A Method for Producing Induced Pluripotent Stem Cell-Derived Cardiomyocytes from Leigh Syndrome Patients for Its Application in Disease Modeling and Drug Validation.

Leigh syndrome (LS), a complex multisystemic disorder, poses significant challenges in genetic medicine due to its intricate pathogenesis and wide-ranging clinical manifestations. Notably, these arise from mutations in either nuclear genetic DNA or mitochondrial DNA, affecting ATP production and resulting in diverse clinical outcomes. The unpredictable trajectory of this disease, ranging from severe developmental delays to early mortality, underscores the need for improved therapeutic solutions. This research pivots toward the novel use of induced pluripotent stem cells (iPSCs) as a promising platform for understanding disease mechanisms and spearheading patient-specific drug discoveries. Given the past successes of iPSCs in delineating organ-specific disorders and the recent endorsement of human iPSC-derived cardiomyocytes (CMs) by the FDA for drug evaluation, our work seeks to bridge this innovation to Leigh syndrome research. We detail a methodological approach to generate iPSCs from LS patients and differentiate them into iPSCs-CMs. Using multi-electrode array (MEA) analyses, we evaluate the field potential of these cells, spotlighting the potential of hiPSC-CM in drug validation and disease modeling. This pioneering approach offers a glimpse into the future of patient-centric therapeutic interventions for Leigh/Leigh-like syndrome.

Phenotypic assessment of Cox10 variants and their implications for Leigh Syndrome.

OBJECTIVES: Cox10 is an enzyme required for the activity of cytochrome c oxidase. Humans who lack at least one functional copy of Cox10 have a form of Leigh Syndrome, a genetic disease that is usually fatal in infancy. As more human genomes are sequenced, new alleles are being discovered; whether or not these alleles encode functional proteins remains unclear. Thus, we set out to measure the phenotypes of many human Cox10 variants by expressing them in yeast cells. RESULTS: We successfully expressed the reference sequence and 25 variants of human Cox10 in yeast. We quantitated the ability of these variants to support growth on nonfermentable media and directly measured cytochrome c oxidase activity. 11 of these Cox10 variants supported approximately half or more the cytochrome c oxidase activity compared to the reference sequence. All of the strains containing those 11 variants also grew robustly using a nonfermentable carbon source. Cells expressing the other variants showed low cytochrome c oxidase activity and failed to grow on nonfermentable media.

IARS2 mutations lead to Leigh syndrome with a combined oxidative phosphorylation deficiency.

BACKGROUND: Leigh syndrome (LS) is a common mitochondrial disease caused by mutations in both mitochondrial and nuclear genes. Isoleucyl-tRNA synthetase 2 (IARS2) encodes mitochondrial isoleucine-tRNA synthetase, and variants in IARS2 have been reported to cause LS. However, the pathogenic mechanism of IARS2 variants is still unclear. METHODS: Two unrelated patients, a 4-year-old boy and a 5-year-old boy diagnosed with LS, were recruited, and detailed clinical data were collected. The DNA of the patients and their parents was isolated from the peripheral blood for the identification of pathogenic variants using next-generation sequencing and Sanger sequencing. The ClustalW program, allele frequency analysis databases (gnomAD and ExAc), and pathogenicity prediction databases (Clinvar, Mutation Taster and PolyPhen2) were used to predict the conservation and pathogenicity of the variants. The gene expression level, oxygen consumption rate (OCR), respiratory chain complex activity, cellular adenosine triphosphate (ATP) production, mitochondrial membrane potential (MMP) and mitochondrial reactive oxygen species (ROS) levels were measured in patient-derived lymphocytes and IARS2-knockdown HEK293T cells to evaluate the pathogenicity of the variants. RESULTS: We reported 2 unrelated Chinese patients manifested with LS who carried biallelic IARS2 variants (c.1_390del and c.2450G > A from a 4-year-old boy, and c.2090G > A and c.2122G > A from a 5-year-old boy), of which c.1_390del and c.2090G > A were novel. Functional studies revealed that the patient-derived lymphocytes carrying c.1_390del and c.2450G > A variants exhibited impaired mitochondrial function due to severe mitochondrial complexes I and III deficiencies, which was also found in IARS2-knockdown HEK293T cells. The compensatory experiments in vitro cell models confirmed the pathogenicity of IARS2 variants since re-expression of wild-type IARS2 rather than mutant IARS2 could rescue complexes I and III deficiency, oxygen consumption, and cellular ATP content in IARS2 knockdown cells. CONCLUSION: Our results not only expand the gene mutation spectrum of LS, but also reveal for the first time the pathogenic mechanism of IARS2 variants due to a combined deficiency of mitochondrial complexes I and III, which is helpful for the clinical diagnosis of IARS2 mutation-related diseases.

