Infantile Encephalopathy and Defective Mitochondrial DNA Translation in Patients with Mutations of Mitochondrial Elongation Factors EFG1 and EFTu

Abstract

Mitochondrial protein translation is a complex process performed within mitochondria by an apparatus composed of mitochondrial DNA (mtDNA)-encoded RNAs and nuclear DNA-encoded proteins. Although the latter by far outnumber the former, the vast majority of mitochondrial translation defects in humans have been associated with mutations in RNA-encoding mtDNA genes, whereas mutations in protein-encoding nuclear genes have been identified in a handful of cases. Genetic investigation involving patients with defective mitochondrial translation led us to the discovery of novel mutations in the mitochondrial elongation factor G1 (EFG1) in one affected baby and, for the first time, in the mitochondrial elongation factor Tu (EFTu) in another one. Both patients were affected by severe lactic acidosis and rapidly progressive, fatal encephalopathy. The EFG1-mutant patient had early-onset Leigh syndrome, whereas the EFTu-mutant patient had severe infantile macrocystic leukodystrophy with micropolygyria. Structural modeling enabled us to make predictions about the effects of the mutations at the molecular level. Yeast and mammalian cell systems proved the pathogenic role of the mutant alleles by functional complementation in vivo. Nuclear-gene abnormalities causing mitochondrial translation defects represent a new, potentially broad field of mitochondrial medicine. Investigation of these defects is important to expand the molecular characterization of mitochondrial disorders and also may contribute to the elucidation of the complex control mechanisms, which regulate this fundamental pathway of mtDNA homeostasis.

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

“…12,20 The clinical symptomatology in our patient is reminiscent of the somewhat milder phenotype described by Valente et al, 21 which was characterized by feeding problems, axial hypotonia, increased limb muscle tone and infantile-onset epilepsy. Notably, liver dysfunction was absent, as in our patient.…”

“…The first patient harbored a homozygous mutation in the GTP-binding domain I (Asn174Ser), 20 and subsequent reports revealed compound heterozygous missense and nonsense mutations affecting other domains of EFG1: Ser321Pro (on the boundary between domains I and II)+Leu607X (predicted to remove domain V) 12 and Met496Arg (in domain III)+Arg47X (predicted to eliminate basically all domains). 21 Mitochondrial translation was impaired in a similar manner in all patients: a severe overall decrease in the rate of mitochondrial translation, with the expression of subunits ND5, ND6, COX I, II and III generally being the lowest. ND3 expression, on the contrary, was often increased compared with control levels.…”

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

“…Compared with wild-type fibroblasts, CI/CII ratios were decreased in mutant fibroblast, both in transfected and untransfected cells. To make cells more dependent on OXPHOS function, 7,8 fibroblasts were grown in DMEM-no glucose supplemented with galactose (5 mM) and fetal bovine serum 10%. In galactose medium, CI/CII ratios were also decreased in mutant cells, but they were partially recovered in transfected cells (Figure 2b).…”

Radioactive Labeling of Mitochondrial Translation Products in Cultured Cells

“…Changes in the normal pattern of mitochondrial protein synthesis, as revealed through radioactive labeling, have served to identify and confi rm causal mutations, and to analyze the pathogenic mechanism of mitochondrial diseases caused by mutations in components of the mitochondrial translation system that are encoded by either the mitochondrial ( 5-10 ) or the nuclear genome (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) …”