Primary Mitochondrial Disorders Research Articles

Biallelic variants in COX18 cause a mitochondrial disorder primarily manifesting as peripheral neuropathy.

Defects in mitochondrial dynamics are a common cause of Charcot-Marie-Tooth disease (CMT), while primary deficiencies in the mitochondrial respiratory chain (MRC) are rare and atypical for this etiology. This study aims to report COX18 as a novel CMT-causing gene. This gene encodes an assembly factor of mitochondrial Complex IV (CIV) that translocates the C-terminal tail of MTCO2 across the mitochondrial inner membrane. Exome sequencing was performed in four affected individuals. The patients and available family members underwent thorough neurological and electrophysiological assessment. The impact of one of the identified variants on splicing, protein levels, and mitochondrial bioenergetics was investigated in patient-derived lymphoblasts. The functionality of the mutant protein was assessed using a Proteinase K protection assay and immunoblotting. Neuronal relevance of COX18 was assessed in a Drosophila melanogaster knockdown model. Exome sequencing coupled with homozygosity mapping revealed a homozygous splice variant c.435-6A>G in COX18 in two siblings with early-onset progressive axonal sensory-motor peripheral neuropathy. By querying external databases, we identified two additional families with rare deleterious biallelic variants in COX18 . All affected individuals presented with axonal CMT and some patients also exhibited central nervous system symptoms, such as dystonia and spasticity. Functional characterization of the c.435-6A>G variant demonstrated that it leads to the expression of an alternative transcript that lacks exon 2, resulting in a stable but defective COX18 isoform. The mutant protein impairs CIV assembly and activity, leading to a reduction in mitochondrial membrane potential. Downregulation of the COX18 homolog in Drosophila melanogaster displayed signs of neurodegeneration, including locomotor deficit and progressive axonal degeneration of sensory neurons. Our study presents genetic and functional evidence that supports COX18 as a newly identified gene candidate for autosomal recessive axonal CMT with or without central nervous system involvement. These findings emphasize the significance of peripheral neuropathy within the spectrum of primary mitochondrial disorders and the role of mitochondrial CIV in the development of CMT. Our research has important implications for the diagnostic workup of CMT patients.

Imbalanced mitochondrial dynamics contributes to the pathogenesis of X-linked adrenoleukodystrophy.

The peroxisomal disease adrenoleukodystrophy (X-ALD) is caused by loss of the transporter of very-long-chain fatty acids (VLCFAs), ABCD1. An excess of VLCFAs disrupts essential homeostatic functions crucial for axonal maintenance, including redox metabolism, glycolysis and mitochondrial respiration. As mitochondrial function and morphology are intertwined, we set out to investigate the role of mitochondrial dynamics in X-ALD models. Using quantitative 3D transmission electron microscopy, we revealed mitochondrial fragmentation in corticospinal axons in Abcd1- mice. In patient fibroblasts, an excess of VLCFAs triggers mitochondrial fragmentation through the redox-dependent phosphorylation of DRP1 (DRP1S616). The blockade of DRP1-driven fission by the peptide P110 effectively preserved mitochondrial morphology. Furthermore, mRNA inhibition of DRP1 not only prevented mitochondrial fragmentation but also protected axonal health in a Caenorhabditis elegans model of X-ALD, underscoring DRP1 as a potential therapeutic target. Elevated levels of circulating cell-free mtDNA in patients' CSF align this leukodystrophy with primary mitochondrial disorders. Our findings underscore the intricate interplay between peroxisomal dysfunction, mitochondrial dynamics and axonal integrity in X-ALD, shedding light on potential avenues for therapeutic intervention.

Novel MRI and histopathological findings in a young Bullmastiff cross dog with mitochondrial fission encephalopathy.

A 9-month-old male Bullmastiff cross dog was presented with a history of progressive proprioceptive ataxia and behavior changes. Neuroanatomical localization was multifocal with forebrain and vestibulo-cerebellum involvement. MRI identified moderate diffuse cerebral sulci widening, dilation of the ventricular system, and rounded, well-defined, bilaterally symmetrical T2W, FLAIR, and T2* hyperintense intra-axial lesions affecting the olivary nuclei. Histopathological examination was indicative of a primary mitochondrial disorder. This was confirmed following genetic analysis which identified mitochondrial fission encephalopathy with a homozygous frameshift variant in the MFF gene. This case report documents diagnostic imaging and histopathological findings not previously reported in dogs affected with mitochondrial fission encephalopathy, suggesting a different selective regional vulnerability of the neurons.

