The genetic puzzle of FAP: exploring novel diagnostic approaches for APC/MUTYH-negative case

Multiple polyposis syndromes include Familial adenomatous polyposis (FAP), Peutz-Jeghers syndrome (PJS), Juvenile polyposis syndrome (JPS), PTEN hamartoma tumor syndrome (PHTS), MUTYH-associated polyposis (MAP), NTHL1-associated polyposis (NAP), Polymerase proofreading-associated polyposis (PPAP), and MBD4-associated polyposis. Common to these syndromes is the presence of polyps in the large intestine and very high risk of developing colorectal cancer (CRC), which can reach up to 100% in the case of FAP. The development of FAP is associated with pathogenic variants of the APC gene. However, pathogenic variants are not always detected in patients with FAP, which poses a significant clinical challenge for both patients and their families, who may be at increased risk for developing the disease. A second strong predisposition to CRC is MAP, characterized by biallelic pathogenic variants in the MUTYH gene, with a phenotype similar to FAP. This mini review focuses on potential approaches to improve the diagnosis of patients in whom pathogenic variants in the APC and MUTYH genes are not detected by routine testing.