Exploring the endonuclease MUS81 as a new factor impacting on cancer chemosensitivity

Background: The use of PARP inhibitors (PARPi) has resulted in significant prolongation of disease-free survival, particularly in patients with cancers characterized by Homologous Recombination Repair (HRR) deficiency. However, 20% of these eligible patients do not respond to PARPi or could develop PARPi resistance; to overcome this variety of problems, research is now considering the possibility of combining PARPi with other therapeutic approaches. Over recent years, our lab has actively investigated the activation and roles of the human MUS81, a structure specific endonuclease involved in the resolution of DNA replication intermediates during G2/M phase. Its role in maintaining genome stability makes it a potential target for cancer therapy. Aim: PARPi in combination with MUS81 depletion could counteract chemoresistance in breast cancer; this hypothesis is rooted in the idea that MUS81 loss can induce defects that evoke PARP activation. Methods: We knocked-out MUS81 by Crispr/Cas9 in cancer-derived, HRR-proficient, breast epithelial cells and in healthy mammary epithelial cells and investigated on the parylation levels, phenotype and possible PARPi sensitization. Results: MUS81 KO results in elevated parylation and DNA damage accumulation in breast cancer cells; these results correlate with more NAD+ consumption, due to an accentuated PARP activation. Cancer cells undergo metabolic alteration and rewiring when MUS81 is knocked out, while there is no effect on NAD+ levels in healthy breast epithelial cells with MUS81 loss. MUS81 KO sensitized greatly the HRR+ cancer cells to the PARP inhibitor Olaparib, while in normal epithelial cell line we had not found significant reduction in terms of cell viability. Conclusions: MUS81 depletion sensitize breast cancer cell to PARPi independently of their HRR status. This makes MUS81 a promising and critical target, since its depletion in combination with PARPi has shown efficacy exclusively on cancer cells, with minimal toxicity in healthy cells. Results: MUS81 KO results in elevated parylation and DNA damage accumulation in breast cancer cells; these results correlate with more NAD+ consumption, due to an accentuated PARP activation. Cancer cells undergo metabolic alteration and rewiring when MUS81 is knocked out, while there is no effect on NAD+ levels in healthy breast epithelial cells with MUS81 loss. MUS81 KO sensitized greatly the HRR+ cancer cells to the PARP inhibitor Olaparib, while in normal epithelial cell line we had not found significant reduction in terms of cell viability. Conclusions: MUS81 depletion sensitize breast cancer cell to PARPi independently of their HRR status. This makes MUS81 a promising and critical target, since its depletion in combination with PARPi has shown efficacy exclusively on cancer cells, with minimal toxicity in healthy cells.