Phosphorylation status of MUS81 is a modifier of Olaparib sensitivity in BRCA2-deficient cells

Abstract

The MUS81 complex is crucial for preserving genome stability through resolution of branched DNA intermediates in mitosis and also for 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 mutant of MUS81 that impairs its function in M-phase, 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 a constitutively-active MUS81 is sufficient to confer PARPi resistance. From a mechanistic point of view, our data indicate that deregulated action of the mitotic active form of MUS81 in S-phase leads to the cleavage of stalled replication forks before their reversal, bypassing fork deprotection, and engaging a Polθ-dependent DSBs repair. 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.