ANTXR1 deficiency promotes fibroblast senescence: implications for GAPO syndrome as a progeroid disorder

ANTXR1 is one of two cell surface receptors mediating the uptake of the anthrax toxin into cells. Despite substantial research on its role in anthrax poisoning and a proposed function as a collagen receptor, ANTXR1's physiological functions remain largely undefined. Pathogenic variants in ANTXR1 lead to the rare GAPO syndrome, named for its four primary features: Growth retardation, Alopecia, Pseudoanodontia, and Optic atrophy. The disease is also associated with a complex range of other phenotypes impacting the cardiovascular, skeletal, pulmonary and nervous systems. Aberrant accumulation of extracellular matrix components and fibrosis are considered to be crucial components in the pathogenesis of GAPO syndrome, contributing to the shortened life expectancy of affected individuals. Nonetheless, the specific mechanisms connecting ANTXR1 deficiency to the clinical manifestations of GAPO syndrome are largely unexplored. In this study, we present evidence that ANTXR1 deficiency initiates a senescent phenotype in human fibroblasts, correlating with defects in nuclear architecture and actin dynamics. We provide novel insights into ANTXR1's physiological functions and propose GAPO syndrome to be reconsidered as a progeroid disorder highlighting an unexpected role for an integrin-like extracellular matrix receptor in human aging.

Nr2f1 heterozygous knockout mice recapitulate neurological phenotypes of Bosch-Boonstra-Schaaf optic atrophy syndrome and show impaired hippocampal synaptic plasticity

Bosch-Boonstra-Schaaf optic atrophy syndrome (BBSOAS) has been identified as an autosomal-dominant disorder characterized by a complex neurological phenotype, with high prevalence of intellectual disability and optic nerve atrophy/hypoplasia. The syndrome is caused by loss-of-function mutations in NR2F1, which encodes a highly conserved nuclear receptor that serves as a transcriptional regulator. Previous investigations to understand the protein's role in neurodevelopment have mostly used mouse models with constitutive and tissue-specific homozygous knockout of Nr2f1. In order to represent the human disease more accurately, which is caused by heterozygous NR2F1 mutations, we investigated a heterozygous knockout mouse model and found that this model recapitulates some of the neurological phenotypes of BBSOAS, including altered learning/memory, hearing defects, neonatal hypotonia and decreased hippocampal volume. The mice showed altered fear memory, and further electrophysiological investigation in hippocampal slices revealed significantly reduced long-term potentiation and long-term depression. These results suggest that a deficit or alteration in hippocampal synaptic plasticity may contribute to the intellectual disability frequently seen in BBSOAS. RNA-sequencing (RNA-Seq) analysis revealed significant differential gene expression in the adult Nr2f1+/- hippocampus, including the up-regulation of multiple matrix metalloproteases, which are known to be critical for the development and the plasticity of the nervous system. Taken together, our studies highlight the important role of Nr2f1 in neurodevelopment. The discovery of impaired hippocampal synaptic plasticity in the heterozygous mouse model sheds light on the pathophysiology of altered memory and cognitive function in BBSOAS.

Receptor for Advanced Glycation End Products and Age-Related Macular Degeneration

PURPOSE

Advanced glycation end products (AGE) exacerbate disease progression through two general mechanisms: modifying molecules and forming nondegradable aggregates, thus impairing normal cellular/tissue functions, and altering cellular function directly through receptor-mediated activation. In the present study receptor for AGE (RAGE)-mediated cellular activation was evaluated in the etiology of human retinal aging and disease.

METHODS

The maculas of human donor retinas from normal eyes and eyes with early age-related macular degeneration (AMD) and advanced AMD with geographic atrophy (GA) were assayed for AGE and RAGE by immunocytochemistry. Cultured ARPE-19 cells were challenged with known ligands for RAGE, AGE, and S100B, to test for activation capacity. Immunocytochemistry, real-time RT-PCR, immunoblot analysis, and the TUNEL assay were used to determine the consequences of RPE cellular activation.

RESULTS

Little to no immunolabeling for AGE or RAGE was found in photoreceptor and RPE cell layers in normal retinas. However, when small drusen were present, AGE and RAGE were identified in the RPE or both the RPE and photoreceptors. In early AMD and GA, the RPE and remnant photoreceptor cells showed intense AGE and RAGE immunolabeling. Both AGE and S100B activated cultured RPE cells, as revealed by upregulated expression of RAGE, NFkappaB nuclear translocation, and apoptotic cell death.

CONCLUSIONS

Immunolocalization of RAGE in RPE and photoreceptors coincided with AGE deposits and macular disease in aged, early AMD, and GA retinas. Further, AGE stimulated RAGE-mediated activation of cultured ARPE-19 cells in a dose-dependent fashion. AGE accumulation, as occurs with normal aging and in disease, may induce receptor-mediated activation of RPE/photoreceptor cells, contributing to disease progression in the aging human retinas.

