Sequence analysis of the complete mitochondrial genome in patients with Leber's hereditary optic neuropathy lacking the three most common pathogenic DNA mutations

Genomics landscape of mitochondrial DNA variations in patients from South Italy affected by mitochondriopathies

Mitochondrial DNA (mtDNA) is a 16,569 base pairs, double-stranded, circular molecule that contains 37 genes coding for 13 subunits of the respiratory chain plus 2 rRNAs and 22 tRNAs. Mutations in these genes have been identified in patients with a variety of disorders affecting every system in the body. The advent of next generation sequencing technologies has provided the possibility to perform the whole mitochondrial DNA sequencing, allowing the identification of disease-causing pathogenic variants in a single platform. In this study, the whole mtDNA of 100 patients from South Italy affected by mitochondrial diseases was analyzed by using an amplicon-based approach and then the enriched libraries were deeply sequenced on the ION Torrent platform (Thermofisher Scientific Waltham, MA, USA). After bioinformatics analysis and filtering, we were able to find 26 nonsynonymous variants with a MAF <1% that were associated with different pathological phenotypes, expanding the mutational spectrum of these diseases. Moreover, among the new mutations found, we have also analyzed the 3D structure of the MT-ATP6 A200T gene variation in order to confirm suspected functional alterations. This work brings light on new variants possibly associated with several mitochondriopathies in patients from South Italy and confirms that deep sequencing approach, compared to the standard methods, is a reliable and time-cost reducing strategy to detect all the variants present in the mitogenome, making the possibility to create a genomics landscape of mitochondrial DNA variations in human diseases.

Leber Hereditary Optic Neuropathy (LHON) in Patients with Presumed Childhood Monocular Amblyopia

BACKGROUND

Most Leber hereditary optic neuropathy (LHON) cases are bilateral and sequential; however, there are rare unilateral examples, or those in which the delay of onset of vision loss between one and the other eye is longer. In the case of presumed childhood amblyopia in one eye, vision loss in the good eye may be the only symptom of bilateral disease, which was unnoticed in the previously amblyopic eye, or a preexisting episode of LHON in the "amblyopic" eye. The clinical decision in such cases may be difficult and suggestive of other forms of atypical optic neuropathy until confirmed by genetic testing.

CASE SERIES

We present three genetically confirmed (MT-ND1:m.3700G>A, MT-ND6:m14484 T>C, and MT-ND4:m.11778G>A) patients with subacute vision loss in the previously good eye, with the other eye believed to be amblyopic from childhood and their features different from what would be expected in true amblyopia. In all, electrophysiology testing showed a bilaterally reduced amplitude of PERG with low VEP P100 wave amplitudes and prolonged peak time in both eyes, also unusual for amblyopia. During follow-up, the pallor of the optic discs progressed in all eyes. Significant thinning of the peripapillary retinal nerve fiber layer (pRNFL; retinal nerve fiber layer around the optic disc) and ganglion cell complex (GCC) in the macular region was present. All three patients had a peculiar history. The first patient was treated for presumed hyperopic amblyopia that did not improve since childhood, experienced visual loss in the good eye at the age of 17, and was negative for the three typical LHON mutations. Extended testing confirmed an atypical pathogenic variant MT-ND1:m.3700G>A in homoplasmy. The second patient with presumed strabismic amblyopia had an unusual presentation of vision loss only at the age of 61, and after the exclusion of other causes, a typical MT-ND4:m.11778G>A pathogenic variant was found in homoplasmy. The third case was peculiar as he had presumed strabismic amblyopia since childhood and had some degree of disc pallor in the amblyopic eye upon presenting with loss of vision in the good eye at the age of 21, and a typical pathogenic variant m14484 T>C, p.Met64Val was subsequently confirmed. However, one year after disease onset, he started to experience significant spontaneous functional improvement in the non-amblyopic up to 1.0 Snellen whilst improvement in the presumed amblyopic eye was modest, suggesting preexisting amblyopia. This interestingly extensive improvement was carefully followed by electrophysiology as well as visual acuity and fields.

CONCLUSIONS

This report shows three different scenarios of presentation of LHON in patients with presumed uniocular amblyopia from childhood. In such cases, the diagnosis may be difficult, and detailed structural and functional evaluation of the optic nerve head is necessary to assess whether an earlier LHON episode was misdiagnosed as amblyopia or whether LHON presented bilaterally on both eyes whilst only being noticed in the previously good eye.

Mitochondrial DNA variants in a cohort from Argentina with suspected Leber's hereditary optic neuropathy (LHON)

The present study investigates the spectrum and analysis of mitochondrial DNA (mtDNA) variants associated with Leber hereditary optic neuropathy (LHON) in an Argentinean cohort, analyzing 3 LHON-associated mitochondrial genes. In 32% of the cases, molecular confirmation of the diagnosis could be established, due to the identification of disease-causing variants. A total of 54 variants were observed in a cohort of 100 patients tested with direct sequencing analysis. The frequent causative mutations m.11778G>A in MT-ND4, m.3460G>A in MT-ND1, and m.14484T>C in MT-ND6 were identified in 28% of the cases of our cohort. Secondary mutations in this Argentinean LHON cohort were m.11253T>C p.Ile165Thr in MT-ND4, identified in three patients (3/100, 3%) and m.3395A>G p.Tyr30Cys in MT-ND1, in one of the patients studied (1%). This study shows, for the first time, the analysis of mtDNA variants in patients with a probable diagnosis of LHON in Argentina. Standard molecular methods are an effective first approach in order to achieve genetic diagnosis of the disease, leaving NGS tests for those patients with negative results.

The evolution of the human mitochondrial bc1 complex- adaptation for reduced rate of superoxide production?

The mitochondrial bc1 complex is a major source of mitochondrial superoxide. While bc1-generated superoxide plays a beneficial signaling role, excess production of superoxide lead to aging and degenerative diseases. The catalytic core of bc1 comprises three peptides -cytochrome b, Fe-S protein, and cytochrome c1. All three core peptides exhibit accelerated evolution in anthropoid primates. It has been suggested that the evolution of cytochrome b in anthropoids was driven by a pressure to reduce the production of superoxide. In humans, the bc1 core peptides exhibit anthropoid-specific substitutions that are clustered near functionally critical sites that may affect the production of superoxide. Here we compare the high-resolution structures of bovine, mouse, sheep and human bc1 to identify structural changes that are associated with human-specific substitutions. Several cytochrome b substitutions in humans alter its interactions with other subunits. Most significantly, there is a cluster of seven substitutions, in cytochrome b, the Fe-S protein, and cytochrome c1 that affect the interactions between these proteins at the tether arm of the Fe-S protein and may alter the rate of ubiquinone oxidation and the rate of superoxide production. Another cluster of substitutions near heme bH and the ubiquinone reduction site, Qi, may affect the rate of ubiquinone reduction and thus alter the rate of superoxide production. These results are compatible with the hypothesis that cytochrome b in humans (and other anthropoid primates) evolve to reduce the rate of production of superoxide thus enabling the exceptional longevity and exceptional cognitive ability of humans.

