When a common biological role does not imply common disease outcomes: Disparate pathology linked to human mitochondrial aminoacyl-tRNA synthetases

FARS2 Deficiency Causes Cardiomyopathy by Disrupting Mitochondrial Homeostasis and the Mitochondrial Quality Control System

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a common heritable heart disease. Although HCM has been reported to be associated with many variants of genes involved in sarcomeric protein biomechanics, pathogenic genes have not been identified in patients with partial HCM. FARS2 (the mitochondrial phenylalanyl-tRNA synthetase), a type of mitochondrial aminoacyl-tRNA synthetase, plays a role in the mitochondrial translation machinery. Several variants of FARS2 have been suggested to cause neurological disorders; however, FARS2-associated diseases involving other organs have not been reported. We identified FARS2 as a potential novel pathogenic gene in cardiomyopathy and investigated its effects on mitochondrial homeostasis and the cardiomyopathy phenotype.

METHODS

FARS2 variants in patients with HCM were identified using whole-exome sequencing, Sanger sequencing, molecular docking analyses, and cell model investigation. Fars2 conditional mutant (p.R415L) or knockout mice, fars2-knockdown zebrafish, and Fars2-knockdown neonatal rat ventricular myocytes were engineered to construct FARS2 deficiency models both in vivo and in vitro. The effects of FARS2 and its role in mitochondrial homeostasis were subsequently evaluated using RNA sequencing and mitochondrial functional analyses. Myocardial tissues from patients were used for further verification.

RESULTS

We identified 7 unreported FARS2 variants in patients with HCM. Heart-specific Fars2-deficient mice presented cardiac hypertrophy, left ventricular dilation, progressive heart failure accompanied by myocardial and mitochondrial dysfunction, and a short life span. Heterozygous cardiac-specific Fars2R415L mice displayed a tendency to cardiac hypertrophy at age 4 weeks, accompanied by myocardial dysfunction. In addition, fars2-knockdown zebrafish presented pericardial edema and heart failure. FARS2 deficiency impaired mitochondrial homeostasis by directly blocking the aminoacylation of mt-tRNAPhe and inhibiting the synthesis of mitochondrial proteins, ultimately contributing to an imbalanced mitochondrial quality control system by accelerating mitochondrial hyperfragmentation and disrupting mitochondrion-related autophagy. Interfering with the mitochondrial quality control system using adeno-associated virus 9 or specific inhibitors mitigated the cardiac and mitochondrial dysfunction triggered by FARS2 deficiency by restoring mitochondrial homeostasis.

CONCLUSIONS

Our findings unveil the previously unrecognized role of FARS2 in heart and mitochondrial homeostasis. This study may provide new insights into the molecular diagnosis and prevention of heritable cardiomyopathy as well as therapeutic options for FARS2-associated cardiomyopathy.

Regulating Lars2 in mitochondria: A potential Alzheimer's therapy by inhibiting tau phosphorylation

Driven by the scarcity of effective treatment options in clinical settings, the present study aimed to identify a new potential target for Alzheimer's disease (AD) treatment. We focused on Lars2, an enzyme synthesizing mitochondrial leucyl-tRNA, and its role in maintaining mitochondrial function. Bioinformatics analysis of human brain transcriptome data revealed downregulation of Lars2 in AD patients compared to healthy controls. During in vitro experiments, the knockdown of Lars2 in mouse neuroblastoma cells (neuro-2a cells) and primary cortical neurons led to morphological changes and decreased density in mouse hippocampal neurons. To explore the underlying mechanisms, we investigated how downregulated Lars2 expression could impede the phosphatidylinositol 3-kinase/protein kinase B (PI3K-AKT) pathway, thereby mitigating AKT's inhibitory effect on glycogen synthase kinase 3 beta (GSK3β). This led to the activation of GSK3β, causing excessive phosphorylation of Tau protein and subsequent neuronal degeneration. During in vivo experiments, knockout of lars2 in hippocampal neurons confirmed cognitive impairment through the Barnes maze test, the novel object recognition test, and nest-building experiments. Additionally, immunofluorescence assays indicated an increase in p-tau, atrophy in the hippocampal region, and a decrease in neurons following Lars2 knockout. Taken together, our findings indicate that Lars2 represents a promising therapeutic target for AD.

Variants in the SARS2 gene cause HUPRA syndrome with atypical features: two case reports and review of the literature

Hyperuricemia, pulmonary hypertension, renal failure in infancy, and alkalosis (HUPRA syndrome) is a rare autosomal recessive mitochondrial disease with a prevalence of <1:1 000 000, due to variations in the seryl-tRNA synthetase (SARS2) gene encoding SARS on chromosome 19 (19q13.2). This study investigated two Palestinian girls from the same village who presented with progressive renal failure during infancy, with atypical clinical manifestations of HUPRA syndrome including leukopenia, anemia, salt wasting, renal failure, marked hyperuricemia, hypercholesterolemia, hyperlactatemia, and hypertriglyceridemia but without pulmonary hypertension or alkalosis. Instead, they showed acidosis on routine follow-up, distinguishing them from previous cases. Using single whole exome sequencing, we identified two homozygous pathogenic variants in the SARS2 gene (c.1175A>G (p.D392G)) and (c.1169A>G (p.D390G)). These cases with their unique phenotypes, expand the SARS2 pathogenic variant spectrum and describe clinical differences between homozygous and compound heterozygous variants.

Clinical, neuroradiological, and molecular characterization of mitochondrial threonyl-tRNA-synthetase (TARS2)-related disorder

PURPOSE

Biallelic variants in TARS2, encoding the mitochondrial threonyl-tRNA-synthetase, have been reported in a small group of individuals displaying a neurodevelopmental phenotype but with limited neuroradiological data and insufficient evidence for causality of the variants.

METHODS

Exome or genome sequencing was carried out in 15 families. Clinical and neuroradiological evaluation was performed for all affected individuals, including review of 10 previously reported individuals. The pathogenicity of TARS2 variants was evaluated using in vitro assays and a zebrafish model.

RESULTS

We report 18 new individuals harboring biallelic TARS2 variants. Phenotypically, these individuals show developmental delay/intellectual disability, regression, cerebellar and cerebral atrophy, basal ganglia signal alterations, hypotonia, cerebellar signs, and increased blood lactate. In vitro studies showed that variants within the TARS2301-381 region had decreased binding to Rag GTPases, likely impairing mTORC1 activity. The zebrafish model recapitulated key features of the human phenotype and unraveled dysregulation of downstream targets of mTORC1 signaling. Functional testing of the variants confirmed the pathogenicity in a zebrafish model.

