Mild Leber hereditary optic neuropathy (LHON) in a Western European family due to the rare Asian m.14502T>C variant in the MT-ND6 gene

The role of mitochondrial DNA copy number in female infertility: a bidirectional two-sample Mendelian randomization study

BACKGROUND

Previous studies on the impact of mitochondrial DNA (mtDNA) copy number on female infertility were limited and inconsistent.

METHODS

The causal relationship between mtDNA copy number and female infertility was evaluated using a bidirectional 2-sample Mendelian randomization (MR) method. Inverse variance weighted (IVW) method was applied for principal analysis, and MR-Egger, weighted median, simple mode, weighted mode method for secondary analyses. Sensitivity analysis was conducted using MR-PRESSO, MR-Egger, Cochran's Q, and leave-one-out tests. Two large-scale GWAS mtDNA copy number datasets were employed for testing and validation to ensure reliable results.

RESULTS

According to the forward MR analysis, genetically predicted mtDNA copy number was not associated with premature ovarian failure (POF) (OR = 1.969, 95% CI 0.571-6.789; p = .283), polycystic ovary syndrome (PCOS) (OR = 0.821, 95% CI 0.314-2.142; p = .686), endometriosis (OR = 1.281, 95% CI 0.962-1.704; p = 0.090), or female infertility (OR = 0.966; 95% CI 0.744-1.253; p = .794) but was associated with intestinal endometriosis (OR = 7.528; 95% CI 1.654-34.262; p = .009) and adenomyosis (OR = 1.710; 95% CI 1.118-2.616; p = .013). Reverse MR studies did not reveal a correlation between female infertility and mtDNA copy number. Similar results were observed in the validation data.

CONCLUSIONS

Our study suggested that there is no causal relationship between mtDNA copy number and female infertility, but there is a causal relationship between mtDNA copy number and intestinal endometriosis and adenomyosis. The genetic evidence provided by this study provides a new perspective for studying the impact of mtDNA copy number on female infertility.

[Rare pathogenic nucleotide variants of mitochondrial DNA associated with Leber's hereditary optic neuropathy]

Patients with Leber Hereditary Optic Neuropathy (LHON) in most cases have one of the three most common mutations: m.11778G>A in the ND4 gene, m.3460G>A in the ND1 gene, or m.14484T>C in the ND6 gene. According to the international Mitomap database, in addition to these three most common mutations, there are 16 other primary mutations that are even more rare. There are nucleotide substitutions that are classified as candidate or conditionally pathogenic mutations. Their involvement in the disease development is not proven due to insufficient research. Moreover, in many publications, the authors describe new primary and potential mitochondrial DNA mutations associated with LHON, which are not yet included in the genetic data bases. This makes it possible to expand the diagnostic spectrum during genetic testing in the future. The advancements in genetic diagnostic technologies allow confirmation of the clinical diagnosis of LHON. The importance of genetic verification of the disease is determined by the existing problem of differential diagnosis of hereditary optic neuropathies with optic neuropathies of a different origin.

Mitochondrial Diabetes is Associated with tRNALeu(UUR) A3243G and ND6 T14502C Mutations

BACKGROUND

Mutations in mitochondrial DNA (mtDNA) are associated with type 2 diabetes mellitus (T2DM). In particular, m.A3243G is the most common T2DM-related mtDNA mutation in many families worldwide. However, the clinical features and pathophysiology of m.A3243G-induced T2DM are largely undefined.

METHODS

Two pedigrees with maternally inherited T2DM were underwent clinical, molecular and biochemical assessments. The mtDNA genes were PCR amplified and sequenced. Mitochondrial adenosine triphosphate (ATP) and reactive oxygen species (ROS) were measured in polymononuclear leukocytes derived from three patients with both the m.A3243G and m.T14502C mutations, three patients with only the m.A3243G mutation and three controls without these mutations. Moreover, GJB2, GJB3 and GJB6 mutations were screened by PCR-Sanger sequencing.

RESULTS

Members of the two pedigrees manifestated variable clinical phenotypes including diabetes and hearing and vision impairments. The age at onset of T2DM varied from 31 to 66 years, with an average of 41 years. Mutational analysis of mitochondrial genomes indicated the presence of the m.A3243G mutation in both pedigrees. Matrilineal relatives in one of the pedigrees harbored the coexisting of m.A3243G and m.T14502C mutations. Remarkably, the m.T14502C mutation, which causes the substitution of a conserved isoleucine for valine at position 58 in ND6 mRNA, may affect the mitochondrial respiratory chain functions. Biochemical analysis revealed that cell lines bearing both the m.A3243G and m.T14502C mutations exhibited greater reductions in ATP levels and increased ROS production compared with those carrying only the m.A3243G mutation. However, we did not find any mutations in the GJB2, GJB3 and GJB6 genes.

CONCLUSION

Our study indicated that mitochondrial diabetes is associated with the tRNA^Leu(UUR) A3243G and ND6 T14502C mutations.