A common Chk1-dependent phenotype of DNA double-strand break suppression in two distinct radioresistant cancer types
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Triple-negative breast cancers (TNBC) are often resistant to treatment with ionizing radiation (IR). We sought to investigate whether pharmacologic inhibition of Chk1 kinase, which is commonly overexpressed in TNBC, preferentially sensitizes TNBC cells to IR.
Ten breast cancer cell lines were screened with small molecule inhibitors against Chk1 and other kinases. Chk1 inhibition was also tested in isogenic KRAS mutant or wild-type cancer cells. Cellular radiosensitization was measured by short-term and clonogenic survival assays and by staining for the DNA double-strand break (DSB) marker γ-H2AX. Radiosensitization was also assessed in breast cancer biopsies using an ex vivo assay. Aurora B kinase-dependent mitosis-like chromatin condensation, a marker of radioresistance, was detected using a specific antibody against co-localized phosphorylation of serine 10 and trimethylation of lysine 9 on histone 3 (H3K9me3/S10p). Expression of CHEK1 and associated genes was evaluated in TNBC and lung adenocarcinoma.
Inhibition of Chk1 kinase preferentially radiosensitized TNBC cells in vitro and in patient biopsies. Interestingly, TNBC cells displayed lower numbers of IR-induced DSBs than non-TNBC cells, correlating with their observed radioresistance. We found that Chk1 suppressed IR-induced DSBs in these cells, which was dependent on H3K9me3/S10p—a chromatin mark previously found to indicate radioresistance in KRAS mutant cancers. Accordingly, the effects of Chk1 inhibition in TNBC were reproduced in KRAS mutant but not wild-type cells. We also observed co-expression of genes in this Chk1 chromatin pathway in TNBC and KRAS mutant lung cancers.
Chk1 promotes an unexpected, common phenotype of chromatin-dependent DSB suppression in radioresistant TNBC and KRAS mutant cancer cells, providing a direction for future investigations into overcoming the treatment resistance of TNBC.
KeywordsTriple-negative breast cancer Radioresistance Radiosensitization Chk1 DNA double-strand breaks
This work was in part supported by the American Cancer Society 123420RSG-12-224-01-DMC (H. Willers), UK Wellcome Trust 102696 (C.H. Benes), the Werner-Otto-Stiftung, Hamburg (P.H. Dinkelborg), and the National Cancer Institute of the National Institutes of Health under Award Number U01CA220714 (H. Willers, C.H. Benes). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
Compliance with ethical standards
Conflict of interest
The following authors declare: Benes, C.H.—funding from Novartis, Amgen, Araxes; Hong, T. S.—consultant/advisory role: Clinical Genomics, EMD Serono, funding from Novartis, Taiho; Juric, D.—consultant/advisory role: Novartis, EMD Serono, Eisai, Genentech. All other authors declare that they have no conflict of interest.
All patient tumor samples were collected under a protocol approved by the institutional review board.
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