Topoisomerase‐1 gene copy aberrations are frequent in patients with breast cancer

Abstract

Topoisomerase-1 (Top1) targeting drugs have shown promising efficacy in patients with metastatic breast cancer (BC). However, these drugs are rather toxic calling for development and validation of predictive biomarkers to increase the therapeutic index. As these drugs are targeting the Top1 protein and since no validated anti-Top1 antibodies for immunohistochemistry have been reported, we raised the hypothesis that TOP1 gene amplifications may serve as a proxy for the Top1 protein and thereby a biomarker of response to treatment with Top1 inhibitors in BC. The aim was to determine the prevalence of TOP1 gene copy gain in BC.The prevalence of TOP1 gene copy gain was investigated by fluorescence in situ hybridization with a TOP1/CEN-20 probemix in normal breast tissue (N 5 100) and in tissue from patients with metastatic BC in a discovery (N 5 100) and a validation cohort (N 5 205). As amplification of 20q including CEN-20 is common in BC a TOP1/CEN-2 probemix was applied to the validation cohort.More than 30% of the patients had gene copy numbers of 4 and ;20% of the patients had TOP1/CEN-20 ratios 1.5. The CEN-2 probe did not add any information.Gain of the TOP1 gene appears to be common in BC making the gene a potential biomarker for response to treatment with Top1 inhibitors. As 20q amplification is a common finding in BC and as no other suitable reference gene has yet been identified, TOP1 copy number may be a more valid method of detecting gain than using a gene/centromere ratio.Irinotecan is a semisynthetic derivative of the naturally occurring camptothecin. It is an inhibitor of the topoisomerase 1 (Top1) enzyme and for almost two decades, it has been a cornerstone in the treatment of colorectal cancer. 1 Irinotecan for the use in breast cancer (BC) has been investigated in several clinical trials. 2 Generally, studies have been small and non-randomized and response rates (RR) for irinotecan monotherapy have been in the range 5-23% whereas irinotecan combined with various chemotherapeutics have shown RR ranging from 14 to 64%. 2 Despite RR comparable to those seen for other second or third-line treatments in metastatic BC, irinotecan has not yet found a place in the treatment of BC. Recently, however, a randomized phase II study evaluated the long-acting Top1 inhibitor Etirinotecan in 70 taxane resistant patients with metastatic BC and found a RR of 29% when the drug was given as monotherapy as second or third-line treatment. This drug, which consists of irinotecan bound to a proprietary polyethylene glycol core, is currently evaluated in a randomized phase III study versus physicians choice of treatment. 3 As for most chemotherapeutics used today, no predictive biomarker exists for response to irinotecan. The predictive role of Top1 in patients with

Two open-label, single arm, non-randomized phase II studies of irinotecan for the treatment of metastatic breast cancer in patients with increased copy number of the topoisomerase I gene

“…Prior to initiating the clinical studies we had investigated the frequency of TOP1 CN gain in normal breast tissue and in samples from primary BC. Based on these results we had anticipated that up to 30% of patients with BC would have increased CN of the TOP1 gene [15]. In the clinical trials only 19% of patients fulfilled the criteria for increased CN.…”

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“…Top1 gene amplification has been reported in about 30% of breast cancers [40] and a tight correlation between mRNA and protein expression was observed in the NCI-60 panel of cell lines [27]. Irinotecan, a semisynthetic derivative of CPT is routinely used in colorectal cancer therapy [41].…”

The Top1 paradox: Friend and foe of the eukaryotic genome

“…The transient Top1-mediated cleavage of the DNA backbone, however, can also initiate mutagenic processes. Top1 is often overexpressed in human tumors [67,68] and an elevated Top1 level is presumably needed to sustain rapid growth. This not only enhances the sensitivity of tumor cells to Top1 inhibitors, it also may drive Top1-mediated instability that contributes to tumor evolution.…”