Primary cilia formation requires the Leigh syndrome-associated mitochondrial protein NDUFAF2.

Mitochondria-related neurodegenerative diseases have been implicated in the disruption of primary cilia function. Mutation in an intrinsic mitochondrial complex I component NDUFAF2 has been identified in Leigh syndrome, a severe inherited mitochondriopathy. Mutations in ARMC9, which encodes a basal body protein, cause Joubert syndrome, a ciliopathy with defects in the brain, kidney, and eye. Here, we report a mechanistic link between mitochondria metabolism and primary cilia signaling. We discovered that loss of NDUFAF2 caused both mitochondrial and ciliary defects in vitro and in vivo and identified NDUFAF2 as a binding partner for ARMC9. We also found that NDUFAF2 was both necessary and sufficient for cilia formation and that exogenous expression of NDUFAF2 rescued the ciliary and mitochondrial defects observed in cells from patients with known ARMC9 deficiency. NAD+ supplementation restored mitochondrial and ciliary dysfunction in ARMC9-deficient cells and zebrafish and ameliorated the ocular motility and motor deficits of a patient with ARMC9 deficiency. The present results provide a compelling mechanistic link, supported by evidence from human studies, between primary cilia and mitochondrial signaling. Importantly, our findings have significant implications for the development of therapeutic approaches targeting ciliopathies.

Distinct Imaging Markers of Leigh's Disease Linked to SURF1 Mutation: A Pediatric Case Study.

BACKGROUND Leigh disease (LD) is a rare progressive mitochondrial neurodegenerative disorder characterized by subacute necrotizing encephalopathy and symmetrical spongiform lesions in the brain. Cytochrome C oxidase deficiencies due to SURF1 Cytochrome C Oxidase Assembly Factor (SURF1) gene mutations are seen only in 15% of LD cases. Consideration of these genetic mutations in young patients is crucial for early diagnosis, intervention, and further genetic counseling. Although only a few cases of the SURF1 mutation have been reported, there are anecdotal case reports describing imaging features. CASE REPORT We report a case of a 2-year-old boy with developmental delay, hypotonia, involuntary movements, shortness of breath, and reduced activity since age 6 months. On blood examination there was mildly elevated lactate levels and increased lactate to pyruvate ratio and cerebrospinal fluid lactate levels. Magnetic resonance imaging findings showed symmetrical lesions in the dentate nucleus, subthalamic nucleus, midbrain (substantia nigra, periaqueductal gray matter), posterolateral pons, and olivary nucleus of the medulla extending into the cervical spinal cord, with mild elevation of the lactate peak on magnetic resonance spectroscopy. CONCLUSIONS These findings prompted further genetic analysis, which indicated a mitochondrial type IV deficiency with the SURF1 gene defect, an intranuclear type 1 mutation (MC4DN1) (OMIM 220110). Treatment is usually supportive with vitamins supplementation and physiotherapy, and genetic counseling of the parents is mandatory.

Hyperkinesias in Leigh-like Syndrome with Complex-I Deficiency Due to m.10191T>C in MT-ND3.

Hyperkinesias in a patient with complex-I deficiency due to the variant m.10191T>C in MT-ND3 have not been previously reported. The patient is a 32 years-old female with multisystem mitochondrial disease due to variant m.10191T>C in MT-ND3, who has been experiencing episodic, spontaneous or induced abnormal movements since age 23. The abnormal movements started as right hemi-athetosis, bilateral dystonia of the legs, or unilateral dystonia of the right arm and leg. They often progressed to severe ballism, involving the trunk, and limbs. The arms were more dystonic than the legs. In conclusion, complex-I deficiency due to the variant m.10191T>C in MT-ND3 may manifest as multisystem disease including hyperkinesias. Neurologists should be aware of hyperkinesias as a manifestation of complex-I deficiency.

