Up-regulation of miR-370-3p restores glioblastoma multiforme level of sensitivity to temozolomide by influencing MGMT expression. light in recent years. Key emerging mechanisms include the involvement of other DNA repair systems, aberrant signaling pathways, autophagy, epigenetic modifications, microRNAs, and extracellular vesicle production. This review aims to provide a comprehensive overview of the clinically relevant molecular mechanisms and their considerable interconnections to better inform efforts to combat TMZ resistance. gene promoter region, is an important factor in TMZ treatment response. Hypermethylation of the promoter results in decreased expression of the MGMT protein and has been shown to correlate with prolonged survival in GBM patients. In contrast, unmethylated tumors (with increased MGMT activity) generally exhibit resistance to TMZ. Therefore, the epigenetic status of has been established as a surrogate marker of intrinsic resistance to TMZ[38,39]. Open in a separate window Physique 2. Mechanism of TMZ metabolism, DNA damage and DNA repair. Under physiological conditions TMZ is usually metabolized to PU-H71 MTIC and then to its active form, a methyl diazonium ion. The electrophilic methyl diazonium ion acts as a methyl donor, transferring its methyl group to negatively charged DNA and creating DNA adducts. This alkylating activity occurs preferentially at N7 of guanine, O3 of adenine and O6 of guanine and, if left unrepaired, results in improper base pairing and single and double-stranded DNA breaks. The primary endogenous DNA repair mechanisms that counteract the DNA damage caused by TMZ, and are thus generally implicated in TMZ resistance, include MGMT, BER and MMR. TMZ: temozolomide; MTIC: metabolite 5-(3-methyltriazen-1-yl) imidazole-4-carboxamide; MGMT: O6-methylguanine-DNA methyltransferase; BER: base excision repair; MMR: mismatch repair Additionally, there is mounting evidence from meta-analysis studies suggesting that MGMT status may be susceptible to change throughout a tumors treatment, progression or recurrence. It has been observed that tumors with initial MGMT methylation have a decreased methylation ratio upon recurrence after treatment with TMZ, suggesting that the reduction in MGMT promoter methylation is usually a mechanism for acquiring therapeutic resistance to TMZ. A deeper understanding of the mechanisms that prompt PU-H71 switch in MGMT methylation and their contribution to TMZ resistance will be fundamental to comprehensive GBM treatment. Recent emerging evidence from clinical trials suggests that a combination treatment of lomustine and TMZ may improve overall survival when used as a first-line treatment for patients with MGMT methylation, further demonstrating the clinical importance of targeting and characterizing DNA repair enzymes early in GBM treatment. Base excision repair Base excision repair (BER) is responsible for repairing single nucleotide modifications, and its mechanism involves several enzymatic reactions carried out by glycosylase, endonuclease, polymerase and DNA ligase[22,24]. The vast majority ( 90%) of N7-guanine and N3-adenine methylation that is induced by TMZ is usually recognized and PU-H71 rapidly repaired by BER mechanisms[42,43]. Several proteins PU-H71 involved in the BER pathway have been associated with promoting resistance to TMZ and confer poor prognosis in patients; these include DNA glycosylase MPG, stem cell factor high-mobility group A2, as well as, DNA polymerase- (pol-B) which protects from DNA-induced cytotoxicity via its lyase activity[42,44,45]. Another protein involved in the BER pathway is usually poly (ADP-ribose) polymerase 1 or PARP1. This protein recognizes breaks in single-stranded DNA and protects cells from accumulating too many apurinic/apyrimidinic sites[46,47]. Tang and in response to standard chemotherapeutic treatment; these MMR mutations have been found to lead to hypermutation in recurrent tumors, particularly in the setting of initial MGMT methylation. Interestingly, MMR proteins also tend to display inverse expression in relation to MGMT, with the combination of methylated MGMT and high MMR activity conferring the best response to TMZ. This relationship may be exploited in the development of TMZ resistance, where a protective DNA repair genotype may be counteracted by a mutation in a different repair system. More detailed MMR characterization is being explored in GBM with the hope of identifying specific MMR proteins as additional prognostic markers[57,58]. There have also been recent efforts to develop a more comprehensive assessment of DNA repair capacity that accounts for MGMT, MMR, BER, nucleotide excision repair and homologous PU-H71 recombination (HR). Although not yet widely implemented, these efforts may help Rabbit polyclonal to Caldesmon.This gene encodes a calmodulin-and actin-binding protein that plays an essential role in the regulation of smooth muscle and nonmuscle contraction.The conserved domain of this protein possesses the binding activities to Ca(2+)-calmodulin, actin, tropomy to more accurately predict chemoresistance. Interplay of DNA repair and molecular pathway mutations Many specific mutations or pathway perturbances have been found to alter the efficacy of these DNA integrity control mechanisms. For example, epidermal growth factor receptor variant III (EGFRvIII), a prominent.