The oncogene epidermal growth factor receptor variant III (EGFRvIII) is frequently expressed in glioblastomas (GBM) but its impact on therapy response is still under controversial debate. Here we wanted to test if EGFRvIII influences the sensitivity towards the alkylating agent temozolomide (TMZ). Therefore, we retrospectively analyzed the survival of 336 GBM patients, demonstrating that under standard treatment, which includes TMZ, EGFRvIII expression is associated with prolonged survival, but only in patients with O6-methylguanine-DNA methyltransferase (MGMT) promoter methylated tumors. Using isogenic GBM cell lines with endogenous EGFRvIII expression we could demonstrate that EGFRvIII increases TMZ sensitivity and results in enhanced numbers of DNA double-strand breaks and a pronounced S/G2-phase arrest after TMZ treatment. We observed a higher expression of DNA mismatch repair (MMR) proteins in EGFRvIII+ cells and patient tumor samples, which was most pronounced for MSH2 and MSH6. EGFRvIII-specific knockdown reduced MMR protein expression thereby increasing TMZ resistance. Subsequent functional kinome profiling revealed an increased activation of p38-and ERK1/2-dependent signaling in EGFRvIII expressing cells, which regulates MMR protein expression downstream of EGFRvIII. In summary, our results demonstrate that the oncoprotein EGFRvIII sensitizes a fraction of GBM to current standard of care treatment through the upregulation of DNA MMR.

Background The oncogene epidermal growth factor receptor variant III (EGFRvIII) is expressed in approximately one third of all glioblastoma (GBM). So far it is not clear if EGFRvIII expression induces replication stress in GBM cells, which might serve as a therapeutical target. Methods Isogenetic EGFRvIII- and EGFRvIII+ cell lines with endogenous EGFRvIII expression were used. Markers of oncogenic and replication stress such as γH2AX, RPA, 53BP1, ATR and Chk1 were analysed using western blot, immunofluorescence and flow cytometry. The DNA fiber assay was performed to analyse replication, transcription was measured by incorporation of EU and genomic instability was investigated by micronuclei and CGH-Array analysis. Immunohistochemistry staining was used to detect replication stress markers and R-loops in human GBM samples. Results EGFRvIII+ cells exhibit an activated replication stress response, increased spontaneous DNA damage, elevated levels of single stranded DNA and reduced DNA replication velocity, which are all indicative characteristics for replication stress. Furthermore, we show here that EGFRvIII expression is linked to increased genomic instability. EGFRvIII expressing cells display elevated RNA synthesis and R-loop formation, which could also be confirmed in EGFRvIII positive GBM patient samples. Targeting replication stress by irinotecan resulted in increased sensitivity of EGFRvIII+ cells. Conclusion This study demonstrates that EGFRvIII expression is associated with increased replication stress, R-loop accumulation and genomic instability. This might contribute to intratumoral heterogeneity but may also be exploited for individualized therapy approaches.