CONCLUSION

We define the clinico-radiological spectrum of TARS2-related mitochondrial disease, unveil the likely involvement of the mTORC1 signaling pathway as a distinct molecular mechanism, and establish a TARS2 zebrafish model as an important tool to study variant pathogenicity.

Disease models of mitochondrial aminoacyl-tRNA synthetase defects

Mitochondrial aminoacyl-tRNA synthetases (mtARS) are enzymes critical for the first step of mitochondrial protein synthesis by charging mitochondrial tRNAs with their cognate amino acids. Pathogenic variants in all 19 nuclear mtARS genes are now recognized as causing recessive mitochondrial diseases. Most mtARS disorders affect the nervous system, but the phenotypes range from multisystem diseases to tissue-specific manifestations. However, the mechanisms behind the tissue specificities are poorly understood, and challenges remain in obtaining accurate disease models for developing and testing treatments. Here, some of the currently existing disease models that have increased our understanding of mtARS defects are discussed.

Recessive aminoacyl-tRNA synthetase disorders: lessons learned from in vivo disease models

Protein synthesis is a fundamental process that underpins almost every aspect of cellular functioning. Intriguingly, despite their common function, recessive mutations in aminoacyl-tRNA synthetases (ARSs), the family of enzymes that pair tRNA molecules with amino acids prior to translation on the ribosome, cause a diverse range of multi-system disorders that affect specific groups of tissues. Neurological development is impaired in most ARS-associated disorders. In addition to central nervous system defects, diseases caused by recessive mutations in cytosolic ARSs commonly affect the liver and lungs. Patients with biallelic mutations in mitochondrial ARSs often present with encephalopathies, with variable involvement of peripheral systems. Many of these disorders cause severe disability, and as understanding of their pathogenesis is currently limited, there are no effective treatments available. To address this, accurate in vivo models for most of the recessive ARS diseases are urgently needed. Here, we discuss approaches that have been taken to model recessive ARS diseases in vivo, highlighting some of the challenges that have arisen in this process, as well as key results obtained from these models. Further development and refinement of animal models is essential to facilitate a better understanding of the pathophysiology underlying recessive ARS diseases, and ultimately to enable development and testing of effective therapies.

LARS2 variants can present as premature ovarian insufficiency in the absence of overt hearing loss

Premature ovarian insufficiency (POI) affects 1 in 100 women and is a leading cause of female infertility. There are over 80 genes in which variants can cause POI, with these explaining only a minority of cases. Whole exome sequencing (WES) can be a useful tool for POI patient management, allowing clinical care to be personalized to underlying cause. We performed WES to investigate two French sisters, whose only clinical complaint was POI. Surprisingly, they shared one known and one novel likely pathogenic variant in the Perrault syndrome gene, LARS2. Using amino-acylation studies, we established that the novel missense variant significantly impairs LARS2 function. Perrault syndrome is characterized by sensorineural hearing loss in addition to POI. This molecular diagnosis alerted the sisters to the significance of their difficulty in following conversation. Subsequent audiology assessment revealed a mild bilateral hearing loss. We describe the first cases presenting with perceived isolated POI and causative variants in a Perrault syndrome gene. Our study expands the phenotypic spectrum associated with LARS2 variants and highlights the clinical benefit of having a genetic diagnosis, with prediction of potential co-morbidity and prompt and appropriate medical care, in this case by an audiologist for early detection of hearing loss.

Aminoacylation-defective bi-allelic mutations in human EPRS1 associated with psychomotor developmental delay, epilepsy, and deafness

Aminoacyl-tRNA synthetases are enzymes that ensure accurate protein synthesis. Variants of the dual-functional cytoplasmic human glutamyl-prolyl-tRNA synthetase, EPRS1, have been associated with leukodystrophy, diabetes and bone disease. Here, we report compound heterozygous variants in EPRS1 in a 4-year-old female patient presenting with psychomotor developmental delay, seizures and deafness. Functional studies of these two missense mutations support major defects in enzymatic function in vitro and contributed to confirmation of the diagnosis.

Translation in Mitochondrial Ribosomes

Mitochondrial protein synthesis is essential for the life of aerobic eukaryotes. Without it, oxidative phosphorylation cannot be coupled. Evolution has shaped a battery of factors and machinery that are key to production of just a handful of critical proteins. In this general concept chapter, we attempt to briefly summarize our current knowledge of the overall process in mitochondria from a variety of species, breaking this down to the four parts of translation: initiation, elongation, termination, and recycling. Where appropriate, we highlight differences between species and emphasize gaps in our understanding. Excitingly, with the current revolution in cryoelectron microscopy and mitochondrial genome editing, it is highly likely that many of these gaps will be resolved in the near future. However, the absence of a faithful in vitro reconstituted system to study mitochondrial translation is still problematic.

A Variant Mutation in the gene encoding Mitochondrial Seryl-tRNA Synthetase Cause HUPRA Syndrome with Atypical Presentation-A Case Report.. [DOI: 10.21203/rs.3.rs-2302489/v1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Background Hyperuricemia, pulmonary hypertension, renal failure, and alkaline intoxication syndrome (HUPRA syndrome) is a rare autosomal recessive mitochondrial disease with prevalence of less than one in a million. Due to mutations in the mitochondrial SARS enzyme encoding seryl-tRNA synthetase on chromosome 19 (19q13.2). Case–Diagnosis/Treatment We investigated two Palestinian girls from the same village presented with progressive renal failure in infancy were diagnosed with this multisystemic disease. presented with atypical clinical manifestations of HUPRA syndrome include leukopenia, anemia, salt wasting resulting in hyponatremia and hypochloremia, renal failure with elevated blood lactate, marked hyperuricemia, hypercholesterolemia and hypertriglyceridemia but without pulmonary hypertension or alkaline intoxication that distinguish them from the rest of the usual cases, instead they showed acidosis in routine follow up. By using single exome sequencing analysis, we identified a two homozygous pathogenic mutation c.1175A>G (p.D392G), c.1169A>G (D390G) in SARS2 gene. This sequence identified a new variant mutation of HUPRA syndrome c.1175A>G (p.D392G) with atypical presentation, that will be added to the literature. Conclusion SARS2 gene with pathogenic homozygous mutation variants were detected in our two patients c.1175A>G (p.D392G), c.1169A>G (D390G) in exon 13, with atypical clinical manifestations of HUPRA syndrome, expanding the spectrum of SARS2 pathogenic variants with its characteristic findings, describing the differences in clinical manifestations between homozygous and compound heterozygous mutations.