Type III 3-methylglutaconic aciduria (optic atrophy plus syndrome, or Costeff optic atrophy syndrome): identification of the OPA3 gene and its founder mutation in Iraqi Jews

Type III 3-methylglutaconic aciduria (MGA) (MIM 258501) is a neuro-ophthalmologic syndrome that consists of early-onset bilateral optic atrophy and later-onset spasticity, extrapyramidal dysfunction, and cognitive deficit. Urinary excretion of 3-methylglutaconic acid and of 3-methylglutaric acid is increased. The disorder has been reported in approximately 40 patients of Iraqi Jewish origin, allowing the mapping of the disease to chromosome 19q13.2-q13.3, by linkage analysis. To isolate the causative gene, OPA3, we sequenced four genes within the critical interval and identified, in the intronic sequence of a gene corresponding to cDNA clone FLJ22187, a point mutation that segregated with the type III MGA phenotype. The FLJ22187-cDNA clone, which we identified as the OPA3 gene, consists of two exons and encodes a peptide of 179 amino acid residues. Northern blot analysis revealed a primary transcript of approximately 5.0 kb that was ubiquitously expressed, most prominently in skeletal muscle and kidney. Within the brain, the cerebral cortex, the medulla, the cerebellum, and the frontal lobe, compared to other parts of the brain, had slightly increased expression. The intronic G-->C mutation abolished mRNA expression in fibroblasts from affected patients and was detected in 8 of 85 anonymous Israeli individuals of Iraqi Jewish origin. Milder mutations in OPA3 should be sought in patients with optic atrophy with later onset, even in the absence of additional neurological abnormalities.

Novel mtDNA mutations and oxidative phosphorylation dysfunction in Russian LHON families

Leber's hereditary optic neuropathy (LHON) is characterized by maternally transmitted, bilateral, central vision loss in young adults. It is caused by mutations in the mitochondrial DNA (mtDNA) encoded genes that contribute polypeptides to NADH dehydrogenase or complex I. Four mtDNA variants, the nucleotide pair (np) 3460A, 11778A, 14484C, and 14459A mutations, are known as "primary" LHON mutations and are found in most, but not all, of the LHON families reported to date. Here, we report the extensive genetic and biochemical analysis of five Russian families from the Novosibirsk region of Siberia manifesting maternally transmitted optic atrophy consistent with LHON. Three of the five families harbor known LHON primary mutations. Complete sequence analysis of proband mtDNA in the other two families has revealed novel complex I mutations at nps 3635A and 4640C, respectively. These mutations are homoplasmic and have not been reported in the literature. Biochemical analysis of complex I in patient lymphoblasts and transmitochondrial cybrids demonstrated a respiration defect with complex-I-linked substrates, although the specific activity of complex I was not reduced. Overall, our data suggests that the spectrum of mtDNA mutations associated with LHON in Russia is similar to that in Europe and North America and that the np 3635A and 4640C mutations may be additional mtDNA complex I mutations contributing to LHON expression.

'Secondary' 4216/ND1 and 13708/ND5 Leber's hereditary optic neuropathy mitochondrial DNA mutations do not further impair in vivo mitochondrial oxidative metabolism when associated with the 11778/ND4 mitochondrial DNA mutation

The pathogenic role of 'secondary' mitochondrial DNA (mtDNA) point mutations, when occurring in patients with Leber's hereditary optic neuropathy (LHON) in association with 'primary' mutations, is still controversial. We used phosphorus magnetic resonance spectroscopy to establish whether two of these 'secondary' LHON mtDNA mutations, 4216/ND1 and 13708/ND5 (haplogroup J), further affect in vivo mitochondrial oxidative metabolism in subjects with the 'primary' 11778/ND4 mtDNA mutation. Brain and skeletal muscle energy metabolism was assessed in 10 subjects homoplasmic for the 11778/ND4 mtDNA mutation and 10 subjects homoplasmic for the same mutation occurring on the haplogroup J mtDNA background. Brain phosphocreatine concentration and phosphorylation potential were significantly reduced and brain inorganic phosphate concentration was significantly increased compared with controls in both groups of 11778/ND4-positive subjects. The degree of reduction in the phosphocreatine concentration and phosphorylation potential and of increase in the inorganic phosphate concentration was, however, similar in the two groups with the 11778/ND4 mtDNA mutation with or without the haplogroup J. Similarly, the rate of muscle phosphocreatine resynthesis after exercise, a sensitive index of the rate of mitochondrial ATP production, was reduced by the same extent in both groups of LHON subjects. This in vivo study does not support synergism of the 4216/ND1 and 13708/ND5 'secondary' mutations with the 11778/ND4 'primary' mutation in determining the deficit of energy metabolism in LHON.

