Biallelic variants in COX18 cause a mitochondrial disorder primarily manifesting as peripheral neuropathy.

Defects in mitochondrial dynamics are a common cause of Charcot-Marie-Tooth disease (CMT), while primary deficiencies in the mitochondrial respiratory chain (MRC) are rare and atypical for this etiology. This study aims to report COX18 as a novel CMT-causing gene. This gene encodes an assembly factor of mitochondrial Complex IV (CIV) that translocates the C-terminal tail of MTCO2 across the mitochondrial inner membrane. Exome sequencing was performed in four affected individuals. The patients and available family members underwent thorough neurological and electrophysiological assessment. The impact of one of the identified variants on splicing, protein levels, and mitochondrial bioenergetics was investigated in patient-derived lymphoblasts. The functionality of the mutant protein was assessed using a Proteinase K protection assay and immunoblotting. Neuronal relevance of COX18 was assessed in a Drosophila melanogaster knockdown model. Exome sequencing coupled with homozygosity mapping revealed a homozygous splice variant c.435-6A>G in COX18 in two siblings with early-onset progressive axonal sensory-motor peripheral neuropathy. By querying external databases, we identified two additional families with rare deleterious biallelic variants in COX18 . All affected individuals presented with axonal CMT and some patients also exhibited central nervous system symptoms, such as dystonia and spasticity. Functional characterization of the c.435-6A>G variant demonstrated that it leads to the expression of an alternative transcript that lacks exon 2, resulting in a stable but defective COX18 isoform. The mutant protein impairs CIV assembly and activity, leading to a reduction in mitochondrial membrane potential. Downregulation of the COX18 homolog in Drosophila melanogaster displayed signs of neurodegeneration, including locomotor deficit and progressive axonal degeneration of sensory neurons. Our study presents genetic and functional evidence that supports COX18 as a newly identified gene candidate for autosomal recessive axonal CMT with or without central nervous system involvement. These findings emphasize the significance of peripheral neuropathy within the spectrum of primary mitochondrial disorders and the role of mitochondrial CIV in the development of CMT. Our research has important implications for the diagnostic workup of CMT patients.

Pyruvate and uridine rescue the metabolic profile of OXPHOS dysfunction.

INTRODUCTION: Primary mitochondrial diseases (PMD) are a large, heterogeneous group of genetic disorders affecting mitochondrial function, mostly by disrupting the oxidative phosphorylation (OXPHOS) system. Understanding the cellular metabolic re-wiring occurring in PMD is crucial for the development of novel diagnostic tools and treatments, as PMD are often complex to diagnose and most of them currently have no effective therapy. OBJECTIVES: To characterize the cellular metabolic consequences of OXPHOS dysfunction and based on the metabolic signature, to design new diagnostic and therapeutic strategies. METHODS: In vitro assays were performed in skin-derived fibroblasts obtained from patients with diverse PMD and validated in pharmacological models of OXPHOS dysfunction. Proliferation was assessed using the Incucyte technology. Steady-state glucose and glutamine tracing studies were performed with LC-MS quantification of cellular metabolites. The therapeutic potential of nutritional supplements was evaluated by assessing their effect on proliferation and on the metabolomics profile. Successful therapies were then tested in a in vivo lethal rotenone model in zebrafish. RESULTS: OXPHOS dysfunction has a unique metabolic signature linked to an NAD+/NADH imbalance including depletion of TCA intermediates and aspartate, and increased levels of glycerol-3-phosphate. Supplementation with pyruvate and uridine fully rescues this altered metabolic profile and the subsequent proliferation deficit. Additionally, in zebrafish, the same nutritional treatment increases the survival after rotenone exposure. CONCLUSIONS: Our findings reinforce the importance of the NAD+/NADH imbalance following OXPHOS dysfunction in PMD and open the door to new diagnostic and therapeutic tools for PMD.

Oxygraphy Versus Enzymology for the Biochemical Diagnosis of Primary Mitochondrial Disease.

Primary mitochondrial disease (PMD) is a large group of genetic disorders directly affecting mitochondrial function. Although next generation sequencing technologies have revolutionized the diagnosis of these disorders, biochemical tests remain essential and functional confirmation of the critical genetic diagnosis. While enzymological testing of the mitochondrial oxidative phosphorylation (OXPHOS) complexes remains the gold standard, oxygraphy could offer several advantages. To this end, we compared the diagnostic performance of both techniques in a cohort of 34 genetically defined PMD patient fibroblast cell lines. We observed that oxygraphy slightly outperformed enzymology for sensitivity (79 ± 17% versus 68 ± 15%, mean and 95% CI), and had a better discriminatory power, identifying 58 ± 17% versus 35 ± 17% as "very likely" for oxygraphy and enzymology, respectively. The techniques did, however, offer synergistic diagnostic prediction, as the sensitivity rose to 88 ± 11% when considered together. Similarly, the techniques offered varying defect specific information, such as the ability of enzymology to identify isolated OXPHOS deficiencies, while oxygraphy pinpointed PDHC mutations and captured POLG mutations that were otherwise missed by enzymology. In summary, oxygraphy provides useful information for the diagnosis of PMD, and should be considered in conjunction with enzymology for the diagnosis of PMD.

Long-term outcomes of very low birth weight infants with spontaneous intestinal perforation: A retrospective case-matched cohort study.

BACKGROUND: Spontaneous intestinal perforation (SIP) is an intestinal complication that occurs in very ill preterms. We investigated whether SIP survivors have worse neurodevelopmental and gastrointestinal outcomes and a poorer quality of life than controls. METHODS: A retrospective case-matched cohort study was performed involving infants treated for SIP in a NICU between August 1994 and April 2014. Controls and SIP patients were matched to gestational age, gender, and birth period. Medical records were reviewed. Telephone surveys were conducted to evaluate the medical condition, quality of life (PedsQL™ 4.0), neuropsychiatric and gastrointestinal outcome. McNemar's and Wilcoxon tests were performed, and generalized linear models were computed. RESULTS: Forty-nine SIP patients were included. The percentages of children with multiple disabilities (40% vs. 17%, OR = 3.3) and requiring physiotherapy (86% vs. 60%, OR = 4.77) were higher in the SIP group than in the control group. Intraventricular hemorrhage (IVH) led to a worse neurodevelopmental outcome regardless of SIP (OR = 8.79 for disability), and female gender was a protective factor against disability (OR = 0.06). Reported quality of life and gastrointestinal comorbidities did not differ between the two groups. CONCLUSION: SIP survivors tend to be at risk of multiple disabilities. IVH and female gender influence the neurodevelopmental outcome regardless of SIP. LEVELS OF EVIDENCE: Level III: case-control study.