It is unclear whether the mutation spectra in WNT genes vary among distinct types of
colorectal tumors. We have analyzed mutations in specific WNT genes in a cohort of 52
colorectal tumors and performed a meta-analysis of previous studies. Notably, significant differences were found among the mutation spectra. We have previously shown that in
familial adenomatous polyposis, APC somatic mutations are selected to provide the "just-right" level of WNT signaling for
tumor formation. Here, we found that APC mutations encompassing at least two
beta-catenin down-regulating motifs (20 a.a. repeats) are significantly more frequent in microsatellite unstable (MSI-H) than in microsatellite stable (MSS)
tumors where truncations retaining less than two repeats are more frequent (P = 0.0009). Moreover, in cases where both APC hits are detected, selection for mutations retaining a cumulative number of two 20 a.a. repeats became apparent in MSI-H
tumors (P = 0.001). This type of mutations were also more frequent in proximal versus distal colonic
tumors, regardless of MSI status (P = 0.0008). Among MSI-H
tumors, CTNNB1 mutations were significantly more frequent in HNPCC than in sporadic lesions (28% versus 6%, P < 10-6) and were preferentially detected in the proximal colon, independently of MSI status (P = 0.017). In conclusion, the observed spectra of WNT gene mutations in
colorectal tumors are likely the result from selection of specific levels of
beta-catenin signaling, optimal for
tumor formation in the context of specific anatomical locations and forms of genetic instability. We suggest that this may underlie the preferential location of MMR deficient
tumors in the proximal colon.