PO-014 Targeting PARP1 in XRCC1 deficient sporadic invasive breast cancer or pre-invasive ductal carcinoma in situ for synthetic lethality and chemoprevention

Abstract

IntroductionPrimary liver cancer is the second largest cause of cancer related mortality worldwide. Deregulation of cellular energetics is an emerging hallmark of cancer. NAD+ is a ubiquitous metabolite that plays a critical role in the regulation of many metabolic pathways. Recent work has shown that boosting NAD+ biosynthesis can protect against the development of hepatocellular carcinoma. Here we have explored whether organelle specific changes in NAD+ are important for this effect.Material and methodsWe have examined transgenic animals overexpressing either NMNAT1 or NMNAT3 to increase NAD+ biosynthesis in the nuclear and mitochondrial compartments respectively. The transgenic mice and WT littermates were injected with the liver carcinogen diethylnitrosamine (DEN) at 2 weeks of age and subsequently placed on a high-fat diet to accelerate tumourigenesis. We also acutely treated 12–16 week old WT and transgenic mice to investigate the changes within 48 hour of DEN treatment. Liver tissue and serum collected from these experiments were subjected to intensive analysis in search for mechanisms of protection.Results and discussionsOverexpression of NMNAT1 significantly reduced the multiplicity of liver tumours, with an average of 5 tumours per mouse, compared to 13.6 in WT. In contrast, NMNAT3 overexpression did not influence tumour multiplicity. However when tumours were established, the transgenic models showed no protection with regards to tumour burden. The mice with higher nuclear NAD+ biosynthesis displayed a 50% reduction in serum levels of the liver damage markers ALT and AST compared to WT littermates. We postulated that increased nuclear NAD+ levels may be important in protecting against tumour initiation events. Treatment of adult mice with an acute dose of DEN revealed that NMNAT1 transgenic mice had a 3-fold lower ALT response compared WT and NMNAT3 transgenic mice. They also had lower levels of oxidised peroxiredoxin 2 and inflammatory marker CCL5, indicating that nuclear NAD+ is protective against genotoxic liver damage. In isolated hepatocytes NMNAT1 mice had lower levels of DNA damage marker pγH2Ax upon DEN treatement and this protection was abolished when co-treated with PARP1 inhibitor Olaparib.ConclusionOur study shows that elevation in nuclear NAD+ are important for protection against liver tumour formation. Increased NMNAT1 activity appears to protects the liver at the tumour initiation event by reduction of oxidative stress and DNA damage by regulating the activity of PARP1.