RSPO3 expands intestinal stem cell and niche compartments and drives tumorigenesis.

Abstract

The gross majority of colorectal cancer cases results from aberrant Wnt/β-catenin signalling through adenomatous polyposis coli (<i>APC)</i> or <i>CTNNB1</i> mutations. However, a subset of human colon tumours harbour, mutually exclusive with <i>APC</i> and <i>CTNNB1</i> mutations, gene fusions in <i>RSPO2</i> or <i>RSPO3,</i> leading to enhanced expression of these R-spondin genes. This suggested that <i>RSPO</i> activation can substitute for the most common mutations as an alternative driver for intestinal cancer. Involvement of RSPO3 in tumour growth was recently shown in <i>RSPO3</i>-fusion-positive xenograft models. The current study determines the extent into which solely a gain in RSPO3 actually functions as a driver of intestinal cancer in a direct, causal fashion, and addresses the in vivo activities of RSPO3 in parallel. We generated a conditional <i>Rspo3</i> transgenic mouse model in which the <i>Rspo3</i> transgene is expressed upon Cre activity. Cre is provided by cross-breeding with <i>Lgr5</i>-GFP-Cre^ERT2 mice. Upon in vivo <i>Rspo3</i> expression, mice rapidly developed extensive hyperplastic, adenomatous and adenocarcinomatous lesions throughout the intestine. RSPO3 induced the expansion of Lgr5⁺ stem cells, Paneth cells, non-Paneth cell label-retaining cells and Lgr4⁺ cells, thus promoting both intestinal stem cell and niche compartments. Wnt/β-catenin signalling was modestly increased upon <i>Rspo3</i> expression and mutant <i>Kras</i> synergised with <i>Rspo3</i> in hyperplastic growth. We provide in vivo evidence that RSPO3 stimulates the crypt stem cell and niche compartments and drives rapid intestinal tumorigenesis. This establishes RSPO3 as a potent driver of intestinal cancer and proposes RSPO3 as a candidate target for therapy in patients with colorectal cancer harbouring <i>RSPO3</i> fusions.