Colorectal cancers show distinct mutation spectra in members of the canonical WNT signaling pathway according to their anatomical location and type of genetic instability.

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.

A missense mutation in the APC tumor suppressor gene disrupts an ASF/SF2 splicing enhancer motif and causes pathogenic skipping of exon 14.

A missense mutation at codon 640 in the APC gene was identified in a familial adenomatous polyposis (FAP) patient, however, its pathological consequence remained unclear. Here we found that this missense mutation interferes at the nucleotide level with an exonic splicing regulatory element and leads to aberrant splicing of the mutant APC transcript rather than exerting its effect through the observed amino acid change. Analysis of the patient RNA revealed complete skipping of exon 14 in transcripts from the mutant APC allele, leading to a frameshift and a premature stop codon. When cloned into a splicing reporter minigene and transfected into colorectal cell lines, the exon 14 point mutation c.1918C>G (pR640G) was found sufficient to cause the observed exon skipping. Bioinformatic analysis predicted the mutation to change SRp55, hnRNP A1 or ASF/SF2 splicing factor binding sites. Using RNA interference methodology these predictions were experimentally validated and revealed that only ASF/SF2 was required for exon 14 inclusion. These research data identify APC mutation c.1918C>G (pR640G) as pathogenic and indicate a mechanism involving disruption of an ASF/SF2 exonic splicing enhancer element. The results allow genetic diagnosis of a hereditary tumour predisposition but also illustrate the need to complement in silico prediction by splicing reporter assays.