Ataxia-telangiectasia mutated (ATM) and RAD3-related (ATR) are key players in the mitotic DNA damage response (DDR) functioning to both recognize double-strand breaks (DSBs) and signal their presence to the cell. ATM and ATR have also more recently been shown to have a role in meiosis where they can influence the number of programmed DSBs that are made by the topoisomerase-like enzyme Spo11. In yeast and mammals, ATM acts as a negative regulator of Spo11-dependent DSB formation. By contrast, yeast ATR appears to function in a positive feedback loop for the formation of DSBs. We are interested in understanding how ATM and ATR work antagonistically to achieve a balance of meiotic DSBs. Efforts to further elucidate the in vivo roles of ATM and ATR in meiosis in mammals have been hampered by the fact that atm-1 mutant mice are infertile, deficient in making mature gametes. Therefore we exploit the facile genetics and superb cytological of the nematode, C. elegans to probe the function of these genes in meiotic.
I have been investigating the roles of C. elegans ATM and ATR orthologs, ATM-1 and ATL-1, respectively, by analyzing the phenotypic consequences of atm-1 and atl-1 mutations and double mutants with components of the DSB machinery. I have been analyzing the effects of these mutations on COs formation through analysis of diakinesis-stage oocytes when chromosomes with COs condense into bivalent, cruciform structures. I also monitor how many DSBs are channeled into homologous recombination by assaying recruitment of RAD-51 to the chromatin by immunolocalization.
In my studies to date, we have found the surprising result that while atm-1 mutants produce more DSBs during meiosis, as in other systems, fewer crossovers (COs) are formed. This finding suggests that ATM-1 not only regulates DSB formation but also influences downstream processing of these breaks. Two exciting possibilities are being explored: atm-1 could modulate repair pathway choice, directing DSBs to CO outcomes; alternatively atm-1 may function in a crossover checkpoint, preventing apoptosis if the correct level of DSBs is achieved. Further study will focus on determining the functions of atm-1 and atl-1 in formation of DSBs and their downstream processing.