Mitochondrial haplogroup background may influence Southeast Asian G11778A Leber hereditary optic neuropathy

Clinical expression and mitochondrial deoxyribonucleic acid study in twins with 14484 Leber’s hereditary optic neuropathy: A case report

BACKGROUND

This study aimed to explore clinical and molecular factors that cause discordance for clinical expression of Leber’s hereditary optic neuropathy (LHON) in a pair of identical twins with the 14484 point mutation.

CASE SUMMARY

Twin patients with the 14484 point mutation were studied for zygosity by using the Short Tandem Repeats Typing system. For the monozygotic twins, the radioactive restriction and densitometric analyses were used to quantitate the heteroplasmy level for the 14484 point mutation. The mitochondrial genome was analyzed to determine influential factors by mitochondrial deoxyribonucleic acid (DNA) sequencing, denaturing high-performance liquid chromatography and next generation sequencing. For the dizygotic twins, the nuclear DNA was analyzed. The twins with 14484 LHON were monozygotic with homoplasmy. No difference in the point mutation in mitochondrial DNA was found. No modifying genes that potentially influenced the disparity in phenotypic expression of LHON were detected in these twins.

CONCLUSION

This 11-year follow-up of monozygotic twins showed additional genetic modifications and epigenetic factors are possibly associated with discordance for LHON.

The Mitochondrial tRNAPhe 625G>A Mutation in Three Han Chinese Families With Cholecystolithiasis

In this study, we assessed three Chinese families with inherited cholecystolithiasis and conducted the clinical, genetic, and molecular characterization of these subjects. Eight of eighteen matrilineal relatives had a clinical phenotype in these three families. Sequence analysis of complete mitochondrial genomes in these probands identified the homoplasmic tRNA^Phe 625 G > A mutation and distinct sets of mtDNA polymorphisms belonging to haplogroups H2, F4b, and M10a. The 625G > A mutation disturbed the classic G-C base-pairings at a highly conserved position 49 in the T-stem of mitochondrial tRNAs. Molecular dynamics simulation showed that the structure of tRNA^phe with 625 G > A mutation was noticeably remodeled while compared with the isoform of the wild type. The occurrence of tRNA^Phe 625 G > A mutation in these various genetically unrelated subjects strongly indicates that this mutation is involved in the pathogenesis of cholecystolithiasis. This is the first evidence that tRNA mutations are associated with cholecystolithiasis, and it provided more insights into the genetic mechanism of cholecystolithiasis.

Association of Mitochondrial DNA Polymorphisms With Pediatric-Onset Cyclic Vomiting Syndrome

BACKGROUND

Cyclic vomiting syndrome (CVS) is a functional gastrointestinal disorder characterized by recurrent stereotypic episodes of vomiting. The pathophysiology of CVS remains obscure. Previous studies have supported the hypotheses of mitochondrial dysfunction. However, data on association studies between mitochondrial DNA (mtDNA) polymorphisms and pediatric-onset CVS are limited and inconsistent. The aims of this study were to describe clinical characteristics, evaluate association of mtDNA polymorphisms 16519T and 3010A with pediatric-onset CVS and identify new mtDNA candidate variants.

METHODS

This study involved Thai patients diagnosed with CVS according to the Rome III or IV criteria before the age of 15 years. Patients' demographic data, clinical characteristics, previous investigations and treatment outcomes were obtained. Blood samples were collected for next-generation (whole exome) sequencing, followed by analysis of chromosome M (mitochondrial. Variants were filtered according to clinical significance using ClinVar and MITOMAP. mtDNA polymorphisms in 148 normal Thai individuals were used as controls.

