BRCA1 transports the DNA damage signal for CDDP-induced centrosome amplification through the centrosomal Aurora A.

Breast cancer gene 1 (BRCA1) plays roles in DNA repair and centrosome regulation and is involved in DNA damage-induced centrosome amplification (DDICA). Here, the centrosomal localization of BRCA1 and the kinases involved in centrosome duplication were analyzed in each cell cycle phase after treatment with DNA crosslinker cisplatin (CDDP). CDDP treatment increased the centrosomal localization of BRCA1 in early S-G2 phase. BRCA1 contributed to the increased centrosomal localization of Aurora A in S phase and that of phosphorylated Polo-like kinase 1 (PLK1) in late S phase after CDDP treatment, resulting in centriole disengagement and overduplication. The increased centrosomal localization of BRCA1 and Aurora A induced by CDDP treatment involved the nuclear export of BRCA1 and BRCA1 phosphorylation by ataxia telangiectasia mutated (ATM). Patient-derived variants and mutations at phosphorylated residues of BRCA1 suppressed the interaction between BRCA1 and Aurora A, as well as the CDDP-induced increase in the centrosomal localization of BRCA1 and Aurora A. These results suggest that CDDP induces the phosphorylation of BRCA1 by ATM in the nucleus and its transport to the cytoplasm, thereby promoting the centrosomal localization Aurora A, which phosphorylates PLK1. The function of BRCA1 in the translocation of the DNA damage signal from the nucleus to the centrosome to induce centrosome amplification after CDDP treatment might support its role as a tumor suppressor.

RACK1 regulates centriole duplication through promoting the activation of polo-like kinase 1 by Aurora A.

Breast cancer gene 1 (BRCA1) contributes to the regulation of centrosome number. We previously identified receptor for activated C kinase 1 (RACK1) as a BRCA1-interacting partner. RACK1, a scaffold protein that interacts with multiple proteins through its seven WD40 domains, directly binds to BRCA1 and localizes to centrosomes. RACK1 knockdown suppresses centriole duplication, whereas RACK1 overexpression causes centriole overduplication in a subset of mammary gland-derived cells. In this study, we showed that RACK1 binds directly to polo-like kinase 1 (PLK1) and Aurora A, and promotes the Aurora A-PLK1 interaction. RACK1 knockdown decreased phosphorylated PLK1 (p-PLK1) levels and the centrosomal localization of Aurora A and p-PLK1 in S phase, whereas RACK1 overexpression increased p-PLK1 level and the centrosomal localization of Aurora A and p-PLK1 in interphase, resulting in an increase of cells with abnormal centriole disengagement. Overexpression of cancer-derived RACK1 variants failed to enhance the Aurora A-PLK1 interaction, PLK1 phosphorylation and the centrosomal localization of p-PLK1. These results suggest that RACK1 functions as a scaffold protein that promotes the activation of PLK1 by Aurora A in order to promote centriole duplication.This article has an associated First Person interview with the first author of the paper.

Evaluating homologous recombination activity in tissues to predict the risk of hereditary breast and ovarian cancer and olaparib sensitivity.

Homologous recombination (HR) repairs DNA damage including DNA double-stranded breaks and alterations in HR-related genes results in HR deficiency. Germline alteration of HR-related genes, such as BRCA1 and BRCA2, causes hereditary breast and ovarian cancer (HBOC). Cancer cells with HR deficiency are sensitive to poly (ADP-ribose) polymerase (PARP) inhibitors and DNA-damaging agents. Thus, accurately evaluating HR activity is useful for diagnosing HBOC and predicting the therapeutic effects of anti-cancer agents. Previously, we developed an assay for site-specific HR activity (ASHRA) that can quantitatively evaluate HR activity and detect moderate HR deficiency. HR activity in cells measured by ASHRA correlates with sensitivity to the PARP inhibitor, olaparib. In this study, we applied ASHRA to lymphoblastoid cells and xenograft tumor tissues, which simulate peripheral blood lymphocytes and tumor tissues, respectively, as clinically available samples. We showed that ASHRA could be used to detect HR deficiency in lymphoblastoid cells derived from a BRCA1 pathogenic variant carrier. Furthermore, ASHRA could quantitatively measure the HR activity in xenograft tumor tissues with HR activity that was gradually suppressed by inducible BRCA1 knockdown. The HR activity of xenograft tumor tissues quantitatively correlated with the effect of olaparib. Our data suggest that ASHRA could be a useful assay for diagnosing HBOC and predicting the efficacy of PARP inhibitors.

BRCA1/ATF1-Mediated Transactivation is Involved in Resistance to PARP Inhibitors and Cisplatin.

