PHOSPHORYLATION STATUS OF MUS81 IS A MODIFIER OF OLAPARIB SENSITIVITY IN BRCA2-DEFICIENT CELLS

The MUS81 complex is crucial for preserving genome stability through resolution of branched DNA intermediates in mitosis and also participates in the processing of deprotected replication forks in BRCA2-deficient cells. Because of the existence of two different MUS81 complexes in mammalian cells that act in M or S-phase, whether and how the PARPi sensitivity of BRCA2-deficient cells is affected by loss of MUS81 function is unclear. Here, using a separation-of-function mutant of MUS81, we show that viability of BRCA2-deficient cells but not their PARPi sensitivity requires a fully-functional MUS81 complex in mitosis. In contrast, expression of an aberrantly active MUS81 is sufficient to confer PARPi resistance. From a mechanistic point of view, our data indicates that deregulated action of the mitotic active form of MUS81 leads to introduction of DSBs upstream reversal of stalled replication forks bypassing fork deprotection and the engagement of the S-phase MUS81 complex. Collectively, our findings describe a novel mechanism leading to PARPi resistance that involves unscheduled MUS81-dependent cleavage of intact, unreversed replication forks. Since this cleavage occurs mimicking the phosphorylated status of S87 of MUS81, our data suggest that hyperphosphorylation of this residue in S-phase might represent a novel biomarker to identify resistance to PARPi.