RésuméL'hyperkinésie d'une patiente atteinte d'un déficit en complexe I dû à la variante m.10191T>C du gène MT-ND3 n'a jamais été rapportée auparavant. La patiente est une femme de 32 ans atteinte d'une maladie mitochondriale multisystémique due à la variante m.10191T>C du gène MT-ND3, qui présente des mouvements anormaux épisodiques, spontanés ou provoqués depuis l'âge de 18 ans. mouvements anormaux épisodiques, spontanés ou provoqués depuis l'âge de 23 ans. Les mouvements anormaux ont commencé par une hémiathétose droite, dystonie bilatérale des jambes ou dystonie unilatérale du bras et de la jambe droite. Ils ont souvent évolué vers un ballisme sévère, impliquant le tronc et les membres. le tronc et les membres. Les bras étaient plus dystoniques que les jambes. En conclusion, le déficit en complexe I dû à la variante m.10191T>C du gène MT-ND3 peut se manifester par une maladie multisystémique comprenant des hyperkinésies. Les neurologues doivent être conscients que l'hyperkinésie est une manifestation du déficit en complexe-I. de la déficience en complexe I.

Exclusion of sulfide:quinone oxidoreductase from mitochondria causes Leigh-like disease in mice by impairing sulfide metabolism.

Leigh syndrome is the most common inherited mitochondrial disease in children and is often fatal within the first few years of life. In 2020, mutations in the gene encoding sulfide:quinone oxidoreductase (SQOR), a mitochondrial protein, were identified as a cause of Leigh syndrome. Here, we report that mice with a mutation in the gene encoding SQOR (SqorΔN/ΔN mice), which prevented SQOR from entering mitochondria, had clinical and pathological manifestations of Leigh syndrome. SqorΔN/ΔN mice had increased blood lactate levels that were associated with markedly decreased complex IV activity and increased hydrogen sulfide (H2S) levels. Because H2S is produced by both gut microbiota and host tissue, we tested whether metronidazole (a broad-spectrum antibiotic) or a sulfur-restricted diet rescues SqorΔN/ΔN mice from developing Leigh syndrome. Daily treatment with metronidazole alleviated increased H2S levels, normalized complex IV activity and blood lactate levels, and prolonged the survival of SqorΔN/ΔN mice. Similarly, a sulfur-restricted diet normalized blood lactate levels and inhibited the development of Leigh syndrome. Taken together, these observations suggest that mitochondrial SQOR is essential to prevent systemic accumulation of H2S. Metronidazole administration and a sulfur-restricted diet may be therapeutic approaches to treatment of patients with Leigh syndrome caused by mutations in SQOR.

Infantile Epileptic Spasms Syndrome Complicated by Leigh Syndrome and Leigh-Like Syndrome: A Retrospective, Nationwide, Multicenter Case Series.

BACKGROUND: Six percent of patients with Leigh syndrome (LS) present with infantile epileptic spasms syndrome (IESS). However, treatment strategies for IESS with LS remain unclear. This retrospective study aimed to evaluate the efficacy and safety of treatment strategies in patients with IESS complicated by LS and Leigh-like syndrome (LLS). METHODS: We distributed questionnaires to 750 facilities in Japan, and the clinical data of 21 patients from 15 hospitals were collected. The data comprised treatment strategies, including adrenocorticotropic hormone (ACTH) therapy, ketogenic diet (KD) therapy, and antiseizure medications (ASMs); effectiveness of each treatment; and the adverse events. RESULTS: The median age at LS and LLS diagnosis was 7 months (range: 0 to 50), whereas that at the onset of epileptic spasms was 7 (range: 3 to 20). LS was diagnosed in 17 patients and LLS in four patients. Seven, two, five, and seven patients received ACTH + ASMs, ACTH + KD + ASMs, KD + ASMs, and ASMs only, respectively. Four (44%) of nine patients treated with ACTH and one (14%) of seven patients treated with KD achieved electroclinical remission within one month of treatment. No patients treated with only ASMs achieved electroclinical remission. Seven patients (33%) achieved electroclinical remission by the last follow-up. Adverse events were reported in four patients treated with ACTH, none treated with KD therapy, and eight treated with ASMs. CONCLUSION: ACTH therapy shows the best efficacy and rapid action in patients with IESS complicated by LS and LLS. The effectiveness of KD therapy and ASMs in this study was insufficient.

