Identification of host tRNAs preferentially recognized by the Plasmodium surface protein tRip.

Malaria is a life-threatening and devastating parasitic disease. Our previous work showed that parasite development requires the import of exogenous transfer RNAs (tRNAs), which represents a novel and unique form of host-pathogen interaction, as well as a potentially druggable target. This import is mediated by tRip (tRNA import protein), a membrane protein located on the parasite surface. tRip displays an extracellular domain homologous to the well-characterized OB-fold tRNA-binding domain, a structural motif known to indiscriminately interact with tRNAs. We used MIST (Microarray Identification of Shifted tRNAs), a previously established in vitro approach, to systematically assess the specificity of complexes between native Homo sapiens tRNAs and recombinant Plasmodium falciparum tRip. We demonstrate that tRip unexpectedly binds to host tRNAs with a wide range of affinities, suggesting that only a small subset of human tRNAs is preferentially imported into the parasite. In particular, we show with in vitro transcribed constructs that tRip does not bind specific tRNAs solely based on their primary sequence, hinting that post-transcriptional modifications modulate the formation of our host/parasite molecular complex. Finally, we discuss the potential utilization of the most efficient tRip ligands for the translation of the parasite's genetic information.

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.

The phenotypic spectrum of germline YARS2 variants: from isolated sideroblastic anemia to mitochondrial myopathy, lactic acidosis and sideroblastic anemia 2.

YARS2 variants have previously been described in patients with myopathy, lactic acidosis and sideroblastic anemia 2 (MLASA2). YARS2 encodes the mitochondrial tyrosyl-tRNA synthetase, which is responsible for conjugating tyrosine to its cognate mt-tRNA for mitochondrial protein synthesis. Here we describe 14 individuals from 11 families presenting with sideroblastic anemia and YARS2 variants that we identified using a sideroblastic anemia gene panel or exome sequencing. The phenotype of these patients ranged from MLASA to isolated congenital sideroblastic anemia. As in previous cases, inter- and intra-familial phenotypic variability was observed, however, this report includes the first cases with isolated sideroblastic anemia and patients with biallelic YARS2 variants that have no clinically ascertainable phenotype. We identified ten novel YARS2 variants and three previously reported variants. In vitro amino-acylation assays of five novel missense variants showed that three had less effect on the catalytic activity of YARS2 than the most commonly reported variant, p.(Phe52Leu), associated with MLASA2, which may explain the milder phenotypes in patients with these variants. However, the other two missense variants had a more severe effect on YARS2 catalytic efficiency. Several patients carried the common YARS2 c.572 G>T, p.(Gly191Val) variant (minor allele frequency =0.1259) in trans with a rare deleterious YARS2 variant. We have previously shown that the p.(Gly191Val) variant reduces YARS2 catalytic activity. Consequently, we suggest that biallelic YARS2 variants, including severe loss-of-function alleles in trans of the common p.(Gly191Val) variant, should be considered as a cause of isolated congenital sideroblastic anemia, as well as the MLASA syndromic phenotype.

Elaborate uORF/IRES features control expression and localization of human glycyl-tRNA synthetase.

The canonical activity of glycyl-tRNA synthetase (GARS) is to charge glycine onto its cognate tRNAs. However, outside translation, GARS also participates in many other functions. A single gene encodes both the cytosolic and mitochondrial forms of GARS but 2 mRNA isoforms were identified. Using immunolocalization assays, in vitro translation assays and bicistronic constructs we provide experimental evidence that one of these mRNAs tightly controls expression and localization of human GARS. An intricate regulatory domain was found in its 5'-UTR which displays a functional Internal Ribosome Entry Site and an upstream Open Reading Frame. Together, these elements hinder the synthesis of the mitochondrial GARS and target the translation of the cytosolic enzyme to ER-bound ribosomes. This finding reveals a complex picture of GARS translation and localization in mammals. In this context, we discuss how human GARS expression could influence its moonlighting activities and its involvement in diseases.

Aminoacylation of Plasmodium falciparum tRNA(Asn) and insights in the synthesis of asparagine repeats.

