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

The pathogenetic mechanism of the deafness-associated mitochondrial DN A (mtDNA) T7445C mutation has been investigated in several lymphoblast oid cell lines from members of a New Zealand pedigree exhibiting the m utation in homoplasmic form and from control individuals. We show here that the mutation Banks the 3' end of the tRNA(Ser(UCN)) gene sequenc e and affects the rate but not the sites of processing of the tRNA pre cursor. This causes an average reduction of similar to 70% in the tRNA (Ser(UCN)) level and a decrease of similar to 45% in protein synthesis rate in the cell lines analyzed. The data show a sharp threshold in t he capacity of tRNA(Ser(UCN)) to support the wild-type protein synthes is rate, which corresponds to similar to 40% of the control level of t his tRNA. Strikingly, a 7445 mutation-associated marked reduction has been observed in the level of the mRNA for the NADH dehydrogenase (com plex I) ND6 subunit gene, which is located similar to 7 kbp upstream a nd is cotranscribed with the tRNA(Ser(UCN)) gene, with strong evidence pointing to a mechanistic link with the tRNA precursor processing def ect. Such reduction significantly affects the rate of synthesis of the ND6 subunit and plays a determinant role in the deafness-associated r espiratory phenotype of the mutant cell lines. In particular, it accou nts for their specific, very significant decrease in glutamate- or mal ate-dependent O-2 consumption Furthermore, several homoplasmic mtDNA m utations affecting subunits of NADH dehydrogenase may play a synergist ic role in the establishment of the respiratory phenotype of the mutan t cells.