Background: CDC73 is an ubiquitously expressed tumour suppressor and member of the transcriptional complex PAF1 (RNA polymerase II-associated factor 1). Mutation and/or down-regulation or loss of nuclear CDC73 has been implicated in parathyroid, renal, breast, gastric, uterine, lung and colorectal cancer. The mechanism(s) that define the tumour suppressive function of CDC73 remain unclear. This study sought to explore possible interactions between CDC73 and the key regulator of apoptosis - p53.

Aims: To determine the role of CDC73 in p53-regulated apoptosis and cell division pathways.

Methods & Results: Endogenous CDC73 was transiently down-regulated using siRNA in cancer cell lines MCF-7 and A2780. p53 expression was elevated in these cell lines upon treatment with actinomycin D at a concentration of 2.5nM for 30 h. Proteins and RNA samples were harvested from cells after 78 and 102 h post-transfection with siRNA. Western blotting analysis showed that down-regulating CDC73 in both MCF-7 and A2780 cells resulted in a decrease of endogenous levels of p53 and p53-targets such as MDM2, BAX and p21. Co-immunoprecipitation using a Myc-tagged-CDC73/p53 overexpression model in MCF-7 cells demonstrated that CDC73 and p53 are likely binding partners. DuoLink Proximity Ligation Assay showed that CDC73 and p53 are in close proximity in MCF-7 cells under conditions that activate p53. The presence of CDC73 at p53 response elements in the promoter regions of p53 target genes MDM2 and CDKN1A was determined using chromatin immunoprecipitation.

Discussion: Our study is the first to demonstrate that CDC73 functionally impacts on transcription and/or expression of p53 targets. Furthermore, we provide evidence that this regulation is mediated at least in part through the physical interaction of CDC73 and p53. This data sheds new light on the mechanism(s) of how CDC73 functions as a tumour suppressor, and identifies this protein as a potential therapeutic target for future cancer treatments.