Age-dependent 6kb deletion in human liver mitochondrial DNA.

Using PCR technique, restriction mapping and DNA sequencing, we analyzed liver mitochondrial DNA (mtDNA) of 2 stillborn babies and 62 Chinese subjects with non-liver disease from 27 to 86 years old. The results showed an age-dependent 6,063 bp deletion in the liver mtDNA of older subjects. We found a TAACAGAC sequence flanking the 5'-end breakpoint at 7,842 nucleotide position and an imperfect repeat sequence CAACATAC flanking the 3'-end breakpoint at 13,905 nucleotide position. The incidence of the deleted mtDNA was found to increase with age. The deleted mtDNA was not detected in the liver of the stillbirth or blood cells of all the subjects. This is the first account that an age-related 6,063 bp deletion occurs in the liver mtDNA of old humans. The occurrence of this and previously reported 4,977 bp deletions is consistent with our recent finding that liver mitochondrial respiratory functions decline with age and support the hypothesis that continuous accumulation of mtDNA mutations is an important contributor to ageing process in the human.     

Multiple mitochondrial DNA deletions in an elderly human individual.

We have used the polymerase chain reaction (PCR) to study deletions in the mitochondrial DNA (mtDNA) of an elderly human individual. An extended set of PCR primers has been utilised to identify 10 mitochondrial DNA deletions in a 69-year-old female subject with no known mitochondrial disease. The particular deletions visualised as PCR products depended on the primer pairs used, such that the more distantly separated PCR primers enabled visualisation of larger deletions. Some deletions were common to the heart, brain and skeletal muscle, whereas others were apparently specific to individual tissues. DNA sequencing analysis of PCR products showed that short direct repeat sequences (5 to 13 bp) flanked all deletion breakpoints; in most cases one copy of the repeat was deleted. It is proposed that the accumulation of such multiple deletions is a general phenomenon during the ageing process.     

Accumulation of mitochondrial DNA deletions in human oral tissues -- effects of betel quid chewing and oral cancer

Accumulation of mitochondrial DNA (mtDNA) mutations in human tissues has been associated with intrinsic aging and environmental insult. Recently, mtDNA mutations have been detected in various tumors, including head and neck tumors. However, the factors affecting the occurrence and accumulation of mtDNA deletions in tumor tissues are poorly understood. In Taiwan, betel quid chewing is a major risk factor for oral cancer. Using polymerase chain reaction (PCR) techniques, we examined large-scale deletions of mtDNA in 53 pairs of tumor and non-tumor oral tissues from the patients with or without betel quid chewing history. The results revealed that irrespective of the history of betel quid chewing, the incidences of the 4977bp deletion and other deletions of mtDNA were lower in the tumor portion as compared with the non-tumor portion. The average proportions of the 4977bp deleted mtDNA in the tumor tissues of the betel quid chewers and non-betel quid chewers were 13- and 5-fold, respectively, lower than those in the corresponding non-tumor tissues. Moreover, the average proportion of 4977bp deleted mtDNA was significantly higher (P<0.05) in the non-tumor oral tissues of the patients with betel quid chewing history than that of the patients without the history of betel quid chewing. These results suggest that betel quid chewing may increase mtDNA mutation in human oral tissues and that accumulation of mtDNA deletions and subsequent cytoplasmic segregation of these mutations during cell division could be an important contributor to the early phase of oral carcinogenesis.     

Ageing-associated 5 kb deletion in human liver mitochondrial DNA

Using PCR technique and restriction mapping, we analyzed liver mitochondrial DNA (mtDNA) of 2 stillborn babies and 55 Chinese subjects from 27 to 86 years old and blood cell mtDNA from 20 subjects of various ages. An ageing-associated 4,977-bp deletion was detected between nucleotide position 8,469 and 13,447 (or between 8,482 and 13,460) in the liver mtDNA of older subjects. In the region containing the junction fragment, we observed a 13 bp repeat "ACCTCCCTCACCA". Moreover, the incidence of the deleted mtDNA of each of the study subjects was found to increase with age. The deletion was found in 5 out of 8 patients of the 31-40 age group and 9 out of 11 patients of the 41-50 age group, and in all the patients over 50 years old. The deletion was not observed in either the mtDNA of the liver of the stillbirth or the blood cells of subjects of all the age groups. These results support our previous contention that liver mitochondrial respiratory functions decline with age and the hypothesis that continuous accumulation of mitochondrial DNA mutation is an important contributor to ageing process.     

