Evolution Meets Disease: Penetrance and Functional Epistasis of Mitochondrial tRNA Mutations

Raquel Moreno-Loshuertos,M Esther Gallardo,Rebeca Acín-Pérez,Acisclo Pérez-Martos,José Antonio Enríquez,Massimo Zeviani,Gustavo Ferrín,Patricio Fernández-Silva,Carlo Viscomi

doi:10.1371/journal.pgen.1001379

Abstract

About half of the mitochondrial DNA (mtDNA) mutations causing diseases in humans occur in tRNA genes. Particularly intriguing are those pathogenic tRNA mutations than can reach homoplasmy and yet show very different penetrance among patients. These mutations are scarce and, in addition to their obvious interest for understanding human pathology, they can be excellent experimental examples to model evolution and fixation of mitochondrial tRNA mutations. To date, the only source of this type of mutations is human patients. We report here the generation and characterization of the first mitochondrial tRNA pathological mutation in mouse cells, an m.3739G>A transition in the mitochondrial mt-Ti gene. This mutation recapitulates the molecular hallmarks of a disease-causing mutation described in humans, an m.4290T>C transition affecting also the human mt-Ti gene. We could determine that the pathogenic molecular mechanism, induced by both the mouse and the human mutations, is a high frequency of abnormal folding of the tRNAIle that cannot be charged with isoleucine. We demonstrate that the cells harboring the mouse or human mutant tRNA have exacerbated mitochondrial biogenesis triggered by an increase in mitochondrial ROS production as a compensatory response. We propose that both the nature of the pathogenic mechanism combined with the existence of a compensatory mechanism can explain the penetrance pattern of this mutation. This particular behavior can allow a scenario for the evolution of mitochondrial tRNAs in which the fixation of two alleles that are individually deleterious can proceed in two steps and not require the simultaneous mutation of both.

Highlights

Mammalian mitochondrial DNA is a double-stranded circular molecule that codes for 13 of the 87 proteins that constitute the OXPHOS system, as well as two rRNAs and the 22 tRNAs required for mitochondrial protein synthesis
In mammals the rRNAs and all the tRNAs required for the mitoribosomes to work are encoded by the mitochondrial DNA (mtDNA)
Half of mtDNA mutations causing diseases occur in tRNA genes while they represent only 10% of the total sequence

Summary

Introduction

Mammalian mitochondrial DNA is a double-stranded circular molecule that codes for 13 of the 87 proteins that constitute the OXPHOS system, as well as two rRNAs and the 22 tRNAs required for mitochondrial protein synthesis. Mutations in mitochondrial DNA are known to be responsible for a wide variety of diseases in humans whose common characteristic is the impairment of the OXPHOS system. Cells carrying pathological mutations in mt-tRNAs usually exhibit impaired respiration and reduced growth rates in medium with galactose instead of glucose. This is due to the fact that mutations in tRNA genes may affect the synthesis of critical subunits of Complexes I, III and IV and two subunits of complex

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