The mitochondrial tRNA(Ala) T5628C variant may have a modifying role in the phenotypic manifestation of the 12S rRNA C1494T mutation in a large Chinese family with hearing loss

We report here the clinical, genetic, and molecular characterization of a large Han Chinese family with aminoglycoside-induced and nonsyndromic hearing loss. Two and 13 of 66 matrilineal relatives suffered from aminoglycoside-induced and nonsyndromic hearing loss, respectively. These matrilineal relatives exhibited a wide range of severity of hearing loss, varying from profound to normal hearing. In the absence of aminoglycosides, the age-at-onset of hearing impairment in these matrilineal relatives ranged from 13 to 50years. Furthermore, these affected matrilineal relatives shared some common features: bilateral hearing loss of high frequencies and symmetries. Sequence analysis of mitochondrial DNA (mtDNA) in the pedigree identified the homoplasmic 12S rRNA C1494T mutation and other 34 variants belonging to Eastern Asian haplogroup F1. Of these, the variant T5628C occurs at an extremely conserved nucleotide (A31) of tRNA(Ala). This variant converted a very conservative A-U to a G-U base-pairing at AC-stem of this tRNA. The disruption of this base-pairing in tRNAs by mtDNA mutations has been associated with several clinical abnormalities. The alteration of structure of the tRNA(Ala) by the T5628C mutation may lead to a failure in tRNA metabolism and lead to impairment of mitochondrial translation, thereby worsening mitochondrial dysfunctions, caused by the C1494T mutation. Therefore, this mtDNA mutation may influence the phenotypic manifestation of the 12S rRNA C1494T mutation in this Chinese pedigree.     

Maternally inherited aminoglycoside-induced and nonsyndromic hearing loss is associated with the 12S rRNA C1494T mutation in three Han Chinese pedigrees

We report here the clinical, genetic and molecular characterization of three Han Chinese pedigrees with maternally transmitted aminoglycoside-induced and nonsyndromic bilateral hearing loss. Clinical evaluation revealed the wide range of severity, age-at-onset and audiometric configuration of hearing impairment in matrilineal relatives in these families. The penetrances of hearing loss in these pedigrees were 28%, 20%, and 15%, with an average of 21%, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrances of hearing loss in these seven pedigrees were 21%, 13% and 8%, with an average of 14%. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the presence of the deafness-associated 12S rRNA C1494T mutation, in addition to distinct sets of mtDNA polymorphism belonging to Eastern Asian haplogroups F1a1, F1a1 and D5a2, respectively. This suggested that the C1494T mutation occurred sporadically and multiplied through evolution of the mtDNA. The absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in their mtDNA suggests that these mtDNA haplogroup-specific variants may not play an important role in the phenotypic expression of the C1494T mutation in those Chinese families. In addition, the lack of significant mutation in the GJB2 gene ruled out the possible involvement of GJB2 in the phenotypic expression of the C1494T mutation in those affected subjects. However, aminoglycosides and other nuclear modifier genes play a modifying role in the phenotypic manifestation of the C1494T mutation in these Chinese families.     

Coexistence of mitochondrial 12S rRNA C1494T and CO1/tRNA(Ser(UCN)) G7444A mutations in two Han Chinese pedigrees with aminoglycoside-induced and non-syndromic hearing loss

Mutations in mitochondrial DNA are one of the important causes of hearing loss. We report here the clinical, genetic, and molecular characterization of two Han Chinese pedigrees with maternally transmitted aminoglycoside-induced and nonsyndromic bilateral hearing loss. Clinical evaluation revealed the wide range of severity, age-at-onset, and audiometric configuration of hearing impairment in matrilineal relatives in these families. The penetrances of hearing loss in these pedigrees were 20% and 18%, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrances of hearing loss in these seven pedigrees were 10% and 15%. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the presence of the deafness-associated 12S rRNA C1494T and CO1/tRNA(Ser(UCN)) G7444A mutations. Their distinct sets of mtDNA polymorphism belonged to Eastern Asian haplogroup C4a1, while other previously identified six Chinese mitochondrial genomes harboring the C1494T mutation belong to haplogroups D5a2, D, R, and F1, respectively. This suggested that the C1494T or G7444A mutation occurred sporadically and multiplied through evolution of the mitochondrial DNA (mtDNA). The absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in their mtDNA suggest that these mtDNA haplogroup-specific variants may not play an important role in the phenotypic expression of the 12S rRNA C1494T and CO1/tRNA(Ser(UCN)) G7444A mutations in those Chinese families. However, aminoglycosides and other nuclear modifier genes play a modifying role in the phenotypic manifestation of the C1494T mutation in these Chinese families.     

