Mitochondrial tRNAThr G15927A mutation may modulate the phenotypic manifestation of ototoxic 12S rRNA A1555G mutation in four Chinese families

Mutational Analysis of Mitochondrial tRNA Genes in 200 Patients with Type 2 Diabetes Mellitus

Objective

Previous studies showed that variants in mitochondrial DNA (mtDNA) are associated with type 2 diabetes mellitus (T2DM). However, the relationships between mitochondrial tRNA (mt-tRNA) variants and T2DM remain poorly understood.

Methods

In this study, we performed a mutational screening of 22 mt-tRNA genes in a cohort of 200 Han Chinese subjects with T2DM and 200 control subjects through PCR–Sanger sequencing. The identified mt-tRNA variants were assessed for their pathogenicity via the phylogenetic approach, structural and functional analysis. Furthermore, two Han Chinese pedigrees with maternally inherited diabetes and deafness (MIDD) were reported by clinical and genetic assessments.

Results

A total of 49 genetic variants in mt-tRNA genes were identified; among them, 31 variants (17 pathogenic/likely pathogenic) were absent in controls, located at extremely conserved nucleotides, may have potential structural and functional significance, thereby considered to be T2DM-associated variants. In addition, sequence analysis of entire mitochondrial genomes of the matrilineal relatives from two MIDD pedigrees revealed the occurrence of tRNA^Leu(UUR) A3243G and T3290C mutations, as well as sets of polymorphisms belonging to mitochondrial haplogroups F2 and D4. However, the lack of any functional variants in connexin 26 gene (GJB2) and tRNA 5-methylaminomethyl-2-thiouridylate (TRMU) suggested that nuclear genes may not play active roles in clinical expression of MIDD in these pedigrees.

Conclusion

Our data indicated that mt-tRNA variants were associated with T2DM, screening for mt-tRNA pathogenic mutations was recommended for early detection and prevention of mitochondrial diabetes.

Leber's Hereditary Optic Neuropathy: the roles of mitochondrial transfer RNA variants

Leber’s Hereditary Optic Neuropathy (LHON) was a common maternally inherited disease causing severe and permanent visual loss which mostly affects males. Three primary mitochondrial DNA (mtDNA) mutations, ND1 3460G>A, ND4 11778G>A and ND6 14484T>C, which affect genes encoding respiratory chain complex I subunit, are responsible for >90% of LHON cases worldwide. Families with maternally transmitted LHON show incomplete penetrance with a male preponderance for visual loss, suggesting the involvement of secondary mtDNA variants and other modifying factors. In particular, variants in mitochondrial tRNA (mt-tRNA) are important risk factors for LHON. These variants decreased the tRNA stability, prevent tRNA aminoacylation, influence the post-transcriptionalmodification and affect tRNA maturation. Failure of mt-tRNA metabolism subsequently impairs protein synthesis and expression, folding, and function of oxidative phosphorylation (OXPHOS) enzymes, which aggravates mitochondrial dysfunction that is involved in the progression and pathogenesis of LHON. This review summarizes the recent advances in our understanding of mt-tRNA biology and function, as well as the reported LHON-related mt-tRNA second variants; it also discusses the molecular mechanism behind the involvement of these variants in LHON.

The Mitochondrial tRNAHis G12192A Mutation May Modulate the Clinical Expression of Deafness-Associated tRNAThr G15927A Mutation in a Chinese Pedigree

BACKGROUND

Mutations in mitochondrial tRNA (mt-tRNA) genes have been found to be associated with both syndromic and non-syndromic hearing impairment. However, the pathophysiology underlying mt-tRNA mutations in clinical expression of hearing loss remains poorly understood.

OBJECTIVE

The aim of this study was to explore the potential association between mttRNA mutations and hearing loss.

METHODS AND RESULTS

We reported here the molecular features of a pedigree with maternally transmitted non-syndromic hearing loss. Among 12 matrilineal relatives, five of them suffered variable degree of hearing impairment, but none of them had any medical history of using aminoglycosides antibiotics (AmAn). Genetic screening of the complete mitochondrial genomes from the matrilineal relatives identified the coexistence of mt-tRNAHis G12192A and mt-tRNAThr G15927A mutations, together with a set of polymorphisms belonging to human mitochondrial haplogroup B5b1b. Interestingly, the G12192A mutation occurred 2-bp from the 3' end of the TψC loop of mt-tRNAHis, which was evolutionarily conserved from various species. In addition, the well-known G15927A mutation, which disrupted the highly conserved C-G base-pairing at the anticodon stem of mt-tRNAThr, may lead to the failure in mt-tRNA metabolism. Furthermore, a significant decreased in ATP production and an increased ROS generation were observed in polymononuclear leukocytes (PMNs) which were isolated from the deaf patients carrying these mt-tRNA mutations, suggested that the G12192A and G15927A mutations may cause mitochondrial dysfunction that was responsible for deafness. However, the absence of any functional mutations/variants in GJB2, GJB3, GJB6 and TRMU genes suggested that the nuclear genes may not play important roles in the clinical expression of non-syndromic hearing loss in this family.

CONCLUSION

Our data indicated that mt-tRNAHis G12192A mutation may increase the penetrance and expressivity of deafness-associated m-tRNAThr G15927A mutation in this family.

A coronary artery disease-associated tRNAThr mutation altered mitochondrial function, apoptosis and angiogenesis

The tissue specificity of mitochondrial tRNA mutations remains largely elusive. In this study, we demonstrated the deleterious effects of tRNAThr 15927G>A mutation that contributed to pathogenesis of coronary artery disease. The m.15927G>A mutation abolished the highly conserved base-pairing (28C-42G) of anticodon stem of tRNAThr. Using molecular dynamics simulations, we showed that the m.15927G>A mutation caused unstable tRNAThr structure, supported by decreased melting temperature and slower electrophoretic mobility of mutated tRNA. Using cybrids constructed by transferring mitochondria from a Chinese family carrying the m.15927G>A mutation and a control into mitochondrial DNA (mtDNA)-less human umbilical vein endothelial cells, we demonstrated that the m.15927G>A mutation caused significantly decreased efficiency in aminoacylation and steady-state levels of tRNAThr. The aberrant tRNAThr metabolism yielded variable decreases in mtDNA-encoded polypeptides, respiratory deficiency, diminished membrane potential and increased the production of reactive oxygen species. The m.15927G>A mutation promoted the apoptosis, evidenced by elevated release of cytochrome c into cytosol and increased levels of apoptosis-activated proteins: caspases 3, 7, 9 and PARP. Moreover, the lower wound healing cells and perturbed tube formation were observed in mutant cybrids, indicating altered angiogenesis. Our findings provide new insights into the pathophysiology of coronary artery disease, which is manifested by tRNAThr mutation-induced alterations.

