Exploring novel treatment strategies for primary mitochondrial disease

Mutations of mitochondrial DNA (mtDNA) cause a wide range of diseases that cause severe disability and shorted lifespan and represent a major challenge in modern medicine. Unfortunately, treatment remains restricted to symptomatic control. The majority of diseases resulting from mtDNA mutations are heteroplasmic - cells and/or tissues contain a mixture of mtDNA species including both normal (WT) and mutant mtDNA. It is widely accepted that disease severity matches mutant load: i.e. patients with a higher mutant load have more severe disease. People with lower mutant burden may even be asymptomatic. The corollary is that a therapy reduces mutant load even a modest amount may have a disporoprtionate benefit.

We have studied patient derived fibroblasts carrying the A3243G mtDNA mutation, the commonest of disease-causing mutations, which associates with MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke like episodes) or MIDD (maternally inherited diabetes and deafness). The cells (with a mutant burden of ~80%) show decreased oxygen consumption, decreased mitochondrial membrane potential, mitochondrial fragmentation, disruption of respiratory chain complexes, and increased rates of free radical generation and increased lactate release. RNAseq showed increased expression of the PI3Kinase/AKT/mTOR pathway in the mutant cells, confirmed by assays of AKT and mTOR phosphorylation (Western Blot) and downstream pathways (e.g S6k). Immunofluorescence of patient derived muscle biopsies also show increased phospho-AKT/AKT ratio. We interpret these changes in signaling patways as an adaptive metabolic response to impaired energy homeostasis. Remarkably, treatment of the cultured cells with inhibitors of PI3K (LY294002), AKT (using MK2206) or mTOR (with rapamycin) for ~2 months, all substantially decreased mutant burden and rescued the mitochondrial bioenergetic phenotype, increasing oxygen consumption, restoring mitochondrial membrane potential and morphology, decreasing lactic acid production and rescuing respiratory chain assembly (Blue native page blots with in situ enzyme assays). The mechanism seems to involve stimulation of mitochondrial autophagy.