Abstract
Abstract Glioblastoma multiforme (GBM) is the most aggressive primary brain tumor in adults. Current therapy includes surgery, radiation and chemotherapy with temozolomide (TMZ). Major determinants of clinical response to TMZ include methylation status of the O6-methylguanine-DNA methyltransferase (MGMT) promoter and mismatch repair (MMR) status. Though the MGMT promoter is methylated in 45% of cases, for the first nine months of follow-up, TMZ does not change survival outcome. Furthermore, MMR deficiency makes little contribution to clinical resistance, suggesting that there exist unrecognized mechanisms of resistance. We generated paired GBM cell lines whose resistance was attributed to neither MGMT nor MMR. We show that, responding to TMZ, these cells exhibit a decoupling of DNA damage response (DDR) from ongoing DNA damages. They display methylation-resistant synthesis in which ongoing DNA synthesis is not inhibited. They are also defective in the activation of the S and G2 phase checkpoint. DDR proteins ATM, Chk2, MDC1, NBS1 and gammaH2AX also fail to form discrete foci. These results demonstrate that failure of DDR may play an active role in chemoresistance to TMZ. DNA damages by TMZ are repaired by MMR proteins in a futile, reiterative process, which activates DDR signaling network that ultimately leads to the onset of cell death. GBM cells may survive genetic insults in the absence of DDR. We anticipate that our findings will lead to more studies that seek to further define the role of DDR in ultimately determining the fate of a tumor cell in response to TMZ and other DNA methylators.
Highlights
Glioblastoma multiforme (GBM) is the most common, aggressive and difficult to treat primary brain tumour in adults with the majority of patients surving less than two years[1]
We report here that these resistant GBM cell lines exhibit methylationresistant DNA synthesis (MDS), defective cell cycle checkpoints, and impaired DNA damage response (DDR) signalling, suggesting that, in the TMZ-resistant GBM cell lines, DDR signaling is decoupled from ongoing DNA alkylation damages by TMZ, which may underlie the phenotypic resistance in the TMZ-resistant GBM cells
With the addition of 25 μmol/L O6BG, U251 (TR) cells showed a clonogenic survival profile similar to that of U251 cells, suggesting that resistance in U251 (TR) cells to TMZ was due to high methylguanine-DNA methyltransferase (MGMT) activities.TMZ markedly inhibited the growth of U251 with an IC50 and IC90 of 50 and 100 μmol/ L, respectively, whereas TMZ exerted no noticeable growth-inhibitory effect on the TMZ-resistant U251 (OTR) (Fig. 1B)
Summary
Glioblastoma multiforme (GBM) is the most common, aggressive and difficult to treat primary brain tumour in adults with the majority of patients surving less than two years[1]. Radiation and adjuvant chemotherapy[2]. Temozolomide (TMZ), an SN1 type monofunctional DNA methylating agent., increases the survival of GBM patients in concurrent therapy with radiation[3]. Resistance to TMZ emerges with prolonged treatment and posts a major therapeutic challenge. One of the major determinants of clinical response to TMZ is the methylation status of the promoter region of O6methylguanine (O6-meG) DNA methyltransferase (MGMT). The hypermethylation of MGMT promoter is seen in approximately 40% to 60% of patients and
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
