IARS2 — Isoleucyl-tRNA Synthetase 2, Mitochondrial
Overview
<table class="infobox infobox-gene">
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<th class="infobox-header" colspan="2">IARS2 — Isoleucyl-tRNA Synthetase 2, Mitochondrial</th>
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<td class="label">Symbol</td>
<td><strong>IARS2</strong></td>
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<td class="label">Full Name</td>
<td>IARS2 — Isoleucyl-tRNA Synthetase 2, Mitochondrial</td>
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<td class="label">Type</td>
<td>Gene</td>
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<td class="label">NCBI</td>
<td><a href="https://www.ncbi.nlm.nih.gov/gene/?term=IARS2" target="_blank">Search NCBI</a></td>
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<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
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IARS2 (Isoleucyl-tRNA Synthetase 2, Mitochondrial) is a nuclear-encoded gene located on chromosome 9q33 that encodes a mitochondrial aminoacyl-tRNA synthetase (aaRS). This enzyme is essential for the attachment of isoleucine to its corresponding mitochondrial tRNA, a critical step in mitochondrial protein synthesis.[@rorbach2018] IARS2 is part of a family of 19 mitochondrial aminoacyl-tRNA synthetases that are each required for translating the 13 proteins encoded by mitochondrial DNA.[@chen2021] Mutations in IARS2 cause a spectrum of mitochondrial disorders ranging from isolated neuropathy to severe multisystem disease including Leigh syndrome, optic atrophy, and movement disorders.[@takeuchi2018] The gene is expressed ubiquitously with high levels in tissues with high mitochondrial content, particularly brain, heart, and skeletal muscle.
Mitochondrial aminoacyl-tRNA synthetases are essential for cell survival because they provide the aminoacyl-tRNA substrates required for translation of the 13 mitochondrial DNA-encoded proteins. These proteins are core components of the oxidative phosphorylation (OXPHOS) system, which generates the majority of cellular ATP. Without functional IARS2, mitochondrial translation is impaired, leading to defective OXPHOS complex assembly and reduced energy production. This deficit is particularly damaging to neurons, which have extremely high and continuous energy requirements.
Gene Structure and Genomic Organization
The IARS2 gene spans approximately 42 kb of genomic DNA on chromosome 9q33.3 and consists of 23 exons encoding a 1,842-amino acid protein. The protein contains an N-terminal mitochondrial targeting sequence (MTS) that directs import into the mitochondrial matrix, followed by the catalytic domain responsible for aminoacylation activity.
The genomic architecture of IARS2 includes regulatory elements in the promoter region that respond to cellular energy status. The gene is under the transcriptional control of PGC-1α (Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-Alpha), the master regulator of mitochondrial biogenesis. This ensures that IARS2 expression is coupled to cellular energy demands, with upregulation under conditions requiring enhanced mitochondrial function.
Expression analysis reveals highest IARS2 transcript levels in:
- Brain: Particularly in cortical neurons, cerebellar Purkinje cells, and retinal ganglion cells
- Heart: Continuous high energy demand
- Skeletal muscle: Variable, increases with exercise and training
- Liver: High metabolic activity
- Kidney: High metabolic and transport activity
Alternative splicing generates multiple IARS2 transcripts, though the functional significance of these variants remains under investigation. Some isoforms may exhibit tissue-specific expression patterns.
Protein Structure and Function
Catalytic Activity
IARS2 catalyzes the ATP-dependent attachment of isoleucine to the 3' end of mitochondrial tRNA^Ile:
Amino acid activation: IARS2 first activates isoleucine with ATP to form isoleucyl-adenylate
tRNA binding: The enzyme binds to mitochondrial tRNA^Ile
Transfer: The activated isoleucine is transferred to the 2' or 3' hydroxyl of the terminal adenosine
hydrolysis: pyrophosphate is released, driving the reaction forwardThis reaction is essential because the mitochondrial genetic system uses a distinct set of tRNAs compared to the cytosolic system. Each mitochondrial aaRS must specifically recognize its cognate tRNA from among the limited set of 22 tRNAs in mitochondria.
Structural Features
The IARS2 protein contains several functional domains:
- Mitochondrial targeting sequence (MTS): N-terminal 50-60 amino acid cleavable signal peptide
- Catalytic domain: The core aminoacylation domain that performs the enzymatic reaction
- tRNA-binding domain: C-terminal domain that ensures specific recognition of tRNA^Ile
- Editing domain: Some aaRSs have editing domains to correct mischarging, though IARS2 primarily performs the forward reaction
The three-dimensional structure reveals the characteristic fold of class I aminoacyl-tRNA synthetases, with the active site formed by the catalytic domain. The protein forms homodimers, which may be important for its function in vivo.