Primary mitochondrial disorders and mimics: Insights from a large French cohort.

The objective of this study was to evaluate the implementation of NGS within the French mitochondrial network, MitoDiag, from targeted gene panels to whole exome sequencing (WES) or whole genome sequencing (WGS) focusing on mitochondrial nuclear-encoded genes. Over 2000 patients suspected of Primary Mitochondrial Diseases (PMD) were sequenced by either targeted gene panels, WES or WGS within MitoDiag. We described the clinical, biochemical, and molecular data of 397 genetically confirmed patients, comprising 294 children and 103 adults, carrying pathogenic or likely pathogenic variants in nuclear-encoded genes. The cohort exhibited a large genetic heterogeneity, with the identification of 172 distinct genes and 253 novel variants. Among children, a notable prevalence of pathogenic variants in genes associated with oxidative phosphorylation (OXPHOS) functions and mitochondrial translation was observed. In adults, pathogenic variants were primarily identified in genes linked to mtDNA maintenance. Additionally, a substantial proportion of patients (54% (42/78) and 48% (13/27) in children and adults, respectively), undergoing WES or WGS testing displayed PMD mimics, representing pathologies that clinically resemble mitochondrial diseases. We reported the largest French cohort of patients suspected of PMD with pathogenic variants in nuclear genes. We have emphasized the clinical complexity of PMD and the challenges associated with recognizing and distinguishing them from other pathologies, particularly neuromuscular disorders. We confirmed that WES/WGS, instead of panel approach, was more valuable to identify the genetic basis in patients with "possible" PMD and we provided a genetic testing flowchart to guide physicians in their diagnostic strategy.

Digenic Leigh syndrome on the background of the m.11778G>A Leber hereditary optic neuropathy variant.

Leigh syndrome spectrum (LSS) is a primary mitochondrial disorder defined neuropathologically by a subacute necrotizing encephalomyelopathy and characterized by bilateral basal ganglia and/or brainstem lesions. LSS is associated with variants in several mitochondrial DNA genes and more than 100 nuclear genes, most often related to mitochondrial complex I (CI) dysfunction. Rarely, LSS has been reported in association with primary Leber hereditary optic neuropathy (LHON) variants of the mitochondrial DNA, coding for CI subunits (m.3460G>A in MT-ND1, m.11778G>A in MT-ND4 and m.14484T>C in MT-ND6). The underlying mechanism by which these variants manifest as LSS, a severe neurodegenerative disease, as opposed to the LHON phenotype of isolated optic neuropathy, remains an open question. Here, we analyse the exome sequencing of six probands with LSS carrying primary LHON variants, and report digenic co-occurrence of the m.11778G > A variant with damaging heterozygous variants in nuclear disease genes encoding CI subunits as a plausible explanation. Our findings suggest a digenic mechanism of disease for m.11778G>A-associated LSS, consistent with recent reports of digenic disease in individuals manifesting with LSS due to biallelic variants in the recessive LHON-associated disease gene DNAJC30 in combination with heterozygous variants in CI subunits.

H+-slip correlated to rotor free-wheeling as cause of F1FO-ATPase dysfunction in primary mitochondrial disorders.

Inborn errors of metabolism are related to mitochondrial disorders caused by dysfunction of the oxidative phosphorylation (OXPHOS) system. Congenital hypermetabolism in the infant is a rare disease belonging to Luft syndrome, nonthyroidal hypermetabolism, arising from a singular example of a defect in OXPHOS. The mitochondria lose coupling of mitochondrial substrates oxidation from the ADP phosphorylation. Since Luft syndrome is due to uncoupled cell respiration responsible for deficient in ATP production that originates in the respiratory complexes, a de novo heterozygous variant in the catalytic subunit of mitochondrial F1FO-ATPase arises as the main cause of an autosomal dominant syndrome of hypermetabolism associated with dysfunction in ATP production, which does not involve the respiratory complexes. The F1FO-ATPase works as an embedded molecular machine with a rotary action using two different motor engines. The FO, which is an integral domain in the membrane, dissipates the chemical potential difference for H+, a proton motive force (Δp), across the inner membrane to generate a torsion. The F1 domain-the hydrophilic portion responsible for ATP turnover-is powered by the molecular rotary action to synthesize ATP. The structural and functional coupling of F1 and FO domains support the energy transduction for ATP synthesis. The dissipation of Δp by means of an H+ slip correlated to rotor free-wheeling of the F1FO-ATPase has been discovered to cause enzyme dysfunction in primary mitochondrial disorders. In this insight, we try to offer commentary and analysis of the molecular mechanism in these impaired mitochondria.