Maternally inherited nonsyndromic hearing loss is associated with the T7511C mutation in the mitochondrial tRNASerUCN gene in a Japanese family

We report here the characterization of a Japanese family with maternally transmitted nonsyndromic hearing loss. Fourteen of 21 matrilineal relatives in this family exhibited early or late-onset/progressive but noncongenital hearing impairment with a wide range of severity, ranging from severe to normal hearing. The age-of-onset varies from 3 to 30 years. Sequence analysis of the complete mitochondrial genome in one matrilineal relative of this family revealed the presence of T7511C mutation and other variants. However, the levels of heteroplasmy of T7511C mutation did not correlate with the severity and age-of-onset of hearing loss in this family. Furthermore, none of other mtDNA variants are evolutionarily conserved and implicated to have significantly functional consequence. The absence of the ND1 T3308C and tRNA(Ala) T5655C mutations in this Japanese family but the presence of these mtDNA mutations in an African family with a high penetrance seems to account for different penetrance between two pedigrees. Incomplete penetrance in this family indicates the involvement of modulatory factors in the phenotypic expression of hearing impairment associated with the T7511C mutation. Here, two known variants G79A and G109A in the GJB2 gene were identified in the hearing-impaired and normal hearing matrilineal relatives of this Japanese family. However, the lack of correlation in the severity and age-of-onset in hearing impairment with homozygous or heterozygous G79A or G109A or combination of both variants in the GJB2 gene in those subjects with hearing impairment and normal hearing indicates that those variants of GJB2 gene may not be a modifier of the phenotypic effects of the T7511C mutation in those subjects. Thus, the phenotypic variability in this family is due to the involvement of other modifier factor(s).     

The ND4 G11696A mutation may influence the phenotypic manifestation of the deafness-associated 12S rRNA A1555G mutation in a four-generation Chinese family

We report here the clinical, genetic and molecular characterization of a large Han Chinese family with aminoglycoside-induced and nonsyndromic hearing loss. The penetrance of hearing loss (affected matrilineal relatives/total matrilineal relatives) in this pedigree was 53%, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrance of hearing loss in this pedigree was 42%. These matrilineal relatives exhibited a wide range of severity of hearing loss, varying from profound to normal hearing. Furthermore, these affected matrilineal relatives shared some common features: bilateral hearing loss of high frequencies and symmetries. Sequence analysis of mitochondrial DNA (mtDNA) in the pedigree identified the homoplasmic 12S rRNA A1555G mutation and other 35 variants belonging to Eastern Asian haplogroup D4. Of these, the V313I (G11696A) mutation in ND4 was associated with vision loss. However, the extremely low penetrance of visual loss, and the mild biochemical defect and the presence of one/167 Chinese controls indicted that the G11696A mutation is itself not sufficient to produce a clinical phenotype. Thus, the G11696A mutation may act in synergy with the primary deafness-associated 12S rRNA A1555G mutation in this Chinese family, thereby increasing the penetrance and expressivity of hearing loss in this Chinese pedigree.     

mtDNA mutation C1494T, haplogroup A, and hearing loss in Chinese

Mutation C1494T in mitochondrial 12S rRNA gene was recently reported in two large Chinese families with aminoglycoside-induced and nonsyndromic hearing loss (AINHL) and was claimed to be pathogenic. This mutation, however, was first reported in a sample from central China in our previous study that was aimed to reconstruct East Asian mtDNA phylogeny. All these three mtDNAs formed a subclade defined by mutation C1494T in mtDNA haplogroup A. It thus seems that mutation C1494T is a haplogroup A-associated mutation and this matrilineal background may contribute a high risk for the penetrance of mutation C1494T in Chinese with AINHL. To test this hypothesis, we first genotyped mutation C1494T in 553 unrelated individuals from three regional Chinese populations and performed an extensive search for published complete or near-complete mtDNA data sets (>3000 mtDNAs), we then screened the C1494T mutation in 111 mtDNAs with haplogroup A status that were identified from 1823 subjects across China. The search for published mtDNA data sets revealed no other mtDNA besides the above-mentioned three carrying mutation C1494T. None of the 553 randomly selected individuals and the 111 haplogroup A mtDNAs was found to bear this mutation. Therefore, our results suggest that C1494T is a very rare event. The mtDNA haplogroup A background in general is unlikely to play an active role in the penetrance of mutation C1494T in AINHL.     