Detection of mitochondrial DNA deletion by a modified PCR method in a 60Co radiation-exposed patient

A new PCR based method was developed to detect deleted mitochondrial DNA (mtDNA). Peripheral blood cell DNA was obtained from a victim who was accidently exposed to a 60Co radiation source in 1990. Using the DNA as template, first PCR was performed to generate multiple products including true deletions and artifacts. The full length product was recovered and used as template of secondary PCR. The suspicious deletion product of mtDNA could be confirmed only if it was yielded by first PCR. Using either original primers or their nested primers, the suspicious deletion product was amplified and authenticated as a true deletion product. The template was recovered and determined to be a deletion by sequencing directly. The results show that a new mtDNA deletion, which spans 889 bp from nt 11688 to nt 12576, was detected in the peripheral blood cells of the victim. It indicates that this new PCR-based method was more efficient at detecting small populations of mtDNA deletion than other routine methods. MtDNA deletion was found in the victim, suggesting the relationship between the deletion and phenotypes of the disease.     

Different copy numbers of apparently identically deleted mitochondrial DNA in tissues from a patient with Kearns-Sayre syndrome detected by PCR

An apparently identical deletion of 4.977 bp in length (position 8,483-13,459) was detectable in the mitochondrial DNA from skeletal muscle, heart muscle, kidney, and liver of a patient with Kearns-Sayre syndrome. The proportion of deleted genome varied from 60% for the skeletal muscle to 15% for heart muscle and kidney, and was below 5% in the liver. The mtDNA heteroplasmy of the liver was only detectable after amplification by PCR. In skeletal and heart muscle histochemical and immunocytochemical findings concerning cytochrome c oxidase were in good correlation with the proportion of deleted mitochondrial DNA.     

Mutations in mitochondrial DNA accumulate differentially in three different human tissues during ageing.

In 60 human tissue samples (encompassing skeletal muscle, heart and kidney) obtained from subjects aged from under 1 to 90 years, we used quantitative PCR procedures to quantify mitochondrial DNA (mtDNA) molecules carrying the 4977 bp deletion (mtDNA4977) and 3243 A-->G base substitution. In addition, the prevalence of multiple mtDNA deletions was assessed in a semi-quantitative manner. For all three tissues, the correlations between the accumulation of the particular mtDNA mutations and age of the subject are highly significant. However, differential extents of accumulation of the two specific mutations in the various tissues were observed. Thus, the mean abundance (percentage of mutant mtDNA out of total mtDNA) of mtDNA4977in a subset of age-matched adults is substantially higher in skeletal muscle than in heart and kidney. However, the mean abundance of the 3243 A-->G mutation in skeletal muscle was found to be lower than that in heart and kidney. Visualisation of arrays of PCR products arising from multiple mtDNA deletions in DNA extracted from adult skeletal muscle, was readily made after 30 cycles of PCR. By contrast, in DNA extracted from adult heart or kidney, amplification for 35 cycles of PCR was required to detect multiple mtDNA deletions. Although such multiple deletions are less abundant in heart and kidney than in skeletal muscle, in all tissue extracts there are unique patterns of bands, even from different tissues of the same subject. The differential accumulation of mtDNA4977, other mtDNA deletions and the 3243 A-->G mutation in the three tissues analysed presumably reflects different metabolic and senescence characteristics of these various tissues.     

Mitochondrial DNA alterations as ageing-associated molecular events.