Gene therapy for mitochondrial disorders.

In this review, we detail the current state of application of gene therapy to primary mitochondrial disorders (PMDs). Recombinant adeno-associated virus-based (rAAV) gene replacement approaches for nuclear gene disorders have been undertaken successfully in more than ten preclinical mouse models of PMDs which has been made possible by the development of novel rAAV technologies that achieve more efficient organ targeting. So far, however, the greatest progress has been made for Leber Hereditary Optic Neuropathy, for which phase 3 clinical trials of lenadogene nolparvovec demonstrated efficacy and good tolerability. Other methods of treating mitochondrial DNA (mtDNA) disorders have also had traction, including refinements to nucleases that degrade mtDNA molecules with pathogenic variants, including transcription activator-like effector nucleases, zinc-finger nucleases, and meganucleases (mitoARCUS). rAAV-based approaches have been used successfully to deliver these nucleases in vivo in mice. Exciting developments in CRISPR-Cas9 gene editing technology have achieved in vivo gene editing in mouse models of PMDs due to nuclear gene defects and new CRISPR-free gene editing approaches have shown great potential for therapeutic application in mtDNA disorders. We conclude the review by discussing the challenges of translating gene therapy in patients both from the point of view of achieving adequate organ transduction as well as clinical trial design.

Protein biomarkers GDF15 and FGF21 to differentiate mitochondrial hepatopathies from other pediatric liver diseases.

Mitochondrial hepatopathies (MHs) are primary mitochondrial genetic disorders that can present as childhood liver disease. No recognized biomarkers discriminate MH from other childhood liver diseases. The protein biomarkers growth differentiation factor 15 (GDF15) and fibroblast growth factor 21 (FGF21) differentiate mitochondrial myopathies from other myopathies. We evaluated these biomarkers to determine if they discriminate MH from other liver diseases in children. Serum biomarkers were measured in 36 children with MH (17 had a genetic diagnosis); 38 each with biliary atresia, α1-antitrypsin deficiency, and Alagille syndrome; 20 with NASH; and 186 controls. GDF15 levels compared to controls were mildly elevated in patients with α1-antitrypsin deficiency, Alagille syndrome, and biliary atresia-young subgroup, but markedly elevated in MH (p<0.001). FGF21 levels were mildly elevated in NASH and markedly elevated in MH (p<0.001). Both biomarkers were higher in patients with MH with a known genetic cause but were similar in acute and chronic presentations. Both markers had a strong performance to identify MH with a molecular diagnosis with the AUC for GDF15 0.93±0.04 and for FGF21 0.90±0.06. Simultaneous elevation of both markers >98th percentile of controls identified genetically confirmed MH with a sensitivity of 88% and specificity of 96%. In MH, independent predictors of survival without requiring liver transplantation were international normalized ratio and either GDF15 or FGF21 levels, with levels <2000 ng/L predicting survival without liver transplantation (p<0.01). GDF15 and FGF21 are significantly higher in children with MH compared to other childhood liver diseases and controls and, when combined, were predictive of MH and had prognostic implications.

Leber's hereditary optic neuropathy: Update on the novel genes and therapeutic options.

A maternal inheritance disorder called Leber's hereditary optic neuropathy (LHON) is the most common primary mitochondrial deoxyribonucleic acid (DNA) disorder. In most studies, there are more male patients than female patients, which contradicts the usual pattern in mitochondrial hereditary diseases. This suggests that nuclear DNA (nDNA) may influence the degeneration of retinal ganglion cells (RGCs) in LHON. The primary cause of this is dysfunction in complex I of the electron transport chain, leading to ineffective adenosine triphosphate (ATP) production. In addition to MT-ND4 or MT-ND1 mutations, genes such as PRICKLE3 , YARS2 , and DNAJC30 , which come from nDNA, also play a role in LHON. These three genes affect the electron chain transport differently. PRICKLE3 interacts with ATP synthase (complex V) at Xp11.23, while YARS2 is a tyrosyl-tRNA synthetase 2 involved in mitochondria . DNAJC30 mutations result in autosomal recessive LHON (arLHON). Understanding how genes impact the disease is crucial for developing new treatments. Idebenone has been approved for treating LHON and has shown safety and efficacy in clinical trials. Mesenchymal stem cell-based therapy has also emerged as a potential treatment for LHON by transferring mitochondria into target cells. Gene therapy research focuses on specific gene mutations, and the wild-type ND4 gene target in the adeno-associated viruses (AAV) vector has shown promise in clinical trials as a potential treatment for LHON.

