Background:

RSPO ligands, activators of the Wnt/β-catenin pathway, are overexpressed in different cancers. The objective of this study was to investigate the role of RSPOs in breast cancer (BC).

Methods:

Expression of RSPO and markers of various cancer pathways were measured in breast tumours and cell lines by qRT–PCR. The effect of RSPO on the Wnt/β-catenin pathway activity was determined by luciferase assay, western blotting, and qRT–PCR. The effect of RSPO2 inhibition on proliferation was determined by using RSPO2 siRNAs. The effect of IWR-1, an inhibitor of the Wnt/β-catenin pathway, was examined on the growth of an RSPO2-positive patient-derived xenograft (PDX) model of metaplastic triple-negative BC.

Results:

We detected RSPO2 and RSPO4 overexpression levels in BC, particularly in triple-negative BC (TNBC), metaplastic BC, and triple-negative cell lines. Various mechanisms could account for this overexpression: presence of fusion transcripts involving RSPO, and amplification or hypomethylation of RSPO genes. Patients with RSPO2-overexpressing tumours have a poorer metastasis-free survival (P= 3.6× 10− 4). RSPO2 and RSPO4 stimulate Wnt/β-catenin pathway activity. Inhibition of RSPO expression in a TN cell line inhibits cell growth, and IWR-1 significantly inhibits the growth of an RSPO2-overexpressing PDX.

Conclusions:

RSPO overexpression could therefore be a new prognostic biomarker and therapeutic target for TNBC.

Main

Breast cancer (BC) is the leading cause of death by cancer in women (Jemal et al, 2007). Fifteen percent of primary BC are triple-negative BC (TNBC) with lack of expression of oestrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (Morris et al, 2007). The majority of TNBC are invasive ductal carcinomas, but metaplastic BC (MBC) constitutes a rare subtype (< 1%). MBC is characterised morphologically by the differentiation of neoplastic epithelium into squamous cells and/or mesenchymal cells. MBC displays a fairly aggressive clinical behaviour, and unlike other forms of TNBC, these tumours do not appear to respond to conventional chemotherapy regimens (Hennessy et al, 2006). Globally, women with TNBC or MBC have a poor prognosis, particularly due to the absence of targeted therapies (Aydiner et al, 2015).