Biallelic pathogenic variants in COX11 are associated with an infantile-onset mitochondrial encephalopathy

Abstract

Primary mitochondrial diseases are a group of genetically and clinically heterogeneous disorders resulting from oxidative phosphorylation (OXPHOS) defects. COX11 encodes a copper chaperone that participates in the assembly of complex IV and has not been previously linked to human disease. In a previous study, we identified that COX11 knockdown decreased cellular adenosine triphosphate (ATP) derived from respiration, and that ATP levels could be restored with coenzyme Q₁₀ (CoQ₁₀ ) supplementation. This finding is surprising since COX11 has no known role in CoQ₁₀ biosynthesis. Here, we report a novel gene-disease association by identifying biallelic pathogenic variants in COX11 associated with infantile-onset mitochondrial encephalopathies in two unrelated families using trio genome and exome sequencing. Functional studies showed that mutant COX11 fibroblasts had decreased ATP levels which could be rescued by CoQ₁₀ . These results not only suggest that COX11 variants cause defects in energy production but reveal a potential metabolic therapeutic strategy for patients with COX11 variants.

Keywords: COX11; OXPHOS; coenzyme Q; mitochondrial disorders.

Figure 1

BN‐PAGE for complex IV and OXPHOS Western blot in COX11 fibroblasts. Mitochondria isolated from Patient X1 and control fibroblasts analyzed by BN‐PAGE immunoblotting for COX2, COX4, (a) and COX1 (b) suggest a decreased level of monomeric CIV more evident in mitochondria solubilized with Triton X‐100, which separates the individual OXPHOS complexes (marked by *); ^‡indicates previous COX1 signal and complex II (SDHA) was used as a loading control. (c) The levels of other OXPHOS subunits in isolated mitochondria were similar to controls (CI‐NDUFA9, CIII‐CORE1, and CV‐ATP5A). (d) Representative SDS‐PAGE and western blot analysis in whole cell lysates with an antibody cocktail targeting individual OXPHOS complex subunits showed consistently lower levels of CIV in patient X1; porin (VDAC1) was used as a loading control. BN, Blue native; CIV, complex IV; OXPHOS, oxidative phosphorylation; BN‐ PAGE, Blue native polyacrylamide gel electrophoresis; SDHA, succinate dehydrogenase complex subunit A; SDS, sodium dodecyl sulfate.

Figure 2

COX11 variation is associated with decreased respiration‐derived ATP levels, which can be restored with CoQ_10. ATP levels were measured by flow cytometry after 6 h of metabolic substrate in patient X1 (COX11) and control fibroblasts expressing a genetically encoded FRET‐based ATP sensor. Basal ATP levels were similar between all conditions. Respiration‐only ATP levels were normalized within each cell line to basal and depleted conditions. Patient XI (COX11) and control fibroblast ATP levels remained high with unrestricted basal metabolism. When metabolism was restricted to respiration‐only by addition of 10 mM 2‐deoxy‐d‐glucose, ATP levels dropped in patient cells that were treated with the vehicle, while patient XI (COX11) cells maintained higher levels of ATP with CoQ₁₀ treatment. Data show mean ± SEM; n= 2−8 replicates of at least 300 cells per condition, compiled from two independent experiments. ***p< 0.001, *<0.05 by two‐way analysis of variance with Tukey's multiple comparisons test. ATP adenosine triphosphate; FRET, fluorescence resonance energy transfer.