Mitochondrial disorders …

Neuromuscular Disorders

Updated 05.27.2026
Released 02.04.2011
Expires For CME 05.27.2029

Mitochondrial disorders

Authors: Georgette Dib MD, Miriam Bekhit MD

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Editor: Nicholas E Johnson MD MSCI FAAN

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Mitochondrial disorders

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Introduction

Overview

Mitochondrial diseases represent one of the most clinically and genetically diverse groups of neurologic disorders. Rapid advances in next-generation sequencing and multi-omic technologies have transformed the diagnostic landscape, enabling earlier detection of pathogenic variants and clearer genotype–phenotype correlations. Recent discoveries have expanded the understanding of how defects in oxidative phosphorylation, mitochondrial dynamics, and organelle quality control contribute to disease. At the same time, emerging targeted therapies, including novel small-molecule approaches and early gene-based strategies, are beginning to reshape the treatment horizon. This overview highlights key developments in diagnosis and management as the field moves toward increasingly precise therapeutic strategies.

Key points

	• Mitochondrial dysfunction can affect any high-energy organ, including the brain, heart, kidney, GI tract, retina, and muscle.
	• Over 350 to 400 genes, mainly nuclear-encoded, have been implicated, with better genotype–phenotype correlations and heteroplasmy detection-enhancing diagnosis, counseling, and mechanistic understanding.
	• Therapeutic advances include FDA-approved agents (elamipretide, idebenone), emerging small-molecule and gene-based strategies, multidisciplinary management, and reproductive technologies, such as mitochondrial replacement therapy.

Description

General overview. Mitochondrial diseases are among the most clinically and genetically heterogeneous groups of human disorders, arising from pathogenic variants in either mitochondrial DNA (mtDNA) or nuclear DNA (nDNA) that disrupt oxidative phosphorylation and other critical mitochondrial pathways. These conditions rank among the most common inherited metabolic disorders, with an overall lifetime risk of approximately one in 1500 (48; 06). Advances in next-generation sequencing have largely replaced muscle biopsy with minimally invasive blood sampling, enabling unbiased genetic diagnosis. Whole-exome and whole-genome sequencing have doubled the number of recognized disease genes, now exceeding 350 to 400, and significantly increased diagnostic yield (04; 43; 62).

Clinical manifestations are highly variable, reflecting the ubiquitous role of mitochondria in energy production. High-energy tissues, including the brain, sensory epithelia, extraocular muscles, cardiac muscle, and skeletal muscle, are most affected. Common features include seizures, stroke-like episodes, hearing loss, retinopathy, external ophthalmoparesis, exercise intolerance, and diabetes mellitus. The presence of two or more such features should prompt evaluation for a mitochondrial disorder. Disease severity ranges from severe childhood-onset conditions to milder adult-onset presentations (48).

Pathogenic mechanisms extend beyond primary oxidative phosphorylation defects to include mtDNA depletion or multiple deletions, disrupted NAD⁺/NADH ratios, and abnormalities in mitochondrial dynamics, including biogenesis and mitophagy. These insights have guided the development of targeted therapeutic strategies (62; 02).

Mitochondrial disorders are classified using clinical, biochemical, and molecular approaches. Clinical/syndromic classification groups patients by recognizable syndromes, such as MELAS, MERRF, Leber hereditary optic neuropathy, Kearns-Sayre, and Leigh syndrome, though many patients present with overlapping or atypical phenotypes (41; 48; 62). Biochemical classification relies on respiratory chain deficiencies or other metabolic abnormalities, but results can be limited by tissue availability and artifacts, and some genetically confirmed patients show normal biochemistry (04; 43; 02). Molecular/genetic classification, increasingly favored, categorizes disorders by underlying gene defect and function, including defects in respiratory chain subunits, assembly factors, mtDNA maintenance, translation, mitochondrial dynamics, or other functions, such as cofactor metabolism (41; 02; 19; 06). This approach enables definitive diagnosis, genetic counseling, and genotype-phenotype correlation, though current practice often integrates all three classification strategies (62; 06).

Mitochondria, remnants of protobacteria according to the endosymbiotic hypothesis, are the primary source of energy for human tissues. They host not only oxidative phosphorylation and the electron transport chain but also metabolic pathways, including the pyruvate dehydrogenase complex, amino acid catabolism, the carnitine cycle, beta-oxidation, and the tricarboxylic acid (Krebs) cycle (66).

Although defects across any of these mitochondrial pathways technically constitute mitochondrial disease, the term “mitochondrial encephalomyopathy” is most often used to describe disorders caused by respiratory chain dysfunction (25; 22). The respiratory chain represents the “business end” of oxidative metabolism, where ATP production occurs. Reducing equivalents generated by the Krebs cycle and beta-oxidation are transferred through a series of protein complexes in the inner mitochondrial membrane that make up the electron transport chain. This chain consists of four multimeric complexes (I–IV) and two mobile electron carriers—coenzyme Q10 (ubiquinone) and cytochrome c.

Electron transfer through these complexes drives proton extrusion from the mitochondrial matrix into the intermembrane space, generating an electrochemical gradient. Complex V (ATP synthase) then harnesses this gradient to synthesize ATP through oxidative phosphorylation. Disruption at any step of this process impairs cellular energy production and underlies the pathophysiology of respiratory chain–related mitochondrial encephalomyopathies.

A defining feature of the respiratory chain is its dual genetic control: mitochondrial DNA (mtDNA) encodes 13 of the approximately 89 protein subunits that constitute the oxidative phosphorylation system, whereas nuclear DNA (nDNA) encodes all remaining components. Notably, complex II (succinate dehydrogenase, which also participates in the TCA cycle), coenzyme Q10, and cytochrome c are encoded exclusively by nDNA. In contrast, complexes I, III, IV, and V contain essential mtDNA-encoded subunits: seven for complex I (ND1, ND2, ND3, ND4, ND4L, ND5, ND6), one for complex III (cytochrome b), three for complex IV (COX I, COX II, COX III), and two for complex V (ATPase 6, ATPase 8) (25; 22).

History. Initially, mitochondrial disorders were described by symptoms, and the first were mitochondrial myopathies (affecting muscles). Mitochondrial myopathies were described in the early 1960s when systematic ultrastructural and histochemical studies revealed excessive proliferation of normal- or abnormal-looking mitochondria in the muscle of patients with weakness or exercise intolerance (75; 76). Because, with the modified Gomori trichrome stain, the areas of mitochondrial accumulation appeared crimson, the abnormal fibers were dubbed “ragged-red fibers” (35) and came to be considered the pathological hallmark of mitochondrial disease.

However, it soon became apparent that in many patients with ragged-red fibers, the myopathy is associated with symptoms and signs of brain involvement, and the term “mitochondrial encephalomyopathies” was introduced. It also became clear that the lack of ragged-red fibers in the biopsy does not exclude a mitochondrial etiology, as exemplified by Leigh syndrome, an encephalopathy of infancy or childhood invariably due to mitochondrial dysfunction but rarely accompanied by ragged-red fibers.

Through the 1970s and 1980s, abnormalities of oxidative phosphorylation were identified (24), so this was the assay often done. Multisystem disorders such as Kearns-Sayre (limb weakness, progressive ataxia, and ophthalmologic abnormalities) and Leigh syndrome (brain MRI findings consistent with encephalopathy) were associated with mitochondrial disease. In the late 1970s, Koeningsberger described mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), as well as myoclonic epilepsy with ragged red fibers (MERRF) (50). Once mitochondrial DNA was characterized in 1988 (44), variations were quickly associated with diseases that had been described. However, not all patients with clear mitochondrial disease (by symptoms or respiratory chain enzyme assays) had abnormal mtDNA, and with the advent of nuclear DNA sequencing, more and more nuclear genes were associated with mitochondrial disease (14). The list of nuclear genes that cause mitochondrial disease continues to expand.

mtDNA. Human mtDNA is a 16,569-kb circular, double-stranded molecule containing 37 genes: two rRNA genes, 22 tRNA genes, and 13 structural genes encoding the respiratory chain subunits listed above.

