Late onset nonsyndromic hearing loss in a Dongxiang Chinese family is associated with the 593T>C variant in the mitochondrial tRNAPhe gene

Deafness-associated tRNAPhe mutation impaired mitochondrial and cellular integrity

Defects in mitochondrial RNA metabolism have been linked to sensorineural deafness that often occurs as a consequence of damaged or deficient inner ear hair cells. In this report, we investigated the molecular mechanism underlying a deafness-associated tRNAPhe 593T > C mutation that changed a highly conserved uracil to cytosine at position 17 of the DHU-loop. The m.593T > C mutation altered tRNAPhe structure and function, including increased melting temperature, resistance to S1 nuclease-mediated digestion, and conformational changes. The aberrant tRNA metabolism impaired mitochondrial translation, which was especially pronounced by decreases in levels of ND1, ND5, CYTB, CO1, and CO3 harboring higher numbers of phenylalanine. These alterations resulted in aberrant assembly, instability, and reduced activities of respiratory chain enzyme complexes I, III, IV, and intact supercomplexes overall. Furthermore, we found that the m.593T > C mutation caused markedly diminished membrane potential, and increased the production of reactive oxygen species in the mutant cell lines carrying the m.593T > C mutation. These mitochondrial dysfunctions led to the mitochondrial dynamic imbalance via increasing fission with abnormal mitochondrial morphology. Excessive fission impaired the process of autophagy including the initiation phase, formation, and maturation of the autophagosome. In particular, the m.593T > C mutation upregulated the PARKIN-dependent mitophagy pathway. These alterations promoted an intrinsic apoptotic process for the removal of damaged cells. Our findings provide critical insights into the pathophysiology of maternally inherited deafness arising from tRNA mutation-induced defects in mitochondrial and cellular integrity.

A systematic review of the monogenic causes of Non-Syndromic Hearing Loss (NSHL) and discussion of Current Diagnosis and Treatment options

Hearing impairment is one of the most widespread inheritable sensory disorder affecting at least 1 in every 1000 born. About two-third of hereditary hearing loss (HHL) disorders are non-syndromic. To provide comprehensive update of monogenic causes of non-syndromic hearing loss (NSHL), literature search has been carried out with appropriate keywords in the following databases-PubMed, Google Scholar, Cochrane library, and Science Direct. Out of 2214 papers, 271 papers were shortlisted after applying inclusion and exclusion criterion. Data extracted from selected papers include information about gene name, identified pathogenic variants, ethnicity of the patient, age of onset, gender, title, authors' name, and year of publication. Overall, pathogenic variants in 98 different genes have been associated with NSHL. These genes have important role to play during early embryonic development in ear structure formation and hearing development. Here, we also review briefly the recent information about diagnosis and treatment approaches. Understanding pathogenic genetic variants are helpful in the management of affected and may offer targeted therapies in future.

Maternally transmitted nonsyndromic hearing impairment may be associated with mitochondrial tRNAAla 5601C>T and tRNALeu(CUN) 12311T>C mutations

BACKGROUND

Sequence alternations in mitochondrial genomes, especially in genes encoding mitochondrial tRNA (mt-tRNA), were the important contributors to nonsyndromic hearing loss (NSHL); however, the molecular mechanisms remained largely undetermined.

METHODS

A maternally transmitted Chinese pedigree with NSHL underwent clinical, genetic, and biochemical assessment. PCR and direct sequence analyses were performed to detect mitochondrial DNA (mtDNA), GJB2, and SLC26A4 gene mutations from matrilineal relatives of this family. Mitochondrial functions including mitochondrial membrane potential (MMP), ATP, and ROS were evaluated in polymononuclear leukocytes (PMNs) derived from three deaf patients and three controls from this pedigree.

