R-spondin (RSPO) proteins constitute a family of four secreted glycoproteins (RSPO1–4) that have appeared as multipotent signaling ligands. The best-known molecular function of RSPOs lie within their capacity to agonize the Wnt/β-catenin signaling pathway. As RSPOs act upon cognate receptors LGR4/5/6 that are typically expressed by stem cells and progenitor cells, RSPO proteins importantly potentiate Wnt/β-catenin signaling especially within these proliferative stem cell compartments. Since multiple organs express LGR4/5/6 receptors and RSPO ligands within their stem cell niches, RSPOs can exert an influential role in stem cell regulation throughout the body. Inherently, over the last decade a multitude of reports implicated the deregulation of RSPOs in cancer development. First, RSPO2 and RSPO3 gene fusions with concomitant enhanced expression have been identified in colon cancer patients, and proposed as an alternative driver of Wnt/β-catenin hyperactivation that earmarks cancer in the colorectal tract. Moreover, the causal oncogenic capacity of RSPO3 overactivation has been demonstrated in the mouse intestine. As a paradigm organ in this field, most of current knowledge about RSPOs in cancer is derived from studies in the intestinal tract. However, RSPO gene fusions as well as enhanced RSPO expression have been reported in multiple additional cancer types, affecting different organs that involve divergent stem cell hierarchies. Importantly, the emerging oncogenic role of RSPO and its potential clinical utility as a therapeutic target have been recognized and investigated in preclinical and clinical settings. This review provides a survey of current knowledge on the role of RSPOs in cancer biology, addressing the different organs implicated, and of efforts made to explore intervention opportunities in cancer cases with RSPO overrepresentation, including the potential utilization of RSPO as novel therapeutic target itself.

R‐spondins (RSPOs) are influential signaling molecules that promote the Wnt/β‐catenin pathway and self‐renewal of stem cells. Currently, RSPOs are emerging as clinically relevant oncogenes, being linked to cancer development in multiple organs. Although this has instigated the rapid development and testing of therapeutic antibodies targeting RSPOs, functional evidence that RSPO causally drives cancer has focused primarily on the intestinal tract. Here, we assess the oncogenic capacity of RSPO in breast cancer in a direct fashion by generating and characterizing a novel mouse model with conditional Rspo3 expression in the mammary gland. We also address the prevalence of RSPO gene alterations in breast cancer patients. We found that a quarter of breast cancer patients harbor RSPO2/RSPO3 copy number amplifications, which are associated with lack of steroid hormone receptor expression and reduced patient survival. Foremost, we demonstrate the causal oncogenic capacity of RSPO3 in the breast, as conditional Rspo3 overexpression consistently drives the development of mammary adenocarcinomas in our novel Rspo3 breast cancer model. RSPO3‐driven mammary tumors typically show poor differentiation, areas of epithelial‐to‐mesenchymal transition, and metastatic potential. Given the reported interplay in the Wnt/β‐catenin pathway, we comparatively analyzed RSPO3‐driven mouse mammary tumors versus classical WNT1‐driven analogues. This revealed that RSPO3‐driven tumors are distinct, as the poorly differentiated tumor morphology and metastatic potential were observed in RSPO3‐driven tumorigenesis exclusively, further substantiated by differentiating gene expression profiles. Co‐expression of Rspo3 and Wnt1 transduced mammary tumors with a mixed phenotype harboring morphological features characteristic of both transgenes. In summary, we report that a quarter of breast cancer patients harbor RSPO2/RSPO3 copy number gains, and these patients have a worse prognosis, whilst providing in vivo evidence that RSPO3 drives poorly differentiated invasive breast cancer in mice. Herewith, we establish RSPO3 as a driver of breast cancer with clinical relevance, proposing RSPO3 as a novel candidate target for therapy in breast cancer. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

No abstract