Pathophysiological insights into ACO2-related inherited disorders through combined functional and multi-omic analyses of ACO2 patient-derived fibroblasts

ACO2 encodes the mitochondrial aconitase, involved in the second step of the tricarboxylic acid cycle. ACO2 variants cause isolated dominant or recessive optic atrophy, characterized by the degeneration of the optic nerves and leading to vision loss with high clinical heterogeneity, which ranges from pauci-symptomatic to legally blind patients. ACO2 variants also cause rare severe syndromic presentations, like infantile cerebellar-retinal degeneration, characterized by developmental delay, truncal hypotonia, ataxia, seizures, optic atrophy and retinal degeneration. As ACO2 pathophysiology still lacks proper characterization and no treatment is available for ACO2-related disorders, we combined a functional study with a multi-omic analysis of patient-derived fibroblasts carrying ACO2 variants. Here, we show that ACO2 variants decrease ACO2 protein levels, ACO2 and cytoplasmic ACO1 mRNA expression and mtDNA levels. Metabolomic analyses identified 14 discriminating metabolites involved in amino acid metabolism, TCA cycle, nucleotide and lipid metabolism. Transcriptomic analyses disclosed a down-regulation of immunity-related pathways and an up-regulation of cell cycle-related pathways. These results provide new insights into the cellular impact of ACO2 deficiency beyond its primary role in the TCA cycle, and identify citrate accumulation as a crucial determinant in ACO2 physiopathology.