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Additional file 2: Fig. S1. Overview of the study. Fig. S2. Quality control. Fig. S3. DNA-RNA sample matching. Fig. S4. Aberrant events per sample. Fig. S5. Rare variants among expression outliers. Fig. S6. Power analysis of overexpression outliers. Fig. S7. Power analysis of underexpression outliers with respect to biological coefficient of variation. Fig. S8. Cases with many mtDNA expression outliers. Fig. S9. Rare variants among splicing outliers. Fig. S10. Splicing prediction algorithms evaluation. Fig. S11. Complex pattern of aberrant splicing. Fig. S12. Analysis of variants called by RNA-seq. Fig. S13. Rare variants leading to outliers. Fig. S14. Diagnostic rate across cohorts.

Dominant spinocerebellar ataxias (SCA) are characterized by genetic heterogeneity. Some mapped and named loci remain without a causal gene identified. Here we applied next generation sequencing (NGS) to uncover the genetic etiology of the SCA25 locus.Whole-exome and whole-genome sequencing were performed in families linked to SCA25, including the French family in which the SCA25 locus was originally mapped. Whole exome sequence data were interrogated in a cohort of 796 ataxia patients of unknown etiology.The SCA25 phenotype spans a slowly evolving sensory and cerebellar ataxia, in most cases attributed to ganglionopathy. A pathogenic variant causing exon skipping was identified in the gene encoding Polyribonucleotide Nucleotidyltransferase PNPase 1 (PNPT1) located in the SCA25 linkage interval. A second splice variant in PNPT1 was detected in a large Australian family with a dominant ataxia also mapping to SCA25. An additional nonsense variant was detected in an unrelated individual with ataxia. Both nonsense and splice heterozygous variants result in premature stop codons, all located in the S1-domain of PNPase. In addition, an elevated type I interferon response was observed in blood from all affected heterozygous carriers tested. PNPase notably prevents the abnormal accumulation of double-stranded mtRNAs in the mitochondria and leakage into the cytoplasm, associated with triggering a type I interferon response.This study identifies PNPT1 as a new SCA gene, responsible for SCA25, and highlights biological links between alterations of mtRNA trafficking, interferonopathies and ataxia. ANN NEUROL 2022;92:122-137.

Mandibuloacral dysplasia syndromes are mainly due to recessive LMNA or ZMPSTE24 mutations, with cardinal nuclear morphological abnormalities and dysfunction. We report five homozygous null mutations in MTX2, encoding Metaxin-2 (MTX2), an outer mitochondrial membrane protein, in patients presenting with a severe laminopathy-like mandibuloacral dysplasia characterized by growth retardation, bone resorption, arterial calcification, renal glomerulosclerosis and severe hypertension. Loss of MTX2 in patients’ primary fibroblasts leads to loss of Metaxin-1 (MTX1) and mitochondrial dysfunction, including network fragmentation and oxidative phosphorylation impairment. Furthermore, patients’ fibroblasts are resistant to induced apoptosis, leading to increased cell senescence and mitophagy and reduced proliferation. Interestingly, secondary nuclear morphological defects are observed in both MTX2-mutant fibroblasts and mtx-2-depleted C. elegans. We thus report the identification of a severe premature aging syndrome revealing an unsuspected link between mitochondrial composition and function and nuclear morphology, establishing a pathophysiological link with premature aging laminopathies and likely explaining common clinical features., Association Française contre les Myopathies (AFM); Deutsche Forschungsgemeinschaft; GENMED Laboratory of Excellence on Medical Genomics, Agence Nationale de la Recherche; Institut National de la Santé et de la Recherche Médicale (INSERM); Aix-Marseille University (AMU) by the RAREMED Amidex Project

Mitochondrial disorders are clinically and genetically heterogeneous, with variants in mitochondrial or nuclear genes leading to varied clinical phenotypes.