In the course of evolution, mtDNA has lost much of its original autonomy and now depends heavily on the nuclear genome to produce factors needed for mtDNA integrity; transcription, translation, and replication (“maintenance”); inner membrane integrity; and mitochondrial dynamics (22). Mitochondrial disease can be inherited through the human mitochondrial DNA (mtDNA), which is inherited through the maternal line because it is a circular double-stranded molecule. It can also be inherited through nuclear inheritance patterns, including X-linked, autosomal recessive, and autosomal dominant manners.

Since 1988, the circle of mtDNA has become crowded with pathogenic mutations, and the principles of mitochondrial genetics should, therefore, be familiar to the practicing physician.

Heteroplasmy and threshold effect. Each cell contains hundreds or thousands of mtDNA copies, which, at cell division, are distributed randomly among daughter cells. In normal tissues, all mtDNA molecules are identical (homoplasmy). Deleterious mtDNA mutations usually (but not always) affect only some (but not all) mtDNAs. The clinical expression of a pathogenic mtDNA mutation is largely determined by the relative proportion of normal and mutant genomes in different tissues. A minimum critical number of mutant mtDNAs is required to cause mitochondrial dysfunction in a particular organ or tissue and mitochondrial disease in an individual (threshold effect). Studies examining a known mtDNA mutation, m.3243 A>G, demonstrated that the amount of heteroplasmy in blood and urine > muscle and blood > muscle and urine seemed to correlate with disease severity (42).

Mitotic segregation. At cell division, the proportion of mutant mtDNAs in daughter cells may shift, and the phenotype may change accordingly. This phenomenon, called “mitotic segregation,” explains how the clinical phenotype may change in certain patients with mtDNA-related disorders as they grow older.

Maternal inheritance. At fertilization, all mtDNA derives from the oocyte. Therefore, the mode of transmission of mtDNA and mtDNA point mutations (single deletions of mtDNA are usually sporadic events) differs from Mendelian inheritance. A mother carrying an mtDNA point mutation will pass it on to all her children (boys and girls), but only her daughters will transmit it to their progeny.

Nuclear DNA (nDNA). In the last several years, there has been an increasing recognition that mitochondrial disease is not just inherited as mtDNA but also is from both parents through genes passed on by the nucleus.

For the most part, most nuclear-inherited mitochondrial disorders are inherited in an autosomal recessive manner, such that both parents are carriers and the affected offspring has two defective copies of the gene causing the mitochondrial disease. However, X-linked and autosomal dominant inheritance also occur. A common example of X-linked is pyruvate dehydrogenase deficiency from mutations in the PDHA1 subunit located on the X chromosome. Disorders from the autosomally located POLG can be inherited in an autosomal dominant and an autosomal recessive manner, depending on the type of variation.

Abnormal oxidative phosphorylation from abnormal respiratory chain enzymes is not the only cause of mitochondrial disease. Mitochondrial oxidative phosphorylation is dependent on numerous processes. The respiratory chain subunits must be encoded. The actual electron channel is usually encoded by mtDNA, and the support system by nuclear DNA. These subunits must be assembled (usually inherited through nuclear DNA). Protein synthesis within the mitochondria is dependent on translation by mitochondrial-specific ribosomal RNA (encoded by mtDNA) as well as transfer RNA (encoded by mtDNA), which are loaded by tRNA synthases (which are encoded by nuclear DNA). Nuclear genes encode mtDNA maintenance and replication. The mitochondria themselves move, join, and separate (fusion, fission, and motility) and are encoded by nuclear genes. Mitochondria are also dependent on having appropriate lipid bilayers that are built from proteins encoded by nuclear genes. As you can see, normal mitochondrial function is a complex orchestration between mtDNA and nuclear DNA and is still not entirely understood.

To best understand testing, a classification using the two major categories, disorders due to defects of mtDNA and disorders due to defects of nDNA, can be used.

Disorders due to defects of mtDNA

Clinically named disorders. Many of the initial mitochondrial disorders were diagnosed based on clinical findings. Several of these were then found to have mtDNA causes. Of important note, as nuclear genes have also been identified, this association is not always true, so the clinical phenotype is important to naming, but the initial description of cause (mtDNA point mutation or deletion) is not always found, and a nuclear mutation is found instead.

Causes of these named disorders include mitochondrial DNA rearrangements (single deletions or duplications) and point mutations.

Disorders initially attributed to mtDNA rearrangements and single deletions of mtDNA. These include three sporadic conditions: Pearson syndrome, Kearns-Sayre syndrome, and progressive external ophthalmoplegia with or without proximal limb weakness.

Pearson syndrome is usually a fatal disorder in infancy characterized by sideroblastic anemia and exocrine pancreas dysfunction.

Kearns-Sayre syndrome is a multisystem disorder of impaired eye movements (progressive external ophthalmoplegia), pigmentary retinopathy, and heart block, with onset before the age of 20. Frequent additional signs include ataxia, dementia, and endocrinopathies (diabetes mellitus, short stature, hypoparathyroidism). Lactic acidosis, elevated CSF protein (over 100mg/dl), and scattered COX-negative ragged-red fibers in the muscle biopsy are typical laboratory abnormalities. This is one of the earlier described disorders.

Progressive external ophthalmoplegia with or without proximal limb weakness. It is often compatible with a normal lifespan. Deletions within the mtDNA vary in size and location, but a “common” deletion of 5 kb is frequently seen in patients and in aged individuals (72; 71).

Point mutations within the mtDNA. These are common, with over 250 pathogenic point mutations having been identified in mtDNA from patients with various disorders. Most are maternally inherited and multisystemic (61), but some are sporadic and tissue-specific. Among the maternally inherited encephalomyopathies, four syndromes are more common.

MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) typically, but not always, presents in children or young adults after normal early development. Symptoms include recurrent vomiting, migraine-like headache, and stroke-like episodes causing cortical blindness, hemiparesis, or hemianopia. These symptoms often are progressive, leading to greater morbidity and, potentially, mortality. MRI of the brain shows “infarcts” that do not correspond to the distribution of major vessels, raising the question of whether the strokes are vascular or metabolic in nature (56). The most common mtDNA mutation is m.3243A>G in the tRNA(Leu)(UUR) gene, but about a dozen other mutations have been associated with MELAS, most notably a mutation (m.13513G> A) in the ND5 gene, which encodes subunit 5 of complex I (74). Notably, most maternal relatives of a typical MELAS patient carry the mutation in low abundance and are either mildly affected or unaffected due to heteroplasmy. In agreement with this observation, two epidemiological studies have reported comparably high prevalence (about one in 750) of the m.3243A>G mutation in the normal population (57; 34), and one of them has found that the prevalence of pathogenic mtDNA mutations is one in 200 individuals in northern England.

MERRF (myoclonus epilepsy with ragged-red fibers), characterized by myoclonus, weakness, ataxia, seizures, hearing loss, dementia, multiple lipomas, and neuropathy (55). Three mtDNA mutations (m.8344A> G, m.8356T> C, m.8363G> A) have been associated with MERRF, and all are in the tRNA(Lys) gene.

The third syndrome comes in two phenotypes: (1) neuropathy, ataxia, and retinitis pigmentosa (NARP) usually affecting young adults and causing retinitis pigmentosa, dementia, seizures, ataxia, proximal weakness, and sensory neuropathy (45) or (2) maternally inherited Leigh syndrome (MILS), a severe infantile encephalopathy with characteristic symmetrical lesions in the basal ganglia and the brainstem (69).

Leber hereditary optic neuropathy is characterized by acute or subacute vision loss in young adults, more frequently males, due to bilateral optic atrophy (16). Three mtDNA point mutations in ND genes are often homoplasmic and account for more than 90% of Leber hereditary optic neuropathy cases. These are m.11778G>A in ND4, m.3460G>A in ND1, and m.14484T>C in ND6.