Leber hereditary optic neuropathy-new insights and old challenges

Leber hereditary optic neuropathy (LHON) is the most common primary mitochondrial DNA (mtDNA) disorder with the majority of patients harboring one of three primary mtDNA point mutations, namely, m.3460G>A (MTND1), m.11778G>A (MTND4), and m.14484T>C (MTND6). LHON is characterized by bilateral subacute loss of vision due to the preferential loss of retinal ganglion cells (RGCs) within the inner retina, resulting in optic nerve degeneration. This review describes the clinical features associated with mtDNA LHON mutations and recent insights gained into the disease mechanisms contributing to RGC loss in this mitochondrial disorder. Although treatment options remain limited, LHON research has now entered an active translational phase with ongoing clinical trials, including gene therapy to correct the underlying pathogenic mtDNA mutation.

Mitochondrial Genetic Heterogeneity in Leber's Hereditary Optic Neuropathy: Original Study with Meta-Analysis

Leber’s hereditary optic neuropathy (LHON) is a mitochondrial disorder that causes loss of central vision. Three primary variants (m.3460G>A, m.11778G>A, and m.14484T>C) and about 16 secondary variants are responsible for LHON in the majority of the cases. We investigated the complete mitochondrial DNA (mtDNA) sequences of 189 LHON patients and found a total of 54 disease-linked pathogenic variants. The primary variants m.11778G>A and m.14484T>C were accountable for only 14.81% and 2.64% cases, respectively. Patients with these two variants also possessed additional disease-associated variants. Among 156 patients who lacked the three primary variants, 16.02% harboured other LHON-associated variants either alone or in combination with other disease-associated variants. Furthermore, we observed that none of the haplogroups were explicitly associated with LHON. We performed a meta-analysis of m.4216T>C and m.13708G>A and found a significant association of these two variants with the LHON phenotype. Based on this study, we recommend the use of complete mtDNA sequencing to diagnose LHON, as we found disease-associated variants throughout the mitochondrial genome.

Next-generation sequencing of the whole mitochondrial genome identifies novel and common variants in patients with psoriasis, type 2 diabetes mellitus and psoriasis with comorbid type 2 diabetes mellitus

Recent studies have shown the role of mitochondrial DNA (mtDNA) variants in the pathogenesis of both psoriasis (Ps) and type 2 diabetes (T2D) amongst different ethnicities. However, no studies have investigated the mtDNA variants present in patients with Ps, T2D, and both Ps and T2D (Ps-T2D) in the Arab population. The entire mitochondrial genomes of Kuwaiti subjects with Ps, T2D, Ps-T2D and healthy controls were sequenced using Ion Torrent next-generation sequencing. A total of 36 novel mutations and 51 previously reported mutations were identified in the patient groups that were absent in the controls. Amongst the novel mutations, eight were non-synonymous and exhibited amino acid changes. Of these, two missense mutations (G5262A and A12397G) in the ND genes were detected in the Ps group and a C15735T missense mutation in the CYB gene was detected in Ps-T2D. Other known sequence variations were seen more frequently in all or certain patient groups compared with the controls (P<0.05). Additionally, the A8701G missense mutation in the ATPase 6 gene missense mutation was also observed in a higher frequency in the Ps group compared with the control. The present study is the first to perform a complete mitochondrial genome sequence analysis of Kuwaiti subjects with Ps, T2D and Ps-T2D, and both novel and known mtDNA variants were discovered. The patient-specific novel non-synonymous mutations may be co-responsible in the determination of these diseases. The higher frequency of certain mtDNA variants in the patients compared with the controls may suggest a role in predisposing patients to these diseases. Further functional analyses are required to reveal the role of the identified mutations in these disease conditions.

Mitochondrial genome variation in male LHON patients with the m.11778G > A mutation

Leber hereditary optic neuropathy (LHON) is a mitochondrial disorder with symptoms limited to a single tissue, optic nerve, resulting in vision loss. In the majority of cases it is caused by one of three point mutations in mitochondrial DNA (mtDNA) but their presence is not sufficient for disease development, since ~50% of men and ~10% women who carry them are affected. Thus additional modifying factors must exist. In this study, we use next generation sequencing to investigate the role of whole mtDNA variation in male Polish patients with LHON and m.11778G > A, the most frequent LHON mutation. We present a possible association between mtDNA haplogroup K and variants in its background, a combination of m.3480A > G, m.9055G > A, m.11299 T > C and m.14167C > T, and LHON mutation. These variants may have a negative effect on m.11778G > A increasing its penetrance and the risk of LHON in the Polish population. Surprisingly, we did not observe associations previously reported for m.11778G > A and LHON in European populations, particularly for haplogroup J as a risk factor, implying that mtDNA variation is much more complex. Our results indicate possible contribution of novel combination of mtDNA genetic factors to the LHON phenotype.

Neurodevelopmental regression, severe generalized dystonia, and metabolic acidosis caused by POLR3A mutations

OBJECTIVE

To expand the clinical phenotype of POLR3A mutations by assessing the functional consequences of a missense and a splicing acceptor mutation.

METHODS

We performed whole-exome sequencing for identification of likely pathogenic mutations in a 9-year-old female patient with severe generalized dystonia, metabolic acidosis, leukocytosis, hypotonia, and dysphagia. Brain MRI showed basal ganglia atrophy and presence of lactate and lipid peaks by [1H]-magnetic resonance spectroscopy. Expression levels of Pol III target genes were measured by quantitative real-time (qRT)-PCR to study the pathogenicity of the biallelic mutations in patient fibroblasts.

RESULTS

The patient is a compound heterozygous for a novel missense c.3721G>A (p.Val1241Met) and the splicing region c.1771-6C>G mutation in POLR3A, the gene coding for the catalytic subunit of RNA polymerase III (Pol III). Aberrant splicing was observed for the c.1771-6C>G mutation. Decreased RNA expression levels of Pol III targets (HNRNPH2, ubiquitin B, lactotransferrin, and HSP90AA1) were observed in patient fibroblasts with rescue to normal levels by overexpression of the wild-type protein but not by the p.Val1241Met variant.

CONCLUSIONS

Mutations in the POLR3A gene cause POLR3A-related hypomyelinating leukodystrophy with or without oligodontia or hypogonadotropic hypogonadism (HLD7, OMIM: 607694) and neonatal progeroid syndrome (OMIM: 264090), both with high phenotypic variability. We demonstrated the pathogenicity of c.1771-6C>G and c.3721G>A mutations causing an early-onset disorder. The phenotype of our patient expands the clinical presentation of POLR3A-related mutations and suggests a new classification that we propose designating as Neurodevelopmental Disorder with Regression, Abnormal Movements, and Increased Lactate.