Homologous recombination (HR)-deficient cells are sensitive to PARP inhibitors through a synthetic lethal effect. We previously developed an HR activity assay named Assay of Site-Specific HR Activity (ASHRA). Here, we evaluated the HR activity of 30 missense variants of BRCA1 by ASHRA and found that several BRCA1 variants showed intermediate HR activity, which was not clearly discerned by our previous analyses using a conventional method. HR activity measured by ASHRA was significantly correlated with sensitivity to olaparib. However, cells expressing the severely HR-deficient BRCA1-C61G variant were resistant to olaparib, and resistance was dependent on high expression of activating transcription factor 1 (ATF1), which binds to BRCA1 and activates the transcription of target genes to regulate cell proliferation. The BRCA1-C61G variant bound to ATF1 and stimulated ATF1-mediated transactivation similar to wild-type BRCA1. High expression of ATF1 conferred resistance to olaparib and cisplatin activating BRCA1/ATF1-mediated transcription without affecting HR activity in BRCA2-knockdown or RAD51-knockdown cells, but not in BRCA1-knockdown cells. These results suggest that ASHRA is a useful method to evaluate HR activity in cells and to predict the sensitivity to PARP inhibitors. The expression level of ATF1 might be an important biomarker of the effect of PARP inhibitors and platinum agents on HR-deficient tumors with the BRCA1-C61G variant or alteration of non-BRCA1 HR factors such as BRCA2 and RAD51. Significance: ASHRA could evaluate HR activity in cells and predict the sensitivity to PARP inhibitors. High expression level of ATF1 may predict the resistance of BRCAness tumors with alterations of non-BRCA1 HR factors to PARP inhibitors and platinum agents.

Evaluation of site-specific homologous recombination activity of BRCA1 by direct quantitation of gene editing efficiency.

Homologous recombination (HR) contributes to the repair of DNA double-strand breaks (DSBs) and inter-strand crosslinks. The HR activity in cancer cells can be used to predict their sensitivity to DNA-damaging agents that cause these damages. To evaluate HR activity, we developed a system called Assay for Site-specific HR Activity (ASHRA), in which cells are transiently transfected with an expression vector for CRISPR/Cas9 and a HR donor sequence containing a marker gene. DSBs are created by Cas9 and then repaired by HR using donor vector sequences homologous to the target gene. The level of genomic integration of the marker gene is quantified by Western blotting, flowcytometry, or quantitative PCR (qPCR). ASHRA detected HR deficiency caused by BRCA1, BARD1, or RAD51 knockdown or introduction of BRCA1 variants. The influence of BRCA1 variants on HR, as determined by qPCR, was consistent with the chemosensitivities of the transfected cells. The qPCR format of ASHRA could measure HR activity in both transcribed and un-transcribed regions. Knockdown of BRCA1 nor BARD1 did not affect HR activity in a transcriptionally inactive site. ASHRA can evaluate HR activity and will be useful for predicting sensitivity to chemotherapy, screening drugs that affect HR, and investigating the mechanisms of HR.

RACK1 regulates centriole duplication by controlling localization of BRCA1 to the centrosome in mammary tissue-derived cells.

Breast cancer gene 1 (BRCA1) is a tumor suppressor that is associated with hereditary breast and ovarian cancer. BRCA1 functions in DNA repair and centrosome regulation together with BRCA1-associated RING domain protein (BARD1), a heterodimer partner of BRCA1. Obg-like ATPase 1 (OLA1) was identified as a protein that interacts with BARD1. OLA1 regulates the centrosome by binding to and collaborating with BRCA1 and BARD1. We identified receptor for activated C kinase (RACK1) as a protein that interacts with OLA1. RACK1 directly bound to OLA1, the N-terminal region of BRCA1, and γ-tubulin, associated with BARD1, and localized the centrosomes throughout the cell cycle. Knockdown of RACK1 caused abnormal centrosomal localization of BRCA1 and abrogated centriole duplication. Overexpression of RACK1 increased the centrosomal localization of BRCA1 and caused centrosome amplification due to centriole overduplication. The number of centrioles in cells with two γ-tubulin spots was higher in cell lines derived from mammary tissue compared to those derived from other tissues. The effects of aberrant RACK1 expression level on centriole duplication were observed in cell lines derived from mammary tissue, but not in those derived from other tissues. Two BRCA1 variants, R133H and E143K, and a RACK1 variant, K280E, associated with cancer, which weakened the BRCA1-RACK1 interaction, interfered with the centrosomal localization of BRCA1 and reduced centrosome amplification induced by overexpression of RACK1. These results suggest that RACK1 regulates centriole duplication by controlling the centrosomal localization of BRCA1 in mammary tissue-derived cells and that this is dependent on the BRCA1-RACK1 interaction.