In this study, we discovered that the ETS gene fusion product, ERG, physically interacts with the enzymes PARP1 and DNA-PKcs. Both PARP1 and DNA-PKcs are required for ERG-mediated transcription and cell invasion, suggesting that both of these co-factors are necessary for ERG-mediated prostate cancer progression. Moreover, therapeutic inhibition of PARP1 preferentially sensitized ETS-overexpressing prostate cancer xenografts compared to ETS negative xenografts. Thus, similar to the successful paradigm of targeting the BCR-ABL gene fusion in CML with the small molecule kinase inhibitor imatinib meyslate (Druker et al., 2001
), one could envision targeting the ETS-PARP1 axis in prostate cancer and possibly other ETS gene fusion-dependent cancers. While directly inhibiting transcription factors, such as ERG, may be difficult, blocking the function of regulatory co-factors, such as PARP1, is more feasible, and may represent a viable treatment paradigm in cancer therapy.
In particular, PARP1 represents a very promising therapeutic target. Based on its role in base excision repair, PARP1 has been explored in both preclinical and clinical settings as a target in tumors with deficiencies in double-stranded DNA repair, such as mutations in BRCA1 and BRCA2 (Bryant et al., 2005
; Farmer et al., 2005
). In these cancers, the inhibition of PARP1 in cells with an inherent defect in homologous repair results in stalled replication forks and subsequent cell death (Bryant et al., 2005
; Farmer et al., 2005
). An initial Phase I trial of the PARP inhibitor Olaparib has suggested an excellent therapeutic response in patients with BRCA1/2 deficient tumors from multiple organ sites with most patients experiencing a large reduction in total tumor volume (Fong et al., 2009
). However, most cancers do not harbor BRCA1/2 mutations; only 5–6% of all breast cancers are associated with an inherited BRCA1/2 gene mutation (Malone et al., 1998
) and only 3% of prostate tumors from an Ashkenazi Jewish population of 832 patients were BRCA1/2-deficient (Gallagher et al., 2010
While PARP inhibitors can exploit the DNA repair defects of BRCA deficient tumors to induce cell death, we now demonstrate that they can also inhibit ETS gene fusion protein activity by preventing ETS transcriptional activity, inhibiting ETS-associated invasion, and enhancing ETS-mediated DNA damage. Future studies will help determine if, as with AR-mediated transcription (Haffner et al., 2010
), ETS-mediated transcription is directly coupled the induction of DNA damage. Importantly, the potentiation of ETS-induced DNA damage by PARP inhibition is of particular clinical interest; analogous to the “synthetic lethality” resulting from PARP inhibition in BRCA1/2-deficient tumors. By suggesting that cancers driven by specific oncogenic transcription factors may respond to PARP inhibition independent of BRCA1/2 status, our data supports the notion that multiple tumor subtypes will be susceptible to PARP pharmacotherapy. It is important to note that the company Sanofi-BiPAR recently released a press report that their phase III trial assessing the addition of their next generation PARP inhibitor, Iniparib, to a gemcitabine-carboplatin regimen for patients with metastatic triple-negative breast cancer, was negative for an overall survival benefit (http://sanofi-aventis.mediaroom.com/index.php?s=43&item=310
). This is in direct contrast to the recently reported phase II trial showing that the addition of Iniparib approximately doubled overall survival in this setting (O'Shaughnessy et al., 2011
), and some questions about specificity have been raised (Carey and Sharpless, 2011
). Nonetheless, these results highlight the importance of target selection; it is expected that ongoing phase III trials assessing chemotherapy with or without PARP inhibitor in BRCA-mutant cancers (instead of a non-specific triple-negative breast cancer population) will be positive, because the patient population is selected based on the presence of the PARP inhibitor target – BRCA mutation (Ellisen, 2011
). Here, we have shown that Olaparib very specifically, and in a dose dependent manner, delays tumor growth of ETS positive, but not ETS negative prostate cancer xenografts.
By exploiting the ETS:PARP1 interaction to selectively target ETS-overexpressing xenografts, our studies significantly expand the total population of patients who could benefit from PARP inhibition. Consequently, the data presented here also have implications on the design of subsequent clinical trials which will follow the recently reported Phase I trial of Olaparib (Fong et al., 2009
). While most trials will undoubtedly be designed to target and subtype BRCA-deficient tumors, trials designed in organ sites that are also known to harbor aberrantly expressed ETS genes, such as breast, melanoma, Ewing’s sarcoma and especially prostate, should also subtype the disease by ETS status. Based on the data presented here, ETS-positive tumors are expected to respond with a higher probability to PARP inhibition than ETS-negative tumors, potentially making ETS status an important predictive biomarker. In line with the observation that PARP inhibitors can significantly increase the mean overall survival of patients with triple negative breast cancer when added onto an existing regimen, our data suggests that the best design for a clinical trial in hormone-refractory metastatic prostate cancer will be to add PARP inhibitors in combination with chemotherapeutics known to potentiate the effects of PARP inhibition such as temozolomide.
Finally, the observation that gene fusions which drive the gross overexpression of ETS genes also induce DNA double strand break formation provides additional mechanistic insight into how ETS gene fusions drive cancer progression. Specifically, by causing DNA double-strand breaks, ETS gene overexpression may also play a role in the gradual evolution of genomic rearrangements. This finding may explain why recurrent ETS gene fusions may have been difficult to discover, because they simply lead to the accumulation of additional gene fusions only some of which will drive disease progression. In fact, this model may partially explain the clinical behavior of prostate cancers which lie dormant for years only to spontaneously become extremely aggressive.