Biochemical features of mtDNA 14484 (ND6/M64V) point mutation associated with Leber's hereditary optic neuropathy

We report the effect on complex I function of the 14484 Leber's hereditary optic neuropathy (LHON) mutation affecting the ND6 subunit gene. The same gene was also reported to carry another mutation, at position 14459, associated with the LHON/dystonia phenotype that induces a reduction of complex I-specific activity and increases the sensitivity to the product decylubiquinol. Given the proximity of both mutations in the ND6 gene, we tested the specific activity of complex I and its sensitivity to myxothiazol and nonylbenzoquinol, both inhibitors at the ubiquinol product site, in platelet submitochondrial particles from nine 14484 homoplasmic individuals, 8 Italians with Caucasian mtDNA haplogroup J (adjunctive 4216 and 13708 mutations), and 1 Tunisian with an African mtDNA haplogroup. The specific activity of complex I was not affected by the 14484 mutation, but the sensitivity to both inhibitors was significantly increased compared with control subjects regardless of the presence of haplogroup J polymorphisms. Analysis of 70 different amino acid sequences of the ND6 subunit indicated that the 14484 mutation affects an amino acid belonging to its most conserved region, which shows local similarities with cytochrome b regions interacting with ubiquinone or ubiquinol in complex III. Our results suggest that both 14484 and 14459 mutations may affect amino acids forming the interaction site of ubiquinol product, and the 14484 mutation produces a biochemical defect resembling in part that already reported for the common 11778/ND4 LHON mutation.

Clinical, biochemical and molecular genetic features of Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) has traditionally been considered a disease causing severe and permanent visual loss in young adult males. In nearly all families with LHON it is associated with one of three pathogenic mitochondrial DNA (mtDNA) mutations, at bp 11778, 3460 or 14484. The availability of mtDNA confirmation of a diagnosis of LHON has demonstrated that LHON occurs with a wider range of age at onset and more commonly in females than previously recognised. In addition, analysis of patients grouped according to mtDNA mutation has demonstrated differences both in the clinical features of visual failure and in recurrence risks to relatives associated with each of the pathogenic mtDNA mutations. Whilst pathogenic mtDNA mutations are required for the development of LHON, other factors must be reponsible for the variable penetrance and male predominance of this condition. Available data on a number of hypotheses including the role of an additional X-linked visual loss susceptibility locus, impaired mitochondrial respiratory chain activity, mtDNA heteroplasmy, environmental factors and autoimmunity are discussed. Subacute visual failure is seen in association with all three pathogenic LHON mutations. However, the clinical and experimental data reviewed suggest differences in the phenotype associated with each of the three mutations which may reflect variation in the disease mechanisms resulting in this common end-point.

Improved brain and muscle mitochondrial respiration with CoQ. An in vivo study by 31P-MR spectroscopy in patients with mitochondrial cytopathies

We used in vivo phosphorus magnetic resonance spectroscopy (31P-MRS) to study the effect of CoQ10 on the efficiency of brain and skeletal muscle mitochondrial respiration in ten patients with mitochondrial cytopathies. Before CoQ, brain [PCr] was remarkably lower in patients than in controls, while [Pi] and [ADP] were higher. Brain cytosolic free [Mg2+] and delta G of ATP hydrolysis were also abnormal in all patients. MRS also revealed abnormal mitochondrial function in the skeletal muscles of all patients, as shown by a decreased rate of PCr recovery from exercise. After six-months of treatment with CoQ (150 mg/day), all brain MRS-measurable variables as well as the rate of muscle mitochondrial respiration were remarkably improved in all patients. These in vivo findings show that treatment with CoQ in patients with mitochondrial cytopathies improves mitochondrial respiration in both brain and skeletal muscles, and are consistent with Lenaz's view that increased CoQ concentration in the mitochondrial membrane increases the efficiency of oxidative phosphorylation independently of enzyme deficit.

Aspects of human bioenergetics as studied in vivo by magnetic resonance spectroscopy