Purpose: The study aimed to define the genotypic and phenotypic spectrum of reversible acute liver failure (ALF) of infancy resulting from biallelic pathogenic TRMU variants and to determine the role of cysteine supplementation in its treatment. Methods: Individuals with biallelic (likely) pathogenic variants in TRMU were studied through an international retrospective collection of de-identified patient data. Results: In 62 individuals, including 30 previously unreported cases, we described 48 (likely) pathogenic TRMU variants, of which, 18 were novel. Of these 62 individuals, 42 were alive at a median age of 6.8 (0.6-22) years after a median follow up of 3.6 (0.1-22) years. The most frequent finding, occurring in all but 2 individuals, was liver involvement. ALF occurred only in the first year of life and was reported in 43 of 62 individuals, 11 of whom received liver transplantation. Loss-of-function TRMU variants were associated with poor survival. Supplementation with at least 1 cysteine source, typically N-acetylcysteine, improved survival significantly. Neurodevelopmental delay was observed in 11 individuals and persisted in 4 of the survivors, but we were unable to determine whether this was a primary or a secondary consequence of TRMU deficiency. Conclusion: In most patients, TRMU-associated ALF is a transient, reversible disease and cysteine supplementation improved survival. © 2022 The Authors, DMB-1805- 0002; 01GM1207; MR/S005021/1; G0800674; National Institutes of Health, NIH: 5U54-NS078059-11, 5U54-NS115198-02; Wellcome Trust, WT: 203105/Z/16/Z; PTC Therapeutics, PTC; Manchester Biomedical Research Centre, BRC; Medical Research Council, MRC: MR/W019027/1; Pathological Society of Great Britain and Ireland; National Health and Medical Research Council, NHMRC: GNT1155244, GNT1164479; Bundesministerium für Bildung und Forschung, BMBF: 01GM1906B, 01KU2016A; Newcastle upon Tyne Hospitals NHS Foundation Trust; State Government of Victoria; Astellas Pharma; Bundesministerium für Bildung und Frauen, BMBF; Medizinische Universität Innsbruck, MUI; King Salman Center for Disability Research, KSCDR: RG-2022-010; Lily Foundation, The Chair in Genomic Medicine awarded to J.C. is generously supported by The Royal Children’s HospitalFoundation The Royal Children's Hospital Foundation . We are grateful to the Crane, Perkins, and Miller families for their generous financial support. We thank the Kinghorn Centre for Clinical Genomics for assistance with production and processing of genome sequencing data. This project was supported by the funding from MitoCanada ( https://mitocanada.org ) as part of the Mitochondrial Functional and Integrative Next Generation Diagnostics (MITO-FIND) study. This work was supported by the European Reference Network for Hereditary Metabolic Disorders (MetabERN). S.W. received funding from ERAPERMED2019-310 Personalized Mitochondrial Medicine (PerMiM): Optimizing diagnostics and treatment for patients with mitochondrial diseases FWF 4704-B. F.S.A. is funded by the National Institutes of Health along with the North American Mitochondrial Disease Consortia (5U54-NS078059-11), the Frontiers of Congenital Disorders of Glycosylation Consortia (FCDGC, 5U54-NS115198-02), Mervar Foundation, Courage for a Cure Foundation , PTC Therapeutics , Astellas Pharma Inc, and Saol Therapeutics. R.M. and R.W.T. are funded by the Wellcome Trust Centre for Mitochondrial Research (203105/Z/16/Z), the Mitochondrial Disease Patient Cohort (United Kingdom) (G0800674), the Medical Research Council International Centre for Genomic Medicine in Neuromuscular Disease (MR/S005021/1), the Medical Research Council (MR/W019027/1), the Lily Foundation , the UK NIHR Biomedical Research Centre for Ageing and Age-related Disease award to the Newcastle upon Tyne Hospitals NHS Foundation Trust , and the UK NHS Highly Specialised Service for Rare Mitochondrial Disorders of Adults and Children. R.W.T. also receives funding from the Pathological Society of Great Britain and Ireland. J.C. is supported by a New South Wales Office of Health and Medical Research Council Sydney Genomics Collaborative grant. We acknowledge funding from the National Health and Medical Research Council ( NHMRC ): project grant GNT1164479 (D.R.T.) and Principal Research Fellowship GNT1155244 (D.R.T.). The research conducted at the Murdoch Children’s Research Institute was supported by the Victorian Government’s Operational Infrastructure Support program. This study was supported by BMBF (German Federal Ministry of Education and Research ) through the German Network for Mitochondrial Diseases ([mitoNET] grant number 01GM1906B), Personalized Mitochondrial Medicine (PerMiM) (grant number 01KU2016A), and E-Rare project GENOMIT (grant number 01GM1207) and the Bavarian State Ministry of Health and Care within its framework of DigiMed Bayern (grant number DMB-1805- 0002). The authors extend their appreciation to the King Salman Center For Disability Research for funding this work through research group number RG-2022-010 (to F.S.A.), The Chair in Genomic Medicine awarded to J.C. is generously supported by The Royal Children's HospitalFoundationThe Royal Children's Hospital Foundation. We are grateful to the Crane, Perkins, and Miller families for their generous financial support. We thank the Kinghorn Centre for Clinical Genomics for assistance with production and processing of genome sequencing data. This project was supported by the funding from MitoCanada (https://mitocanada.org) as part of the Mitochondrial Functional and Integrative Next Generation Diagnostics (MITO-FIND) study. This work was supported by the European Reference Network for Hereditary Metabolic Disorders (MetabERN). S.W. received funding from ERAPERMED2019-310 Personalized Mitochondrial Medicine (PerMiM): Optimizing diagnostics and treatment for patients with mitochondrial diseases FWF 4704-B. F.S.A. is funded by the National Institutes of Health along with the North American Mitochondrial Disease Consortia (5U54-NS078059-11), the Frontiers of Congenital Disorders of Glycosylation Consortia (FCDGC, 5U54-NS115198-02), Mervar Foundation, Courage for a CureFoundation, PTC Therapeutics, Astellas Pharma Inc, and Saol Therapeutics. R.M. and R.W.T. are funded by the Wellcome Trust Centre for Mitochondrial Research (203105/Z/16/Z), the Mitochondrial Disease Patient Cohort (United Kingdom) (G0800674), the Medical Research Council International Centre for Genomic Medicine in Neuromuscular Disease (MR/S005021/1), the Medical Research Council (MR/W019027/1), the LilyFoundation, the UK NIHR Biomedical Research Centre for Ageing and Age-related Disease award to the Newcastle upon Tyne Hospitals NHS Foundation Trust, and the UK NHS Highly Specialised Service for Rare Mitochondrial Disorders of Adults and Children. R.W.T. also receives funding from the Pathological Society of Great Britain and Ireland. J.C. is supported by a New South Wales Office of Health and Medical Research Council Sydney Genomics Collaborative grant. We acknowledge funding from the National Health and Medical Research Council (NHMRC): project grant GNT1164479 (D.R.T.) and Principal Research Fellowship GNT1155244 (D.R.T.). The research conducted at the Murdoch Children's Research Institute was supported by the Victorian Government's Operational Infrastructure Support program. This study was supported by BMBF (German Federal Ministry of Education and Research) through the German Network for Mitochondrial Diseases ([mitoNET] grant number 01GM1906B), Personalized Mitochondrial Medicine (PerMiM) (grant number 01KU2016A), and E-Rare project GENOMIT (grant number 01GM1207) and the Bavarian State Ministry of Health and Care within its framework of DigiMed Bayern (grant number DMB-1805- 0002). The authors extend their appreciation to the King Salman Center For Disability Research for funding this work through research group number RG-2022-010 (to F.S.A.), Conceptualization: G.F.V. S.W.; Data Curation: G.F.V. S.W. Y.M.-G. Y.E.L. R.G.F. J.A.M. H.B. L.D.S. H.Pr. A.Pec. F.S.A. J.J.B. G.B. I.B. N.B. B.B. J.C. E.C. D.C. A.M.D. N.D. A.D.M. F.D. E.A.E. M.E. W.F. P.G. R.D.G. E.G. C.H. J.H. V.K. M.Ko. M.Ke. A.K. D.L. R.M. M.G.M. K.Mo. T.M. K.Mu. E.N. A.Pen. H.Pe. D.P.-A. A.R. R.S. F.S. M.Sc. M.Shag. M.Shar. C.S.-A. C.S. I.S. M.St. R.W.T. D.R.T. E.L.T. J.-S.W. D.W.; Methodology: G.F.V. S.W. R.G.F. J.A.M.; Visualization: G.F.V. S.W. H.B. J.S.; Writing-original draft: G.F.V. S.W.; Writing-review and editing: G.F.V. S.W. Y.M.-G. Y.E.L. R.G.F. J.A.M. H.B. L.D.S. H.Pr. A.Pec. F.S.A. J.J.B. G.B. I.B. N.B. B.B. J.C. E.C. D.C. A.M.D. N.D. A.D.M. F.D. E.A.E. M.E. W.F. P.G. R.D.G. E.G. C.H. J.H. V.K. M.Ko. M.Ke. A.K. D.L. R.M. M.G.M. K.Mo. T.M. K.Mu. E.N. A.Pen. H.Pe. D.P.-A. A.R. R.S. F.S. M.Sc. M.Shag. M.Shar. C.S.-A. C.S. I.S. M.St. R.W.T. D.R.T. E.L.T. J.-S.W. D.W. This study was conducted in accordance with the guidelines of the Institutional Review Board of the Medical University of Innsbruck and the 1975 Declaration of Helsinki.29 Participants gave written informed consent for genetic investigations according to local regulations.

