Mus81-Eme1–dependent aberrant processing of DNA replication intermediates in mitosis impairs genome integrity

Abstract

Chromosome instability (CIN) underpins cancer evolution and is associated with drug resistance and poor prognosis. Understanding the mechanistic basis of CIN is thus a priority. The structure-specific endonuclease Mus81-Eme1 is known to prevent CIN. Intriguingly, however, here we show that the aberrant processing of late replication intermediates by Mus81-Eme1 is a source of CIN. Upon depletion of checkpoint kinase 1 (Chk1), Mus81-Eme1 cleaves under-replicated DNA engaged in mitotic DNA synthesis, leading to chromosome segregation defects. Supplementing cells with nucleosides allows the completion of mitotic DNA synthesis, restraining Mus81-Eme1–dependent DNA damage in mitosis and the ensuing CIN. We found no correlation between CIN arising from nucleotide shortage in mitosis and cell death, which were selectively linked to DNA damage load in mitosis and S phase, respectively. Our findings imply the possibility of optimizing Chk1-directed therapies by inducing cell death while curtailing CIN, a common side effect of chemotherapy.

Human MUS81: A Fence-Sitter in Cancer

“…DDR-directed therapies have been introduced in clinical trials recently (Pilié et al, 2019 ). However, chromosomal instability (CIN) is the dominant drawback of this strategy, which conversely underpins evolution and growth of tumor cells (Bakhoum and Cantley, 2018 ; Calzetta et al, 2020 ). Thus, knowledge of the interactions among multiple DDR pathways and understanding the discrepant functions of DDR-relevant proteins in various cancers contribute to efficient cancer therapy and needs to be further elucidated.…”

Sister chromatid exchanges induced by perturbed replication are formed independently of homologous recombination factors

“…Coordinated cleavage of replication forks could be proceeded by TRAIP/p97-dependent unloading of the CMG helicase (Deng et al, 2019), leaving stretches of vulnerable ssDNA at the leading strands. Although the responsible nuclease in processing under-replicated DNA upon PARP inhibition remains elusive, MUS81 has been shown to be active during mitosis (Calzetta et al, 2020; García-Luis and Machín, 2014; Wild et al, 2019; Wyatt et al, 2013), to localize to under-replicated DNA in mitosis (Di Marco et al, 2017), and to act on stalled replication forks in mitosis (Fugger et al, 2021; Zimmermann et al, 2013). However, our data showed that depletion of MUS81 alone was not sufficient to reduce SCEs, possibly due to redundancy with other endonucleases that are active during mitosis (Chan and West, 2014; Wyatt et al, 2013).…”

Structural Chromosome Instability: Types, Origins, Consequences, and Therapeutic Opportunities

“…However, under certain conditions, the same endonucleases can promote CIN instead of preventing it. For example, in CHK1 depleted samples, MUS81 and its cofactor EME1 augment bulky chromosome bridges and MN frequency in a manner that is associated with nucleoside-shortage mediated aberrant processing of UR-DNA [ 72 , 73 ].…”

“…For example, CHK1 inhibitors induce cell death and accumulation of both bulky chromosome bridges and MN. A mitosis-specific pathway involving MUS81 endonuclease, its partner EME1, and MiDAS components RAD52 and POLD3 promote the accumulation of such CIN markers, with no effect on the extent of cell death [ 72 ]. While it is still unclear whether precluding acute CIN during cancer treatments suffices to prevent the acquisition of genome instability, it is tempting to speculate that the limitation of acute CIN could benefit the outcome of cancer treatments.…”