Selective degradation of tRNASer(AGY) is the primary driver for mitochondrial seryl-tRNA synthetase-related disease

Mitochondrial translation is of high significance for cellular energy homeostasis. Aminoacyl-tRNA synthetases (aaRSs) are crucial translational components. Mitochondrial aaRS variants cause various human diseases. However, the pathogenesis of the vast majority of these diseases remains unknown. Here, we identified two novel SARS2 (encoding mitochondrial seryl-tRNA synthetase) variants that cause a multisystem disorder. c.654-14T > A mutation induced mRNA mis-splicing, generating a peptide insertion in the active site; c.1519dupC swapped a critical tRNA-binding motif in the C-terminus due to stop codon readthrough. Both mutants exhibited severely diminished tRNA binding and aminoacylation capacities. A marked reduction in mitochondrial tRNASer(AGY) was observed due to RNA degradation in patient-derived induced pluripotent stem cells (iPSCs), causing impaired translation and comprehensive mitochondrial function deficiencies. These impairments were efficiently rescued by wild-type SARS2 overexpression. Either mutation caused early embryonic fatality in mice. Heterozygous mice displayed reduced muscle tissue-specific levels of tRNASers. Our findings elucidated the biochemical and cellular consequences of impaired translation mediated by SARS2, suggesting that reduced abundance of tRNASer(AGY) is a key determinant for development of SARS2-related diseases.

Aminoacyl-tRNA synthetases in human health and disease

The Aminoacyl-tRNA Synthetases (aaRSs) are an evolutionarily ancient family of enzymes that catalyze the esterification reaction linking a transfer RNA (tRNA) with its cognate amino acid matching the anticodon triplet of the tRNA. Proper functioning of the aaRSs to create aminoacylated (or “charged”) tRNAs is required for efficient and accurate protein synthesis. Beyond their basic canonical function in protein biosynthesis, aaRSs have a surprisingly diverse array of non-canonical functions that are actively being defined. The human genome contains 37 genes that encode unique aaRS proteins. To date, 56 human genetic diseases caused by damaging variants in aaRS genes have been described: 46 are autosomal recessive biallelic disorders and 10 are autosomal dominant monoallelic disorders. Our appreciation of human diseases caused by damaging genetic variants in the aaRSs has been greatly accelerated by the advent of next-generation sequencing, with 89% of these gene discoveries made since 2010. In addition to these genetic disorders of the aaRSs, anti-synthetase syndrome (ASSD) is a rare autoimmune inflammatory myopathy that involves the production of autoantibodies that disrupt aaRS proteins. This review provides an overview of the basic biology of aaRS proteins and describes the rapidly growing list of human diseases known to be caused by genetic variants or autoimmune targeting that affect both the canonical and non-canonical functions of these essential proteins.

Mitochondrial Neurodegeneration

Mitochondria are cytoplasmic organelles, which generate energy as heat and ATP, the universal energy currency of the cell. This process is carried out by coupling electron stripping through oxidation of nutrient substrates with the formation of a proton-based electrochemical gradient across the inner mitochondrial membrane. Controlled dissipation of the gradient can lead to production of heat as well as ATP, via ADP phosphorylation. This process is known as oxidative phosphorylation, and is carried out by four multiheteromeric complexes (from I to IV) of the mitochondrial respiratory chain, carrying out the electron flow whose energy is stored as a proton-based electrochemical gradient. This gradient sustains a second reaction, operated by the mitochondrial ATP synthase, or complex V, which condensates ADP and Pi into ATP. Four complexes (CI, CIII, CIV, and CV) are composed of proteins encoded by genes present in two separate compartments: the nuclear genome and a small circular DNA found in mitochondria themselves, and are termed mitochondrial DNA (mtDNA). Mutations striking either genome can lead to mitochondrial impairment, determining infantile, childhood or adult neurodegeneration. Mitochondrial disorders are complex neurological syndromes, and are often part of a multisystem disorder. In this paper, we divide the diseases into those caused by mtDNA defects and those that are due to mutations involving nuclear genes; from a clinical point of view, we discuss pediatric disorders in comparison to juvenile or adult-onset conditions. The complementary genetic contributions controlling organellar function and the complexity of the biochemical pathways present in the mitochondria justify the extreme genetic and phenotypic heterogeneity of this new area of inborn errors of metabolism known as ‘mitochondrial medicine’.

A case of QARS1 associated epileptic encephalopathy and review of epilepsy in aminoacyl-tRNA synthetase disorders

INTRODUCTION

Mutations in QARS1, which encodes human glutaminyl-tRNA synthetase, have been associated with epilepsy, developmental regression, progressive microcephaly and cerebral atrophy. Epilepsy caused by variants in QARS1 is usually drug-resistant and intractable. Childhood onset epilepsy is also reported in various aminoacyl-tRNA synthetase disorders. We describe a case with a milder neurological phenotype than previously reported with QARS1 variants and review the seizure associations with aminoacyl-tRNA synthetase disorders.

CASE REPORT

The patient is a 4-year-old girl presenting at 6 weeks of age with orofacial dyskinesia and hand stereotypies. She developed focal seizures at 7 months of age. Serial electroencephalograms showed shifting focality. Her seizures were controlled after introduction of carbamazepine. Progress MRI showed very mild cortical volume loss without myelination abnormalities or cerebellar atrophy. She was found to have novel compound heterozygous variants in QARS1 (NM_005051.2): c.[1132C > T];[1574G > A], p.[(Arg378Cys)];[(Arg525Gln)] originally classified as "variants of uncertain significance" and later upgraded to "likely pathogenic" based on functional testing and updated variant database review. Functional testing showed reduced solubility of the corresponding QARS1 mutants in vitro, but only mild two-fold loss in catalytic efficiency with the c.1132C > T variant and no noted change in tRNA^Gln aminoacylation with the c.1574G > A variant.

CONCLUSION

We describe two QARS1 variants associated with overall conserved tRNA aminoacylation activity but characterized by significantly reduced QARS protein solubility, resulting in a milder clinical phenotype. 86% of previous patients reported with QARS1 had epilepsy and 79% were pharmaco-resistant. We also summarise literature regarding epilepsy in aminoacyl-tRNA synthetase disorders, which is also often early onset, severe and drug-refractory.

