Molecular Epidemiology Investigation Of Enlarged Vestibular Aqueduct Associated With SLC26A4 Gene

Enlarged vestibular aqueduct(EVA) is the commest inner ear malformation, which acounts for about 1-12% in the children with sensorineural hearing loss. Most patients associated with EVA clinically showed fluctuating or progressive sensorineural/ mixed(in the low-tone frequency) hearing loss. The hearing loss level varies from moderate to profound, which is severer in high-tone frequencies. The hearing loss of patients with EVA is always present before speech develops, which results in the disability of communication with speech for most patients. Hearing drop is always induced by factors that could make the intracranial pressure increased, such as cold, fever, microtrauma on the head et al. If we could make a accurate diagnosis to these patients earlier and take out some measures in time, the hearing loss onset may be delayed and the birth rate of these patients may be decreased.Previous study revealed that EVA is inherited in an autosome recessive manner. In 1999, Abe et al localized the related gene to chromosome region 7q31. At the same year, Usami et al discovered that SLC26A4 (PDS) was the exact gene responsible for EVA as well as Pendred syndrome. SLC26A4 contains 21 exons with an open reading frame of 2343 base pairs. Pendrin, the expression product of SLC26A4, is expressed in the non-sensory epithelia and acts as a chlorate transporter. It may assist in maintaining proper ionic composition of the endolymph, which is a prerequisite for normal inner ear function.To investigate the molecular pathomechanism of EVA, screening of SLC26A4 gene in patients with EVA has been carried out extensively. Up to date, more than 124 SLC26A4 mutations have been detected, including missense mutation, frame shift mutation, splice site mutation, nonsense mutation and small deletion mutation et al (http: www.hgmd.cf.ac.uk/ac/search.php). These mutations covered all the 21 exons of SLC26A4 gene. Studies in different races demonstrate that the frequencies and nature of mutations varied according to racial background. Each racial group has its own distinctive, diverse series of SLC26A4 mutations, which may include one or a few prevalent founder mutation. China contains approximately a quarter of the global population. However, there were only a few studies about SLC26A4 mutations in this population and the mutations characters were not disclosed. In the present study, 101 nuclear families associated with EVA were recruited from Otolaryngology Clinic at PLA General Hospital. All the 21 exons of SLC26A4 gene were screened in these subjects. Results of the screening disclosed a unique SLC26A4 mutation spectrum different from other populations. Furthermore, in order to investigate the incidence of EVA in patients with severe or profound hearing level, 159 non-syndromic hearing loss students were enrolled from deafness school in Gansu provience. Following the screening of SLC26A4 gene, high resolution computerized tomography(HRCT) scan on temporal bones were performed to students with mutant alleles. The incidence of EVA was evaluated by the screening of SLC26A4 gene combined with HRCT. Details were described as following:Part one: Studys of clinical traits and SLC26A4 mutation spectrum in patients associated with enlarged vestibular aqueductA total of 101 nuclear families (6 multiplex families(each has two patients) and 95 simplex families(each has only one patient)) with EVA were recruited,including 107 patients associated with EVA. The onset age of most patients was no more than 6 years old. Audiometric evaluation revealed that 73.3% of all patients had a hearing loss level of severe or profound. Air-bone gap at low-tone frequency was detected among 69.1% of patients with audiograms.In order to disclosed the inheritance mode and the SLC26A4 mutation spectrum of EVA, 95 patients in simplex families and 12 patients in multiplex families were analysed respectively:All exons of SLC26A4 were screened in patients with EVA. All 12 patients in the multiplex families have two mutant alleies. In these families, a total of 6 mutations were identified, including 3 novel mutatients(G209E, E303Q and 1746delG) and 3 reported(IVS7-2A>G, H723R, ). In the pedigree of family 673 and 701, two patients were in two continued generation. SLC26A4 mutations analysis indicated that SLC26A4 mutations were inherited in autosomal recessive manner. Results of pedigrees and SLC26A4 mutations revealed the "Pseudo dominant" inheritance in family 673 and 701. This results suggested that carrier frequncy of SLC26A4 mutation is very high.In the simplex families, mutations were identified in 93/95 patients, which accounted for 97.9% of all. 88/95(88.4%) patients have two mutant alleies and 9/95(9.5%) families have only one mutant allele. Mutations were not detected in the other 2 (2.1%) families. A total of 38 mutations were identified, including 23 novel mutations and 15 recurrent mutations. Of all these mutations, IVS7-2A>G is the commonest, which accounts for 57.63% of all mutant alleles(102/177).. Besides exon8 containing IVS7-2A>G, four exons (exonl9, 10, 17 and 15 ) were found having more mutant alleies than others. Almost all patients detected at least one mutant allele localized in these regions. The high incidence of patients with SLC26A4 mutation(especially patients with two mutant alleies), various mutations of SLC26A4 gene found only in Chinese, the commonest mutation and thecommonest mutation exons special to Chinese constructe the unique spectrum of SLC26A4 mutation in Chinese.Part two: Incidence studys of SLC26A4 mutation among Patients with severe-profound Hearing LossIn this section, 159 patients with severe-profound hearing loss were enrolled from deafness school in Gansu province. All the subjects are prelingual hearing loss and disabled in speech. Five common exons of SLC26A4 were detected in these subjects. Mutations were detected in 33/159(20.8%) subjects, including 21 subjects with two mutant alleles and 12 subjects with one mutant allele. HRCT scan were performed to 17 subjects with two mutant alleles and 8 subjects with one mutant allele. All subjects with two mutant alleles had EVA, however, one subject with one mutant allele showed normal inner ear. From results here, we can presume that 4 subjects with two mutant alleles, who were failed to follow up, must have EVA. So, in this cohort, 28/159 subjects may have EVA, which account for 17.6%. The result is much higher than that reported by Park et al. If patients with GJB2 and 12sRNA mutations were included, the incidence of EVA decreased to 14.8%, which is still higher than that of GJB2(l\.6%) in this cohort. Gene responsible for EVA may be the most frequent in this population with severe or profound non-syndromic hearing loss.