Altered 2-thiouridylation impairs mitochondrial translation in reversible infantile respiratory chain deficiency

Rita Horvath,Aurora Gomez-Duran,Ulrike Schara,Veronika Boczonadi,Angela Pyle,Patrick F Chinnery,Mar Tulinius,Paul M Smith

doi:10.1093/hmg/ddt309

Abstract

Childhood-onset mitochondrial encephalomyopathies are severe, relentlessly progressive conditions. However, reversible infantile respiratory chain deficiency (RIRCD), due to a homoplasmic mt-tRNAGlu mutation, and reversible infantile hepatopathy, due to tRNA 5-methylaminomethyl-2-thiouridylate methyltransferase (TRMU) deficiency, stand out by showing spontaneous recovery, and provide the key to treatments of potential broader relevance. Modification of mt-tRNAGlu is a possible functional link between these two conditions, since TRMU is responsible for 2-thiouridylation of mt-tRNAGlu, mt-tRNALys and mt-tRNAGln. Here we show that down-regulation of TRMU in RIRCD impairs 2-thiouridylation and exacerbates the effect of the mt-tRNAGlu mutation by triggering a mitochondrial translation defect in vitro. Skeletal muscle of RIRCD patients in the symptomatic phase showed significantly reduced 2-thiouridylation. Supplementation with l-cysteine, which is required for optimal TRMU function, rescued respiratory chain enzyme activities in human cell lines of patients with RIRCD as well as deficient TRMU. Our results show that l-cysteine supplementation is a potential treatment for RIRCD and for TRMU deficiency, and is likely to have broader application for the growing group of intra-mitochondrial translation disorders.

Highlights

Mitochondrial diseases are a large and clinically heterogeneous group of disorders that result from deficiencies in cellular energy production and affect at least 1 in 5000 of the population
Gene expression levels of both EARS2 and MTO1 were reduced in respiratory chain deficiency (RIRCD) compared with controls, and further decrease was detected after TRMU depletion, suggesting that deficient 2-thiolation may alter these other important mt-tRNAGlu modifying factors (Fig. 3E)
The clinical phenotypes are usually early-onset, severe and often fatal, implying the importance of mitochondrial translation from birth [15]. Some of these conditions affect multiple tissues; tissue-specific manifestations have been reported for several mt-tRNA aminoacyl synthetases or mt-tRNA-modifying genes [13,14]

Summary

INTRODUCTION

Mitochondrial diseases are a large and clinically heterogeneous group of disorders that result from deficiencies in cellular energy production and affect at least 1 in 5000 of the population. Affected children uniformly present with severe muscle weakness, often requiring assisted ventilation in the first days or weeks of life If they survive the first months of life, they improve spontaneously, and recover fully by 2 or 3 years of age. Previous data provide strong evidence for a pathogenic role of m.14674T.C/G, they do not explain why all patients develop severe isolated myopathy in the neonatal period and, most importantly, what triggers the timed spontaneous recovery Another unanswered question is why clinical symptoms manifest only in 30% of individuals carrying the homoplasmic m.14674T.C/G [3]. The age-dependent, partially reversible clinical presentation and the impairment of mt-tRNAGlu strongly suggest a possible pathophysiological link underpinning the spontaneous improvement in these mitochondrial conditions. Defining the common mechanism would suggest new avenues for treatment in these reversible disorders, but could have more general relevance for the growing group of intra-mitochondrial translation defects

RESULTS

DISCUSSION

Findings

MATERIALS AND METHODS