R-spondin gain-of-function in colorectal cancer – a comparison between mature tumours and precursor polyps


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Sam Kleeman1,Roland Arnold1,on behalf of S-CORT Consortium2,Tim Maughan3,Simon Leedham1
1Wellcome Trust Centre for Human Genetics,2S-CORT Consortium,3CRUK/MRC Oxford Institute for Radiation Oncology

Abstract

Background

R-spondins (RSPO) bind to LGR-family receptors to disinhibit ligand-dependent Wnt signalling. Recurrent fusions affecting RSPO2/3 are thought to occur in 4-10% of colorectal cancers and ~30% of traditional serrated adenomas (Sekine et al, JPath, 2016). Recently, porcupine inhibitors and anti-RSPO3 antibodies have been shown to induce rapid tumour regression in RSPO-mutant tumours in vivo. The aim of this study was to characterise the relationship between RSPO fusions and overexpression in colorectal polyps in comparison to tumours.

Method

We examined RSPO expression and identified RSPO fusions in RNA sequencing of 54 polyps (10 TSA, 15 SSA, 29 TVA) and 622 colorectal cancers (TCGA). RSPO fusions were identified by searching for breakpoint motifs in reads aligning to fusion partners with positive calls validated by STAR-Fusion.

Results

RSPO2 fusions were not detected in either cohort while PTPRK-RSPO3 fusions were detected in 3 TSA polyps and 11 tumours (2%). As expected, all RSPO-mutant samples had high RSPO3 expression (>2 SD above the mean). Furthermore, despite absence of RSPO fusions, 14 additional tumours had high RSPO2 and/or RSPO3 expression. Compared to RSPO-mutant cases, these samples showed significant upregulation of a cancer-associated fibroblast score (p<0.001) and were exclusively CMS4 (mesenchymal) subtype tumours (100% versus 20%). Consistent with this, in RSPO-wildtype CRCs, CMS4 tumours have significantly increased expression of both RSPO2 (fold change>6, p<0.001) and RSPO3 (fold change>2, p<0.001) versus each other subtype.

Conclusion

These findings suggest there is a continuum of RSPO gain-of-function in a subset of colorectal tumours ranging from epithelial RSPO fusions that arise in precursor TSA lesions to stromal RSPO3 secretion in mesenchymal subtype tumours. As a result, emerging therapies targeting ligand-dependent Wnt signalling could be effective in a proportion of CMS4 tumours known to be associated with a poor prognosis.