Further delineation of short-chain enoyl-CoA hydratase deficiency in the Pacific population.

Short-chain enoyl-coA hydratase (SCEH) deficiency due to biallelic pathogenic ECHS1 variants was first reported in 2014 in association with Leigh syndrome (LS) and increased S-(2-carboxypropyl)cysteine excretion. It is potentially treatable with a valine-restricted, high-energy diet and emergency regimen. Recently, Simon et al. described four Samoan children harbouring a hypomorphic allele (c.489G > A, p.Pro163=) associated with reduced levels of normally-spliced mRNA. This synonymous variant, missed on standard genomic testing, is prevalent in the Samoan population (allele frequency 0.17). Patients with LS and one ECHS1 variant were identified in NZ and Australian genomic and clinical databases. ECHS1 sequence data were interrogated for the c.489G > A variant and clinical data were reviewed. Thirteen patients from 10 families were identified; all had Pacific ancestry including Samoan, Maori, Cook Island Maori, and Tokelauan. All developed bilateral globus pallidi lesions, excluding one pre-symptomatic infant. Symptom onset was in early childhood, and was triggered by illness or starvation in 9/13. Four of 13 had exercise-induced dyskinesia, 9/13 optic atrophy and 6/13 nystagmus. Urine S-(2-carboxypropyl)cysteine-carnitine and other SCEH-related metabolites were normal or mildly increased. Functional studies demonstrated skipping of exon four and markedly reduced ECHS1 protein. These data provide further support for the pathogenicity of this ECHS1 variant which is also prevalent in Maori, Cook Island Maori, and Tongan populations (allele frequency 0.14-0.24). It highlights the need to search for a second variant in apparent heterozygotes with an appropriate phenotype, and has implications for genetic counselling in family members who are heterozygous for the more severe ECHS1 alleles. SYNOPSIS: Short-chain enoyl-CoA hydratase deficiency is a frequent cause of Leigh-like disease in Maori and wider-Pacific populations, due to the high carrier frequency of a hypomorphic ECHS1 variant c.489G > A, p.[Pro163=, Phe139Valfs*65] that may be overlooked by standard genomic testing.

The Blood-Brain Barrier Is Unaffected in the Ndufs4-/- Mouse Model of Leigh Syndrome.

Mitochondrial dysfunction plays a major role in physiological aging and in many pathological conditions. Yet, no study has explored the consequence of primary mitochondrial deficiency on the blood-brain barrier (BBB) structure and function. Addressing this question has major implications for pharmacological and genetic strategies aimed at ameliorating the neurological symptoms that are often predominant in patients suffering from these conditions. In this study, we examined the permeability of the BBB in the Ndufs4-/- mouse model of Leigh syndrome (LS). Our results indicated that the structural and functional integrity of the BBB was preserved in this severe model of mitochondrial disease. Our findings suggests that pharmacological or gene therapy strategies targeting the central nervous system in this mouse model and possibly other models of mitochondrial dysfunction require the use of specific tools to bypass the BBB. In addition, they raise the need for testing the integrity of the BBB in complementary in vivo models.

Synthetic aporphine alkaloids are potential therapeutics for Leigh syndrome.