RESULTS

Four of nine matrilineal relatives developed hearing loss at the variable age of onset. Two putative pathogenic mutations, m.5601C>T in tRNA^Ala and m.12311T>C in tRNA^Leu(CUN) , were identified via PCR-Sanger sequencing, as well as 34 variants that belonged to mtDNA haplogroup G2b2. Intriguingly, m.5601C>T mutation resided at very conserved nucleotide in the TψC loop of tRNA^Ala (position 59), while the T-to-C substitution at position 12311 located at position 48 in the variable stem of tRNA^Leu(CUN) and was believed to alter the aminoacylation and the steady-state level of tRNA. Biochemical analysis revealed the impairment of mitochondrial functions including the significant reductions of ATP and MMP, whereas markedly increased ROS levels were found in PMNs derived from NSHL patients with m.5601C>T and m.12311T>C mutations. However, we did not detect any mutations in GJB2 and SLC26A4 genes.

CONCLUSION

Our data indicated that mt-tRNA^Ala m.5601C>T and tRNA^Leu(CUN) 12311T>C mutations were associated with NSHL.

Insight of the mitochondrial genomes of the Orang Asli and Malays: The heterogeneity and the disease-associated variants

Orang Asli are the oldest inhabitants in Peninsular Malaysia that forms as a national minority while the Malays are the majority. The study aimed to screen the mitochondrial genomes of the Orang Asli and the Malays to discover the disease-associated variants. A total of 99 Orang Asli from six tribes (Bateq, Cheq Wong, Orang Kanaq, Kensiu, Lanoh, and Semai) were recruited. Mitochondrial genome sequencing was conducted using a next-generation sequencing platform. Furthermore, we retrieved mitochondrial DNA sequences from the Malays for comparison. The clinical significance, pathogenicity prediction and frequency of variants were determined using online tools. Variants associated with mitochondrial diseases were detected in the 2 populations. A high frequency of variants associated with mitochondrial diseases, breast cancer, prostate cancer, and cervical cancer were detected in the Orang Asli and modern Malays. As medicine evolves to adopt prediction and prevention of diseases, this study highlights the need for intervention to adopt genomics medicine to strategise better healthcare management as a way forward for Precision Health.

Mutational analysis of mitochondrial tRNA genes in 138 patients with Leber's hereditary optic neuropathy

INTRODUCTION

Mutations in mitochondrial DNA (mtDNA) are the most important causes for Leber's hereditary optic neuropathy (LHON). Of these, three primary mtDNA mutations account for more than 90% cases of this disease. However, to date, little is known regarding the relationship between mitochondrial tRNA (mt-tRNA) variants and LHON.

AIM

In this study, we aimed to investigate the association between mt-tRNA variants and LHON.

METHODOLOGY

One hundred thirty-eight LHON patients lacking three primary mutations (ND1 3460G > A, ND4 11778Gxs > A, and ND6 14484 T > C), as well as 266 controls were enrolled in this study. PCR-Sanger sequencing was performed to screen the mt-tRNA variants. Moreover, the phylogenetic analysis, pathogenicity scoring system, as well as mitochondrial functions were performed.

RESULTS

We identified 8 possible pathogenic variants: tRNA^Phe 593 T > C, tRNA^Leu(UUR) 3275C > T, tRNA^Gln 4363 T > C, tRNA^Met 4435A > G, tRNA^Ala 5587 T > C, tRNA^Glu 14693A > G, tRNA^Thr 15927G > A, and 15951A > G, which may change the structural and functional impact on the corresponding tRNAs, and subsequently lead to a failure in tRNA metabolism. Furthermore, significant reductions in mitochondrial ATP and MMP levels and an overproduction of ROS were observed in cybrid cells containing these mt-tRNA variants, suggesting that these variants may lead to mitochondrial dysfunction which was responsible for LHON.

CONCLUSION

Our study indicated that mt-tRNA variants were associated with LHON, and screening for mt-tRNA variants were recommended for early detection, diagnosis, and prevention of maternally inherited LHON.

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.