Overexpression of TopBP1 leads to transformation with a TP53 mutation of non-tumorigenic breast epithelial cells

DNA topoisomerase IIβ - binding protein 1 (TopBP1) is a mediator protein that regulates the cell cycle checkpoint signaling pathway. A plethora of studies suggests high TopBP1 levels are positively associated with various cancers. Although TopBP1 transcript, as well as protein expression levels, are high in breast cancers, its role in breast tumorigenesis is not yet explored. In our studies, we observed that TopBP1 levels are high in premalignant and malignant cells of the MCF10A cancer progression series compared to the non-tumorigenic MCF10A cells. In order to establish the role of TopBP1 in tumorigenesis, TopBP1 overexpression in non-tumorigenic MCF10A, and stable knock-down in malignant MCF10CA1a cells were performed and grown in Matrigel ™ as breast spheroids. Overexpression of TopBP1 in MCF10A spheroids induced hyperproliferation, disruption of polarity and cell-cell junctions. Moreover, TopBP1 overexpressing 3D dissociated cells exhibited EMT-like phenotype and tumorigenic properties such as increased cell migration, invasion, colony formation capabilitiy and anchorage-independent growth, indicating acquisition of cellular transformation. Finally, we demonstrated TopBP1 overexpressing cells to form tumors in athymic mice thereby confirming their tumorigenic potential. We also confirmed that overexpression of TopBP1 led to a mutation in TP53 and other genomic insults. To summarise, we observed that ectopic expression of TopBP1 transforms MCF10A breast epithelial cells. These transformed cells harbour phenotypic and genotypic characteristics similar to that of malignant cells.