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Chromosomal Instability in Microsatellite-Unstable and Stable Colon Cancer

Authors' Affiliations: ¹ Comprehensive Cancer Center, Departments of ² Medicine, ³ Laboratory Medicine, and ⁴ Pathology, University of California San Francisco, San Francisco, California; and ⁵ Department of Laboratory Medicine and Pathology, Mayo Clinic College of Medicine, Rochester, Minnesota

Requests for reprints: Frederic M. Waldman, University of California San Francisco Comprehensive Cancer Center, San Francisco, CA 94143-0808. Phone: 415-476-3821; Fax: 415-476-8218; E-mail: waldman{at}cc.ucsf.edu.

Purpose: The genomic instability in colon cancer can be divided into at least two major types, microsatellite instability (MSI) or chromosomal instability (CIN). Although initially felt to be mutually exclusive, recent evidence suggests that there may be overlap between the two. The aim of this study was to identify chromosomal alterations at high resolution in sporadic colon cancers with high-level microsatellite instability (MSI-H) and to compare them to those present in a set of matched microsatellite stable (MSS) tumors.

Experimental Design: Array-based comparative genomic hybridization was used to analyze a set of 23 sporadic MSI-H and 23 MSS colon cancers matched for location, gender, stage, and age. The arrays consisted of 2,464 bacterial artificial chromosome clones.

Results: MSI and MSS colon cancers differed significantly with respect to frequency and type of chromosomal alterations. The median fraction of genome altered was lower among MSI-H tumors than MSS tumors (2.8% versus 30.7%, P = 0.00006). However, the MSI-H tumors displayed a range of genomic alterations, from the absence of detectable alterations to extensive alterations. Frequent alterations in MSI-H tumors included gains of chromosomes 8, 12, and 13, and loss of 15q14. In contrast, the most frequent alterations in MSS tumors were gains of 7, 13, 8q, and 20, and losses of 8p, 17p, and 18. A small, previously uncharacterized, genomic deletion on 16p13.2, found in 35% of MSI-H and 21% of MSS tumors, was confirmed by fluorescence in situ hybridization.

Conclusion: MSI and CIN are not mutually exclusive forms of genomic instability in sporadic colon cancer, with MSI tumors also showing varying degrees of CIN.

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