Comparison of two cases of Familial Adenomatous Polyposis with the same APC genotype and different phenotypes.

Familial adenomatous polyposis (FAP) is a colorectal cancer (CRC) predisposition syndrome characterized by the presence of numerous colorectal adenomatous polyps, resulting from a single germline, heterozygous, likely pathogenic/pathogenic (LP/P) variant in the APC gene, an important tumor suppressor encoding gene. Classic FAP is considered in individuals with a germline LP/P variant in APC and have ≥100 colorectal adenomatous polyps beginning on average in adolescence, while attenuated FAP typically presents with fewer colorectal adenomatous polyps (10-<100 polyps) in adulthood. Both forms can feature extracolonic manifestations, such as desmoid tumors, thyroid cancer, and osteomas. Reported genotype-phenotype correlations in FAP provide valuable insights for healthcare providers, but variable expressivity persists even among individuals sharing a genotype. In this case study, we report two patients with the same pathogenic APC variant [c.4348C>T, p.Arg1450Ter] and different presentations and clinical findings. Case 1 describes a 21-year-old male with an extensive family history of cancer who was diagnosed with FAP at age 4. Case 2 describes a 22-year-old female with no family history who was diagnosed with FAP after she initially presented with a desmoid tumor. Despite genetic similarities, their clinical courses significantly differed, emphasizing the variable expressivity of FAP. These cases highlight the importance of individual management and surveillance, as genotype alone may not predict clinical outcomes. They also underscore the need for further research into factors that influence FAP expressivity.