Whole Mitochondrial Genome Analysis in Serbian Cases of Leber's Hereditary Optic Neuropathy

Leber’s hereditary optic neuropathy (LHON) is a maternally inherited disorder that affects central vision in young adults and is typically associated with mitochondrial DNA (mtDNA) mutations. This study is based on a mutational screening of entire mtDNA in eight Serbian probands clinically and genetically diagnosed with LHON and four of their family members, who are asymptomatic mutation carriers. All obtained sequence variants were compared to human mtDNA databases, and their potential pathogenic characteristics were assessed by bioinformatics tools. Mitochondrial haplogroup analysis was performed by MITOMASTER. Our study revealed two well-known primary LHON mutations, m.11778G>A and m.3460G>A, and one rare LHON mutation, m.8836A>G. Various secondary mutations were detected in association with the primary mutations. MITOMASTER analysis showed that the two well-known primary mutations belong to the R haplogroup, while the rare LHON m.8836A>G was detected within the N1b haplogroup. Our results support the need for further studies of genetic background and its role in the penetrance and severity of LHON.

Four novel mutations in the mitochondrial ND4 gene of complex I in patients with multiple sclerosis

Multiple sclerosis (MS) is an immune-mediated neurological, inflammatory disease of the central nervous system. Recent studies have suggested that genetic variants in mitochondrial DNA (mtDNA)-encoded complexes of respiratory chain, particularly, complex I (NADH dehydrogenase), contribute to the pathogenicity of MS among different ethnicities, and targeting mitochondrial function may represent a novel approach for MS therapy. In this study, we sequenced ND genes (ND1, ND2, ND3, ND4, ND4L, ND5 and ND6) encoding subunits of complex I in 124 subjects, 60 patients with relapsing-remitting MS and 64 healthy individuals, in order to identify potential novel mutations in these patients. We found several variants in ND genes in both the patients and controls, and specific variants only in patients with MS. While the majority of these variants were synonymous, 4 variants in the ND4 gene were identified as missense mutations in patients with MS. Of these, m.11150G>A was observed in one patient, whereas m.11519A>C, m.11523A>C and m.11527C>T were observed in another patient. Functional analysis predicted the mutations, m.11519A>C, m.11523A>C and m.11150G>A, as deleterious with a direct impact on ND4 protein stability and complex I function, whereas m.11527C>T mutation had no effect on ND4 protein stability. However, the 3 mutations, m.11519A>C, m.11523A>C and m.11527C>T, which were observed in the same patient, were predicted to cause a cumulative destabilizing effect on ND4 protein, and could thus disrupt complex I function. On the whole, this study identified 4 novel mutations in the mtDNA-encoded ND4 gene in patients with MS, which could lead to complex I dysfunction, and further confirmed the implication of mtDNA mutations in the pathogenicity of MS. The identified novel mutations in patients with MS may be ethnic-related and may prove to be significant in personalized treatment.

Mammalian Mitochondrial Complex I Structure and Disease-Causing Mutations

Complex I has an essential role in ATP production by coupling electron transfer from NADH to quinone with translocation of protons across the inner mitochondrial membrane. Isolated complex I deficiency is a frequent cause of mitochondrial inherited diseases. Complex I has also been implicated in cancer, ageing, and neurodegenerative conditions. Until recently, the understanding of complex I deficiency on the molecular level was limited due to the lack of high-resolution structures of the enzyme. However, due to developments in single particle cryo-electron microscopy (cryo-EM), recent studies have reported nearly atomic resolution maps and models of mitochondrial complex I. These structures significantly add to our understanding of complex I mechanism and assembly. The disease-causing mutations are discussed here in their structural context.

Mitochondrial G8292A and C8794T mutations in patients with Niemann-Pick disease type C

Niemann-Pick disease type C (NP-C) is a neurovisceral lipid storage disorder. At the cellular level, the disorder is characterized by accumulation of unesterified cholesterol and glycolipids in the lysosomal/late endosomal system. NP-C is transmitted in an autosomal recessive manner and is caused by mutations in either the NPC1 (95% of families) or NPC2 gene. The estimated disease incidence is 1 in 120,000 live births, but this likely represents an underestimate, as the disease may be under-diagnosed due to its highly heterogeneous presentation. Variants of adenosine triphosphatase (ATPase) subunit 6 and ATPase subunit 8 (ATPase6/8) in mitochondrial DNA (mtDNA) have been reported in different types of genetic diseases including NP-C. In the present study, the blood samples of 22 Iranian patients with NP-C and 150 healthy subjects as a control group were analyzed. The DNA of the blood samples was extracted by the salting out method and analyzed for ATPase6/8 mutations using polymerase chain reaction sequencing. Sequence variations in mitochondrial genome samples were determined via the Mitomap database. Analysis of sequencing data confirmed the existence of 11 different single nucleotide polymorphisms (SNPs) in patients with NP-C1. One of the most prevalent polymorphisms was the A8860G variant, which was observed in both affected and non-affected groups and determined to have no significant association with NP-C incidence. Amongst the 11 polymorphisms, only one was identified in the ATPase8 gene, while 9 including A8860G were observed in the ATPase6 gene. Furthermore, two SNPs, G8292A and C8792A, located in the non-coding region of mtDNA and the ATPase6 gene, respectively, exhibited significantly higher prevalence rates in NP-C1 patients compared with the control group (P<0.01). The present study suggests that there may be an association between mitochondrial ATPase6/8 mutations and the incidence of NP-C disease. In addition, the mitochondrial SNPs identified maybe pathogenic mutations involved in the development and prevalence of NP-C. Furthermore, these results suggest a higher occurrence of mutations in ATPase6 than in ATPase8 in NP-C patients.

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.

Age-Related Accumulation of Somatic Mitochondrial DNA Mutations in Adult-Derived Human iPSCs

The genetic integrity of iPSCs is an important consideration for therapeutic application. In this study, we examine the accumulation of somatic mitochondrial genome (mtDNA) mutations in skin fibroblasts, blood, and iPSCs derived from young and elderly subjects (24-72 years). We found that pooled skin and blood mtDNA contained low heteroplasmic point mutations, but a panel of ten individual iPSC lines from each tissue or clonally expanded fibroblasts carried an elevated load of heteroplasmic or homoplasmic mutations, suggesting that somatic mutations randomly arise within individual cells but are not detectable in whole tissues. The frequency of mtDNA defects in iPSCs increased with age, and many mutations were non-synonymous or resided in RNA coding genes and thus can lead to respiratory defects. Our results highlight a need to monitor mtDNA mutations in iPSCs, especially those generated from older patients, and to examine the metabolic status of iPSCs destined for clinical applications.

