Abstract 3812: Targeting BRCA1-deficient breast cancer by inhibition of the DEAD box RNA helicase DDX3X
Authors: Voss Marise R. Heerma van , Vesuna Farhad , Bol Guus M. , Diest Paul J. van , Raman Venu 
Published: May 2015

Introduction: The DEAD box RNA helicase DDX3X (DDX3) has been demonstrated to have a pro-oncogenic role in breast cancer. Thus, abrogating DDX3 activity was explored as a viable anti-cancer strategy using a small molecule inhibitor of DDX3, referred to as RK-33. RK-33 was found to inhibit non-homologous end joining (NHEJ), a DNA double strand break (DSB) repair mechanism. Therefore, we set out to determine the utility of RK-33 in breast cancers of BRCA1 mutation carriers, as they exhibit deficiencies in homologous recombination, an additional DSB repair mechanism. We hypothesized that BRCA1-deficient breast cancers would be more dependent on NHEJ to maintain genomic stability and thus inhibiting DDX3 activity by RK-33 would be an efficacious treatment strategy.

Methods: We evaluated DDX3 protein expression levels by immunohistochemistry in 102 BRCA1 and 29 BRCA2 germline mutation carriers and compared these to those of 345 sporadic breast cancer patients. In addition, DDX3 expression in two BRCA1-deficient (SUM149PT and HCC1937) and two BRCA1-proficient breast cancer cell lines (MCF-7 and MDA-MB-231) was quantified by immunoblotting. Furthermore, we compared the cell killing abilities of RK-33 in these BRCA1 pro- and deficient cell lines, as well as in MCF-7 cells before and after knockdown of BRCA1 by shRNA.

Results: High cytoplasmic DDX3 expression is less frequent in BRCA1 (24.5%) and BRCA2 (20.7%) related breast cancer, when compared to sporadic breast cancer (34.2%; OR 0.63; 95% CI 0.38-1.03; P = 0.065). This difference is statistically significant when correcting for higher grade and basal-like subtype in the group of BRCA1 mutation carriers (OR 0.33; 95% CI 0.14-0.77; P = 0.01). The BRCA1 pro- and deficient cell lines expressed similar levels of DDX3 and had a comparable sensitivity to RK-33 with IC-50 values ranging between 2 and 7 uM. Knockdown of BRCA1 in MCF-7 did not result in an increased sensitivity to RK-33.

Conclusion: DDX3 expression is lower in breast cancer in BRCA1 mutation carriers, when compared to sporadic breast cancers of similar grade and molecular subtype. BRCA1 pro- and deficient breast cancer cell lines are equally sensitive to inhibition of DDX3 by RK-33. This indicates that abrogating NHEJ activity by RK-33 results in cell death irrespective of additional deficiencies in the DNA repair pathway. Given that RK-33 showed good in vitro efficacy to kill all breast cancer cell lines, additional research is required to evaluate the use of RK-33 as a targeted chemotherapy agent for breast cancer.

Citation Format: Marise R. Heerma van Voss, Farhad Vesuna, Guus M. Bol, Paul J. van Diest, Venu Raman. Targeting BRCA1-deficient breast cancer by inhibition of the DEAD box RNA helicase DDX3X. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3812. doi:10.1158/1538-7445.AM2014-3812