Mitochondrial diseases are mainly caused by dysfunction of mitochondrial respiratory chain complexes and have a variety of genetic variants or phenotypes. There are only a few approved treatments, and fundamental therapies are yet to be developed. Leigh syndrome (LS) is the most severe type of progressive encephalopathy. We previously reported that apomorphine, an anti- "off" agent for Parkinson's disease, has cell-protective activity in patient-derived skin fibroblasts in addition to strong dopamine agonist effect. We obtained 26 apomorphine analogs, synthesized 20 apomorphine derivatives, and determined their anti-cell death effect, dopamine agonist activity, and effects on the mitochondrial function. We found three novel apomorphine derivatives with an active hydroxy group at position 11 of the aporphine framework, with a high anti-cell death effect without emetic dopamine agonist activity. These synthetic aporphine alkaloids are potent therapeutics for mitochondrial diseases without emetic side effects and have the potential to overcome the low bioavailability of apomorphine. Moreover, they have high anti-ferroptotic activity and therefore have potential as a therapeutic agent for diseases related to ferroptosis.

NDUFV1-Related Mitochondrial Complex-1 Disorders: A Retrospective Case Series and Literature Review.

BACKGROUND: Pathogenic variants in the NDUFV1 gene disrupt mitochondrial complex I, leading to neuroregression with leukoencephalopathy and basal ganglia involvement on neuroimaging. This study aims to provide a concise review on NDUFV1-related disorders while adding the largest cohort from a single center to the existing literature. METHODS: We retrospectively collected genetically proven cases of NDUFV1 pathogenic variants from our center over the last decade and explored reported instances in existing literature. Magnetic resonance imaging (MRI) patterns observed in these patients were split into three types-Leigh (putamen, basal ganglia, thalamus, and brainstem involvement), mitochondrial leukodystrophy (ML) (cerebral white matter involvement with cystic cavitations), and mixed (both). RESULTS: Analysis included 44 children (seven from our center and 37 from literature). The most prevalent comorbidities were hypertonia, ocular abnormalities, feeding issues, and hypotonia at onset. Children with the Leigh-type MRI pattern exhibited significantly higher rates of breathing difficulties, whereas those with a mixed phenotype had a higher prevalence of dystonia. The c.1156C>T variant in exon 8 of the NDUFV1 gene was the most common variant among individuals of Asian ethnicity and is predominantly associated with irritability and dystonia. Seizures and Leigh pattern of MRI of the brain was found to be less commonly associated with this variant. Higher rate of mortality was observed in children with Leigh-type pattern on brain MRI and those who did not receive mitochondrial cocktail. CONCLUSIONS: MRI phenotyping might help predict outcome. Appropriate and timely treatment with mitochondrial cocktail may reduce the probability of death and may positively impact the long-term outcomes, regardless of the genetic variant or age of onset.

Biallelic Variants of MRPS36 Cause a New Form of Leigh Syndrome.

BACKGROUND: The MRPS36 gene encodes a recently identified component of the 2-oxoglutarate dehydrogenase complex (OGDHC), a key enzyme of the Krebs cycle catalyzing the oxidative decarboxylation of 2-oxoglutarate to succinyl-CoA. Defective OGDHC activity causes a clinically variable metabolic disorder characterized by global developmental delay, severe neurological impairment, liver failure, and early-onset lactic acidosis. METHODS: We investigated the molecular cause underlying Leigh syndrome with bilateral striatal necrosis in two siblings through exome sequencing. Functional studies included measurement of the OGDHC enzymatic activity and MRPS36 mRNA levels in fibroblasts, assessment of protein stability in transfected cells, and structural analysis. A literature review was performed to define the etiological and phenotypic spectrum of OGDHC deficiency. RESULTS: In the two affected brothers, exome sequencing identified a homozygous nonsense variant (c.283G>T, p.Glu95*) of MRPS36. The variant did not affect transcript processing and stability, nor protein levels, but resulted in a shorter protein lacking nine residues that contribute to the structural and functional organization of the OGDHC complex. OGDHC enzymatic activity was significantly reduced. The review of previously reported cases of OGDHC deficiency supports the association of this enzymatic defect with Leigh phenotypic spectrum and early-onset movement disorder. Slightly elevated plasma levels of glutamate and glutamine were observed in our and literature patients with OGDHC defect. CONCLUSIONS: Our findings point to MRPS36 as a new disease gene implicated in Leigh syndrome. The slight elevation of plasma levels of glutamate and glutamine observed in patients with OGDHC deficiency represents a candidate metabolic signature of this neurometabolic disorder. © 2024 International Parkinson and Movement Disorder Society.