A somatic T15091C mutation in the Cytb gene of mouse mitochondrial DNA dominantly induces respiration defects

Our previous studies provided evidence that mammalian mitochondrial DNA (mtDNA) mutations that cause mitochondrial respiration defects behave in a recessive manner, because the induction of respiration defects could be prevented with the help of a small proportion (10%-20%) of mtDNA without the mutations. However, subsequent studies found the induction of respiration defects by the accelerated accumulation of a small proportion of mtDNA with various somatic mutations, indicating the presence of mtDNA mutations that behave in a dominant manner. Here, to provide the evidence for the presence of dominant mutations in mtDNA, we used mouse lung carcinoma P29 cells and examined whether some mtDNA molecules possess somatic mutations that dominantly induce respiration defects. Cloning and sequence analysis of 40-48 mtDNA molecules from P29 cells was carried out to screen for somatic mutations in protein-coding genes, because mutations in these genes could dominantly regulate respiration defects by formation of abnormal polypeptides. We found 108 missense mutations existing in one or more of 40-48 mtDNA molecules. Of these missense mutations, a T15091C mutation in the Cytb gene was expected to be pathogenic due to the presence of its orthologous mutation in mtDNA from a patient with cardiomyopathy. After isolation of many subclones from parental P29 cells, we obtained subclones with various proportions of T15091C mtDNA, and showed that the respiration defects were induced in a subclone with only 49% T15091C mtDNA. Because the induction of respiration defects could not be prevented with the help of the remaining 51% mtDNA without the T15091C mutation, the results indicate that the T15091C mutation in mtDNA dominantly induced the respiration defects.

Analysis of mitochondrial DNA in Tibetan gastric cancer patients at high altitude

The highest risk areas of gastric cancer are currently Japan, Korea and China; Qinghai, a high-altitude area, has one of the highest gastric cancer rates in China. The incidence of gastric cancer is higher in the Tibetan ethnic group compared to that in the Han ethnic group in Qinghai. This study was conducted to determine the clinical characteristics of mitochondrial DNA (mtDNA) mutations and copy numbers among Tibetans with gastric cancer residing at high altitudes and investigate the association between adaptations to hypoxic conditions and oncogenesis. A total of 23 Tibetan gastric cancer patients and 40 matched controls were recruited in this study. Leukocyte mtDNA genes and copy numbers were analyzed. The haplogroups were classified based on mitochondrial gene sequences. A total of 56.5% of the study participants had used alcohol at some point in their lives and 73.9% were positive for Helicobacter pylori (H. pylori). Eight mutations in 8 mitochondrial genes were identified in 43.4% of the Tibetan cancer patient group. There were no significant differences in leukocyte mtDNA copy number levels based on smoking status, alchohol consumption, obesity or H. pylori infection between the control and cancer groups. Statistical differences were also not found between gastric cancer patients with and those without mtDNA mutations. The majority of Tibetan patients with gastric cancer belonged to the mitochondrial haplogroup M9. In conclusion, Tibetans with gastric cancer residing at high altitudes exhibited a wide spectrum of mtDNA mutations. However, leukocyte mtDNA copy numbers in stage II gastric cancer were not statistically different compared to those in healthy Tibetans.

New MT-ND1 pathologic mutation for Leber hereditary optic neuropathy

BACKGROUND

Mutations causing Leber hereditary optic neuropathy are usually homoplasmic, show incomplete penetrance, and many of the affected positions are not well conserved through evolution. A large percentage of patients harbouring these mutations have no family history of disease. Moreover, the transfer of the mutation in the cybrid model is frequently not accompanied by the transfer of the cellular, biochemical and molecular phenotype. All these features make difficult their classification as the etiologic factors for this disease. We report a patient who exhibits typical clinical features of Leber hereditary optic neuropathy but lacks all three of the most common mitochondrial DNA mutations.

METHODS

The diagnosis was made based on clinical studies. The mitochondrial DNA was completely sequenced, and the candidate mutation was analysed in more than 18 000 individuals around the world, its conservation index was estimated in more than 3100 species from protists to mammals, its position was modelled in the crystal structure of a bacteria ortholog subunit, and its functional consequences were studied in a cybrid model.

RESULTS

Genetic analysis revealed an m.3472T>C transition in the MT-ND1 gene that changes a phenylalanine to leucine at position 56. Bioinformatics, molecular-genetic analysis and functional studies suggest that this transition is the etiological factor for the disorder.

CONCLUSIONS

This mutation expands the spectrum of deleterious changes in mitochondrial DNA-encoded complex I polypeptides associated with this pathology and highlights the difficulties in assigning pathogenicity to new homoplasmic mutations that show incomplete penetrance in sporadic Leber hereditary optic neuropathy patients.

The mitochondrial ATPase6 gene is more susceptible to mutation than the ATPase8 gene in breast cancer patients

Background

Breast cancer is the most common malignancy in women throughout the world. Mitochondria play important roles in cellular energy production, free radical generation and apoptosis. Identification of mitochondrial DNA mutations and/or polymorphisms as cancer biomarkers is rapidly developing in molecular oncology research.

Methods

In this study, the DNA alterations of the mitochondrial ATPase 6 and 8 genes were investigated in 49 breast cancer patients using PCR amplification and direct DNA sequencing on mtDNA. A possible association between these variants and tumorigenesis was assessed. Furthermore, the impact of non-synonymous substitutions on the amino acid sequence was evaluated using the PolyPhen-2 software.

Results

Twenty eight distinct somatic mitochondrial DNA variants were detected in tumor tissues but not in the corresponding adjacent non-tumor tissues. Among these variants, 9 were observed for the first time in breast cancer patients. The mtDNA variants of A8384 (T7A), T8567C (I14T), G8572A (G16S), A9041G (H172R) and G9055A (A177T) showed the most significant effects probably due to damaging changes to the resulting protein. Furthermore, non-synonymous amino acid changing variants were more frequent in the ATPase6 gene compared to the ATPase8 gene.

Conclusion

Our results showed that the ATPase6 gene is more susceptible to variations in breast cancer and may play an important role in tumorigenesis by changing the energy metabolism level in cancer cells.

Mitochondrial NADH dehydrogenase polymorphisms are associated with breast cancer in Poland

Complex I NADH-oxidoreductase-ubiquinone transports reducing equivalents from the reduced form of NADH to ubiquinone (coenzyme Q-CoQ). The purpose of this study was to analyze mutations in MT-ND1, MT-ND2, MT-ND3 and MT-ND6 genes and their effect on the biochemical properties, structure and functioning of proteins in patients with breast tumours. In research materials, in 50 patients, 28 total polymorphisms and five mutations were detected. Most detected polymorphisms (50 %, 14/28) were observed in MT-ND2 gene. Most of them were silent mutations. Five polymorphisms (m.G3916A, m.C4888T, m.A4918G, m.C5363T, m.C10283T) do not exist in the database. A total of five mutations in 13 patients (13/50) were detected, including two not described in the literature: m.C4987G and m.T10173C. It cannot be excluded that, through the mutations and polymorphism impact on the protein structure, they may cause mitochondrial dysfunction and contribute to the appearance of other changes in mtDNA. The results of our study indicate the presence of homological changes in the sequence of mtDNA in both breast cancer and in some mitochondrial diseases. Mutations in the examined genes in breast cancer may affect the cell and cause its dysfunction, as is the case in mitochondrial diseases.