Hereditary hearing loss; about the known and the unknown

Hereditary hearing loss is both clinically and genetically very heterogeneous. Despite the large number of genes that have been associated with the condition, many cases remain unexplained. Novel gene associations with hearing loss are to be expected but also are defects of regulatory regions of the genome which are currently not routinely addressed in molecular genetic testing and research. Inheritance patterns other than monogenic might be more common than assumed in isolated cases and diagnoses might have been missed because of misinterpretation of identified DNA variants. This review summarizes current insights in the genetics of hearing loss, the next steps that are being taken in research, and their challenges. Furthermore, genotype-phenotype correlations and modifying factors are discussed as these are instrumental in counselling hearing impaired individuals and/or their family members.

Contribution of the tRNAIle 4317A→G mutation to the phenotypic manifestation of the deafness-associated mitochondrial 12S rRNA 1555A→G mutation

The 1555A→G mutation in mitochondrial 12S rRNA has been associated with aminoglycoside-induced and non-syndromic deafness in many individuals worldwide. Mitochondrial genetic modifiers are proposed to influence the phenotypic expression of m.1555A→G mutation. Here, we report that a deafness-susceptibility allele (m.4317A→G) in the tRNA^Ile gene modulates the phenotype expression of m.1555A→G mutation. Strikingly, a large Han Chinese pedigree carrying both m.4317A→G and m.1555A→G mutations exhibited much higher penetrance of deafness than those carrying only the m.1555A→G mutation. The m.4317A→G mutation affected a highly conserved adenine at position 59 in the T-loop of tRNA^Ile We therefore hypothesized that the m.4317A→G mutation alters both structure and function of tRNA^Ile Using lymphoblastoid cell lines derived from members of Chinese families (three carrying both m.1555A→G and m.4317A→G mutations, three harboring only m.1555A→G mutation, and three controls lacking these mutations), we found that the cell lines bearing both m.4317A→G and m.1555A→G mutations exhibited more severe mitochondrial dysfunctions than those carrying only the m.1555A→G mutation. We also found that the m.4317A→G mutation perturbed the conformation, stability, and aminoacylation efficiency of tRNA^Ile These m.4317A→G mutation-induced alterations in tRNA^Ile structure and function aggravated the defective mitochondrial translation and respiratory phenotypes associated with the m.1555A→G mutation. Furthermore, mutant cell lines bearing both m.4317A→G and m.1555A→G mutations exhibited greater reductions in the mitochondrial ATP levels and membrane potentials and increasing production of reactive oxygen species than those carrying only the m.1555A→G mutation. Our findings provide new insights into the pathophysiology of maternally inherited deafness arising from the synergy between mitochondrial 12S rRNA and tRNA mutations.

A hypertension-associated mitochondrial DNA mutation introduces an m1G37 modification into tRNAMet, altering its structure and function

Defective nucleotide modifications of mitochondrial tRNAs have been associated with several human diseases, but their pathophysiology remains poorly understood. In this report, we investigated the pathogenic molecular mechanism underlying a hypertension-associated 4435A→G mutation in mitochondrial tRNA^Met The m.4435A→G mutation affected a highly conserved adenosine at position 37, 3' adjacent to the tRNA's anticodon, which is important for the fidelity of codon recognition and stabilization. We hypothesized that the m.4435A→G mutation introduced an m¹G37 modification of tRNA^Met, altering its structure and function. Primer extension and methylation activity assays indeed confirmed that the m.4435A→G mutation created a tRNA methyltransferase 5 (TRMT5)-catalyzed m¹G37 modification of tRNA^Met We found that this mutation altered the tRNA^Met structure, indicated by an increased melting temperature and electrophoretic mobility of the mutated tRNA compared with the wildtype molecule. We demonstrated that cybrid cell lines carrying the m.4435A→G mutation exhibited significantly decreased efficiency in aminoacylation and steady-state levels of tRNA^Met, as compared with those of control cybrids. The aberrant tRNA^Met metabolism resulted in variable decreases in mitochondrial DNA (mtDNA)-encoded polypeptides in the mutant cybrids. Furthermore, we found that the m.4435A→G mutation caused respiratory deficiency, markedly diminished mitochondrial ATP levels and membrane potential, and increased the production of reactive oxygen species in mutant cybrids. These results demonstrated that an aberrant m¹G37 modification of mitochondrial tRNA^Met affected the structure and function of its tRNA and consequently altered mitochondrial function. Our findings provide critical insights into the pathophysiology of maternally inherited hypertension, which is manifested by the deficient tRNA nucleotide modification.

Leber's hereditary optic neuropathy caused by a mutation in mitochondrial tRNAThr in eight Chinese pedigrees

PURPOSE

The purpose of this study was to investigate the pathophysiology underlying Leber's hereditary optic neuropathy (LHON)-associated mitochondrial tRNA mutation.

METHODS

Severn hundred ninety-seven Han Chinese subjects underwent clinical and genetic evaluation and analysis of mitochondrial DNA (mtDNA). The cybrid cell lines were constructed by transferring mitochondria from lymphoblastoid cell lines derived from a Chinese family into mtDNA-less (ρ^o) cells. These cell lines were assayed by tRNA Northern blot and Western blot analyses, respiratory enzymatic activities, the rate of ATP production and the generation of reactive oxygen species.

RESULTS

The tRNA^Thr 15927G>A mutation was identified in eight probands with suggestively maternal inheritance among 352 Han Chinese probands lacking these known LHON-associated mtDNA mutations. The m.15927G>A mutation affected a highly conserved guanine at position 42 at the anticodon-stem of tRNA^Thr, destabilizing the conservative base pairing (28C-42G). We therefore hypothesized that the m.15927G>A mutation, and altered the structure and function of tRNA^Thr. Northern blot analysis revealed 60% decrease in the steady-state level of tRNA^Thr in the mutant cell lines. Western blot analysis showed the variable reductions of 4 mtDNA encoding proteins, especially for marked decrease of ND1 and CYTB observed in mutant cell lines. Furthermore, we demonstrated that the m.15927G>A mutation decreased the activities of mitochondrial complexes I and III, markedly diminished mitochondrial ATP levels, and increased the production of reactive oxygen species in the mutant cells.

CONCLUSIONS

Our data demonstrated the first mitochondrial tRNA mutation leading to LHON. Our findings may provide new insights into the understanding of pathophysiology of LHON.