Clinical evaluation and mitochondrial DNA sequence analysis in two Chinese families with aminoglycoside-induced and non-syndromic hearing loss

We report here the clinical, genetic, and molecular characterization of two Chinese pedigrees with aminoglycoside-induced and non-syndromic hearing impairment. Clinical evaluation revealed the variable phenotype of hearing impairment including audiometric configuration in these subjects. Penetrances of hearing loss in BJ105 and BJ106 pedigrees are 67% and 33%, respectively. In particular, three of 10 affected matrilineal relatives of BJ105 pedigree had aminoglycoside-induced hearing loss, while seven affected matrilineal relatives in BJ105 pedigree and six affected matrilineal relatives in BJ106 pedigree did not have a history of exposure to aminoglycosides. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the identical homoplasmic A1555G mutation and distinct sets of mtDNA variants belonging to haplogroups F3 and M7b. These variants showed no evolutionary conservation, implying that mitochondrial haplotype may not play a significant role in the phenotypic expression of the A1555G mutation in these Chinese pedigrees. However, aminoglycosides and nuclear backgrounds appear to be major modifier factors for the phenotypic manifestation of the A1555G mutation in these Chinese families.     

Extremely low penetrance of hearing loss in four Chinese families with the mitochondrial 12S rRNA A1555G mutation

Mutations in mitochondrial DNA (mtDNA) have been found to be associated with sensorineural hearing loss. We report here the clinical, genetic, and molecular characterization of four Chinese pedigrees with aminoglycoside-induced and nonsyndromic hearing impairment. Clinical evaluation revealed the variable phenotype of hearing impairment including audiometric configuration in these subjects, although these subjects share some common features: bilateral and sensorineural hearing impairment. Strikingly, these Chinese pedigrees exhibited extremely low penetrance of hearing loss (5.2%, 4.8%, 4.2%, and 13.3%, respectively, and with an average 8% penetrance). In particular, four of all five affected matrilineal relatives of these pedigrees had aminoglycoside-induced hearing loss. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the distinct sets of mtDNA polymorphism, in addition to the identical homoplasmic A1555G mutation, associated with hearing impairment in many families from different genetic backgrounds. The fact that mtDNA of those pedigrees belonged to different haplogroups R9a, N9a, D4a, and D4 suggested that the A1555G mutation occurred sporadically and multiplied through evolution of the mtDNA in China. However, there was the absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in these Chinese families. These data imply that the nuclear background or/and mitochondrial haplotype may not play a significant role in the phenotypic expression of the A1555G mutation in these Chinese pedigrees. However, aminoglycoside appears to be a major modifier factor for the phenotypic manifestation of the A1555G mutation in these Chinese families.     

线粒体tRNAThr G15927A突变可能影响耳聋相关的12S rRNA A1555G突变的表型表达

摘要: 对1个中国汉族耳聋家系进行了临床和分子遗传学特征分析。家系中听力下降的母系成员表现为程度不等、听力图形态不同的听力损害, 但同为双侧对称的感觉神经性耳聋。该家系耳聋外显率很高, 包括药物致聋的耳聋外显率为75%, 而非药物致聋的外显率为41.7%。对母系成员进行线粒体DNA(mtDNA)全序列扩增分析, 发现了耳聋相关12S rRNA A1555G同质性突变位点和多态性位点, 属于东亚人群B5b单体型。在这些变异位点中, mtDNA 15927位点的G-A碱基变化破坏tRNA^Thr反密码子结构上十分保守的C-G碱基对, 这可能加重由A1555G突变造成的线粒体功能缺陷。这表明tRNAThrG15927A突变可能增强携带12S rRNA A1555G的中国汉族耳聋家系的外显率和表现度。     

Mitochondrial haplotype and phenotype of 13 Chinese families may suggest multi-original evolution of mitochondrial C1494T mutation