FARS2 deficiency in Drosophila reveals the developmental delay and seizure manifested by aberrant mitochondrial tRNA metabolism

Mutations in genes encoding mitochondrial aminoacyl-tRNA synthetases are linked to diverse diseases. However, the precise mechanisms by which these mutations affect mitochondrial function and disease development are not fully understood. Here, we develop a Drosophila model to study the function of dFARS2, the Drosophila homologue of the mitochondrial phenylalanyl–tRNA synthetase, and further characterize human disease-associated FARS2 variants. Inactivation of dFARS2 in Drosophila leads to developmental delay and seizure. Biochemical studies reveal that dFARS2 is required for mitochondrial tRNA aminoacylation, mitochondrial protein stability, and assembly and enzyme activities of OXPHOS complexes. Interestingly, by modeling FARS2 mutations associated with human disease in Drosophila, we provide evidence that expression of two human FARS2 variants, p.G309S and p.D142Y, induces seizure behaviors and locomotion defects, respectively. Together, our results not only show the relationship between dysfunction of mitochondrial aminoacylation system and pathologies, but also illustrate the application of Drosophila model for functional analysis of human disease-causing variants.

Decoding the impact of disease-causing mutations in an essential aminoacyl-tRNA synthetase

Aminoacyl-tRNA synthetases are housekeeping enzymes that catalyze the specific attachment of amino acids onto cognate tRNAs, providing building blocks for ribosomal protein synthesis. Owing to the absolutely essential nature of these enzymes, the possibility that mutations in their sequence could be the underlying cause of diseases had not been foreseen. However, we are learning of patients bearing familial mutations in aminoacyl-tRNA synthetases at an exponential rate. In a recent issue of JBC, Jin et al. analyzed the impact of two such mutations in the very special bifunctional human glutamyl-prolyl-tRNA synthetase and convincingly decode how these mutations elicit the integrated stress response.

Discovery and implications of polygenicity of common diseases

The sequencing of the human genome has allowed the study of the genetic architecture of common diseases: the number of genomic variants that contribute to risk of disease and their joint frequency and effect size distribution. Common diseases are polygenic, with many loci contributing to phenotype, and the cumulative burden of risk alleles determines individual risk in conjunction with environmental factors. Most risk loci occur in noncoding regions of the genome regulating cell- and context-specific gene expression. Although the effect sizes of most risk alleles are small, their cumulative effects in individuals, quantified as a polygenic (risk) score, can identify people at increased risk of disease, thereby facilitating prevention or early intervention.

Metabolic impact of pathogenic variants in the mitochondrial glutamyl-tRNA synthetase EARS2

Glutamyl-tRNA synthetase 2 (encoded by EARS2) is a mitochondrial aminoacyl-tRNA synthetase required to translate the 13 subunits of the electron transport chain encoded by the mitochondrial DNA. Pathogenic EARS2 variants cause combined oxidative phosphorylation deficiency, subtype 12 (COXPD12), an autosomal recessive disorder involving lactic acidosis, intellectual disability, and other features of mitochondrial compromise. Patients with EARS2 deficiency present with variable phenotypes ranging from neonatal lethality to a mitigated disease with clinical improvement in early childhood. Here, we report a neonate homozygous for a rare pathogenic variant in EARS2 (c.949G>T; p.G317C). Metabolomics in primary fibroblasts from this patient revealed expected abnormalities in TCA cycle metabolites, as well as numerous changes in purine, pyrimidine, and fatty acid metabolism. To examine genotype-phenotype correlations in COXPD12, we compared the metabolic impact of reconstituting these fibroblasts with wild-type EARS2 versus four additional EARS2 variants from COXPD12 patients with varying clinical severity. Metabolomics identified a group of signature metabolites, mostly from the TCA cycle and amino acid metabolism, that discriminate between EARS2 variants causing relatively mild and severe COXPD12. Taken together, these findings indicate that metabolomics in patient-derived fibroblasts may help establish genotype-phenotype correlations in EARS2 deficiency and likely other mitochondrial disorders.

Phenotypic diversity of brain MRI patterns in mitochondrial aminoacyl-tRNA synthetase mutations

BACKGROUND AND PURPOSE

Mitochondrial aminoacyl-tRNA synthetases-encoded by ARS2 genes-are evolutionarily conserved enzymes that catalyse the attachment of amino acids to their cognate tRNAs, ensuring the accuracy of the mitochondrial translation process. ARS2 gene mutations are associated with a wide range of clinical presentations affecting the CNS.

METHODS

Two senior neuroradiologists analysed brain MRI of 25 patients (age range: 3 d-25 yrs.; 11 males; 14 females) with biallelic pathogenic variants of 11 ARS2 genes in a retrospective study conducted between 2002 and 2019.

RESULTS

Though several combinations of brain MRI anomalies were highly suggestive of specific aetiologies (DARS2, EARS2, AARS2 and RARS2 mutations), our study detected no MRI pattern common to all patients. Stroke-like lesions were associated with pathogenic SARS2 and FARS2 variants. We also report early onset cerebellar atrophy and calcifications in AARS2 mutations, early white matter involvement in RARS2 mutations, and absent involvement of thalami in EARS2 mutations. Finally, our findings show that normal brain MRI results do not exclude the presence of ARS2 mutations: 5 patients with normal MRI images were carriers of pathogenic IARS2, YARS2, and FARS2 variants.

CONCLUSION

Our study extends the spectrum of brain MRI anomalies associated with pathogenic ARS2 variants and suggests ARS2 mutations are largely underdiagnosed.

Mitochondrial Aminoacyl-tRNA Synthetase and Disease: The Yeast Contribution for Functional Analysis of Novel Variants

In most eukaryotes, mitochondrial protein synthesis is essential for oxidative phosphorylation (OXPHOS) as some subunits of the respiratory chain complexes are encoded by the mitochondrial DNA (mtDNA). Mutations affecting the mitochondrial translation apparatus have been identified as a major cause of mitochondrial diseases. These mutations include either heteroplasmic mtDNA mutations in genes encoding for the mitochondrial rRNA (mtrRNA) and tRNAs (mttRNAs) or mutations in nuclear genes encoding ribosomal proteins, initiation, elongation and termination factors, tRNA-modifying enzymes, and aminoacyl-tRNA synthetases (mtARSs). Aminoacyl-tRNA synthetases (ARSs) catalyze the attachment of specific amino acids to their cognate tRNAs. Differently from most mttRNAs, which are encoded by mitochondrial genome, mtARSs are encoded by nuclear genes and then imported into the mitochondria after translation in the cytosol. Due to the extensive use of next-generation sequencing (NGS), an increasing number of mtARSs variants associated with large clinical heterogeneity have been identified in recent years. Being most of these variants private or sporadic, it is crucial to assess their causative role in the disease by functional analysis in model systems. This review will focus on the contributions of the yeast Saccharomyces cerevisiae in the functional validation of mutations found in mtARSs genes associated with human disorders.