Genetic Epidemiology of Mitochondrial Pathogenic Variants Causing Nonsyndromic Hearing Loss in a Large Cohort of South Indian Hearing Impaired Individuals

Mitochondria play a critical role in the generation of metabolic energy in the form of ATP. Tissues and organs that are highly dependent on aerobic metabolism are involved in mitochondrial disorders including nonsyndromic hearing loss (NSHL). Seven pathogenic variants leading to NSHL have so far been reported on two mitochondrial genes: MT-RNR1 encoding 12SrRNA and MT-TS1 encoding tRNA for Ser((UCN)) . We screened 729 prelingual NSHL subjects to determine the prevalence of MT-RNR1 variants at position m.961, m.1555A>G and m.1494C>T, and MT-TS1 m.7445A>G, m.7472insC m.7510T>C and m.7511T>C variants. Mitochondrial pathogenic variants were found in eight probands (1.1%). Five of them were found to have the m.1555A>G variant, two others had m.7472insC and one proband had m.7444G>A. The extended relatives of these probands showed variable degrees of hearing loss and age at onset. This study shows that mitochondrial pathogenic alleles contribute to about 1% prelingual hearing loss. This study will henceforth provide the reference for the prevalence of mitochondrial pathogenic alleles in the South Indian population, which to date has not been estimated. The m.1555A>G variant is a primary predisposing genetic factor for the development of hearing loss. Our study strongly suggests that mitochondrial genotyping should be considered for all hearing impaired individuals and particularly in families where transmission is compatible with maternal inheritance, after ruling out the most common variants.

Mutational Analysis of Mitochondrial tRNA Genes in Patients with Asthma

BACKGROUND

Mitochondria are autonomous cellular organelles that oversee a variety of functions such as metabolism, energy production, calcium buffering, and cell fate determination. Most recently, mitochondrial dysfunction caused by mitochondrial mutations played important roles in the pathogenesis of asthma. However, the frequency of mitochondrial tRNA mutations in asthma is largely unknown.

METHODS

Overall, 200 patients with asthma and 100 healthy control subjects were recruited between Jan 2015 and Dec 2015 at the Guangming New District People's Hospital, Shenzhen, Guangdong Province, China. We first performed PCR amplification of the mitochondrial tRNA genes and subsequently sequenced the PCR products, and we used the pathogenicity scoring system to evaluate the potential role of these mutations.

RESULTS

Two patients carrying the tRNA^Thr G15927A mutation, three patients carrying the tRNA^Ala T5655C mutation and one patient carrying the tRNA^Glu A14693G mutation, these mutations were absent in healthy controls. Moreover, these mutations located at positions highly conserved between different species, and may cause a failure in mitochondrial tRNA metabolism, consequently result in mitochondrial dysfunction that responsible for asthma. In addition, the pathogenicity scoring system showed that these mutations should be regarded as "pathogenic".

CONCLUSION

Mitochondrial tRNA mutations caused the mitochondrial dysfunction may be involved in the pathogenesis of asthma. Thus, this study provided novel insight into the molecular mechanism underlying mitochondrial tRNA mutations in asthma. Moreover, screening for the mitochondrial tRNA mutations was advised for the diagnosis of patients with asthma.

Mutational analysis of mitochondrial tRNA genes in patients with lung cancer

Mutations in mitochondrial tRNA (mt-tRNA) genes have been found to be associated with various diseases including lung cancer. To understand the possible relationship between mtRNA mutations and lung cancer, we sequenced the 22 mt-tRNA genes from 200 lung cancer blood samples, as well as 100 healthy subjects. As a result, five mutations were identified including the tRNA^Ala T5655C, tRNA^Arg T10454C, tRNA^Leu(CUN) A12330G, tRNA^Ser(UCN) T7505C and tRNA^Thr G15927A. These mutations were absent in the healthy subjects. These mutations and polymorphisms were localized at the highly conserved nucleotides of the corresponding mitochondrial tRNAs, which are critical for the tRNA steady state level and may result in failure in the tRNA metabolism. Moreover, through the application of the pathogenicity scoring system, we found that only the T10454C mutation should be classified as a “neutral polymorphism,” while the other mutations were regarded as “definitely pathogenic.” Taken together, our data indicate that tRNA genes are the hot-spots for pathogenic mutations associated with lung cancer. Our findings may provide valuable information for pathophysiology, management and genetic counseling of lung cancer.

Mitochondrial DNA mutations associated with aminoglycoside induced ototoxicity

Aminoglycosides (AmAn) are widely used for their great efficiency against gram-negative bacterial infections. However, they can also induce ototoxic hearing loss, which has affected millions of people around the world. As previously reported, individuals bearing mitochondrial DNA mutations in the 12S rRNA gene, such as m.1555A>G and m.1494C>T, are more prone to AmAn-induced ototoxicity. These mutations cause human mitochondrial ribosomes to more closely resemble bacterial ribosomes and enable a stronger aminoglycoside interaction. Consequently, exposure to AmAn can induce or worsen hearing loss in these individuals. Furthermore, a wide range of severity and penetrance of hearing loss was observed among families carrying these mutations. Studies have revealed that these mitochondria mutations are the primary molecular mechanism of genetic susceptibility to AmAn ototoxicity, though nuclear modifier genes and mitochondrial haplotypes are known to modulate the phenotypic manifestation.

Identification of a mitochondrial 12S rRNA A1555G mutation causing aminoglycoside-induced deafness in a large Thai family

Background

Hearing loss is among the most frequent sensory disorders. Preventable causes include medications given to genetically susceptible individuals. Several families around the world with an A1555G mitochondrial mutation who became profoundly deaf after receiving aminoglycosides have been described. However, none has been reported in Thailand.

Objectives

To identify the cause of hearing loss of a large Thai family with 11 members who reportedly turned deaf after receiving antibiotics.

Methods

We obtained blood samples from 5 members; 4 of whom had hearing loss. Mutation analyses were performed using molecular techniques including polymerase chain reaction, Sanger sequencing, and restriction fragment length polymorphism.

Results

All 4 affected members were found to harbor the same A1555G mitochondrial mutation, while the unaffected had only the wild-type A.

Conclusions

We have identified the mitochondrial mutation leading to aminoglycoside-induced hearing loss in a Thai population. Raising awareness for medical practitioners of this genetic susceptibility in Thailand is warranted. Avoidance of certain medications in these individuals would prevent this acquired permanent hearing loss.

