Glioblastoma multiforme (GBM) is a brain tumor characterized by high heterogeneity, diffuse infiltration, aggressiveness, and formation of recurrences. Patients with this kind of tumor suffer from cognitive, emotional, and behavioral problems, beyond exhibiting dismal survival rates. Current treatment comprises surgery, radiotherapy, and chemotherapy with the methylating agent, temozolomide (TMZ). GBMs harbor intrinsic mutations involving major pathways that elicit the cells to evade cell death, adapt to the genotoxic stress, and regrow. Ionizing radiation and TMZ induce, for the most part, DNA damage repair, autophagy, stemness, and senescence, whereas only a small fraction of GBM cells undergoes treatment-induced apoptosis. Particularly upon TMZ exposure, most of the GBM cells undergo cellular senescence. Increased DNA repair attenuates the agent-induced cytotoxicity; autophagy functions as a pro-survival mechanism, protecting the cells from damage and facilitating the cells to have energy to grow. Stemness grants the cells capacity to repopulate the tumor, and senescence triggers an inflammatory microenvironment favorable to transformation. Here, we highlight this mutational background and its interference with the response to the standard radiochemotherapy. We discuss the most relevant and recent evidence obtained from the studies revealing the molecular mechanisms that lead these cells to be resistant and indicate some future perspectives on combating this incurable tumor.

Therapy of malignant glioma depends on the induction of O⁶-methylguanine by the methylating agent temozolomide (TMZ). However, following TMZ exposure, most glioma cells evade apoptosis and become senescent and are thereby protected against further anticancer therapy. This protection is thought to be dependent on the senescent cell anti-apoptotic pathway (SCAP). Here we analyzed the factors involved in the SCAP upon exposure to TMZ in glioblastoma cell lines (LN-229, A172, U87MG) and examined whether inhibition of these factors could enhance TMZ-based toxicity by targeting senescent cells. We observed that following TMZ treatment, c-IAP2 and Bcl-2 were upregulated. Inhibition of these SCAP factors using non-toxic concentrations of the small molecule inhibitors, BV6 and venetoclax, significantly increased cell death, as measured 144 h after TMZ exposure. Most importantly, BV6 and venetoclax treatment of senescent cells strongly increased cell death after an additional 120 h. Moreover, Combenefit analyses revealed a significant synergy combining BV6 and venetoclax. In contrast to BV6 and venetoclax, AT406, embelin, and TMZ itself, teniposide and the PARP inhibitor pamiparib did not increase cell death in senescent cells. Based on these data, we suggest that BV6 and venetoclax act as senolytic agents in glioblastoma cells upon TMZ exposure.

Therapy-induced senescence (TIS) is a major challenge in cancer therapy as senescent cancer cells provoke local and systemic inflammation and might be the cause of recurrences. Elucidation of pathways leading to TIS is of utmost importance for establishing strategies to counteract this. Previously we have shown that temozolomide (TMZ), an alkylating drug used forefront in glioma therapy, causes majorly cellular senescence, which is triggered by the primary damage O⁶-methylguanine, activating the mismatch repair dependent ATR/ATM-CHK1/CHK2-p53 damage response pathway. The downstream pathways leading to TIS remained to be explored. Here, we show that TMZ-induced TIS in glioma cells does not require activation of the DREAM complex, but is bound on a G2-specific response. We show that the CDK inhibitor p21^CIP1 does not interact with CDK4, but with CDK1 and CDK2 causing abrogation of the B-Myb and FOXM1-signaling pathway and subsequently arrest of cells in the G2-phase. The induced G2-arrest is incomplete as DNA synthesis can be resumed leading to endoreduplications. This process, which is inhibited by the CDK4-blocking drug palbociclib, is preceded by reactivation of the G1/S-specific E2F1-signaling pathway due to lack of functional DREAM activation. These findings provide an explanation for the polyploidization and giant cell phenotype of anticancer drug-induced senescent cells. Incomplete DREAM activation may also explain the observation that downregulation of DNA repair is a transient phenomenon, which goes along with the entrance of cells into the senescent state.

Xeroderma Pigmentosum variant (XP-V) is an autosomal recessive disease with an increased risk to develop cutaneous neoplasms in sunlight exposed regions. These cells are deficient in the translesion synthesis DNA polymerase eta. Eleven skin tumors from a genetic cluster of XP-V patients had their exome sequenced. Mutational signatures identified for most tumors were related to ultraviolet exposure, such as C>T transitions targeted to pyrimidine dimers. However, four samples carry different mutational signatures, with C>A mutations associated with tobacco usage. Basal cell carcinomas showed a distinct C>A mutation spectra reflecting a novel mutational signature. Higher levels for retroposon insertions were detected in the XP-V tumors, compared to non-XP skin tumors. The results reveal other possible causes for XP-V tumors and the involvement of polymerase eta in suppressing retrotransposition. The expected high mutation burden, found in most of these tumors, renders these XP patients good candidates for immunotherapy with checkpoint blockers.