atm Mutation and Oxidative Stress Enhance the Pre-Cancerous Effects of UHRF1 Overexpression in Zebrafish Livers

SIMPLE SUMMARY: Mutation of the ataxia-telangiectasia mutated (atm) gene in humans and mice renders them susceptible to tumors due to both its role as a DNA damage sensor acting in pre-malignant cells to activate Tp53 and to its role in sensing and reducing oxidative stress. The oncogene UHRF1 is overexpressed in many cancers and we previously reported that UHRF1 overexpression in zebrafish hepatocytes activates a tumor suppressive pathway dependent on Tp53, resulting in senescence and a small liver which later is bypassed resulting in liver cancer. We tested the hypothesis that Atm was involved in the precancerous small liver phenotype caused by UHRF1 overexpression by generating atm zebrafish mutants. We show that atm mutation and high ROS levels enhanced, whereas antioxidant treatment suppressed, the small liver phenotype in UHRF1 overexpressing larvae. This suggests that the pre-cancerous small liver phenotype caused by UHRF1 overexpression is due to oxidative stress, which is mitigated by Atm. ABSTRACT: The ataxia-telangiectasia mutated (atm) gene is activated in response to genotoxic stress and leads to activation of the tp53 tumor suppressor gene which induces either senescence or apoptosis as tumor suppressive mechanisms. Atm also serves non-canonical functions in the response to oxidative stress and chromatin reorganization. We previously reported that overexpression of the epigenetic regulator and oncogene Ubiquitin Like with PHD and Ring Finger Domains 1 (UHRF1) in zebrafish hepatocytes resulted in tp53-dependent hepatocyte senescence, a small liver and larval lethality. We investigated the role of atm on UHRF1-mediated phenotypes by generating zebrafish atm mutants. atm(−/−) adults were viable but had reduction in fertility. Embryos developed normally but were protected from lethality caused by etoposide or H(2)O(2) exposure and failed to fully upregulate Tp53 targets or oxidative stress response genes in response to these treatments. In contrast to the finding that Tp53 prevents the small liver phenotype caused by UHRF1 overexpression, atm mutation and exposure to H(2)O(2) further reduced the liver size in UHRF1 overexpressing larvae whereas treatment with the antioxidant N-acetyl cysteine suppressed this phenotype. We conclude that UHRF1 overexpression in hepatocytes causes oxidative stress, and that loss of atm further enhances this, triggering elimination of these precancerous cells, leading to a small liver.