BackgroundThe spectrum of mitochondrial disease is genetically and phenotypically diverse, resulting from pathogenic variants in over 400 genes, with aerobic energy metabolism defects as a common denominator. Such heterogeneity poses a significant challenge in making an accurate diagnosis, critical for precision medicine.MethodsIn an international collaboration initiated by the European Network for Mitochondrial Diseases (GENOMIT) we recruited 2,023 pediatric patients at 11 specialist referral centers between October 2010 and January 2021, accumulating exome sequencing and HPO-encoded phenotype data. An exome-wide search for variants in known and potential novel disease genes, complemented by functional studies, followed ACMG guidelines.Results1,109 cases (55%) received a molecular diagnosis, of which one fifth have potential disease-modifying treatments (236/1,109, 21%). Functional studies enabled diagnostic uplift from 36% to 55% and discovery of 62 novel disease genes. Pathogenic variants were identified within genes encoding mitochondrial proteins or RNAs in 801 cases (72%), while, given extensive phenotype overlap, the remainder involved proteins targeted to other cellular compartments. To delineate genotype-phenotype associations, our data was complemented with registry and literature data to develop “GENOMITexplorer”, an open access resource detailing patient- (n=3,940), gene- (n=427), and variant-level (n=1,492) associations (prokischlab.github.io/GENOMITexplorer/).ConclusionsReaching a molecular diagnosis was essential for implementation of precision medicine and clinical trial eligibility, underlining the need for genome-wide screening given inability to accurately define mitochondrial diseases clinically. Key to diagnostic success were functional studies, encouraging early acquisition of patient- derived tissues and routine integration of high-throughput functional data to improve patient care by uplifting diagnostic rate.

