Changying Li

Poly (ADP-ribose) polymerase inhibitors (PARPi) demonstrate effective treatment outcomes in ovarian cancer patients with BRCA1/2 mutations or homologous recombination (HR) repair deficiencies, leveraging the principle of synthetic lethality. However, PARPi resistance and HR proficiency remain significant challenges in the clinical management of PARPi, necessitating the development of novel strategies for PARPi therapy in ovarian cancer. Our previous research identified PAK1's involvement in replication stress-induced cytotoxicity. Nonetheless, whether PAK1 also affects HR repair and PARPi sensitivity in ovarian cancer remains unresolved. In this research, we found that the expression of PAK1 correlated with an unfavorable prognosis in ovarian cancer. Depletion of PAK1, introduction of a kinase-dead mutation, or treatment with the inhibitor IPA-3 could reduce HR repair efficiency and increase ovarian cancer cell sensitivity to the PARP inhibitor olaparib. The combination of olaparib and IPA-3 synergistically increased olaparib-induced DNA replication stress and double-stranded breaks. Using cell line-derived xenograft, patient-derived organoid, and patient-derived xenograft models, we discovered that IPA-3 potentiated the therapeutic efficacy of olaparib both in vivo and ex vivo. Collectively, our findings suggest that targeting PAK1 might offer a new avenue for increasing the sensitivity of olaparib and improving the outcomes of ovarian cancer patients.

Figure S2 shows TGS1 is associated with poor prognosis in a variety of cancers

The clinical information for all patient samples in TMA slices

Figure S9 shows sinefungin treatment does not impair HR repair after TGS1 knockdown

Figure S8 shows sinefungin treatment enhances cellular sensitivity to olaparib in PAAD cells

Figure S1 shows TGS1 is upregulated in PAAD and associated with poor prognosis

Figure S6 shows high TGS1 expression is correlated with olaparib resistance in PAAD patient-derived xenografts

Pancreatic adenocarcinoma remains one of the most lethal malignancies, primarily due to its robust resistance to genotoxic therapies, such as chemotherapy and radiotherapy. Understanding the mechanisms underlying this resistance is essential to improve clinical outcomes. In this study, we identified trimethylguanosine synthase 1 (TGS1), previously known for its role in RNA modification, as a critical mediator of homologous recombination (HR) repair that specifically contributes to resistance in pancreatic adenocarcinoma. TGS1 was significantly overexpressed in pancreatic adenocarcinoma tissues, correlating strongly with advanced disease stages, therapy resistance, and poor patient prognosis. Following DNA damage, ATM kinase phosphorylated TGS1 at serine residues S389 and S531, which mediated its direct interaction with BRCA1 and subsequent recruitment of BRCA1 to DNA damage sites. The phosphorylation-dependent interaction enhanced HR repair efficiency, enabling cancer cells to survive genotoxic stress. Depletion or pharmacologic inhibition of TGS1 induced HR deficiency and markedly enhanced sensitivity to DNA-damaging agents, particularly PARP inhibitors, in pancreatic adenocarcinoma cell lines in vitro and in both cell line– and patient-derived xenograft models in vivo. Collectively, these findings uncover an ATM–TGS1–BRCA1 signaling axis that promotes DNA repair and resistance to genotoxic therapies in pancreatic cancer, positioning TGS1 as a promising predictive biomarker and therapeutic target to enhance treatment efficacy.Significance:The ATM–TGS1–BRCA1 signaling axis is a critical regulator of DNA repair that can be targeted to overcome genotoxic therapy resistance, providing a promising therapeutic strategy that could transform pancreatic cancer treatment.

Figure S3 shows TGS1 promotes BRCA1 recruitment and HR repair independent of methyltransferase activity

Figure S10 shows sinefungin treatment induces olaparib-sensitivity in PAAD patient-derived xenografts