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A predominant increase in the APC gene isoform with exon 9a in a case of attenuated familial adenomatous polyposis

To the Editor: Familial adenomatous polyposis (FAP) is an autosomal dominantly inherited cancer-predisposition syndrome classically characterized by the progressive development of more than 100 colorectal adenomatous polyps and extracolonic manifestations. The disorder is caused by a germline mutation of the adenomatous polyposis coli (APC) gene mapped at 5q21 (1). In contrast to classical FAP, attenuated FAP (AAPC) with fewer than 100 colorectal adenomatous polyps has been assigned to have germline mutations in the 5ƒ region, exon 9 and the 3ƒ region of the APC gene (2–4) An exon 9 splicedonor mutation at position 5 in intron 9 and a frameshift mutation in the alternatively spliced region of exon 9 have also been shown to be associated with late-onset FAP (5, 6). These findings suggest that an altered proportion of the products of the alternatively spliced-APC gene may lead to a phenotype of the disease. Here, we described an AAPC patient associated with a cribriform-morular variant of papillary thyroid carcinoma. Molecular analysis revealed a predominant accumulation of a transcript containing APC exon 9a. A 34-year-old female patient presented to the Kuma Hospital for a cribriform-morular variant of papillary thyroid carcinoma, which is observed frequently in FAP-associated thyroid carcinoma. The diagnosis of AAPC was based on the presence of 85 colonic adenomatous polyps. Her paternal grandmother had died of pancreas cancer. Her sister was affected by a colonic adenomatous polyp and an adenomatous goiter of the thyroid. APC molecular screening was carried out using the protein truncation test (PTT) described in the literature (7), but it failed to demonstrate a truncated mutation in each exon of the APC gene. The alterations identified by two independent PTTs were a signal of decreased intensity representing a segment of wild-type APC protein (codons 1 to 479), as well as a signal of significantly increased intensity corresponding to the smaller product of