The DNA Damage Response (DDR) is a complex network comprising sensors, transducers, and effectors that collaboratively manage cell cycle checkpoints, repair processes and apoptosis. When DNA damage occurs, DDR components detect the impairment, pause the cell cycle, and coordinate the necessary repair mechanisms. If the damage is irreparable, the cell may initiate apoptosis to eliminate the potentially harmful cells. Repair mechanisms include base excision repair, nucleotide excision repair, homologous recombination (HR), and non-homologous end joining (Jackson & Bartek, 2009).

Loss-of-function mutations in DNA damage response proteins can lead to an accumulation of mutations and genomic instability, which may drive normal cells toward oncogenesis (Tian et al., 2015). For example, mutations in HR repair genes such as BRCA1 or BRCA2 (BRCA1/2) impair the cell's ability to effectively repair DNA double strand breaks and can subsequently lead to malignant transformation (Rempel et al., 2022). However, other repair mechanisms, such as base excision repair and translesion synthesis, can still operate and partially compensate for the deficiencies in HR. These alternative mechanisms are upregulated in BRCA1/2-deficient cells and can help mitigate the impact of accumulated mutations and support cancer cell viability (Voutsadakis & Stravodimou, 2023).

Synthetic lethality occurs when the simultaneous loss of function of two genes leads to cell death, while the loss of either gene alone does not significantly impair cell viability (O'Neil, Bailey, & Hieter, 2017). This concept has been exploited in cancer therapy, where tumors with a specific genetic deficiency are targeted with a drug that inhibits a compensatory pathway, leading to selective cancer cell death. For instance, cancers with deficiencies in HR due to BRCA1/2 mutations are exquisitely sensitive to Poly(ADP-Ribose) polymerase (PARP) inhibitors. PARP is crucial for base excision repair, and its inhibition exacerbates the repair deficits in these cancers, leading to increased cell death (Jurkovicova, Neophytou, Gasparovic, & Goncalves, 2022).

Several proteins involved in the DDR are currently being investigated as potential synthetic lethality targets for new cancer treatments. For instance, ATM, ATR, Wee1, WRN, PARG, POLQ, PMTRT, PKMYT1, and ACL1 are novel synthetic lethal drug candidates currently in preclinical and clinical development aimed at modulating the DDR machinery (Li et al., 2023).

Ubiquitin-specific protease 1 (USP1) is a deubiquitinating enzyme (DUB) that plays a key role in regulating the DNA damage response (Kim et al., 2009; Murai et al., 2011). USP1 deubiquitinates its substrates, mono-ubiquitinated FANCD2-FANCI, and PCNA, to regulate interstrand crosslink (ICL) repair and translesion synthesis (TLS), respectively (Huang et al., 2006; Nijman et al., 2005) (Fig. 1). USP1 expression and activity is upregulated in BRCA1-deficient tumors, osteosarcoma, colorectal cancer, and other tumor cells (Lim et al., 2018; Williams et al., 2011; Xu et al., 2019), suggesting that these cancers are hyper-reliant on USP1. Importantly, synthetic lethality between USP1 and BRCA1/2 has been demonstrated experimentally, setting the stage for the development of USP1 inhibitors (Lim et al., 2018). Additionally, the inhibition of both PARP and USP1 has shown strong synergy in BRCA1/2 mutant tumors (Cadzow et al., 2024; Simoneau et al., 2023). This research has spurred the discovery of USP1 inhibitors such as RO7623066 (KSQ-4279), SIM0501, XL309–101, and HSK39775. These inhibitors have advanced into phase 1/2 clinical trials, either as monotherapies or in combination with PARP inhibitors, with promising early results emerging (Yap et al., 2024).

This review provides a comprehensive overview of the fundamental mechanisms of USP1 function and inhibition, as well as the current state of preclinical and clinical development of USP1 inhibitors.