A Deafness- and Diabetes-associated tRNA Mutation Causes Deficient Pseudouridinylation at Position 55 in tRNAGlu and Mitochondrial Dysfunction

Several mitochondrial tRNA mutations have been associated with maternally inherited diabetes and deafness. However, the pathophysiology of these tRNA mutations remains poorly understood. In this report, we identified the novel homoplasmic 14692A→G mutation in the mitochondrial tRNA^Glu gene among three Han Chinese families with maternally inherited diabetes and deafness. The m.14692A→G mutation affected a highly conserved uridine at position 55 of the TΨC loop of tRNA^Glu The uridine is modified to pseudouridine (Ψ55), which plays an important role in the structure and function of this tRNA. Using lymphoblastoid cell lines derived from a Chinese family, we demonstrated that the m.14692A→G mutation caused loss of Ψ55 modification and increased angiogenin-mediated endonucleolytic cleavage in mutant tRNA^Glu The destabilization of base-pairing (18A-Ψ55) caused by the m.14692A→G mutation perturbed the conformation and stability of tRNA^Glu An approximately 65% decrease in the steady-state level of tRNA^Glu was observed in mutant cells compared with control cells. A failure in tRNA^Glu metabolism impaired mitochondrial translation, especially for polypeptides with a high proportion of glutamic acid codons such as ND1, ND6, and CO2 in mutant cells. An impairment of mitochondrial translation caused defective respiratory capacity, especially reducing the activities of complexes I and IV. Furthermore, marked decreases in the levels of mitochondrial ATP and membrane potential were observed in mutant cells. These mitochondrial dysfunctions caused an increasing production of reactive oxygen species in the mutant cells. Our findings may provide new insights into the pathophysiology of maternally inherited diabetes and deafness, which is primarily manifested by the deficient nucleotide modification of mitochondrial tRNA^Glu.

Genetic and clinical analysis of nonsyndromic hearing impairment in pediatric and adult cases

Previous studies have linked GJB2 gene and mitochondrial DNA (mtDNA) mutations to nonsyndromic hearing impairment (NSHI), but no study in China has yet investigated these mutations across all age groups. To fill the gap, this study ascertained 263 patients with NSHI between ages 2 months and 60 years and analyzed the presence of GJB2 gene and mtDNA A1555G/C1494T mutations by polymerase chain reaction (PCR) and DNA sequencing. A total of 20 types of mutations were detected for the GJB2 gene. The GJB2 gene and mtDNA A1555G/C1494T mutations were detected in 18.63 and 11.41% cases, respectively. At the first hospital visit, GJB2 gene mutations were detected in 5.97% of adult patients (>18 years) and 22.96% pediatric patients (<18 years) (χ² =9.506, p = 0.002), and mtDNA A1555G/C1494T mutations were detected in 31.34% of adult patients and 4.59% of pediatric patients (χ² = 35.359, p <0.001). When patients were classified by age at onset of deafness, significantly more (20.68%) pediatric patients had GJB2 gene mutations than did adult patients (0.0%) (χ² = 4.685; p = 0.006). Mitochondrial DNA A1555G/C1494T mutations were detected in 15.38% of adult-onset and 8.86% pediatric-onset patients, respectively. Interestingly, most GJB2 gene mutation carriers experienced NSHI onset within the first year of life (65.31%), while mtDNA A1555G/C1494T mutation carriers experienced onset at any age. Therefore, GJB2 gene mutations appear to contribute to congenital deafness, while mtDNAA1555G/C1494T mutations contribute mainly to acquired deafness in Chinese individuals. Both newborn hearing screening and genetic testing are important to diagnose and treat deafness.

A Hypertension-Associated tRNAAla Mutation Alters tRNA Metabolism and Mitochondrial Function

In this report, we investigated the pathophysiology of a novel hypertension-associated mitochondrial tRNA^Ala 5655A → G (m.5655A → G) mutation. The destabilization of a highly conserved base pairing (A1-U72) at the aminoacyl acceptor stem by an m.5655A → G mutation altered the tRNA^Ala function. An in vitro processing analysis showed that the m.5655A → G mutation reduced the efficiency of tRNA^Ala precursor 5′ end cleavage catalyzed by RNase P. By using cybrids constructed by transferring mitochondria from lymphoblastoid cell lines derived from a Chinese family into mitochondrial DNA (mtDNA)-less (ρ^o) cells, we showed a 41% reduction in the steady-state level of tRNA^Ala in mutant cybrids. The mutation caused an improperly aminoacylated tRNA^Ala, as suggested by aberrantly aminoacylated tRNA^Ala and slower electrophoretic mobility of mutated tRNA. A failure in tRNA^Ala metabolism contributed to variable reductions in six mtDNA-encoded polypeptides in mutant cells, ranging from 21% to 37.5%, with an average of a 29.1% reduction, compared to levels of the controls. The impaired translation caused reduced activities of mitochondrial respiration chains. Furthermore, marked decreases in the levels of mitochondrial ATP and membrane potential were observed in mutant cells. These caused increases in the production of reactive oxygen species in the mutant cybrids. The data provide evidence for the association of the tRNA^Ala 5655A → G mutation with hypertension.

Analysis of mitochondrial alterations in Brazilian patients with sensorineural hearing loss using MALDI-TOF mass spectrometry

BACKGROUND

Mutations in the mitochondrial DNA (mtDNA) have been associated with aminoglycoside-induced and nonsyndromic deafness in different populations. In the present study, we investigated the contribution of mutations in mitochondrial genes to the etiology of hearing loss in a Brazilian sample.

METHODS

Using mass spectrometry genotyping technology, combined with direct sequencing, 50 alterations previously described in 14 mitochondrial genes were screened in 152 patients with sensorineural hearing loss and in104 normal hearing controls.

RESULTS

Fifteen known mitochondrial alterations were detected (G709A, A735G, A827G, G988A, A1555G, T4363C, T5628C, T5655C, G5821A, C7462T, G8363A, T10454C, G12236A, T1291C, G15927A). Pathogenic mutations in MT-RNR1 and MT-TK genes were detected in 3 % (5/152) of the patients with hearing loss.

CONCLUSIONS

This study contributed to show the spectrum of mitochondrial variants in Brazilian patients with hearing loss. Frequency of A1555G was relatively high (2.6 %), indicating that this mutation is an important cause of hearing loss in our population. This work reports for the first time the investigation and the detection of the tRNA(Lys) G8363A mutation in Brazilian patients with maternally inherited sensorineural hearing loss.

A meta-analysis and systematic review of the prevalence of mitochondrially encoded 12S RNA in the general population: Is there a role for screening neonates requiring aminoglycosides?

BACKGROUND

This was a meta-analysis and systematic review to determine the global prevalence of the mitochondrially encoded 12S RNA (MT-RNR1) genetic mutation in order to assess the need for neonatal screening prior to aminoglycoside therapy.

MATERIALS AND METHODS

A comprehensive search of MEDLINE, EMBASE, Ovid, Database of Abstracts of Reviews of Effect, Cochrane Library, Clinical Evidence and Cochrane Central Register of Trials was performed including cross-referencing independently by 2 assessors. Selections were restricted to human studies in English. Meta-analysis was done with MetaXL 2013.

RESULTS

Forty-five papers out of 295 met the criteria. Pooled prevalence in the general population for MT-RNR1 gene mutations (A1555G, C1494T, A7445G) was 2% (1-4%) at 99%.