Mutations in mitochondrial DNA (mtDNA) are associated with sensorineural hearing loss. In this study, we traced the origin of the 12S rRNA C1494T mutation through analysis of the clinical, genetic, and molecular characteristics of 13 Han Chinese pedigrees with aminoglycoside-induced and non-syndromic bilateral hearing loss that were selected by C1494T screening in 3133 subjects with non-syndromic hearing impairment from 27 regions of China (13/3133). Clinical evaluation revealed the variable phenotypes of hearing impairment including severity, age-of-onset, and audiometric configuration in these subjects. Through the whole mitochondrial genome DNA sequence analysis, we identified two evolutionarily conservative variants in protein-coding genes: tRNA^Ala T 5628C and tRNA^Tyr A5836G mutations. However, the pedigrees with these mutations did not have a higher or lower penetrance of deafness than in other pedigrees. These results suggested that both T 5628C and A5836G mutations might not significantly modify the manifestation of the C1494T mutation. Sequencing analysis of the whole mitochondrial genome of the probands showed that 13 pedigrees from seven different provinces were classified into 10 haplogroups by the distinct sets of mtDNA polymorphisms, including haplogroups A, B, D, D4, D4b2, F1, M, M7c, N9a1, and H2b. This result suggested that the C1494T mutation occurred sporadically with multi-origins through the evolution of the mtDNA in China, and these mtDNA haplogroup-specific variants may not play an important role in the phenotypic expression of the C1494T mutation in these Chinese families with different penetrance of hearing loss. In addition, the lack of a significant mutation in the GJB2 gene ruled out the possible involvement of GJB2 in the phenotypic expression of the C1494T mutation in those affected subjects. Therefore, the aminoglycosides is solo well-established factor to contribute to the deafness manifestation of the C1494T mutation, and prevention by avoiding the administration of aminoglycosides in individuals carrying C1494T mutation is the most effective way to protect their vulnerability to deafness.     

The coexistence of mitochondrial ND6 T14484C and 12S rRNA A1555G mutations in a Chinese family with Leber's hereditary optic neuropathy and hearing loss

We report here the clinical, genetic and molecular characterization of one three-generation Han Chinese family with Leber's hereditary optic neuropathy (LHON) and hearing loss. Four of 14 matrilineal relatives exhibited the moderate central vision loss at the average age of 12.5 years. Of these, one subject exhibited both LHON and mild hearing impairment. Sequence analysis of the complete mitochondrial genomes in the pedigree showed the presence of homoplasmic LHON-associated ND6 T14484C mutation, deafness-associated 12S rRNA A1555 mutation and 47 other variants belonging to Eastern Asian haplogroup H2. None of other mitochondrial variants was evolutionarily conserved and functional significance. Therefore, the coexistence of the A1555G mutation and T14484C mutations in this Chinese family indicate that the A1555G mutation may play a synergistic role in the phenotypic manifestation of LHON associated ND6 T14484C mutation. However, the incomplete penetrance of vision and hearing loss suggests the involvement of nuclear modifier genes and environmental factors in the phenotypic expression of these mtDNA mutations.     

Extremely low penetrance of deafness associated with the mitochondrial 12S rRNA mutation in 16 Chinese families: implication for early detection and prevention of deafness

Mutations in mitochondrial DNA (mtDNA) have been found to be associated with sensorineural hearing loss. We report here the clinical, genetic, and molecular characterization of 16 Chinese pedigrees (a total of 246 matrilineal relatives) with aminoglycoside-induced impairment. Clinical evaluation revealed the variable phenotype of hearing impairment including audiometric configuration in these subjects, although these subjects share some common features: being bilateral and sensorineural hearing impairment. Strikingly, these Chinese pedigrees exhibited extremely low penetrance of hearing loss, ranging from 4% to 18%, with an average of 8%. In particular, nineteen of 246 matrilineal relatives in these pedigrees had aminoglycoside-induced hearing loss. Mutational analysis of the mtDNA in these pedigrees showed the presence of homoplasmic 12S rRNA A1555G mutation, which has been associated with hearing impairment in many families worldwide. The extremely low penetrance of hearing loss in these Chinese families carrying the A1555G mutation strongly supports the notion that the A1555G mutation itself is not sufficient to produce the clinical phenotype. Children carrying the A1555G mutation are susceptible to the exposure of aminoglycosides, thereby inducing or worsening hearing impairment, as in the case of these Chinese families. Using those genetic and molecular approaches, we are able to diagnose whether children carry the ototoxic mtDNA mutation. Therefore, these data have been providing valuable information and technology to predict which individuals are at risk for ototoxicity, to improve the safety of aminoglycoside therapy, and eventually to decrease the incidence of deafness.     

