## 4 The Importance of Biomarkers in Monitoring RT Toxicity
### Cell-Intrinsic Radiosensitivity Biomarkers
#### 4.1.3 Other Biomarkers of the DDR Pathway  
Promising biomarkers for assessing radiosensitivity in various cancer types are also the MRN complex and the ATM (ataxia-telangiectasiamutated) protein. The MRN complex, consisting of Mre11, Rad50, and Nbs1/Xrs2 proteins, is an important player in DNA repair and maintenance of genome stability. This complex acts as a sensor and mediator of DNA damage and is involved in multiple DNA repair mechanisms, including HR and NHEJ [81]. While the MRN complex has a crucial role in DNA damage sensing and repair processes, ATM is a key regulator of the cellular response to DSBs. Upon detection of DSBs, ATM is activated by the MRN complex, undergoing autophosphorylation and activation and leading to a signaling cascade initiation that regulates various cellular processes (including DNA repair, cell cycle checkpoint activation, and apoptosis) [82]. Several studies have demonstrated the correlation between a dysfunctional MRN complex, especially the MRE11 protein, and increased radiosensitivity, suggesting its potential utility as a predictive biomarker for RT outcomes for better treatment decisions. The MRN complex and the ATM expression are commonly measured by IF for visualizing their localization and dynamics in response to DNA damage. Seminal contributions have been made by [PERSON] et al., 2007 by analyzing the tumor samples from 224 premenopausal women with early breast cancer. The study pointed out that high expression of the MRN complex, but not of the ATM, is necessary for tumor cell killing by RT. Therefore, a reduced expression of the MRN complex predicts a poor therapeutic effect of RT in patients with early breast cancer [83]. On the contrary, [PERSON] et al., 2020 have shown that the methylation status of the ATM promoter in hepatocellular carcinoma (HCC) has a predictive value for RT outcome, as assessed by performing PCR (qPCR) and immunohistochemistry (IHC) in 50 paired HCC and adjacent normal tissues, and 68 locally advanced HCC biopsy tissues [84]. [PERSON] et al., 2012 performed an immunochemical analysis of breast cancer tissues obtained from 254 surgically treated female patients with confirmed pathology of invasive ductal carcinoma and showed that high MRE11 expression in breast cancer cells was associated with malignant behavior, higher recurrence rates, lymph node metastasis, resistance to RT and chemotherapy, and decreased patient survival [85]. The study by [PERSON] et al., 2018 having analyzed surgical specimens collected from 265 patients treated with CRT or neoadjuvant RT followed by surgery for rectal cancer, concluded that high tissue expression levels of the three MRN complex proteins are prognostic indicators in rectal cancer and in response to preoperative therapy [86]. Therefore, the incorporation of the MRN complex and the ATM protein as biomarkers for the radiosensitivity evaluation holds great promise for improving personalized cancer treatment and for optimizing RT procedures and outcomes. As the MRN complex is essential for DSB repair and telomere maintenance, it is postulated that defective function and low expression of its components lead to DNA damage and malignant transformation. Also, owing to its ability to repair DSBs, it is likely that the levels of MRN expression influence the response of cancer cells to chemotherapy and RT in terms of apoptosis. Generally, conflicting data were obtained in different studies; some studies have associated high expression of MRN and its components with poorer outcome and treatment resistance to chemo-radiation [87; 88; 89], while others have found the opposite [90]. In parallel to MRE11, ATM overexpression has a significant relationship with poor disease-free survival in rectal cancer and lymph node positivity [86], suggesting that the observed poor overall survival was likely due to the aggressiveness and metastatic properties of the tumor; the latter's progression could be reasonably attributed to the increased MRE11 levels that stabilize RAD50 and NBS1 and recruit ATM [91].