Inhibition of ATR Reverses a Mitochondrial Respiratory Insufficiency

Diseases that affect the mitochondrial electron transport chain (ETC) often manifest as threshold effect disorders, meaning patients only become symptomatic once a certain level of ETC dysfunction is reached. Cells can invoke mechanisms to circumvent reaching their critical ETC threshold, but it is an ongoing challenge to identify such processes. In the nematode Caenorhabditis elegans, severe reduction of mitochondrial ETC activity shortens life, but mild reduction actually extends it, providing an opportunity to identify threshold circumvention mechanisms. Here, we show that removal of ATL-1, but not ATM-1, worm orthologs of ATR and ATM, respectively, key nuclear DNA damage checkpoint proteins in human cells, unexpectedly lessens the severity of ETC dysfunction. Multiple genetic and biochemical tests show no evidence for increased mutation or DNA breakage in animals exposed to ETC disruption. Reduced ETC function instead alters nucleotide ratios within both the ribo- and deoxyribo-nucleotide pools, and causes stalling of RNA polymerase, which is also known to activate ATR. Unexpectedly, atl-1 mutants confronted with mitochondrial ETC disruption maintain normal levels of oxygen consumption, and have an increased abundance of translating ribosomes. This suggests checkpoint signaling by ATL-1 normally dampens cytoplasmic translation. Taken together, our data suggest a model whereby ETC insufficiency in C. elegans results in nucleotide imbalances leading to the stalling of RNA polymerase, activation of ATL-1, dampening of global translation, and magnification of ETC dysfunction. The loss of ATL-1 effectively reverses the severity of ETC disruption so that animals become phenotypically closer to wild type.