Mitochondrial Diabetes Is Associated with the ND4 G11696A Mutation

Mitochondrial DNA variants in the pathogenesis and metabolic alterations of diabetes mellitus

Mitochondrial DNA (mtDNA) variants considerably affect diabetes mellitus by disturbing mitochondrial function, energy metabolism, oxidative stress response, and even insulin secretion. The m.3243 A > G variants is associated with maternally inherited diabetes and deafness (MIDD), where early onset diabetes and hearing loss are prominent features. Other types of mtDNA variants involve genes ND4 and tRNA Ala genes that increase susceptibility to type 2 diabetes. Understanding these variants will provide a basis for developing targeted therapy to improve mitochondrial function and metabolic health. This article reviews the impact of mtDNA variants in diabetes, specifically with regards to the m.3243 A > G variant effects on mitochondrial function and insulin secretion and other mtDNA variants that contribute to diabetes susceptibility, particularly ND4 and tRNA Ala gene variants. Data from extant literature were synthesised to obtain an understanding of how mtDNA variants affect diabetes pathogenesis. The main defect for MIDD is the m.3243 A > G variant, which comprises enhanced susceptibility to metabolic syndrome and type 2 diabetes, followed by mitochondrial dysfunction, insulin resistance, and beta-cell dysfunction. Other mtDNA variants have also been reported to enhance diabetes susceptibility through mitochondrial dysfunction and insulin resistance. Increased production of reactive oxygen species (ROS) resulting from mitochondrial malfunction adds to metabolic and tissue damage. This happens in tissues crucial to glucose homeostasis, and it represents an important contribution of mitochondrial dysfunction to metabolic disturbances in diabetes. These mechanisms would underlie the rationale for developing targeted therapies to preserve mitochondrial function and, hence improve the metabolic health of diabetic patients.

Screening for Mitochondrial tRNA Variants in 200 Patients with Systemic Lupus Erythematosus

INTRODUCTION

Systemic lupus erythematosus (SLE) is a common autoimmune disease with unknown etiology. Recently, a growing number of evidence suggested that mitochondrial dysfunctions played active roles in the pathogenesis of SLE, but its detailed mechanism remains largely undetermined. The aim of this study was to analyze the frequencies of mitochondrial tRNA (mt-tRNA) variants in Chinese individuals with SLE.

METHODS

We carried out a mutational screening of mt-tRNA variants in a cohort of 200 patients with SLE and 200 control subjects by PCR-Sanger sequencing. The potential pathogenicity of mt-tRNA variants was evaluated by phylogenetic conservation and haplogroup analyses. In addition, trans-mitochondrial cybrid cell lines were established, and mitochondrial functions including ATP, reactive oxygen species (ROS), mitochondrial DNA (mtDNA) copy number, mitochondrial membrane potential (MMP), superoxide dismutase (SOD), and mt-RNA transcription were analyzed in cybrids with and without these putative pathogenic mt-tRNA variants.

RESULTS

We identified five possible pathogenic variants: tRNAVal G1606A, tRNALeu(UUR) A3243G, tRNAIle A4295G, tRNAGly T9997C, and tRNAThr A15924G that only found in SLE patients but were absent in controls. Interestingly, these variants were located at extremely conserved nucleotides of the corresponding tRNAs and may alter tRNAs' structure and function. Furthermore, cells carrying these tRNA variants had much lower levels of ATP, mtDNA copy number, MMP, and SOD than controls; by contrast, the levels of ROS increased significantly (p < 0.05 for all). Furthermore, a significant reduction in mt-ND1, ND2, ND3, ND5, and A6 mRNA expression was observed in cells with these mt-tRNA variants, while compared with controls. Thus, failures in tRNA metabolism caused by these variants would impair mitochondrial translation and subsequently lead to mitochondrial dysfunction that was involved in the progression and pathogenesis of SLE.

CONCLUSIONS

Our study suggested that mt-tRNA variants were important causes for SLE, and screening for mt-tRNA pathogenic variants was recommended for early detection and prevention for this disorder.

