The A4435G mutation is localized at 3′ end adjacent to the anticodon (position 37) of tRNA
Met.
26 In fact, the adenine at this position of tRNA
Met is extraordinarily conserved from bacteria to human mitochondria.
27 Almost all A37 in tRNAs are modified, by such processes as thiolation and methylation.
28 Indeed, this modified nucleotide contributes to the high fidelity of codon recognition, the structural formation and stabilization of functional tRNAs.
39 In
Escherichia coli, nucleotide modifications at positions 37 and 34 are responsible for the stabilization of the canonical loop structure in the anticodon domain of tRNA
Lys.
40 Also, it has been shown that the modification of A37 stabilizes the 3′ stacking features of the anticodon, thereby improving its interaction with the codon.
41 Thedeficient modification of A37 decreases the activity of the corresponding tRNA
42 and increases +1 frameshifts for tRNA
Phe,
43 whereas the A-to-G substitution at position 37 leads to a 10-fold reduction in the section of tRNAs at the A-site.
44 Most recently, the T4291C mutation at the anticodon region of mitochondrial tRNA
Ile has been associated with hypertension, hypercholesterolemia, and hypomagnesemia.
45 The lack of modification at U34 of anticodon in the tRNA
Lys has been observed in cells carrying the MERRF-associated A8344G mutation in this gene.
46 In the current study, compared with control cells lacking those mutations, a ∼50% reduction in the level of tRNA
Met was observed in cells carrying both the G11778A and A4435G mutations, whereas there was no significant reduction in the level of tRNA
Met in cells carrying the G11778A mutation. The lower level of tRNA
Met in cells carrying the A4435G mutation most probably results from a defect in nucleotide modification at position 37 of tRNA
Met. As a result, a failure in mitochondrial tRNA metabolism, caused by the A4435G mutation, may lead to impairment of mitochondrial translation, especially for seven subunits, including ND4 of NADH (complex I) encoded by mtDNA. Thus, the mitochondrial dysfunction, particularly in deficient activities of complex I, caused by ND4 G11778 mutation, would be worsened by the A4435G mutation, similar to other secondary LHON missense mutations or G7444A mutation. Therefore, the A4435G mutation may have a modifying role in increasing the penetrance and expressivity of the primary LHON-associated G11778A mutation in this Chinese family.