BIRC3 is a novel driver of therapeutic resistance in Glioblastoma

Therapeutic resistance is a hallmark of glioblastoma multiforme (GBM) making disease recurrence inevitable. Despite advances in the multimodal strategies of surgical resection, radiotherapy (RT) and chemotherapy with Temozolomide (TMZ), the median survival for newly diagnosed patients hovers around 14 months1. Moreover, the prognosis is markedly dismal for patients with recurrent GBM, where median survival of 3–9 months with standard chemotherapy2,3 and 6-month progression-free survival rates of 15–16%2,4,5 are often realized. Evasion from apoptosis is central to cancers in general6, and GBM is no exception. Genome-wide analysis of GBM reveals pervasive aberrations in multiple apoptotic pathways7. For instance several critical anti-apoptotic signaling pathways such as the Epidermal Growth Factor Receptor (EGFR), Platelet Derived Growth Factor Receptor (PDGFR), Phosphatidylinositide 3-kinase [PI3K], and Signal Transducer and Activator of Transcription (STAT3) are highly activated in GBM7. Furthermore, aberrancies of the anti-apoptosis BCL-2 family7,8, mutations in apoptosis-related tumor suppressor proteins such as TP537, and increase expression of Inhibitor of Apoptosis Proteins (IAP)9,10,11 collectively skew the apoptotic balance in GBM towards cell survival mechanisms, which all lead to therapeutic failure. Given the central role of anti-apoptosis signaling, strategies that define and target anti-apoptosis mechanisms could potentially ameliorate therapeutic resistance in GBM. IAPs are characterized by the presence of baculoviral IAP repeat (BIR) domains12,13,14, highly up-regulated in GBMs9,10,11, and known to promote cellular survival in cancers through regulation of apoptosis15. Therefore, IAPs are emerging as attractive pharmacologic targets for ameliorating therapeutic resistance in cancers. Besides cell death16, IAPs also play a role in immunity and inflammation17. The human IAP family is composed of eight members: Neuronal IAP (NAIP), cellular IAP1 (c-IAP1) [BIRC2], cellular IAP2 (c-IAP2) [BIRC3], X-chromosome linked IAP (XIAP) [BIRC4], survivin [BIRC5], Apollon/Bruce [BIRC6)], Melanoma IAP (ML-IAP), and IAP-like Protein 2 (ILP-2)16. Only BIRC2, BIRC3, and BIRC4 regulate caspase activity18. BIRC4 directly inhibits caspases 3,7 and 919,20,21,22,23,24, whereas the BIRC2 and BIRC3 proteins indirectly regulate caspase activation through E3 ligase activity, TNF-signaling and NFkB signaling25. The central role of IAPs within the terminal segment of apoptosis has profound therapeutic and prognostic implications (Supplementary Figure 1). Since IAPs interact at the level of caspases, IAPs could serve as the definitive convergence point for signaling pathways that promote apoptosis evasion. Therefore, identifying and targeting critical IAPs that contribute to apoptotic evasion in GBM is a very rationale strategy. Higher expressions of IAP’s have been documented in malignant gliomas and often correlated with poor prognosis9,10,11. There is also preclinical evidence that targeting IAPs with small molecule inhibitors can reverse therapeutic resistance in GBM26,27. However, no studies to date have characterized the mechanistic impact of IAPs on therapeutic resistance and also on long-term survival in GBM. We therefore sought to understand the role of IAP expression on survival in a large cohort of GBM patients. We were interested in the role of IAP in the current standard GBM therapy of TMZ and RT. Detailed understanding of such mechanisms could permit optimized synergy between IAP targeting and standard therapy. Such a targeting strategy of downstream convergence signaling nodes could potentially overcome the current shortcomings of targeted GBM therapies that focus on upstream pathways. The Cancer Genome Atlas (TCGA) provides a unique opportunity to examine GBM on a larger scale both clinically and molecularly since TCGA contains expression data from over 527 unique GBM samples7. Using TCGA data in this study, we identified BIRC3 as a critical determinant of survival in GBM patients. BIRC3 was the only IAP among several IAPs whose differential expression was significantly related to the 5-year survival in patients with GBM. Lower expression levels of BIRC3 were associated with a markedly favorable outcome. Given the above observations, we sought to further delineate the unique mechanistic role of BIRC3 in GBM therapeutic resistance in this study. For the first time, we demonstrate that BIRC3 expression increases secondary to acquisition of TMZ and RT resistance. In addition, BIRC3 emerges as a novel driver of therapeutic resistance in GBM. Furthermore, for the first time, we mechanistically implicate BIRC expression as a downstream signaling node for STAT3 and PI3K in response to GBM therapy. BIRC3 therefore emerges as a rational target with translational implications.