Fifteen novel mutations in the mitochondrial NADH dehydrogenase subunit 1, 2, 3, 4, 4L, 5 and 6 genes from Iranian patients with Leber's hereditary optic neuropathy (LHON)

Leber's hereditary optic neuropathy (LHON) is an optic nerve dysfunction resulting from mutations in mitochondrial DNA (mtDNA), which is transmitted in a maternal pattern of inheritance. It is caused by three primary point mutations: G11778A, G3460A and T14484C; in the mitochondrial genome. These mutations are sufficient to induce the disease, accounting for the majority of LHON cases, and affect genes that encode for the different subunits of mitochondrial complexes I and III of the mitochondrial respiratory chain. Other mutations are secondary mutations associated with the primary mutations. The purpose of this study was to determine MT-ND variations in Iranian patients with LHON. In order to determine the prevalence and distribution of mitochondrial mutations in the LHON patients, their DNA was studied using PCR and DNA sequencing analysis. Sequencing of MT-ND genes from 35 LHON patients revealed a total of 44 nucleotide variations, in which fifteen novel variations-A14020G, A13663G, C10399T, C4932A, C3893G, C10557A, C12012A, C13934T, G4596A, T12851A, T4539A, T4941A, T13255A, T14353C and del A 4513-were observed in 27 LHON patients. However, eight patients showed no variation in the ND genes. These mutations contribute to the current database of mtDNA polymorphisms in LHON patients and may facilitate the definition of disease-related mutations in human mtDNA. This research may help to understand the disease mechanism and open up new diagnostic opportunities for LHON.

Frequency and pattern of heteroplasmy in the complete human mitochondrial genome

Determining the levels of human mitochondrial heteroplasmy is of utmost importance in several fields. In spite of this, there are currently few published works that have focused on this issue. In order to increase the knowledge of mitochondrial DNA (mtDNA) heteroplasmy, the main goal of this work is to investigate the frequency and the mutational spectrum of heteroplasmy in the human mtDNA genome. To address this, a set of nine primer pairs designed to avoid co-amplification of nuclear DNA (nDNA) sequences of mitochondrial origin (NUMTs) was used to amplify the mitochondrial genome in 101 individuals. The analysed individuals represent a collection with a balanced representation of genders and mtDNA haplogroup distribution, similar to that of a Western European population. The results show that the frequency of heteroplasmic individuals exceeds 61%. The frequency of point heteroplasmy is 28.7%, with a widespread distribution across the entire mtDNA. In addition, an excess of transitions in heteroplasmy were detected, suggesting that genetic drift and/or selection may be acting to reduce its frequency at population level. In fact, heteroplasmy at highly stable positions might have a greater impact on the viability of mitochondria, suggesting that purifying selection must be operating to prevent their fixation within individuals. This study analyses the frequency of heteroplasmy in a healthy population, carrying out an evolutionary analysis of the detected changes and providing a new perspective with important consequences in medical, evolutionary and forensic fields.

Respiratory complex III dysfunction in humans and the use of yeast as a model organism to study mitochondrial myopathy and associated diseases

The bc1 complex or complex III is a central component of the aerobic respiratory chain in prokaryotic and eukaryotic organisms. It catalyzes the oxidation of quinols and the reduction of cytochrome c, establishing a proton motive force used to synthesize adenosine triphosphate (ATP) by the F1Fo ATP synthase. In eukaryotes, the complex III is located in the inner mitochondrial membrane. The genes coding for the complex III have a dual origin. While cytochrome b is encoded by the mitochondrial genome, all the other subunits are encoded by the nuclear genome. In this review, we compile an exhaustive list of the known human mutations and associated pathologies found in the mitochondrially-encoded cytochrome b gene as well as the fewer mutations in the nuclear genes coding for the complex III structural subunits and accessory proteins such as BCS1L involved in the assembly of the complex III. Due to the inherent difficulties of studying human biopsy material associated with complex III dysfunction, we also review the work that has been conducted to study the pathologies with the easy to handle eukaryotic microorganism, the yeast Saccharomyces cerevisiae. Phenotypes, biochemical data and possible effects due to the mutations are also discussed in the context of the known three-dimensional structure of the eukaryotic complex III. This article is part of a Special Issue entitled: Respiratory complex III and related bc complexes.

Rare primary mitochondrial DNA mutations and probable synergistic variants in Leber's hereditary optic neuropathy

BACKGROUND

Leber's hereditary optic neuropathy (LHON) is a maternally inherited blinding disorder, which in over 90% of cases is due to one of three primary mitochondrial DNA (mtDNA) point mutations (m.11778G>A, m.3460G>A and m.14484T>C, respectively in MT-ND4, MT-ND1 and MT-ND6 genes). However, the spectrum of mtDNA mutations causing the remaining 10% of cases is only partially and often poorly defined.

METHODOLOGY/PRINCIPAL FINDINGS

In order to improve such a list of pathological variants, we completely sequenced the mitochondrial genomes of suspected LHON patients from Italy, France and Germany, lacking the three primary common mutations. Phylogenetic and conservation analyses were performed. Sixteen mitochondrial genomes were found to harbor at least one of the following nine rare LHON pathogenic mutations in genes MT-ND1 (m.3700G>A/p.A132T, m.3733G>A-C/p.E143K-Q, m.4171C>A/p.L289M), MT-ND4L (m.10663T>C/p.V65A) and MT-ND6 (m.14459G>A/p.A72V, m.14495A>G/p.M64I, m.14482C>A/p.L60S, and m.14568C>T/p.G36S). Phylogenetic analyses revealed that these substitutions were due to independent events on different haplogroups, whereas interspecies comparisons showed that they affected conserved amino acid residues or domains in the ND subunit genes of complex I.

CONCLUSIONS/SIGNIFICANCE

Our findings indicate that these nine substitutions are all primary LHON mutations. Therefore, despite their relative low frequency, they should be routinely tested for in all LHON patients lacking the three common mutations. Moreover, our sequence analysis confirms the major role of haplogroups J1c and J2b (over 35% in our probands versus 6% in the general population of Western Europe) and other putative synergistic mtDNA variants in LHON expression.