We outline the relevant capabilities of in vivo phosphorus MR spectroscopy by discussing some aspects of normal human biochemistry as studied by this technique. The transport of inorganic phosphate from cytosol into mitochondria in the human skeletal muscle was studied by exploiting a new experimental protocol. We found that Pi was transported into mitochondria in the absence of ATP biosynthesis and in the presence of a pH gradient. The control of CoQ on the efficiency of oxidative phosphorylation in the skeletal muscle and brain was studied by administering CoQ to patients with mitochondrial cytopathies due to known enzyme defects. Before CoQ we had detected a relevant reduction of mitochondrial functionality in the skeletal muscle as shown by the reduced rate of phosphocreatine recovery from exercise, and in the occipital lobes by reduced [phosphocreatine] and a high [ADP] and [Pi]. After CoQ all brain variables were remarkably improved. Treatment with CoQ also improved the rate of muscle phosphocreatine recovery from exercise. Our in vivo findings support the hypothesis that the concentration of CoQ rather than the rate of its lateral diffusion in the mitochondrial membrane controls the efficiency of oxidative phosphorylation. Other experiments were undertaken to clarify the functional relationship between cytosolic free [Mg2+] and cell bioenergetics in the intact human brain. In the same group of patients with mitochondrial cytopathies we found decreased delta G of ATP hydrolysis and low cytosolic free [Mg2+]. Treatment with CoQ resulted in improved brain bioenergetics and increased free [Mg2+]. These findings strongly indicate that decreased free magnesium was secondary to defective mitochondrial respiration, and support the hypothesis that cytosolic free [Mg2+] is regulated in the intact brain cell to equilibrate, at least in part, any changes in rapidly available free energy.

The influence of nuclear background on the biochemical expression of 3460 Leber's hereditary optic neuropathy

The role of mitochondrial DNA (mtDNA) mutations in the pathogenesis of Leber's hereditary optic neuropathy (LHON) has yet to be characterized. Several clinical features of the disease imply that nuclear genes might also be involved in its expression. We have confirmed the presence of a severe NADH:coenzyme Q1 reductase (complex I) defect in association with the A3460G mtDNA LHON mutation in cultured fibroblasts compared with age-matched controls. This defect was not seen in clonal fibroblasts with 0% mutant mtDNA developed from a heteroplasmic A3460G LHON subject, confirming the association between the A3460G mutation and the complex I defect. Cybrids prepared from the fusion of enucleated fibroblasts homoplasmic for the A3460G mutation with 206 (osteosarcoma) cells lacking mtDNA (p0) also had a severe deficiency of complex I activity. However, in A3460G LHON fusion cybrids containing a different nuclear background, A549 p0 (lung derived), this biochemical defect was not apparent in all the clones studied. These results suggest that the nuclear environment can influence the expression of the biochemical defect in LHON patients with the A3460G mutation.

GCAP1 (Y99C) mutant is constitutively active in autosomal dominant cone dystrophy

GCAP1 stimulates photoreceptor guanylate cyclase (GC) in bleached vertebrate photoreceptors when [Ca2+]free decreases but is inactivated when cytoplasmic [Ca2+]free increase after dark adaptation. A Y99C mutation in GCAP1 has recently been found to be associated with autosomal dominant cone dystrophy. We show that the GCAP1(Y99C) mutant and native GCAP1 are highly effective in stimulation of photoreceptor GC1. The Ca2+ sensitivity of the mutant GCAP1, however, is markedly altered, causing reduced but persistent stimulation of GC1 under physiological dark conditions. These results are consistent with a model in which enhanced GC activity in dark-adapted cones leads to elevated levels of cytoplasmic cGMP. Alterations in physiological cGMP levels are also associated with other retinal degenerations, including Leber's congenital amaurosis.

Brain metabolic profiles obtained by proton MRS in two forms of mitochondriopathies: Leber's hereditary optic neuropathy and chronic progressive external ophthalmoplegia

The status of brain metabolism has been evaluated using monovoxel short echo time (20 ms) 1H magnetic resonance spectroscopy in 6 patients with two forms of mitochondrial disorders without clinical cerebral involvement: 2 patients with Leber's hereditary optic neuropathy (LHON) and 4 patients with chronic progressive external ophthalmoplegia (CPEO). Patients with LHON displayed normal spectra. In all patients with CPEO, the brain metabolic profiles were abnormal, with no single uniform pattern. No typical cerebral metabolic profile was found even when these disorders were classified either by syndrome or by biochemical defect. No lactate signal was detected. The metabolic alterations observed in CPEO patients contrasted with the absence of clinical signs of encephalopathy. The absence of a typical metabolic profile reflects the large variability in the clinical expression of biochemical defects in mitochondriopathies, and the lack of convergence between genetic deletions, biochemical anomalies and clinical syndromes.

Energy charge is not decreased in lymphocytes of patients with Leber's hereditary optic neuropathy with the 11,778 mutation

OBJECTIVES

A defect in mitochondrial energy conservation is strongly suggested to be involved in the pathogenesis of Leber's hereditary optic neuropathy (LHON). The authors therefore compared the energy charge in lymphocytes among patients with LHON, their asymptomatic maternal lineages, and normal control subjects.

MATERIALS AND METHODS

Blood samples were obtained from 7 patients, 10 asymptomatic maternal relatives, and 16 normal subjects. Molecular analysis confirmed that all had the homoplasmic 11,778 point mutation in the mtDNA of their blood cells. The concentrations of adenosine triphosphate (ATP), diphosphate (ADP), and monophosphate (AMP) were determined by high-performance liquid chromatography. The energy charge was calculated as (ATP + 1/2 ADP)/(ATP + ADP + AMP).

