The WRN and MUS81 proteins limit cell death and genome instability following oncogene activation

Abstract

Oncogene-induced replication stress is recognized as the primary cause of accumulation of DNA damage and genome instability in precancerous cells. Although the molecular mechanisms responding to such type of replication perturbation are not fully characterized, it has been speculated that their dysfunction may enhance genome instability and accelerate tumor progression. Here, we show that the WRN protein, a member of the human RecQ helicases, is necessary to sustain replication fork progression in response to oncogene-induced replication stress. Loss of WRN affects cell cycle progression and results in enhanced accumulation of double-strand breaks and instability at common fragile sites in cells experiencing oncogene-induced replication stress. Moreover, we demonstrate that double-strand breaks, observed upon oncogene over-expression, depend on the MUS81 endonuclease, which represents a parallel pathway collaborating with WRN to prevent cell death. Overall, our findings give insights into the mechanisms protecting replication forks in cells experiencing oncogene-induced replication stress, and identify factors that, when mutated or dysfunctional, may enhance genome instability in precancerous cells. In addition, because concomitant depletion of WRN and MUS81 causes synthetic sickness in cells growing under oncogene-induced replication stress, our results support the possibility of targeting cancer cells with an impaired replication fork recovery pathway by a specific inactivation of the other parallel pathway.

The Structure-Specific Endonucleases MUS81 and SEND1 Are Essential for Telomere Stability in Arabidopsis

“…HeLa cells depleted in the RecQ helicase WRN (Werner syndrome ATP-dependent helicase) accumulate DSB in a MUS81-dependent manner after HU treatment. However, simultaneous depletion of WRN and RAD51 does not suppress these DSBs, suggesting that MUS81 operates upstream of RAD51 in the processing of toxic replication intermediates (Franchitto et al, 2008;Murfuni et al, 2012;Murfuni et al, 2013). Furthermore, MUS81 also plays a role in the cleavage of RFs remaining at later (G2/M) stages of the cell cycle at common fragile sites, and its absence leads to an increase in anaphase bridges due to improper disjunction of sister chromatids (Naim et al, 2013;Ying et al, 2013;Pepe and West, 2014).…”

Holliday junction processing enzymes as guardians of genome stability

“…This is particularly pronounced in the absence of alternative replication fork-processing pathways, such as that mediated by the WRN RecQ-helicase [55]. The synthetic sickness of WRN-and MUS81-deficient cells in the presence of oncogene-induced replication stress raises the intriguing possibility of therapeutic intervention by targeting cancer cells lacking one pathway with specific inactivation of its parallel pathway.…”

“…MUS81-dependent fragile-site breakage is further evident in the presence of oncogene-induced replication stress [55]. Oncogene activation affects replication initiation directly or indirectly (via CDK regulation) and leads to deregulated origin firing, impaired replication fork progression, and DSB formation [56].…”

“…MUS81-mediated fork cleavage in cyclin E-or cyclin D1-overexpressing cells has a positive effect on cell survival [55,58], highlighting its importance for overcoming endogenous replication stress. This is particularly pronounced in the absence of alternative replication fork-processing pathways, such as that mediated by the WRN RecQ-helicase [55].…”

MYC and the Control of DNA Replication

“…Furthermore, Emu-MYC mice with deficient WRN helicase activity show a significant delay in lymphoma onset, coinciding with signs of increased DNA damage and senescence in the tumor cells (Moser et al 2012). The role of WRN is not restricted to limiting MYC-induced replication stress, as deregulation of other oncogenes capable of inducing replication stress (i.e., cyclin E and E2F1) leads to cell death in absence of WRN, or in absence of MUS81, an endonuclease required to process stalled forks and certain replication intermediates in response to replication stress (Franchitto et al 2008;Murfuni et al 2013;Neelsen et al 2013). …”