Maternally inherited aminoglycoside-induced and nonsyndromic deafness is associated with the novel C1494T mutation in the mitochondrial 12S rRNA gene in a large Chinese family

We report here the characterization of a large Chinese family with maternally transmitted aminoglycoside-induced and nonsyndromic deafness. In the absence of aminoglycosides, some matrilineal relatives in this family exhibited late-onset/progressive deafness, with a wide range of severity and age at onset. Notably, the average age at onset of deafness has changed from 55 years (generation II) to 10 years (generation IV). Clinical data reveal that the administration of aminoglycosides can induce or worsen deafness in matrilineal relatives. The age at the time of drug administration appears to be correlated with the severity of hearing loss experienced by affected individuals. Sequence analysis of mitochondrial DNA in this pedigree identified a homoplasmic C-to-T transition at position 1494 (C1494T) in the 12S rRNA gene. The C1494T mutation is expected to form a novel U1494-1555A base pair, which is in the same position as the C1494-1555G pair created by the deafness-associated A1555G mutation, at the highly conserved A site of 12S rRNA. Exposure to a high concentration of paromomycin or neomycin caused a variable but significant average increase in doubling time in lymphoblastoid cell lines derived from four symptomatic and two asymptomatic individuals in this family carrying the C1494T mutation when compared to four control cell lines. Furthermore, a significant decrease in the rate of total oxygen consumption was observed in the mutant cell lines. Thus, our data strongly support the idea that the A site of mitochondrial 12S rRNA is the primary target for aminoglycoside-induced deafness. These results also strongly suggest that the nuclear background plays a role in the aminoglycoside ototoxicity and in the development of the deafness phenotype associated with the C1494T mutation in the mitochondrial 12S rRNA gene.     

Very low penetrance of hearing loss in seven Han Chinese pedigrees carrying the deafness-associated 12S rRNA A1555G mutation

Mutations in mitochondrial DNA (mtDNA) have been found to be associated with sensorineural hearing loss. We report here the clinical, genetic and molecular characterizations of seven Han Chinese pedigrees with aminoglycoside-induced and nonsyndromic bilateral hearing loss. Clinical evaluation revealed the variable phenotype of hearing impairment including severity, age-at-onset and audiometric configuration in these subjects. The penetrance of hearing loss in these pedigrees ranged from 3% to 29%, with an average of 13.6%, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrances of hearing loss in these seven pedigrees varied from 0% to 17%, with an average of 5.3%. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the presence of the deafness-associated 12S rRNA A1555G mutation, in addition to distinct sets of mtDNA polymorphism belonging to East Asian haplogroups B4, D4, D5 and F1, respectively. This suggested that the A1555G mutation occurred sporadically and multiplied through evolution of the mtDNA in China. Despite the presence of several evolutionary conservative variants in protein-encoding genes, there was the absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in these seven Chinese families. These suggest that these mtDNA haplogroup-specific variants may not play an important role in the phenotypic expression of the A1555G mutation in those Chinese families with very low penetrance of hearing loss. However, aminoglycosides appear to be a major modifier factor for the phenotypic manifestation of the A1555G mutation in these Chinese families.     

Mutational analysis of the mitochondrial 12S rRNA gene in Chinese pediatric subjects with aminoglycoside-induced and non-syndromic hearing loss

Mutations in mitochondrial DNA (mtDNA) have been found to be associated with sensorineural hearing loss. We report here a systematic mutational screening of the mitochondrial 12S rRNA gene in 128 Chinese pediatric subjects with sporadic aminoglycoside-induced and non-syndromic hearing loss. We show that aminoglycoside ototoxicity accounts for 48% of cases of hearing loss in this Chinese pediatric population. Of the known deafness-associated mutations in this gene, the incidence of the A1555G mutation is ~13% and ~2.9% in this Chinese pediatric population with aminoglycoside-induced and non-syndromic hearing loss, respectively. Furthermore, mutations at position 961 in the 12S rRNA gene account for ~1.7% and 4.4% of cases of aminoglycoside-induced and non-syndromic hearing loss in this Chinese clinical population, respectively. The T1095C mutation has been identified in one maternally inherited family with aminoglycoside-induced and non-syndromic hearing loss. However, the C1494T mutation was not detected in this clinical population. In addition, three variants, A827G, T1005C and A1116G, in the 12S rRNA gene, localized at highly conserved sites, may play a role in the pathogenesis of aminoglycoside ototoxicity. These data strongly suggest that the mitochondrial 12S rRNA is a hot-spot for deafness-associated mutations in the Chinese population.Z. Li and R. Li contributed equally to this work.     