Genome sequencing revealed an extreme AT-rich genome and a profusion of asparagine repeats associated with low complexity regions (LCRs) in proteins of the malarial parasite Plasmodium falciparum. Despite their abundance, the function of these LCRs remains unclear. Because they occur in almost all families of plasmodial proteins, the occurrence of LCRs cannot be associated with any specific metabolic pathway; yet their accumulation must have given selective advantages to the parasite. Translation of these asparagine-rich LCRs demands extraordinarily high amounts of asparaginylated tRNA(Asn). However, unlike other organisms, Plasmodium codon bias is not correlated to tRNA gene copy number. Here, we studied tRNA(Asn) accumulation as well as the catalytic capacities of the asparaginyl-tRNA synthetase of the parasite in vitro. We observed that asparaginylation in this parasite can be considered standard, which is expected to limit the availability of asparaginylated tRNA(Asn) in the cell and, in turn, slow down the ribosomal translation rate when decoding asparagine repeats. This observation strengthens our earlier hypothesis considering that asparagine rich sequences act as "tRNA sponges" and help cotranslational folding of parasite proteins. However, it also raises many questions about the mechanistic aspects of the synthesis of asparagine repeats and about their implications in the global control of protein expression throughout Plasmodium life cycle.

Misfolded human tRNA isodecoder binds and neutralizes a 3' UTR-embedded Alu element.

Several classes of small noncoding RNAs are key players in cellular metabolism including mRNA decoding, RNA processing, and mRNA stability. Here we show that a tRNA(Asp) isodecoder, corresponding to a human tRNA-derived sequence, binds to an embedded Alu RNA element contained in the 3' UTR of the human aspartyl-tRNA synthetase mRNA. This interaction between two well-known classes of RNA molecules, tRNA and Alu RNA, is driven by an unexpected structural motif and induces a global rearrangement of the 3' UTR. Besides, this 3' UTR contains two functional polyadenylation signals. We propose a model where the tRNA/Alu interaction would modulate the accessibility of the two alternative polyadenylation sites and regulate the stability of the mRNA. This unique regulation mechanism would link gene expression to RNA polymerase III transcription and may have implications in a primate-specific signal pathway.

RNA aptamers developed against tRip: A preliminary approach targeting tRNA entry in Plasmodium.

Malaria is caused by Plasmodium parasites that multiply inside host cells and can be lethal when P. falciparum is involved. We identified tRip as a membrane protein that facilitates the import of exogenous transfer RNA (tRNA) into the parasite. tRip encompasses a tRNA binding domain exposed on the parasite surface. We used the SELEX approach to isolate high-affinity and specific tRip-binding RNA motifs from a library of random 25 nucleotide-long sequences. In five rounds of combined negative and positive selections, an enriched pool of aptamers was obtained; sequencing revealed that they were all different in their primary sequence; only by comparing their structure predictions did most of the selected aptamers reveal a conserved 5-nucleotide motif sequence. We showed that the integral motif is essential for tRip-binding while the rest of the molecule can be significantly reduced or mutated as long as the motif is presented in a single-stranded region. Such RNA aptamers bind in place of the original tRNA substrate and act as an efficient competitor, suggesting that they can block tRip function and slow parasite development.

Mutation of the mitochondrial tyrosyl-tRNA synthetase gene, YARS2, causes myopathy, lactic acidosis, and sideroblastic anemia--MLASA syndrome.

Mitochondrial respiratory chain disorders are a heterogeneous group of disorders in which the underlying genetic defect is often unknown. We have identified a pathogenic mutation (c.156C>G [p.F52L]) in YARS2, located at chromosome 12p11.21, by using genome-wide SNP-based homozygosity analysis of a family with affected members displaying myopathy, lactic acidosis, and sideroblastic anemia (MLASA). We subsequently identified the same mutation in another unrelated MLASA patient. The YARS2 gene product, mitochondrial tyrosyl-tRNA synthetase (YARS2), was present at lower levels in skeletal muscle whereas fibroblasts were relatively normal. Complex I, III, and IV were dysfunctional as indicated by enzyme analysis, immunoblotting, and immunohistochemistry. A mitochondrial protein-synthesis assay showed reduced levels of respiratory chain subunits in myotubes generated from patient cell lines. A tRNA aminoacylation assay revealed that mutant YARS2 was still active; however, enzyme kinetics were abnormal compared to the wild-type protein. We propose that the reduced aminoacylation activity of mutant YARS2 enzyme leads to decreased mitochondrial protein synthesis, resulting in mitochondrial respiratory chain dysfunction. MLASA has previously been associated with PUS1 mutations; hence, the YARS2 mutation reported here is an alternative cause of MLASA.