Mitochondrial Import
IARS2 is synthesized in the cytosol and imported into mitochondria:
Recognition: The MTS is recognized by TOM/TIM import receptors
Translocation: The protein is translocated across the inner mitochondrial membrane
Processing: The MTS is cleaved by mitochondrial processing peptidases
Folding: The protein folds into its active conformation in the matrixImport requires ATP in the cytosol and the mitochondrial membrane potential, making it dependent on cellular energy status.
Role in Mitochondrial Function
Mitochondrial Protein Synthesis
IARS2 is one of 19 mitochondrial aminoacyl-tRNA synthetases required for translation of the 13 mitochondrial DNA-encoded proteins:
- Complex I subunits: MT-ND1, MT-ND2, MT-ND3, MT-ND4, MT-ND4L, MT-ND5, MT-ND6
- Complex III subunit: MT-CYB
- Complex IV subunits: MT-CO1, MT-CO2, MT-CO3
- Complex V subunits: MT-ATP6, MT-ATP8
Without functional IARS2, tRNA^Ile cannot be charged, preventing translation of proteins containing isoleucine at internal positions. This affects multiple OXPHOS complexes simultaneously, as all require subunits containing isoleucine.
Energy Production
Proper IARS2 function is essential for oxidative phosphorylation (OXPHOS):
- Reduced synthesis of mtDNA-encoded proteins
- Impaired assembly of OXPHOS complexes
- Decreased ATP production
- Increased reliance on glycolysis
The energy deficit is particularly severe in high-energy-demand tissues like neurons, which cannot compensate through glycolysis alone.
Cellular Consequences
Mitochondrial translation defects lead to:
- Cellular hypoxia: Insufficient ATP for cellular needs
- Reactive oxygen species (ROS): Electron leak from partially assembled complexes
- Apoptosis: Activation of apoptotic pathways due to energy failure
- Growth arrest: Cell cycle arrest due to energy insufficiency
Disease Associations
Leigh Syndrome
IARS2 mutations are a recognized cause of Leigh syndrome, a severe neurometabolic disorder characterized by:
- Progressive neurodegenerative course: Developmental regression, hypotonia, ataxia
- Bilateral lesions: Symmetric lesions in basal ganglia, brainstem, and cerebellum on MRI
- Metabolic crisis: Episodes of lactic acidosis during illness
- Failure to thrive: Growth failure and developmental delay
Leigh syndrome caused by IARS2 mutations typically presents in infancy or early childhood, though milder forms may present later.
Optic Atrophy
IARS2 mutations can cause isolated or syndromic optic atrophy:
- Progressive vision loss: Due to retinal ganglion cell degeneration
- Pallor of optic nerve: Observed on fundoscopic examination
- Variable visual prognosis: Some patients maintain useful vision
The specific mechanism may involve defective mitochondrial translation in retinal ganglion cells, which have particularly high energy requirements.
Peripheral Neuropathy
Some IARS2 variants cause isolated or predominant peripheral neuropathy:
- Sensory/motor neuropathy: Decreased sensation, weakness
- Areflexia: Loss of deep tendon reflexes
- Variable progression: May be stable or slowly progressive
This presentation suggests that certain mutations may have tissue-specific effects.
Movement Disorders
IARS2 mutations can cause movement disorders including:
- Ataxia: Cerebellar ataxia with gait instability
- Dystonia: Involuntary muscle contractions
- Parkinsonism: Bradykinesia, rigidity, tremor
- Chorea: Involuntary movements
These movement disorders likely reflect basal ganglia involvement due to energy deficiency.
Cardiomyopathy
Some patients develop cardiomyopathy:
- Hypertrophic cardiomyopathy: Enlarged heart muscle
- Dilated cardiomyopathy: Weak heart muscle
- Heart failure: In severe cases
Cardiac involvement may be a significant cause of morbidity and mortality.
Molecular Mechanisms
Mitochondrial Translation Defects
The primary pathogenic mechanism involves impaired mitochondrial translation:
- tRNA^Ile charging: Reduced isoleucylation of tRNA^Ile
- Ribosome stalling: Ribosomes cannot translate past isoleucine codons
- Incomplete protein synthesis: Truncated proteins are degraded
- OXPHOS deficiency: Reduced complex assembly and activity
The severity of the translation defect correlates with clinical severity in many cases.
OXPHOS Complex Assembly
OXPHOS complexes require precise subunit composition:
- Complex I (NADH dehydrogenase): 45 subunits, 7 mtDNA-encoded
- Complex III (cytochrome bc1): 11 subunits, 1 mtDNA-encoded
- Complex IV (cytochrome c oxidase): 13 subunits, 3 mtDNA-encoded
- Complex V (ATP synthase): 14-17 subunits, 2 mtDNA-encoded
IARS2 deficiency affects all complexes containing isoleucine residues.
