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Genetics of Colorectal Cancer (PDQ®): Genetics - Health Professional Information [NCI] - Major Genetic Syndromes

Table 5. Extracolonic Tumor Risks in Familial Adenomatous Polyposis continued...

Most APC mutations that occur between codon 169 and codon 1393 result in the classic FAP phenotype.[27,28,29] There has been much interest in correlating the location of the mutation within the gene with the clinical phenotype, including the distribution of extracolonic tumors, polyposis severity, and congenital hypertrophy of the retinal pigment epithelium. The most consistent observations are that attenuated polyposis and the less classic forms of FAP are associated with mutations that occur in or before exon 4 and in the latter two-thirds of exon 15,[28] and that retinal lesions are rarely associated with mutations that occur before exon 9.[29,32] Exon 9 mutations have also been associated with attenuated polyposis. Additionally, individuals with exon 9 mutations tend not to have duodenal adenomas.[33]

Density of colonic polyposis

Researchers have found that dense carpeting of colonic polyps, a feature of classic FAP, is seen in most patients with APC mutations, particularly those mutations that occur between codons 169 and 1393. At the other end of the spectrum, sparse polyps are features of patients with mutations occurring at the extreme ends of the APC gene or in exon 9. (Refer to the Attenuated Familial Adenomatous Polyposis (AFAP) section of this summary for more information.)

Extracolonic tumors

Desmoid tumors

Desmoid tumors are proliferative, locally invasive, nonmetastasizing, fibromatous tumors in a collagen matrix. Although they do not metastasize, they can grow very aggressively and be life threatening.[34] Desmoids may occur sporadically, as part of classical FAP, or in a hereditary manner without the colon findings of FAP.[15,35] Desmoids have been associated with hereditary APC gene mutations even when not associated with typical adenomatous polyposis of the colon.[35,36]

Most studies have found that 10% of FAP patients develop desmoids, with reported ranges of 8% to 38%. The incidence varies with the means of ascertainment and the location of the mutation in the APC gene.[35,37,38]APC mutations occurring between codons 1445 and 1578 have been associated with an increased incidence of desmoid tumors in FAP patients.[32,36,39,40] Desmoid tumors with a late onset and a milder intestinal polyposis phenotype (hereditary desmoid disease) have been described in patients with mutations at codon 1924.[35]

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