Application of Genome Sequencing from Blood to Diagnose Mitochondrial Diseases

Mitochondrial diseases can be caused by pathogenic variants in nuclear or mitochondrial DNA-encoded genes that often lead to multisystemic symptoms and can have any mode of inheritance. Using a single test, Genome Sequencing (GS) can effectively identify variants in both genomes, but it has not yet been universally used as a first-line approach to diagnosing mitochondrial diseases due to related costs and challenges in data analysis. In this article, we report three patients with mitochondrial disease molecularly diagnosed through GS performed on DNA extracted from blood to demonstrate different diagnostic advantages of this technology, including the detection of a low-level heteroplasmic pathogenic variant, an intragenic nuclear DNA deletion, and a large mtDNA deletion. Current technical improvements and cost reductions are likely to lead to an expanded routine diagnostic usage of GS and of the complementary “Omic” technologies in mitochondrial diseases.

Novel SARS2 variants identified in a Chinese girl with HUPRA syndrome

Background

Hyperuricemia, pulmonary hypertension, renal failure, and alkaline intoxication syndrome (HUPRA syndrome) is a rare autosomal recessive mitochondrial disease. SARS2 gene encoding seryl‐tRNA synthetase is the only pathogenic gene of HUPRA syndrome. All the previously reported cases with HUPRA syndrome were detected for homozygous mutation.

Methods

We identified compound heterozygous mutations causing HUPRA syndrome using whole‐exome sequencing, and verifed pathogenicity with ACMG standards. All the previously published cases with SARS2 mutations were reviewed.

Results

SARS2 gene compound heterozygotes variants were detected in this Chinese patient (c.667G>A/c.1205G>A). Bioinformatics studies and protein models predict that a new variant (c.667G>A) is likely to be pathogenic. A total of six patients, five of whom were previously reported with HUPRA syndrome, were analyzed. All of the six had typical clinical manifestations of HUPRA syndrome, except the Chinese girl who had no pulmonary hypertension or alkaline intoxication. The shrunken kidney was more prominent in our proband. The average survival time for previously reported patients was 17 months, and the Chinese girl was 70 months. Three mutation variants were found, including five homozygous mutants, three of which were Palestinian (c.1169A > G), two of which were from a Spanish family (c.1205G> A), and one was a new variant (c.667G>A/c.1205G>A).

Conclusion

We found a new pathogenic form (compound heterozygous mutation) causing HUPRA syndrome, and identified a novel pathogenic site (c.667G>A) of the SARS2 gene, expanding the spectrum of SARS2 pathogenic variants. The mild phenotype in complex heterozygous mutations is described.

An animal model for mitochondrial tyrosyl-tRNA synthetase deficiency reveals links between oxidative phosphorylation and retinal function

Mitochondria maintain a distinct pool of ribosomal machinery, including tRNAs and tRNAs activating enzymes, such as mitochondrial tyrosyl-tRNA synthetase (YARS2). Mutations in YARS2, which typically lead to the impairment of mitochondrial protein synthesis, have been linked to an array of human diseases including optic neuropathy. However, the lack of YARS2 mutation animal model makes us difficult to elucidate the pathophysiology underlying YARS2 deficiency. To explore this system, we generated YARS2 knockout (KO) HeLa cells and zebrafish using CRISPR/Cas9 technology. We observed the aberrant tRNA^Tyr aminoacylation overall and reductions in the levels in mitochondrion- and nucleus-encoding subunits of oxidative phosphorylation system (OXPHOS), which were especially pronounced effects in the subunits of complex I and complex IV. These deficiencies manifested the decreased levels of intact supercomplexes overall. Immunoprecipitation assays showed that YARS2 bound to specific subunits of complex I and complex IV, suggesting the posttranslational stabilization of OXPHOS. Furthermore, YARS2 ablation caused defects in the stability and activities of OXPHOS complexes. These biochemical defects could be rescued by the overexpression of YARS2 cDNA in the YARS2^KO cells. In zebrafish, the yars2^KO larva conferred deficient COX activities in the retina, abnormal mitochondrial morphology, and numbers in the photoreceptor and retinal ganglion cells. The zebrafish further exhibited the retinal defects affecting both rods and cones. Vision defects in yars2^KO zebrafish recapitulated the clinical phenotypes in the optic neuropathy patients carrying the YARS2 mutations. Our findings highlighted the critical role of YARS2 in the stability and activity of OXPHOS and its pathological consequence in vision impairments.

Distinct pathogenic mechanisms of various RARS1 mutations in Pelizaeus-Merzbacher-like disease

Mutations of the genes encoding aminoacyl-tRNA synthetases are highly associated with various central nervous system disorders. Recurrent mutations, including c.5A>G, p.D2G; c.1367C>T, p.S456L; c.1535G>A, p.R512Q and c.1846_1847del, p. Y616Lfs*6 of RARS1 gene, which encodes two forms of human cytoplasmic arginyl-tRNA synthetase (hArgRS), are linked to Pelizaeus-Merzbacher-like disease (PMLD) with unclear pathogenesis. Among these mutations, c.5A>G is the most extensively reported mutation, leading to a p.D2G mutation in the N-terminal extension of the long-form hArgRS. Here, we showed the detrimental effects of R512Q substitution and ΔC mutations on the structure and function of hArgRS, while the most frequent mutation c.5A>G, p.D2G acted in a different manner without impairing hArgRS activity. The nucleotide substitution c.5A>G reduced translation of hArgRS mRNA, and an upstream open reading frame contributed to the suppressed translation of the downstream main ORF. Taken together, our results elucidated distinct pathogenic mechanisms of various RARS1 mutations in PMLD.

Mitochondrial translation defects and human disease

In eukaryotic cells, mitochondria perform the essential function of producing cellular energy in the form of ATP via the oxidative phosphorylation system. This system is composed of 5 multimeric protein complexes of which 13 protein subunits are encoded by the mitochondrial genome: Complex I (7 subunits), Complex III (1 subunit),Complex IV (3 subunits), and Complex (2 subunits). Effective mitochondrial translation is necessary to produce the protein subunits encoded by the mitochondrial genome (mtDNA). Defects in mitochondrial translation are known to cause a wide variety of clinical disease in humans with high-energy consuming organs generally most prominently affected. Here, we review several classes of disease resulting from defective mitochondrial translation including disorders with mitochondrial tRNA mutations, mitochondrial aminoacyl-tRNA synthetase disorders, mitochondrial rRNA mutations, and mitochondrial ribosomal protein disorders.