RESULTS

Forty-eight children were enrolled in the clinical study, and 30 participated in the genetic analysis. The median age at onset and median age at diagnosis was 3.0 (1.5-5.6) and 6.3 (3.0-8.6) years, respectively. Maternal history of migraine was positive in 16.7%. About 45.7% (21 of 46) of the patients achieved complete clinical remission, with the mean symptom duration of 5.9 ± 3.3 years. The prevalence of mtDNA variants 16519T and 3010A among the patient group and Thai general population (control) were as follows: 40.0% (12/30) vs. 27.7% (P = 0.18) and 6.7% (2/30) vs. 0.7% (P = 0.07), respectively. Five known pathogenic variants were identified in 6 patients, including mtDNA 8528C in one patient who also had infantile hypertrophic cardiomyopathy. Six likely pathogenic variants were found but without statistical significance. We identified 11 variants with significant prevalence in the patient group. Though, these variants were classified as variants of unknown significance (VUS), several of them were located in mt functional regions and therefore they deserve further investigations as new candidates for association with pediatric CVS.

CONCLUSION

There were no associations of mtDNA polymorphisms 16519T and 3010A with CVS in our pediatric cohort. Five pathogenic variants and 11 VUS were found associated with pediatric-onset CVS.

Emerging model systems and treatment approaches for Leber's hereditary optic neuropathy: Challenges and opportunities

Leber's hereditary optic neuropathy (LHON) is a mitochondrial disease mainly affecting retinal ganglion cells (RGCs). The pathogenesis of LHON remains ill-characterized due to a historic lack of effective disease models. Promising models have recently begun to emerge; however, less effective models remain popular. Many such models represent LHON using non-neuronal cells or assume that mutant mtDNA alone is sufficient to model the disease. This is problematic because context-specific factors play a significant role in LHON pathogenesis, as the mtDNA mutation itself is necessary but not sufficient to cause LHON. Effective models of LHON should be capable of demonstrating processes that distinguish healthy carrier cells from diseased cells. In light of these considerations, we review the pathophysiology of LHON as it relates to old, new and future models. We further discuss treatments for LHON and unanswered questions that might be explored using these new model systems.

Incomplete penetrance in mitochondrial optic neuropathies

Incomplete penetrance characterizes the two most frequent inherited optic neuropathies, Leber's Hereditary Optic Neuropathy (LHON) and dominant optic atrophy (DOA), due to genetic errors in the mitochondrial DNA (mtDNA) and the nuclear DNA (nDNA), respectively. For LHON, compelling evidence has accumulated on the complex interplay of mtDNA haplogroups and environmental interacting factors, whereas the nDNA remains essentially non informative. However, a compensatory mechanism of activated mitochondrial biogenesis and increased mtDNA copy number, possibly driven by a permissive nDNA background, is documented in LHON; when successful it maintains unaffected the mutation carriers, but in some individuals it might be hampered by tobacco smoking or other environmental factors, resulting in disease onset. In females, mitochondrial biogenesis is promoted and maintained within the compensatory range by estrogens, partially explaining the gender bias in LHON. Concerning DOA, none of the above mechanisms has been fully explored, thus mtDNA haplogroups, environmental factors such as tobacco and alcohol, and further nDNA variants may all participate as protective factors or, on the contrary, favor disease expression and severity. Next generation sequencing, complemented by transcriptomics and proteomics, may provide some answers in the next future, even if the multifactorial model that seems to apply to incomplete penetrance in mitochondrial optic neuropathies remains problematic, and careful stratification of patients will play a key role for data interpretation. The deep understanding of which factors impinge on incomplete penetrance may shed light on the pathogenic mechanisms leading to optic nerve atrophy, on their possible compensation and, thus, on development of therapeutic strategies.