Mitochondrial DNA (mtDNA) is a naked double-stranded circular extrachromosomal genetic element continuously exposed to the matrix that contains great amounts of reactive oxygen species and free radicals. The age-dependent decline in the capability and capacity of mitochondria to dispose these oxy-radicals will render mtDNA more vulnerable to mutations during the ageing process. During the past 3 years, more than 10 different types of deletions have been identified in the mtDNA of various tissues of old humans. Some of them were found only in a certain tissue but some others appeared in more than one organ or tissue. The 4977-bp deletion is the most prevalent and abundant one among these deletions. Skeletal muscle is the target tissue of most ageing-associated mtDNA deletions and has often been found to carry multiple deletions. The onset age of the various deletions in mtDNA varies greatly with individual and type of the deletion. The 4977-bp deletion has been independently demonstrated to occur in the mtDNA of various tissues of the human in the early third decade of life. However, the 7436-bp deletion was only detected in the heart mtDNA of human subjects in their late thirties. The others appeared only in older humans over 40 years old. No apparent sex difference was found in the onset age of these ageing-associated mtDNA deletions. The various ageing-associated deletions could be classified into two groups. Most of the deletions belong to the first group, in which the 5'- and 3'-end breakpoints of the deletion are flanked by 4-bp or longer direct repeats. The deletion in the second group occurs less frequently and shows no distinct repeat sequences flanking the deletion sites. These two groups of mtDNA deletions may occur by different mechanisms. The first group is most probably caused by internal recombination or slippage mispairing during replication of mtDNA by the D-loop mechanism. The deleted mtDNA and the deleted DNA fragment may be further degraded or escape from the mitochondria and get translocated into the nucleus. The latter route has been substantiated by many observations of inserted mtDNA sequences in the nuclear DNA. Thus, the fragments of migrating mtDNA may change the information content and expression level of certain nuclear genes and thereby promote the ageing process or cause cancer. Similar ageing-associated alterations of mtDNA have also been observed in aged animals and plants. I suggest that mtDNA deletions and other mutations to be discovered are molecular events generally associated with the ageing process.     

Large-scale mitochondrial DNA deletions in skeletal muscle of patients with end-stage renal disease

End-stage renal disease (ESRD) is associated with enhanced oxidative stress. This disease state provides a unique system for investigating the deleterious effect of exogenous sources of free radicals and reactive oxygen species (ROS) on mitochondrial DNA (mtDNA). To test the hypothesis that uremic milieu might cause more severe damage to mtDNA, we investigated the prevalence and abundance of mtDNA deletions in the skeletal muscles of ESRD patients. The results showed that the frequencies of occurrence of the 4977 bp and 7436 bp deletions of mtDNA in the muscle tissues of the older ESRD patients were higher than those of the younger patients. The frequency of occurrence of the 4977 bp-deleted mtDNA in the muscle was 33.3% for the patients in the age group of < 40 years, 66.6% in the 41-60-year-old group, 100% in the 61-80-year-old group, and 100% in patients >80 years of age, respectively. Only 22% of the normal aged controls carried the 4977 bp mtDNA deletion, whereas 77% (17/22) of the ESRD patients exhibited the mtDNA deletion. Using a semiquantitative PCR method, we determined the proportion of the 4977 bp-deleted mtDNA from the muscles that had been confirmed to harbor the deletion. We found that the proportions of the 4977 bp-deleted mtDNA in the muscle were significantly higher than those of the aged matched controls. Using long-range PCR techniques, a distinctive array of mtDNA deletions was demonstrated in the muscle of uremic patients. In summary, we found diverse and multiple mtDNA deletions in the skeletal muscles of ESRD patients. These deletions are more prevalent and abundant in ESRD patients than those found in normal populations. Accumulation of uremic toxins and impaired free radical scavenging systems may be responsible for the increased oxidative stress in ESRD patients. Such stress may result in oxidative damage and aging-associated mutation of the mitochondrial genome.     

Increase of deleted mitochondrial DNA in the striatum in Parkinson's disease and senescence

A mutant mitochondrial DNA (mtDNA) with a 4,977-bp deletion was detected in the parkinsonian brain by using the polymerase chain reaction. Although the deleted mtDNA was detectable even in the brain of aged controls, the proportion of deleted mtDNA to normal mtDNA in the striatum was higher in the parkinsonian patients than in the controls. In both the parkinsonian patients and the aged controls, the proportion was higher in the striatum than in the cerebral cortex. These results indicate that age-related accumulation of deleted mtDNA is accelerated in the parkinsonian striatum and suggest that the deletion contributes to pathophysiological processes underlying Parkinson's disease.     

