Familial Nonmedullary Thyroid Carcinoma (FNMTC) makes up to 5ââ?¬â??10% of all thyroid cancers, also including those FNMTC\noccurring as a minor component of familial cancer syndromes, such as Familial Adenomatous Polyposis (FAP).We give evidence\nthat this extracolonic manifestation of FAP is determined by the same germline mutation of the APC gene responsible for colonic\npolyps and cancer but also shows some unusual features (F :Mratio = 80 : 1, absence of LOH for APC in the thyroid tumoral tissue,\nand indolent biological behaviour, despite frequent multicentricity and lymph nodal involvement), suggesting that the APC gene\nconfers only a generic susceptibility to thyroid cancer, but perhaps other factors, namely, modifier genes, sex-related factors, or\nenvironmental factors, are also required for its phenotypic expression. This great variability is against the possibility of classifying\nall FNMTC as a single entity, not only with a unique or prevalent causative genetic factor, but also with a unique or common\nbiological behavior and a commonly dismal prognosis. A new paradigm is also suggested that could be useful (1) for a proper\nclassification of FAP associated PTC within the larger group of FNMTC and (2) for making inferences to sporadic carcinogenesis,\nbased on the lesson from FAP.
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