Skeletal Phenotypes Due to Abnormalities in Mitochondrial Protein Homeostasis and Import

Mitochondrial disease represents a collection of rare genetic disorders caused by mitochondrial dysfunction. These disorders can be quite complex and heterogeneous, and it is recognized that mitochondrial disease can affect any tissue at any age. The reasons for this variability are not well understood. In this review, we develop and expand a subset of mitochondrial diseases including predominantly skeletal phenotypes. Understanding how impairment ofdiverse mitochondrial functions leads to a skeletal phenotype will help diagnose and treat patients with mitochondrial disease and provide additional insight into the growing list of human pathologies associated with mitochondrial dysfunction. The underlying disease genes encode factors involved in various aspects of mitochondrial protein homeostasis, including proteases and chaperones, mitochondrial protein import machinery, mediators of inner mitochondrial membrane lipid homeostasis, and aminoacylation of mitochondrial tRNAs required for translation. We further discuss a complex of frequently associated phenotypes (short stature, cataracts, and cardiomyopathy) potentially explained by alterations to steroidogenesis, a process regulated by mitochondria. Together, these observations provide novel insight into the consequences of impaired mitochondrial protein homeostasis.

Novel compound heterozygous TARS2 variants in a Chinese family with mitochondrial encephalomyopathy: a case report

Background

Mitochondrial encephalomyopathy caused by bi-allelic deleterious variants in TARS2 is rare. To date, only two pedigrees were reported in the literature and the connection between the gene and disease needs further study.

Case presentation

We report one infant who presented with limb hypertonia, epilepsy, developmental delay, and increased serum lactate from a non-consanguineous Chinese family. Whole-genome sequencing was performed to help to underlie the cause. We identified compound heterozygous variants c.470C > G, p.Thr157Arg and c.2143G > A, p.Glu715Lys in TARS2 and the variants were confirmed by Sanger sequencing. The patient was diagnosed with combined oxidative phosphorylation deficiency 21 according to the Online Mendelian Inheritance in Man (OMIM) database based on the clinical data and the deleterious effect of the two variants in TARS2 predicted by in silico tools.

Conclusions

We presented one case diagnosed with combined oxidative phosphorylation deficiency 21 based on clinical characteristics and genetic analysis. This is the first case in China and the fourth case in the world based on our document retrieval. This study facilitates the understanding of combined oxidative phosphorylation deficiency disease and demonstrates that the next-generation sequencing has a high potential to study inherited disease with high phenotypic heterogeneity and genetic heterogeneity including mitochondrial diseases such as combined oxidative phosphorylation deficiency.

Intimate Relations-Mitochondria and Ageing

Mitochondrial dysfunction is associated with ageing, but the detailed causal relationship between the two is still unclear. We review the major phenomenological manifestations of mitochondrial age-related dysfunction including biochemical, regulatory and energetic features. We conclude that the complexity of these processes and their inter-relationships are still not fully understood and at this point it seems unlikely that a single linear cause and effect relationship between any specific aspect of mitochondrial biology and ageing can be established in either direction.

Localization and RNA Binding of Mitochondrial Aminoacyl tRNA Synthetases

Mitochondria contain a complete translation machinery that is used to translate its internally transcribed mRNAs. This machinery uses a distinct set of tRNAs that are charged with cognate amino acids inside the organelle. Interestingly, charging is executed by aminoacyl tRNA synthetases (aaRS) that are encoded by the nuclear genome, translated in the cytosol, and need to be imported into the mitochondria. Here, we review import mechanisms of these enzymes with emphasis on those that are localized to both mitochondria and cytosol. Furthermore, we describe RNA recognition features of these enzymes and their interaction with tRNA and non-tRNA molecules. The dual localization of mitochondria-destined aaRSs and their association with various RNA types impose diverse impacts on cellular physiology. Yet, the breadth and significance of these functions are not fully resolved. We highlight here possibilities for future explorations.

Identification of a Novel Variant in EARS2 Associated with a Severe Clinical Phenotype Expands the Clinical Spectrum of LTBL

The EARS2 nuclear gene encodes mitochondrial glutamyl-tRNA synthetase, a member of the class I family of aminoacyl-tRNA synthetases (aaRSs) that plays a crucial role in mitochondrial protein biosynthesis by catalyzing the charging of glutamate to mitochondrial tRNA(Glu). Pathogenic EARS2 variants have been associated with a rare mitochondrial disorder known as leukoencephalopathy with thalamus and brainstem involvement and high lactate (LTBL). The targeted sequencing of 150 nuclear genes encoding respiratory chain complex subunits and proteins implicated in the oxidative phosphorylation (OXPHOS) function was performed. The oxygen consumption rate (OCR), and the extracellular acidification rate (ECAR), were measured. The enzymatic activities of Complexes I-V were analyzed spectrophotometrically. We describe a patient carrying two heterozygous EARS2 variants, c.376C>T (p.Gln126*) and c.670G>A (p.Gly224Ser), with infantile-onset disease and a severe clinical presentation. We demonstrate a clear defect in mitochondrial function in the patient’s fibroblasts, suggesting the molecular mechanism underlying the pathogenicity of these EARS2 variants. Experimental validation using patient-derived fibroblasts allowed an accurate characterization of the disease-causing variants, and by comparing our patient’s clinical presentation with that of previously reported cases, new clinical and radiological features of LTBL were identified, expanding the clinical spectrum of this disease.

De Novo and Bi-allelic Pathogenic Variants in NARS1 Cause Neurodevelopmental Delay Due to Toxic Gain-of-Function and Partial Loss-of-Function Effects

Aminoacyl-tRNA synthetases (ARSs) are ubiquitous, ancient enzymes that charge amino acids to cognate tRNA molecules, the essential first step of protein translation. Here, we describe 32 individuals from 21 families, presenting with microcephaly, neurodevelopmental delay, seizures, peripheral neuropathy, and ataxia, with de novo heterozygous and bi-allelic mutations in asparaginyl-tRNA synthetase (NARS1). We demonstrate a reduction in NARS1 mRNA expression as well as in NARS1 enzyme levels and activity in both individual fibroblasts and induced neural progenitor cells (iNPCs). Molecular modeling of the recessive c.1633C>T (p.Arg545Cys) variant shows weaker spatial positioning and tRNA selectivity. We conclude that de novo and bi-allelic mutations in NARS1 are a significant cause of neurodevelopmental disease, where the mechanism for de novo variants could be toxic gain-of-function and for recessive variants, partial loss-of-function.