A genetically encoded tool to increase cellular NADH/NAD+ ratio in living cells.

Impaired redox metabolism is a key contributor to the etiology of many diseases, including primary mitochondrial disorders, cancer, neurodegeneration and aging. However, mechanistic studies of redox imbalance remain challenging due to limited strategies that can perturb redox metabolism in various cellular or organismal backgrounds. Most studies involving impaired redox metabolism have focused on oxidative stress; consequently, less is known about the settings where there is an overabundance of NADH reducing equivalents, termed reductive stress. Here we introduce a soluble transhydrogenase from Escherichia coli (EcSTH) as a novel genetically encoded tool to promote reductive stress in living cells. When expressed in mammalian cells, EcSTH, and a mitochondrially targeted version (mitoEcSTH), robustly elevated the NADH/NAD+ ratio in a compartment-specific manner. Using this tool, we determined that metabolic and transcriptomic signatures of the NADH reductive stress are cellular background specific. Collectively, our novel genetically encoded tool represents an orthogonal strategy to promote reductive stress.

Current management of primary mitochondrial disorders in EU countries: the European Reference Networks survey.

Primary mitochondrial diseases (PMDs) are rare diseases for which diagnosis is challenging, and management and training programs are not well defined in Europe. To capture and assess care needs, five different European Reference Networks have conducted an exploratory survey. The survey covering multiple topics relating to PMDs was sent to all ERNs healthcare providers (HCPs) in Europe. We have collected answers from 220 members based in 24/27 European member states and seven non-European member states. Even though most of the responders are aware of neurogenetic diseases, difficulties arise in the ability to deliver comprehensive genetic testing. While single gene analysis is widely available in Europe, whole exome and genome sequencing are not easily accessible, with considerable variation between countries and average waiting time for results frequently above 6months. Only 12.7% of responders were happy with the ICD-10 codes for classifying patients with PMDs discharged from the hospital, and more than 70% of them consider that PMDs deserve specific ICD codes to improve clinical management, including tailored healthcare, and for reimbursement reasons. Finally, 90% of responders declared that there is a need for further education and training in these diseases. This survey provides information on the current difficulties in the care of PMDs in Europe. We believe that the results of this survey are important to help rare disease stakeholders in European countries identify key care and research priorities.

A - 190 Neuropsychological Profile of an Adolescent with Primary Mitochondrial Disorder.

This case describes a 16-year-old Hispanic male presenting with progressive brain atropy and upper motor neuron symptoms. He was diagnosed with a primary mitochondrial disorder (MT-TL1; variant m.3255G > A). Language and history produced difficulties in establishing onset. There is no literature describing the neuropsychological outcomes of this rare disorder. The patient spoke Spanish exclusively until 4. At 12, the patient had a CT after a fall noting ventriculomegaly without transependymal migration of fluid. MRIs, neurological examinations, and whole exome sequencing were ordered. An unknown neurodegenerative process was suspected. Parents felt cognitive changes started with the fall, but care team suspected onset as early as kindergarten due to reports of difficulty learning to read. Neuropsychological evaluation found IQ scores in the exceptionally low range, with relatively preserved adaptive skills. Significant fine motor and language deficits were noted, with low average to below average verbal memory, reading, and spelling, and exceptionally low visual memory and calculation. Executive functioning skills were variable. Extensive history-taking revealed that while the patient had struggled to learn to read English in kindergarten, he had taught himself to read in Spanish at home without difficulty concurrently. Although m.3255G > A mutation is the most frequent of MT-TL1 mutations, literature has only described its mutation associated with myopathy, maternal inherited diabetes and deafness, focal segmental glomerulosclerosis, mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). No literature has defined neuropsychological outcomes of this rare disorder. Further, this case highlights the importance of understanding how language and education intersect in English Language Learner children.

