The CHEK2 gene serves a canonical role in the DNA damage response (DDR) pathway encoding the regulatory kinase CHK2 in the homologous recombination (HR) repair of double-strand breaks (DSB). Although CHEK2 is traditionally considered a tumor suppressor gene, recent studies suggest additional functions. Across several cohort studies, CHEK2 expression was negatively correlated with the efficacy of immune checkpoint inhibitors (ICI), which target the interaction between effector immune and tumor cells. This review explores two possible explanations for this observed phenomenon: the first relating to the canonical role of CHEK2, and the second introducing a novel role of the CHEK2 gene in immunomodulation of the tumor microenvironment (TME). DDR mutations have been implicated in increased levels of tumor mutation burden (TMB), often manifesting as neoepitope expression on the tumor cell surface recognized by effector immune cells. As a result, impaired DNA repair due to CHEK2 loss of function, either from germline deleterious variants or acquired mutations, results in the recruitment of CD8+ cytotoxic T-cells and subsequent efficacy of ICI treatment. However, functional loss of CHEK2 may be directly involved in potentiating the immune response through canonical inflammatory and anti-tumor pathways, acting through the cGAS-STING pathway. Although the exact mechanism by which CHEK2 modulates immune responses is still under investigation, combination therapy with CHEK1/2 inhibition and ICI immunotherapy has shown benefit in preclinical studies of several solid tumors.

Radiation therapy (RT) is the standard of care for glioblastoma but is not curative. Triggering the cGAS/stimulator of interferon genes (STING) pathway with potent agonists, such as 8803, exerts activity across high-grade glioma preclinical models. To determine if the combination of 8803 with RT warrants consideration in the up-front treatment setting and to clarify the underlying mechanisms of therapeutic activity, C57BL/6J mice harboring intracerebral CT-2A or QPP8v gliomas were treated with RT, intratumoral 8803, or both. The treatment with the combination resulted in 80% long-term survival in the CT-2A model but not in the radiation-resistant QPP8v model. This therapeutic effect was maintained in Sting^–/– CT-2A cells, highlighting the direct role of the immune system in mediating the survival benefit. Single-cell RNA-Seq identified increased nitric oxide synthase 2 (Nos2) in inflammatory tumor-associated macrophages; however, the therapeutic effect was maintained in Nos2^–/– mice. Additionally, 8803 reprogrammed the blood-brain barrier (BBB) by altering the Pecam and Cd147 pathways in endothelial cells; intracranial injection of 8803 induced bihemispheric BBB opening for up to 24 hours. Sting activation was visualized longitudinally using 3’-deoxy-3’-[¹⁸F]-fluorothymidine ([¹⁸F]-FLT) PET, which peaked 72–96 hours after 8803 administration. In summary, 8803 combined with RT triggers distinctive antiglioma immune reactivity, facilitates BBB opening, and warrants consideration for up-front clinical trials in glioblastoma, where treatment effects can be monitored using [¹⁸F]-FLT PET imaging.

Abstract