Because mitochondria are present in all tissues, syndromes associated with mtDNA mutations can affect every system in the body, including the eye (optic atrophy, retinitis pigmentosa, cataracts), hearing (sensorineural deafness), endocrine system (short stature, diabetes mellitus, hypoparathyroidism), heart (familial cardiomyopathies, conduction blocks), gastrointestinal tract (exocrine pancreas dysfunction, intestinal pseudo-obstruction, gastroesophageal reflux), and kidney (renal tubular acidosis, nephrotic syndrome). Any combination of the symptoms and signs listed above should raise the suspicion of a mitochondrial disorder, especially if there is evidence of maternal transmission.

On the other hand, point mutations in mtDNA protein-coding genes often escape the rules of mitochondrial genetics in that they affect single individuals and single tissues, most commonly skeletal muscle (05). Thus, patients with exercise intolerance, myalgia, and sometimes recurrent myoglobinuria may have isolated defects of complex I, complex III, or complex IV due to pathogenic mutations in genes encoding ND subunits, cytochrome b, or COX subunits. The lack of maternal inheritance and the involvement of muscle alone suggest that mutations arose de novo in myogenic stem cells after germ-layer differentiation (“somatic mutations”) (23).

Respiratory chain oxidative phosphorylation enzymes. Oxidative phosphorylation by the respiratory chain utilizes reducing substances to produce an electron gradient that drives the ATP synthase. Abnormalities of specific complexes of the respiratory chain (eg, complex 1/COX1) can be seen in biopsy specimens of affected tissues. Historically, muscle biopsies were used for diagnosis. Biochemical testing of a tissue that shows abnormalities of oxidative phosphorylation may not be abnormal in the genes that encode the complexes because abnormal enzyme activity can also be seen in secondary mitochondrial disease (eg, abnormal TCA cycle, pathways of abnormal amino acid catabolism), physiological or psychiatric stress, or abnormal biopsy collection (64). Biopsy enzyme assays and staining are more likely to be useful in confirming a genetic diagnosis or if a genetic diagnosis cannot be identified (64).

Disorders due to defects of nDNA

Disorders caused by pathogenic variants in nuclear DNA (nDNA) account for approximately 90% of mitochondrial disease genes, with more than 350 to 400 nuclear genes now recognized as potential causes (41; 03; 66; 62). These conditions differ substantially from mtDNA-related disorders in inheritance patterns, clinical presentation, and pathogenic mechanisms.

Inheritance patterns

nDNA-related mitochondrial diseases follow Mendelian inheritance, predominantly autosomal recessive, though autosomal dominant and X-linked forms also occur (41; 03; 66; 62). This contrasts sharply with mtDNA disorders, which are maternally inherited or sporadic (84).

Because most nDNA disorders are autosomal recessive, affected siblings typically exhibit remarkably similar phenotypes, often becoming symptomatic in infancy or early childhood with uniformly severe, progressive courses. In one comparative study, seven of eight children with nuclear gene mutations died during the first decade of life, whereas siblings with mtDNA mutations displayed a broader clinical spectrum, with onset ranging from childhood to adulthood (except for Leigh syndrome) and comparatively milder progression (84).

Clinical presentation and disease course

nDNA mutations most commonly present in infancy or childhood and are frequently fatal, although adult-onset forms are recognized (84; 66). Although many phenotypes overlap with mtDNA-related disease—such as cardiomyopathy, encephalopathy, and Leigh syndrome—some presentations are particularly associated with nuclear defects.

GRACILE syndrome is caused by BCS1L mutations and causes growth retardation, aminoaciduria, cholestasis, iron overload, lactic acidosis, and early death (84).

LBSL (leukoencephalopathy with brainstem and spinal cord involvement and high brain lactate) is caused by DARS2 mutations (84).

Mohr–Tranebjaerg syndrome (deafness–dystonia) is caused by TIMM8A mutations (84).

nDNA mutations typically exhibit minimal intrafamilial variability with uniformly severe phenotypes, whereas mtDNA mutations show marked variation in onset and severity due to heteroplasmy and tissue-specific distribution (84).

Categories of nuclear gene defects

Structural subunits of respiratory chain complexes. Pathogenic variants have been identified in nuclear-encoded subunits of complexes I, II (entirely nuclear-encoded), III, and IV. These mutations produce isolated biochemical deficiencies of the affected complex, with normal mtDNA copy number and no mtDNA deletions (41). Complex II defects are rare but have been associated with Leigh syndrome and tumors, such as carotid body paragangliomas and pheochromocytomas (84).

Assembly factors and chaperone proteins. Numerous nDNA genes encode assembly factors required for the proper formation of the respiratory chain complexes (41). Examples include SCO1 mutations causing complex IV deficiency with encephalomyopathy and hepatopathy, and BCS1L mutations resulting in complex III deficiency and GRACILE syndrome (84).

mtDNA maintenance genes. Mutations in nuclear genes involved in mtDNA replication, transcription, and maintenance lead to secondary mtDNA abnormalities—multiple deletions or mtDNA depletion—a key pathogenic distinction from primary mtDNA mutations (41).

Key gene categories include mtDNA replication machinery (POLG, POLG2, TWNK, TFAM, RNASEH1, MGME1, DNA2); nucleotide pool maintenance (TK2, DGUOK, SUCLG1, SUCLA2, ABAT, RRM2B, TYMP, SLC25A4, AGK, MPV17); and mitochondrial dynamics (OPA1, MFN2, FBXL4) (84).

Autosomal dominant mutations in POLG and TWNK cause multiple mtDNA deletions with variable impairment of complexes I, III, IV, and V. Complex II remains preserved because its components are exclusively nuclear-encoded (41). Autosomal recessive variants may cause either mtDNA depletion or multiple deletions. Clinical presentations range from severe infantile myopathy or hepatocerebral disease to adult-onset ophthalmoplegia, myopathy, encephalopathy, polyneuropathy, reduced fertility, and parkinsonism (84).

Coenzyme Q10 (CoQ10) biosynthesis and secondary CoQ10 deficiency. CoQ10 deficiency represents a heterogeneous but often treatable mitochondrial disorder.

Primary CoQ10 biosynthesis gene defects occur in PDSS1, PDSS2, COQ2, COQ4, COQ6, COQ9, and ADCK3/CABC1.

Clinical phenotypes include the following (84; 10):

(1) Myopathy with recurrent myoglobinuria
(2) Cerebellar atrophy with encephalopathy
(3) Isolated myopathy with ragged-red fibers
(4) Infantile multisystem encephalomyopathy
(5) Nephrotic syndrome ± neurologic disease
(6) Variable multisystem involvement from COQ4 mutations

Secondary CoQ10 deficiency occurs with mutations in APTX (AOA1), ETFDH, and ANO10.

Therapeutic relevance. Many patients show clinical improvement with high-dose CoQ10 supplementation.

Toxic indirect assembly defects. Some metabolic disorders cause toxic inhibition of respiratory chain function. Ethylmalonic encephalomyopathy, caused by ETHE1 mutations, leads to the accumulation of sulfide, a potent inhibitor of COX. Clinical features include chronic diarrhea, vasculopathy, encephalopathy, and elevated ethylmalonic acid.

Mitochondrial protein import disorders. Mitochondrial import machinery is essential for the trafficking of nuclear-encoded mitochondrial proteins. Mohr–Tranebjaerg syndrome, caused by TIMM8A mutations, results in progressive deafness, dystonia, cortical blindness, and psychiatric abnormalities. These disorders demonstrate that mitochondrial dysfunction can arise even when respiratory chain components are intact.

Inner mitochondrial membrane lipid defects. Respiratory chain function requires proper membrane lipid composition, particularly cardiolipin. Examples include the following:

AGK	Sengers syndrome	Cardiomyopathy, cataracts, lactic acidosis
CHKB deficiency		“Megaconial myopathy” with giant mitochondria
LPIN1		Childhood recurrent rhabdomyolysis
SERAC1	MEGDEL syndrome	Dystonia–encephalopathy with deafness
TAZ	Barth syndrome	Cardiomyopathy, neutropenia, skeletal myopathy

Defects in mtDNA translation and intergenomic communication. These disorders affect translation of the 13 mtDNA-encoded respiratory chain proteins.