#3320 GENETIC TESTING FOR MOLECULAR DIAGNOSIS OF NEPHROLITHIASIS AND NEPHROCALCINOSIS IN ADULT PATIENTS: A SINGLE-CENTER COHORT

Abstract Background and Aims Hereditary factors represent an important cause of nephrolithiasis which will generate stone disease in adult. Molecular analysis in patients with nephrolithiasis (NL) and/or nephrocalcinosis (NC) for a genetic mutation has become more accessible and the benefits are highlighted by an increasing number of publications. The aim of the paper was to study genetic screening for 36 NL and/or NC patients in order to identify the cases that can be confirmed by mutations in known kidney stone genes. Method Between 2020–2022 we included 35 adult patients with NL/NC with onset on pediatric age or in young adults, plus one of the following criteria: family history for NL/NC, indicative phenotype, recurrent NL. All the patients were diagnosed with NL and/or NC by ultrasound or computed tomography scan. Inform consent was signed and dated before blood samples and we performed genetic testing using nephrolithiasis panel (that include 45 genes) from 2 laboratories. In addition, we performed clinical assessment in a multidisciplinary team, underwent metabolic assessment, and caring out the genealogical tree. Results The study included 18 females and 17 males. The mean age of studied patients was 34.9 ± 10.3 years (range 18 – 54 years), although mean age of NL/NC diagnosis was 19.4 ± 12.0 years (range 0.5 – 34 years). 29 patients presented NL of any type, 12 patients presented both NL and NC, and 3 patients isolated NC. All the patients presented positive family history of NL/NC, 17 (48.5%) patients had pediatric age of onset, 17 (48.5%) patients presented indicative phenotype and 19 (54.2%) recurrent stone disease. Causative monogenic mutations were detected in 25 of 35 NL/NC. We identified 20 deleterious variants in 12 out of 45 analyzed genes. Genetic testing was positive with a definite diagnosis (had pathogenic variant) in 17 (48.6%) of cases, while 3 (8.6%) patients presented likely pathogenic variants, and 5 (14.3%) patients had variants of uncertain significance (VUS). In our cohort, the most common cause of kidney stone disease was cystine nephrolithiasis in 8 (22.8%) patients, followed by hereditary distal renal tubular acidosis in 4 patients, Dent disease in 3 patients, primary hyperoxaluria type 1 in 2 patients, familial hypomagnesaemia with hypercalciuria and nephrocalcinosis in 2 patients. Other causes of kidney stone disease included: renal hypouricemia type 1, hereditary hypophosphatemic rickets with hypercalciuria, primary mitochondrial disorders, autosomal dominant familial idiopathic hypercalciuria, Bartter syndrome type 3, and autosomal dominant tubulo-interstitial disease with hyperuricemia. Pathogenic mutations were detected in the following 5 dominant disease genes: SLC7A9 (4 patients), SLC4A1 (3 patients), ADCY10 (1 patient), HNF1B (2 patients), POLG (1 patient). Also, we identified pathogenic mutations in the following 7 recessive disease genes: CLCN5 (3 patients), AGXT (2 patients), CLDN16 (2 patients), SLC3A1 (3 patients), SLC34A3 (1 patient), WDR72 (1 patient), CLCNKB (1 patient). The mean eGFR for the study group was 71.2 ± 37.6 ml/min/1.72 m2. Seventeen patients presented eGFR &amp;lt; 60 ml/min/1.73 m2: seven patients CKD stage 3, four patients CKD stage 4, two patients CKD stage 5, and two patients were with renal replacement therapy. Conclusion Genetic kidney stone disease is an underdiagnosed condition. Although 48.5% of the patients had NL/NC onset on pediatric age, the molecular diagnosis was performed in adulthood, and, for some of them, when they suffer of advanced kidney disease. In our cohort, genetic testing had a high rate of positive molecular diagnosis of NL/NC due to selection criteria. Five of our patients presented VUS, but with disease-specific phenotype. We emphasize the importance of reporting these cases to generate additional evidence that could allow the reclassification of these variants. We conclude that the molecular diagnosis improves patient management, prevent or delay chronic kidney disease, offer possibility of genetic counseling and an extended screening to the family. Thus, our study showed the potential benefits of genetic testing, especially in high-risk groups for stone disease.

Methodologies in Mitochondrial Testing: Diagnosing a Primary Mitochondrial Respiratory Chain Disorder.