CONCLUSION

Routine screening for MT-RNR1 mutations in the general population prior to treatment with aminoglycosides appear desirable but poorly supported by the weak level of evidence available in the literature. Routine screening in high-risk (Chinese and Spanish) populations appear justified.

Complete mitochondrial genome analysis and clinical documentation of a five-generational Indian family with mitochondrial 1555A>G mutation and postlingual hearing loss

Hearing loss is the most common sensory disorder and is genetically heterogeneous. Apart from nuclear gene mutations, a number of inherited mitochondrial mutations have also been implicated. The m.1555A>G mutation in the mitochondrial MT-RNR1 gene is reported as the most common mutation causing nonsyndromic hearing loss in various ethnic populations. We report here for the first time the clinical, genetic and molecular characterisation of a single large five-generational Tamil-speaking South Indian family with maternally inherited nonsyndromic postlingual hearing loss. Molecular analysis led to identification of m.1555A>G in 28 maternal relatives with variable degree of phenotypic expression. The penetrance of hearing loss among the maternal relatives in this family was 55%. Sequence analysis of the complete mitochondrial genome in 36 members of this pedigree identified 25 known variants and one novel variant co-transmitted along with m.1555A>G mutation. The mtDNA haplotype analysis revealed that the maternal relatives carry the R*T2 haplotype similar to Europeans and South Asians. Sequencing of the coding exon of GJB2 nuclear gene did not show any pathogenic mutations. The results suggest that other nuclear or environmental modifying factors could have played a role in the differential expression of mutation m.1555A>G in postlingual hearing loss in this family.

Mitochondrial tRNA variants in Chinese subjects with coronary heart disease

BACKGROUND

Coronary heart disease is the leading cause of death worldwide. Mitochondrial genetic determinants for the development of this disorder remain less explored.

METHODS AND RESULTS

We performed a clinical and genetic evaluation and mutational screening of 22 mitochondrial tRNA genes in a cohort of 80 genetically unrelated Han Chinese subjects and 125 members of 4 families with coronary heart disease and 512 Chinese control subjects. This analysis identified 16 nucleotide changes among 9 tRNA genes. Of these, the T5592C mutation creates a highly conservative base pairing (5G-68C) on the acceptor stem of tRNA(Gln), whereas the G15927A mutation destabilizes a highly conserved base pairing (28C-42G) in the anticodon stem of tRNA(Thr). However, the other tRNA variants were polymorphisms. The pedigrees of BJH24 carrying the T5592C mutation, BJH15, and BJH45 harboring the G15927A mutation exhibited maternal transmission of coronary heart disease. Sequence analysis of their mitochondrial genomes revealed the presence of T5592C or G15927A mutation but the absence of other functionally significant mutations in all matrilineal relatives of these families.

CONCLUSIONS

Our previous observations showed that altered structures of tRNAs by these mtDNA mutations caused mitochondrial dysfunction. These may be the first evidence that mtDNA mutations increase the risk of coronary heart disease. Our findings may provide new insights into the pathophysiology of this disorder.

Functional recurrent mutations in the human mitochondrial phylogeny: dual roles in evolution and disease

Mutations frequently reoccur in the human mitochondrial DNA (mtDNA). However, it is unclear whether recurrent mtDNA nodal mutations (RNMs), that is, recurrent mutations in stems of unrelated phylogenetic nodes, are functional and hence selectively constrained. To answer this question, we performed comprehensive parsimony and maximum likelihood analyses of 9,868 publicly available whole human mtDNAs revealing 1,606 single nodal mutations (SNMs) and 679 RNMs. We then evaluated the potential functionality of synonymous, nonsynonymous and RNA SNMs and RNMs. For synonymous mutations, we have implemented the Codon Adaptation Index. For nonsynonymous mutations, we assessed evolutionary conservation, and employed previously described pathogenicity score assessment tools. For RNA genes’ mutations, we designed a bioinformatic tool which compiled evolutionary conservation and potential effect on RNA structure. While comparing the functionality scores of nonsynonymous and RNA SNMs and RNMs with those of disease-causing mtDNA mutations, we found significant difference (P < 0.001). However, 24 RNMs and 67 SNMs had comparable values with disease-causing mutations reflecting their potential function thus being the best candidates to participate in adaptive events of unrelated lineages. Strikingly, some functional RNMs occurred in unrelated mtDNA lineages that independently altered susceptibility to the same diseases, thus suggesting common functionality. To our knowledge, this is the most comprehensive analysis of selective signatures in the mtDNA not only within proteins but also within RNA genes. For the first time, we discover virtually all positively selected RNMs in our phylogeny while emphasizing their dual role in past evolutionary events and in disease today.

Coronary heart disease is associated with a mutation in mitochondrial tRNA

Coronary heart disease (CHD) is the leading cause of death worldwide. Mitochondrial genetic determinant for the development of CHD remains poorly explored. We report there the clinical, genetic, molecular and biochemical characterization of a four-generation Chinese family with maternally inherited CHD. Thirteen of 32 adult members in this family exhibited variable severity and age-at-onset of CHD. Mutational analysis of their mitochondrial genomes identified the tRNA(Thr) 15927G>A mutation belonging to the Eastern Asian haplogroup B5. The anticipated destabilization of a highly conserved base-pairing (28C-42G) by the 15927G>A mutation affects structure and function of tRNA(Thr). Northern analysis revealed ≈80% decrease in the steady-state level of tRNA(Thr) in the mutant cell lines carrying the 15927G>A mutation. The 15927G>A mutation changed the conformation of tRNA(Thr), as suggested by slower electrophoretic mobility of mutated tRNA with respect to the wild-type molecule. In addition, ∼39% reduction in aminoacylated efficiency of tRNA(Thr) was observed in mutant cells derived from this Chinese family. An in vivo mitochondrial protein labeling analysis showed ∼53% reduction in the rate of mitochondrial translation in mutant cells. The impaired mitochondrial protein synthesis leads to defects in overall respiratory capacity or malate/glutamate-promoted respiration or succinate/glycerol-3-phosphate-promoted respiration, or N,N,N',N'-tetramethyl-pphenylenediamine/ascorbate-promoted respiration in mutant cells. An increasing production of reactive oxygen species was observed in the mutant cells carrying the 15927G>A mutation. These results provide the direct evidence that the tRNA(Thr) 15927G>A mutation is associated with CHD. Our findings may provide new insights into pathophysiology and intervention targets of this disorder.