Aminoacyl-tRNA synthetases

The aminoacyl-tRNA synthetases are an essential and universally distributed family of enzymes that plays a critical role in protein synthesis, pairing tRNAs with their cognate amino acids for decoding mRNAs according to the genetic code. Synthetases help to ensure accurate translation of the genetic code by using both highly accurate cognate substrate recognition and stringent proofreading of noncognate products. While alterations in the quality control mechanisms of synthetases are generally detrimental to cellular viability, recent studies suggest that in some instances such changes facilitate adaption to stress conditions. Beyond their central role in translation, synthetases are also emerging as key players in an increasing number of other cellular processes, with far-reaching consequences in health and disease. The biochemical versatility of the synthetases has also proven pivotal in efforts to expand the genetic code, further emphasizing the wide-ranging roles of the aminoacyl-tRNA synthetase family in synthetic and natural biology.

The expanding LARS2 phenotypic spectrum: HLASA, Perrault syndrome with leukodystrophy, and mitochondrial myopathy

LARS2 variants are associated with Perrault syndrome, characterized by premature ovarian failure and hearing loss, and with an infantile lethal multisystem disorder: Hydrops, lactic acidosis, sideroblastic anemia (HLASA) in one individual. Recently we reported LARS2 deafness with (ovario) leukodystrophy. Here we describe five patients with a range of phenotypes, in whom we identified biallelic LARS2 variants: three patients with a HLASA-like phenotype, an individual with Perrault syndrome whose affected siblings also had leukodystrophy, and an individual with a reversible mitochondrial myopathy, lactic acidosis, and developmental delay. Three HLASA cases from two unrelated families were identified. All were males with genital anomalies. Two survived multisystem disease in the neonatal period; both have developmental delay and hearing loss. A 55-year old male with deafness has not displayed neurological symptoms while his female siblings with Perrault syndrome developed leukodystrophy and died in their 30s. Analysis of muscle from a child with a reversible myopathy showed reduced LARS2 and mitochondrial complex I levels, and an unusual form of degeneration. Analysis of recombinant LARS2 variant proteins showed they had reduced aminoacylation efficiency, with HLASA-associated variants having the most severe effect. A broad phenotypic spectrum should be considered in association with LARS2 variants.

Mutations in aARS genes revealed by targeted next-generation sequencing in patients with mitochondrial diseases

Mitochondrial diseases are a clinically heterogeneous group of multisystemic disorders that arise as a result of various mitochondrial dysfunctions. Autosomal recessive aARS deficiencies represent a rapidly growing group of severe rare inherited mitochondrial diseases, involving multiple organs, and currently without curative option. They might be related to defects of mitochondrial aminoacyl t-RNA synthetases (mtARS) that are ubiquitous enzymes involved in mitochondrial aminoacylation and the translation process. Here, using NGS analysis of 281 nuclear genes encoding mitochondrial proteins, we identified 4 variants in different mtARS in three patients from unrelated Tunisian families, with clinical features of mitochondrial disorders. Two homozygous variants were found in KARS (c.683C>T) and AARS2 (c.1150-4C>G), respectively in two patients, while two heterozygous variants in EARS2 (c.486-7C>G) and DARS2 (c.1456C>T) were concomitantly found in the third patient. Bio-informatics investigations predicted their pathogenicity and deleterious effects on pre-mRNA splicing and on protein stability. Thus, our results suggest that mtARS mutations are common in Tunisian patients with mitochondrial diseases.

Breaking a single hydrogen bond in the mitochondrial tRNAPhe -PheRS complex leads to phenotypic pleiotropy of human disease

Various pathogenic variants in both mitochondrial tRNA^Phe and Phenylalanyl‐tRNA synthetase mitochondrial protein coding gene (FARS2) gene encoding for the human mitochondrial PheRS have been identified and associated with neurological and/or muscle‐related pathologies. An important Guanine‐34 (G34)A anticodon mutation associated with myoclonic epilepsy with ragged red fibers (MERRF) syndrome has been reported in hmit‐tRNA^Phe. The majority of G34 contacts in available aaRSs‐tRNAs complexes specifically use that base as an important tRNA identity element. The network of intermolecular interactions providing its specific recognition also largely conserved. However, their conservation depends also on the invariance of the residues in the anticodon binding domain (ABD) of human mitochondrial Phenylalanyl‐tRNA synthetase (hmit‐PheRS). A defect in recognition of the anticodon of tRNA^Phe may happen not only because of G34A mutation, but also due to mutations in the ABD. Indeed, a pathogenic mutation in FARS2 has been recently reported in a 9‐year‐old female patient harboring a p.Asp364Gly mutation. Asp364 is hydrogen bonded (HB) to G34 in WT hmit‐PheRS. Thus, there are two pathogenic variants disrupting HB between G34 and Asp364: one is associated with G34A mutation, and the other with Asp364Gly mutation. We have measured the rates of tRNA^Phe aminoacylation catalyzed by WT hmit‐PheRS and mutant enzymes. These data ranked the residues making a HB with G34 according to their contribution to activity and the signal transduction pathway in the hmit‐PheRS‐tRNA^Phe complex. Furthermore, we carried out extensive MD simulations to reveal the interdomain contact topology on the dynamic trajectories of the complex, and gaining insight into the structural and dynamic integrity effects of hmit‐PheRS complexed with tRNA^Phe.

Database

Structural data are available in PDB database under the accession number(s): 3CMQ, 3TUP, 5MGH, 5MGV.

Mitochondrial aminoacyl-tRNA synthetases

In all eukaryotic cells, protein synthesis occurs not only in the cytosol, but also in the mitochondria. Translation of mitochondrial genes requires a set of aminoacyl-tRNA synthetases, many of which are often specialized for organellar function. These enzymes have evolved unique mechanisms for tRNA recognition and for ensuring fidelity of translation. Mutations of human mitochondrial synthetases are associated with a wide range of pathogenic phenotypes, both highlighting the importance of their role in maintaining the cellular "powerhouse" and suggesting additional cellular roles.

Ubiquitously Expressed Proteins and Restricted Phenotypes: Exploring Cell-Specific Sensitivities to Impaired tRNA Charging

Aminoacyl-tRNA synthetases (ARS) are ubiquitously expressed, essential enzymes that charge tRNA with cognate amino acids. Variants in genes encoding ARS enzymes lead to myriad human inherited diseases. First, missense alleles cause dominant peripheral neuropathy. Second, missense, nonsense, and frameshift alleles cause recessive multisystem disorders that differentially affect tissues depending on which ARS is mutated. A preponderance of evidence has shown that both phenotypic classes are associated with loss-of-function alleles, suggesting that tRNA charging plays a central role in disease pathogenesis. However, it is currently unclear how perturbation in the function of these ubiquitously expressed enzymes leads to tissue-specific or tissue-predominant phenotypes. Here, we review our current understanding of ARS-associated disease phenotypes and discuss potential explanations for the observed tissue specificity.

