Creation of Yeast Models for Evaluating the Pathogenicity of Mutations in the Human Mitochondrial Gene MT-ATP6 and Discovering Therapeutic Molecules

Numerous diseases in humans have been associated with mutations of the mitochondrial genome (mtDNA). This genome encodes 13 protein subunits of complexes involved in oxidative phosphorylation (OXPHOS), a process that provides aerobic eukaryotes with the energy-rich adenosine triphosphate molecule (ATP). Mutations of the mtDNA may therefore have dramatic consequences especially in tissues and organs with high energy demand. Evaluating the pathogenicity of these mutations may be difficult because they often affect only a fraction of the numerous copies of the mitochondrial genome (up to several thousands in a single cell), which is referred to as heteroplasmy. Furthermore, due to its exposure to reactive oxygen species (ROS) produced in mitochondria, the mtDNA is prone to mutations, and some may be simply neutral polymorphisms with no detrimental consequences on human health. Another difficulty is the absence of methods for genetically transforming human mitochondria. Face to these complexities, the yeast Saccharomyces cerevisiae provides a convenient model for investigating the consequences of human mtDNA mutations in a defined genetic background. Owing to its good fermentation capacity, it can survive the loss of OXPHOS, its mitochondrial genome can be manipulated, and genetic heterogeneity in its mitochondria is unstable. Taking advantage of these unique attributes, we herein describe a method we have developed for creating yeast models of mitochondrial ATP6 gene mutations detected in patients, to determine how they impact OXPHOS. Additionally, we describe how these models can be used to discover molecules with therapeutic potential.

Novel insights into the functional metabolic impact of an apparent de novo m.8993T>G variant in the MT-ATP6 gene associated with maternally inherited form of Leigh Syndrome

In this study, we report a novel perpective of metabolic consequences for the m.8993T>G variant using fibroblasts from a proband with clinical symptoms compatible with Maternally Inherited Leigh Syndrome (MILS). Definitive diagnosis was corroborated by mitochondrial DNA testing for the pathogenic variant m.8993T>G in MT-ATP6 subunit by Sanger sequencing. The long-range PCR followed by massively parallel sequencing method detected the near homoplasmic m.8993T>G variant at 83% in the proband's fibroblasts and at 0.4% in the mother's fibroblasts. Our results are compatible with very low levels of germline heteroplasmy or an apparent de novo mutation. Our mitochondrial morphometric analysis reveals severe defects in mitochondrial cristae structure in the proband's fibroblasts. Our live-cell mitochondrial respiratory analyses show impaired oxidative phosphorylation with decreased spare respiratory capacity in response to energy stress in the proband's fibroblasts. We detected a diminished glycolysis with a lessened glycolytic capacity and reserve, revealing a stunted ability to switch to glycolysis upon full inhibition of OXPHOS activities. This dysregulated energy reprogramming results in a defective interplay between OXPHOS and glycolysis during an energy crisis. Our study sheds light on the potential pathophysiologic mechanism leading to chronic energy crisis in this MILS patient harboring the m.8993T>G variant.