Distinct Clinical Phenotypes Associated with a Mutation in the Mitochondrial Translation Elongation Factor EFTs

Abstract

The 13 polypeptides encoded in mitochondrial DNA (mtDNA) are synthesized in the mitochondrial matrix on a dedicated protein-translation apparatus that resembles that found in prokaryotes. Here, we have investigated the genetic basis for a mitochondrial protein-synthesis defect associated with a combined oxidative phosphorylation enzyme deficiency in two patients, one of whom presented with encephalomyopathy and the other with hypertrophic cardiomyopathy. Sequencing of candidate genes revealed the same homozygous mutation (C997T) in both patients in TSFM, a gene coding for the mitochondrial translation elongation factor EFTs. EFTs functions as a guanine nucleotide exchange factor for EFTu, another translation elongation factor that brings aminoacylated transfer RNAs to the ribosomal A site as a ternary complex with guanosine triphosphate. The mutation predicts an Arg333Trp substitution at an evolutionarily conserved site in a subdomain of EFTs that interacts with EFTu. Molecular modeling showed that the substitution disrupts local subdomain structure and the dimerization interface. The steady-state levels of EFTs and EFTu in patient fibroblasts were reduced by 75% and 60%, respectively, and the amounts of assembled complexes I, IV, and V were reduced by 35%-91% compared with the amounts in controls. These phenotypes and the translation defect were rescued by retroviral expression of either EFTs or EFTu. These data clearly establish mutant EFTs as the cause of disease in these patients. The fact that the same mutation is associated with distinct clinical phenotypes suggests the presence of genetic modifiers of the mitochondrial translation apparatus.

Functional consequences of mitochondrial tRNATrp and tRNAArg mutations causing combined OXPHOS defects

“…24 They found COI and COIII translation to be most severely disturbed, but in contrast to our results ATP6 showed a profoundly decreased synthesis as well. Increased synthesis of ATP6 and ATP8, as we observed for our tRNA Trp and tRNA Arg mutations, has been reported before [29][30][31][32] and has been suggested to be caused by the fact that bicistronic mRNAs can be translated more efficiently than monocistronic mRNAs, thereby giving the translation of ATP6 and ATP8 an advantage. 32 Many patients with a tRNA Trp mutation, including our patient, show developmental delay, gastrointestinal symptoms and failure to thrive.…”

Molecular-genetic characterization and rescue of a TSFM mutation causing childhood-onset ataxia and nonobstructive cardiomyopathy

“…Despite pulse labeling of mitochondrial translation products only revealed a moderate reduction in mitochondrial protein synthesis, all of these patients died of encephalomyopathy, hypertrophic cardiomyopathy or liver failure before the age of 2 months. [10][11][12] Surprisingly, the patient here reported, presenting ataxia and hypertrophic cardiomyopathy, was alive at the age of 23 years. His mutation also decreased the EFTs levels, but the EFTu levels were normal.…”

“…3,10 Seven patients from five different pedigrees harbored, in homozygosity, a EFTs (p.(Arg312Trp)) amino acid substitution. Despite pulse labeling of mitochondrial translation products only revealed a moderate reduction in mitochondrial protein synthesis, all of these patients died of encephalomyopathy, hypertrophic cardiomyopathy or liver failure before the age of 2 months.…”

Mutation in subdomain G' of mitochondrial elongation factor G1 is associated with combined OXPHOS deficiency in fibroblasts but not in muscle

“…Sequence analysis of all mitochondrial translation factors additionally revealed a homozygous mutation in elongation factor Ts (TSFM) in one patient: Arg333Trp, which has been reported before in two unrelated patients. 23 All three patients presented with extensive lactic acidosis and muscular hypotonia within 3 days after birth, displayed combined complex I, III and IV deficiencies in fibroblasts as well as muscle tissue, and died in the first few months of life. Whereas the mutation in TSFM was associated with either mitochondrial encephalomyopathy or hypertrophic cardiomyopathy in the previously published cases, our patient was affected by both.…”