The Molecular Genetic Basis For Hereditary Late-onset Hearing Impairment

The term Late-onset hearing impairment is defined as any hearing loss that is not present at birth, but identified at a later date. Approximately two to three per 1,000 children will be identified as having childhood acquired or late onset hearing loss. Late-onset hearing impairment is one of the major chronic conditions experienced by older individuals. Both environmental and hereditary factors impact on late-onset hearing impairment. A variety of environmental factors can be considered. Examples of the common factors include infection, trauma, noise exposure or teratogens, etc. And the hereditary factors refer to the deafness-causing genes or their deafness-causing mutations. However the relative contribution of each is not known. As the environmental factors related to hearing impairment have been gradually controlled as our society progress these years, the hereditary factors have turned to be the primary factors leading to late-onset hearing loss. And now, most types of inherited late-onset hearing impairment appear to be non-syndromic autosomal dominant inheritance. Moreover, almost all of the DFNA loci have the auditory phenotype of late-onset hearing loss. These suggest that late-onset hearing impairment and DFNA have close relationship with each other, and DFNA can be good research models for understanding the molecular genetic basis for late-onset hearing impairment. So the map-based cloning (positional cloning) of DFNA which has phenotype of late-onset hearing loss may help elucidate the molecular genetic basis for late-onset hearing impairment, and identify the effect of hereditary factors on late-onset hearing loss, so that we could get more knowledge about diagnosing and treating the late-onset hearing loss in Chinese people. This study used the large pedigrees with plenty of genetic information collected by the Otolaryngology Institute of PLA General Hospital for map-based genediscovery by the method of positional cloning. This thesis comprised two parts and the details were described as following:Part one: Mapping and cloning of the causative gene in the Chinese DFNA family with late-onset hearing impairmentThe purpose of this study was to fine-mapping and causative gene cloning in the Chinese late-onset DFNA4 family (family Z002) which was located by Prof. Qiuju Wang in 2002. Adding five family members in this study on the basis of previous gene mapping study, including one patients with hereditary late-onset hearing impairment and four members with normal hearing for fine-mapping with nine serried markers locating in the 19.92cM region flanked by markers D19S223 and D19S571. Maximum two-point LOD score of 5.30 (0=0) occurred at Marker D19S879. Recombination breakpoints, which was observed in individual 37 who was newly added in this study, were between markers D19S902 and D19S879 defining the proximal border, and between D19S907 and D19S571 defining the distal border. The new critical interval is 5.36cM between these markers (D19S902 and D19S907) which was obviously narrowed contrast to previous 19.92cM. Within the refined interval of the Chinese DFNA4 family, the public databases list 128 genes, greatly less than the previous 454 genes. In this critical region, choosing the known DFNA4 gene MYH14 gene as the first candidate gene for mutation screening. Using the SNPs of MYH14 gene detected in the family to construct haplotype. The results of the screening and the haplotype analysis together excluded MYH14 as the causing gene of family Z002, and narrowing the critical region again to 2.5Mb (physical distance) between marker D19S902 and MYH4 gene. Among other genes in the region, several appear to be promising DFNA4 candidates due to similarities with animal models and with other causative genes involved in hearing disability. We excluded KCNN4,KCNA7, KPTN and ERCC1 as candidate genes for family Z002. This study showed genetically extremely heterogeneous of late-onset DFNA, suggested there was a new deafness gene in our Chinese family. It is hopeful to clone the new DFAN4 gene in family Z002 in the future.Part one: Mapping the disease loci for the two Chinese DFNA family (Z1318 family and W727family) with late-onset hearing impairmentThe purpose of this study was to investigate the two typical pedigrees with genetic late-onset hearing impairment in our country and to discover the causative loci/genes in the two families. These two large Chinese DFNA families, named Z1318 and W727, were clinically studied first, the study was the base for the next gene mapping and cloning. Two Chinese families (Z1318 and W727), with late-onset non-syndromic hearing loss were ascertained. The mode of inheritance of the families should be autosomal dominant according to their pedigrees. Thetwo families shared some common phenotypic features: being bilateral and havingpostlingual, progressive, moderate to moderate severe sensorineural hearing impairment involving higher frequencies principally. However, there appeared to be some differences in the pattern of hearing loss between the two families. Hearing impairment of affected members in family Z1318 had been present since adolescence (10 to 20 years of age), mainly affecting the high frequencies of 2-8 kHz, and causing a down-sloping audiograms. While, in family W727, the age-of-onset varies from 6 to 30 years, the hearing loss began in the high frequencies, and lower frequencies became involved with increasing age, thus causing a flat-type audiogram after age 50 years. After the Clinical Genetics Investigation of the two Chinese families, we exclude the common deafness genes GJB2 or A1555G as the causative gene of the two families, respectively. Then weuse candidate positional cloning and positional cloning approach to family Z1318 and family W727, respectively. The genescan and linkage analysis mapped family W727 into the 10.35cM region flanked by markers D9S269 and D9S1684. This region was partially overlapped with locus DFNA47 which was located by an Italian pedigree in 2003. However, the gene for this locus remained undisclosed. We would do gene cloning in the consensus region of Chinese and Italian pedigrees. Moreover, Dr. Li and Dr. Yuan in our group mapped two late-onset families named FOB and 686 in 9q31.3-q34.3 in 2005 and 9p13.2-9p13.3 in 2006, respectively. And there was another locus named DFNA36 (9q13-q21) also locating on chromosome 9 except for DFA42 (9p21-p22). So several pedigrees located on the same chromosome in the different regions, this suggested that chromosome 9 was a hot spot region of causative genes associated with late-onset hearing impairment. And there was no overlap between family W727 and family F013 or family 686, so W727 was another new Chinese pedigree located on chromosome 9. Positional cloning in family W727 is hopeful to clone a new deafness gene and to build the theory of molecular genetic basis for late-onset haring loss.