Mitonuclear conflict and cooperation govern the integration of genotypes, phenotypes and environments

The mitonuclear genome is the most successful co-evolved mutualism in the history of life on Earth. The cross-talk between the mitochondrial and nuclear genomes has been shaped by conflict and cooperation for more than 1.5 billion years, yet this system has adapted to countless genomic reorganizations by each partner, and done so under changing environments that have placed dramatic biochemical and physiological pressures on evolving lineages. From putative anaerobic origins, mitochondria emerged as the defining aerobic organelle. During this transition, the two genomes resolved rules for sex determination and transmission that made uniparental inheritance the dominant, but not a universal pattern. Mitochondria are much more than energy-producing organelles and play crucial roles in nutrient and stress signalling that can alter how nuclear genes are expressed as phenotypes. All of these interactions are examples of genotype-by-environment (GxE) interactions, gene-by-gene (GxG) interactions (epistasis) or more generally context-dependent effects on the link between genotype and phenotype. We provide evidence from our own studies in Drosophila, and from those of other systems, that mitonuclear interactions—either conflicting or cooperative—are common features of GxE and GxG. We argue that mitonuclear interactions are an important model for how to better understand the pervasive context-dependent effects underlying the architecture of complex phenotypes. Future research in this area should focus on the quantitative genetic concept of effect size to place mitochondrial links to phenotype in a proper context.

This article is part of the theme issue ‘Linking the mitochondrial genotype to phenotype: a complex endeavour’.

Mitochondrial Dysfunction in Primary Ovarian Insufficiency

Primary ovarian insufficiency (POI) is defined by the loss or dysfunction of ovarian follicles associated with amenorrhea before the age of 40. Symptoms include hot flashes, sleep disturbances, and depression, as well as reduced fertility and increased long-term risk of cardiovascular disease. POI occurs in ∼1% to 2% of women, although the etiology of most cases remains unexplained. Approximately 10% to 20% of POI cases are due to mutations in a single gene or a chromosomal abnormality, which has provided considerable molecular insight into the biological underpinnings of POI. Many of the genes for which mutations have been associated with POI, either isolated or syndromic cases, function within mitochondria, including MRPS22, POLG, TWNK, LARS2, HARS2, AARS2, CLPP, and LRPPRC. Collectively, these genes play roles in mitochondrial DNA replication, gene expression, and protein synthesis and degradation. Although mutations in these genes clearly implicate mitochondrial dysfunction in rare cases of POI, data are scant as to whether these genes in particular, and mitochondrial dysfunction in general, contribute to most POI cases that lack a known etiology. Further studies are needed to better elucidate the contribution of mitochondria to POI and determine whether there is a common molecular defect in mitochondrial function that distinguishes mitochondria-related genes that when mutated cause POI vs those that do not. Nonetheless, the clear implication of mitochondrial dysfunction in POI suggests that manipulation of mitochondrial function represents an important therapeutic target for the treatment or prevention of POI.

Biophysical characterization Of Alpers encephalopathy associated mutants of human mitochondrial phenylalanyl-tRNA synthetase

Mutations in nucleus-encoded mitochondrial aminoacyl-tRNA synthetases (mitaaRSs) lead to defects in mitochondrial translation affecting the expression and function of 13 subunits of the respiratory chain complex leading to diverse pathological conditions. Mutations in the FARS2 gene encoding human mitochondrial phenylalanyl-tRNA synthetase (HsmitPheRS) have been found to be associated with two different clinical representations, infantile Alpers encephalopathy and spastic paraplegia. Here we have studied three pathogenic mutants (Tyr144Cys, Ile329Thr, and Asp391Val) associated with Alpers encephalopathy to understand how these variants affect the biophysical properties of the enzyme. These mutants have already been reported to have reduced aminoacylation activity. Our study established that the mutants are significantly more thermolabile compared to the wild-type enzyme with reduced solubility in vitro. The presence of aggregation-prone insoluble HsmitPheRS variants could have a detrimental impact on organellar translation, and potentially impact normal mitochondrial function. © 2019 IUBMB Life, 71(8): 1141-1149, 2019 © 2019 IUBMB Life, 71(8):1141-1149, 2019.

Siblings with lethal primary pulmonary hypoplasia and compound heterozygous variants in the AARS2 gene: further delineation of the phenotypic spectrum

Variants in the mitochondrial alanyl-tRNA synthetase 2 gene AARS2 (OMIM 612035) are associated with infantile mitochondrial cardiomyopathy or later-onset leukoencephalopathy with premature ovarian insufficiency. Here, we report two newborn siblings who died soon after birth with primary pulmonary hypoplasia without evidence of cardiomyopathy. Whole-exome sequencing detected the same compound heterozygous AARS2 variants in both siblings (c.1774C>T, p.Arg592Trp and c.647dup, p.Cys218Leufs*6) that have previously been associated with infantile mitochondrial cardiomyopathy. Segregation analysis in the family confirmed carrier status of the parents and an unaffected sibling. To our knowledge, this is the first report of primary pulmonary hypoplasia in the absence of cardiomyopathy associated with recessive AARS2 variants and further defines the phenotypic spectrum associated with this gene.

Aminoacyl-tRNA synthetases and tRNAs in human disease: an introduction to the JBC Reviews thematic series

Aminoacyl-tRNA synthetases (ARSs) catalyze the attachment of specific amino acids to cognate tRNAs for use in protein synthesis. This historical function of ARSs and tRNAs is fairly well understood. However, ARSs and tRNAs also perform noncanonical functions that are continuing to be unveiled at a rapid pace. The expanded functions of these essential molecules of life range from roles in retroviral replication to stimulation of mammalian target of rapamycin (mTOR) activity; DNA repair, splicing, and transcriptional and translational regulation; and other aspects of cellular homeostasis. Furthermore, mutations in tRNAs and synthetases are known to drive human maladies, such as the neurodegenerative disorder Charcot-Marie-Tooth disease along with other central nervous system dysfunctions and cancer. This series of reviews focuses on the diseases that result from natural variations in human cytoplasmic tRNAs, as well as from mutations in mitochondrial tRNAs and ARSs. Ultimately, the exciting work in this rapidly emerging area may lead to new therapies for microbial and parasitic infections, cancer, and neurodegenerative diseases.