Colorectal cancer represents the second leading cause of cancer-related death in the United States (1
). Its development is due to the accumulation of mutations of a number of tumor suppressor genes and oncogenes, as well as DNA repair genes controlling genomic stability (2
). Patients with Familial Adenomatous Polyposis (FAP), a hereditary disease characterized by the presence of multiple polyps in the colon, carry germline mutations in one allele of the adenomatous polyposis coli (APC
) tumor suppressor gene. Colon cancer in FAP patients requires at least one additional somatic genetic event at the other APC
allele in such individuals, consistent with the “two-hit” hypothesis for cancer arising from defective tumor suppressor genes that are recessive at the cellular level (3
). The APC
gene is also mutated in most cases of sporadic colorectal cancer.
Alterations in the gene expression pattern associated with single-hit mutations of the APC
tumor suppressor gene are likely to represent the earliest molecular changes during colon carcinogenesis. The idea that there might be “one-hit” effects of APC
mutation in heterozygous cells was supported years ago by Kopelovich et al. (4
) who observed profound genetic alterations in morphologically normal skin fibroblasts that were derived from FAP patients, including increased sensitivity to transformation by the KiMSV and SV40 viruses, altered cytoskeletal actin distribution and increased expression of p53. Danes et al. reported increased tetraploidy in morphologically normal colonic epithelial cells derived from FAP patients of the Gardner syndrome variant (6
). Both of these reports indicated “one-hit” effects. These observations have recently been extended using RNA expression profiles in morphologically normal renal epithelial cells that were derived from patients affected with tuberous sclerosis complex and von Hippel-Lindau syndrome (7
). Some or all of these early changes may have a direct bearing on subsequent tumor induction, since similar expression profiles have been identified in the corresponding cancer cells (4
). Thus, even a small growth advantage could increase the number of one-hit cells available for conversion to two-hit tumor cells that represent the paradigm for adenoma formation providing some selective advantage during colorectal cancer development.
Proteomics provides a direct approach to study thousands of proteins in a given tissue at the same time, facilitating the determination of critical pathways during cancer development. We have chosen to study the colonic crypt proteome by two-dimensional (2D) gel electrophoresis, since this approach also provides global quantitative protein isoform information (8
). We demonstrated that, compared with the proteome profile of morphologically normal crypts sampled at about 10 to 20 cm from the tumor of sporadic colorectal cancer patients (9
), the morphologically normal FAP colonic crypts exhibit an altered proteome in a gene-specific manner, consistent with detectable single-hit effect associated with heterozygosity for a mutant APC
tumor suppressor gene. These protein aberrations appear to be directly related to cancer and may represent some of the earliest known biomarkers during colorectal cancer development, potentially leading to the identification of molecular targets and agents that might inhibit or delay adenoma-carcinoma transition.