Worm Breeder's Gazette 16(1): 34 (October 1, 1999)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
MRC Group on the Molecular Biology of Membrane Proteins, Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
We are isolating and characterizing nuclear and mitochondrial DNA (mtDNA) Caenorhabditis elegans mutants with an impaired mitochondrial respiratory chain (MRC). The MRC is made up of 5 protein complexes, and its biogenesis requires the coordinate expression of genes from both the nuclear and the mitochondrial genomes. The MRC is the major source of ATP and thus, defective mitochondrial energy production is increasingly being recognized as a contributor to many human diseases including diabetes, myopathies, neuromuscular, and heart diseases. We have identified and cloned 3 MRC mutations. The first mutation is a 1.2-kb deletion that removes 4 of the 6 exons in the nuo-1 gene encoding the active site subunit of complex I. This mutation is homozygous lethal, leading to an L3 arrest phenotype. The second mutation is a 0.7-kb deletion that removes 3 of the 6 exons in the atp-2 gene encoding the active site subunit of complex V. This mutation is also homozygous lethal, leading to an L3 arrest phenotype. Despite the developmental arrest, atp-2/atp-2 animals have a similar lifespan compared to that of N2. We believe that the mutation is a null mutation, since both atp-2/atp-2 and atp-2/sDf121 animals (sDf121 is a deficiency that covers atp-2) have the same phenotype. RNA interference experiments with atp-2 dsRNA result in significant levels of embryonic arrest. We speculate that the more severe phenotype is due to the absence of a maternal contribution of mRNA which allows animals to develop to the L3 stage. We are currently attempting to measure the levels of respiratory activity in this mutant. The third mutation (uaDf5) is a 3.1-kb mtDNA deletion that removes 4 MRC and 7 tRNA genes. uaDf5 animals are heteroplasmic, having varying proportions of wildtype and mutant mtDNAs. We have maintained the heteroplasmy for over 50 generations and have demonstrated maternal inheritance of the mtDNA deletion. In addition, we have shown a stochastic inheritance pattern for the uaDf5 mtDNA. All uaDf5 animals appear to be aphenotypic despite high (>80%) proportions of mutant mtDNA. We are currently attempting to elevate the proportion of uaDf5 mtDNA in hope of crossing a threshold level where the mutation would become pathogenic. We have examined the effect of ethidium bromide (EtBr) on N2 worms. EtBr is a DNA-intercalating dye known to inhibit mtDNA replication and to deplete mtDNA in yeast and in mammalian cell lines. Interestingly, when N2 gravid adults are exposed to EtBr, L3 arrested F1 progeny are observed. The concentration of EtBr at which 50% of the animals arrest at L3 is about 35 ug/ml. Despite the developmental arrest, these animals appear to have a lifespan similar to control N2. Furthermore, the arrested animals exhibit a progressive decrease in the steady-state level of mtDNA with age. The observations that both nuclear mutants and the mtDNA-depleted animals arrest at L3 lead us to speculate that pathogenic uaDf5 animals are likely to have an L3 arrest phenotype as well. Finally, we speculate that the L3 arrest phenotype is a consequence of impaired energy production. In support of this hypothesis, we have determined that development from L3 to L4 is associated with a 3-fold increase in mtDNA copy number, as well as a significant increase in the levels of ATP-2.