Marc Ferré

Hereditary optic neuropathies share a common mitochondrial coupling defect

Hereditary optic neuropathies are heterogeneous diseases characterized by the degeneration of retinal ganglion cells leading to optic nerve atrophy and impairment of central vision. We found a common coupling defect of oxidative phosphorylation in fibroblasts of patients affected by autosomal dominant optic atrophy (mutations of OPA1), autosomal dominant optic atrophy associated with cataract (mutations of OPA3), and Lebers hereditary optic neuropathy, a disorder associated with point mutations of mitochondrial DNA complex I genes. Interestingly, the energetic defect was significantly more pronounced in Lebers hereditary optic neuropathy and autosomal dominant optic atrophy patients with a more complex phenotype, the so‐called plus phenotype. Ann Neurol 2008

Molecular screening of 980 cases of suspected hereditary optic neuropathy with a report on 77 novel OPA1 mutations.

We report the results of molecular screening in 980 patients carried out as part of their work‐up for suspected hereditary optic neuropathies. All the patients were investigated for Lebers hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (ADOA), by searching for the ten primary LHON‐causing mtDNA mutations and examining the entire coding sequences of the OPA1 and OPA3 genes, the two genes currently identified in ADOA. Molecular defects were identified in 440 patients (45% of screened patients). Among these, 295 patients (67%) had an OPA1 mutation, 131 patients (30%) had an mtDNA mutation, and 14 patients (3%), belonging to three unrelated families, had an OPA3 mutation. Interestingly, OPA1 mutations were found in 157 (40%) of the 392 apparently sporadic cases of optic atrophy. The eOPA1 locus‐specific database now contains a total of 204 OPA1 mutations, including 77 novel OPA1 mutations reported here. The statistical analysis of this large set of mutations has led us to propose a diagnostic strategy that should help with the molecular work‐up of optic neuropathies. Our results highlight the importance of investigating LHON‐causing mtDNA mutations as well as OPA1 and OPA3 mutations in cases of suspected hereditary optic neuropathy, even in absence of a family history of the disease.

Mitochondrial coupling defect in Charcot-Marie-Tooth type 2A disease.

Mutations of the mitofusin 2 gene (MFN2) may account for at least a third of the cases of Charcot–Marie–Tooth disease type 2 (CMT2). This study investigates mitochondrial cellular bioenergetics in MFN2‐related CMT2A.

Resveratrol induces a mitochondrial complex I-dependent increase in NADH oxidation responsible for sirtuin activation in liver cells.

Background: The mechanism of action of resveratrol on sirtuin and mitochondrial metabolism remains elusive. Results: Resveratrol increases the mitochondrial NAD+ level by direct stimulation of complex I, leading to a SIRT3-dependent increase in substrate supplies. Conclusion: Our results link the direct stimulation by resveratrol of NADH oxidation to the SIRT3 activation. Significance: Mitochondrial NAD+/NADH ratio is a critical parameter mediating resveratrol effect on mitochondrial function. Resveratrol (RSV) has been shown to be involved in the regulation of energetic metabolism, generating increasing interest in therapeutic use. SIRT1 has been described as the main target of RSV. However, recent reports have challenged the hypothesis of its direct activation by RSV, and the signaling pathways remain elusive. Here, the effects of RSV on mitochondrial metabolism are detailed both in vivo and in vitro using murine and cellular models and isolated enzymes. We demonstrate that low RSV doses (1–5 μm) directly stimulate NADH dehydrogenases and, more specifically, mitochondrial complex I activity (EC50 ∼1 μm). In HepG2 cells, this complex I activation increases the mitochondrial NAD+/NADH ratio. This higher NAD+ level initiates a SIRT3-dependent increase in the mitochondrial substrate supply pathways (i.e. the tricarboxylic acid cycle and fatty acid oxidation). This effect is also seen in liver mitochondria of RSV-fed animals (50 mg/kg/day). We conclude that the increase in NADH oxidation by complex I is a crucial event for SIRT3 activation by RSV. Our results open up new perspectives in the understanding of the RSV signaling pathway and highlight the critical importance of RSV doses used for future clinical trials.

Mitochondrial complex I deficiency in GDAP1-related autosomal dominant Charcot-Marie-Tooth disease (CMT2K)

Mutations in GDAP1, an outer mitochondrial membrane protein responsible for recessive Charcot-Marie-Tooth disease (CMT4A), have also been associated with CMT2K, a dominant form of the disease. The three CMT2K patients we studied carried a novel dominant GDAP1 mutation, C240Y (c.719G > A). Mitochondrial respiratory chain complex I activity in fibroblasts from CMT2K patients was 40% lower than in controls, whereas the tubular mitochondria were 33% larger in diameter and the mitochondrial mass was 20% greater. Thus, besides the regulatory role GDAP1 plays in mitochondrial network dynamics, it may also be involved in energy production and in the control of mitochondrial volume.

Hereditary spastic paraplegia-like disorder due to a mitochondrial ATP6 gene point mutation.