Mitochondrial DNA sequence variation is associated with free-living activity energy expenditure in the elderly

The decline in activity energy expenditure underlies a range of age-associated pathological conditions, neuromuscular and neurological impairments, disability, and mortality. The majority (90%) of the energy needs of the human body are met by mitochondrial oxidative phosphorylation (OXPHOS). OXPHOS is dependent on the coordinated expression and interaction of genes encoded in the nuclear and mitochondrial genomes. We examined the role of mitochondrial genomic variation in free-living activity energy expenditure (AEE) and physical activity levels (PAL) by sequencing the entire (~16.5 kilobases) mtDNA from 138 Health, Aging, and Body Composition Study participants. Among the common mtDNA variants, the hypervariable region 2 m.185G>A variant was significantly associated with AEE (p=0.001) and PAL (p=0.0005) after adjustment for multiple comparisons. Several unique nonsynonymous variants were identified in the extremes of AEE with some occurring at highly conserved sites predicted to affect protein structure and function. Of interest is the p.T194M, CytB substitution in the lower extreme of AEE occurring at a residue in the Qi site of complex III. Among participants with low activity levels, the burden of singleton variants was 30% higher across the entire mtDNA and OXPHOS complex I when compared to those having moderate to high activity levels. A significant pooled variant association across the hypervariable 2 region was observed for AEE and PAL. These results suggest that mtDNA variation is associated with free-living AEE in older persons and may generate new hypotheses by which specific mtDNA complexes, genes, and variants may contribute to the maintenance of activity levels in late life.

The altered activity of complex III may contribute to the high penetrance of Leber's hereditary optic neuropathy in a Chinese family carrying the ND4 G11778A mutation

The ND4 G11778A mutation is the most common mitochondrial DNA mutation leading to Leber's hereditary optic neuropathy (LHON). Despite considerable clinical evidences, the modifier role of nuclear background and mitochondrial haplotypes in phenotypic manifestation of LHON remains poorly understood. We investigated the effect of these modifiers on bioenergetics in lymphoblastoid cell lines derived from five affected subjects of one Chinese family carrying the G11778A mutation and five Chinese controls. Significant reductions in the activities of complexes I and III were observed in mutant cell lines from the Chinese family, whereas the mutant cell lines from other families carrying the same mutation exhibited only reduced activity of complex I. The reduced activities of complexes I and III caused remarkably higher reductions of ATP synthesis in mutant cell lines from the Chinese family than those from other families. The deficient respiration increased generation of reactive oxygen species. The defect in complex III activity, likely resulting from the mitochondrial haplotype or nuclear gene alteration, worsens mitochondrial dysfunction caused by the G11778A mutation, thereby causing extremely high penetrance and expressivity of optic neuropathy in this Chinese family. Our data provide the first experimental evidence that altered activity of complex III modulates the phenotypic manifestation of LHON-associated G11778A mutation. Thus, our findings may provide new insights into the pathophysiology of LHON.

Is 8860 variation a rare polymorphism or associated as a secondary effect in HCM disease?

INTRODUCTION

mtDNA defects, both deletions and point mutations, have been associated with hypertrophic cardiomyopathies. The aim of this study was to establish a spectrum for mtDNA mutations in Iranian hypertrophic cardiomyopathy (HCM) patients.

MATERIAL AND METHODS

The control group was chosen among the special medical centre visitors who did not have hypertrophic cardiomyopathy or any related heart disease. Hypertrophic cardiomyopathy (HCM) is widely accepted as a pluricausal or multifactorial disease. Because of the linkage between energy metabolism in the mitochondria and cardiac muscle contraction, it is reasonable to assume that mitochondrial abnormalities may be responsible for some forms of HCM. Point mutations and deletions in the two hot spot regions of mtDNA were investigated by PCR and sequencing methods.

RESULTS

Some unreported point mutations have been found in this study but no deletion was detected. Meanwhile some of these point mutations have been investigated among HCM patients for the first time.

CONCLUSIONS

A8860G transition was detected in a high proportion, raising the question whether this rare polymorphism is associated as a secondary effect in HCM disease.

Mitochondrial optic neuropathies - disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

Leber's Hereditary Optic Neuropathy-Gene Therapy: From Benchtop to Bedside

Leber's hereditary optic neuropathy (LHON) is a maternally transmitted disorder caused by point mutations in mitochondrial DNA (mtDNA). Most cases are due to mutations in genes encoding subunits of the NADH-ubiquinone oxidoreductase that is Complex I of the electron transport chain (ETC). These mutations are located at nucleotide positions 3460, 11778, or 14484 in the mitochondrial genome. The disease is characterized by apoplectic, bilateral, and severe visual loss. While the mutated mtDNA impairs generation of ATP by all mitochondria, there is only a selective loss of retinal ganglion cells and degeneration of optic nerve axons. Thus, blindness is typically permanent. Half of the men and 10% of females who harbor the pathogenic mtDNA mutation actually develop the phenotype. This incomplete penetrance and gender bias is not fully understood. Additional mitochondrial and/or nuclear genetic factors may modulate the phenotypic expression of LHON. In a population-based study, the mtDNA background of haplogroup J was associated with an inverse relationship of low-ATP generation and increased production of reactive oxygen species (ROS). Effective therapy for LHON has been elusive. In this paper, we describe the findings of pertinent published studies and discuss the controversies of potential strategies to ameliorate the disease.

Mitochondrial haplogroups are associated with risk of neuroretinal disorder in HIV-positive patients

Although highly active antiretroviral therapy has improved survivorship dramatically and decreased the incidence of cytomegalovirus retinitis among patients with AIDS, other ophthalmic complications continue to occur. One complication observed in approximately 12% of HIV-infected patients is a presumed neuroretinal disorder (NRD), manifested as decreased contrast sensitivity and associated with vague subjective complaints of hazy vision. Pathologically, patients with AIDS even without ocular opportunistic infections have loss of optic nerve axons, suggestive of mitochondrial dysfunction. We explored whether variation in mitochondrial DNA was associated with time to NRD in HIV-infected patients in the Longitudinal Study of Ocular Complications of AIDS cohort. Within the Western European, or "N", mitochondrial DNA macrohaplogroup, haplogroup J, was associated with 80% decrease in the risk of progression to NRD during the study (hazard ratio = 0.20, P = 0.039) and suggested an independent association with protection against NRD in a cross-section of all patients taken at enrollment (1.5% vs. 8.9% in patients with vs. without haplogroup J, respectively, P = 0.05). These data suggest that mitochondrial genotype may influence propensity to develop HIV-associated NRD in patients with AIDS.

Specific polymorphic variation in the mitochondrial genome and increased in-hospital mortality after severe trauma

OBJECTIVE

To determine whether specific genetic variations in the mtDNA that impact energy production and free-radical generation are potential new risk factors for in-hospital mortality after severe trauma.

SUMMARY BACKGROUND DATA

Each of the 3 mitochondrial DNA polymorphisms selected for this study (at positions 4216, 10398, 4917) alter the amino acid sequence of different key subunits of Complex I in the electron transport chain. They have been previously implicated in phenotypes involving tissues with high-energy demand, such as the brain and retina.

