Germline pathogenic variants in RNF43 in patients with and without serrated polyposis syndrome.

Serrated Polyposis Syndrome (SPS) is characterized by multiple and/or large serrated polyps in the colon and an increased risk of colorectal cancer (CRC). The etiology is largely unknown, but in a subset of patients with SPS, monoallelic pathogenic variants in RNF43 are detected. To date, however, the penetrance and phenotypic spectrum of patients carrying pathogenic variants (PV) in RNF43 are poorly described. We present eight patients both with and without serrated polyps from four unrelated families with likely pathogenic variants (LPV) in RNF43 and compare the results to current literature. The patients were referred to genetic counseling due to suspicion of hereditary cancer. They underwent genetic testing with custom NGS gene panels including RNF43 as part of a routine genetic work-up. Three LPVs, one multi-exon deletion and two nonsense variants, were detected in four families. Family I had a history of CRC and serrated polyps, but in the three other families (II‒IV) there was no history of CRC or serrated polyps. Colonoscopies in the probands of these families did not reveal any serrated polyps and/or CRC despite some of them being relatively old. Our findings suggest that the penetrance of RNF43-related disease is much lower than previously thought, and raise questions about the connection between RNF43 and disease. The results highlight the complexity of genetic counseling in RNF43 positive families- particularly in families without polyposis. Further research is needed to elucidate the role of RNF43 in the risk of SPS and CRC.