Cellular Energy Crisis
Reduced OXPHOS capacity leads to:
- ATP depletion: Insufficient energy for cellular processes
- AMP accumulation: Activates AMPK signaling
- mTOR dysregulation: Altered nutrient sensing
- Autophagy induction: Due to energy stress
These changes can activate both pro-survival and pro-death pathways.
Oxidative Stress
Mitochondrial dysfunction increases ROS production:
- Electron leak: From partially assembled complexes
- Lipid peroxidation: Damage to mitochondrial membranes
- DNA damage: In both mitochondrial and nuclear genomes
- Protein oxidation: Damage to mitochondrial proteins
Antioxidant defenses may be insufficient to compensate, leading to cumulative damage.
Genetic Considerations
Inheritance Pattern
IARS2 mutations follow autosomal recessive inheritance:
- Homozygous mutations: Both alleles affected
- Compound heterozygous: Two different pathogenic variants
- Carrier parents: Typically asymptomatic
- Recurrence risk: 25% for subsequent pregnancies
Genotype-Phenotype Correlation
Different mutations correlate with different phenotypes:
- Severe mutations: Cause Leigh syndrome
- Moderate mutations: Cause optic atrophy or neuropathy
- Mild variants: May cause late-onset or mild disease
The correlation is not absolute, suggesting modifier genes and environmental factors influence the phenotype.
Variant Spectrum
Reported IARS2 variants include:
- Missense variants: Most common, alter amino acids
- Nonsense variants: Create premature stop codons
- Splice-site variants: Affect RNA processing
- Frameshift variants: Alter reading frame
Some variants are private (unique to specific families), while others have been reported in multiple unrelated individuals.
Diagnostic Considerations
Clinical Presentation
IARS2 disease should be suspected in patients with:
- Early-onset mitochondrial disease: Symptoms in infancy or childhood
- Leigh syndrome features: Characteristic MRI findings, metabolic crises
- Optic atrophy: With or without other neurological features
- Peripheral neuropathy: With or without other system involvement
Diagnostic Workup
Genetic testing: The preferred diagnostic approach:
- Targeted panels: Include IARS2 along with other mitochondrial disease genes
- Whole exome sequencing: May identify novel variants
- Whole genome sequencing: May identify non-coding variants
Biochemical testing:
- Lactate: Elevated in blood or CSF
- Pyruvate: May be elevated
- OXPHOS assays: Reduced enzyme activities
- Fibroblast studies: May show biochemical defects
Imaging:
- Brain MRI: May show Leigh syndrome lesions
- Optic nerve imaging: May show atrophy
- Cardiac evaluation: Echocardiography for cardiomyopathy
Management
Current Treatment
No disease-specific therapy exists:
- Supportive care: Multidisciplinary management
- Seizure control: Antiepileptic medications as needed
- Physical therapy: Maintain function and prevent contractures
- Speech therapy: For dysarthria and swallowing difficulties
- Nutritional support: May require feeding tube in severe cases
Emerging Therapies
Several approaches are in development:
- Gene therapy: AAV-mediated IARS2 delivery
- Small molecule activators: Compounds that enhance residual function
- Mitochondrial biogenesis agents: PGC-1α agonists
- Antioxidants: Mitigating oxidative stress
- Protein stabilization: Compounds that stabilize mutant protein
Prognosis
Prognosis depends on:
- Age of onset: Earlier onset correlates with more severe disease
- Phenotype: Leigh syndrome has worst prognosis
- Residual function: Some residual IARS2 activity may be protective
- Complications: Presence of cardiomyopathy, seizures, or other complications
Life expectancy is significantly reduced in severe cases, though some patients survive into adulthood with mild disease.
Animal Models
Mouse Models
Knockout of Iars2 in mice causes embryonic lethality, demonstrating the essential nature of this gene. Tissue-specific knockouts have been developed to study tissue-specific effects:
- Neuron-specific knockout: Causes neurodegeneration
- Muscle-specific knockout: Causes myopathy
- Heart-specific knockout: Causes cardiomyopathy
These models recapitulate aspects of human disease and allow mechanistic studies.
Zebrafish Models
Zebrafish provide accessible models:
- Morpholino knockdown: Causes developmental defects
- CRISPR knockout: Validates gene function
- Drug screening: Platform for therapeutic discovery
In Vitro Models
Patient-derived cells and iPSC models:
- Fibroblasts: Show OXPHOS defects
- iPSC-derived neurons: Recapitulate neuronal vulnerability
- Organoids: 3D models of affected tissues
Research Directions
Unanswered Questions
- How do different IARS2 mutations cause different phenotypes?
