Investigation of a pathogenic mtDNA microdeletion reveals a translation-dependent deadenylation decay pathway in human mitochondria

termini: -CCAtrinucleotide is added to mt-tRNAs, whilst mt-rRNAs and mt-mRNAs are oligo- or polyadenylated,respectively. The cis-acting elements, enzymes and indeed the mechanisms involved in these processesare still largely uncharacterized. Further, the function of polyadenylation in promoting stability, translation ordecay of human mt-mRNA is unclear. A microdeletion has been identified in a patient presenting with mtDNAdisease. Loss of these two residues removes the termination codon for MTATP6 and sets MTCO3immediately in frame. Accurate processing at this site still occurs, but there is a markedly decreased steady-state level of RNA14, the ATPase 8- and 6-encoding bi-cistronic mRNA unit, establishing that an mtDNAmutation can cause dysregulation of mRNA stability. Analysis of the polyadenylation profile of the processedRNA14 at steady state revealed substantial abnormalities. The majority of mutated RNA14 terminated withshort poly (A) extensions and a second, partially truncated population, was also present. Initial maturation ofmutated RNA14 was unaffected, but deadenylation occurred rapidly. Inhibition of mitochondrial proteinsynthesis showed that the deadenylation was dependent on translation. Finally, deadenylation was shown toenhance mRNA decay, explaining the decrease in steady-state RNA14. An hypothesis is presented todescribe how an mtDNA mutation that results in the loss of a termination codon causes enhanced mt-mRNAdecay by translation-dependent deadenylation.INTRODUCTIONHuman mtDNA, a circular duplex of 16569bp, is present inmulticopy in all nucleated cells of the body. It encodes 13polypeptides, all of which are believed to be essential com-ponents of the enzymatic complexes that couple cellularrespiration to ATP synthesis. In addition, the genome encodes22 tRNA and 2 rRNA molecules that are required for intra-mitochondrial protein synthesis. Mitochondrial transfection inmammals remains intractable and in the absence of faithfulreconstitution systems many aspects of mitochondrial geneexpressionarenotwellunderstood.Ithasbeenknownforover20years that the genome is almost fully transcribed from bothstrands (1). These polycistronic RNA units are rapidly processedinto discrete RNA species, which are immediately maturedto produce mt-rRNAs and mt-mRNAs that are oligo- andpolyadenylated, or mt-tRNA species that carry a 3