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Flat Colorectal Cancers Are Genetically Determined and Progress to Invasion without Going through a Polypoid Stage

¹ Curriculum in Genetics and Molecular Biology, Departments of ² Genetics and ³ Medicine, Division of Gastroenterology and Hepatology, ⁴ Center for Gastrointestinal Biology and Disease, ⁵ Department of Pathology and Laboratory Medicine, ⁶ Curriculum in Toxicology, and ⁷ Lineberger Cancer Center, University of North Carolina, Chapel Hill, North Carolina

Requests for reprints: David Threadgill, Department of Genetics, CB#7264, 111 Mason Farm Road, University of North Carolina, Chapel Hill, NC 27599. Phone: 919-843-6472; Fax: 919-966-3292; E-mail: dwt{at}med.unc.edu.

Growing evidence suggests that flat colorectal cancers (CRC) account for 10% to 20% of all CRCs and that these are frequently associated with more advanced pathologies. However, controversy exists as to the origin and progression of flat CRCs compared with the more common polypoid-type morphology. We report using the azoxymethane mouse model for human CRC that KK/HIJ and I/LNJ mice develop different frequencies of flat and polypoid tumors; 83% of colon tumors in I/LNJ mice are flat compared with only 19% in KK/HIJ mice, indicating a strong genetic predisposition to the development of specific CRC morphologies. Like polypoid tumors, all flat tumors show a significant increase in the level of nuclear β-catenin (CATNNB1), supported by similar frequencies of mutations in the phosphorylation domain–coding region (codons 32–41) of Catnnb1. However, in contrast to previous reports, tumors bearing higher "oncogenic potential" do not cluster in codon 41 of Catnnb1. There are no differences between flat and polypoid tumors in the frequency of mutations in codons 12 and 13 of Kras or codon 624 of Braf. Similarly, there are no differences between tumor morphologies in their location along the proximal-to-distal colonic axis or in the relative quantity of intratumor stromal myofibrobasts as marked by the expression of -smooth muscle actin. Using a combination of serial colonoscopic and histologic analyses, we definitively show that flat CRCs do not develop de novo but progress through a flat adenomatous stage to invasive carcinoma without transit through an intermediary polypoid stage. [Cancer Res 2007;67(24):11594–600]