Combination treatment using DDX3 and PARP inhibitors induces synthetic lethality in BRCA1-proficient breast cancer

Marise R. Heerma van Voss, Justin D. Brilliant, Farhad Vesuna, Guus M. Bol, Elsken van der Wall, Paul J. van Diest, Venu Raman

Research output: Contribution to journalArticlepeer-review


Triple-negative breast cancers have unfavorable outcomes due to their inherent aggressive behavior and lack of targeted therapies. Breast cancers occurring in BRCA1 mutation carriers are mostly triple-negative and harbor homologous recombination deficiency, sensitizing them to inhibition of a second DNA damage repair pathway by, e.g., PARP inhibitors. Unfortunately, resistance against PARP inhibitors in BRCA1-deficient cancers is common and sensitivity is limited in BRCA1-proficient breast cancers. RK-33, an inhibitor of the RNA helicase DDX3, was previously demonstrated to impede non-homologous end-joining repair of DNA breaks. Consequently, we evaluated DDX3 as a therapeutic target in BRCA pro- and deficient breast cancers and assessed whether DDX3 inhibition could sensitize cells to PARP inhibition. High DDX3 expression was identified by immunohistochemistry in breast cancer samples of 24% of BRCA1 (p = 0.337) and 21% of BRCA2 mutation carriers (p = 0.624), as compared to 30% of sporadic breast cancer samples. The sensitivity to the DDX3 inhibitor RK-33 was similar in BRCA1 pro- and deficient breast cancer cell lines, with IC50 values in the low micromolar range (2.8–6.6 μM). A synergistic interaction was observed for combination treatment with RK-33 and the PARP inhibitor olaparib in BRCA1-proficient breast cancer, with the mean combination index ranging from 0.59 to 0.62. Overall, we conclude that BRCA pro- and deficient breast cancers have a similar dependency upon DDX3. DDX3 inhibition by RK-33 synergizes with PARP inhibitor treatment, especially in breast cancers with a BRCA1-proficient background.

Original languageEnglish (US)
Article number33
JournalMedical Oncology
Issue number3
StatePublished - Mar 1 2017


  • BRCA
  • Breast cancer
  • DDX3
  • DEAD box RNA helicase
  • DNA damage repair
  • PARP inhibitor

ASJC Scopus subject areas

  • Hematology
  • Oncology
  • Cancer Research

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