A Heteroplasmic MT-CO2 m.8024G > A Variant Is Associated with Mitochondrial Bioenergetic Deficiency and Optic Atrophy

Leber's hereditary optic neuropathy (LHON) is a hereditary neurodegenerative disorder caused by pathogenic mitochondrial DNA (mtDNA) variants. While MT-CO2 defects are implicated in neurodegeneration, their direct association with optic atrophy has not been reported. We identify a heteroplasmic MT-CO2 variant, m.8024G > A (p.Glu147Lys), in a patient with progressive optic atrophy and explore its potential association with mitochondrial dysfunction. A 13-year-old male with progressive unilateral-then-bilateral vision loss underwent comprehensive ophthalmic/neurological evaluation, trio whole-exome sequencing, and mtDNA sequencing. The pathogenicity of the identified variant was assessed in patient-derived fibroblasts using mitochondrial stress tests, ATP/ROS assays, enzymatic profiling, BN-PAGE, mitochondrial membrane potential, mtDNA copy number, ultrastructural microscopy, and immunoblotting. Functional analyses revealed that this variant, which reduces the expression of mtDNA-encoded electron transport chain (ETC) subunits and induces severe Complex IV deficiency, reduced cellular oxygen consumption rate (OCR), impaired ATP synthesis, decreased mtDNA copy number, and elevated reactive oxygen species (ROS) production. Concurrently, mutant cells exhibited enhanced mitophagy with preserved flux, a compensatory response to persistent mitochondrial damage. Unlike canonical optic neuropathy associated with homoplasmic mtDNA mutations, this heteroplasmic variant is linked to mitochondrial dysfunction potentially related to tissue-specific heteroplasmy and altered mitophagic responses. We report a heteroplasmic m.8024G > A mutation in MT-CO2 associated with childhood-onset isolated optic atrophy. Functional analyses in patient fibroblasts show that this variant is associated with MT-CO2 structural perturbation, Complex IV dysfunction, altered mitophagy, and mitochondrial energy failure-supporting its potential pathogenic relevance. This study expands the genotypic and phenotypic spectrum of mitochondrial optic neuropathies and provides mechanistic insights into the pathogenesis of heteroplasmic mtDNA variant-related disease.