Metabolomics hallmarks OPA1 variants correlating with their in vitro phenotype and predicting clinical severity

Abstract

Interpretation of variants of uncertain significance is an actual major challenge. We addressed this question on a set of OPA1 missense variants responsible for variable severity of neurological impairments. We used targeted metabolomics to explore the different signatures of OPA1 variants expressed in Opa1 deleted mouse embryonic fibroblasts (Opa1−/− MEFs), grown under selective conditions. Multivariate analyses of data discriminated Opa1+/+ from Opa1−/− MEFs metabolic signatures and classified OPA1 variants according to their in vitro severity. Indeed, the mild p.I382M hypomorphic variant was segregating close to the wild-type allele, while the most severe p.R445H variant was close to Opa1−/− MEFs, and the p.D603H and p.G439V alleles, responsible for isolated and syndromic presentations, respectively, were intermediary between the p.I382M and the p.R445H variants. The most discriminant metabolic features were hydroxyproline, the spermine/spermidine ratio, amino acid pool and several phospholipids, emphasizing proteostasis, endoplasmic reticulum (ER) stress and phospholipid remodeling as the main mechanisms ranking OPA1 allele impacts on metabolism. These results demonstrate the high resolving power of metabolomics in hierarchizing OPA1 missense mutations by their in vitro severity, fitting clinical expressivity. This suggests that our methodological approach can be used to discriminate the pathological significance of variants in genes responsible for other rare metabolic diseases and may be instrumental to select possible compounds eligible for supplementation treatment.

Drug repositioning as a therapeutic strategy for neurodegenerations associated with OPA1 mutations

“…It is of interest that the substantial slowdown of autophagy in these cells is in line with the results of our metabolomics analysis evidencing also a significant fall in the cellular amino acids content, more severe in the R445H than in D603H MEFs ( 71 ). As stated above, ORMs 0, 2 and 14 were able to enhance the ATP content and, although ORM0 less efficiently, to induce the LC3-I cleavage in R445H MEFs, indicating re-activation of the autophagic process.…”

A plasma metabolomic signature of Leber hereditary optic neuropathy showing taurine and nicotinamide deficiencies

“…However, we have previously shown a sharp drop in the concentration of taurine in the aqueous humor of patients with glaucoma where taurine is considered to play an osmolytic role ( 39 ). We also previously found a drop in taurine concentration in mouse embryonic fibroblasts in which OPA1 gene has been inactivated and have found that the severity of the OPA1 was tightly correlated to the taurine level in these cells ( 40 ). Taurine is present in high concentration in oxidative tissues, where it plays a role in stabilizing the inner mitochondrial membrane gradient and the equilibrium between NAD/NADH and GSH/GSSG (oxidized and reduced forms of glutathione) ( 41 ).…”

Molecular Mechanisms behind Inherited Neurodegeneration of the Optic Nerve

“…Alterations of purine metabolism (xanthine, hypoxanthine, and inosine), lipid metabolism (1-oleyl-rac-glycerol, rac-glycerol-1-myristate and glycerate, phosphocholine, and choline), and of few aminoacids (cystine, aspartate, glutamic acid, and urocanate) have been described in plasma from patients carrying OPA1 mutations [ 98 ]. Concordantly, variations in several phospholipids concentration have been reported in mouse embryonic fibroblasts (MEF) Opa1 -/- [ 99 ], MEF Opa1 -/- expressing different OPA1 mutants [ 100 ], and in the Opa1 delTTAG/+ mouse model, specifically in the optic nerve and in plasma [ 101 ]. Thus, while alterations in purine metabolism and reduced glutamic acid are recurrent in mitochondrial diseases or in the presence of ATP depletion [ 102 , 103 , 104 , 105 ], changes in phospholipids, confirmed in three different models, may be the effect of mitochondrial membrane remodeling due to OPA1 dysfunction, in particular the defective mitochondrial fusion.…”