The metabolomic signature of Leber’s hereditary optic neuropathy reveals endoplasmic reticulum stress

Leber’s hereditary optic neuropathy(MIM#535000), the commonest mitochondrial DNA-related disease, is caused by mutations affecting mitochondrial complex I. The clinical expression of the disorder, usually occurring in young adults, is typically characterized by subacute, usually sequential, bilateral visual loss, resulting from the degeneration of retinal ganglion cells. As the precise action of mitochondrial DNA mutations on the overall cell metabolism in Leber’s hereditary optic neuropathyis unknown, we investigated the metabolomic profile of the disease. High performance liquid chromatography coupled with tandem mass spectrometry was used to quantify 188 metabolites in fibroblasts from 16 patients with Leber’s hereditary optic neuropathyand eight healthy control subjects. Latent variable-based statistical methods were used to identify discriminating metabolites. One hundred and twenty-four of the metabolites were considered to be accurately quantified. A supervised orthogonal partial least squares discriminant analysis model separating patients with Leber’s hereditary optic neuropathyfrom control subjects showed good predictive capability (Q2cumulated = 0.57). Thirty-eight metabolites appeared to be the most significant variables, defining a Leber’s hereditary optic neuropathymetabolic signature that revealed decreased concentrations of all proteinogenic amino acids, spermidine, putrescine, isovaleryl- carnitine, propionyl- carnitineand five sphingomyelinspecies, together with increased concentrations of 10 phosphatidylcholine species. This signature was not reproduced by the inhibition of complex I with rotenoneor piericidinA in control fibroblasts. The importance of sphingomyelinsand phosphatidylcholines in the Leber’s hereditary optic neuropathysignature, together with the decreased amino acid pool, suggested an involvement of the endoplasmic reticulum. This was confirmed by the significantly increased phosphorylation of PERK and eIF2α, as well as the greater expression of C/EBP homologous protein and the increased XBP1splicing, in fibroblasts from affected patients, all these changes being reversed by the endoplasmic reticulumstress inhibitor, TUDCA ( tauroursodeoxycholic acid). Thus, our metabolomic analysis reveals a pharmacologically-reversible endoplasmic reticulumstress in complex I-related Leber’s hereditary optic neuropathyfibroblasts, a finding that may open up new therapeutic perspectives for the treatment of Leber’s hereditary optic neuropathywith endoplasmic reticulum-targeting drugs. * Abbreviations : CHOP : C/EBP homologous protein eIF2α : eukaryotic initiation factor2α ER : endoplasmic reticulumLHON : Leber’s hereditary optic neuropathy OPLS-DA : orthogonal partial least squares-discriminant analysis PCA : principal component analysis PERK : protein kinase RNA-activated(PKR)-like ER kinase TUDCA : tauroursodeoxycholic acidVIP : variable importance in the projection