TopBP1 contains transcriptional regulatory domains and regulates gene pathways involved in breast cancer

TopBP1 is a nuclear protein with eight BRCT domains and is involved in many aspects of nucleic acid metabolism: it is involved in the initiation of DNA replication in the Xenopus in vitro replication system by assisting loading of polymerase onto the replication complex; it is a substrate for ATM/ATR and is essential for the ATR DNA damage signalling pathway, and is also probably involved in the actual DNA repair process; it acts as a transcriptional cofactor for E2F1 where it regulates the apoptotic function of this protein. In addition, the yeast homologues of TopBP1, Dbp11 (Saccharomyces cerevisiae) and Cut5 (Schizo-saccharomyces pombe), are also involved in replication and repair processes. TopBP1 also shares functions with BRCA1; both are involved in regulating an intact G₂/M checkpoint, they colocalise to sites of DNA damage, they share sequence homology (even outside the BRCT domains), they are substrates for ATM/ATR, and they can regulate expression of the c-myc gene. All of these properties of TopBP1 led to us investigating whether TopBP1 plays a role in breast cancer.

There is a polymorphism in TopBP1 that gives an increased risk of breast cancer [1], and work from our laboratory has demonstrated that TopBP1 is aberrantly expressed in a significant number of human breast cancers [2]. Clearly the role of TopBP1 in replication and genome maintenance would mean that disturbance of expression could result in genomic instability contributing towards cancer. Our studies have focused on an additional aspect of TopBP1 that could contribute to the transformed phenotype; gene regulation. We have identified several chromatin modification domains on TopBP1 that could contribute not only to transcriptional regulation but also to the replication and repair functions of this protein [3]. Using siRNA knockdown of TopBP1 in MCF7 cells, we identified genes that are regulated by TopBP1. Following knockdown of TopBP1, the short-term growth of the MCF7 cells was not affected. This was surprising as it has been predicted that TopBP1 is essential for DNA replication and our results demonstrate that this is not the case in all cell lines (we have tested other lines in which TopBP1 is essential for S phase). However, even though these cells cycled for several days, they did not survive long term, presumably due to accumulated damage following replication in the absence of TopBP1. Using this MCF7 system we carried out microarray experiments that revealed the absence of TopBP1 alters the expression of genes involved in many cellular pathways implicated in breast cancer, including the oestrogen signalling pathway, and the mitogen-activated protein kinase signalling network. Future work will focus on determining how TopBP1 regulates these pathways and what cellular interacting partners TopBP1 requires for chromatin modification. Such studies will increase our understanding of breast cancer and assist in developing diagnostic and prognostic gene profiling for breast cancer management.