RESULTS

The mean energy charges of lymphocytes were 0.871 +/- 0.049 in patients with LHON, 0.884 +/- 0.061 in their asymptomatic maternal relatives, and 0.885 +/- 0.061 in normal controls, respectively. No statistically significant difference was found among the three groups.

CONCLUSIONS

Although the study did not find the anticipated change in energy charge in peripheral blood cells, this neither confirms nor rejects the notion that a defect in the mitochondrial oxidative phosphorylation system is involved in the pathogenesis of LHON.

Acquired mitochondrial impairment as a cause of optic nerve disease

BACKGROUND

Blindness from an optic neuropathy recently occurred as an epidemic affecting 50,000 patients in Cuba (CEON) and had clinical features reminiscent of both tobacco-alcohol amblyopia (TAA) and Leber's hereditary optic neuropathy (Leber's; LHON). Selective damage to the papillomacular bundle was characteristic, and many patients also developed a peripheral neuropathy. Identified risk factors included vitamin deficiencies as well as exposure to methanol and cyanide. In all 3 syndromes, there is evidence that singular or combined insults to mitochondrial oxidative phosphorylation are associated with a clinically characteristic optic neuropathy.

PURPOSE

First, to test the hypothesis that a common pathophysiologic mechanism involving impairment of mitochondria function and, consequently, axonal transport underlies both genetic optic nerve diseases such as Leber's and acquired toxic and nutritional deficiency optic neuropathies. According to this hypothesis, ATP depletion below a certain threshold leads to a blockage of orthograde axonal transport of mitochondria, which, in turn, leads to total ATP depletion and subsequent cell death. Second, to address several related questions, including (1) How does impaired energy production lead to optic neuropathy, particularly since it seems to relatively spare other metabolically active tissues, such as liver and heart? (2) Within the nervous system, why is the optic nerve, and most particularly the papillomacular bundle, so highly sensitive? Although there have been previous publications on the clinical features of the Cuban epidemic of blindness, the present hypothesis and the subsequent questions have not been previously addressed.

METHODS

Patients in Cuba with epidemic optic neuropathy were personally evaluated through a comprehensive neuro-ophthalmologic examination. In addition, serum, lymphocytes for DNA analysis, cerebrospinal fluid (CSF), sural nerves, and eyes with attached optic nerves were obtained from Cuban patients, as well as from Leber's patients, for study. Finally, we developed an animal model to match the low serum folic acid and high serum formate levels found in the CEON patients, by administering to rats low doses of methanol after several months of a folic acid-deficient diet. Optic nerves and other tissues obtained from these rats were analyzed and compared with those from the Cuban patients.

RESULTS

Patients from the Cuban epidemic of optic neuropathy with clinical evidence of a selective loss of the papillomacular bundle did much better once their nutritional status was corrected and exposure to toxins ceased. Patients with CEON often demonstrated low levels of folic acid and high levels of formate in their blood. Histopathologic studies demonstrated losses of the longest fibers (in the sural nerve) and those of smallest caliber (papillomacular bundle) in the optic nerve, with intra-axonal accumulations just anterior to the lamina cribrosa. Our animal model duplicated the serologic changes (low folic acid, high formate) as well as these histopathologic changes. Furthermore, ultrastructural examination of rat tissues demonstrated mitochondrial changes that further matched those seen on ultrastructural examination of tissues from patients with Leber's.

CONCLUSION

Mitochondria can be impaired either genetically (as in Leber's) or through acquired insults (such as nutritional or toxic factors). Either may challenge energy production in all cells of the body. While this challenge may be met through certain compensatory mechanisms (such as in the size, shape, or number of the mitochondria), there exists in neurons a threshold which, once passed, leads to catastrophic changes. This threshold may be that point at which mitochondrial derangement leads to such ATP depletion that axonal transport is compromised, and decreased mitochondrial transport results in even further ATP depletion. Neurons are singularly dependent on the axonal transport of mitochondria. (

mtDNA mutations confer cellular sensitivity to oxidant stress that is partially rescued by calcium depletion and cyclosporin A

The complete mechanism by which pathogenic mtDNA mutations cause cellular pathophysiology and in some cases cell death is unclear. Oxidant stress is especially toxic to excitable nerve and muscle cells, cells that are often affected in mitochondrial disease. The sensitivity of cells bearing the LHON, MELAS, and MERRF mutations to oxidant stress was determined. All were significantly more sensitive to H2O2 exposure than their nonmutant cybrid controls, the order of sensitivity was MELAS > LHON > MERRF > controls. Depletion of Ca2+ from the medium protected all cell lines from oxidant stress, consistent with the hypothesis that death induced by oxidant stress is Ca(2+)-dependent. A potential downstream target of Ca2+ is the mitochondrial permeability transition, MPT, which is inhibited by cyclosporin A. Treatment of MELAS, LHON, and MERRF cells with cyclosporin A caused significant rescue from oxidant exposure, and in each case significantly greater rescue of mutant than control cells. The pronounced oxidant-sensitivity of mutant cells, and their protection by Ca2+ depletion and CsA, has potential implications for both the pathophysiological mechanism and therapy of these mitochondrial genetic diseases.

