Identification of signalling pathways downstream of BRCA1 and p63

BRCA1 was identified in 1994 as one of the genes predisposing to early-onset breast and ovarian cancer. It is currently estimated that 5 to 10% of all breast and ovarian cancer cases are inherited and the breast cancer susceptibility genes, BRCA1 and BRCA2, have been identified as being responsible for up to 21 to 40% of these cases. Although the exact function of BRCA1 remains to be defined, roles in DNA damage repair, cell cycle checkpoint control, transcriptional regulation and, more recently, ubiquitination have been inferred. p63 was identified as a positively regulated BRCA1 target gene through microarray analysis, and the functional significance of the BRCA1/p63 signalling axis was investigated.

Knockdown of BRCA1 and p63 leads to enhanced proliferation of breast cancer cell lines and increased stem cell numbers as assayed for by mammosphere culture and Aldefluor assay. Expression of BRCA1 or p63 in a background of low BRCA1 and p63 results in decreased cell proliferation. We therefore examined co-regulated targets of BRCA1 and p63 mediating growth control. S100A2, a tumour suppressor, is a known p63 target. Knockdown of BRCA1 and p63 leads to the loss of S100A2 expression. BRCA1 and p63 were found to be localised to the S100A2 promoter by chromatin immunoprecipitation assay. Loss of p63 resulted in recruitment of BRCA1 to the S100A2 promoter. In a p63 and BRCA1 null background, expression of S100A2 results in a reduction of cell proliferation. Conversely, loss of S100A2 in a BRCA1 and p63 expressing background leads to increased proliferation.

We have explored the regulation of signalling pathways by p63 and BRCA1 that are involved in growth control, differentiation and stem cell regulation. We will identify potential regulators of these pathways using microarray analysis to elucidate p63 and BRCA1 co-regulated targets.