MUS81 (MUS81 Structure-Specific Endonuclease Subunit)

In the absence of specific FANCJ-targeting compounds, we evaluated the phenotypic and molecular effects of pharmacological inhibition of the MUS81 endonuclease, which functions downstream of FANCJ in restarting stalled replication forks. When combined with CHK1 inhibition, we observed synergistic effects on neuroblastoma cell growth and survival, supporting further development of on-target MUS81 inhibitors for in vivo preclinical testing and future clinical trials aimed at overcoming replication stress resistance in high-risk neuroblastoma.

One key regulator of nuclease activity is the scaffold protein SLX4, which plays important roles in repairing DNA damage induced by mitomycin C (MMC) and camptothecin (CPT) as well as in the resolution of stalled replication forks...These findings suggest that SLX4 and its associate nucleases are confined within PML NBs and that the optimal protein level of SLX4 is maintained by the coordinated activities of RNF4 and USP7. Our findings provide insight into how cells effectively control the potentially harmful activities of nucleases in the absence of DNA damage by a spatial regulatory mechanism.

Depletion of RAD18, fork remodelers, or the endonuclease MUS81 rescues these phenotypes, indicating that mH2A1.2-ubiquitination prevents toxic RAD18 engagement at MUS81-dependent double-strand breaks (DSBs), which arise at collapsed forks in difficult-to-replicate sites experiencing prolonged arrest. Our findings highlight the detrimental consequences of inappropriate DNA processing by MUS81 at these loci.

Notably, this degradation can be mitigated by expression of human PrimPol, which is absent in yeast. These findings suggest that the essential role of S-phase checkpoints upon DNA damage is to safeguard stalled replication forks from aberrant processing, thereby preserving nascent DNA integrity.

In vivo, combined treatment with intermittent ATRi and continuous PARPi showed greater inhibition of tumor growth than ATRi alone in SMARCA4-deficient lung adenocarcinoma xenograft models. These findings demonstrate that PARPi-induced heterochromatin amplifies RS and ATRi susceptibility, providing a potential rationale for therapeutic strategies targeting SMARCA4-deficient tumors.

Targeted panel sequencing in olaparib-sensitive models lacking BRCA1/2 alterations revealed a MUS81 variant as a possible mechanism underlying PARPi sensitivity. Combined, we show that ex vivo RAD51 analysis effectively predicts in vivo olaparib response and revealed a subset of PARPi-sensitive, HR-deficient ovarian cancer PDX models, lacking a BRCA1/2 alteration.

Differences in DNA methylation among the groups were found in TWIST1, GATA4, MUS81, and NTRK1 genes (TWIST1: atypical hyperplasia 67.5%, benign hyperplasia 2.5%, normal endometrium 22.5%; P < 0.00001; GATA4: atypical hyperplasia 95%, benign hyperplasia 65%, normal endometrium 22.5%; P < 0.00001; MUS81: atypical hyperplasia 57.5%, benign hyperplasia 22.5%, normal endometrium 5%; P < 0.00001; NTRK1: atypical hyperplasia 65%, benign hyperplasia 27.5%, normal endometrium 10%; P < 0.00001). Higher methylation rates were observed for the tumor suppressor genes of TWIST1, GATA4, MUS81, and NTRK1 in samples with atypical endometrial hyperplasia compared to samples with normal endometrial tissue, and higher methylation rates were found in samples with atypical endometrial hyperplasia compared to samples of benign endometrial hyperplasia. DNA methylation of TWIST1, GATA4, MUS81, and NTRK1 is involved in the pathogenesis of atypical endometrial hyperplasia.

Here we report the fragment-based discovery of novel small molecule MUS81 inhibitors with sub-μM biochemical activity. These inhibitors were used to develop a novel crystal system, providing the first structural insight into the inhibition of MUS81 with small molecules.

Further cell-based studies showed that UMI-77 could impair bleomycin-induced DNA damage repair, and significantly sensitized multiple cancer cell lines to the DNA-damaging agents. Finally, in a mouse xenograft tumor model, UMI-77 significantly enhanced the chemotherapeutic efficacy of etoposide with little adverse physiological effects. Our work offers a new avenue to combat chemoresistance in cancer treatment, and suggests that UMI-77 could be further developed as a promising candidate in cancer treatment.

Consequently, cells with defective BLM and BRCA1-BARD1 accumulate catastrophic levels of chromosome breakage and micronucleation, leading to cell death. Thus, we reveal mechanistic insights into SLX4 recruitment to DNA lesions, with potential clinical implications for treating BRCA1-deficient tumors.

We recently defined roles for EEPD1 in restarting stressed replication forks after oxidative DNA damage, and for TATDN2 in mitigating replication stress caused by R-loop accumulation in BRCA1-defective cells. We also discuss how insights into biological responses to genome-wide replication stress can inform novel cancer treatment strategies that exploit synthetic lethal relationships among replication stress response factors.

RAD52, in turn, recruits XPF/ERCC1 to remove DNA secondary structures at DSB ends, enabling HR/break-induced replication (BIR) at CFS-ATs. We propose that the specific requirement of MutSβ in processing DNA secondary structures at CFS-ATs underlies its crucial role in promoting MiDAS and maintaining CFS integrity.