BackgroundLack of functional evidence hampers variant interpretation, leaving a large proportion of cases with a suspected Mendelian disorder without genetic diagnosis after genome or whole exome sequencing (WES). Research studies advocate to further sequence transcriptomes to directly and systematically probe gene expression defects. However, collection of additional biopsies, and establishment of lab workflows, analytical pipelines, and defined concepts in clinical interpretation of aberrant gene expression are still needed for adopting RNA-sequencing (RNA-seq) in routine diagnostics.MethodsWe implemented an automated RNA-seq protocol and a computational workflow with which we analyzed skin fibroblasts of 303 individuals with a suspected mitochondrial disease which previously underwent WES.ResultsWe detected on average 12,500 genes per sample including around 60% disease genes - a coverage substantially higher than with whole blood, supporting the use of skin biopsies. We prioritized genes demonstrating aberrant expression, aberrant splicing, or mono-allelic expression. The pipeline required less than one week from sample preparation to result reporting and provided a median of eight disease-associated genes per patient for inspection. A genetic diagnosis was established for 16% of the 205 WES-inconclusive cases. Detection of aberrant expression was a major contributor to diagnosis including instances of 50% reduction, which, together with mono-allelic expression, allowed for the diagnosis of dominant disorders caused by haploinsufficiency. Moreover, calling aberrant splicing and variants from RNA-seq data enabled detecting and validating splice-disrupting variants, of which the majority fell outside WES-covered regions.ConclusionTogether, these results show that streamlined experimental and computational processes can accelerate the implementation of RNA-seq in routine diagnostics.One sentence summaryImplementation of RNA-seq as a complementary tool in standard diagnostics achieves a 16% in diagnosis rate over whole exome sequencing.

Not available.

The original version of this Article contained errors in the author affiliations. The affiliations of Nathalie Escande-Beillard and Bruno Reversade with ‘Medical Genetics Department, Koç University, School of Medicine (KUSoM), Istanbul, Turkey’ were inadvertently omitted. This has now been corrected in both the PDF and HTML versions of the Article.