Key genes include the following: EFG1, EFTu, TSFM, MRPS16, PUS1, and numerous mitochondrial aminoacyl-tRNA synthetases (DARS2, AARS, RARS2, etc) (Schapira 2012; 41; 84).

Associated phenotypes include the following:

• Infantile encephalomyopathy
• Cardiomyopathy
• Sideroblastic anemia
• LBSL (DARS2)
• Charcot–Marie–Tooth neuropathy (AARS)
• Pontocerebellar hypoplasia (RARS2)
• Lactic acidosis and encephalopathy

Pathogenic mechanisms

Pathogenic mechanisms differ fundamentally between nDNA and mtDNA disorders.

nDNA defects typically cause direct impairment of respiratory chain complex assembly or function; secondary mtDNA abnormalities (depletion or multiple deletions) affecting multiple complexes; disrupted mitochondrial dynamics, biogenesis, or quality control; and imbalances in mitochondrial nucleotide pools (Schapira 2012; 41; 84).

mtDNA defects operate through heteroplasmy and threshold effects (typically 60% to 90% mutation load for biochemical expression); mitotic segregation and clonal expansion; tissue-specific distribution of mutant mtDNA; and maternal inheritance shaped by the genetic bottleneck (84).

Diagnostic implications

Biochemical patterns help differentiate nuclear from mtDNA disorders. nDNA mutations affecting respiratory chain subunits or coenzyme Q10 biosynthesis typically produce isolated complex deficiencies with normal mtDNA copy number (41). In contrast, mutations in nDNA genes responsible for mtDNA maintenance result in combined deficiencies of complexes I, III, IV, and V with measurable mtDNA depletion or multiple deletions; complex II remains preserved (41). mtDNA mutations variably affect complexes I, III, IV, or V, depending on the specific mutation, but complex II is always unaffected (84).

Pathogenesis

The literature emphasizes that mitochondrial disease pathogenesis extends far beyond isolated defects in cellular energy production, with multiple interconnected mechanisms contributing to dysfunction (33; 62). The historical assumption that ATP deficiency alone explains disease manifestations has been challenged by inconsistent detection of measurable energy deficits in vitro and in vivo (33).

The current understanding recognizes several primary pathogenic mechanisms. Defective oxidative phosphorylation (OXPHOS) affects ATP synthesis (33; 66). Other mechanisms include increased reactive oxygen species production or decreased antioxidant protection; loss of mitochondrial membrane potential; impaired mitochondrial calcium handling; activation of apoptotic pathways; and aberrant calcium homeostasis and dysregulated apoptosis (70; 33).

Genetic complexity continues to expand, with defects identified in more than 350 genes across the nuclear and mitochondrial genomes. These mutations disrupt diverse mitochondrial functions, including OXPHOS structural subunits and assembly factors, mtDNA maintenance and replication, mitochondrial translation, lipid metabolism, and mitochondrial dynamics (fusion and fission) (03; 84).

Multi-omic studies have revealed novel mechanistic insights. For example, mutations in SLC25A46 implicate defects in the mitochondrial contact site and cristae organizing system (MICOS), altered mitochondrial dynamics, and impaired lipid homeostasis through interactions with the endoplasmic reticulum membrane protein complex. These findings underscore that abnormal mitochondrial ultrastructure, disrupted mtDNA segregation, and dysfunctional OXPHOS often arise from complex interplay among mitochondrial proteins rather than isolated pathway defects (03).

Tissue specificity is a hallmark of mitochondrial disease, with high–energy demand organs—particularly muscle and brain—most frequently affected, although the biological basis of this selectivity remains incompletely understood (48; 63).

Classic mitochondrial syndromes

There are several well-defined clinical syndromes (70).

MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) is characterized by stroke-like episodes in nonvascular distributions.

MERRF (myoclonic epilepsy and ragged red fibers) features myoclonus and seizures.

Leber hereditary optic neuropathy is characterized by isolated optic nerve involvement.

Kearns-Sayre syndrome is characterized by chronic progressive external ophthalmoplegia (CPEO), retinopathy, and cardiac conduction defects.

NARP is characterized by neuropathy, ataxia, and retinitis pigmentosa.

Leigh syndrome (maternally inherited Leigh syndrome, MILS) is a subacute necrotizing encephalomyelopathy.

Organ-specific manifestations

Tissues with high energy requirements are preferentially affected, though the mechanisms underlying tissue selectivity remain incompletely understood.

Neurologic. Epileptic seizures, stroke-like episodes, ataxia, peripheral neuropathy, dementia, migraine, movement disorders (parkinsonism, dystonia, chorea), and spasticity (20; 48).

Ophthalmologic. External ophthalmoplegia, ptosis, optic atrophy, and pigmentary retinopathy (20).

Cardiac. Cardiomyopathy, heart block, and Wolff-Parkinson-White arrhythmia (70).

Endocrine. Diabetes mellitus and deafness (maternally inherited diabetes and deafness syndrome), growth problems, and hypoparathyroidism (20; 63).

Muscular. Proximal myopathy, exercise intolerance, and myoglobinuria (70; 20).

Other systems. Sensorineural hearing loss, hepatopathies, nephropathies, gastrointestinal problems (33; 13).

Clinical recognition

Deep phenotyping is critical for recognizing mitochondrial disease (62). Although some patients fit classic syndromes, most present with nonclassical multisystem involvement. Common clinical features include ptosis, external ophthalmoplegia, proximal myopathy, exercise intolerance, cardiomyopathy, sensorineural deafness, optic atrophy, pigmentary retinopathy, and diabetes mellitus (20). The presence of more than two such manifestations should raise suspicion for a mitochondrial disorder (48).

Genotype-phenotype correlations

The m.3243A>G variant, one of the most prevalent mtDNA mutations, exemplifies phenotypic diversity in mitochondrial disease. It commonly presents with diabetes and deafness, myopathy, cardiac disease, stroke-like episodes, and gastrointestinal disturbances. Disease heterogeneity results from factors, including mtDNA heteroplasmy, mtDNA copy number, and nuclear genetic modifiers (13).

Mitochondrial diseases can present at any age, with both mtDNA and nuclear DNA disorders now recognized across the lifespan. A high incidence of mid- and late-pregnancy loss is also a common feature (20).

Therapy

Treatment for mitochondrial disorders remains largely symptomatic and supportive, as no curative therapies exist for most conditions. Optimal management requires a coordinated, multidisciplinary approach tailored to the specific manifestations affecting each organ system. Although supportive care continues to form the cornerstone of treatment, a limited number of targeted, disease-modifying therapies have emerged. Notably, elamipretide (Forzinity™) received accelerated FDA approval on September 19, 2025, making it the first cardiolipin-directed therapeutic for a primary mitochondrial disorder, indicated for Barth syndrome in patients weighing at least 30 kg. Shortly after, on November 3, 2025, doxecitine and doxribtimine (Kygevvi™) received FDA approval for thymidine kinase 2 deficiency (TK2d) in adults and pediatric patients with symptom onset at or before age 12 years. This represents the first approved nucleoside replacement therapy to address the root cause of a mitochondrial DNA depletion syndrome (80; 12).

General supportive measures

Vitamin and cofactor supplementation remains one of the most used therapeutic approaches in mitochondrial medicine, although the quality of evidence varies substantially by compound and genotype. In clinical practice, supplements are often prescribed as part of a “mitochondrial cocktail,” typically combining electron carriers, antioxidants, metabolic cofactors, and alternative energy substrates. However, this term is nonspecific, and there is no universally accepted formulation, dose, or monitoring strategy. These include coenzyme Q10, idebenone, riboflavin, thiamine, alpha-lipoic acid, carnitine, creatine, vitamin C, vitamin E, and N-acetylcysteine (Table 1).

Consensus recommendations support a pragmatic approach: treat documented deficiency states, use genotype-specific cofactors when a biologically responsive disorder is suspected, and introduce supplements individually, when possible, to assess tolerability and clinical effect. The Mitochondrial Medicine Society recommends that CoQ10 be offered to most patients with mitochondrial disease, that apha-lipoic acid and riboflavin be considered, that folinic acid be used in patients with CNS disease or documented CSF folate deficiency, and that L-carnitine be reserved for documented deficiency with monitoring (29; 54). Drugs with potential mitochondrial toxicity, like valproate, aminoglycosides, and certain anesthetics, should be avoided or used cautiously; valproate is particularly contraindicated or strongly discouraged in POLG-related disease because of the risk of severe hepatotoxicity.

