Two Siblings with Homozygous Pathogenic Splice-Site Variant in Mitochondrial Asparaginyl-tRNA Synthetase (NARS2)

Abstract

A homozygous missense mutation (c.822G>C) was found in the gene encoding the mitochondrial asparaginyl-tRNA synthetase (NARS2) in two siblings born to consanguineous parents. These siblings presented with different phenotypes: one had mild intellectual disability and epilepsy in childhood, whereas the other had severe myopathy. Biochemical analysis of the oxidative phosphorylation (OXPHOS) complexes in both siblings revealed a combined complex I and IV deficiency in skeletal muscle. In-gel activity staining after blue native-polyacrylamide gel electrophoresis confirmed the decreased activity of complex I and IV, and, in addition, showed the presence of complex V subcomplexes. Considering the consanguineous descent, homozygosity mapping and whole-exome sequencing were combined revealing the presence of one single missense mutation in the shared homozygous region. The c.822G>C variant affects the 3' splice site of exon 7, leading to skipping of the whole exon 7 and a part of exon 8 in the NARS2 mRNA. In EBV-transformed lymphoblasts, a specific decrease in the amount of charged mt-tRNA(Asn) was demonstrated as compared with controls. This confirmed the pathogenic nature of the variant. To conclude, the reported variant in NARS2 results in a combined OXPHOS complex deficiency involving complex I and IV, making NARS2 a new member of disease-associated aaRS2.

Epileptic phenotypes in children with early‐onset mitochondrial diseases

“…These cases come in addition to the reported early-onset epileptic encephalopathy related to mutations in aaRS genes[25][26][27][28] and confirm the high epileptogenic potential of these mutations. These cases come in addition to the reported early-onset epileptic encephalopathy related to mutations in aaRS genes[25][26][27][28] and confirm the high epileptogenic potential of these mutations.…”

Mutations in the mitochondrial cysteinyl-tRNA synthase gene,CARS2,lead to a severe epileptic encephalopathy and complex movement disorder

“…Aminoacylation of mt-tRNA-Cys was analysed as described previously 22. Briefly, total RNA was extracted from fibroblasts using Trizol reagent (Life Technologies) following the manufacturer's instructions, with the final pellet resuspended in 10 mM NaOAc at pH 5.0 and kept at 4°C to preserve the aminoacylation state.…”

Predicting the pathogenicity of aminoacyl-tRNA synthetase mutations

“…For example, mutations in mitochondrial ARS enzymes that cause recessive diseases can directly affect protein solubility [74] or impair mitochondrial protein translation [4,5,46,50,56–59,63,75–79], consistent with a loss-of-function effect. With respect to ARS mutations that cause dominant peripheral neuropathy, structural analyses have revealed that the majority of GARS mutations affect residues within the dimer interface [80].…”