Impact of disease-related mitochondrial mutations on tRNA structure and function

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Abstract

Over 150 mutations with documented pathogenicity have been identified within the human mitochondrial genome. More than half of the disease-related mutations are located within tRNA genes, a remarkable trend, given that these sequences comprise only 10% of the genome. The discovery of diseases correlated with mitochondrial tRNA mutations provides the first example of a class of pathologies related to RNA function, and the study of these tRNAs provides an interesting opportunity to explore the relationship between physiology and tRNA function. Investigations of both cellular and molecular effects have provided important insights into the structural and functional defects caused by the mutations. The picture that emerges from varied studies is that the effects of tRNA mutations are probably multifaceted and complex, but can be traced to the destabilization of structural features that destroy the native tRNA fold required for all aspects of function.

Section snippets

An overview of human mitochondrial tRNAs

tRNAs are essential components of protein synthesis because they function to transport amino acids to the ribosome, match them to the codons of mRNAs and facilitate their incorporation into polypeptides with high fidelity [6]. The human mitochondrial genome contains a minimal set of tRNAs. The 16.6 kb circular genome encodes 22 tRNAs, along with 13 proteins that correspond to respiratory chain subunits, and two rRNAs 1, 7. The tRNAs encoded in the mitochondria are the only ones used in protein

tRNALeu(UUR): loss of native structure hinders function

The gene encoding human mitochondrial tRNALeu(UUR) harbors a significant portion of the known pathogenic mutations within the human mitochondrial genome. Of the 150 documented mutations, 20 occur within the tRNALeu(UUR) gene (see http://www.mitomap.org; Figure 3). The mutations (or single-base deletions) affect 18 of the 76 nucleotides within this tRNA. Varied clinical consequences result from mutations in tRNALeu(UUR), including diabetes, myopathies and encephalopathies (see //www.mitomap.org

tRNAIle: weak structures amplify effects of mutations

The gene encoding tRNAIle contains ten pathogenic mutations associated with cardiomyopathy and ophthalmoplegia 4, 47. Interestingly, the four ophthalmoplegia mutations all introduce CA mispairs into stem regions of this tRNA, whereas the six cardiomyopathy mutations are in varied locations (two produce GU mispairs, three are substitutions of unpaired bases and one replaces a CA mispair in the wild-type structure with a Watson–Crick UA pair; Figure 3). Although a link between a certain type of

tRNALys: importance of post-transcriptional modifications

The human mitochondrial tRNALys is an example of a mitochondrial tRNA with a minimized structure (Figure 3). The D-loop of this molecule is proposed to consist of only three bases, which limits the number of contacts stabilizing the tertiary fold. Post-transcriptional modifications appear to play a significant role in stabilizing the structure of human mitochondrial tRNALys and might contribute to the cellular effects of the pathogenic mutations identified within this gene. A study of cybrid

tRNASer(UCN): decreased tRNA levels with pathogenic mutations

Lowered steady-state tRNA levels have been correlated with two deafness-related mutations within the human mitochondrial tRNASer(UCN) gene. The U7445C mutation affects a nucleotide proximal to the site of 3′ end processing (Figure 3), and in vitro studies indicate that cleavage by 3′-tRNase is significantly inhibited [21]. The defect in processing would affect the levels of tRNASer(UCN). In addition, analysis of cybrid cell lines containing the 7472C insertion that affects tRNASer(UCN) appears

Concluding remarks

Recent efforts to catalog the effects of disease-related mutations affecting mitochondrial tRNAs have highlighted the sensitivity of the function of these important biomolecules to subtle structural perturbations. Disruptions in elements of secondary structure or the loss of contacts stabilizing tertiary structure can significantly impair function, as revealed by in vitro and in vivo studies of protein synthesis efficiency, aminoacylation, post-transcriptional modification and processing. The

Acknowledgements

We acknowledge the NIH (GM063890–01A2) for financial support of the work on human mitochondrial tRNAs conducted in our laboratory.

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