Isoleucyl-tRNA synthetase levels modulate the penetrance of a homoplasmic m.4277T>C mitochondrial tRNAIle mutation causing hypertrophic cardiomyopathy

Abstract

The genetic and epigenetic factors underlying the variable penetrance of homoplasmic mitochondrial DNA mutations are poorly understood. We investigated a 16-year-old patient with hypertrophic cardiomyopathy harboring a homoplasmic m.4277T>C mutation in the mt-tRNA(Ile) (MTTI) gene. Skeletal muscle showed multiple respiratory chain enzyme abnormalities and a decreased steady-state level of the mutated mt-tRNA(Ile). Transmitochondrial cybrids grown on galactose medium demonstrated a functional effect of this mutation on cell viability, confirming pathogenicity. These findings were reproduced in transmitochondrial cybrids, harboring a previously described homoplasmic m.4300A>G MTTI mutation. The pathogenic role of the m.4277T>C mutation may be ascribed to misfolding of the mt-tRNA molecule, as demonstrated by the altered electrophoretic migration of the mutated mt-tRNA. Indeed, structure and sequence analyses suggest that thymidine at position 4277 of mt-tRNA(Ile) is involved in a conserved tertiary interaction with thymidine at position 4306. Interestingly, the mutation showed variable penetrance within family members, with skeletal muscle from the patient's clinically unaffected mother demonstrating normal muscle respiratory chain activities and steady-state levels of mt-tRNA(Ile), while homoplasmic for the m.4277T>C mutation. Analysis of mitochondrial isoleucyl-tRNA synthetase revealed significantly higher expression levels in skeletal muscle and fibroblasts of the unaffected mother when compared with the proband, while the transient over-expression of the IARS2 gene in patient transmitochondrial cybrids improved cell viability. This is the first observation that constitutively high levels of aminoacyl-tRNA synthetases (aaRSs) in human tissues prevent the phenotypic expression of a homoplasmic mt-tRNA point mutation. These findings extend previous observations on aaRSs therapeutic effects in yeast and human.

Mutations in the mitochondrial tRNASer(AGY) gene are associated with deafness, retinal degeneration, myopathy and epilepsy

“…Mosaic patterns of COX-deficiency have previously been reported for pathogenic homoplasmic mt-tRNA mutations 29,30 and their phenotypic expression is considered to be dependent on a complex interaction with environmental and/or nuclear genetic factors. 31,32 The m.12264C4T mutation is obviously mild in nature and must require accumulation to near homoplasmic levels before a disruption of the electron transport is observed.…”

Mutations in mitochondrial DNA causing tubulointerstitial kidney disease

“…This situation is similar to the organ specific effect of mutations in mt tRNA Ile , which primarily cause hypertrophic cardiomyopathy [20][21] and of mutations in NADH dehydrogenase causing Leber’s hereditary optic neuropathy [22]. Taken together, the contrasting effects of these mutations should offer a powerful route to understanding how mitochondrial defects cause disease in different tissues.…”

Modification of the wobble uridine in bacterial and mitochondrial tRNAs reading NNA/NNG triplets of 2-codon boxes

“…Compensatory mechanisms of translational defects can include the presence of mutant tRNAs and the overexpression of either specific tRNAs or tRNA synthetases. 73,[88][89][90][91][92][93][94] Notably, acute infantile liver failure associated with TRMU mutations is reversible in some patients through a yet unknown mechanism. 3,95 Even though a compensatory mechanism of the translation defect may exist, the OXPHOS system function may be impaired, as observed in patient cells carrying MTO1 and TRMU mutations, and in TRMU-knocked-down cells.…”