Mutations in the mitochondrial 12S rRNA gene in elderly Chinese people

Clinical and molecular findings in a Chinese family with a de novo mitochondrial A1555G mutation

Background

The mitochondrial 12S rRNA A1555G mutation is the most prevalent deafness-causing mitochondrial DNA (mtDNA) mutation and is inherited maternally. Studies have suggested that A1555G mutations have multiple origins, although there is no direct evidence of this. Here, we identified a family with a de novo A1555G mutation.

Method

Based on detailed mtDNA analyses of the family members using next-generation sequencing with 1% sensitivity to mutated mtDNA, the level of heteroplasmy in terms of the A1555G mutation in blood DNA samples was quantified.

Results

An individual harbored a heterogeneous A1555G mutation, at 28.68% heteroplasmy. The individual’s son was also a heterogeneous carrier, with 7.25% heteroplasmy. The individual’s brother and mother did not carry the A1555G mutation, and both had less than 1% mitochondrial 12S rRNA A1555G heteroplasmy.

Conclusion

The A1555G mutation arose de novo in this family. This is the first report of a family with a de novo A1555G mutation, providing direct evidence of its multipoint origin. This is important for both diagnostic investigations and genetic counselling.

A Mitochondrial DNA Variant Elevates the Risk of Gallstone Disease by Altering Mitochondrial Function

BACKGROUND AND AIMS

Gallstone disease (cholelithiasis) is a cholesterol-related metabolic disorders with strong familial predisposition. Mitochondrial DNA (mtDNA) variants accumulated during human evolution are associated with some metabolic disorders related to modified mitochondrial function. The mechanistic links between mtDNA variants and gallstone formation need further exploration.

METHODS

In this study, we explored the possible associations of mtDNA variants with gallstone disease by comparing 104 probands and 300 controls in a Chinese population. We constructed corresponding cybrids using trans-mitochondrial technology to investigate the underlying mechanisms of these associations. Mitochondrial respiratory chain complex activity and function and cholesterol metabolism were assessed in the trans-mitochondrial cell models.

RESULTS

Here, we found a significant association of mtDNA 827A>G with an increased risk of familial gallstone disease in a Chinese population (odds ratio [OR]: 4.5, 95% confidence interval [CI]: 2.1-9.4, P=1.2×10-4). Compared with 827A cybrids (haplogroups B4a and B4c), 827G cybrids (haplogroups B4b and B4d) had impaired mitochondrial respiratory chain complex activity and function and activated JNK and AMPK signaling pathways. Additionally, the 827G cybrids showed disturbances in cholesterol transport and accelerated development of gallstones. Specifically, cholesterol transport through the transporter ABCG5/8 was increased via activation of the AMPK signaling pathway in 827G cybrids.

CONCLUSIONS

Our findings reveal that mtDNA 827A>G induces aberrant mitochondrial function and abnormal cholesterol transport, resulting in increased occurrence of gallstones. The results provide an important biological basis for the clinical diagnosis and prevention of gallstone disease in the future.

Mitochondrial mutations in non-syndromic hearing loss at UAE

INTRODUCTION

Hearing loss (HL) is a common sensory disorder over the world, and it has been estimated that genetic etiology is involved in more than 50% of the cases in developed countries. Both nuclear and mitochondrial genes were reported as responsible for hereditary HL. Mitochondrial mutations leading to HL have so far been reported in the MT-RNR1 gene, mitochondrially encoded 12S rRNA.

METHODS

To study the molecular contribution of mitochondrial 12S rRNA gene mutations in UAE-HL, a cohort of 74 unrelated UAE patients with no gap junction protein beta 2 (GJB2) mutations were selected for mitochondrial 12S rRNA gene mutational screening using Sanger sequencing and whole-exome sequencing. Detected DNA variants were analyzed by bioinformatics tools to predict their pathogenic effects.

RESULTS

Our analysis revealed the presence of two known deafness mutations; m.669T > C and m.827A > G in two different deaf individuals. Furthermore, whole-exome sequencing was done for these two patients and showed the absence of any nuclear mutations. Our study supports the pathogenic effect of the m.669T > C and m.827A > G mutations and showed that mitochondrial mutations have a contribution of 2.7% in our cohort.