Table 1. Vitamins and Cofactors Used In Mitochondrial Diseases

Supplement	Main rationale	Best-supported use	Evidence limitations
CoQ10 or ubiquinol	Electron carrier, antioxidant	Primary CoQ10 deficiency; empiric use in primary mitochondrial disease	Limited RCT benefit outside CoQ10 deficiency
Riboflavin	FAD/FMN precursor; flavoprotein support	ACAD9, ETFDH, selected complex I disease	Mostly case series and genotype-specific reports
Thiamine	Pyruvate and TCA-cycle cofactor	SLC19A3 and thiamine-responsive disorders	Limited evidence outside responsive genotypes
Folinic acid	CNS folate replacement	Kearns-Sayre syndrome, low CSF 5-MTHF	No large controlled trials
L-carnitine	Fatty-acid transport, acyl buffering	Documented carnitine deficiency	Not recommended universally
Creatine	Phosphocreatine energy buffering	Selected mitochondrial myopathy	Mixed trial results
α-Lipoic acid	Antioxidant, dehydrogenase cofactor	Adjunctive empiric therapy	Limited clinical efficacy data
Vitamins C/E	Antioxidant support	Adjunctive empiric therapy	No strong disease-specific evidence
NAC	Glutathione precursor	Selected redox-stress phenotypes	Limited controlled data
Niacin/NAD+ precursors	NAD+ restoration, redox support	Investigational mitochondrial myopathy strategy	Dosing and long-term benefit uncertain
Idebenone	CoQ analogue, complex I bypass	Leber hereditary optic neuropathy, especially early treatment	Variable response; limited broader efficacy

Exercise training. Exercise therapy has shown benefits in some patients; it is one of the best-supported nonpharmacologic interventions. Aerobic training and resistance exercise may improve oxidative capacity, functional endurance, and muscle strength in select patients, including adults with mitochondrial myopathy. Resistance training may also promote “gene shifting” in disorders with heteroplasmic mtDNA variants by stimulating satellite-cell fusion and muscle regeneration, although this remains phenotype-dependent and is not a substitute for disease-specific therapy (53; 28).

Nutritional management with individually tailored diets and physical training programs should be offered to all patients (53).

Treatment of the underlying organ system abnormalities

Neurologic manifestations. Stroke-like episodes in MELAS and related m.3243A>G-spectrum disorders require urgent metabolic and neurologic management. L-arginine is commonly used acutely and prophylactically because nitric oxide deficiency is thought to contribute to impaired cerebral microvascular regulation in MELAS. Reviews and consensus-based recommendations support intravenous L-arginine in the acute setting and oral arginine for secondary prevention, although definitive randomized evidence remains limited. Citrulline, another nitric oxide precursor, is biologically plausible and may increase nitric oxide production more efficiently in some metabolic contexts, but clinical outcome data remain less mature (54; 28).

Epilepsy should be treated according to seizure type and genotype. Valproate should be avoided in POLG disease and used with great caution in suspected mitochondrial hepatocerebral disease. Levetiracetam, lamotrigine, lacosamide, benzodiazepines, and other non-valproate antiseizure medications are commonly used, with careful attention to hepatic function, sedation, respiratory status, and drug interactions (53).

Peripheral neuropathy, migraine, movement disorders, dystonia, ataxia, and cognitive symptoms require standard symptomatic therapy, rehabilitation, and avoidance of iatrogenic mitochondrial stress. Dichloroacetate reduces lactate by activating pyruvate dehydrogenase, but its clinical role is limited by neuropathy risk and lack of convincing neurologic benefit in MELAS (30).

Ophthalmologic manifestations. Leber hereditary optic neuropathy can be treated with idebenone, the principal approved disease-modifying therapy for this disease. Clinical benefit is greatest when therapy is started early, particularly within the first year after vision loss, and continued long enough to assess delayed recovery. Response remains incomplete and variable, probably reflecting genotype, disease duration, retinal ganglion-cell reserve, and differences in intracellular quinone-reducing capacity (08; 82). Lenadogene intravitreal gene therapy is pending approval from the European Medicines Agency. The vitamin-E related compound vatiquinone (EPI-743, alpha-tocotrienol quinone) showed arrested disease progression and vision improvement in small trials, with some patients achieving total recovery of visual acuity (29; 28).

Dominant optic atrophy showed some improvement with idebenone in a small pilot trial (82).

Cardiac manifestations. Barth syndrome can now be treated with elamipretide (Forzinity™), which received FDA approval in September 2025 for patients weighing at least 30 kg. Elamipretide is a cardiolipin protector that targets mitochondrial dysfunction (31).

Cardiomyopathy management includes standard heart failure therapies, with coenzyme Q10 supplementation potentially beneficial in primary or secondary coenzyme Q10 deficiency (08).

Gastrointestinal and hepatic manifestations. Mitochondrial neurogastrointestinal encephalomyopathy due to thymidine phosphorylase deficiency can be treated with the following (28; 82).

	• Allogeneic hematopoietic stem-cell transplantation, which can potentially halt disease progression (though morbidity and mortality are high if the general condition is compromised, requiring timely diagnosis).
	• Orthotopic liver transplantation.
	• Platelet infusions to transiently restore circulating thymidine phosphorylase.
	• Enzyme replacement therapy encapsulated in red cell ghosts for patients whose condition precludes stem cell transplantation.

Metabolic and endocrine manifestations. Diabetes mellitus requires standard glycemic control with insulin or oral hypoglycemic agents, avoiding metformin in patients with significant lactic acidosis risk.

Lactic acidosis has been treated with dichloroacetate, which showed statistically significant improvement in venous lactate in pediatric trials, though clinical benefit was limited, and adult trials showed peripheral nerve toxicity (30; 54).

Renal disease may include tubulopathy, Fanconi syndrome, focal segmental glomerulosclerosis, cystic disease, and renal failure. Treatment is supportive and phenotype-specific, including electrolyte replacement, bicarbonate when indicated, angiotensin-converting enzyme inhibitors or angiotensin receptor blockers for proteinuric disease, and renal replacement therapy or transplant evaluation in selected cases.

Muscular manifestations. Exercise intolerance and myopathy can benefit from:

	• Aerobic exercise and resistance training, which improved muscle strength and post-exercise lactate in some studies, though benefits are lost with cessation.
	• Gene shifting approach using resistance training to stimulate muscle regeneration, potentially lowering heteroplasmy levels through satellite cell fusion.
	• Creatine monohydrate showed mixed results, with one trial reporting improved muscle strength but two subsequent trials showing no benefit (54; 08).

NAD+ deficiency in mitochondrial myopathy showed improvement with niacin (vitamin B3) supplementation, which restored NAD+ levels and improved muscle strength, performance, and metabolome in a pilot study (28).

Genotype- and pathway-targeted therapies

Thymidine kinase 2 deficiency (TK2d): nucleoside replacement therapy. TK2 deficiency is an autosomal recessive mitochondrial DNA maintenance disorder caused by biallelic pathogenic variants in TK2, which encodes mitochondrial thymidine kinase 2. Deficient TK2 activity impairs phosphorylation of pyrimidine deoxynucleosides, disrupts mitochondrial dNTP pools, and causes mtDNA depletion or multiple mtDNA deletions. Clinically, TK2 deficiency ranges from severe infantile encephalomyopathy to childhood-onset and adult-onset progressive myopathy with ptosis, ophthalmoparesis, dysphagia, proximal weakness, respiratory insufficiency, and loss of ambulation (21).

The therapeutic rationale for deoxycytidine/deoxythymidine replacement is substrate enhancement. Exogenous pyrimidine nucleosides can be phosphorylated through residual TK2 activity and cytosolic salvage pathways, thereby replenishing mitochondrial nucleotide pools and supporting mtDNA maintenance. This approach was developed from preclinical Tk2 mouse studies and then translated through compassionate-use cohorts, retrospective multicenter analyses, and prospective clinical programs (52).