The role of mitochondrial DNA mutations in hearing loss

Mutations in mitochondrial DNA (mtDNA) are one of the most important causes of hearing loss. Of these, the homoplasmic A1555G and C1494T mutations at the highly conserved decoding site of the 12S rRNA gene are well documented as being associated with either aminoglycoside-induced or nonsyndromic hearing loss in many families worldwide. Moreover, five mutations associated with nonsyndromic hearing loss have been identified in the tRNA(Ser(UCN)) gene: A7445G, 7472insC, T7505C, T7510C, and T7511C. Other mtDNA mutations associated with deafness are mainly located in tRNA and protein-coding genes. Failures in mitochondrial tRNA metabolism or protein synthesis were observed from cybrid cells harboring these primary mutations, thereby causing the mitochondrial dysfunctions responsible for deafness. This review article provides a detailed summary of mtDNA mutations that have been reported in deafness and further discusses the molecular mechanisms of these mtDNA mutations in deafness expression.

Mitochondrial COX2 G7598A mutation may have a modifying role in the phenotypic manifestation of aminoglycoside antibiotic-induced deafness associated with 12S rRNA A1555G mutation in a Han Chinese pedigree

Recent studies suggest that certain mitochondrial haplogroup markers and some specific variants in mitochondrial haplogroup may also influence the phenotypic expression of particular mitochondrial disorders. In this report, the clinical, genetic, and molecular characterization were identified in a Chinese pedigree with the aminoglycoside antibiotic (AmAn)-induced deafness and nonsyndromic hearing loss (NSHL). The pathogenic gene responsible for this hereditary NSHL pedigree was determined by Microarray chip, which possessed the nine NSHL hot-spot mutations, including GJB2 (35delG, 176dell6bp, 235de1C, and 299delAT), GJB3 (538C>T), SLC26A4 (IVS7-2A>G and 2168A>G), and mitochondrial DNA (mtDNA) 12S rRNA (C1494T and A1555G). Only the homoplasmic A1555G mutation was detected, which was confirmed by direct sequencing. Also, real-time amplification refractory mutation system quantitative polymerase chain reaction methodology was performed to calculate the A1555G mutation load. The proband's complete mtDNA genome were amplified and direct sequencing was performed to determine the mitochondrial haplogroup and private mutations. The proband's mitochondrial haplogroup belonges to M7b1 and a private mutation MTCOX2 G7598A (p.Ala 5 Thr) is found. Phylogenetic analysis of COX2 polypeptide sequences demonstrates that the alanine residue is relatively conserved, but owing to the missense mutation (p.Ala 5 Thr), its side chain hydrophobicity will be changed, and what is more, as it is adjacent to a glutamine residue, which is highly conserved and hydrophilic, in an evolutionary stable domain; G7598A (p.Ala 5 Thr) may alter the protein secondary structure and physiological function of COX2 and, thus, aggravate the mitochondrial dysfunction conferred by the A1555G mutation. Furthermore, the G7598A mutation is absent in 100 unrelated healthy controls; therefore, G7598A (p.Ala 5 Thr) in the mitochondrial haplogoup M7b1 may have a modifying role, enhancing its penetrance and severity, in the AmAn-induced deafness and NSHL associated with 12S rRNA A1555G mutation in the Han Chinese pedigree.

Screening of mitochondrial mutations in Tunisian patients with mitochondrial disorders: an overview study

To investigate the spectrum of common mitochondrial mutations in Tunisia during the years of 2002-2012, 226 patients with mitochondrial disorders were clinically diagnosed with hearing loss, Leigh syndrome (LS), diabetes, cardiomyopathy, Kearns-Sayre syndrome (KSS), Pearson syndrome (PS), myopathy, mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS) and Wolfram syndrome. Restriction fragment length polymorphism (PCR-RFLP), radioactive PCR, single specific primer-PCR (SSP-PCR) analysis and PCR-sequencing methods were used to identify the mutations. Two cases with m.1555A>G mutation and two families with the novel 12S rRNA m.735A>G transition were detected in patients with hearing loss. Three cases with m.8993T>G mutation, two patients with the novel m.5523T>G and m.5559A>G mutations in the tRNA(Trp) gene, and two individuals with the undescribed m.9478T>C mutation in the cytochrome c oxidase subunit III (COXIII) gene were found with LS. In addition, one case with hypertrophic cardiomyopathy and deafness presented the ND1 m.3395A>G mutation and the tRNA(Ile) m.4316A>G variation. Besides, multiple mitochondrial deletions were detected in patients with KSS, PS, and Wolfram syndrome. The m.14709T>C mutation in the tRNA(Glu) was reported in four maternally inherited diabetes and deafness patients and a novel tRNA(Val) m.1640A>G mutation was detected in a MELAS patient.

Molecular and clinical characterization of the variable phenotype in Korean families with hearing loss associated with the mitochondrial A1555G mutation

Hearing loss, which is genetically heterogeneous, can be caused by mutations in the mitochondrial DNA (mtDNA). The A1555G mutation of the 12S ribosomal RNA (rRNA) gene in the mtDNA has been associated with both aminoglycoside-induced and non-syndromic hearing loss in many ethnic populations. Here, we report for the first time the clinical and genetic characterization of nine Korean pedigrees with aminoglycoside-induced and non-syndromic hearing loss. These Korean families carry in the A1555G mutation of 12S rRNA gene and exhibit variable penetrance and expressivity of hearing loss. Specifically, the penetrance of hearing loss in these families ranged between 28.6% and 75%, with an average of 60.8%. These results were higher than the 29.8% penetrance that was previously reported in a Chinese population but similar to the 65.4% and 54.1% penetrance observed in a large Arab-Israeli population and nineteen Spanish pedigrees, respectively. The mutational analysis of the complete mtDNA genome in these families showed that the haplogroups of the Korean population, which belongs to the eastern Asian population, were similar to those of the Chinese population but different from the Spanish population, which belongs to the European-Caucasian population. The mtDNA variants that may act as modifier factors were also found to be similar to the Chinese population. Although the mtDNA haplogroups and variants were similar to the eastern Asian population, we did find some differing phenotypes, although some subjects had the same variants. This result suggests that both the ethnic background and environmental factors lead to a variable phenotype of the A1555G mutation.

Novel mutations affecting the secondary structure of MT-RNR1 gene: a causal relationship with profound nonsyndromic hearing impairment

Mutations in mitochondrial DNA (mtDNA) are one of the most important causes of sensorineural hearing loss, especially in the MT-RNR1 gene. In the present study we have performed mutational screening for m.1555A>G and a region of the MT-RNR1 gene in 303 unrelated patients (including family members of 25 probands) with nonsyndromic hearing loss and 200 controls. Three homoplasmic variants, namely, m.1453A>G, 1462G>A, and 1508C>T, were identified in addition to the known deafness-associated m.1555A>G mutation in the MT-RNR1 gene. All the variants were detected only in the patients and not in the controls. m.1555A>G was detected in three probands amounting to 1.0%. Prediction of RNA secondary structure showed changes in all the three variants, the most severe being in m.1453A>G that was inherited in a typical maternal pattern in two families. Screening of GJB2 and GJB6 genes in all these probands revealed cosegregation of the p.W24X mutation (GJB2) in one family with m.1453A>G. Only the proband carrying the p.W24X mutation in a homozygous state expressed the condition while heterozygous and normal homozygous relatives had normal hearing in spite of having the mutation in MT-RNR1. The conservation index (CI) of m.1453A>G was found to be 82%, suggesting it to be a possibly deleterious mutation. Functional studies using cell lines derived from muscle tissue of these patients may reveal the pathogenic mechanism of deafness in them.

