PO-321 Causes and consequences of WDHD1 overexpression in breast cancer

Introduction WDHD1 (WD repeat and HMG-box DNA binding protein 1) controls DNA replication, sister-chromatid cohesion and centromere integrity. These phenomena are often defective in breast cancer cells. This could lead to mutations, chromosomal aberrations and aneuploidy, which could promote breast cancer development and therapy resistance. This prompted us to investigate WDHD1 misexpression and its consequences in breast cancer. Material and methods We performed systematic genomic, transcriptomic and proteomic analyses of WDHD1 expression in breast cancer using publicly available datasets, cell lines, primary cells and immunohistochemistry (IHC) on 547 invasive breast carcinomas with survival data up to 35 years post-diagnosis (Queensland Follow-Up resource). Functional studies included computational approaches, chromatin immunoprecipitations (ChIP), shRNA-mediated knockdown, various in vitro assays and in vivo xenograft modelling. Results and discussions We find that WDHD1 mRNA and protein levels are significantly elevated in breast cancer. IHC identified WDHD1 as a novel prognostic marker. WDHD1 protein expression correlates strongly with grade, ER status, HER2 status, triple-negative status, lymphovascular invasion, lymphocytic infiltrate, central scarring or fibrosis, tumour border, Ki-67 expression and prognostic sub-groups. Somatic copy number gains contribute to WDHD1 overexpression, but more importantly, we establish that WDHD1 is an E2F target gene. Hence, defective RB pathway regulation directly promotes WDHD1 overexpression. Knockdown of WDHD1 decreases cell viability and proliferation, but it does not promote apoptosis. It increases genomic instability in vitro and reduces tumour growth in xenograft experiments. Conclusion We identify WDHD1 mRNA and protein overexpression as novel markers for poor breast cancer prognosis. WDHD1 overexpression is primarily caused by RB pathway defects, which are common in breast tumours. In vitro and in vivo experiments suggest that overexpression of WDHD1 may represent a novel breast cancer therapeutic target.