Detection of mitochondrial DNA deletions by a screening procedure using the Polymerase Chain Reaction

We describe an accurate procedure for a rapid diagnosis of heteroplasmic mtDNA deletions based on the polymerase chain reaction (PCR). For a selective amplification of deleted mtDNA across the breakpoints of the deletion, we used seven combinations of primers surrounding the most common deleted region between the two origins of mtDNA replication. This procedure was performed on muscle biopsies of twenty patients harboring a single mtDNA deletion and one patient with multiple mtDNA deletions. The results were compared with Southern-blotting analysis. We conclude that this PCR procedure is a sensitive and convenient screening method for the detection of mtDNA deletions. (Mol Cell Biochem 174: 221–225, 1997)     

Molecular investigations of mitochondrial deletions: Evaluating the usefulness of different genetic tests

Deletions in mitochondrial DNA are a common cause of mitochondrial disorders. The molecular diagnosis of mtDNA deletions for years was based on Southern hybridization later replaced by PCR methods such as PCR with primers specific for a particular deletion (mainly the so-called common deletion of 4977bp) and long PCR. In order to evaluate the usefulness of MLPA (Multiplex Ligation-dependent Probe Amplification) in molecular diagnosis of large scale mtDNA deletions we compare four diagnostic methods: Southern hybridization, PCR, long-PCR and MLPA in a group of 16 patients with suspected deletions. Analysis was performed on blood, muscle and in one case hepatic tissue DNA. The MLPA was not able to confirm all the deletions detected by PCR methods, but due to its relative ease of processing, minimal equipment, low costs and the additional possibility to detect frequent point mtDNA mutations in one assay it is worth considering as a screening method. We recommend to always confirm MLPA results by PCR methods.     

Detection of mitochondrial DNA deletions in the cochlea and its structural elements from archival human temporal bone tissue

Large-scale deletions of mitochondrial DNA (mtDNA) have been associated with aging and disease in post-mitotic tissues. These post-mitotic tissues, including skeletal muscle, heart and brain, are heavily dependent on intact functional mitochondria. The cochlear tissues are known to contain an abundance of mitochondria. This observation stimulated a search for mtDNA deletions in the cochlea and its elements using a sensitive nested PCR methodology and long range PCR to explain the functional deficits observed in age-related hearing loss. The presence of the so-called “common” deletion (CD) was detected in cochlear tissue from two individuals with age-related hearing loss, 73 and 78 years of age. Three additional deletions, that to our knowledge have not been previously reported, were also identified in these two individuals, including a 5354 bp deletion flanked with a 3 bp repeat, a 9682 bp deletion flanked by a 10 bp repeat and a 5142 bp deletion without a flanking repeat. The 9682 and 5142 bp deletions were also detected in an individual 39 years of age with normal hearing, however, these two deletions were not detected in a normal hearing individual 9 years of age. In contrast, the 5354 bp deletion was detected in all four of the individuals studied. To localize the deletions within the cochlea, the cochlear elements were removed by laser capture microdissection (LCM) and the mtDNA from these tissues was studied. The 5142 and 5354 bp deletions were detected in the organ of corti, spiral ligament, and ganglion cells, but not in the stria vascularis. These findings correlate with the reduction in the number of spiral ganglion cells and outer hair cells, and the normal stria vascularis volume observed in this individual. All four of these deletions involve the cytochrome c oxidase (COX) subunit III gene, encoded by mtDNA. These observations suggest that multiple mtDNA deletions may contribute to a deficit in mitochondrial function in the cochlea and result in hearing loss if a level of physiological significance is reached.     

Direct sequencing of deleted mitochondrial DNA in myopathic patients

To investigate the mechanism of mitochondrial DNA deletion in human diseases, we amplified the deleted mitochondrial DNA of five patients with mitochondrial myopathy by using the polymerase chain reaction, and directly sequenced the crossover regions of the deleted mitochondrial DNA without cloning. In Patient 1, a 7-bp directly repeated sequence of 5'-ATCCCCA-3' was found at the boundaries of deleted segment spanning 7,039 bp between the ATPase 6 and the cytochrome b genes. In Patients 2, 3, and 4, a 13-bp sequence of 5'-ACCTCCCTCACCA-3' was found in the boundaries of deleted segment spanning 4,977 bp between the ATPase 8 and the ND5 genes. In Patient 5, a 3-bp sequence of 5'-CCT-3' was found in the boundaries of deleted segment spanning 3,717 bp between the ATPase 6 and the ND5 genes. Similar directly repeated sequences may contribute to mitochondrial DNA deletions in human degenerative diseases.     

