Importance Cellular mechanisms underlying mitochondrial dysfunction, a hallmark feature of many neurodegenerative conditions, remain incompletely understood, and their true diversity is unknown. Objective To identify and functionally validate novel genetic variants causative of Leigh syndrome. Design We performed whole genome sequencing (WGS) and first-degree relative genotyping on two unrelated adult subjects with brain MRI abnormalities evoking Leigh syndrome. Blue native polyacrylamide gel electrophoresis (BN-PAGE) and respiratory chain enzymatic activity assays were performed to screen for respiratory complex assembly and/or oxidative phosphorylation impairments. Cells obtained from patient dermal and muscular biopsies were immortalized and later genetically corrected to evaluate cellular response to metabolic stress. Setting Subjects were recruited from The Neuro (McGill University), Rizk Hospital (Lebanese American University), and Centre Hospitalier Universitaire Sainte-Justine (University of Montreal). Research connections were established through the White Matter Rounds Network and GeneMatcher. Participants Four subjects representing three families with undiagnosed Leigh syndrome (age range 10-40 years) were ultimately recruited. Main outcome(s) and Measure(s) DNA sequencing uncovered a new autosomal recessive Leigh syndrome-associated gene that was functionally validated. Results Bi-allelic pathogenic variants in AMPD2 were detected in all subjects. BN-PAGE of patient skeletal muscle mitochondria captured an isolated complex V assembly defect in the context of mTOR activation, while the accompanying enzymological assays reported decreased activities of complexes I and IV. Opposite to controls, patient-derived cell lines lacked AMPD2 protein, attributing null status to the variants detected. During metabolic challenge, mutant cells suffered from mitochondrial hyperfusion and high-order cytosolic IMPDH2 oligomerization, implying simultaneous ATP accumulation and GTP deficiency. However, both complex V assembly and mTOR status were impervious to these conditions. All cellular phenotypes observed collectively reverted upon exogenous introduction of wild-type AMPD2. Conclusions and Relevance The recognition of AMPD2-related Leigh syndrome (AMPD2-LS) as a novel entity provides strong evidence for classifying AMPD2 deficiency as a mitochondrial disease. Our data suggest that respiratory capacity is significantly modulated by AMPD2, a cytosolic enzyme selectively regulating complex V assembly through an elusive process. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement JS has received scholarships from the Canadian Institutes of Health Research (CIHR) and the Fonds de Recherche du Quebec en Sante (FRQS). RLP has received a Research Scholar Junior 1 award from the FRQS, research funds for this study from the CIHR (grant 202309PJT-506913), the Canadian Radiological Foundation, and Hoffman-La Roche Limited. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The Institutional Review Board of The Neuro (Montreal Neurological Institute-Hospital) gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data in the present study are available upon reasonable request to the authors.

HaemophiliaVolume 27, Issue 5 p. e638-e640 LETTER TO THE EDITORToken Access A full molecular picture of F8 intron 1 inversion created with optical genome mapping Somayyeh Fahiminiya, Corresponding Author somayyeh.fahiminiya@mail.mcgill.ca orcid.org/0000-0002-7538-3355 Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, Canada Correspondence Somayyeh Fahiminiya and Julie Gauthier, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, QC, Canada. Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, QC, Canada. Email: somayyeh.fahiminiya@mail.mcgill.ca and julie.m.gauthier.hsj@ssss.gouv.qc.caSearch for more papers by this authorGeorges-Etienne Rivard, orcid.org/0000-0002-2861-9441 Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Hematology-Oncology, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, CanadaSearch for more papers by this authorPatrick Scott, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, CanadaSearch for more papers by this authorAlexandre Montpetit, Centre d'expertise et de services Génome Québec, Montreal, Québec, CanadaSearch for more papers by this authorFrançois Bacot, Centre d'expertise et de services Génome Québec, Montreal, Québec, CanadaSearch for more papers by this authorJean St-Louis, orcid.org/0000-0003-1183-533X Division of Hematology-Oncology, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, CanadaSearch for more papers by this authorGrant A. Mitchell, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, CanadaSearch for more papers by this authorWilliam D. Foulkes, Department of Human Genetics, McGill University, Montreal, Québec, CanadaSearch for more papers by this authorJean-Francois Soucy, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, CanadaSearch for more papers by this authorJulie Gauthier, Corresponding Author julie.m.gauthier.hsj@ssss.gouv.qc.ca Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, Canada Correspondence Somayyeh Fahiminiya and Julie Gauthier, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, QC, Canada. Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, QC, Canada. Email: somayyeh.fahiminiya@mail.mcgill.ca and julie.m.gauthier.hsj@ssss.gouv.qc.caSearch for more papers by this author Somayyeh Fahiminiya, Corresponding Author somayyeh.fahiminiya@mail.mcgill.ca orcid.org/0000-0002-7538-3355 Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, Canada Correspondence Somayyeh Fahiminiya and Julie Gauthier, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, QC, Canada. Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, QC, Canada. Email: somayyeh.fahiminiya@mail.mcgill.ca and julie.m.gauthier.hsj@ssss.gouv.qc.caSearch for more papers by this authorGeorges-Etienne Rivard, orcid.org/0000-0002-2861-9441 Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Hematology-Oncology, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, CanadaSearch for more papers by this authorPatrick Scott, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, CanadaSearch for more papers by this authorAlexandre Montpetit, Centre d'expertise et de services Génome Québec, Montreal, Québec, CanadaSearch for more papers by this authorFrançois Bacot, Centre d'expertise et de services Génome Québec, Montreal, Québec, CanadaSearch for more papers by this authorJean St-Louis, orcid.org/0000-0003-1183-533X Division of Hematology-Oncology, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, CanadaSearch for more papers by this authorGrant A. Mitchell, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, CanadaSearch for more papers by this authorWilliam D. Foulkes, Department of Human Genetics, McGill University, Montreal, Québec, CanadaSearch for more papers by this authorJean-Francois Soucy, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, CanadaSearch for more papers by this authorJulie Gauthier, Corresponding Author julie.m.gauthier.hsj@ssss.gouv.qc.ca Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, Québec, Canada Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, Québec, Canada Correspondence Somayyeh Fahiminiya and Julie Gauthier, Molecular Diagnostic Laboratory, CHU Sainte-Justine, Montreal, QC, Canada. Division of Medical Genetics, Department of Pediatrics, CHU Sainte-Justine and Université de Montréal, Montreal, QC, Canada. Email: somayyeh.fahiminiya@mail.mcgill.ca and julie.m.gauthier.hsj@ssss.gouv.qc.caSearch for more papers by this author First published: 07 July 2021 https://doi.org/10.1111/hae.14375Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume27, Issue5September 2021Pages e638-e640 RelatedInformation