Progressive dystonia with optic atrophy in a Jewish-Iraqi family

The combination of progressive dystonia and optic atrophy is extremely rare and its morphological, metabolic and genetic basis is unknown. In a family of 9 children (8 males) born to consanguineous Israeli-Jewish-Iraqi parents, we identified four brothers who developed the syndrome at the end of the first decade. Patients had hemi or bilateral dystonia associated with striatal, mainly putaminal, atrophy on CT and MRI, various degrees of optic atrophy, minimal corticospinal tract involvement, normal intelligence and no peripheral nervous system or systemic abnormalities. No causative metabolic defect was identified. None of the several known mitochondrial DNA mutations associated with Leber's hereditary optic neuropathy (LHON) or with LHON with dystonia were detected. Likewise, linkage to the idiopathic torsion dystonia region on chromosome 9q34 was excluded. It is suggested that this in our patients might be due to a yet unidentified genomic, autosomal recessive mutation.

Clinical and brain bioenergetics improvement with idebenone in a patient with Leber's hereditary optic neuropathy: a clinical and 31P-MRS study

We used phosphorus magnetic resonance spectroscopy (31P-MRS) to study in vivo brain and muscle bioenergetics in a male patient with Leber's hereditary optic neuropathy (LHON) and mtDNA mutation at 11,778 bp who developed spastic paraparesis with white matter lesions on brain MR imaging. The study was performed before and during treatment with idebenone (135 mg t.i.d.) and after withdrawal. Clinical amelioration and worsening were associated with parallel changes in brain and skeletal muscle bioenergetics following the administration or withdrawal of idebenone. Reversal of paraparesis by idebenone was paralleled by normalization of 31P-MRS, serum lactate and central motor conduction. Extra-ocular neurological dysfunction in LHON may be amenable to treatment by appropriate quinones.

Changes in mitochondrial complex I activity and coenzyme Q binding site in Leber's hereditary optic neuropathy (LHON)

The complex I function in sub-mitochondrial particles was studied in platelets from patients and healthy carriers with 11778/ND4 or 3460/ND1 mtDNA point mutations associated with LHON. Both 11778/ND4 and 3460/ND1 mutations induced rotenone resistance and 11778/ND4 showed an increased K(m) for ubiquinol-2 with respect to the control group. It was concluded that even with different pathogenic mechanisms both mutations affect the quinone binding site of complex I.

Catalytic activity of complex I in cell lines that possess replacement mutations in the ND genes in Leber's hereditary optic neuropathy

Short-chain ubiquinone analogues act as electron acceptors and as inhibitors in the lymphoblast mitochondria of ND1/3460 mutants, which indicates structural changes in the ubiquinone-binding domain of Complex I in this mutant. The ND4/11778 mutant and two secondary ND5 mutants studied are associated with reductions of at least 50, 35 and 30% in the catalytic rate constant, respectively. However, the efficiency of oxidative phosphorylation is unaffected in all these ND mutants. The rate of respiration is only slightly limited by Complex I in lymphoblast mitochondria. Consequently, there is a 30-35% reduction in the electron flow through Complex I compared with that through Complex II, and an increased lactate/pyruvate ratio, in the ND1 and ND4 mutants, but these factors were unaffected in the secondary ND5 mutants. Energy metabolism is thus less severely affected in the secondary mutants than in the primary mutants, which supports the division into these two categories. An increased ubiquinone-10 content in the mitochondrial membrane of all the mutants, and enhanced succinate dehydrogenase and citrate synthase activities in the ND4 mutant, are proposed to be compensatory changes. The efficiency of these changes and the level of kinetic limitation of respiration by Complex I in each tissue are proposed to determine the clinical development of the disease.

Respiration and growth defects in transmitochondrial cell lines carrying the 11778 mutation associated with Leber's hereditary optic neuropathy

Mitochondrial DNA from two genetically unrelated patients carrying the mutation at position 11778 that causes Leber's hereditary optic neuropathy has been transferred with mitochondria into human mtDNA-less rho0206 cells. As analyzed in several transmitochondrial cell lines thus obtained, the mutation, which is in the gene encoding subunit ND4 of the respiratory chain NADH dehydrogenase (ND), did not affect the synthesis, size, or stability of ND4, nor its incorporation into the enzyme complex. However, NADH dehydrogenase-dependent respiration, as measured in digitonin-permeabilized cells, was specifically decreased by approximately 40% in cells carrying the mutation. This decrease, which was significant at the 99.99% confidence level, was correlated with a significantly reduced ability of the mutant cells to grow in a medium containing galactose instead of glucose, indicating a clear impairment in their oxidative phosphorylation capacity. On the contrary, no decrease in rotenone-sensitive NADH dehydrogenase activity, using a water-soluble ubiquinone analogue as electron acceptor, was detected in disrupted mitochondrial membranes. This is the first cellular model exhibiting in a foreign nuclear background mitochondrial DNA-linked biochemical defects underlying the optic neuropathy phenotype.