Leber Hereditary Optic Neuropathy: A Mitochondrial Disease Unique in Many Ways

Leber hereditary optic neuropathy (LHON) was the first mitochondrial disease to be identified as being caused by mutations in the mitochondrial DNA (mtDNA). This disease has been studied extensively in the past two decades, particularly in Brazilian, Chinese and European populations; and many primary mutations have been reported. However, the disease is enigmatic with many unique features, and there still are several important questions to be resolved. The incomplete penetrance, the male-biased disease expression and the prevalence in young adults all defy a proper explanation. It has been reported that the development of LHON is affected by the interaction between mtDNA mutations, mtDNA haplogroup background, nuclear genes, environmental factors and epigenetics. Furthermore, with the help of new animal models for LHON that have been created in recent years, we are continuing to learn more about the mechanism of this disease. The stage has now been reached at which there is a good understanding of both the genetic basis of the disease and its epidemiology, but just how the blindness that follows from the death of cells in the optic nerve can be prevented remains to be a pharmacological challenge. In this chapter, we summarize the progress that has been made in various recent studies on LHON, focusing on the molecular pathogenic mechanisms, clinical features, biochemical effects, the pharmacology and its treatment.

Profiling the mitochondrial proteome of Leber's Hereditary Optic Neuropathy (LHON) in Thailand: down-regulation of bioenergetics and mitochondrial protein quality control pathways in fibroblasts with the 11778G>A mutation

Leber's Hereditary Optic Neuropathy (LHON) is one of the commonest mitochondrial diseases. It causes total blindness, and predominantly affects young males. For the disease to develop, it is necessary for an individual to carry one of the primary mtDNA mutations 11778G>A, 14484T>C or 3460G>A. However these mutations are not sufficient to cause disease, and they do not explain the characteristic features of LHON such as the higher prevalence in males, incomplete penetrance, and relatively later age of onset. In order to explore the roles of nuclear encoded mitochondrial proteins in development of LHON, we applied a proteomic approach to samples from affected and unaffected individuals from 3 pedigrees and from 5 unrelated controls. Two-dimensional electrophoresis followed by MS/MS analysis in the mitochondrial lysate identified 17 proteins which were differentially expressed between LHON cases and unrelated controls, and 24 proteins which were differentially expressed between unaffected relatives and unrelated controls. The proteomic data were successfully validated by western blot analysis of 3 selected proteins. All of the proteins identified in the study were mitochondrial proteins and most of them were down regulated in 11778G>A mutant fibroblasts. These proteins included: subunits of OXPHOS enzyme complexes, proteins involved in intermediary metabolic processes, nucleoid related proteins, chaperones, cristae remodelling proteins and an anti-oxidant enzyme. The protein profiles of both the affected and unaffected 11778G>A carriers shared many features which differed from those of unrelated control group, revealing similar proteomic responses to 11778G>A mutation in both affected and unaffected individuals. Differentially expressed proteins revealed two broad groups: a cluster of bioenergetic pathway proteins and a cluster involved in protein quality control system. Defects in these systems are likely to impede the function of retinal ganglion cells, and may lead to the development of LHON in synergy with the primary mtDNA mutation.

Epidermal nerve fiber density, oxidative stress, and mitochondrial haplogroups in HIV-infected Thais initiating therapy

OBJECTIVE

We explored associations between mitochondrial DNA (mtDNA) haplogroups, epidermal nerve fiber density (ENFD), and HIV-associated sensory neuropathy (HIV-SN) in a randomized trial of Thai patients initiating antiretroviral therapy (ART).

DESIGN

The South East Asia Research Collaboration with Hawaii 003 study evaluated toxicity of nucleoside reverse transcriptase inhibitors (stavudine vs. zidovudine vs. tenofovir). We present secondary analyses of mtDNA haplogroups and ENFD changes.

METHODS

ENFD, peripheral blood mononuclear cell mitochondrial complex I and IV, and 8-oxo-deoxyguanine (8-oxo-dG) were quantified. Peripheral blood mononuclear cell mtDNA sequences were obtained for haplogroup determination. Multivariate regression of ENFD change was performed.