Oxidative damage to mitochondrial DNA in atrial muscle of patients with atrial fibrillation

Atrial fibrillation (AF) is the most common cause of arrhythmia and is an aging-related disease encountered in clinical practice. The electrophysiological remolding with Ca(2+) overloading and cellular structure changes were found in cardiomyocytes of AF patients. In previous studies, increased oxidative stress and oxidative damage was found in cardiomyocytes during the ischemia/reperfusion injury. Besides, mitochondrial DNA (mtDNA) deletion and mtDNA proliferation occur frequently in affected tissues of patients with certain degenerative diseases and during aging of the human. However, it remains unclear whether high oxidative stress and alteration of mtDNA play a role in the pathophysiology of AF. In this study, we first screened for large-scale deletions of mtDNA in the atrial muscle of AF patients by long-range polymerase chain reaction (PCR). The results showed that large-scale deletions between nucleotide positions 7900 and 16500 of mtDNA occurred at a high frequency. Among them, the 4977 bp deletion was the most frequent and abundant one, and the mean proportion of mtDNA with the 4977 bp deletion in the atrial muscle of the patients with AF was 3.75-fold higher than that of the patients without AF (p <.005). Furthermore, quantitative PCR was performed to evaluate lesions in mtDNA caused by oxidative damage. We found that the degree of mtDNA damage in the patients with AF was greater than that of the patients without AF (3.29 vs.1.60 per 10 kb, p <.0005). The 8-OHdG, which is one of the most common products of oxidative damage to DNA, was also found at a higher frequency in mtDNA of patients with AF as compared with those without AF. In addition, the mtDNA content was found to increase significantly in the patients with AF (p =.0051). The level of mtDNA lesion and the mtDNA content was positively correlated (r = 0.44). These results suggest that oxidative injury and deletion of mtDNA in cardiac muscle are increased in the patients with AF, which may contribute to the impairment of bioenergetic function of mitochondria and induction of the oxidative vicious cycle involved in the pathogenesis of atrial myopathy in AF.     

Detection and quantification of mitochondrial DNA deletions in individual cells by real-time PCR

Defects of mitochondrial DNA (mtDNA) are an important cause of disease and play a role in the ageing process. There are multiple copies of the mitochondrial genome in a single cell. In many patients with acquired or inherited mtDNA mutations, there exists a mixture of mutated and wild type genomes (termed heteroplasmy) within individual cells. As a biochemical and clinical defect is only observed when there are high levels of mutated mtDNA, a crucial investigation is to determine the level of heteroplasmic mutations within tissues and individual cells. We have developed an assay to determine the relative amount of deleted mtDNA using real-time fluorescence PCR. This assay detects the vast majority of deleted molecules, thus eliminating the need to develop specific probes. We have demonstrated an excellent correlation with other techniques (Southern blotting and three- primer competitive PCR), and have shown this technique to be sensitive to quantify the level of deleted mtDNA molecules in individual cells. Finally, we have used this assay to investigate patients with mitochondrial disease and shown in individual skeletal muscle fibres that there exist different patterns of abnormalities between patients with single or multiple mtDNA deletions. We believe that this technique has significant advantages over other methods to quantify deleted mtDNA and, employed alongside our method to sequence the mitochondrial genome from single cells, will further our understanding of the role of mtDNA mutations in human disease and ageing.     

Detection and characterization of mitochondrial DNA rearrangements in Pearson and Kearns-Sayre syndromes by long PCR

We used a strategy based on long PCR (polymerase chain reaction) for detection and characterization of mitochondrial DNA (mtDNA) rearrangements in two patients with clinical signs suggesting Pearson syndrome and Kearns-Sayre syndrome (KSS), respectively, and one patient with myopathic symptoms of unidentified origin. Mitochondrial DNA rearrangements were detected by amplification of the complete mitochondrial genome (16.6 kb) using long PCR with primers located in essential regions of the mitochondrial genome and quantified by three-primer PCR. Long PCR with deletion-specific primers was used for identification and quantitative estimation of the different forms of rearranged molecules, such as deletions and duplications. We detected significant amounts of a common 7.4-kb deletion flanked by a 12-bp direct repeat in all tissues tested from the patient with Pearson syndrome. In skeletal muscle from the patient with clinical signs of KSS we found significant amounts of a novel 3.7-kb rearrangement flanked by a 4-bp inverted repeat that was present in the form of deletions as well as duplications. In the patient suffering from myopathic symptoms of unidentified origin we did not detect rearranged mtDNA in blood but found low levels of two rearranged mtDNA populations in skeletal muscle, a previously described 7-kb deletion flanked by a 7-bp direct repeat and a novel 6.6-kb deletion with no repeat. These two populations, however, were unlikely to be the cause of the myopathic symptoms as they were present at low levels (10–40 ppm). Using a strategy based on screening with long PCR we were able to detect and characterize high as well as low levels of mtDNA rearrangements in three patients. Received: 10 March 1997 / Accepted: 20 May 1997     

Rearranged mitochondrial genomes are present in human oocytes.

