Rapamycin rescues mitochondrial dysfunction in cells carrying the m.8344A > G mutation in the mitochondrial tRNALys

Abstract

Background Myoclonus, Epilepsy and Ragged-Red-Fibers (MERRF) is a mitochondrial encephalomyopathy due to heteroplasmic mutations in mitochondrial DNA (mtDNA) most frequently affecting the tRNALys gene at position m.8344A > G. Defective tRNALys severely impairs mitochondrial protein synthesis and respiratory chain when a high percentage of mutant heteroplasmy crosses the threshold for full-blown clinical phenotype. Therapy is currently limited to symptomatic management of myoclonic epilepsy, and supportive measures to counteract muscle weakness with co-factors/supplements. Methods We tested two therapeutic strategies to rescue mitochondrial function in cybrids and fibroblasts carrying different loads of the m.8344A > G mutation. The first strategy was aimed at inducing mitochondrial biogenesis directly, over-expressing the master regulator PGC-1α, or indirectly, through the treatment with nicotinic acid, a NAD+ precursor. The second was aimed at stimulating the removal of damaged mitochondria through prolonged rapamycin treatment. Results The first approach slightly increased mitochondrial protein expression and respiration in the wild type and intermediate-mutation load cells, but was ineffective in high-mutation load cell lines. This suggests that induction of mitochondrial biogenesis may not be sufficient to rescue mitochondrial dysfunction in MERRF cells with high-mutation load. The second approach, when administered chronically (4 weeks), induced a slight increase of mitochondrial respiration in fibroblasts with high-mutation load, and a significant improvement in fibroblasts with intermediate-mutation load, rescuing completely the bioenergetics defect. This effect was mediated by increased mitochondrial biogenesis, possibly related to the rapamycin-induced inhibition of the Mechanistic Target of Rapamycin Complex 1 (mTORC1) and the consequent activation of the Transcription Factor EB (TFEB). Conclusions Overall, our results point to rapamycin-based therapy as a promising therapeutic option for MERRF.

Progressive Myoclonus Epilepsy: A Scoping Review of Diagnostic, Phenotypic and Therapeutic Advances

“…Several innovative therapeutic approaches are being considered for MERRF. In one study, researchers attempted to induce mitochondrial biogenesis in cells with various degrees of heteroplasmy [103]. Two approaches were used: (1) directly by the overexpression of the master regulator PGC-1-α or (2) indirectly using nicotinic acid, which is an NAD+ precursor for the ETC, to improve mitochondrial respiration.…”

“…These results suggest that this approach may not be feasible for patients with a high mutation load. These researchers also attempted to stimulate the removal of damaged mitochondria through the prolonged use of rapamycin, which induces the inhibition of mechanistic target of rapamycin complex 1 (mTORC1) and downstream activation of transcription factor EB (TFEB), which mediates increased mitochondrial biogenesis [103]. This treatment was applied for 4 weeks and did induce a slight increase in mitochondrial respiration in the fibroblasts with a high mutation load.…”

Mitochondrial Mutations Can Alter Neuromuscular Transmission in Congenital Myasthenic Syndrome and Mitochondrial Disease

“…These were the only genes associated with mtDNA depletion syndrome, but their high levels of NMJ dysfunction point to the possibility that other genetic causes of this syndrome may suffer from transmission abnormalities as well. Additionally, both RR2MB and m.8344A>G mutations have been linked to impaired oxidative phosphorylation indicating that NMJ dysfunction may be due to compromised ATP synthesis [106][107][108][109]. However, many other mitochondrial diseases impair oxidative phosphorylation and ATP production and do not present with NMJ dysfunction, indicating that more research is needed in this area.…”

Glutamine metabolism in diseases associated with mitochondrial dysfunction

“…Entry of glutamine into the TCA cycle as aKG has also been shown to enhance mTORC1 signaling, which is known to occur in the setting of mitochondrial disease ( Yang et al, 2017 ; Johnson et al, 2019 ). The causal link between glutamine, aKG, mTOR, is complex; the role of mTOR in mitochondrial disease in whole organisms appears to be defined more by immune activity than metabolism, but cultured cell models support a cell-autonomous role for mTOR dysregulation, and this remains an active area of research ( Johnson et al, 2019 ; Johnson et al, 2013 ; Stokes et al, 2022 ; Capristo et al, 2022 ).…”