Cells employ a series of repair mechanisms to maintain genome integrity following exposure to endogenous or exogenous damage to DNA, including therapy-induced DNA damage in cancer cells (Chatterjee and Walker, 2017). Among these, the DNA double-strand break (DSB) is the most lethal DNA lesion induced by radiotherapy. In G0 and G1 phases, DSBs are primarily repaired by nonhomologous end joining (NHEJ), a potentially error-prone DNA repair mechanism (Zhao et al., 2020). Although NHEJ remains available for repair throughout the S and G2 phases, DSBs occurring during these cell cycle phases may alternatively undergo repair via homologous recombination (HR), a relatively error-free mechanism that requires an intact sister chromatid to serve as a template to repair with fidelity (Chapman et al., 2012).

Progression of DSB repair through HR over NHEJ requires DSB resection, where 3’ single-strand DNA overhangs are generated to serve as a platform for HR initiation (He and Chowdhury, 2021, Zhao et al., 2021). The human C-terminal binding protein interacting protein (CtIP) serves as a prominent nuclease collaborating with its partner, the MRE11–RAD50–NBS1 (MRN) complex, to promote DSB resection (Deshpande et al., 2020, Han et al., 2021, Takeda et al., 2007). Both NBS1 and BRCA1, another key factor in HR with resection activity, are binding proteins of CtIP (Cruz-Garcia et al., 2014, Wang et al., 2013). Beyond radiation-induced DSBs, HR is activated at stalled and collapsed replication forks, such as those induced by cisplatin and poly ADP ribose (PARP) inhibitors. Tumors deficient in HR are highly sensitive to these agents, whereas robust HR repair is a potential mechanism underlying both inherent and acquired therapeutic resistance (Bouwman and Jonkers, 2012, Javle and Curtin, 2011, Kumar-Sinha and Chinnaiyan, 2018, Zhou et al., 2021). There is an urgent need for novel tumor-specific strategies to overcome resistance to PARP inhibitors, platinum agents, and other DNA-damaging therapies (Norouzi-Barough et al., 2018).

Spleen associated tyrosine kinase (Syk) is a non-receptor tyrosine kinase which mediates signal transduction downstream of a variety of transmembrane receptors, including classical immunoreceptors such as the B-cell receptor (BCR). Syk plays diverse roles in the regulation of several biological processes, including innate and adaptive immunity, cell adhesion, osteoclast maturation, platelet activation, and vascular development (Mocsai et al., 2010). However, its function in cancer is multifaceted, with reports indicating both tumor-promoting and tumor-suppressing activities (Krisenko and Geahlen, 2015, Singh et al., 2021, Zhang et al., 2012). For instance, while Syk promotes cell survival in most BCR hematopoietic malignancies (Krisenko and Geahlen, 2015), its reduced expression and activity have been linked to the malignant progression of CD19+CD10- pro-B cell acute lymphoblastic lymphoma (Goodman et al., 2001). In addition, Syk expression in pre-treatment ovarian cancer specimens has previously been associated with ovarian cancer cell invasiveness by mediating actin filament assembly and dynamics and microtubule-associated proteins through phosphorylation of cortactin, cofilin and tubulins (Yu et al., 2018). In contrast, loss of Syk expression has been associated with a more malignant breast cancer phenotype (Blancato et al., 2014, Toyama et al., 2003).

In this study, we unveil the overexpression of Syk in some high-grade ovarian cancer (HGSOC), and estrogen receptor (ER) negative, progesterone receptor (PR) negative, and human epidermal growth factor receptor 2 (HER2) gene negative breast cancer (triple negative breast cancer, [TNBC]). We demonstrated that Syk expression promotes HR and confers resistance to DNA targeted therapy. Mechanistically, we show that Syk can be activated and recruited to DNA DSBs in Syk-expressing tumor cells in an ATM-dependent manner, leading to phosphorylation of CtIP and subsequent promotion of CtIP-mediated end-resection and HR. Augmented Syk-mediated DSB resection and HR result in increased resistance to PARP inhibition, radiation and cisplatin, specifically in Syk-expressing tumor cells, which can be abrogated by Syk inhibition. Collectively, our findings underscore the potential of targeting the Syk-mediated ATM-Syk-CtIP pathway as a promising strategy to overcome therapeutic resistance driven by proficient HR repair.