The inheritance of functional mitochondria depends on faithful replication and transmission of mitochondrial DNA (mtDNA). A large and heterogeneous group of human disorders is associated with mitochondrial genome quantitative and qualitative anomalies. Several nuclear genes have been shown to account for these severe OXPHOS disorders. However, in several cases, the disease-causing mutations still remain unknown. Caenorhabditis elegans has been largely used for studying various biological functions because this multicellular organism has short life cycle and is easy to grow in the laboratory. Mitochondrial functions are relatively well conserved between human and C.elegans, and heteroplasmy exists in this organism as in human. C. elegans therefore represents a useful tool for studying mtDNA maintenance. Suppression by RNA interference of genes involved in mtDNA replication such as polg-1, encoding the mitochondrial DNA polymerase, results in reduced mtDNA copy number but in a normal phenotype of the F1 worms. By combining RNAi of genes involved in mtDNA maintenance and EtBr exposure, we were able to reveal a strong and specific phenotype (developmental larval arrest) associated to a severe decrease of mtDNA copy number. Moreover, we tested and validated the screen efficiency for human orthologous genes encoding mitochondrial nucleoid proteins. This allowed us to identify several genes that seem to be closely related to mtDNA maintenance in C. elegans. This work reports a first step in the further development of a large-scale screening in C. elegans that should allow to identify new genes of mtDNA maintenance whose human orthologs will obviously constitute new candidate genes for patients with quantitative or qualitative mtDNA anomalies.
Screening of Caenorhabditis elegans genes possibly involved in the mitochondrial genome maintenance was performed using our previous validated method of RNAi combined with ethidium bromide. This was to knock down C. elegans genes homologous to yeast genes known to be involved in mtDNA stability but of unknown molecular function or to identify transient components that could play important role on the stability of mtDNA in a temporal and/or spatial manner. C. elegans homologs for 11 genes among 27 yeast genes for which deletion leads to a rho0 state were found, however, only 5 genes were present in the RNAi library. Out of these 5 genes, 1 gene (homolog of GEM1) gave a clear L3 arrest on RNAi and ethidium bromide indicating its involvement on mtDNA stability. Four other genes homologs of MTG2, YER087W, AVL9 and RRG3 did not lead to L3 arrest even though their deletion in Saccharomyces cerevisiae leads to rho0 state. Although MTG2 has been reported to be important in the function and structure on mtDNA stability in yeast, our results did not support those findings in C. elegans. The human homolog of this gene (MIRO1) can be considered as a candidate gene involved in mtDNA stability and sequenced in patients with mtDNA depletion diseases.
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