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Different Genetic Features Associated with Colon and Rectal Carcinogenesis

¹ Department of Experimental Oncology and Unit of Experimental Molecular Pathology, Department of Pathology, ² Unit of Experimental Molecular Pathology, Department of Pathology, ³ Department of Experimental Oncology, ⁴ Colorectal Surgery Unit and ⁵ Preventive-Predictive Medicine Unit, ⁶ Istituto Nazionale per lo Studio e la Cura dei Tumori, and Federazione Italiana Ricerca Cancro, Institute of Molecular Oncology, Milan, Italy

ABSTRACT

Purpose: The issue of whether colon and rectal cancer should be considered as a single entity or two distinct entities is still debated, and there is a need to improve studies addressing the heterogeneity of the pathogenetic pathway leading to sporadic colorectal cancers (SCRCs) as well as to identify biological and/or molecular differences between colon and rectal cancers.

Experimental Design: Specimens of SCRCs were analyzed for somatic mutations in APC, K-ras, and TP53 genes and loss-of-heterozygosity of chromosome 18.

Results: Eleven SCRCs showed microsatellite instability. APC mutation frequency was significantly lower in microsatellite instability (MIN+) than in MIN– SCRCs. All MIN– SCRCs showed ß-catenin overexpression. A combined analysis of the biomarkers revealed two pathways mainly represented by MIN– SCRCs and differently followed on the basis of tumor location, APC-K-ras-TP53-Ch18q and APC-TP53-Ch18q.

Conclusions: The APC-ß-catenin pathway is inactivated in MIN– SCRCs and represents the first hit of SCRC development. Two preferential pathways followed by SCRCs occur, one K-ras dependent, in agreement with the Fearon and Vogelstein model, and the other K-ras independent. Significant differences between colon and rectal tumors occur in our series of MIN– SCRCs. The different pathways observed and their distribution can be summarized as follows: (a) K-ras mutations were more commonly detected in colon than in rectum; (b) the number of mutations detected was significantly higher in colon than in rectal tumors; and (c) a mutational pattern restricted to the APC gene was more common in rectal than in colon tumors. This molecular characterization can be translated into a clinical setting to improve diagnosis and to direct a rationale pharmacological treatment.

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