Evidence suggests that the DNA end-binding protein p53-binding protein 1 (53BP1) is down-regulated in subsets of breast cancer. Circulating tumor cells (CTCs) provide accessible "biopsy material" to track cell traits and functions and their alterations during treatment. Here, we prospectively monitored the 53BP1 status in CTCs from 67 metastatic breast cancer (MBC) patients with HER2-CTCs and known hormone receptor (HR) status of the primary tumor and/or metastases before, during, and at the end of chemotherapeutic treatment with Eribulin. Nuclear 53BP1 staining and genomic integrity were evaluated by immunocytochemical and whole-genome-amplification-based polymerase chain reaction (PCR) analysis, respectively. Comparative analysis of CTCs from patients with triple-negative and HR+ tumors revealed elevated 53BP1 levels in CTCs from patients with HR+ metastases, particularly following chemotherapeutic treatment. Differences in nuclear 53BP1 signals did not correlate with genomic integrity in CTCs at baseline or with nuclear γH2AX signals in MBC cell lines, indicating that 53BP1 detected features beyond DNA damage. Kaplan-Meier analysis revealed an increasing association between nuclear 53BP1-positivity and progression-free survival (PFS) during chemotherapy until the final visit. Our data suggest that 53BP1 detection in CTCs could be a useful marker to capture dynamic changes of chemotherapeutic responsiveness in triple-negative and HR+ MBC.Cancers 2020, 12, 930 2 of 18 13.3 months after diagnosis of metastatic sites, TNBC has a particularly poor prognosis. Often patients develop a resistance to first-line chemotherapy very rapidly, resulting in a median duration of first-line palliative chemotherapy of 11.9 weeks [1].Triple-negative tumors are characterized by the lack of HR and HER2 expression, but aside from this common feature this subgroup includes very heterogeneous tumors. A mutation in BRCA1 or 2 is found in up to 20% of triple-negative metastatic patients [2]. The proteins encoded by BRCA genes are critically involved in DNA double-strand break (DSB) repair, more specifically, in the error-free pathway of homologous recombination repair (HRR) [3]. In TNBC, a high prevalence of gene mutations as well as epigenetic changes result in BRCAness, compromising safe DNA repair through HRR [2,4,5]. The DNA damage response factor 53BP1 is crucial in protecting DNA ends in BRCA1-defective cells from resection and entry into error-prone repair pathways [6][7][8]. It has been demonstrated that 53BP1 expression in breast cancer is associated with poor prognosis, particularly in TNBC frequently showing BRCA1 dysfunction [7,9].Due to the lack of predictive targets, chemotherapy was the only treatment option available for a long time. This results in an urgent clinical need for new therapies. During the last years, new drugs such as the Poly(ADP-ribose) polymerase (PARP)-inhibitors targeting HRR-defective tumors were studied in several clinical trials. Two different phase III trials (OlympiaAD and EMBRACA) showed an impro...

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