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.

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 tRNAGln 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.

Decreased aminoacylation in pathology-related mutants of mitochondrial tRNATyr is associated with structural perturbations in tRNA architecture.

A growing number of human pathologies are ascribed to mutations in mitochondrial tRNA genes. Here, we report biochemical investigations on three mt-tRNA(Tyr) molecules with point substitutions associated with diseases. The mutations occur in the atypical T- and D-loops at positions homologous to those involved in the tertiary interaction network of canonical tRNAs. They do not correspond to tyrosine identity positions and likely do not contact the mitochondrial tyrosyl-tRNA synthetase during the aminoacylation process. The impact of these substitutions on mt-tRNA(Tyr) tyrosylation and structure was investigated using the corresponding tRNA transcripts. In vitro tyrosylation efficiency is decreased 600-fold for mutant A22G (mitochondrial gene mutation T5874C), 40-fold for G15A (C5877T), and is without significant effect on U54C (A5843G). Comparative solution probings with lead and nucleases on mutant and wild-type tRNA(Tyr) molecules reveal a greater sensitivity to single-strand specific probes for mutants G15A and A22G. For both transcripts, the mutation triggers a structural destabilization in the D-loop that propagates toward the anticodon arm and thus hinders efficient tyrosylation. Further probing analysis combined with phylogenetic data support the participation of G15 and A22 in the tertiary network of human mt-tRNA(Tyr) via nonclassical Watson-Crick G15-C48 and G13-A22 pairings. In contrast, the pathogenic effect of the tyrosylable mutant U54C, where structure is only marginally affected, has to be sought at another level of the tRNA(Tyr) life cycle.

Structural Analysis of RNA by Small-Angle X-ray Scattering.

Over the past two decades small-angle X-ray scattering (SAXS) has become a popular method to characterize solutions of biomolecules including ribonucleic acid (RNA). In an integrative structural approach, SAXS is complementary to crystallography, NMR, and electron microscopy and provides information about RNA architecture and dynamics. This chapter highlights the practical advantages of combining size-exclusion chromatography and SAXS at synchrotron facilities. It is illustrated by practical case studies of samples ranging from single hairpins and tRNA to a large IRES. The emphasis is also put on sample preparation which is a critical step of SAXS analysis and on optimized protocols for in vitro RNA synthesis ensuring the production of mg amount of pure and homogeneous molecules.

Crystal structure of human mitochondrial tyrosyl-tRNA synthetase reveals common and idiosyncratic features.

We report the structure of a strictly mitochondrial human synthetase, namely tyrosyl-tRNA synthetase (mt-TyrRS), in complex with an adenylate analog at 2.2 A resolution. The structure is that of an active enzyme deprived of the C-terminal S4-like domain and resembles eubacterial TyrRSs with a canonical tyrosine-binding pocket and adenylate-binding residues typical of class I synthetases. Two bulges at the enzyme surface, not seen in eubacterial TyrRSs, correspond to conserved sequences in mt-TyrRSs. The synthetase electrostatic surface potential differs from that of other TyrRSs, including the human cytoplasmic homolog and the mitochondrial one from Neurospora crassa. The homodimeric human mt-TyrRS shows an asymmetry propagating from the dimer interface toward the two catalytic sites and extremities of each subunit. Mutagenesis of the catalytic domain reveals functional importance of Ser200 in line with an involvement of A73 rather than N1-N72 in tyrosine identity.

Biallelic variants in LARS2 and KARS cause deafness and (ovario)leukodystrophy.

