RETRACTED ARTICLE: Correlation of Promoter Methylation in the MGMT Gene with Glioma Risk and Prognosis: a Meta-Analysis

Genetic and Epigenetic Modulation of Drug Resistance in Cancer: Challenges and Opportunities

Cancer is a complex disease that has the ability to develop resistance to traditional therapies. The current chemotherapeutic treatment has become increasingly sophisticated, yet it is not 100% effective against disseminated tumours. Anticancer drugs resistance is an intricate process that ascends from modifications in the drug targets suggesting the need for better targeted therapies in the therapeutic arsenal. Advances in the modern techniques such as DNA microarray, proteomics along with the development of newer targeted drug therapies might provide better strategies to overcome drug resistance. This drug resistance in tumours can be attributed to an individual's genetic differences, especially in tumoral somatic cells but acquired drug resistance is due to different mechanisms, such as cell death inhibition (apoptosis suppression) altered expression of drug transporters, alteration in drug metabolism epigenetic and drug targets, enhancing DNA repair and gene amplification. This review also focusses on the epigenetic modifications and microRNAs, which induce drug resistance and contributes to the formation of tumour progenitor cells that are not destroyed by conventional cancer therapies. Lastly, this review highlights different means to prevent the formation of drug resistant tumours and provides future directions for better treatment of these resistant tumours.

Primary and recurrent diffuse astrocytomas: genomic profile comparison reveals acquisition of biologically relevant aberrations

Background

Diffuse astrocytomas are characterized by their highly variable biological behavior. The possibility that tumors develop novel aberrations, with relevant biological properties, is often neglected. In this study, we present two cases of diffuse astrocytoma in which additional cytogenetic and epigenetic markers with potential influence on cell proliferation or differentiation were detected at relapse.

Findings

The biopsies taken from the primary and recurrent tumors of two patients were analyzed with molecular methods to detect copy number variations (CNVs), gene mutations and epigenetic changes. Both cases were characterized by the R132H mutation in the isocitrate dehydrogenase 1 (IDH1) gene. Features typical of astrocytomas, such as copy-neutral loss of heterozygosity at 17p and the deletion of the cyclin-dependent kinase inhibitor 2A (CDKN2A) gene, were also detected in both cases. These markers were present in the primary and recurrent lesions. Other aberrations, predominantly deletions or amplifications of chromosomal segments and the hypermethylation of gene promoters, were detected in the recurrent lesions.

Conclusions

The IDH1 mutation was the primary event, as previously reported. According to our observations, the methylation of promoters constituted later events, which may have further disrupted cell proliferation and/or differentiation, together with additional CNVs.

A new functional classification system (FGA/B) with prognostic value for glioma patients

Despite advances in multimodal treatments, malignant gliomas remain characterized by a short survival time. Surgical treatment is accepted to be the first line of therapy, with recent studies revealing that maximal possible tumor reduction exerts significant impact on patient outcome. Consideration of tumor localization in relation to functionally eloquent brain areas has been gaining increasing importance. Despite existing assessment methods, the availability of a simple but reliable preoperative grading based on functional data would therefore prove to be indispensable for the prediction of postoperative outcome and hence for overall survival in glioma patients. We performed a clinical investigation comprising 322 patients with gliomas and developed a novel classification system of preoperative tumor status, which considers tumor operability based on two graduations (Friedlein Grading - FG): FGA with lesions at safe distance to eloquent regions which can be completely resected, and FGB referring to tumors which can only be partially resected or biopsied. Investigation of outcome revealed that FGA were characterized by a significantly longer overall survival time compared to FGB. We offer the opportunity to classify brain tumors in a dependable and reproducible manner. The FGA/B grading method provides high prognostic value with respect to overall survival time in relation to the extent of location-dependent tumor resection.

High CD133 Expression Is Associated with Worse Prognosis in Patients with Glioblastoma

The CD133 antigen has been identified as a putative stem cell marker in gliomas. However, the prognostic significance of CD133 expression in glioblastoma patients remained controversial. A meta-analysis of published data was performed to comprehensively assess the prognostic role of CD133 expression in glioblastoma patients. Publications assessing the prognostic significance of CD133 expression in glioblastoma patients were identified in PubMed, Embase, and Web of Science up to November 2014. The pooled hazard ratio (HR) with 95% confidence interval (95% CI) was calculated using meta-analysis to evaluate the prognostic significance of CD133 expression in glioblastoma. Ten studies with a total of 715 glioblastoma patients were included into the meta-analysis. Overall, high CD133 expression was associated with poorer overall survival in patients with glioblastoma (HR = 1.96, 95% CI 1.46-2.64, P < 0.001). In addition, high CD133 expression was also associated with poorer progression-free survival in patients with glioblastoma (HR = 2.03, 95% CI 1.43-2.88, P < 0.001). Meta-analyses of studies with high quality showed that high CD133 expression was associated with both poorer overall survival (HR = 2.39, 95% CI 1.77-3.23, P < 0.001) and poorer progression-free survival (HR = 2.17, 95% CI 1.60-2.94, P < 0.001) in patients with glioblastoma. Meta-analysis of studies with adjusted estimates further showed that high CD133 expression was an independent prognostic factor of glioblastoma. High CD133 expression is associated with worse prognosis in patients with glioblastoma. More prospective studies with well-design are needed to confirm this finding.

Drug resistance in cancer: an overview

Cancers have the ability to develop resistance to traditional therapies, and the increasing prevalence of these drug resistant cancers necessitates further research and treatment development. This paper outlines the current knowledge of mechanisms that promote or enable drug resistance, such as drug inactivation, drug target alteration, drug efflux, DNA damage repair, cell death inhibition, and the epithelial-mesenchymal transition, as well as how inherent tumor cell heterogeneity plays a role in drug resistance. It also describes the epigenetic modifications that can induce drug resistance and considers how such epigenetic factors may contribute to the development of cancer progenitor cells, which are not killed by conventional cancer therapies. Lastly, this review concludes with a discussion on the best treatment options for existing drug resistant cancers, ways to prevent the formation of drug resistant cancers and cancer progenitor cells, and future directions of study.