Aiyana Jade Adams

Background: Glioblastoma (GBM) is the most lethal and recurrent primary brain tumor in adults, classified as a WHO Grade IV astrocytoma.¹⁻³ Despite aggressive multimodal treatment, median survival remains only 12–15 months, largely due to adaptive resistance mechanisms.¹⁻³ One of the most well-established predictors of TMZ response is methylation of the MGMT (O6-methylguanine-DNA methyltransferase) promoter. Methylation silences MGMT expression, sensitizing tumors to TMZ-induced DNA alkylation. However, MGMT expression often reemerges in recurrent tumors, leading to therapy failure and aggressive relapse. This phenotypic switch has been attributed to therapy-induced epigenetic remodeling, but the underlying mechanism remained poorly understood until recently.⁴⁻⁶

Objective: This review explores the role of the PKC–DNMT1–MGMT signaling axis in mediating epigenetic reprogramming under temozolomide pressure and evaluates how this mechanism contributes to both treatment resistance and possible primary tumor initiation.

Methods: A comprehensive literature search was conducted through PubMed (2017–2023) using keywords: “glioblastoma,” “MGMT methylation,” “PKC,” “DNMT1,” “temozolomide resistance,” and “epigenetic memory.” Articles were selected based on mechanistic insight into the regulation of MGMT in primary and recurrent GBM.

Results: Temozolomide exposure activates protein kinase C (PKC), which phosphorylates DNA methyltransferase 1 (DNMT1), the enzyme responsible for maintaining MGMT promoter methylation. This phosphorylation reduces DNMT1 activity, resulting in MGMT promoter demethylation and re-expression, driving resistance even in tumors initially classified as methylated.⁴,⁶ Specific PKC isoforms, notably PKCζ, exert stronger phosphorylating effects on DNMT1, suggesting isoform-specific therapeutic targets.⁷ DNMT1 suppression disrupts broader epigenetic stability, leading to mesenchymal transition, stemness, and invasiveness.⁸  Feldheim et al. showed that long-term TMZ exposure reduced MGMT promoter methylation and selected for glioma stem-like cells with increased migration and proliferation.⁵ These effects persisted even after TMZ withdrawal, supporting the concept of heritable epigenetic memory. In parallel, some recurrent tumors acquire MGMT gene fusions with constitutively active promoters, bypassing methylation control and enabling sustained MGMT expression.⁹ These distinct mechanisms—epigenetic vs. genomic—highlight the need for dual-layered resistance profiling in clinical care to improve prognosis and stratify therapies effectively.⁶,⁹  To counteract resistance, Han et al. demonstrated that CRISPRoff—a programmable epigenome editor using a deactivated Cas9 fused to KRAB and DNMT3A—can restore MGMT promoter methylation in resistant GBM cell lines.¹⁰ This method not only resensitized cells to TMZ but also showed durable suppression of MGMT across multiple cell generations. These findings position epigenetic editing as a highly specific and reversible therapeutic strategy, in contrast to cytotoxic approaches.  Although current data focus primarily on recurrent disease, early dysregulation of the PKC–DNMT1–MGMT axis may also contribute to primary tumor development. Specifically, PKC hyperactivation or DNMT1 suppression in glial precursors could initiate epigenetic instability, creating a cellular environment permissive to MGMT dysregulation and malignant transformation.⁸,¹¹ This hypothesis warrants further investigation using experimental models of early gliomagenesis.

Conclusion: The PKC–DNMT1–MGMT axis represents a mechanistically detailed and clinically actionable model for acquired resistance in GBM. In addition to driving epigenetic memory and recurrence, this pathway may also contribute to early tumor evolution—a hypothesis that warrants further investigation. Future therapies that integrate isoform-selective PKC inhibition, DNMT1 preservation, or epigenetic reprogramming may stabilize the methylome, prolong drug sensitivity, and intercept glioblastoma’s evolutionary resistance trajectory at its molecular roots.

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