Mitochondrial tRNA mutations associated with deafness

Mitochondrial tRNA mutations are one of the important causes of both syndromic and non-syndromic deafness. Of those, syndromic deafness-associated tRNA mutations such as tRNA(Leu(UUR)) 3243A>G are often present in heteroplasmy, while non-syndromic deafness-associated tRNA mutations including tRNA(Ser(UCN)) 7445A>G often occur in homplasmy or in high levels of heteroplasmy. These tRNA mutations are the primary mutations leading to hearing loss. However, other tRNA mutations such as tRNA(Thr) 15927G>A and tRNA(Ser(UCN)) 7444G>A may act in synergy with the primary mitochondrial DNA mutations, modulating the phenotypic manifestation of the primary mitochondrial DNA mutations. Theses tRNA mutations cause structural and functional alteration. A failure in tRNA metabolism caused by these tRNA mutations impaired mitochondrial translation and respiration, thereby causing mitochondrial dysfunctions responsible for deafness. These data offer valuable information for the early diagnosis, management and treatment of maternally inherited deafness.

Mitochondrial 12S rRNA mutations associated with aminoglycoside ototoxicity

The mitochondrial 12S rRNA is a hot spot for mutations associated with both aminoglycoside-induced and nonsyndromic hearing loss. Of those, the homoplasmic 1555A>G and 1494C>T mutations at the highly conserved decoding region of the 12S rRNA have been associated with hearing loss worldwide. In particular, these two mutations account for a significant number of cases of aminoglycoside ototoxicity. The 1555A>G or 1494C>T mutation is expected to form a novel 1494C-G1555 or 1494U-A1555 base-pair at the highly conserved A-site of 12S rRNA. These transitions make the human mitochondrial ribosomes more bacteria-like and alter binding sites for aminoglycosides. As a result, the exposure to aminoglycosides can induce or worsen hearing loss in individuals carrying one of these mutations. Biochemical characterization demonstrated an impairment of mitochondrial protein synthesis and subsequent defects in respiration in cells carrying the A1555G or 1494C>T mutation. Furthermore, a wide range of severity, age-at-onset and penetrance of hearing loss was observed within and among families carrying these mutations. Nuclear modifier genes, mitochondrial haplotypes and aminoglycosides should modulate the phenotypic manifestation of the 12S rRNA 1555A>G and 1494C>T mutations. Therefore, these data provide valuable information and technology: (1) to predict which individuals are at risk for ototoxicity; (2) to improve the safety of aminoglycoside antibiotic therapy; and (3) eventually to decrease the incidence of hearing loss.

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.

Maternally inherited hearing loss is associated with the novel mitochondrial tRNA Ser(UCN) 7505T>C mutation in a Han Chinese family

Mutations in mitochondrial DNA (mtDNA) have been found to be one of the most important causes of sensorineural hearing loss. We report here a clinical, genetic, molecular and biochemical characterization of a Han Chinese pedigree with maternally transmitted nonsyndromic hearing impairment. Seven of nine matrilineal relatives exhibited a variable severity and age-at-onset (8 years old) of hearing loss. Mutational analysis of mtDNA identified the novel homoplasmic tRNA(Ser(UCN)) 7505T>C mutation and other 37 variants belonging to haplogroup F1. The 7505T>C mutation, which is absent in 449 Chinese controls, is located at a highly conserved base-pairing (10A-20U) of tRNA(Ser(UCN)). The abolishment of 10A-20U base-pairing likely alters the tRNA(Ser(UCN)) metabolism. Functional significant of this mutation was supported by approximately 65% reductions in the level of tRNA(Ser(UCN)) observed in the lymphoblastoid cell lines carrying the 7505T>C mutation, compared with the wild-type cell lines. This reduced tRNA level is below the proposed threshold to support a normal respiration in lymphoblastoid cells. Furthermore, the highly conserved tRNA(Ala) 5587T>C and Cytb C93Y variants may have a modifying role of deafness expression associated with the 7505T>C mutation. However, genotyping analysis of nuclear modifier gene TRMU and the prominent deafness-cause gene GJB2 failed to detect any mutations in the member of this family. These data strongly indicate that the novel tRNA(Ser(UCN)) 7505T>C mutation is involved in maternally transmitted hearing loss. However, other genetic, epigenetic or environmental factors may contribute to the phenotypic variability of this family. Our findings will be helpful for counseling families of maternally inherited hearing loss.

Very low penetrance of Leber's hereditary optic neuropathy in five Han Chinese families carrying the ND1 G3460A mutation

We report here the clinical, genetic, and molecular characterization of five Han Chinese families with Leber's hereditary optic neuropathy (LHON). Strikingly, there were very low penetrances of visual impairment in these Chinese families, ranging from 4.2% to 22.2%, with an average of 10.2%. In particular, only 7 (4 males/3 females) of 106 matrilineal relatives in these families exhibited the variable severity and age-at-onset in visual dysfunction. The age-at-onset for visual impairment in matrilineal relatives in these families, varied from 20 to 25 years, with an average of 21.8 years old. Molecular analysis of mitochondrial genomes identified the homoplasmic ND1 G3460A mutation and distinct sets of variants, belonging to the Asian haplogroups B5b, C4a1, D5, F1, and R9, respectively. This suggests that the G3640A mutation occurred sporadically and multiplied through evolution of the mtDNA in China. However, there was the absence of known secondary LHON-associated mtDNA mutations in these Chinese families. Very low penetrance of visual loss in these five Chinese pedigrees strongly indicated that the G3640A mutation was itself insufficient to develop the optic neuropathy. The absence of secondary LHON mtDNA mutations suggest that these mtDNA haplogroup-specific variants may not play an important role in the phenotypic expression of the G3640A mutation in those Chinese families with low penetrance of vision loss. However, nuclear modifier genes, epigenetic and environmental factors appear to be modifier factors for the phenotypic manifestation of the G3640A mutation in these Chinese families.

