MTO1 Mutations are Associated with Hypertrophic Cardiomyopathy and Lactic Acidosis and Cause Respiratory Chain Deficiency in Humans and Yeast

Enrico Baruffini,Massimo Zeviani,Laura Melchionda,Daniele Ghezzi,Eleonora Lamantea,Holger Prokisch,Suresh Vijayaraghavan,Claudia Donnini,Federica Invernizzi,Emma L Blakely,Robert Kopajtich,Anett Walther,Tobias B Haack,Saikat Santra,Cristina Dallabona,Alberto Burlina,Robert W Taylor,Ileana Ferrero,John W Yarham,Tim M Strom,Helen Roper

doi:10.1002/humu.22393

Abstract

We report three families presenting with hypertrophic cardiomyopathy, lactic acidosis, and multiple defects of mitochondrial respiratory chain (MRC) activities. By direct sequencing of the candidate gene MTO1, encoding the mitochondrial-tRNA modifier 1, or whole exome sequencing analysis, we identified novel missense mutations. All MTO1 mutations were predicted to be deleterious on MTO1 function. Their pathogenic role was experimentally validated in a recombinant yeast model, by assessing oxidative growth, respiratory activity, mitochondrial protein synthesis, and complex IV activity. In one case, we also demonstrated that expression of wt MTO1 could rescue the respiratory defect in mutant fibroblasts. The severity of the yeast respiratory phenotypes partly correlated with the different clinical presentations observed in MTO1 mutant patients, although the clinical outcome was highly variable in patients with the same mutation and seemed also to depend on timely start of pharmacological treatment, centered on the control of lactic acidosis by dichloroacetate. Our results indicate that MTO1 mutations are commonly associated with a presentation of hypertrophic cardiomyopathy, lactic acidosis, and MRC deficiency, and that ad hoc recombinant yeast models represent a useful system to test the pathogenic potential of uncommon variants, and provide insight into their effects on the expression of a biochemical phenotype.

Highlights

Mitochondrial disorders are a group of syndromes associated with severe dysfunction of oxidative phosphorylation (OXPHOS), the main energy bioreactor of cells
A partial but significant reduction in maximal respiration rate (MRR), spare respiratory capacity (SRC), and oxygen consumption rate (OCR)/extracellular acidification rate (ECAR) was measured by SeaHorse microscale oxygraphy in fibroblast cell lines of Pt1 and Pt2
To confirm the pathogenic role of p.Thr411Ile and p.Arg477His mutations predicted by in silico analysis, we introduced the corresponding mutant alleles in the paromomycin-resistant yeast strain disrupted in MTO1

Summary

Introduction

Mitochondrial disorders are a group of syndromes associated with severe dysfunction of oxidative phosphorylation (OXPHOS), the main energy bioreactor of cells. Cardiomyocytes, with their extremely high request of energy, are one of the major targets of OXPHOS impairment, and infantile hypertrophic cardiomyopathy is a key clinical feature in many mitochondrial disorders. We have recently reported the first patients affected by hypertrophic cardiomyopathy and lactic acidosis carrying mutations in MTO1 (MIM #614702) [Ghezzi et al, 2012]. Muscle and fibroblasts showed decreased activities of mitochondrial respiratory chain (MRC) complex I (CI) and CIV. The third patient, homozygous for the c.1282G>A (p.Ala428Thr) mutation, had early-onset cardiac hypertrophy with severe lactic acidosis, and defective CI + CIV activities in

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Conclusion