Mitochondria are cytosolic organelles within most eukaryotic cells. Mitochondria generate the majority of cellular energy in the form of adenosine triphosphate (ATP) through oxidative phosphorylation (OxPhos). Pathogenic variants in mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) lead to defects in OxPhos and physiological malfunctions (Nat Rev Dis Primer 2016;2:16080.). Patients with primary mitochondrial disorders (PMD) experience heterogeneous symptoms, typically in multiple organ systems, depending on the tissues affected by mitochondrial dysfunction. Because of this heterogeneity, clinical diagnosis is challenging (Annu Rev Genomics Hum Genet 2017;18:257-75.). Laboratory diagnosis of mitochondrial disease depends on a multipronged analysis that can include biochemical, histopathologic, and genetic testing. Each of these modalities has complementary strengths and limitations in diagnostic utility. The primary focus of this review is on diagnosis and testing strategies for primary mitochondrial diseases. We review tissue samples utilized for testing, metabolic signatures, histologic findings, and molecular testing approaches. We conclude with future perspectives on mitochondrial testing. This review offers an overview of the current biochemical, histologic, and genetic approaches available for mitochondrial testing. For each we review their diagnostic utility including complementary strengths and weaknesses. We identify gaps in current testing and possible future avenues for test development.

Pathophysiology and Management of Fatigue in Neuromuscular Diseases

Fatigue is a major determinant of quality of life and motor function in patients affected by several neuromuscular diseases, each of them characterized by a peculiar physiopathology and the involvement of numerous interplaying factors. This narrative review aims to provide an overview on the pathophysiology of fatigue at a biochemical and molecular level with regard to muscular dystrophies, metabolic myopathies, and primary mitochondrial disorders with a focus on mitochondrial myopathies and spinal muscular atrophy, which, although fulfilling the definition of rare diseases, as a group represent a representative ensemble of neuromuscular disorders that the neurologist may encounter in clinical practice. The current use of clinical and instrumental tools for fatigue assessment, and their significance, is discussed. A summary of therapeutic approaches to address fatigue, encompassing pharmacological treatment and physical exercise, is also overviewed.

Optimisation of AAV-NDI1 Significantly Enhances Its Therapeutic Value for Correcting Retinal Mitochondrial Dysfunction

AAV gene therapy for ocular disease has become a reality with the market authorisation of LuxturnaTM for RPE65-linked inherited retinal degenerations and many AAV gene therapies currently undergoing phase III clinical trials. Many ocular disorders have a mitochondrial involvement from primary mitochondrial disorders such as Leber hereditary optic neuropathy (LHON), predominantly due to mutations in genes encoding subunits of complex I, to Mendelian and multifactorial ocular conditions such as dominant optic atrophy, glaucoma and age-related macular degeneration. In this study, we have optimised the nuclear yeast gene, NADH-quinone oxidoreductase (NDI1), which encodes a single subunit complex I equivalent, creating a candidate gene therapy to improve mitochondrial function, independent of the genetic mutation driving disease. Optimisation of NDI1 (ophNdi1) substantially increased expression in vivo, protected RGCs and increased visual function, as assessed by optokinetic and photonegative response, in a rotenone-induced murine model. In addition, ophNdi1 increased cellular oxidative phosphorylation and ATP production and protected cells from rotenone insult to a significantly greater extent than wild type NDI1. Significantly, ophNdi1 treatment of complex I deficient patient-derived fibroblasts increased oxygen consumption and ATP production rates, demonstrating the potential of ophNdi1 as a candidate therapy for ocular disorders where mitochondrial deficits comprise an important feature.

Current and Emerging Therapies for Leber Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is the most common primary mitochondrial DNA disorder, presenting typically as a sequential, painless, subacute, optic neuropathy in young males. Despite the very limited therapeutic options in LHON, recent developments involving novel pharmacological agents and emerging gene therapy interventions have shown promising results for improved visual outcomes. A synthetic analogue of coenzyme Q (idebenone) is the most common medical treatment in LHON. In a multicentre, double-blind randomized, placebo-controlled clinical trial (Rescue of Hereditary Optic Disease Outpatient Study [RHODOS]), a dose of 900 mg/day of idebenone for 24 weeks was found to be well tolerated and safe. In a follow-up study (RHODOS-OFU), the visual acuity of 70% of patients enrolled in RHODOS was reassessed 30 months after discontinuation of idebenone. Results from this study suggested that visual acuity continued to improve even after discontinuation of the drug. Gene therapy has recently emerged as a potential treatment for LHON. RESCUE and REVERSE were two phase III clinical trials of viral-mediated gene therapy using lenadogene nolparvovec intravitreal injections in patients with early-stage LHON. Results in these trials have shown long-term safety and bilateral visual acuity improvement after unilateral intravitreal injections at 96 weeks, and sustained visual improvement after 3 years of treatment. The most recent phase III clinical trial in LHON (REFLECT) has shown significant improvement of vision after bilateral intravitreal injections of lenadogene nolparvovec compared with unilateral injections. These promising results suggest that, in the near future, LHON might become the first mitochondrial disorder to benefit from gene therapy.