Functional analysis of the novel mitochondrial tRNATrp and tRNASer(AGY) variants associated with type 2 diabetes mellitus

BACKGROUND

Mutations in mitochondrial tRNA (mt-tRNA) genes that result in mitochondrial dysfunction play important roles in type 2 diabetes mellitus (T2DM). We pre-viously reported a large Chinese pedigree with maternally inherited T2DM that harbors novel mt-tRNA Trp A5514G and tRNA Ser(AGY) C12237T variants, however, the effects of these mt-tRNA variants on T2DM progression are largely unknown.

AIM

To assess the potential pathogenicity of T2DM-associated m.A5514G and m.C12237T variants at genetic, molecular, and biochemical levels.

METHODS

Cytoplasmic hybrid (cybrid) cells carrying both m.A5514G and m.C12237T variants, and healthy control cells without these mitochondrial DNA (mtDNA) variants were generated using trans-mitochondrial technology. Mitochondrial features, including mt-tRNA steady-state level, levels of adenosine triphosphate (ATP), mitochondrial membrane potential (MMP), reactive oxygen species (ROS), mtDNA copy number, nicotinamide adenine dinucleotide (NAD+)/NADH ratio, enzymatic activities of respiratory chain complexes (RCCs), 8-hydroxy-deo-xyguanine (8-OhdG), malondialdehyde (MDA), and superoxide dismutase (SOD) were examined in cell lines with and without these mt-tRNA variants.

RESULTS

Compared with control cells, the m.A5514G variant caused an approximately 35% reduction in the steady-state level of mt-tRNA Trp (P < 0.0001); however, the m.C12237T variant did not affect the mt-tRNA Ser(AGY) steady-state level (P = 0.5849). Biochemical analysis revealed that cells with both m.A5514G and m.C12237T variants exhibited more severe mitochondrial dysfunctions and elevated oxidative stress than control cells: ATP, MMP, NAD+/NADH ratio, enzyme activities of RCCs and SOD levels were markedly decreased in mutant cells (P < 0.05 for all measures). By contrast, the levels of ROS, 8-OhdG and MDA were significantly increased (P < 0.05 for all measures), but mtDNA copy number was not affected by m.A5514G and m.C12237T variants (P = 0.5942).

CONCLUSION

The m.A5514G variant impaired mt-tRNA Trp metabolism, which subsequently caused mitochondrial dysfunction. The m.C12237T variant did not alter the steady-state level of mt-tRNA Ser(AGY), indicating that it may be a modifier of the m.A5514G variant. The m.A5514G variant may exacerbate the pathogenesis and progression of T2DM in this Chinese pedigree.

Maternally Inherited Essential Hypertension May Be Associated with the Mutations in Mitochondrial tRNAGlu Gene

Background

Mitochondrial DNA (mtDNA) mutations are associated with essential hypertension (EH), but the molecular mechanism remains largely unknown.

Objective

The aim of this study is to explore the association between mtDNA mutations and EH.

Methods

Two maternally inherited families with EH are underwent clinical, genetic and biochemical assessments. mtDNA mutations are screened by PCR-Sanger sequencing and phylogenetic, and bioinformatics analyses are performed to evaluate the pathogenicity of mtDNA mutations. We also generate cytoplasmic hybrid (cybrid) cell lines to analysis mitochondrial functions.

Results

Matrilineal relatives exhibit variable degree of clinical phenotypes. Molecular analysis reveals the presence of m.A14693G and m.A14696G mutations in two pedigrees. Notably, the m.A14693G mutation occurs at position 54 in the TψC loop of tRNAGlu, a position which is critical for post-transcriptionally modification of tRNAGlu. While the m.A14696G mutation creates a novel base-pairing (51C-64G). Bioinformatic analysis shows that these mutations alter tRNAGlu secondary structure. Additionally, patients with tRNAGlu mutations exhibit markedly decreased in mtDNA copy number, mitochondrial membrane potential (MMP) and ATP, whereas the levels of reactive oxygen species (ROS) increase significantly.

Conclusion

The m.A14696G and m.A14693G mutations lead to failure in tRNAGlu metabolism and cause mitochondrial dysfunction that is responsible for EH.