Kygevvi™ contains doxecitine and doxribtimine, both pyrimidine nucleosides, and is indicated for TK2 deficiency in adults and children with symptom onset at or before 12 years. The FDA label recommends baseline ALT, AST, and bilirubin testing, titration from 260 mg/kg/day to 520 mg/kg/day and then 800 mg/kg/day as tolerated, and oral administration in three equally divided daily doses with food (81). The most common adverse reactions are diarrhea, abdominal pain, vomiting, and elevated ALT/AST; gastrointestinal toxicity may require dose reduction, interruption, or discontinuation (39; 30).

The FDA’s public approval summary states that treated patients were compared with an untreated external control group assembled from the literature and retrospective natural-history data. In the matched survival analysis, three of 78 treated patients died compared with 28 of 78 controls, and mean survival at 10 years was 9.6 years versus 5.7 years. The European Medicines Agency has also described Kygevvi as a prescription therapy for genetically confirmed TK2 deficiency with onset at or before 12 years, taken orally three times daily, while noting uncertainty in the magnitude of effect because randomized placebo-controlled trials are difficult in this ultra-rare disease.

Important limitations should be acknowledged. Nucleoside therapy appears most effective for the myopathic and respiratory components of TK2 deficiency. CNS benefit may be limited in severe infantile encephalomyopathic presentations, likely because of blood-brain barrier exposure, tissue-specific salvage pathway biology, and irreversible early neurodevelopmental injury. Therefore, early diagnosis, deep phenotyping, respiratory monitoring, swallowing assessment, and genotype-informed prognostication remain essential.

GDF-15 as a biomarker of treatment response was validated in a study of 24 TK2d patients, showing that baseline GDF-15 levels (elevated 30-fold in children and 6-fold in adults) significantly declined with treatment, correlating with clinical improvements. The decline was greater in the pediatric group, which included the most severe patients and showed the greatest clinical benefit (27).

Broader deoxynucleoside therapy. The success of TK2-directed nucleoside replacement has stimulated interest in applying deoxycytidine/deoxythymidine therapy to other mtDNA depletion or maintenance disorders. Early open-label and interim phase 2 studies have explored this approach in POLG-related disease and other mtDNA depletion syndromes, including those associated with FBXL4, SUCLG1, SUCLA2, and RRM2B. These data remain preliminary and should not be generalized outside trials or expert-supervised compassionate-use settings, but they support the broader concept that nucleotide-pool manipulation may become a therapeutic class for selected mtDNA maintenance disorders (11; 37).

Sonlicromanol. Sonlicromanol is an oral, brain-penetrant redox-modulating and anti-inflammatory agent under development for primary mitochondrial disease, especially m.3243A>G-associated disease. A 2025 publication in Brain described sonlicromanol as a chromanol piperidine that improves cellular redox status, activates the thioredoxin/peroxiredoxin system, attenuates lipid peroxidation, and may reduce ferroptosis and inflammation (77). The phase 2b program included a randomized dose-selection study followed by a 52-week extension in patients with m.3243A>G-spectrum disease. Khondrion announced FDA IND clearance for a pivotal phase 3 trial in adults with m.3243A>G primary mitochondrial disease, with planned endpoints focused on chronic fatigue and muscle weakness. Sonlicromanol, therefore, represents one of the more advanced non-replacement pharmacologic candidates in mitochondrial medicine, but it remains investigational (77; 37).

KL1333. KL1333 is an oral small molecule intended to modulate NAD+/NADH biology and improve fatigue and functional capacity in primary mitochondrial disease. The FALCON phase 2 study is a randomized, double-blind, placebo-controlled trial evaluating KL1333 over 48 weeks in adults with genetically confirmed primary mitochondrial disease due to mtDNA mutations or deletions, with endpoints centered on fatigue, daily function, and lower-extremity strength and endurance. Because enrollment excludes predominant neurodegenerative disease and nuclear DNA disorders, its results will be most applicable to adult mtDNA-related fatigue or myopathy phenotypes (01).

Mavodelpar. Formerly REN001, mavodelpar is a peroxisome proliferator-activated receptor delta agonist developed to enhance fatty-acid oxidation and mitochondrial oxidative metabolism in primary mitochondrial myopathy. The pivotal STRIDE phase 2b study tested 100 mg once daily over 24 weeks in adults with primary mitochondrial myopathy due to mtDNA defects, using the change in 12-minute walk distance as the primary endpoint. After STRIDE failed to support continued development, the sponsor suspended the STRIDE AHEAD extension and other mavodelpar development activities. This negative program is important for the literature review because it underscores challenges in endpoint selection, placebo response, heterogeneity, and the need to align mechanisms with genotypes and phenotypes (67).

NAD+ restaturation energy. NAD+ augmentation remains biologically attractive because mitochondrial disease is often associated with altered redox state, impaired sirtuin signaling, and secondary metabolic stress. Niacin, nicotinamide riboside, nicotinamide mononucleotide, and related strategies have been investigated in small studies. Pilot data suggest possible improvements in muscle NAD+ levels, metabolomic profiles, and physical performance in mitochondrial myopathy, but optimal agent, dose, duration, target population, and clinical endpoints remain uncertain. These approaches should be presented as investigational or adjunctive rather than established disease-modifying therapy (06).

Elamipretide.

Mechanism of action. Elamipretide is a mitochondria-targeting tetrapeptide that selectively binds cardiolipin in the inner mitochondrial membrane, stabilizing cristae ultrastructure, reducing reactive oxygen species production, and enhancing ATP synthesis. It additionally modulates mitochondrial membrane electrostatic potentials and facilitates the assembly of cardiolipin-dependent proteins central to mitochondrial physiology (54).

Clinical trial evidence. The clinical development program for elamipretide in primary mitochondrial myopathy has followed a sequential phase structure with progressively larger and more rigorous trials. The MMPOWER-1 phase 1/2 trial (2018) demonstrated that the highest intravenous dose (0.25 mg/kg/h) increased 6-minute walk test (6MWT) distance by 64.5 meters versus 20.4 meters for placebo (p=0.053), with a statistically significant dose-dependent effect (p=0.014) [3][4]. The MMPOWER-2 crossover trial (2020) showed a numerically favorable 19.8-meter improvement in 6MWT that did not reach statistical significance (p=0.0833), but did demonstrate nominally significant improvements in patient-reported fatigue outcomes, including the Primary Mitochondrial Myopathy Symptom Assessment total fatigue score (p=0.0006) and Neuro-QoL Fatigue scale (p=0.0115).

The MMPOWER-3 phase 3 trial (2023) enrolled 218 adults with genetically confirmed primary mitochondrial myopathy. Elamipretide was administered at 40 mg/day subcutaneously and compared with placebo over 24 weeks. The trial failed to meet either of its primary endpoints: the difference in 6MWT distance was -3.2 meters (95% CI -18.7 to 12.3; p=0.69), and the difference in Primary Mitochondrial Myopathy Symptom Assessment total fatigue score did not reach statistical significance (-0.07; 95% CI -0.10 to 0.26; p=0.37). No serious adverse events or deaths were attributed to the study drug, and injection site reactions--the most common adverse event, occurring in approximately 80% of participants in MMPOWER-2--were predominantly mild.

Genotype-specific subgroup analysis. Despite the negative primary outcome, pre-specified and post hoc subgroup analyses revealed clinically meaningful genotype-dependent effects. Patients with pathogenic variants in nuclear DNA-encoded mtDNA replisome genes, specifically POLG and TWNK, demonstrated a trend toward improved 6MWT performance (25.2 ± 8.7 meters versus 2.0 ± 8.6 meters for placebo; p=0.06). Among the subset with chronic progressive external ophthalmoplegia and replisome gene variants, the improvement was statistically significant (37.3 ± 9.5 meters versus -8.0 ± 10.7 meters for placebo; p=0.0024). In contrast, patients harboring primary mtDNA pathogenic variants or single large-scale mtDNA deletions, who constituted 74% of the overall trial population, derived no discernible benefit. Pharmacokinetic analyses identified a weak positive correlation between plasma elamipretide concentration and 6MWT improvement in the nuclear DNA cohort, providing modest mechanistic support for the subgroup findings (46; 08).