We describe two sporadic and two familial cases with loss-of-function variants in PRPS1, which is located on the X chromosome and encodes phosphoribosyl pyrophosphate synthetase 1 (PRS-1). We illustrate the clinical variability associated with decreased PRS-1 activity, ranging from mild isolated hearing loss to severe encephalopathy. One of the variants we identified has already been reported with a phenotype similar to our patient's, whereas the other three were unknown. The clinical and biochemical information we provide will hopefully contribute to gain insight into the correlation between genotype and phenotype of this rare condition, both in females and in males. Moreover, our observation of a new family in which hemizygous males display hearing loss without any neurological or ophthalmological symptoms prompts us to suggest analysing PRPS1 in cases of isolated hearing loss. Eventually, PRPS1 variants should be considered as a differential diagnosis of mitochondrial disorders.

Currently, genetic testing of mitochondrial DNA mutations includes screening for single-nucleotide variants, several base pair insertions or deletions, large-scale deletions, or relative depletion of total mitochondrial DNA content. Within the last decade, next-generation sequencing (NGS) has resulted in remarkable advances in the field of mitochondrial diseases (MD) and has become a routine step of the diagnostic workup.We aimed to present an overview of current technologies employed in molecular diagnosis of mitochondrial DNA diseases. We report on the recent contributions of NGS testing to the diagnosis and understanding of MD.The progress of NGS technologies allows the simultaneous detection of mutations and quantification of the heteroplasmy level, ensuring sensitivity and specificity requested for the detection of mitochondrial DNA point mutations. NGS protocols enabling the simultaneous analysis of mitochondrial and nuclear DNA are now efficient and cost-saving approaches, and have become the gold-standard technique in diagnostic laboratories.

BackgroundBiallelic variants in PNPT1 cause a mitochondrial disease of variable severity. PNPT1 (polynucleotide phosphorylase) is a mitochondrial protein involved in RNA processing where it has a dual role in the import of small RNAs into mitochondria and in preventing the formation and release of mitochondrial double-stranded RNA into the cytoplasm. This, in turn, prevents the activation of type I interferon response. Detailed neuroimaging findings in PNPT1-related disease are lacking with only a few patients reported with basal ganglia lesions (Leigh syndrome) or non-specific signs.Objective and methodsTo document neuroimaging data in six patients with PNPT1 highlighting novel findings.ResultsTwo patients exhibited striatal lesions compatible with Leigh syndrome; one patient exhibited leukoencephalopathy and one patient had a normal brain MRI. Interestingly, two unrelated patients exhibited cystic leukoencephalopathy resembling RNASET2-deficient patients, patients with Aicardi-Goutières syndrome (AGS) or congenital CMV infection.ConclusionWe suggest that similar to RNASET2, PNPT1 be searched for in the setting of cystic leukoencephalopathy. These findings are in line with activation of type I interferon response observed in AGS, PNPT1 and RNASET2 deficiencies, suggesting a common pathophysiological pathway and linking mitochondrial diseases, interferonopathies and immune dysregulations.

UDP-glucose (UDP-Glc) is synthesized by UGP2-encoded UDP-Glc pyrophosphorylase (UGP) and is required for glycoconjugate biosynthesis and galactose metabolism because it is a uridyl donor for galactose-1-P (Gal1P) uridyltransferase. Chinese hamster lung fibroblasts harboring a hypomrphic UGP(G116D) variant display reduced UDP-Glc levels and cannot grow if galactose is the sole carbon source. Here, these cells were cultivated with glucose in either the absence or presence of galactose in order to investigate glycoconjugate biosynthesis and galactose metabolism. The UGP-deficient cells display &lt, 5% control levels of UDP-Glc/UDP-Gal and &gt, 100-fold reduction of [6-3H]galactose incorporation into UDP-[6-3H]galactose, as well as multiple deficits in glycoconjugate biosynthesis. Cultivation of these cells in the presence of galactose leads to partial restoration of UDP-Glc levels, galactose metabolism and glycoconjugate biosynthesis. The Vmax for recombinant human UGP(G116D) with Glc1P is 2000-fold less than that of the wild-type protein, and UGP(G116D) displayed a mildly elevated Km for Glc1P, but no activity of the mutant enzyme towards Gal1P was detectable. To conclude, although the mechanism behind UDP-Glc/Gal production in the UGP-deficient cells remains to be determined, the capacity of this cell line to change its glycosylation status as a function of extracellular galactose makes it a useful, reversible model with which to study different aspects of galactose metabolism and glycoconjugate biosynthesis.

