Increased Expression of DNA Repair Gene RAD51 Associated with Shorter Overall Survival in Glioblastoma

Abstract

The diagnosis of Glioblastoma (GBM) carries a poor prognosis. Despite surgical resection and high-dose adjuvant radiation, most treatment failures occur locally. We conducted a retrospective clinical-genetic study to identify factors which may mediate resistance of GBM to radiation, such as increased repair of DNA damage. We quantified the expression levels of key DNA repair enzymes in patient-derived tissue samples and examined their correlation with overall survival (OS). We retrospectively identified adult patients diagnosed with GBM, with tissue available in our institution’s archive, who were treated at our institution with maximal safe resection followed by adjuvant temozolomide and radiation (60 Gy). Total mRNA was extracted from formalin-fixed paraffin embedded (FFPE) tissue. mRNA gene expression profiles were obtained utilizing a digital gene expression panel. We compared the mRNA expression profiles of the GBM samples with a cohort of non-neoplastic temporal lobe FFPE tissue obtained from surgical resections for epilepsy. Normalized mRNA counts and differential expression (DE) comparisons were generated using analysis software. Kaplan-Meier (KM) and multivariate Cox proportional hazard (CPH) models were generated to examine factors associated with OS. Twenty-four GBM and 12 non-neoplastic brain tissue samples with sufficient quantity and quality of RNA were identified, processed, and analyzed. DE comparison identified 7 DNA repair genes that were significantly up-regulated in the GBM samples relative to the non-neoplastic brain tissue (more than 8-fold difference with adjusted p values all < 0.01). A multivariate CPH model that included age, tumor size, KPS, extent of resection, and the 7 differentially expressed DNA repair genes identified increased levels of RAD51 as being independently associated with an increased risk of death (HR = 10.11; p = 0.026), and RAD51 expression levels were over 9-fold higher in GBM samples versus non-neoplastic brain tissue. KM OS models demonstrated that GBM patients with high levels of RAD51 (above the cohort’s median) had significantly shorter OS compared to patients with low levels of RAD51 (10.6 months vs 18.2 months, KM log-rank p = 0.03). Within this small population of GBM patients that were uniformly treated with the current standard of care, we found that expression levels of RAD51, a key component of the homologous recombination DNA repair pathway, was significantly correlated with OS. These findings suggest that RAD51 mRNA levels could be used as a novel prognostic biomarker to improve outcome predictions in patients with GBM, and more importantly, inhibition of RAD51 protein activity during radiation could offer a new therapeutic target to improve the treatment of this deadly disease.