Regulatory status and future directions. In September 2025, elamipretide (Forzinity™) received FDA approval for the treatment of Barth syndrome in patients weighing at least 30 kilograms, representing the first approved cardiolipin-directed therapeutic for a primary mitochondrial disorder. Based on the subgroup findings from MMPOWER-3, a follow-up phase 3 trial (NuPOWER) is being designed specifically for patients with mtDNA replisome gene disorders, particularly those with a chronic progressive external ophthalmoplegia phenotype, to prospectively evaluate efficacy in this genetically defined population (53).

Idebenone.

Mechanism of action. Idebenone is a synthetic short-chain analogue of ubiquinone that bypasses complex I and II, shuttling electrons directly to complex III. Its activity is critically dependent on NAD(P)H:quinone oxidoreductase 1 (NQO1), the major enzyme involved in idebenone activation.

Clinical evidence. Idebenone is the only disease-modifying treatment approved by the European Medicines Agency for Leber hereditary optic neuropathy. The recommended dose is 300 mg three times daily with meals. The response rate is 34% after 1 year and 52% after 2 years, though response is only partial in most cases, and about half of patients appear unresponsive (28).

The RHODOS randomized placebo-controlled trial failed to demonstrate superiority over placebo in primary endpoints, but cumulative evidence from retrospective data and real-world expanded access programs supported regulatory approval. Treatment should be initiated as soon as possible, ideally within the first year of disease onset, and continued for at least 1 year to assess benefit.

Safety and limitations. Idebenone has a narrow therapeutic window and an NQO1-dependent toxicity profile (31). In NQO1-deficient cells, idebenone causes a marked decrease in viability, ROS production, and deleterious effects on ATP levels. This NQO1 dependence may explain the high variability in therapeutic outcomes between patients. Idebenone-induced toxicity has been demonstrated in mouse retina ex vivo, correlating with variation in NQO1 expression between retinal cell types.

In cortical neurons (which have poor NQO1 expression), idebenone reduced respiratory capacity rather than supporting it, whereas astrocytes (with higher NQO1) showed stimulated respiration and Complex I bypass activity. This fundamental difference may explain the limited efficacy in some neurodegenerative disease trials.

Patient selection. The specific dependence on NQO1 activity suggests that patient NQO1 expression levels may predict response, though this is not currently used clinically. Access to idebenone is limited, as it is only available for prescription in a few countries. (54; 31).

Gene therapy and mitochondrial genome editing

Gene therapy is advancing fastest for nuclear-encoded mitochondrial diseases and selected ophthalmologic mtDNA disorders. AAV-based gene replacement has shown preclinical success in multiple nuclear-gene mitochondrial disease models, including disorders affecting mitochondrial translation, mtDNA maintenance, and cardiolipin remodeling. Translation is limited by vector tropism, immune responses, durability, dosing, organ targeting, and the need to reach high-energy tissues such as skeletal muscle, heart, and brain (83).

For mtDNA disorders, strategies include allotopic expression, selective destruction of mutant mtDNA using mitochondrially targeted nucleases, and newer base-editing approaches that do not rely on classic CRISPR-Cas9 import into mitochondria. Allotopic expression has been most clinically developed in Leber hereditary optic neuropathy, particularly MT-ND4 disease. MitoTALENs, zinc-finger nucleases, meganucleases, DddA-derived cytosine base editors, and related platforms have demonstrated proof-of-concept heteroplasmy shifting in preclinical systems. However, clinical translation requires rigorous assessment of off-target editing, tissue distribution, durability, immunogenicity, and the threshold of heteroplasmy shift required for clinical benefit (59).

Allotopic expression for Leber hereditary optic neuropathy (lenadogene nolparvovec). The most advanced gene therapy program targets Leber hereditary optic neuropathy, the mtDNA-based optic neuropathy. Phase 3 clinical trials of lenadogene nolparvovec (an AAV2-based therapy expressing the MT-ND4 gene allotopically for the m.11778G> A mutation) demonstrated efficacy and good tolerability, representing the greatest progress achieved so far in mitochondrial gene therapy (28).

AAV-based gene replacement for nuclear-encoded defects. Recombinant AAV-based gene replacement approaches for nuclear gene disorders have been successfully demonstrated in more than 10 preclinical mouse models of primary mitochondrial diseases, made possible by novel rAAV technologies that achieve more efficient organ-specific targeting. Clinical translation is ongoing, though still early-stage for most conditions.

mtDNA editing: nucleases and base editors. Targeting pathogenic mutations directly in mitochondrial DNA remains a major frontier. Methods include refinements to nucleases that degrade mtDNA molecules with pathogenic variants--transcription activator-like effector nucleases (TALENs), zinc-finger nucleases, and meganucleases (mitoARCUS) (09). CRISPR-Cas9 gene editing has been achieved in vivo in mouse models of nuclear gene-related primary mitochondrial diseases. CRISPR-free gene editing approaches show great potential for mtDNA disorders (59).

Researchers have developed a precise and flexible way to manipulate mtDNA in living cells using new enzymatic technology, laying the groundwork for future gene therapies aimed at correcting mitochondrial mutations at their source (11).

Mitochondrial DNA base-editing tools have established novel disease models and represent a new possibility toward personalized gene therapies for the treatment of mtDNA-based disorders. These are presently preclinical but advancing rapidly (09).

Mitochondrial replacement therapy. In mitochondrial replacement therapy, the nuclear genome from a mother carrying a deleterious mtDNA mutation is physically transferred, through micromanipulation techniques, into an enucleated oocyte from another healthy female with no mtDNA mutations. This is a preventive strategy; it stops transmission of mitochondrial disease to the next generation rather than treating an existing patient. It is legal and practiced in the United Kingdom under regulatory oversight and has been performed in a limited number of cases.

Mitochondrial transplantation

Advances in mitochondrial biology have led to the development of mitochondrial transplantation as a novel and promising therapeutic strategy. Transplanted mitochondria exert therapeutic effects through restoring ATP production, attenuating oxidative stress, modulating inflammatory responses, reducing cellular apoptosis, promoting cell repair and regeneration, facilitating neural circuit reconstruction, and exhibiting antitumor properties.

Strategies using healthy mitochondria to replenish or replace damaged mitochondria have shown promise in preclinical trials across various diseases. Clinical application is still nascent but represents a genuinely novel therapeutic paradigm distinct from gene therapy (37).

One promising approach delivers wild-type mtDNA genes using viral vectors that remain in the cytosol, with mitochondrial targeting signals facilitating protein transport into mitochondria. This bypasses the challenge of mitochondrial membrane impermeability (21).

Gene therapy approaches are in development, including allotopic expression (delivering wild-type mtDNA genes via viral vectors) and mitochondrial base editing, though mitochondrial membrane impermeability poses challenges.

Breakthroughs

POLG mutation therapy. Researchers at the University of Gothenburg have identified a molecule that helps more mitochondria function properly in diseases caused by POLG mutations. POLG regulates the production of DNA polymerase gamma, an enzyme that copies mitochondrial DNA--and mutations in it can cause brain damage and life-threatening liver problems in young children, as well as muscle weakness, epilepsy, and organ failure in later childhood (83).

MELAS drug showing broader promise. Researchers at Children's Hospital of Philadelphia found that a drug currently in clinical trials for MELAS was safe and effective in multiple preclinical zebrafish models of Leigh syndrome and other forms of mitochondrial disease, suggesting it may have broader utility than originally anticipated (32; 59).

mtDNA editing advances. Key pathogenic mechanisms--OXPHOS defects, heteroplasmy, impaired mtDNA repair, disrupted dynamics/mitophagy, and inflammatory signaling--are now being individually targeted with emerging therapeutic approaches (59).

Challenges

Despite significant momentum, the field faces structural obstacles (06).