Mitochondrial translation is essential for the biogenesis of the mitochondrial oxidative phosphorylation system (OXPHOS) that synthesizes the bulk of ATP for the cell. Mutations in either mitochondrial DNA or in nuclear genes that encode mitochondrial translation factors can result in impaired OXPHOS biogenesis and mitochondrial diseases with variable clinical presentations. Mutations in the FARS2 gene encoding the mitochondrial phenylalanyl-tRNA synthetase are commonly linked to either early-onset epileptic mitochondrial encephalopathy or spastic paraplegia. Here, we expand the genetic spectrum of FARS2-linked disease with three patients carrying novel compound heterozygous variants in the FARS2 gene and presenting with spastic tetraparesis, axial hypotonia and myoclonic epilepsy in two cases.

Mitochondrial translation is essential for the biogenesis of the mitochondrial oxidative phosphorylation system (OXPHOS) that synthesizes the bulk of ATP for the cell. Hypomorphic and loss-of-function variants in either mitochondrial DNA or in nuclear genes that encode mitochondrial translation factors can result in impaired OXPHOS biogenesis and mitochondrial diseases with variable clinical presentations.Compound heterozygous or homozygous missense and frameshift variants in the FARS2 gene, that encodes the mitochondrial phenylalanyl-tRNA synthetase, are commonly linked to either early-onset epileptic mitochondrial encephalopathy or spastic paraplegia. Here, we expand the genetic spectrum of FARS2-linked disease with three patients carrying novel compound heterozygous variants in the FARS2 gene and presenting with spastic tetraparesis, axial hypotonia and myoclonic epilepsy in two cases.

Aminoacyl-tRNA synthetases are ubiquitous enzymes, which universally charge tRNAs with their cognate amino acids for use in cytosolic or organellar translation. In humans, mutations in mitochondrial tRNA synthetases have been linked to different tissue-specific pathologies. Mutations in the KARS gene, which encodes both the cytosolic and mitochondrial isoform of lysyl-tRNA synthetase, cause predominantly neurological diseases that often involve deafness, but have also been linked to cardiomyopathy, developmental delay, and lactic acidosis. Using whole exome sequencing, we identified two compound heterozygous mutations, NM_001130089.1:c.683C>T p.(Pro228Leu) and NM_001130089.1:c.1438del p.(Leu480TrpfsX3), in a patient presenting with sensorineural deafness, developmental delay, hypotonia, and lactic acidosis. Nonsense-mediated mRNA decay eliminated the truncated mRNA transcript, rendering the patient hemizygous for the missense mutation. The c.683C>T mutation was previously described, but its pathogenicity remained unexamined. Molecular characterization of patient fibroblasts revealed a multiple oxidative phosphorylation deficiency due to impaired mitochondrial translation, but no evidence of inhibition of cytosolic translation. Reintroduction of wild-type mitochondrial KARS, but not the cytosolic isoform, rescued this phenotype confirming the disease-causing nature of p.(Pro228Leu) exchange and demonstrating the mitochondrial etiology of the disease. We propose that mitochondrial translation deficiency is the probable disease culprit in this and possibly other patients with mutations in KARS.

Diaphragmatic dysfunction has been reported in congenital myopathies, muscular dystrophies, and occasionally, mitochondrial respiratory chain deficiency. Using a minimally invasive procedure in 3 young girls, 1 with a heteroplasmic MT-CYB mutation and 2 with biallelic pathogenic TK2 variants, we provided functional evidence of diaphragmatic dysfunction with global respiratory muscle weakness in mitochondrial respiratory chain deficiency. Analysis of respiratory muscle performance using esogastric pressures revealed paradoxical breathing and severe global inspiratory and expiratory muscle weakness with a sniff esophageal inspiratory pressure and a gastric pressure during cough averaging 50% and 40% of predicted values, respectively. This diaphragmatic dysfunction was responsible for severe undiagnosed nocturnal hypoventilation, requiring noninvasive ventilation. Our results underline the interest of this minimally invasive procedure for the evaluation of respiratory muscle performance and its potential value for the monitoring of future clinical trials in respiratory chain deficiency.