CONCLUSIONS

This is the first report of mtDNA mutations in the UAE which revealed that both variants m.669T > C and m.827A > G should be included in the molecular diagnosis of patients with maternally inherited HL in UAE.

Mitochondrial mutations associated with hearing and balance disorders

Hearing and balance disorders are related to the inner ear and are among the major cause of falls in older adults. Hearing loss that commonly occurs with aging (aka presbyacusis) can result from noise exposure, smoking, ototoxic drugs and genetic factors such as mutations in nuclear and mitochondrial genes. Mutations in mitochondrial DNA (mtDNA) have been reported to play an important role in cell function by providing energy, as well as, cell death (apoptosis). This study aims to systematically review mitochondrial mutations associated with presbyacusis and suggests preventive measurements to improve the quality of life in older adults.

Association of genetic variations in the mitochondrial DNA control region with presbycusis

Background

The prominent role of mitochondria in the generation of reactive oxygen species, cell death, and energy production contributes to the importance of this organelle in the intracellular mechanism underlying the progression of the common sensory disorder of the elderly, presbycusis. Reduced mitochondrial DNA (mtDNA) gene expression and coding region variation have frequently been reported as being associated with the development of presbycusis. The mtDNA control region regulates gene expression and replication of the genome of this organelle. To comprehensively understand of the role of mitochondria in the progression of presbycusis, we compared variations in the mtDNA control region between subjects with presbycusis and controls.

Methods

A total of 58 presbycusis patients and 220 control subjects were enrolled in the study after examination by the otolaryngologist and audiology tests. Variations in the mtDNA control region were investigated by polymerase chain reaction and Sanger sequencing.

Results

A total of 113 sequence variants were observed in mtDNA, and variants were detected in 100% of patients, with 84% located in hypervariable regions. The frequencies of the variants, 16,223 C>T, 16,311 T>C, 16,249 T>C, and 15,954 A>C, were significantly different between presbycusis and control subjects.

Conclusion

The statistically significant difference in the frequencies of four nucleotide variants in the mtDNA control region of presbycusis patients and controls is in agreement with previous experimental evidence and supports the role of mitochondria in the intracellular mechanism underlying presbycusis development. Moreover, these variants have potential as diagnostic markers for individuals at a high risk of developing presbycusis. The data also suggest the possible presence of changes in the mtDNA control region in presbycusis, which could alter regulatory factor binding sites and influence mtDNA gene expression and copy number.

The potential role for use of mitochondrial DNA copy number as predictive biomarker in presbycusis

OBJECTIVES

Age-related hearing impairment, or presbycusis, is the most common communication disorder and neurodegenerative disease in the elderly. Its prevalence is expected to increase, due to the trend of growth of the elderly population. The current diagnostic test for detection of presbycusis is implemented after there has been a change in hearing sensitivity. Identification of a pre-diagnostic biomarker would raise the possibility of preserving hearing sensitivity before damage occurs. Mitochondrial dysfunction, including the production of reactive oxygen species and induction of expression of apoptotic genes, participates in the progression of presbycusis. Mitochondrial DNA sequence variation has a critical role in presbycusis. However, the nature of the relationship between mitochondrial DNA copy number, an important biomarker in many other diseases, and presbycusis is undetermined.

METHODS

Fifty-four subjects with presbycusis and 29 healthy controls were selected after ear, nose, throat examination and pure-tone audiometry. DNA was extracted from peripheral blood samples. The copy number of mitochondrial DNA relative to the nuclear genome was measured by quantitative real-time polymerase chain reaction.

RESULTS

Subjects with presbycusis had a lower median mitochondrial DNA copy number than healthy subjects and the difference was statistically significant (P=0.007). Mitochondrial DNA copy number was also significantly associated with degree of hearing impairment (P=0.025) and audiogram configuration (P=0.022).

CONCLUSION

The findings of this study suggest that lower mitochondrial DNA copy number is responsible for presbycusis through alteration of mitochondrial function. Moreover, the significant association of mitochondrial DNA copy number in peripheral blood samples with the degree of hearing impairment and audiogram configuration has potential for use as a standard test for presbycusis, providing the possibility of the development of an easy-to-use biomarker for the early detection of this condition.

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.