METHODS

Seven hundred forty-five consecutive patients admitted to the trauma intensive care unit at Vanderbilt University Medical Center between April 11, 2005, and February 27, 2006, were potentially eligible for this study. Under an Institutional Review Board-approved protocol (which excluded patients <18 years of age and prisoners), 666 patients had DNA extracted from a blood sample. Detailed demographic and clinical covariates were also obtained (including age, gender, ethnicity, lactate measurements, and injury severity score). A flurogenic 5' nuclease allelic discrimination Taqman assay and the ABI 7900HT Sequence Detection System (v2.1) was used to genotype the T4216C, A10398G, and A4917G polymorphisms. The primary outcome was in-hospital mortality.

RESULTS

Multivariate logistic regression analysis revealed that the 4216T allele was a significant independent predictor of in-hospital mortality (OR = 2.63, 95% CI 1.14-6.07, P = 0.02) after adjustment for age, gender, injury severity score, highest lactate level, mechanism of injury, and the 10398 polymorphism.

CONCLUSIONS

Variation in the mtDNA, specifically the 4216T allele, appears to increase the risk of in-hospital mortality after severe injury.

Analysis of mitochondrial DNA sequences in childhood encephalomyopathies reveals new disease-associated variants

Background

Mitochondrial encephalomyopathies are a heterogeneous group of clinical disorders generally caused due to mutations in either mitochondrial DNA (mtDNA) or nuclear genes encoding oxidative phosphorylation (OXPHOS). We analyzed the mtDNA sequences from a group of 23 pediatric patients with clinical and morphological features of mitochondrial encephalopathies and tried to establish a relationship of identified variants with the disease.

Methodology/Principle Findings

Complete mitochondrial genomes were amplified by PCR and sequenced by automated DNA sequencing. Sequencing data was analyzed by SeqScape software and also confirmed by BLASTn program. Nucleotide sequences were compared with the revised Cambridge reference sequence (CRS) and sequences present in mitochondrial databases. The data obtained shows that a number of known and novel mtDNA variants were associated with the disease. Most of the non-synonymous variants were heteroplasmic (A4136G, A9194G and T11916A) suggesting their possibility of being pathogenic in nature. Some of the missense variants although homoplasmic were showing changes in highly conserved amino acids (T3394C, T3866C, and G9804A) and were previously identified with diseased conditions. Similarly, two other variants found in tRNA genes (G5783A and C8309T) could alter the secondary structure of Cys-tRNA and Lys-tRNA. Most of the variants occurred in single cases; however, a few occurred in more than one case (e.g. G5783A and A10149T).

Conclusions and Significance

The mtDNA variants identified in this study could be the possible cause of mitochondrial encephalomyopathies with childhood onset in the patient group. Our study further strengthens the pathogenic score of known variants previously reported as provisionally pathogenic in mitochondrial diseases. The novel variants found in the present study can be potential candidates for further investigations to establish the relationship between their incidence and role in expressing the disease phenotype. This study will be useful in genetic diagnosis and counseling of mitochondrial diseases in India as well as worldwide.

Epidemiology and penetrance of Leber hereditary optic neuropathy in Finland

We have performed an entire-population-based survey of the epidemiology and penetrance of Leber hereditary optic neuropathy (LHON) in Finland - a country that is among the best-studied genetic isolates in the world. During our long-term clinical follow-up period since 1970, we have so far identified 36 LHON families in Finland, comprised of almost 1000 family members. Counting the unaffected family members has been possible thanks to accessible genealogical records, and this has improved the accuracy of our penetrance figures by minimizing the sample bias. Our results, although confirming some well-known features of LHON, indicate that the overall penetrance of LHON is lower than previously estimated, and that affected females have a higher incidence of affected offspring compared to the unaffected females. The prevalence of LHON in Finland is 1:50 000, and one in 9000 Finns is a carrier of one of the three LHON primary mutations.

High penetrance of sequencing errors and interpretative shortcomings in mtDNA sequence analysis of LHON patients

For identifying mutation(s) that are potentially pathogenic it is essential to determine the entire mitochondrial DNA (mtDNA) sequences from patients suffering from a particular mitochondrial disease, such as Leber hereditary optic neuropathy (LHON). However, such sequencing efforts can, in the worst case, be riddled with errors by imposing phantom mutations or misreporting variant nucleotides, and moreover, by inadvertently regarding some mutations as novel and pathogenic, which are actually known to define minor haplogroups. Under such circumstances it remains unclear whether the disease-associated mutations would have been determined adequately. Here, we re-analyse four problematic LHON studies and propose guidelines by which some of the pitfalls could be avoided.

Leber's hereditary optic neuropathy: a case report

BACKGROUND

Leber's hereditary optic neuropathy (LHON) is a bilateral optic neuropathy of mitochondrial inheritance that produces significant painless, central vision loss and dyschromatopsia. LHON usually occurs in young males between the ages of 15 and 30 years and manifests an episode of subacute or acute vision loss in one eye, with the opposite eye becoming involved weeks to months later. Approximately 80% to 90% of all LHON patients are male. While the disease usually presents itself around the third decade of life, its onset ranges anywhere from 5 to 80 years.

CASE REPORT

We report a case of an uncooperative 12-year-old Hispanic boy who was brought to our group practice following referral from an outside optometrist for amblyopia therapy. Following the workup by the binocular vision clinician, a neuro-ophthalmic consultation was obtained, eventually leading to the diagnosis and confirmation of LHON.

CONCLUSION

Leber's hereditary optic neuropathy may manifest signs and symptoms that mimic common ophthalmic entities. Teenage males often are reluctant to report its subtle clinical findings, making its discovery even more challenging. LHON should be kept in mind as a possibility for anyone who manifests unexplained visual loss.

Amélioration rapide de la fonction visuelle après perte visuelle aiguë secondaire à une neuropathie optique de Leber

INTRODUCTION

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder affecting the optic nerves in which the typical clinical presentation is subacute, painless, sequential visual loss in young adult males. Patients with LHON who have atypical clinical features may be initially misdiagnosed.

OBSERVATION

An 8-year-old boy developed an acute severe bilateral optic neuropathy associated with pain and mild optic disc edema. Molecular genetic testing of his mitochondrial DNA revealed two point mutations, T14484C and G15257A. His vision began to improve within one month of onset of visual loss and eventually recovered to 20/15 in both eyes. Four years previously, his oldest sister had acutely lost vision in both eyes at age 12 years. Her young age, female gender, the bilateral visual loss associated with pain, optic disc edema, absent family history of visual loss and negative workup were felt to be most consistent with a clinical diagnosis of idiopathic optic neuritis of childhood. Her visual recovery which began within two months of visual loss further supported the diagnosis. She was retrospectively re-diagnosed with LHON after her younger brother was genetically confirmed.