Mitochondrial tRNAGlu 14693G variant may modulate the phenotypic manifestation of deafness-associated 12S rRNA A1555G mutation in a Hah Chinese family

Mutations in mitochondrial 12S rRNA gene are one of the most important causes of aminoglycoside-induced and nonsyndromic hearing loss. Here we report the characterization of one Han Chinese pedigree with aminoglycoside-induced and nonsyndromic hearing loss.This Chinese family carrying the 12S rRNA A1555G mutation exhibited high penetrance and expressivity of hearing impairment. In particular, penetrances of hearing loss in this family pedigree were 43.8% and 25%, respectively, when aminoglycoside-induced heating loss was included or excluded. Mutational analysis of entire mitochondrial genomes in this family showed the homoplasmic A1555G mutation and a set of variants belonging to haplogroup Y2. Of these, the A14693G variant occurred at the extremely conserved nucleotide (conventional position 54) of the TψC-loop of tRNAGlu and was absent in 156 Chinese controls. Nucleotides at position 54 of tRNAs are often modified, thereby contributing to the structural formation and stabilization of functional tRNAs. Thus, the structural alteration of tRNA by the A14693G variant may lead to a failure in tRNA metabolism and impair mitochondrial protein synthesis, thereby worsening mitochondrial dysfunctions altered by the A1555G mutation. Therefore, the tRNAalu A14693G variant may have a potential modifier role in increasing the penetrance and expressivity of the deafness-associated AI555G mutation in this Chinese pedigree.     

Only male matrilineal relatives with Leber's hereditary optic neuropathy in a large Chinese family carrying the mitochondrial DNA G11778A mutation

We report here the characterization of a five-generation large Chinese family with Leber's hereditary optic neuropathy (LHON). Very strikingly, six affected individuals of 38 matrilineal relatives (17 females/21 males) are exclusively males in this Chinese family. These matrilineal relatives in this family exhibited late-onset/progressive visual impairment with a wide range of severity, ranging from blindness to normal vision. The age of onset in visual impairment varies from 17 to 30 years. Sequence analysis of the complete mitochondrial genome in this pedigree revealed the presence of the G11778A mutation in ND4 gene and 29 other variants. This mitochondrial genome belongs to the Southern Chinese haplogroup B5b. We showed that the G11778A mutation is present at near homoplasmy in matrilineal relatives of this Chinese family but not in 164 Chinese controls. Incomplete penetrance of LHON in this family indicates the involvement of modulatory factors in the phenotypic expression of visual dysfunction associated with the G11778A mutation. However, none of other mtDNA variants are evolutionarily conserved and implicated to have significantly functional consequence. Thus, nuclear modifier gene(s) or environmental factor(s) seem to account for the penetrance and phenotypic variability of LHON in this Chinese family carrying the G11778A mutation.     

Mitochondrial tRNA^Glu A14693G variant may modulate the phenotypic manifestation of deafness-associated 12S rRNA A1555G mutation in a Han Chinese family

Mutations in mitochondrial 12S rRNA gene are one of the most important causes of aminoglycoside-induced and nonsyndromic hearing loss. Here we report the characterization of one Han Chinese pedigree with aminoglycoside-induced and nonsyndromic hearing loss. This Chinese family carrying the 12S rRNA A1555G mutation exhibited high penetrance and expressivity of heating impairment. In particular, penetrances of hearing loss in this family pedigree were 43.8% and 25%, respectively, when aminoglycoside-induced heating loss was included or excluded. Mutational analysis of entire mitochondrial genomes in this family showed the homoplasmic A1555G mutation and a set of variants belonging to haplogroup Y2. Of these, the A14693G variant occurred at the extremely conserved nucleotide （conventional position 54） of the TψC-loop of tRNA^Clu and was absent in 156 Chinese controls. Nucleotides at position 54 of tRNAs are often modified, thereby contributing to the structural formation and stabilization of functional tRNAs. Thus, the structural alteration of tRNA by the A14693G variant may lead to a failure in tRNA metabolism and impair mitochondrial protein synthesis, thereby worsening mitochondrial dysfunctions altered by the A1555G mutation. Therefore, the tRNA^Glu A14693G variant may have a potential modifier role in increasing the penetrance and expressivity of the deafness-associated A1555G mutation in this Chinese pedigree.     

