The Deafness-Associated Mitochondrial DNA Mutation at Position 7445, Which Affects tRNA^Ser(UCN) Precursor Processing, Has Long-Range Effects on NADH Dehydrogenase Subunit ND6 Gene Expression

Abstract

The pathogenetic mechanism of the deafness-associated mitochondrial DNA (mtDNA) T7445C mutation has been investigated in several lymphoblastoid cell lines from members of a New Zealand pedigree exhibiting the mutation in homoplasmic form and from control individuals. We show here that the mutation flanks the 3 end of the tRNA Ser(UCN) gene sequence and affects the rate but not the sites of processing of the tRNA precursor. This causes an average reduction of ϳ70% in the tRNA Ser(UCN) level and a decrease of ϳ45% in protein synthesis rate in the cell lines analyzed. The data show a sharp threshold in the capacity of tRNA Ser(UCN) to support the wild-type protein synthesis rate, which corresponds to ϳ40% of the control level of this tRNA. Strikingly, a 7445 mutation-associated marked reduction has been observed in the level of the mRNA for the NADH dehydrogenase (complex I) ND6 subunit gene, which is located ϳ7 kbp upstream and is cotranscribed with the tRNA Ser(UCN) gene, with strong evidence pointing to a mechanistic link with the tRNA precursor processing defect. Such reduction significantly affects the rate of synthesis of the ND6 subunit and plays a determinant role in the deafness-associated respiratory phenotype of the mutant cell lines. In particular, it accounts for their specific, very significant decrease in glutamate-or malate-dependent O 2 consumption. Furthermore, several homoplasmic mtDNA mutations affecting subunits of NADH dehydrogenase may play a synergistic role in the establishment of the respiratory phenotype of the mutant cells.

Cosegregation of the G7444A mutation in the mitochondrial COI/tRNASer(UCN) genes with the 12S rRNA A1555G mutation in a Chinese family with aminoglycoside‐induced and nonsyndromic hearing loss

“…In particular, mtDNA mutations at positions 4216 and 13708 labeled as second LHON mutations were implicated to increase the penetrance of the deafness-associated A7445G mutation [Guan et al, 1998], while the ND1 T3308C and tRNA Ala T5655C mutations likely contribute to the higher penetrance of deafness in an African pedigree than Japanese and French families carrying the T7511C mutation [Li et al, 2004c]. Apart from the A1555G and G7444A mutations, 36 variants in this mitochondrial genome, belonging to the Eastern Asian haplogroup D4a [Yao et al, 2002], showed no evolutionary conservation.…”

“…Alternatively, the G7444A mutation is adjacent to the site of 3' end endonucleolytic processing of the L-strand RNA precursor, spanning tRNA Ser(UCN) and ND6 mRNA [Guan et al, 1998;Levinger et al, 2004]. Our previous data showed that the A7445G mutation in the precursor of tRNA Ser(UCN) led to a failure in the processing of the L-strand RNA precursor, thereby causing a marked decrease of the steady-state levels of tRNA Ser(UCN) and ND6 mRNA [Guan et al, 1998;Li et al, 2005b]. Thus, the G7444A mutation, similar to the A7445G mutation, may also cause a defect in the processing of the L-strand RNA precursor, thus causing mitochondrial dysfunctions.…”

Mitochondrial DNA Sequence Variants Associated With Blood Pressure Among 2 Cohorts of Older Adults

“…More than half of pathogenic mtDNA mutations are located in genes controlling tRNA expression 41. Such mutations lead to reduced tRNA steady‐state levels, decreased mitochondrial protein synthesis, and destabilized tRNA secondary or tertiary structure42, 43 and impaired oxidative phosphorylation and oxygen consumption 43. Elsewhere, tRNA‐thr m.15294T>C and other tRNA‐thr variants have been associated with dilated cardiomyopathy 44, 45.…”

Mitochondrial rRNA and tRNA and hearing function

“…These mutations often occur in homoplasmy or in high levels of heteroplasmy, indicating a high threshold for pathogenicity. It is believed that mutations in this gene can cause a failure in tRNA metabolism, thereby npg leading to a decrease in the amount of affected tRNAs, which subsequently results in insufficient mitochondrial protein synthesis and the respiration defects [85].…”

“…The A7445G mutation is a silent change of both the last nucleotide of the COI gene on the heavy strand and the nucleotide immediately adjacent to the 3′ end of the tRNA Ser(UCN) gene on the light strand. It is possible that the mutation may affect normal processing of the light-strand polycistronic RNA and lead to significant decreases in the levels of both tRNA Ser(UCN) and cotranscripted ND6 mRNA, which subsequently disturbs both mitochondrial protein synthesis and respiration of the mutant cells [85]. A recent study has indicated that the biochemical phenotype associated with the A7445G mutation was modified by nuclear background [87].…”