CONCLUSION

We describe two siblings with LHON whose time course from onset of visual loss to onset of visual recovery was unusually rapid. Because of other atypical features for LHON such as their young age at presentation, bilateral simultaneous visual loss and associated periorbital pain, their clinical profile appeared more typical of a demyelinating disease such as childhood optic neuritis. The first affected sibling (sister) was initially misdiagnosed. This report emphasizes that the clinical spectrum of LHON is variable and thus, LHON should be considered in any patient with an acute bilateral optic neuropathy, even in the absence of a positive family history. Correct diagnosis of this maternally inherited disorder is important for assessment of visual prognosis and appropriate genetic counseling.

The ND1 gene of complex I is a mutational hot spot for Leber's hereditary optic neuropathy

A novel mitochondrial DNA (mtDNA) transition (3733G-->A) inducing the E143 K amino acid change at a very conserved site of the NADH dehydrogenase subunit 1 (ND1) was identified in a family with six maternally related individuals with Leber's hereditary optic neuropathy (LHON) and in an unrelated sporadic case, all negative for known mutations and presenting with the canonical phenotype. The transition was not detected in 1,082 control mtDNAs and was heteroplasmic in several individuals from both pedigrees. In addition, the mtDNAs of the two families were found to belong to different haplogroups (H and X), thus confirming that the 3733G-->A mutation occurred twice independently. Phosphorus magnetic resonance spectroscopy disclosed an in vivo brain and skeletal muscle energy metabolism deficit in the four examined patients. Muscle biopsy from two patients showed slight mitochondrial proliferation with abnormal mitochondria. Biochemical investigations in platelets showed partially insensitive complex I to rotenone inhibition. We conclude that the 3733G-->A transition is a novel cause of LHON and, after those at positions 3460 and 4171, is the third ND1 mutation to be identified in multiple unrelated families. This finding shows that, in addition to ND6, the ND1 subunit gene is also a mutational hot spot for LHON.

Sequence analysis of the complete mitochondrial genome in patients with mitochondrial encephaloneuromyopathies lacking the common pathogenic DNA mutations

The purpose of this study was to identify novel mitochondrial deoxyribonucleic acid (mtDNA) mutations in a series of patients with clinical and/or morphological features of mitochondrial dysfunction, but still no genetic diagnosis. A heterogeneous group of clinical disorders is caused by mutations in mtDNA that damage respiratory chain function of cell energy production. We developed a method to systematically screen the entire mitochondrial genome. The sequence-data were obtained with a rapid automated system. In the six mitochondrial genomes analysed we found 20 variants of the revised Cambridge reference sequence [Nat. Genet. 23 (1999) 147]. In skeletal muscle nineteen novel mtDNA variants were homoplasmic, suggesting secondary pathogenicity or co-responsibility in determination of the disease. In one patient we identified a novel heteroplasmic mtDNA mutation which presumably has a pathogenic role. This screening is therefore useful to extend the mtDNA polymorphism database and should facilitate definition of disease-related mutations in human mtDNA.

Techniques and pitfalls in the detection of pathogenic mitochondrial DNA mutations

Mutations in the mitochondrial DNA (mtDNA) are now recognized as major contributors to human pathologies and possibly to normal aging. A large number of rearrangements and point mutations in protein coding and tRNA genes have been identified in patients with mitochondrial disorders. In this review, we discuss genotype-phenotype correlations in mitochondrial diseases and common techniques used to identify pathogenic mtDNA mutations in human tissues. Although most of these approaches employ standard molecular biology tools, the co-existence of wild-type and mutated mtDNA (mtDNA heteroplasmy) in diseased tissues complicates both the detection and accurate determination of the size of the mutated fractions. To address these problems, novel approaches were developed and are discussed in this review.

Mitochondrial dysfunction as a cause of optic neuropathies

Mitochondria are increasingly recognized as central players in the life and death of cells and especially of neurons. The energy-dependence of retinal ganglion cells (RGC) and their axons, which form the optic nerve, is singularly skewed. In fact, while mitochondria are very abundant in the initial, unmyelinated part of the axons anterior to the lamina cribrosa, their number suddenly decreases as the myelin sheath begins more posteriorly. The vascular system also presents different blood-brain barrier properties anterior and posterior to the lamina, possibly reflecting the different metabolic needs of the optic nerve head (unmyelinated) and of the retrobulbar optic nerve (myelinated). Mitochondrial biogenesis occurs within the cellular somata of RGC in the retina. It needs the coordinated interaction of nuclear and mitochondrial genomes. Mitochondria are then transported down the axons and distributed where they are needed. These locations are along the unmyelinated portion of the nerve, under the nodes of Ranvier in the retrobulbar nerve, and at the synaptic terminals. Efficient transportation of mitochondria depends on multiple factors, including their own energy production, the integrity of the cytoskeleton and its protein components (tubulin, etc.), and adequate myelination of the axons. Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria. Leber's hereditary optic neuropathy (LHON) is the paradigm of mitochondrial optic neuropathies where a primary role for mitochondrial dysfunction is certified by maternal inheritance and association with specific mutations in the mitochondrial DNA (mtDNA). Clinical phenocopies of this pathology are represented by the wide array of optic neuropathies associated with vitamin depletion, toxic exposures, alcohol and tobacco abuse, and use of certain drugs. Moreover, the recent identification of mutations in the nuclear gene OPA1 as the causative factor in dominant optic atrophy (DOA, Kjer's type) brought the unexpected finding that this gene encodes for a mitochondrial protein, suggesting that DOA and LHON may be linked by similar pathogenesis. Polymorphisms in this very same gene may be associated with normal tension glaucoma (NTG), which might be considered a genetically determined optic neuropathy that again shows similarities with both LHON and DOA. Exciting new developments come from first examples of mitochondrial optic neuropathies in animal models that are genetically determined or are the result of ingenious engineering of mitochondrial gene expression, or from biochemical manipulations of the respiratory complexes. Even more exciting is the first successful attempt to correct the LHON-related complex I dysfunction by the allotopic nuclear expression of the recoded mitochondrial gene. There is hope that the genetic complexities, biochemical dysfunctions, and integrated anatomical-physiological cellular relationships will soon be precisely delineated and that promising therapeutic and prophylactic strategies will be proposed.

Clinical variability in maternally inherited leber hereditary optic neuropathy with the G14459A mutation

Spasticity and dystonia have been associated with mitochondrial (mt) DNA mutations at A11696G, G14459A, and T14596A. We describe the clinical features and molecular analysis of two Caucasian pedigrees with the 14,459 guanosine (G) --> adenine (A) transition. The maternally inherited Leber hereditary optic neuropathy (LHON) phenotypes showed extreme clinical variability and the only screening test that was abnormal in the patient with spasticity/dystonia was a high T2 signal in the putamen bilaterally. The male patient in the second pedigree showed features of optic neuropathy without spasticity/dystonia. These results further support that the 14,459 G --> A transition mutation is causally related to LHON and spasticity/dystonia.