Heterogeneity. Over 400 genes can cause mitochondrial disease, each producing a distinct biochemical and clinical profile. No single therapy is likely to help all patients.

Heteroplasmy. The coexistence of normal and mutant mtDNA within a cell means that disease severity is a moving target that can shift with cellular stress, age, or treatment.

Endpoint definition. Translating scientific breakthroughs into effective clinical treatments remains challenging, reflecting the complexity of these conditions. Small, genetically heterogeneous patient populations make placebo-controlled trials exceptionally difficult to power adequately, as shown by the repeated phase III trial failures of elamipretide and PPAR-delta agonists.

Delivery to mitochondria. Genetic modifications targeting mtDNA are constrained by the mitochondrial double membrane, hindering the development of therapeutic approaches. Optimizing editing efficiency and minimizing off-target effects remain critical challenges.

Acting on pathogenesis

Removing noxious compounds. The most logical intervention in any inborn error of metabolism appears to be removing noxious compounds. In most mitochondrial encephalomyopathies, the obvious culprit is lactic acid. Bicarbonate therapy is common and almost “automatic” but should be used prudently (68). Dichloroacetate inhibits pyruvate dehydrogenase kinase, keeping pyruvate dehydrogenase in the dephosphorylated, active form, thus favoring pyruvate metabolism and lactate oxidation. Although oral dichloroacetate was well tolerated in randomized studies of children with congenital lactic acidosis and heterogeneous mitochondrial diseases, it did not improve neurologic outcomes in patients with MELAS and was associated with evidence of peripheral neuropathy (47). In mitochondrial neurogastrointestinal encephalomyopathy, the toxic metabolites that accumulate in blood as a direct consequence of the thymidine phosphorylase defect are thymidine and deoxyuridine (78). Hemodialysis was only transiently effective in lowering blood levels, whereas the effect of allogeneic stem cell transplantation was more substantial and permanent, as discussed above.

Administration of electron acceptors. This is most effective in disorders due to primary defects of such acceptors, best exemplified by primary CoQ10 deficiencies. However, at least one child with Leigh syndrome and CoQ10 deficiency due to mutations in the PDSS2 gene did not respond to CoQ10 administration, possibly because therapy was started too late, because the dose was inadequate, or because the energy defect was too drastic (51). In patients with secondary CoQ10 deficiency, the response to supplementation is generally good but unpredictable and often variable. For example, patients with the myopathic form of glutaric aciduria type II due to electron transfer flavoprotein dehydrogenase deficiency may need both CoQ10 and riboflavin for optimal response (40). In patients with complex I deficiency due to mutations in ACAD9, a flavoprotein-containing protein, administration of riboflavin has been beneficial (38). Ubiquinol is the preferred form of CoQ10 due to its improved bioavailability, but much of the dosing has been determined with ubiquinone (07).

“Cocktails” of vitamins and cofactors. These are the most widely used therapies in clinical practice. Although there is little evidence of their benefit (15), vitamin and cofactor supplementation has been used based on the underlying pathophysiology, and with an intelligent approach, it may show some benefit (07).

In the hope of facilitating ATP production by a sluggish respiratory chain, electron flux is “boosted” by the addition of electron acceptors (CoQ10, vitamin C, vitamin K, succinate). Alternatively, we try to boost the synthesis of a naturally occurring high-phosphate compound, phosphocreatine, by administering creatine. L-carnitine is prescribed because plasma-free carnitine tends to be lower and esterified carnitine higher than normal, probably reflecting a partial impairment of beta-oxidation. Folic acid deserves special mention because early observations had shown that it was decreased in the CSF of patients with Kearns-Sayre syndrome, and a study documented both clinical and neuroradiological improvement in a child after 1 year of monotherapy with 2.2 mg folinic acid/kg daily, suggesting that early and aggressive administration of this compound should be tried in this devastating condition (65).

Riboflavin and magnesium are both growing in terms of acceptance as therapies for migraines associated with mitochondrial disease (07).

A compound, a parabenzoquinone labeled EPI-743, is being tested in infants and children with Leigh syndrome (36; 58), but has not been shown to be as promising as initially thought.

To improve ATP synthesis, creatine monohydrate supplementation has been used, but the only two randomized studies came to different conclusions, possibly due to the difference in muscle phosphocreatine concentration between the two groups (79; 49).

Scavenging excessive reactive oxygen species. This is probably the most common approach not just to mitochondrial diseases due to respiratory chain defects but also to late-onset neurodegenerative disorders, including amyotrophic lateral sclerosis, Parkinson disease, and Alzheimer disease, in which there is direct or indirect evidence of oxidative stress. Antioxidants used in clinical practice include vitamin E, CoQ10, idebenone, glutathione, and dihydrolipoate.

As mentioned above, CoQ10 is useful in primary CoQ10 deficiencies, but it is also widely prescribed to patients with respiratory chain disorders. Although anecdotal reports (too numerous to be cited here) are generally positive, we still lack a rigorous double-blind trial. However, clinical experience teaches that CoQ10 needs to be administered at high doses (no less than 300 mg daily in adults), which fortunately has been shown to be well tolerated in numerous studies of large cohorts.

Importantly, initial studies of idebenone had suggested a beneficial effect only on the cardiopathic component of Friedreich ataxia, but a standardized study showed a dose-related beneficial effect also on the neurologic component of the disease (60; 73). Idebenone is now a recommended treatment for Leber hereditary optic neuropathy (18; 17).

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Contributors

All contributors' financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.

Authors

Georgette Dib MD

Dr. Dib of Cleveland Clinic has no relevant financial relationships to disclose.
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Miriam Bekhit MD

Dr. Bekhit of Cleveland Clinic Foundation has no relevant financial relationships to disclose.
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Editor

Nicholas E Johnson MD MSCI FAAN

Dr. Johnson of Virginia Commonwealth University received consulting fees and/or research grants from AMO Pharma, Avidity, Dyne, Novartis, Pepgen, Sanofi Genzyme, Sarepta Therapeutics, Takeda, and Vertex, consulting fees and stock options from Juvena, and honorariums from Biogen Idec and Fulcrum Therapeutics as a drug safety monitoring board member.
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Former Authors

Salvatore DiMauro MD
Kimberly A Chapman MD PhD
Emma Ciafaloni MD FAAN

Patient Profile

Age range of presentation: 0 month to 65+ years
Sex preponderance: male=female
Heredity: heredity may be a factor

heredity typical
Population groups selectively affected: None selectively affected
Occupation groups selectively affected: None selectively affected

ICD & OMIM codes

ICD-10: Mitochondrial myopathy, not elsewhere classified: G71.3

Leigh syndrome: G31.82

Alpers syndrome: G31.81

Kearns-Sayre syndrome: H49.81

Leber hereditary optic neuropathy: H47.22

Other specified metabolic disorders: E88.8
ICD-11: Other specified mitochondrial myopathies: 8C73.Y

Mitochondrial myopathies, unspecified: 8C73.Z

Leigh syndrome: 5C53.24

Progressive external ophthalmoplegia: 9C82.0

Disorders of mitochondrial oxidative phosphorylation: 5C53.2

Other specified inborn errors of energy metabolism: 5C53.Y

Other specified metabolic disorders: 5D0Y

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Mitochondrial disorders

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Mitochondrial disorders

Cite this article

Introduction

Overview

Key points

Description

Disorders due to defects of mtDNA

Disorders due to defects of nDNA

Inheritance patterns

Clinical presentation and disease course

Categories of nuclear gene defects

Pathogenic mechanisms

Diagnostic implications

Pathogenesis

Classic mitochondrial syndromes

Organ-specific manifestations

Clinical recognition

Genotype-phenotype correlations

Therapy

General supportive measures

Table 1. Vitamins and Cofactors Used In Mitochondrial Diseases

Treatment of the underlying organ system abnormalities

Genotype- and pathway-targeted therapies

Gene therapy and mitochondrial genome editing

Mitochondrial transplantation

Breakthroughs

Challenges

Acting on pathogenesis

Conclusion

References

Contributors

Authors

Editor

Former Authors

Patient Profile

ICD & OMIM codes

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