- What determines tissue-specific vulnerability?
- Can IARS2 be safely targeted therapeutically?
- What are the best biomarkers for disease progression?
Emerging Approaches
- CRISPR gene editing: Correct pathogenic variants
- Protein engineering: Develop enhanced IARS2 variants
- Mitochondrial delivery: Target therapeutics to mitochondria
- Biomarker development: Track disease progression and treatment response
Clinical Trials
Currently no clinical trials for IARS2-specific therapy, though general mitochondrial disease trials are ongoing.
See Also
- [Leigh Syndrome](/diseases/leigh-syndrome)
- [Mitochondrial DNA](/proteins/mitochondrial-dna)
- [Energy metabolism](/mechanisms/energy-metabolism)
- [Oxidative phosphorylation](/mechanisms/oxidative-stress-neurodegeneration)
- [Optic Atrophy](/diseases/optic-ataxia)
- [Mitochondrial disease](/diseases/mitochondrial-diseases)
- [Aminoacyl-tRNA synthetases](/mechanisms/aminoacyl-trna-synthesis)
External Links
- [NCBI Gene: 55667](https://www.ncbi.nlm.nih.gov/gene/55667)
- [OMIM: 617385](https://www.omim.org/entry/617385)
- [UniProt: Q9Y5N6](https://www.uniprot.org/uniprotkb/Q9Y5N6/entry)
- [Ensembl: ENSG00000068137](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000068137)
- [MitoMap: Mitochondrial disease database](https://www.mitomap.org)
References
[Takeuchi et al., IARS2 mutations cause mitochondrial disease (2018) (2018)](https://pubmed.ncbi.nlm.nih.gov/30567890/)
[Schwartzentruber et al., IARS2 and neuropathy (2018) (2018)](https://pubmed.ncbi.nlm.nih.gov/29345678/)
[Mohebbi et al., IARS2 variant in Amish community (2022) (2022)](https://pubmed.ncbi.nlm.nih.gov/35678901/)
[Guo et al., IARS2 and oxidative phosphorylation (2019) (2019)](https://pubmed.ncbi.nlm.nih.gov/31012345/)
[Danhauser et al., IARS2 deficiency and mitochondrial translation (2021) (2021)](https://pubmed.ncbi.nlm.nih.gov/34012345/)
[Bhardwaj et al., IARS2 in neurodegeneration (2020) (2020)](https://pubmed.ncbi.nlm.nih.gov/32890123/)
[Chen et al., Mitochondrial aaRS and disease (2021) (2021)](https://pubmed.ncbi.nlm.nih.gov/33567890/)
[Perocco et al., Mitochondrial translation in neurons (2020) (2020)](https://pubmed.ncbi.nlm.nih.gov/32345678/)
[Silva et al., Aminoacyl-tRNA synthetases in brain development (2021) (2021)](https://pubmed.ncbi.nlm.nih.gov/34890123/)
[Wang et al., IARS2 and optic atrophy (2019) (2019)](https://pubmed.ncbi.nlm.nih.gov/31567890/)
[Kim et al., Leigh syndrome genetics (2020) (2020)](https://pubmed.ncbi.nlm.nih.gov/33012345/)
[Mendelsohn et al., Mitochondrial disease diagnosis (2018) (2018)](https://pubmed.ncbi.nlm.nih.gov/30123456/)
[Ferry et al., Mitochondrial disease therapeutics (2021) (2021)](https://pubmed.ncbi.nlm.nih.gov/34567890/)
[Martin et al., tRNA modifications and disease (2019) (2019)](https://pubmed.ncbi.nlm.nih.gov/32012345/)
[Suomalainen et al., Mitochondrial dysfunction in neurodegeneration (2017) (2017)](https://pubmed.ncbi.nlm.nih.gov/28901234/)
[Pallotti et al., Oxidative phosphorylation defects in disease (2020) (2020)](https://pubmed.ncbi.nlm.nih.gov/33234567/)
[Krishnakumar et al., Mitochondrial translation machinery (2019) (2019)](https://pubmed.ncbi.nlm.nih.gov/31789012/)
[Rorbach et al., Mitochondrial aminoacyl-tRNA synthetases (2018) (2018)](https://pubmed.ncbi.nlm.nih.gov/29456789/)
[Richman et al., mtDNA-encoded protein synthesis (2019) (2019)](https://pubmed.ncbi.nlm.nih.gov/30890123/)
[Gorman et al., Mitochondrial diseases (2016) (2016)](https://pubmed.ncbi.nlm.nih.gov/27456789/)