OBJECTIVE: To describe the leukodystrophy caused by pathogenic variants in LARS2 and KARS, encoding mitochondrial leucyl transfer RNA (tRNA) synthase and mitochondrial and cytoplasmic lysyl tRNA synthase, respectively. METHODS: We composed a group of 5 patients with leukodystrophy, in whom whole-genome or whole-exome sequencing revealed pathogenic variants in LARS2 or KARS. Clinical information, brain MRIs, and postmortem brain autopsy data were collected. We assessed aminoacylation activities of purified mutant recombinant mitochondrial leucyl tRNA synthase and performed aminoacylation assays on patients' lymphoblasts and fibroblasts. RESULTS: Patients had a combination of early-onset deafness and later-onset neurologic deterioration caused by progressive brain white matter abnormalities on MRI. Female patients with LARS2 pathogenic variants had premature ovarian failure. In 2 patients, MRI showed additional signs of early-onset vascular abnormalities. In 2 other patients with LARS2 and KARS pathogenic variants, magnetic resonance spectroscopy revealed elevated white matter lactate, suggesting mitochondrial disease. Pathology in one patient with LARS2 pathogenic variants displayed evidence of primary disease of oligodendrocytes and astrocytes with lack of myelin and deficient astrogliosis. Aminoacylation activities of purified recombinant mutant leucyl tRNA synthase showed a 3-fold loss of catalytic efficiency. Aminoacylation assays on patients' lymphoblasts and fibroblasts showed about 50% reduction of enzyme activity. CONCLUSION: This study adds LARS2 and KARS pathogenic variants as gene defects that may underlie deafness, ovarian failure, and leukodystrophy with mitochondrial signature. We discuss the specific MRI characteristics shared by leukodystrophies caused by mitochondrial tRNA synthase defects. We propose to add aminoacylation assays as biochemical diagnostic tools for leukodystrophies.

Binding of human SLBP on the 3'-UTR of histone precursor H4-12 mRNA induces structural rearrangements that enable U7 snRNA anchoring.

In metazoans, cell-cycle-dependent histones are produced from poly(A)-lacking mRNAs. The 3' end of histone mRNAs is formed by an endonucleolytic cleavage of longer precursors between a conserved stem-loop structure and a purine-rich histone downstream element (HDE). The cleavage requires at least two trans-acting factors: the stem-loop binding protein (SLBP), which binds to the stem-loop and the U7 snRNP, which anchors to histone pre-mRNAs by annealing to the HDE. Using RNA structure-probing techniques, we determined the secondary structure of the 3'-untranslated region (3'-UTR) of mouse histone pre-mRNAs H4-12, H1t and H2a-614. Surprisingly, the HDE is embedded in hairpin structures and is therefore not easily accessible for U7 snRNP anchoring. Probing of the 3'-UTR in complex with SLBP revealed structural rearrangements leading to an overall opening of the structure especially at the level of the HDE. Electrophoretic mobility shift assays demonstrated that the SLBP-induced opening of HDE actually facilitates U7 snRNA anchoring on the histone H4-12 pre-mRNAs 3' end. These results suggest that initial binding of the SLBP functions in making the HDE more accessible for U7 snRNA anchoring.

Plasmodium apicoplast tyrosyl-tRNA synthetase recognizes an unusual, simplified identity set in cognate tRNATyr.

The life cycle of Plasmodium falciparum, the agent responsible for malaria, depends on both cytosolic and apicoplast translation fidelity. Apicoplast aminoacyl-tRNA synthetases (aaRS) are bacterial-like enzymes devoted to organellar tRNA aminoacylation. They are all encoded by the nuclear genome and are translocated into the apicoplast only after cytosolic biosynthesis. Apicoplast aaRSs contain numerous idiosyncratic sequence insertions: An understanding of the roles of these insertions has remained elusive and they hinder efforts to heterologously overexpress these proteins. Moreover, the A/T rich content of the Plasmodium genome leads to A/U rich apicoplast tRNA substrates that display structural plasticity. Here, we focus on the P. falciparum apicoplast tyrosyl-tRNA synthetase (Pf-apiTyrRS) and its cognate tRNATyr substrate (Pf-apitRNATyr). Cloning and expression strategies used to obtain an active and functional recombinant Pf-apiTyrRS are reported. Functional analyses established that only three weak identity elements in the apitRNATyr promote specific recognition by the cognate Pf-apiTyrRS and that positive identity elements usually found in the tRNATyr acceptor stem are excluded from this set. This finding brings to light an unusual behavior for a tRNATyr aminoacylation system and suggests that Pf-apiTyrRS uses primarily negative recognition elements to direct tyrosylation specificity.

Idiosyncratic behaviour of tRNA-like structures in translation of plant viral RNA genomes.

