Screening for colorectal cancer

Estrogen and colon cancer: current issues

Carcinoma of the colon is common and its incidence varies according to the geographical location and dietary habits. The aims of this paper are, first, to review the current epidemiological data on the incidence and mortality of colon cancer in postmenopausal women using hormone replacement therapy (HRT); second, to review the published data on the prevalence of estrogen receptors in healthy and malignant colonic tissue; and third, to examine the available evidence of gene silencing as applicable to this and other neoplastic conditions. Estrogen use confers overall protection, with a reduction in the incidence of colon adenoma and carcinoma of about 30%. Estrogen use reduces the colon cancer-related mortality. The risk of colon cancer is decreased among current and recent users of postmenopausal HRT but the molecular mechanisms involved remain unclear. Estrogen acts either on a single major transformation step in the oncogenetic process, or is involved in multiple events that avert the course of this transformation. Aberrant methylation of the CpG islands in the promoter regions of the estrogen receptor gene, as well as of other genes, is equivalent to the silencing of that gene, with the consequence of inactivation, or reduced expression, of a number of genes downstream, including tumor suppressor genes. This epigenetic mechanism, when reversed, suppresses the growth of cancer cells in vitro and in vivo. The ubiquitous distribution of the estrogen receptor genes and their isoforms, in a tissue-specific manner, opens new avenues for the understanding of cellular behavior in health and disease.

Risk of colorectal cancer after breast cancer

BACKGROUND

History of breast cancer has been reported as a risk factor for colorectal cancer in women. In view of the ambiguous nature of existing evidence and the growing interest in targeted colorectal cancer prevention, we sought to quantify this risk.

METHODS

We used the Surveillance Epidemiology and End Results (SEER) database to estimate risk of colorectal cancer after breast-cancer diagnosis in women with first incident breast cancer between 1974 and 1995. Observed colon and rectal cancer risk was compared with that expected in the general population. We stratified comparisons by age at breast-cancer diagnosis, stage of cancer, ethnic origin of patient, and follow-up time.

FINDINGS

Overall, women with previous breast cancer were 5% less likely (95% CI 1-9) to develop colon and 13% less likely (6-19) to develop rectal cancer than women in the general population. Stratified analyses suggested that the risk reductions observed for colon and rectal cancer were most pronounced for women with breast cancer diagnosed after age 65 years, in white women, women with local stage breast cancer, and women diagnosed in the later study years (1990-94).

INTERPRETATIONS

Breast cancer does not increase subsequent colorectal cancer risk, and reduced risk was seen for certain subgroups of women. Because no biologically plausible endogenous protective factor has been identified, we suggest that reduced risk could stem from an accumulation of exposures that increase breast-cancer frequency but protect against colorectal cancer.

Ribozyme-mediated inactivation of mutant K-ras oncogene in a colon cancer cell line

Mutation of c-K-ras oncogene is an important step in progression of colon cancer. We used a hammerhead ribozyme (KrasRz) against mutated K-ras gene transcripts (codon 12, GTT) to inactivate mutant K-ras function in the colon cancer cell line SW480, harbouring a mutant K-ras gene. The beta-actin promoter-driven KrasRz sequence (pHbeta/KrasRz) was introduced into these cells (SW480/KrasRz), and we evaluated its effects on growth of the colon cancer. The gene expression of angiogenesis-related molecules (vascular endothelial growth factor and thrombospondin) was also estimated in SW480/KrasRz. KrasRz specifically and efficiently cleaved the mutant K-ras mRNA but not wild-type mRNA in vitro. SW480/KrasRz showed decreased growth rate under tissue culture conditions (P< 0.01, Dunnett's test). The xenotransplantability of SW480/KrasRz (XeSW480/KrasRz) was significantly decreased in nude mice (P< 0.05, Fisher's exact test). Tumour volume of the xenografts XeSW480/KrasRz was significantly smaller than that of XeSW480/DisKrasRz (P< 0.01, Dunnett's test). Gene expression of VEGF was suppressed in SW480/KrasRz, while TSP1 gene expression was enhanced. The SW480/KrasRz cells showed apoptosis-related features including nuclear condensation and DNA fragmentation. These results suggested that the hammerhead ribozyme-mediated inactivation of the mutated K-ras mRNA induced growth suppression, apoptosis and alteration of angiogenic factor expression.