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    • Materials and methods Between January and December patients

    2021-05-07

    Materials and methods Between January 2006 and December 2008, 34 patients with GBM were treated with postoperative three-dimensional (3D) radiotherapy at Tartu University Hospital or North Estonian Medical Centre. Characteristics of patients are listed in Table 1.
    Results
    Discussion Radiotherapy causes a variety of DNA lesions, including single-strand brakes (SSB) and double-strand brakes (DSB) [11]. The lethal lesion is probably an unrepaired or misrepaired DSB produced as part of a complex lesion [12]. In the present study, we assessed 2 DNA-repair enzymes (PARP-1, DNA-PK) in GBM tissue before standard radiotherapy. PARP-1 is an enzyme of PARP superfamily that is responsible for most of PARP activity [13]. The most well-known role of PARP-1 is the detection of SSB [14]. After binding to radiation-induced SSB (damage detection), activated PARP recruits repair enzymes (X-ray repair cross-complementing group 1, DNA polymerase-β, DNA ligase III) that are involved in PF-431396 excision repair (BER) pathway. Recruited enzymes process broken DNA ends, synthesize missing DNA and seal the gap in DNA [15], [16]. DNA-PK plays an important role in DNA DSB repair by the nonhomologous end joining (NHEJ) pathway [17]. DNA-PK is a kinase that binds to DNA DSB, phosphorylates, and activates DNA-binding proteins (X-ray repair cross-complementing protein 4, DNA ligase IV) [18]. Due to interaction of these enzymes, double strand break ends are directly ligated [18], [19]. Since DNA repair enzyme inhibitors enhance the cytotoxic effects of DNA-damaging agents (radiation, chemotherapy), their role in cancer therapy is increasingly explored [20]. Present study revealed a weak constitutive expression of PARP-1 and DNA-PK in normal brain tissue. Similarly, constitutive expression of PARP-1 and DNA-PK within normal brain has been documented in previous human studies [21], [22]. A primary function of PARP-1 and DNA-PK under basal condition is the detection of DNA damage and the facilitation of DNA repair to maintain genomic integrity [23], [24]. Similarly to our findings, the high expression of DNA repair enzymes in GBM tissue has been described in other studies [22], [25]. In the present study, the median survival of the entire study group was 10.0months. This is in a good accordance with previous studies where postoperative radiotherapy has resulted in median survival of 9–11.6months [2], [26], [27]. Since GBM patients had different tumor levels of PARP-1 and DNA-PK, we searched if differences in protein expression influence the treatment outcome. Present study showed that survival did not depend on PARP-1 expression in tumor tissue, since median survival of GBM patients with high or low levels of PARP-1 did not differ. In contrast, survival was significantly influenced by the expression level of DNA-PK, showing much shorter median survival of patients with high tumor levels (9.0months) compared to patients with low levels (13.0months). Moreover, multivariate analysis showed that next to the well-established prognostic factor KPS [28], DNA-PK expression emerged as a significant independent predictor for overall survival. This suggests that GBM patients with high tumor levels of DNA-PK are more resistant and respond less to standard radiotherapy. Up to the date of writing, there are no published clinical studies that would have compared treatment outcome of GBM patients after radiotherapy depending on tumor PARP-1 and DNA-PK expression levels. Therefore, the data of the present study cannot be compared with similar studies. In spite of that, there are some preclinical studies where inhibition of these DNA repair enzymes (resulting in low levels of these enzymes) has been tested in GBM cell cultures. The effect of PARP-1 inhibitor Olaparib in the combination with radiotherapy has been evaluated in four GBM cell lines (T98G, U373-MG, UVW, U87-MG) [29]. In three GBM cell lines, decrease in surviving fraction of tumor cells, radiosensitization and delayed repair of radiation-induced DNA-brakes were seen. However, in U87-MG cell line, Olaparib had no effect on radiation sensitivity in two independent studies [29], [30]. This shows that the inhibition of PARP-1 might be insufficient to increase the radiosensitivity of GBM, which is also suggested in our study.