Tobacco mosaic virus (TMV) and Nemesia ring necrosis virus (NeRNV) belong to the Tobamoviridae and Tymoviridae families, respectively. Although their RNAs present different 5'-untranslated regions and different family-specific genomic organizations, they share common 3'-ends organized into three consecutive pseudoknot structures followed by a histidylatable tRNA-like structure (TLS). We investigate here whether the histidine residue becomes incorporated into viral proteins and if the TLSs of TMV and NeRNV play a role in viral translation. Our results indicate that, regardless of the genomic context, the histidine moiety does not become incorporated in proteins via ribosomal translation, and that disruption of the TLS in either viral RNA does not perturb the viral translation patterns. In the light of the present data and of previous results on tymoviral TLSVal and bromoviral TLSTyr showing differential effects on translation, we suggest that the key role for the TLS in promoting translation initiation appears to be dictated by the TLS architecture and identity.

Tyrosyl-tRNA synthetase: the first crystallization of a human mitochondrial aminoacyl-tRNA synthetase.

Human mitochondrial tyrosyl-tRNA synthetase and a truncated version with its C-terminal S4-like domain deleted were purified and crystallized. Only the truncated version, which is active in tyrosine activation and Escherichia coli tRNA(Tyr) charging, yielded crystals suitable for structure determination. These tetragonal crystals, belonging to space group P4(3)2(1)2, were obtained in the presence of PEG 4000 as a crystallizing agent and diffracted X-rays to 2.7 A resolution. Complete data sets could be collected and led to structure solution by molecular replacement.

Atypical archaeal tRNA pyrrolysine transcript behaves towards EF-Tu as a typical elongator tRNA.

The newly discovered tRNA(Pyl) is involved in specific incorporation of pyrrolysine in the active site of methylamine methyltransferases in the archaeon Methanosarcina barkeri. In solution probing experiments, a transcript derived from tRNA(Pyl) displays a secondary fold slightly different from the canonical cloverleaf and interestingly similar to that of bovine mitochondrial tRNA(Ser)(uga). Aminoacylation of tRNA(Pyl) transcript by a typical class II synthetase, LysRS from yeast, was possible when its amber anticodon CUA was mutated into a lysine UUU anticodon. Hydrolysis protection assays show that lysylated tRNA(Pyl) can be recognized by bacterial elongation factor. This indicates that no antideterminant sequence is present in the body of the tRNA(Pyl) transcript to prevent it from interacting with EF-Tu, in contrast with the otherwise functionally similar tRNA(Sec) that mediates selenocysteine incorporation.

LARS2 Variants Associated with Hydrops, Lactic Acidosis, Sideroblastic Anemia, and Multisystem Failure.

Pathogenic variants in mitochondrial aminoacyl-tRNA synthetases result in a broad range of mitochondrial respiratory chain disorders despite their shared role in mitochondrial protein synthesis. LARS2 encodes the mitochondrial leucyl-tRNA synthetase, which attaches leucine to its cognate tRNA. Sequence variants in LARS2 have previously been associated with Perrault syndrome, characterized by premature ovarian failure and hearing loss (OMIM #615300). In this study, we report variants in LARS2 that are associated with a severe multisystem metabolic disorder. The proband was born prematurely with severe lactic acidosis, hydrops, and sideroblastic anemia. She had multisystem complications with hyaline membrane disease, impaired cardiac function, a coagulopathy, pulmonary hypertension, and progressive renal disease and succumbed at 5 days of age. Whole exome sequencing of patient DNA revealed compound heterozygous variants in LARS2 (c.1289C>T; p.Ala430Val and c.1565C>A; p.Thr522Asn). The c.1565C>A (p.Thr522Asn) LARS2 variant has previously been associated with Perrault syndrome and both identified variants are predicted to be damaging (SIFT, PolyPhen). Muscle and liver samples from the proband did not display marked mitochondrial respiratory chain enzyme deficiency. Immunoblotting of patient muscle and liver showed LARS2 levels were reduced in liver and complex I protein levels were reduced in patient muscle and liver. Aminoacylation assays revealed p.Ala430Val LARS2 had an 18-fold loss of catalytic efficiency and p.Thr522Asn a 9-fold loss compared to wild-type LARS2. We suggest that the identified LARS2 variants are responsible for the severe multisystem clinical phenotype seen in this baby and that mutations in LARS2 can result in variable phenotypes.