The Study Of Molecular Mechanism In Mitochondrial DNA Mutations Related Hearing Loss

Hearing loss is the most common disability of mankind, which severely impairs people's everyday life, communication and social productivity. According to the World Health Organization, there are about 250 millions of people with more than moderate hearing loss and this number is 20.57 million in China. The number of deafness people caused by mitochondrial DNA (mtDNA) mutation is estimated to be 5‰ of all the people with hearing impairment. Thus there are about 1.25 million deaf people on the world and about 102850 people in China with mtDNA mutations related hearing loss. Therefore, researching the hearing loss associated with mtDNA mutation is not only one of important aspects in investigating the natural world but also has great social and economical significance. The purpose of present study is to investigate the mechanism of deafness associated with mtDNA mutation and provide some theory and suggesting for preventing and therapying this kind of hearing loss. There are mainly two kinds of mutation that can cause hearing loss- base mutation and recombination of large fragments. In order to comprehensively study the molecular mechanism of deafness associated with mtDNA mutation, we carried out our research as the following three parts.Chapter one: the clinical data collection of patients with inherited hearing lossWe collected clinical data of patients with inherited hearing loss in the outpatient department of the PLA General Hospital. Some standards such as the inclusion criteria and exclusion criteria, the calibrating criteria for audiometric assessment instruments, criteria for testing technique of hearing loss audiometry, the severity criteria for hearing loss were setup before the collection. This work is part of the collaborated project between the Otolaryngology Department of PLA General Hospital and Mitochondrial Genetics Laboratory of Division and Program in Human Genetics, Cincinnati Children's Hospital Medical Center. We collected the clinical data of 188 patients during March 2003 and January 2004, of which there are 34 aminoglycoside-induced hearing loss patients. Besides, we collected 582 blood samples of the family members of those patients. All the samples were screened for the mtDNA 12S rRNA mutation and 7 patients with A1555G mutation, 3 with T1095C mutation were discovered. Besides, we found a family with Kearns-Sayre Syndrome and maternal inherited vision loss. The second and the third part of the research were based on this part of work.Chapter two: the study of the molecular mechanism in non-syndromic hearing loss caused by mitochondrial DNAmutation.We studied the molecular and genetic characteristic of four families with A1555G mutation. The phenotypes of these families are distinct by their extremely low penetrance (5.2 %, 4.8 %, 4.2 % and 13.3% respectively, and with an average 8 %). There are often only one or two hearing loss patients and no other deaf maternal relatives although they also have A1555G mutation with them. The analysis of the mtDNA sequence demonstrated that besides A1555G mutation, their mtDNA polymorphism is distinct. Their mtDNA belong to different haplogroups R9a, N9a, D4a and D4 respectively. These results suggestthat the A1555G occurred sporadically and increased during the evolution of the Chinese mtDNA. However, there aren't mutation of functional significance and secondary LHON mutations in these mtDNA genomes. These data implies that nuclear genome and/ or mtDNA haplotype doesn't play important role in the phenotypic expression of the A1555G mutation in these Chinese families. Aminoglycosides may be the major modifier factor for the phenotypic manifestation of the A1555G mutation in these Chinese families.As some of the patients with aminoglycoside-induced hearing loss don't have known mtDNA mutation, we tried to find potential novel mtDNA mutation that may cause hearing loss on the base of molecular and genetic characteristic. We found three patients with T1095C mutation in the mtDNA genome. T1095C has been believed to be associated with hearing loss from different genetic background. We didn't find this mutation in 364 Chinese controls besides these three patients. The presence in genetic unrelated families of T1095C strongly indicated that this mutation is pathologic in the occurrence of hearing loss. There are also some mutations on highly evolutionary conserved sites in these patients' mtDNA genome, which may contribute to the expression of the hearing loss phenotype. Theses results suggest that T1095C may be associated with aminoglycoside-induced and non-syndromic hearing loss.Chapter three: the study of molecular mechanism in mitochondrial DNA mutation related syndromic hearing loss: KearnsSayresyndrome.Besides point mutation, there is another form of mtDNA mutation that can induce syndromic hearing loss, the mtDNA rearrangement including large fragment deletion and duplication. There is a patient with Kearns-Sayre Syndrome...