Human mitochondrial leucyl-tRNA synthetase corrects mitochondrial dysfunctions due to the tRNALeu(UUR) A3243G mutation, associated with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like symptoms and diabetes

Mutations in mitochondrial tRNA genes are associated with a wide spectrum of human diseases. In particular, the tRNA(Leu(UUR)) A3243G mutation causes mitochondrial encephalomyopathy, lactic acidosis, and stroke-like symptoms (MELAS) and 2% of cases of type 2 diabetes. The primary defect in this mutation was an inefficient aminoacylation of the tRNA(Leu(UUR)). In the present study, we have investigated the molecular mechanism of the A3243G mutation and whether the overexpression of human mitochondrial leucyl-tRNA synthetase (LARS2) in the cytoplasmic hybrid (cybrid) cells carrying the A3243G mutation corrects the mitochondrial dysfunctions. Human LARS2 localizes exclusively to mitochondria, and LARS2 is expressed ubiquitously but most abundantly in tissues with high metabolic rates. We showed that the alteration of aminoacylation tRNA(Leu(UUR)) caused by the A3243G mutation led to mitochondrial translational defects and thereby reduced the aminoacylated efficiencies of tRNA(Leu(UUR)) as well as tRNA(Ala) and tRNA(Met). We demonstrated that the transfer of human mitochondrial leucyl-tRNA synthetase into the cybrid cells carrying the A3243G mutation improved the efficiency of aminoacylation and stability of mitochondrial tRNAs and then increased the rates of mitochondrial translation and respiration, consequently correcting the mitochondrial dysfunction. These findings provide new insights into the molecular mechanism of maternally inherited diseases and a step toward therapeutic interventions for these disorders.

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 approximately 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 30years old, with the average of 14.5years. 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.

Phenotypic expression of maternally inherited deafness is affected by RNA modification and cytoplasmic ribosomal proteins

The homoplasmic mitochondrial A1555G mutation in the 12S rRNA gene leads to a mitochondrial translation disorder associated with deafness. The absence of disease in non-cochlear tissues in all patients, and in the cochlea in some patients, is not well understood. We used a system-based approach, including whole genome expression and biological function analysis, to elucidate the pathways underlying tissue specificity and clinical severity of this condition. Levels of over 48K RNA transcripts from EBV-transformed lymphoblasts of deaf and hearing individuals with the A1555G mutation and controls were obtained. Differentially expressed transcripts were functionally grouped using gene set enrichment analysis. Over 50 RNA binding proteins were differentially expressed between deaf and hearing individuals with the A1555G mutation (P-value of 2.56E-7), confirming previous genetic data implicating this pathway in the determination of the severity of hearing loss. Unexpectedly, the majority of cytoplasmic ribosomal genes were up-regulated in a coordinated fashion in individuals with the A1555G mutation versus controls (P-value of 3.91E-135). This finding was verified through real time RT-PCR, and through measuring of protein levels by flow cytometry. Analysis of expression levels of other differentially expressed genes suggests that this coordinated over-expression of cytoplasmic ribosomal proteins might occur through the Myc/Max pathway. We propose that expression levels of RNA binding proteins help determine the severity of the cochlear phenotype, and that coordinated up-regulation of the cytoplasmic translation apparatus operates as a compensation mechanism in unaffected tissues of patients with maternal deafness associated with the A1555G mutation.

Failures in mitochondrial tRNAMet and tRNAGln metabolism caused by the novel 4401A>G mutation are involved in essential hypertension in a Han Chinese Family

We report here on the clinical, genetic, and molecular characterization of 1 Han Chinese family with maternally transmitted hypertension. Three of 7 matrilineal relatives in this 4-generation family exhibited the variable degree of essential hypertension at the age at onset, ranging from 35 to 60 years old. Sequence analysis of the complete mitochondrial DNA in this pedigree identified the novel homoplasmic 4401A>G mutation localizing at the spacer immediately to the 5' end of tRNA(Met) and tRNA(Gln) genes and 39 other variants belonging to the Asian haplogroup C. The 4401A>G mutation was absent in 242 Han Chinese controls. Approximately 30% reductions in the steady-state levels of tRNA(Met) and tRNA(Gln) were observed in 2 lymphoblastoid cell lines carrying the 4401A>G mutation compared with 2 control cell lines lacking this mutation. Failures in mitochondrial metabolism are apparently a primary contributor to the reduced rate of mitochondrial translation and reductions in the rate of overall respiratory capacity, malate/glutamate-promoted respiration, succinate/glycerol-3-phosphate-promoted respiration, or N,N,N',N'-tetramethyl-p-phenylenediamine/ascorbate-promoted respiration in lymphoblastoid cell lines carrying the 4401A>G mutation. The homoplasmic form, mild biochemical defect, late onset, and incomplete penetrance of hypertension in this family suggest that the 4401A>G mutation itself is insufficient to produce a clinical phenotype. Thus, the other modifier factors, eg, nuclear modifier genes and environmental and personal factors, may also contribute to the development of hypertension in these subjects carrying this mutation. These data suggest that mitochondrial dysfunctions, caused by the 4401A>G mutation, are involved in the development of hypertension in this Chinese pedigree.

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 hearing impairment. In particular, penetrances of hearing loss in this family pedigree were 43.8% and 25%, respectively, when aminoglycoside-induced hearing 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 TPsiC-loop of tRNA(Glu) 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 transfer RNAMet 4435A>G mutation is associated with maternally inherited hypertension in a Chinese pedigree

Mitochondrial DNA mutations have been associated with cardiovascular disease. We report here the clinical, genetic, and molecular characterization of 1 Han Chinese family with suggestively maternally transmitted hypertension. Matrilineal relatives in this family exhibited the variable degree of hypertension at the age at onset of 44 to 55 years old. Sequence analysis of entire mitochondrial DNA in this pedigree identified the known homoplasmic 4435A>G mutation, which is located immediately at the 3 prime end to the anticodon, corresponding with the conventional position 37 of tRNA(Met), and 35 other variants belonging to the Asian haplogroup B5a. The adenine (A37) at this position of tRNA(Met) is extraordinarily conserved from bacteria to human mitochondria. This modified A37 was shown to contribute to the high fidelity of codon recognition, the structural formation, and stabilization of functional tRNAs. In fact, a 40% reduction in the levels of tRNA(Met) was observed in cells carrying the 4435A>G mutation. As a result, a failure in mitochondrial tRNA metabolism, caused by the 4435A>G mutation, led to approximately 30% reduction in the rate of mitochondrial translation. However, the homoplasmic form, mild biochemical defect, and late onset of hypertension in subjects carrying the 4435A>G mutation suggest that the 4435A>G mutation itself is insufficient to produce a clinical phenotype. The other modifier factors, such as nuclear modifier genes, environmental, and personal factors may also contribute to the development of hypertension in the subjects carrying this mutation. Our findings imply that the 4435A>G mutation may act as an inherited risk factor for the development of hypertension in this Chinese pedigree.