Mitochondrial disorders in neuro-ophthalmology

The pathogenesis of mitochondrial disorders relevant to neuro-ophthalmology has been further clarified by identifying and correlating to disease manifestations the three primary pathogenic mutations of Leber's hereditary optic neuropathy, and by correlating genetic, biochemical, and histochemical abnormalities in the mitochondrial myopathies and encephalomyopathies.

Defective brain and muscle energy metabolism shown by in vivo 31P magnetic resonance spectroscopy in nonaffected carriers of 11778 mtDNA mutation

In vivo phosphorus magnetic resonance spectroscopy (31P-MRS) showed defective brain and muscle energy metabolism in three affected siblings in a family with Leber's hereditary optic neuropathy (LHON) with the 11778 mtDNA mutation. We studied 14 nonaffected members of the same pedigree by 31P-MRS and molecular genetics. Nine of 14 individuals studied had the 11778 mtDNA mutation, with various degrees of heteroplasmy. A decreased brain energy reserve, as shown by low phosphocreatine content and phosphorylation potential and high [ADP], was present in eight of these nine subjects with the 11778 mutation. A low rate of postexercise phosphocreatine recovery in muscle was present in six of the nine mutated individuals. Normal MRS findings in the brain of one and the muscle of three carriers were accompanied by a low percentage of mutated mtDNA. All subjects without mutation had normal brain and muscle MRS. 31P-MRS disclosed defective bioenergetics in the brain or muscle or both of all asymptomatic carriers studied from our pedigree.

MtDNA mutations associated with Leber's hereditary optic neuropathy: studies on cytoplasmic hybrid (cybrid) cells

Leber's hereditary optic neuropathy (LHON) has been associated with "primary" and "secondary" mtDNA missense point mutations, and a synergistic role has been proposed for secondary mutations. No previous study has investigated the effects of LHON primary or primary plus secondary mutations on the respiratory competence of cell lines. We constructed and compared cybrid cell lines obtained from two unrelated LHON patients both carrying the common 11778/ND4 primary mutation. One of the patients also carried the 13708/ND5 and 4216/ND1 secondary mutations. The cybrid clones were evaluated for growth efficiency, oxygen consumption, complexes I, III and IV enzymatic activity and mitochondrial protein synthesis. Complex activity and mitochondrial protein synthesis were not significantly changed in cybrid clones from the patients. Oxygen consumption was significantly decreased in all clones carrying the 11778/ND4 primary mutation demonstrating its pathogenic role in impairing cell respiration. Clones also carrying the secondary mutations showed an even lower oxygen consumption and a significantly higher doubling time, suggesting that the co-presence of the secondary mutations could be relevant in further reducing the cell fitness.

Mitochondrial DNA mutations in human degenerative diseases and aging

A wide variety of mitochondrial DNA (mtDNA) mutations have recently been identified in degenerative diseases of the brain, heart, skeletal muscle, kidney and endocrine system. Generally, individuals inheriting these mitochondrial diseases are relatively normal in early life, develop symptoms during childhood, mid-life, or old age depending on the severity of the maternally-inherited mtDNA mutation; and then undergo a progressive decline. These novel features of mtDNA disease are proposed to be the product of the high dependence of the target organs on mitochondrial bioenergetics, and the cumulative oxidative phosphorylation (OXPHOS) defect caused by the inherited mtDNA mutation together with the age-related accumulation mtDNA mutations in post-mitotic tissues.

Defective brain energy metabolism shown by in vivo 31P MR spectroscopy in 28 patients with mitochondrial cytopathies

We studied brain energy metabolism by phosphorus magnetic resonance spectroscopy (31P MRS) in 28 patients with mitochondrial cytopathies, and 20 normal control subjects. Fourteen patients had myopathy alone, six had only mild brain symptoms, and eight showed different degrees of brain involvement. Brain 31P MRS showed a low phosphocreatine content in all patients, accompanied by a high inorganic phosphate in 14 of 28 patients. The average value of the Pi concentration in the patient group was significantly (p = 0.009) different from the control group. The cytosolic pH was normal. From these data were derived a high concentration of ADP (calculated from the creatine kinase equilibrium), a high percent value of V/Vmax for ATP biosynthesis, and a low phosphorylation potential, all features showing a derangement of brain energy metabolism, in all patients with mitochondrial cytopathies. 31P MRS proved to be sensitive enough to disclose a deficit of mitochondrial functionality not only in the affected patients, but also in those without clinically evident brain symptoms.