Interferon-gamma contributes to disease progression in the Ndufs4(-/-) model of Leigh syndrome.

AIM: Leigh syndrome (LS), the most common paediatric presentation of genetic mitochondrial dysfunction, is a multi-system disorder characterised by severe neurologic and metabolic abnormalities. Symmetric, bilateral, progressive necrotizing lesions in the brainstem are defining features of the disease. Patients are often symptom free in early life but typically develop symptoms by about 2 years of age. The mechanisms underlying disease onset and progression in LS remain obscure. Recent studies have shown that the immune system causally drives disease in the Ndufs4(-/-) mouse model of LS: treatment of Ndufs4(-/-) mice with the macrophage-depleting Csf1r inhibitor pexidartinib prevents disease. While the precise mechanisms leading to immune activation and immune factors involved in disease progression have not yet been determined, interferon-gamma (IFNγ) and interferon gamma-induced protein 10 (IP10) were found to be significantly elevated in Ndufs4(-/-) brainstem, implicating these factors in disease. Here, we aimed to explore the role of IFNγ and IP10 in LS. METHODS: To establish the role of IFNγ and IP10 in LS, we generated IFNγ and IP10 deficient Ndufs4(-/-)/Ifng(-/-) and Ndufs4(-/-)/IP10(-/-) double knockout animals, as well as IFNγ and IP10 heterozygous, Ndufs4(-/-)/Ifng(+/-) and Ndufs4(-/-)/IP10(+/-), animals. We monitored disease onset and progression to define the impact of heterozygous or homozygous loss of IFNγ and IP10 in LS. RESULTS: Loss of IP10 does not significantly impact the onset or progression of disease in the Ndufs4(-/-) model. IFNγ loss significantly extends survival and delays disease progression in a gene dosage-dependent manner, though the benefits are modest compared to Csf1r inhibition. CONCLUSIONS: IFNγ contributes to disease onset and progression in LS. Our findings suggest that IFNγ targeting therapies may provide some benefits in genetic mitochondrial disease, but targeting IFNγ alone would likely yield only modest benefits in LS.

Biallelic NDUFA4 Deletion Causes Mitochondrial Complex IV Deficiency in a Patient with Leigh Syndrome.

Oxidative phosphorylation involves a complex multi-enzymatic mitochondrial machinery critical for proper functioning of the cell, and defects herein cause a wide range of diseases called "primary mitochondrial disorders" (PMDs). Mutations in about 400 nuclear and 37 mitochondrial genes have been documented to cause PMDs, which have an estimated birth prevalence of 1:5000. Here, we describe a 4-year-old female presenting from early childhood with psychomotor delay and white matter signal changes affecting several brain regions, including the brainstem, in addition to lactic and phytanic acidosis, compatible with Leigh syndrome, a genetically heterogeneous subgroup of PMDs. Whole genome sequencing of the family trio identified a homozygous 12.9 Kb deletion, entirely overlapping the NDUFA4 gene. Sanger sequencing of the breakpoints revealed that the genomic rearrangement was likely triggered by Alu elements flanking the gene. NDUFA4 encodes for a subunit of the respiratory chain Complex IV, whose activity was significantly reduced in the patient's fibroblasts. In one family, dysfunction of NDUFA4 was previously documented as causing mitochondrial Complex IV deficiency nuclear type 21 (MC4DN21, OMIM 619065), a relatively mild form of Leigh syndrome. Our finding confirms the loss of NDUFA4 function as an ultra-rare cause of Complex IV defect, clinically presenting as Leigh syndrome.

Primary mitochondrial diseases: The intertwined pathophysiology of bioenergetic dysregulation, oxidative stress and neuroinflammation.

