncoding regions (eg, promoter and intronic mutations); or (3) mutations in other genes that might act in trans in combination with mutations of SLC26A4 to cause hearing loss with EVA. Consistent with these hypothesis, several genomic deletions spanning multiple exons of SLC26A4 have been described previously.[31–33] Most of them are identified in single cases, and multiexon deletions and duplications are estimated to account for approximately 1.8% of missing SLC26A4 mutations in EVA patients overall. In our previous study, a 7666 bp deletion in SLC6A4 gene spanning the first three exon and start codon was identified in a consanguineous family with non-syndromic EVA. The same multiexon deletion was also detected in 4 out of 22 (18%) Chinese probands with non-syndromic EVA carrying 1 mutant allele of SLC26A4. Although multiexon deletions account for a rather low percentage of the overall causes of EVA, clinical testing for multiexon deletions may be warranted for patients with monoallelic SLC26A4 mutations.
Mutations in noncoding regions of SLC26A4 gene also contribute to the pathogenesis of EVA. For instance, mutations in the promoter or enhancer region may decrease gene expression and mutations in the introns of SLC26A4 may create cryptic splice sites. Consistent with this hypothesis, a mutation within the core promoter element of SLC26A4 gene c.-103T→C has been identified. This cis-regulatory element within the SCL26A4 promoter has a unique head-to-head binding motif for FOXI1 (forkhead box I1), a forkhead transcription factor that activates the transcription of SLC26A4 in the endolymphatic sac and duct. The c.-103T→C mutation disrupts the binding of FOXI1 to the SLC26A4 promoter and abolishes FOXI1-mediated transcriptional activation.
Mutations of FOXI1 itself have also been shown to contribute to the disease phenotype. FOXI1–/– mice are profoundly deaf and have abnormality in the inner ear that resembles human EVA. Additionally, the epithelium of endolymphatic duct and sac of FOXI1–/– mice completely lacks the expression of pendrin. Mutation screening in probands with either non-syndromic EVA or Pendred syndrome identified five FOXI1 mutations that led to reduced transcriptional activation ability in six individuals. One of these patients has a double heterozygous inheritance pattern, carrying one single heterozygous mutation in both SLC26A4 and FOXI1.
Mutations in an inward-rectifier potassium channel encoding gene, KCNJ10, have also been causally related to EVA. Studies in SLC26A4−/− mice have shown that in stria vascularis malfunction of pendrin leads to increased oxidative stress and a reduction of KCNJ10 protein expression.[39,40] Since expression of functional KCNJ10 is critical for the generation of the endocochlear potential, which is necessary for hearing, these observations support the hypothesis that mutations of KCNJ10 might be an etiologic contributor to hearing loss associated with EVA. Consistent with this, two patients were identified with segregated heterozygous mutations in both KCNJ10 and SLC26A4 in a study screening 89 patients with non-syndromic EVA or Pendred syndrome carrying monoallelic SLC26A4 mutations. The KCNJ10 p.R348C and p.P194H mutations identified in this study were damaging to channel activity and led to reduced potassium conductance, suggesting a possible digenic interaction between KCNJ10 and SLC26A4 in the etiology of hearing impairment associated with EVA.
The role of FOXI1 and KCNJ10 mutations in hearing loss with EVA, however, seem to be limited, as illustrated by several subsequent studies systematically testing FOXI1 and KCNJ10 mutations in individuals affected by Pendred syndrome or non-syndromic EVA.[29,30,42–44] Overall, possible pathogenic variants in FOXI1 and KCNJ10 were detected in 1.3% and 3.1% EVA patients respectively. In addition, it has been shown that EVA in some families does not segregate in an autosomal recessive pattern, further supporting that hearing loss with EVA is a complex genetic disease.
EVA-associated hearing loss is a relatively common but enigmatic disorder in children. Studies with mouse models indicate that disruption of fluid absorption by endolymphatic sac that depends on Cl−/HCO3− exchanger SLC26A4 during morphogenesis of the inner ear might be the root cause of hearing loss with EVA. Although mutations in SLC26A4, FOXI1, and KCNJ10 together account for a large portion of EVA cases, the etiologic basis of hearing loss in patients without SLC26A4, FOXI1 or KCNJ10 mutations remains unknown. Further investigation is needed to elucidate the underlying etiologies of these patients, thereby providing proper genetic counseling and potential interventions to prevent or reverse EVA-associated hearing loss.
XY participated in literature retrieval, manuscript drafting and writing of the main part in the manuscript. TY designed and wrote the manuscript. HW reviewed and modified the manuscript. All authors approved the final version of the paper.
Conflicts of interest
The authors declare that they have no conflicts of interest.
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