RESULTS

Paired ENFD was available from 118 patients. Median age, CD4 cell count, and height at entry were 34 years, 172 cells/μl, and 162 cm, respectively. Major haplogroups included M (42%), F (21%), and B (16%). Baseline ENFD, CD4 cell count, randomized ART, and biomarkers did not differ by haplogroup. Haplogroup B patients were older (P=0.02) at baseline, and had an increase in median ENFD (+1.5 vs. -2.9 fibers/mm; P=0.03) and 8-oxo-dG break frequency (+0.05 vs. 0.00; P=0.05) compared to other haplogroups. In a multivariate model, haplogroup B was associated with increased ENFD (β=3.5, P=0.009) at week 24, whereas older age (P=0.02), higher baseline CD4 cell count, (P=0.03), higher complex I level (P=0.03), and higher ENFD (P<0.001) at baseline were all associated with decreased ENFD. Three of the six HIV-SN cases were haplogroup B (P=0.05).

CONCLUSIONS

Thai persons belonging to mtDNA haplogroup B had increased ENFD and 8-oxo-dG on ART, and were more likely to develop HIV-SN. These results suggest that mtDNA variation influences early oxidative damage and ENFD changes.

Mitochondrial DNA haplogroup background affects LHON, but not suspected LHON, in Chinese patients

Recent studies have shown that mtDNA background could affect the clinical expression of Leber hereditary optic neuropathy (LHON). We analyzed the mitochondrial DNA (mtDNA) variation of 304 Chinese patients with m.11778G>A (sample #1) and of 843 suspected LHON patients who lack the three primary mutations (sample #2) to discern mtDNA haplogroup effect on disease onset. Haplogroup frequencies in the patient group was compared to frequencies in the general Han Chinese population (n = 1,689; sample #3). The overall matrilineal composition of the suspected LHON population resembles that of the general Han Chinese population, suggesting no association with mtDNA haplogroup. In contrast, analysis of these LHON patients confirms mtDNA haplogroup effect on LHON. Specifically, the LHON sample significantly differs from the general Han Chinese and suspected LHON populations by harboring an extremely lower frequency of haplogroup R9, in particular of its main sub-haplogroup F (#1 vs. #3, P-value = 1.46×10⁻¹⁷, OR = 0.051, 95% CI: 0.016–0.162; #1 vs. #2, P-value = 4.44×10⁻¹⁷, OR = 0.049, 95% CI: 0.015–0.154; in both cases, adjusted P-value <10⁻⁵) and higher frequencies of M7b (#1 vs. #3, adjusted P-value = 0.001 and #1 vs. #2, adjusted P-value = 0.004). Our result shows that mtDNA background affects LHON in Chinese patients with m.11778G>A but not suspected LHON. Haplogroup F has a protective effect against LHON, while M7b is a risk factor.

Is mitochondrial tRNA(phe) variant m.593T>C a synergistically pathogenic mutation in Chinese LHON families with m.11778G>A?

Mitochondrial transfer RNA (mt-tRNA) mutations have been reported to be associated with a variety of diseases. In a previous paper that studied the mtDNA background effect on clinical expression of Leber's hereditary optic neuropathy (LHON) in 182 Chinese families with m.11778G>A, we found a strikingly high frequency (7/182) of m.593T>C in the mitochondrially encoded tRNA phenylalanine (MT-TF) gene in unrelated LHON patients. To determine the potential role of m.593T>C in LHON, we compared the frequency of this variant in 479 LHON patients with m.11778G>A, 843 patients with clinical features of LHON but without the three known primary mutations, and 2374 Han Chinese from the general populations. The frequency of m.593T>C was higher in LHON patients (14/479) than in suspected LHON subjects (12/843) or in general controls (49/2374), but the difference was not statistically significant. The overall penetrance of LHON in families with both m.11778G>A and m.593T>C (44.6%) was also substantially higher than that of families with only m.11778G>A (32.9%) (P = 0.083). Secondary structure prediction of the MT-TF gene with the wild type or m.593T>C showed that this nucleotide change decreases the free energy. Electrophoretic mobility of the MT-TF genes with the wild type or m.593T>C transcribed in vitro further confirmed the change of secondary structure in the presence of this variant. Although our results could suggest a modest synergistic effect of variant m.593T>C on the LHON causing mutation m.11778G>A, the lack of statistical significance probably due to the relatively small sample size analyzed, makes necessary more studies to confirm this effect.