Genomic instability, defined as an elevated rate of acquisition of genetic mutations, has been postulated as a pivotal player in tumor development, progression, and resistance to treatment [15]. P53 gene inactivation was observed in a majority of human cancers [16]. In the early G1 phase, cell exposure to physiological and environmental stress conditions drives the expression of RRM2B and subsequent contribution to DNA repair [17]. This subunit showed anti-ROS potential by suppressing oxidative stress and has an essential role in vital cellular activities such as DNA replication and mitochondrial DNA maintenance [11]. Thus, malignancy suppressing the potential of RRM2B has been originally suggested due to its p53-inducible nature and mechanisms as mentioned earlier. However, the precise role of RRM2B in human cancer progression and metastasis is incompletely understood and found to vary in different tumor types and stages. According to prior analytic findings, RRM2B could suppress the invasion of pancreatic, oropharyngeal, prostate, and colon cancer cells [18]. Genetic variation of RRM2B in non-small cell lung cancer (NSCLC), Hepatocellular carcinoma (HCC), gastric and urothelial carcinoma have been previously reported [19–22]. Tian et al. reported down-regulation of RRM2B in HCC commonly. Also, cell migration, invasion, and metastasis in HCC have been suggested to be inhibited by RRM2B in vitro and in vivo [21]. On the contrary, widespread overexpression of RRM2B in transgenic mice induces NSCLC [22]. However, no correlation was found between RRM2B expression and grade, stage, and histological type of gastric cancer [19]. Nevertheless, RRM2B is highly expressed in some human cancers as noted above, experimental suppression of RRM2B expression impairs cancer cell proliferation in vitro [23]. A possible explanation for the controversial role of the RRM2B gene in different cancers might be dependent on the time of gene expression. Before forming cancer cells, p53R2 provides dNTPs for DNA repair and increases expression of P21 while decreasing the expression of cyclin D in wild-type cells to arrest the cell cycle to repair damaged DNA. After the formation of malignancy and their increasing demands for nutrients and support, p53R2 may contribute to cancer cell progression, especially when p21 presents in the cytoplasm [17].
The current study set out to evaluate the expression level of RRM2B gene and protein in CRC patients compared to normal controls. This investigation has shown that RRM2B gene expression was significantly lower in CRC tumor tissue (51% lower, p < 0.001), strengthening the idea that down-regulation of the RRM2B gene may contribute to poor prognosis. In accordance with the present finding, previous studies have shown markedly better survival in CRC patients who had elevated RRM2B expression [24]. If so, the detection of RRM2B-negative cells may be helpful to identify patients at high risk of CRC progression.
Several studies confirmed the usefulness of biomarkers such as APC, MLH1, MSH2, VEGF, p53, IMP3 to be served as promising tools for patient prognosis and treatment choice [25]. Notably, all prognostic molecular tools up to now were based on the identification of the gene profiles of individual tumors. Contrary to prior research that focused on measuring nuclear and cytosolic RRM2B levels, our project determines, for the first time to our knowledge, the serum level of RRM2B protein. Therefore, it can be speculated that only a single serum specimen from suspected individuals could explain the colon tumorigenesis. Besides, our results indicate significantly different RRM2B serum levels before and after surgery, supporting the possibility of using serum RRM2B level as a biomarker for patient follow-up. We observed declined serum RRM2B protein after surgery. This discrepancy could be attributed to the elimination of the source of RRM2B protein expression by surgery. Due to the short-time follow-up nature of our study, evaluating whether the altered level of serum RRM2B could impress the patient prognosis and survival was not clearly described here. Further research should determine the possible role of this biomarker in overall survival. Although we report the significant decrease of RRM2B gene expression in CRC patients, our results showed no difference between CRC patients before surgery and the healthy group regarding serum RRM2B protein levels.
This study does not demonstrate any difference in RRM2B gene expression between patients with stage II and III. Besides, advanced stages of CRC and occurrence of metastasis are not found to alter RRM2B protein expression. We also explore the effect of RRM2B gene expression on tumor metastasis and found no association (Table 2, P = 0.254). These outcomes differ from earlier findings which revealed a negative relation between RRM2B expression and cancer cell invasiveness, lymph node involvement, and metastasis [18, 24].
Prior researches have mentioned gender as an essential factor influencing survival results among CRC patients. Females have been reported to have significantly better overall survival (OS) and cancer-specific survival (CSS) than males [26]. This result seems consistent with our finding, revealing higher expression of RRM2B protein in females after surgery. Hence, elevated levels of this biomarker may contribute to the better survival of females. However, both genders do not show any significant difference in the expression of the RRM2B gene in their tumor tissue (Table 1, P = 0.258).
It has been proved that RRM2 plays a critical role in tumor invasiveness and malignancy [27]. As a Subsidiary finding, we report significantly higher levels of RRM2 than RRM2B in tumor cells (P < 0.001). Although RRM2 and RRM2B are highly homologous in their gene sequences, their expressional levels and subcellular localizations are differently regulated in cells. Similar to our results, immunohistochemistry studies of Ding Y. et al. showed that RRM2 gene expression levels were higher than that of the other two subunits in almost all the studied cancer types, especially CRC. Therefore, dysregulated expression of RR small subunit genes, and the related enzyme activities, operate under different mechanisms related to aspects of cancer progression or patient survival in colorectal tumor cells [28].
As reported by several studies, up-regulation of RRM2B expression could result in resistance to treatment. Also, cell hypersensitivity to DNA-damaging agents was observed in the absence of RRM2B [23, 29–32]. Hence, evaluation of RRM2B expression, in addition to its prognostic role, seems to help physicians to make treatment decisions.