Mitochondrial haplotypes may modulate the phenotypic manifestation of the deafness-associated 12S rRNA 1555A>G mutation

Mitochondrial 12S rRNA 1555A>G mutation is one of the important causes of aminoglycoside-induced and nonsyndromic deafness. Our previous investigations showed that the A1555G mutation was a primary factor underlying the development of deafness but was insufficient to produce deafness phenotype. However, it has been proposed that mitochondrial haplotypes modulate the phenotypic manifestation of the 1555A>G mutation. Here, we performed systematic and extended mutational screening of 12S rRNA gene in a cohort of 1742 hearing-impaired Han Chinese pediatric subjects from Zhejiang Province, China. Among these, 69 subjects with aminoglycoside-induced and nonsyndromic deafness harbored the homoplasmic 1555A>G mutation. These translated to a frequency of ～3.96% for the 1555A>G mutation in this hearing–impaired population. Clinical and genetic characterizations of 69 Chinese families carrying the 1555A>G mutation exhibited a wide range of penetrance and expressivity of hearing impairment. The average penetrances of deafness were 29.5% and 17.6%, respectively, when aminoglycoside-induced hearing loss was included or excluded. Furthermore, the average age-of-onset for deafness without aminoglycoside exposure ranged from 5 and 30 years old, with the average of 14.5 years. Their mitochondrial genomes exhibited distinct sets of polymorphisms belonging to ten Eastern Asian haplogroups A, B, C, D, F, G, M, N, R and Y, respectively. These indicated that the 1555A>G mutation occurred through recurrent origins and founder events. The haplogroup D accounted for 40.6% of the patient’s mtDNA samples but only 25.8% of the Chinese control mtDNA samples. Strikingly, these Chinese families carrying mitochondrial haplogroup B exhibited higher penetrance and expressivity of hearing loss. In addition, the mitochondrial haplogroup specific variants: 15927G>A of haplogroup B5b, 12338T>C of haplogroup F2, 7444G>A of haplogroup B4, 5802T>C, 10454T>C, 12224C>T and 11696G>A of D4 haplogroup, 5821G>A of haplogroup C, 14693A>G of haplogroups Y2 and F, and 15908T>C of Y2 may enhance the penetrace of hearing loss in these Chinese families. Moreover, the absence of mutation in nuclear modifier gene TRMU suggested that TRMU may not be a modifier for the phenotypic expression of the 1555A>G mutation in these Chinese families. These observations suggested that mitochondrial haplotypes modulate the variable penetrance and expressivity of deafness among these Chinese families.     

Mitochondrial 12S rRNA variants in 1642 Han Chinese pediatric subjects with aminoglycoside-induced and nonsyndromic hearing loss

In this report, we investigated the frequency and spectrum of mitochondrial 12S rRNA variants in a large cohort of 1642 Han Chinese pediatric subjects with aminoglycoside-induced and nonsyndromic hearing loss. Mutational analysis of 12S rRNA gene in these subjects identified 68 (54 known and 14 novel) variants. The frequencies of known 1555A>G and 1494C>T mutations were 3.96% and 0.18%, respectively, in this cohort with nonsyndromic and aminoglycoside-induced hearing loss. Prevalence of other putative deafness-associated mutation at positions 1095 and 961 were 0.61% and 1.7% in this cohort, respectively. Furthermore, the 745A>G, 792C>T, 801A>G, 839A>G, 856A>G, 1027A>G, 1192C>T, 1192C>A, 1310C>T, 1331A>G, 1374A>G and 1452T>C variants conferred increased sensitivity to ototoxic drugs or nonsyndromic deafness as they were absent in 449 Chinese controls and localized at highly conserved nucleotides of this rRNA. However, other variants appeared to be polymorphisms. Moreover, 65 Chinese subjects carrying the 1555A>G mutation exhibited bilateral and sensorineural hearing loss. A wide range of severity, age-of-onset and audiometric configuration was observed among these subjects. In particular, the sloping and flat-shaped patterns were the common audiograms in individuals carrying the 1555A>G mutation. The phenotypic variability in subjects carrying these 12S rRNA mutations indicated the involvement of nuclear modifier genes, mitochondrial haplotypes, epigenetic and environmental factors in the phenotypic manifestation of these mutations. Therefore, our data demonstrated that mitochondrial 12S rRNA is the hot spot for mutations associated with aminoglycoside ototoxicity.