Targeting RSPO3 Reduces Stem Cell Function in PTPRK–RSPO3 Colon Tumors

Molecular characterization of colorectal cancer has shown that the majority exhibit WNT pathway activation, but effective WNT pathway inhibitors acting at the level of or downstream of β-catenin have been difficult to develop. A subset of colorectal cancer tumors with WNT pathway activation was found to harbor a fusion involving protein tyrosine phosphatase, receptor type, K (PTPRK) and R-spondin 3 (RSPO3). R-spondins have previously been shown to amplify WNT signaling, prompting Storm and colleagues to generate function-blocking antibodies against RSPO2 and RSPO3 to treat two colorectal cancer patient–derived xenografts (PDX) harboring the PTPRK–RSPO3 fusion. Treatment with anti-RSPO3 resulted in growth inhibition of both PDXs, which were shown by histopathologic analysis to be differentiated, contain large amounts of mucus, and exhibit reduced proliferation. Consistent with these results, RNA sequencing of treated tumors revealed that two markers of intestinal stem cells, leucine-rich repeat containing G protein–coupled receptor 5 (LGR5) and achaete-scute family BHLH transcription factor 2 (ASCL2), were among the most significantly downregulated genes, while WNT target genes were modestly downregulated, suggesting that RSPO3 amplifies WNT signaling to meet stem-cell requirements. In addition, markers of differentiation were upregulated in treated cells. In normal intestine, the combination of anti-RSPO2 and anti-RSPO3 profoundly reduced regeneration after irradiation whereas single-antibody treatment had minimal effect, indicating that RSPO2 and RSPO3 both regulate normal stem cell function and are functionally redundant. In line with this finding, treatment of mice bearing serially transplanted PDX tumors with anti-RSPO3 resulted in a marked reduction of both tumor engraftment and cells positive for CD133 and CD44, two markers of colorectal cancer–initiating cells. Together, these results suggest that therapeutically targeting RSPO3 promotes differentiation of the stem-cell compartment of RSPO3 fusion–positive tumors, thus reducing WNT pathway activity, and may induce fewer side effects due to the functional redundancy of R-spondins in normal colon.

Storm EE, Durinck S, de Sousa e Melo F, Tremayne J, Kljavin N, Tan C, et al. Targeting PTPRK–RSPO3 colon tumours promotes differentiation and loss of stem-cell function. Nature 2016;529:97–100.