Mitochondrial disorders in muscle

The successful analysis of mutations in mitochondrial DNA has been a major step forward in our understanding of a number of baffling clinical syndromes, and the application of new technology to analyze and study mitochondrial DNA mutations has led to new insights into the pathogenesis of these disorders. Moreover, the field of mitochondrial DNA diseases is now expanding beyond the limited purview of relatively rare disorders to encompass clinical phenotypes that may be much more frequent in the population.

Nonviability of cells with oxidative defects in galactose medium: a screening test for affected patient fibroblasts

Diagnosis of respiratory chain defects in cultured skin fibroblasts is a difficult diagnostic procedure. We investigated the feasibility of using survival of skin fibroblasts in culture medium with galactose as the major carbon source as a method of quickly diagnosing cell lines that were compromised in oxidative metabolism. We found that cells from patients with most forms of cytochrome oxidase deficiency, cells with complex I deficiency, cells with multiple respiratory chain defects and cells with severe pyruvate dehydrogenase (PDH) complex deficiency failed to survive when subcultured into galactose (5 mM) medium. Cells from patients with Lebers hereditary optic neuropathy (LHON), Kearns-Sayre syndrome (KSS), myoclonus-epilepsy-lactic acidosis-stroke (MELAS), the hepatic form of cytochrome oxidase deficiency, and mild PDH complex deficiency survived well in galactose (5 mM)-containing medium. This could be used as a rapid screening test for skin fibroblasts with major oxidative defects.

Evidence for a metabolic trigger for Leber's hereditary optic neuropathy. A case report

A 9-year-old girl with recently diagnosed juvenile onset diabetes mellitus presented with signs and symptoms of bilateral optic neuropathy. Leber's hereditary optic neuropathy was suspected on the basis of a strong family history. Subsequent mitochondrial DNA testing was positive. Visual recovery occurred once the diabetes was well controlled. This case suggests that such metabolic compromise that occurs in diabetes may precipitate the clinical expression of Leber's optic neuropathy.

Leber's hereditary optic neuropathy: genetic, biochemical, and phosphorus magnetic resonance spectroscopy study in an Italian family

Three siblings of a family affected with Leber's hereditary optic neuropathy (LHON) showed a mitochondrial DNA mutation at position 11778. The lactate response to a standardized effort was increased in only one case. Muscle biopsies and biochemistry of muscle and platelet mitochondrial enzymes were normal. All patients showed an altered energy metabolism during exercise and during recovery after exercise on phosphorus 31-magnetic resonance spectroscopy (31P-MRS) of muscle. Brain 31P-MRS showed a decreased energy reserve (decreased PCr/Pi ratio) in all patients. 31P-MRS noninvasively demonstrated an altered mitochondrial energy metabolism in muscle and, for the first time, in the brains of LHON patients.

[Two siblings of Leber's congenital amaurosis with an increase in very long chain fatty acid in blood: relationship between peroxisomal disorders and Leber's congenital amaurosis]

We reported two siblings of Leber's congenital amaurosis associated with increased level of very long chain fatty acid (VLCFA) in blood. Case 1, a 3 1/2-year-old boy had congenital blindness, severe psychomotor retardation, hepatomegaly, profound hypotonia, loss of deep tendon reflexes, muscular atrophy and weakness, and non-convulsive status epilepticus characterized by a sudden respiratory failure, and also showed a flat electroretinogram, non-pigmentary retinal degeneration, severe atrophy of the brain stem and cerebellum, hepatic fibrosis, decreased motor and sensory conduction velocities and atlanto-axial instability. Sural nerve biopsy revealed severely decreased number of total myelinated fibers without remarkable demyelination or remyelination. Case 2, an elder sister of case 1, with pigmentary retinal degeneration, hepatomegaly and pericarditis had died at 3 months. Autopsy revealed hypomyelination and heterotopy of the cerebral white matter, hepatic fibrosis, renal microcysts and normal adrenal cytoarchitecture. In case 1, the level of VLCFA was increased twofold and sevenfold of controls in serum and in red cell membrane, respectively. Phytanic or trihydroxycholestanoic acid was not detected in the serum and bile. Normal shaped peroxisomes were definitely recognized in biopsied liver by means of electronmicroscopic histochemistry. From the above findings, these patients was thought to be a new variant of peroxisomal disorders relating to degradation of VLCFA, other than Zellweger syndrome, infantile Refsum disease and infantile adrenoleukodystrophy. It was concluded that peroxisomal functions should be studied in cases of Leber's congenital amaurosis.