Hereditary spastic paraplegia refers to a genetically heterogeneous syndrome. We identified five members of a family suffering from a late-onset spastic paraplegia-like disorder, carrying the homoplasmic m.9176 T>C mutation in the mitochondrial ATP6 gene. The clinical severity of the disease observed in the family was correlated with the biochemical and assembly defects of the ATP synthase. The m.9176 T>C mutation has been previously associated to Leigh syndrome or familial bilateral striatal necrosis. Other factors such as modifying genes may be involved in the phenotypic expression of the disease. The family belongs to the mitochondrial haplogroup J, previously shown to play a role in modulating the phenotype of mitochondrial diseases and be associated with longevity. Moreover other nuclear modifying genes or environmental factors may contribute to the disease phenotype. This finding extends the genetic heterogeneity of the hereditary spastic paraplegia together with the clinical spectrum of mutations of the ATP6 gene.

Prevalence of rare mitochondrial DNA mutations in mitochondrial disorders

Abstract Background Mitochondrial DNA (mtDNA) diseases are rare disorders whose prevalence is estimated around 1 in 5000. Patients are usually tested only for deletions and for common mutations of mtDNA which account for 5–40% of cases, depending on the study. However, the prevalence of rare mtDNA mutations is not known. Methods We analysed the whole mtDNA in a cohort of 743 patients suspected of manifesting a mitochondrial disease, after excluding deletions and common mutations. Both heteroplasmic and homoplasmic variants were identified using two complementary strategies (Surveyor and MitoChip). Multiple correspondence analyses followed by hierarchical ascendant cluster process were used to explore relationships between clinical spectrum, age at onset and localisation of mutations. Results 7.4% of deleterious mutations and 22.4% of novel putative mutations were identified. Pathogenic heteroplasmic mutations were more frequent than homoplasmic mutations (4.6% vs 2.8%). Patients carrying deleterious mutations showed symptoms before 16 years of age in 67% of cases. Early onset disease (<1 year) was significantly associated with mutations in protein coding genes (mainly in complex I) while late onset disorders (>16 years) were associated with mutations in tRNA genes. MTND5 and MTND6 genes were identified as ‘hotspots’ of mutations, with Leigh syndrome accounting for the large majority of associated phenotypes. Conclusions Rare mitochondrial DNA mutations probably account for more than 7.4% of patients with respiratory chain deficiency. This study shows that a comprehensive analysis of mtDNA is essential, and should include young children, for an accurate diagnosis that is now accessible with the development of next generation sequencing technology.

Loss-of-function mutations in WDR73 are responsible for microcephaly and steroid-resistant nephrotic syndrome: Galloway-Mowat syndrome.

Galloway-Mowat syndrome is a rare autosomal-recessive condition characterized by nephrotic syndrome associated with microcephaly and neurological impairment. Through a combination of autozygosity mapping and whole-exome sequencing, we identified WDR73 as a gene in which mutations cause Galloway-Mowat syndrome in two unrelated families. WDR73 encodes a WD40-repeat-containing protein of unknown function. Here, we show that WDR73 was present in the brain and kidney and was located diffusely in the cytoplasm during interphase but relocalized to spindle poles and astral microtubules during mitosis. Fibroblasts from one affected child and WDR73-depleted podocytes displayed abnormal nuclear morphology, low cell viability, and alterations of the microtubule network. These data suggest that WDR73 plays a crucial role in the maintenance of cell architecture and cell survival. Altogether, WDR73 mutations cause Galloway-Mowat syndrome in a particular subset of individuals presenting with late-onset nephrotic syndrome, postnatal microcephaly, severe intellectual disability, and homogenous brain MRI features. WDR73 is another example of a gene involved in a disease affecting both the kidney glomerulus and the CNS.

Idebenone increases mitochondrial complex I activity in fibroblasts from LHON patients while producing contradictory effects on respiration

BackgroundLebers hereditary optic neuropathy (LHON) is caused by mutations in the complex I subunits of the respiratory chain. Although patients have been treated with idebenone since 1992, the efficacy of the drug is still a matter of debate.MethodsWe evaluated the effect of idebenone in fibroblasts from LHON patients using enzymatic and polarographic measurements.ResultsComplex I activity was 42% greater in treated fibroblasts compared to controls (p = 0.002). Despite this complex I activity improvement, the effects on mitochondrial respiration were contradictory, leading to impairment in some cases and stimulation in others.ConclusionThese results indicate that idebenone is able to compensate the complex I deficiency in LHON patient cells with variable effects on respiration, indicating that the patients might not be equally likely to benefit from the treatment.

Reversible optic neuropathy with OPA1 exon 5b mutation

A new c.740G>A (R247H) mutation in OPA1 alternate spliced exon 5b was found in a patient presenting with bilateral optic neuropathy followed by partial, spontaneous visual recovery. R247H fibroblasts from the patient and his unaffected father presented unusual highly tubular mitochondrial network, significant increased susceptibility to apoptosis, oxidative phosphorylation uncoupling, and altered OPA1 protein profile, supporting the pathogenicity of this mutation. These results suggest that the clinical spectrum of the OPA1‐associated optic neuropathies may be larger than previously described, and that spontaneous recovery may occur in cases harboring an exon 5b mutation. Ann Neurol 2008