Human mitochondrial aminoacyl-tRNA synthetases (mt-aaRSs) are key enzymes in the mitochondrial protein translation system and catalyze the charging of amino acids on their cognate tRNAs. Mutations in their nuclear genes are associated with pathologies having a broad spectrum of clinical phenotypes, but with no clear molecular mechanism(s). For example, mutations in the nuclear genes encoding mt-AspRS and mt-ArgRS are correlated with the moderate neurodegenerative disorder leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL) and with the severe neurodevelopmental disorder pontocerebellar hypoplasia type 6 (PCH6), respectively. Previous studies have shown no or only minor impacts of these mutations on the canonical properties of these enzymes, indicating that the role of the mt-aaRSs in protein synthesis is mostly not affected by these mutations, but their effects on the mitochondrial localizations of aaRSs remain unclear. Here, we demonstrate that three human aaRSs, mt-AspRS, mt-ArgRS, and LysRS, each have a specific sub-mitochondrial distribution, with mt-ArgRS being exclusively localized in the membrane, LysRS exclusively in the soluble fraction, and mt-AspRS being present in both. Chemical treatments revealed that mt-AspRs is anchored in the mitochondrial membrane through electrostatic interactions, whereas mt-ArgRS uses hydrophobic interactions. We also report that novel mutations in mt-AspRS and mt-ArgRS genes from individuals with LBSL and PCH6, respectively, had no significant impact on the mitochondrial localizations of mt-AspRS and mt-ArgRS. The variable sub-mitochondrial locations for these three mt-aaRSs strongly suggest the existence of additional enzyme properties, requiring further investigation to unravel the mechanisms underlying the two neurodegenerative disorders.

Background Primary microcephaly and profound global developmental delay have been considered the core clinical phenotype in patients with bi-allelic PRUNE1 mutations. Methods Linkage analysis and whole-exome sequencing (WES) in a multiplex family and extraction of further cases from a WES repository containing 571 children with severe developmental disabilities and neurologic symptoms. Results We identified bi-allelic PRUNE1 mutations in twelve children from six unrelated families. All patients who survived beyond the first 6 months of life had early-onset global developmental delay, bilateral spastic paresis, dysphagia and difficult-to-treat seizures, while congenital or later-evolving microcephaly was not a consistent finding. Brain MRI showed variable anomalies with progressive cerebral and cerebellar atrophies and T2-hyperintense brain stem lesions. Peripheral neuropathy was documented in five cases. Disease course was progressive in all patients and eight children died in the first or early second decade of life. In addition to the previously reported missense mutation p.(Asp106Asn), we observed a novel homozygous missense variant p.(Leu172Pro) and a homozygous contiguous gene deletion encompassing most of the PRUNE1 gene and part of the neighboring BNIPL gene. Conclusions PRUNE1 deficiency causes severe early-onset disease affecting the central and peripheral nervous systems. Microcephaly is probably not a universal feature.

Biogenesis of the mitochondrial oxidative phosphorylation system, which produces the bulk of ATP for almost all eukaryotic cells, depends on the translation of 13 mtDNA-encoded polypeptides by mitochondria-specific ribosomes in the mitochondrial matrix. These mitoribosomes are dual-origin ribonucleoprotein complexes, which contain mtDNA-encoded rRNAs and tRNAs and ∼80 nucleus-encoded proteins. An increasing number of gene mutations that impair mitoribosomal function and result in multiple OXPHOS deficiencies are being linked to human mitochondrial diseases. Using exome sequencing in two unrelated subjects presenting with sensorineural hearing impairment, mild developmental delay, hypoglycemia, and a combined OXPHOS deficiency, we identified mutations in the gene encoding the mitochondrial ribosomal protein S2, which has not previously been implicated in disease. Characterization of subjects' fibroblasts revealed a decrease in the steady-state amounts of mutant MRPS2, and this decrease was shown by complexome profiling to prevent the assembly of the small mitoribosomal subunit. In turn, mitochondrial translation was inhibited, resulting in a combined OXPHOS deficiency detectable in subjects' muscle and liver biopsies as well as in cultured skin fibroblasts. Reintroduction of wild-type MRPS2 restored mitochondrial translation and OXPHOS assembly. The combination of lactic acidemia, hypoglycemia, and sensorineural hearing loss, especially in the presence of a combined OXPHOS deficiency, should raise suspicion for a ribosomal-subunit-related mitochondrial defect, and clinical recognition could allow for a targeted diagnostic approach. The identification of MRPS2 as an additional gene related to mitochondrial disease further expands the genetic and phenotypic spectra of OXPHOS deficiencies caused by impaired mitochondrial translation.