Abstract 1318: Proteasome and Fanconi Anemia/BRCA pathways as prognostic determinants of glioblastoma response to radiation and temozolomide: a proposed clinical trial

Abstract

Abstract OBJECTIVE: Despite the standard regimen of maximal resection followed by concurrent ionizing radiation (IR) and temozolomide (TMZ), long-term survivors of patients afflicted with glioblastoma (GBM) remain rare. We sought to identify molecular determinants of resistance to these agents using a combined functional genomic and bioinformatic approach. METHODS: Parallel RNAi screens were performed to identify gene silencings that modulate IR and TMZ resistance. A viability-based screen was performed in multiple glioma lines with the Hannon-Elledge library of 74,705 short hairpin RNAs targeting nearly all human genes. A focused screen was also performed to identify modulators of the Fanconi Anemia (FA)/BRCA pathway, a pathway we previously identified as a determinant of IR and TMZ resistance. The most promising candidates were confirmed by independent RNAi and pharmacologic inhibition in vitro and in vivo xenografts. Correlations between relative expression levels of the candidate genes and patient survival were made using data from seven series of clinically annotated GBM gene expression profiles. RESULTS: PSMA1, a core subunit of the 20S proteasome, was a top hit in both screens. Proteasome inhibition by either RNAi or pharmacologic inhibition sensitized GBM cell lines to IR and TMZ in both in vitro and in vivo xenograft models. This inhibition also resulted in decreased FANCD2 monoubiquitination, a marker for FA pathway activation. Using our collated set of 531 patients, we observed that patients with low (< mean) PSMA1 showed higher OS vs. those with high expression, median OS = 14.8 vs. 12.2 months (log-rank P = 0.017). This correlation was recapitulated with several other proteasome genes. Since recent studies revealed distinct GBM subtypes, we tested whether the prognostic value of proteasome gene expression was subtype-specific. We found that the difference was only significant in the proneural subtype, median OS = 16.5 vs. 9.4 mo. (P = 0.012). Realizing that proteasome inhibition exerts phenotypes beyond FA pathway inhibition, we determined the extent to which FA/BRCA expression impacts prognosis. High FA/BRCA expression correlated with lower OS only for proneural tumors (P = 0.015); this observation suggested that proteasome function impacts tumor survival, in large part, by modulating the FA/BRCA pathway. CONCLUSION: Proteasome inhibition sensitizes GBM cells to IR and TMZ and results in impaired FA pathway activation. Our combined functional genomic and bioinformatic analysis suggests a clinical trial: patients with proneural gliomas with high FA/BRCA expression should be treated with proteasome inhibitors such as bortezomib to augment standard therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1318.