Frequent frameshift mutations in 2 mononucleotide repeats of RNF43 gene and its regional heterogeneity in gastric and colorectal cancers

RNF43, an E3 ligase, inhibits Wnt signaling by removing Wnt receptors and behaves as a candidate tumor suppressor. Recent studies identified that RNF43 gene was frequently mutated in gastric (GC), colorectal (CRC), and endometrial cancers with high microsatellite instability (MSI-H). The aim of this study is to explore whether RNF43 gene is mutated in GC and CRC in Korean patients and whether the mutations show regional intratumoral heterogeneity (ITH). We analyzed 2 exonic repeats (C6 and G7) of RNF43 in 78 GCs and 130 CRCs by single-strand conformation polymorphism and DNA sequencing analyses. Also, we analyzed regional ITH of RNF43 mutation in 16 CRCs. We found RNF43 frameshift mutation in MSI-H (50/118), the incidence of which was significantly higher than that in microsatellite stable/low microsatellite instability (1/90). GCs showed a significantly higher incidence of the mutation than CRCs (66.7% of GC and 32.9% of CRC with MSI-H). Also, we found that all of the 7 CRCs with the mutations harbored mutational ITH. By immunohistochemistry, we observed that loss of RNF43 expression was significantly more common in those with RNF43 frameshift mutation than those with wild-type RNF43. Our data indicate that RNF43 gene harbored not only exceedingly high mutations but also mutational ITH, which together might play a role in tumorigenesis of GC and CRC. We suggest that regional analysis is required for a more comprehensive evaluation of the mutation status in these tumors.