Recognizing Mitochondrial Hepatopathy in Acute Fatty Liver of Pregnancy

Acute fatty liver of pregnancy (AFLP) is a rare cause of catastrophic liver dysfunction and failure in late pregnancy. Defective mitochondrial fatty acid oxidation (FAO) seems to be the underlying pathogenic mechanism. Stressors of both late pregnancy (increased maternal dependence on fats as energy source) and fetus with homozygous defect in mitochondrial FAO, precipitate AFLP in a woman who was previously asymptomatic. Mitochondrial disorders exhibit significant clinical heterogeneity, but many of these primary and secondary disorders have liver as the primary organ affected. The liver injury pattern in these patients also has a varied spectrum and course. Unlike other primary mitochondrial disorders affecting the liver, AFLP represents a potentially completely reversible form of liver injury. Thus, AFLP behaves as a secondary mitochondrial disorder precipitated by inciting factors from fetus and placenta. In this review, we attempt to highlight the varied aspects of mitochondrial dysfunction in AFLP. Mitochondrial injury is the mainstay of pathogenesis and can be recognized in typical clinical features and histopathological findings in patients with AFLP. We also present the consequent impact of recognizing underlying mitochondrial injury on clinical diagnosis and management strategies employed in AFLP.

Cellular and computational models reveal environmental and metabolic interactions in MMUT-type methylmalonic aciduria.

Methylmalonyl-coenzyme A (CoA) mutase (MMUT)-type methylmalonic aciduria is a rare inherited metabolic disease caused by the loss of function of the MMUT enzyme. Patients develop symptoms resembling those of primary mitochondrial disorders, but the underlying causes of mitochondrial dysfunction remain unclear. Here, we examined environmental and genetic interactions in MMUT deficiency using a combination of computational modeling and cellular models to decipher pathways interacting with MMUT. Immortalized fibroblast (hTERT BJ5ta) MMUT-KO (MUTKO) clones displayed a mild mitochondrial impairment in standard glucose-based medium, but they did not to show increased reliance on respiratory metabolism nor reduced growth or viability. Consistently, our modeling predicted MUTKO specific growth phenotypes only for lower extracellular glutamine concentrations. Indeed, two of three MMUT-deficient BJ5ta cell lines showed a reduced viability in glutamine-free medium. Further, growth on 183 different carbon and nitrogen substrates identified increased NADH (nicotinamide adenine dinucleotide) metabolism of BJ5ta and HEK293 MUTKO cells compared with controls on purine- and glutamine-based substrates. With this knowledge, our modeling predicted 13 reactions interacting with MMUT that potentiate an effect on growth, primarily those of secondary oxidation of propionyl-CoA, oxidative phosphorylation and oxygen diffusion. Of these, we validated 3-hydroxyisobutytyl-CoA hydrolase (HIBCH) in the secondary propionyl-CoA oxidation pathway. Altogether, these results suggest compensation for the loss of MMUT function by increasing anaplerosis through glutamine or by diverting flux away from MMUT through the secondary propionyl-CoA oxidation pathway, which may have therapeutic relevance.

Solutions to a Radical Problem: Overview of Current and Future Treatment Strategies in Leber's Hereditary Opic Neuropathy.

Leber's Hereditary Optic Neuropathy (LHON) is the most common primary mitochondrial DNA disorder. It is characterized by bilateral severe central subacute vision loss due to specific loss of Retinal Ganglion Cells and their axons. Historically, treatment options have been quite limited, but ongoing clinical trials show promise, with significant advances being made in the testing of free radical scavengers and gene therapy. In this review, we summarize management strategies and rational of treatment based on current insights from molecular research. This includes preventative recommendations for unaffected genetic carriers, current medical and supportive treatments for those affected, and emerging evidence for future potential therapeutics.