Novel Insights into DNA Double-Strand Break Repair and Its Cancer Implications

Despite the aggressive treatment with DNA damage-inducing agents, glioblastomas (GBM) inevitably develop therapy resistance, leading to relapse and patient mortality. Cancer cells that survive therapy acquire additional damage-induced oncogenic changes that likely facilitate therapy resistance and tumor recurrence. To understand which damage-induced oncogenic alterations may promote tumor recurrence, we previously irradiated brains of mice harboring deletions of key tumor suppressors frequently lost in GBM. The most significant acquired alteration was amplification of the Met tyrosine kinase. We find that Met-expressing cells display cancer stem cell properties, augmented tumorigenesis, up-regulation of numerous DNA damage response (DDR) proteins, and an extended G2/M arrest. We hypothesize that Met expression drives therapy resistance and may be a potential target for radiosensitizing GBM. An alternative sensitization approach could involve direct inhibition of key DDR proteins, specifically in the homologous recombination (HR) double-strand break (DSB) repair pathway which is implicated in radioresistance of GBM stem cells. One indispensable step of HR is DNA-end resection, primarily executed by the exonuclease EXO1. We found that an EXO1 construct lacking the C-terminus and containing only the nuclease domain does not localize to DSBs, causing severe resection and repair defects. We hypothesized that the C-terminus of EXO1 serves as a platform for proteins to regulate EXO1's function. We found that the C-terminus interacts with BLM helicase, and it contains four Ser/Thr-Pro sites that are phosphorylated by CDKs1/2 to promote resection. We are currently examining whether CDK phosphorylation of EXO1 modulates the duration of the G2/M checkpoint since proper DNA repair requires a halt in the cell cycle. We are using CRISPR technology to generate EXO1 knock-out cells that will be complemented with WT or CDK-mutant EXO1 for checkpoint studies. We hypothesize that CDK phosphorylation of EXO1 serves to regulate resection and sustain the G2/M checkpoint. To further elucidate the role of EXO1 in maintaining genomic stability, we examined a cancer-associated SNP in EXO1 and found that it causes resection and DSB repair defects which may contribute to genomic instability and cancer progression. Overall, we provide novel insights into multiple aspects of DSB repair and identify potential targets for cancer therapy.