Simple SummaryMetastatic disease is the leading cause of death in cancer patients. In our earlier clinical study of breast cancer patients, it was shown that metastasis after neoadjuvant chemotherapy and adjuvant hormone therapy occurred only in patients with WNT signalling in the tumour due to activator gene amplifications (15 genes identified) and/or negative regulator gene deletions (7 genes identified) of WNT signalling pathway genes. If, on the contrary, activator deletions and negative regulator amplifications are noted, these patients do not metastasise. We hypothesized that spontaneous exit of tumour cells from postchemotherapy shock (replication arrest, senescence) is associated with ectopic expression of WNT signalling pathway genes due to activator amplifications and negative regulator deletions. In the present work, we have confirmed this assumption on cell cultures.AbstractBackground: the present study aims to prove or disprove the hypothesis that the state of copy number aberration (CNA) activation of WNT signalling pathway genes accounts for the ability of differentiated tumour cells to emerge from postchemotherapy shock. Methods: In the first step, the CNA genetic landscape of breast cancer cell lines BT-474, BT-549, MDA-MB-231, MDA-MD-468, MCF7, SK-BR-3 and T47D, which were obtained from ATCC, was examined to rank cell cultures according to the degree of ectopic activation of the WNT signalling pathway. Then two lines of T47D with ectopic activation and BT-474 without activation were selected. The differentiated EpCAM+CD44-CD24-/+ cells of these lines were subjected to IL6 de-differentiation with formation of mammospheres on the background of cisplatin and WNT signalling inhibitor ICG-001. Results: it was found that T47D cells with ectopic WNT signalling activation after cisplatin exposure were dedifferentiated to form mammospheres while BT-474 cells without ectopic WNT-signalling activation did not form mammospheres. The dedifferentiation of T47D cells after cisplatin exposure was completely suppressed by the WNT signalling inhibitor ICG-001. Separately, ICG-001 reduced, but did not abolish, the ability to dedifferentiate in both cell lines. Conclusions: these data support the hypothesis that the emergence of differentiated tumour cells from postchemotherapy shock after chemotherapy is due to ectopic activation of WNT signalling pathway genes.