OBJECTIVES AND SCOPE: Primary mitochondrial diseases (PMDs) are rare genetic disorders resulting from mutations in genes crucial for effective oxidative phosphorylation (OXPHOS) that can affect mitochondrial function. In this review, we examine the bioenergetic alterations and oxidative stress observed in cellular models of primary mitochondrial diseases (PMDs), shedding light on the intricate complexity between mitochondrial dysfunction and cellular pathology. We explore the diverse cellular models utilized to study PMDs, including patient-derived fibroblasts, induced pluripotent stem cells (iPSCs) and cybrids. Moreover, we also emphasize the connection between oxidative stress and neuroinflammation. INSIGHTS: The central nervous system (CNS) is particularly vulnerable to mitochondrial dysfunction due to its dependence on aerobic metabolism and the correct functioning of OXPHOS. Similar to other neurodegenerative diseases affecting the CNS, individuals with PMDs exhibit several neuroinflammatory hallmarks alongside neurodegeneration, a pattern also extensively observed in mouse models of mitochondrial diseases. Based on histopathological analysis of postmortem human brain tissue and findings in mouse models of PMDs, we posit that neuroinflammation is not merely a consequence of neurodegeneration but a potential pathogenic mechanism for disease progression that deserves further investigation. This recognition may pave the way for novel therapeutic strategies for this group of devastating diseases that currently lack effective treatments. SUMMARY: In summary, this review provides a comprehensive overview of bioenergetic alterations and redox imbalance in cellular models of PMDs while underscoring the significance of neuroinflammation as a potential driver in disease progression.

Alternative splicing expands the clinical spectrum of NDUFS6-related mitochondrial disorders.

PURPOSE: We describe 3 families with Charcot-Marie-Tooth neuropathy (CMT), harboring a homozygous NDUFS6 NM_004553.6:c.309+5G>A variant previously linked to fatal Leigh syndrome. We aimed to characterize clinically and molecularly the newly identified patients and understand the mechanism underlying their milder phenotype. METHODS: The patients underwent extensive clinical examinations. Exome sequencing was done in 4 affected individuals. The functional effect of the c.309+5G>A variant was investigated in patient-derived EBV-transformed lymphoblasts at the complementary DNA, protein, and mitochondrial level. Alternative splicing was evaluated using complementary DNA long-read sequencing. RESULTS: All patients presented with early-onset, slowly progressive axonal CMT, and nystagmus; some exhibited additional central nervous system symptoms. The c.309+5G>A substitution caused the expression of aberrantly spliced transcripts and negligible levels of the canonical transcript. Immunoblotting showed reduced levels of mutant isoforms. No detectable defects in mitochondrial complex stability or bioenergetics were found. CONCLUSION: We expand the clinical spectrum of NDUFS6-related mitochondrial disorders to include axonal CMT, emphasizing the clinical and pathophysiologic overlap between these 2 clinical entities. This work demonstrates the critical role that alternative splicing may play in modulating the severity of a genetic disorder, emphasizing the need for careful consideration when interpreting splice variants and their implications on disease prognosis.

Identification of a novel MT-ND3 variant and restoring mitochondrial function by allotopic expression of MT-ND3 gene.

Mitochondrial diseases are caused by nuclear, or mitochondrial DNA (mtDNA) variants and related co-factors. Here, we report a novel m.10197G > C variant in MT-ND3 in a patient, and two other patients with m.10191 T > C. MT-ND3 variants are known to cause Leigh syndrome or mitochondrial complex I deficiency. We performed the functional analyses of the novel m.10197G > C variant that significantly lowered MT-ND3 protein levels, causing complex I assembly and activity deficiency, and reduction of ATP synthesis. We adapted a previously described re-engineering technique of delivering mitochondrial genes into mitochondria through codon optimization for nuclear expression and translation by cytoplasmic ribosomes to rescue defects arising from the MT-ND3 variants. We constructed mitochondrial targeting sequences along with the codon-optimized MT-ND3 and imported them into the mitochondria. To achieve the goal, we imported codon-optimized MT-ND3 into mitochondria in three patients with m.10197G > C and m.10191 T > C missense variants in the MT-ND3. Nuclear expression of the MT-ND3 gene partially restored protein levels, complex I deficiency, and significant improvement of ATP production indicating a functional rescue of the mutant phenotype. The codon-optimized nuclear expression of mitochondrial protein and import inside the mitochondria can supplement the requirements for ATP in energy-deficient mitochondrial disease patients.