Using quantitative PCR, we have determined that a human oocyte contains approximately 100,000 mitochondrial genomes (mtDNAs). We have also found that some oocytes harbor measurable levels (up to 0.1%) of the so-called common deletion, an mtDNA molecule containing a 4,977-bp rearrangement that is present in high amounts in many patients with "sporadic" Kearns-Sayre syndrome (KSS) and progressive external ophthalmoplegia (PEO). This is the first demonstration that rearranged mtDNAs are present in human oocytes, and it provides experimental support for the supposition that pathogenic deletions associated with the ontogeny of sporadic KSS and PEO can be transmitted in the female germ line, from mother to child. The relevance of these finding to the accumulation of extremely low levels of deleted mtDNAs in both somatic and germ-line tissues during normal human aging is also discussed.     

Accumulation of mitochondrial DNA deletions in myotubes cultured from muscles of patients with mitochondrial myopathies

 Myoblast cultures were established from muscle biopsies of two patients harboring heteroplasmic mitochondrial (mt) DNA deletions. The accumulation kinetics of the deleted mtDNA was followed during myoblast to myotube differentiation. The percent- age of deleted mtDNA was determined by quantitative PCR in myoblasts, myotubes, and muscle biopsies. The deleted form accounted for 65% of the mtDNA present in a muscle biopsy from a patient harboring a 5.6-kb deletion. The percentage of deleted mtDNA was 1.2% in myoblasts and increased progressively after differentiation, up to 12% at 21 days after the commitment time. In a second patient harboring a 2.8-kb deletion, the percentage of deleted mtDNA increased much more slowly: from 0.07% in myoblasts to 0.21% after 22 days of differentiation, as compared with 45% in the muscle biopsy. Thus, a three- and ten-fold increase, respectively, in the fraction of deleted mtDNA occurred during the differentiation of myoblasts to myotubes from the two patients. The faster accumulation of deleted mtDNA in the first patient’s cells was linked to an earlier myoblast to myotube differentiation, suggesting that the level of deleted mtDNA is inversely related to the rate of cell proliferation. Received: 16 April 1996/Accepted: 29 July 1996     

Contribution of common deletion to total deletion burden in mitochondrial DNA from inner ear of d-galactose-induced aging rats

Mitochondrial DNA (mtDNA) mutations, especially deletions, have been suggested to play an important role in aging and degenerative diseases. In particular, the common deletion in humans and rats (4977bp and 4834bp deletion, respectively) has been shown to accumulate with age in post-mitotic tissues with high energetic demands. Among numerous deletions, the common deletion has been proposed to serve as a molecular marker for aging and play a critical role in presbyacusis. However, so far no previous publication has quantified the contribution of common deletion to the total burden of mtDNA deletions in tissues during aging process. In the present study, we established a rat model with various degrees of aging in inner ear induced by three different doses of d-galactose (d-gal) administration. Firstly, multiple mtDNA deletions in inner ear were detected by nested PCR and long range PCR. In addition to the common deletion, three novel mtDNA deletions were identified. All four deletions, located in the major arc of mtDNA, are flanked by direct repeats and involve the cytochrome c oxidase (COX) subunit III gene, encoded by mtDNA. Additionally, absolute quantitative real-time PCR assay was used to detect the level of common deletion and total deletion burden of mtDNA. The quantitative data show that the common deletion is the most frequent type of mtDNA deletions, exceeding 67.86% of the total deletion burden. Finally, increased mtDNA copy number, reduced COX activity and mosaic ultrastructural impairments in inner ear were identified in d-gal-induced aging rats. The increase of mtDNA replication may contribute to the accelerated accumulation of mtDNA deletions, which may result in impairment of mitochondrial function in inner ear. Taken together, these findings suggest that the common deletion may serve as an ideal molecular marker to assess the mtDNA damage in inner ear during aging.