SummaryThe mammalian adult small intestinal epithelium is a rapidly self-renewing tissue that is maintained by a pool of cycling stem cells intermingled with Paneth cells at the base of crypts. These crypt base stem cells exclusively express Lgr5 and require Wnt3 or, in its absence, Wnt2b. However, the Frizzled (Fzd) receptor that transmits these Wnt signals is unknown. We determined the expression profile of Fzd receptors in Lgr5+ stem cells, their immediate daughter cells, and Paneth cells. Here we show Fzd7 is enriched in Lgr5+ stem cells and binds Wnt3 and Wnt2b. Conditional deletion of the Fzd7 gene in adult intestinal epithelium leads to stem cell loss in vivo and organoid death in vitro. Crypts of conventional Fzd7 knockout mice show decreased basal Wnt signaling and impaired capacity to regenerate the epithelium following deleterious insult. These observations indicate that Fzd7 is required for robust Wnt-dependent processes in Lgr5+ intestinal stem cells.

The APC tumour suppressor gene is the most commonly mutated gene in colorectal cancer (CRC). Loss of Apc in intestinal stem cells (ISCs) drives aberrant Wnt signalling and adenoma formation in mice 1 . We previously showed that a reduction in WNT-ligand secretion increases the ability of Apc-mutant ISCs to colonise a crypt (fixation) and accelerate tumourigenesis 2 . Here, we investigate key mechanistic processes whereby Apc-mutant cells gain a clonal advantage over wild-type counterparts to achieve fixation. We find that Apc-mutant cells are enriched for transcripts encoding several secreted Wnt antagonists, with Notum being the most highly expressed. Indeed, conditioned medium from Apc-mutant cells suppresses the growth of wild-type organoids in a Notum-dependent manner. Furthermore, Notum-secreting mutant clones actively inhibit the proliferation of surrounding wild-type crypt cells and drive their differentiation, thereby outcompeting them from the niche. Importantly, genetic or pharmacological inhibition of Notum is sufficient to abrogate the expansion of Apcmutant cells and their ability to form intestinal adenomas. Taken together, we demonstrate Notum as a key mediator during the early stages of mutation fixation, which can be targeted to restore wild-type cell competition and thus, offer novel preventative strategies for high-risk patients. MainThe colonic epithelium displays one of the highest mutation rates of all tissues 3,4 , with lossof-function mutations in the APC tumour suppressor considered a key early event in colorectal cancer (CRC) initiation 5 . For a mutation to be maintained within a crypt, it needs to become "fixed", by mutant cells outcompeting wild-type intestinal stem cells (ISC) from the crypt 6,7 .Previous studies revealed that Apc loss (or Kras activation) confer a clonal advantage to ISCs 7,8, increasing their probability of fixation/winning within the crypt and, in the case of Apc mutation, driving adenoma formation. Even though APC-deficient clones have an increased probability of "winning", they can still be stochastically eliminated from the ISC pool i.e. lose.This suggests uncovering the molecular mechanisms by which APC-deficient cells outcompete wild-type cells could lead to novel chemo-preventative approaches.APC is a negative regulator of Wnt signalling that functions as an integral part of the destruction complex, which directs the phosphorylation and degradation of β-catenin 9 . Since Apc-mutant tumours exhibit constitutive Wnt-pathway activation, we first sought to identify genes differentially upregulated in Apc-mutant cells relative to the normal intestinal epithelium.For this, we performed transcriptomic analysis of tumours that develop in VillinCre ER ;Apc fl/+ (hereafter VilCre ER ;Apc fl/+ ) mice following the sporadic loss of the remaining copy of Apc 10 , akin to human CRC 11 . As expected, Wnt-target genes were highly upregulated in these Apcmutant tumours (Extended Data Fig. 1a). The most highly upregulated gene was Notum (Fig. 1a), which encodes a secreted WNT...

A subset of patients with gastric cancer have mutations in genes that participate in or regulate Wnt signaling at the level of ligand (Wnt) receptor (Fzd) binding. Moreover, increased Fzd expression is associated with poor clinical outcome. Despite these findings, there are no in vivo studies investigating the potential of targeting Wnt receptors for treating gastric cancer, and the specific Wnt receptor transmitting oncogenic Wnt signaling in gastric cancer is unknown. Here, we use inhibitors of Wnt/Fzd (OMP-18R5/vantictumab) and conditional gene deletion to test the therapeutic potential of targeting Wnt signaling in preclinical models of intestinal-type gastric cancer and ex vivo organoid cultures. Pharmacologic targeting of Fzd inhibited the growth of gastric adenomas in vivo. We identified Fzd7 to be the predominant Wnt receptor responsible for transmitting Wnt signaling in human gastric cancer cells and mouse models of gastric cancer, whereby Fzd7-deficient cells were retained in gastric adenomas but were unable to respond to Wnt signals and consequently failed to proliferate. Genetic deletion of Fzd7 or treatment with vantictumab was sufficient to inhibit the growth of gastric adenomas with or without mutations to Apc. Vantictumab is currently in phase Ib clinical trials for advanced pancreatic, lung, and breast cancer. Our data extend the scope of patients that may benefit from this therapeutic approach as we demonstrate that this drug will be effective in treating patients with gastric cancer regardless of APC mutation status. Significance: The Wnt receptor Fzd7 plays an essential role in gastric tumorigenesis irrespective of Apc mutation status, therefore targeting Wnt/Fzd7 may be of therapeutic benefit to patients with gastric cancer.