The aim of the study was to answer the question whether the immune system is to some extent responsible for the increased risk of cancer incidence when carrying certain mutation. CHEK2 I157T is the most frequent CHEK2 variant in the general Polish population (about 5%) (Cybulski et al., 2006). Meta-analyses conducted by Liu et al. showed that the presence of the CHEK2 I157T variant is associated with an increased risk of breast cancer and colorectal cancer (2012). It was also found that CHEK2 I157T variant doubles the risk of papillary thyroid carcinoma and predisposes to more frequent kidney cancer, low grade ovary cancer and prostate cancer (Cybulski et al., 2004; Lizis-Kolus et al., 2010). Moreover, it was suggested that the risk of breast cancer in carriers of a deleterious CHEK2 mutation (1100delC, del5395) is increased if the second allele is the I157T variant (Cybulski et al., 2009).
The cytokines we analyzed can be traditionally attributed to several clusters, such as pro-inflammatory (IL-1β, IL-6, TNF-α, TNF-β), anti-inflammatory (IL-10), Th1-related (IL-2, IFN-γ), Th2-related (IL-4, IL-5, IL-13), and Th17-related (IL-17). Our results showed altered levels of two interleukins, IL-2 and IL-6, among the CHEK2 I157T carriers group. IL-2 is known to be a crucial cytokine involved in immune-mediated elimination of cancer cells. Its activity leads to the stimulation of innate and adaptive immune cells. IL-2 exerts its effects by binding to the IL-2 receptor (IL-2R), which is composed of three subunits: IL-2Ra (CD25), IL-2Rb (CD122), and IL-2Rg (CD132). Signal transduction through the IL-2R is mediated by the interaction of CD122 and CD132, which form the IL-2 medium-affinity receptor. This receptor is expressed on natural killer (NK) cells, monocytes, macrophages, and resting CD4 + and CD8 + T cells. The trimeric high-affinity IL-2R (CD25/CD122/CD132) is found on activated T and NK cells and is constitutively expressed on CD4 + FoxP3 + regulatory T cells (Tregs) (Nirschl et al., 2022). IL-2 stimulates the proliferation and activation of T cells and NK cells, and it promotes the differentiation of CD8 + T cells into effector and memory cells. Recombinant human IL-2 is approved for the treatment of metastatic melanoma and renal cell carcinoma as a high-dose therapy, but this treatment is associated with serious side effects, including vascular leakage syndrome and hypotension, which limit its practical use (Nirschl et al., 2022; Sharma et al., 2020).
To explain the difference in IL-2 levels, we propose a mechanism based on the observation that Cabin1 undergoes phosphorylation by Chk2. Cabin1 acts as an inhibitor of calcineurin within the signaling pathway governing T cell activation. In T cells, when Chk2, in its activated wild-type form, phosphorylates Cabin1, it triggers the subsequent degradation of this protein. This degradation event diminishes the degree of inhibition imposed on calcineurin (Choi et al., 2013). Consequently, this reduction in inhibition contributes to heightened NFAT nuclear translocation, ultimately resulting in escalated IL-2 production. In the instance of the I157T variant, known for its documented decrease in overall hydrophobic characteristics and weakened interactions with its binding partners (Han et al., 2013), a plausible scenario emerges whereby Cabin1 may not be phosphorylated at the requisite level. Therefore, the stability of Cabin1 within T cells could increase, culminating in a dampened rate of NFAT nuclear translocation and a consequent reduction in IL-2 production.
IL-6 is a cytokine involved in cell signaling, that plays a role in the development and progression of cancer. IL-6 has been shown to be involved in the growth, survival, invasion, and metastasis of cancer cells. It can promote angiogenesis, the formation of new blood vessels, which is necessary for tumor growth. IL-6 can also stimulate the immune system, particularly the activation and proliferation of T cells and B cells, and promote inflammation, which can contribute to the development of cancer. In addition, IL-6 can inhibit apoptosis in cancer cells, allowing them to survive and proliferate. IL-6 has been shown to be elevated in the serum of patients with various types of cancer, including breast cancer, prostate cancer, and lung cancer. Inhibition of IL-6 signaling has been shown to have anticancer effects in preclinical studies, and drugs targeting IL-6 are currently being tested in clinical trials for the treatment of cancer (Kumari et al., 2016).
In the case of IL-6, it has been observed that wild-type Chk2 possesses the capability to downregulate the NF-κB pathway in macrophages, as demonstrated by Xie et al. (2022). On the contrary, Chk2 plays a pivotal role in the activation of p53 transcription through its phosphorylation at serine 366. Conventionally, the activation of the NF-κB pathway opposes the action of the transcription factor p53; the former is associated with apoptosis induction, while the latter confers cellular resistance to programmed cell death (Carrà et al., 2020). Given that the I157T variant is structurally stable and functions as a dominant-negative mutant, even in the presence of the wild-type allele, Chk2 activity becomes disrupted. In this context, a plausible scenario arises wherein the I157T variant exhibits significantly diminished phosphorylation activity towards p53. Consequently, this alteration leads to the predominance of the NF-κB pathway in the cellular equilibrium. NF-κB activation, in turn, is recognized as a key element in inducing IL-6 production. Al-Rakan et al. found that CHEK2 plays a role in the regulation of the secretory phenotype of breast stromal fibroblasts and their activation. It was demonstrated that under normal conditions Chk2 inhibits the procarcinogenic effects of these cells and has a non-cell-autonomous tumor suppressive function through the repression of expression and secretion of IL-6 (2016).
We acknowledge that this study serves as a preliminary contribution to further investigations into the relationship between the occurrence of specific genetic variants associated with an increased susceptibility to cancer and the immunological state of the organism. In future research endeavors, the expansion of the study cohort and the analysis of the interplay between genes and physiology at the transcriptomic level will be of great importance. Such comprehensive exploration may ultimately provide insights into the fundamental question of why certain mutation carriers manifest tumor development over the course of their lives while others remain unaffected.