Association of aneuploidy in index adenomas with metachronous colorectal adenoma development and a comparison

Hydrophobic bile acids, genomic instability, Darwinian selection, and colon carcinogenesis

Sporadic colon cancer is caused predominantly by dietary factors. We have selected bile acids as a focus of this review since high levels of hydrophobic bile acids accompany a Western-style diet, and play a key role in colon carcinogenesis. We describe how bile acid-induced stresses cause cell death in susceptible cells, contribute to genomic instability in surviving cells, impose Darwinian selection on survivors and enhance initiation and progression to colon cancer. The most likely major mechanisms by which hydrophobic bile acids induce stresses on cells (DNA damage, endoplasmic reticulum stress, mitochondrial damage) are described. Persistent exposure of colon epithelial cells to hydrophobic bile acids can result in the activation of pro-survival stress-response pathways, and the modulation of numerous genes/proteins associated with chromosome maintenance and mitosis. The multiple mechanisms by which hydrophobic bile acids contribute to genomic instability are discussed, and include oxidative DNA damage, p53 and other mutations, micronuclei formation and aneuploidy. Since bile acids and oxidative stress decrease DNA repair proteins, an increase in DNA damage and increased genomic instability through this mechanism is also described. This review provides a mechanistic explanation for the important link between a Western-style diet and associated increased levels of colon cancer.

Comparison of p53 and bcl-2 expression in initial, synchronous, and metachronous colorectal adenomas

To investigate the significance of p53 and bcl-2 expression in metachronous colorectal adenomas arising in the remaining colon after carcinoma resection, we analyzed p53 and bcl-2 expression immunohistochemically in initial adenomas (type I), synchronous adenomas with concurrent carcinoma (type II), and metachronous adenomas arising after resection of initial adenomas (type III) or carcinoma (type IV). The incidence of p53 immunoreactivity in type IV adenomas with mild dysplasia was significantly higher than that in type I, type II, or type III adenomas with mild dysplasia. bcl-2 immunoreactivity was more frequently detected in type IV adenomas with mild dysplasia than in type I or type III adenomas with mild dysplasia. Coexpression of p53 and bcl-2 was detected in 16% of the type IV adenomas, this being a significantly higher frequency than that seen in the type I, type II, or type III adenomas. These results suggest that the evaluation of p53 and bcl-2 in metachronous adenomas in the remaining colon after resection of carcinoma may be a useful biologic marker for assessing the risk of cancer development.

Immunohistochemical analysis of pericryptal fibroblast sheath and proliferating epithelial cells in human colorectal adenomas and carcinomas with adenoma components

In order to examine stromal-epithelial interaction during the oncogenic progression of large bowel tumors, the association between pericryptal fibroblast sheath (PCFS) and expression of Ki-67 antigen was evaluated in 87 cases of colorectal adenoma and 95 cases of carcinoma with an adenoma component (CWA). For the immunohistochemistry, anti-alpha-smooth muscle actin antibody (alpha-SMA) and anti-Ki-67 antigen antibody (MIB-1) were used. In adenomas and adenoma components of CWA, the quantity of neoplastic glands with PCFS was reduced relative to the progression of histological atypia. Pericryptal fibroblast sheath was virtually absent from invasive carcinoma areas of CWA. Increased expression of Ki-67 correlated with the degree of histological atypia of adenomas. A significant reverse correlation was also seen between Ki-67 expression and PCFS-positive glands in adenoma components of CWA. These findings suggest that the prevalence of PCFS and Ki-67 expression are important indicators of colorectal neoplasia progression. The significant reduction of PCFS in colorectal epithelial neoplasms reflects progression in the adenoma-carcinoma sequence.

The interrelationship between tubular and papillary sectors of tubulo-villous colorectal adenomas: comparative morphometric analysis and evaluation of cell proliferation

In a series of 100 colorectal adenomas, 23 tubulo-villous adenomas were individualized through the identification of papillae as structures persisting for more than 100 microm in serial sections with a connective axis lined with epithelial cells. In these adenomas, the tubular and villous areas with the highest dysplasia were selected, and a morphometric analysis was undertaken to assess the Index of Structural Atypia, the Nucleo-Glandular Index, and the Nuclear Stratification Index. The AgNor count and the proliferating cell nuclear antigen (PCNA) Label Index (LI) also were performed. The overall mean of each of these indexes was significantly higher in the villous sector than in the tubular one (P < .001). In 16 cases, the semi-objective method of dysplasia gradation showed a superior degree in the papillary sector, whereas it showed an equal degree in the remaining seven lesions. The AgNOR count was significantly different in all cases, with higher values in villous sectors (P < .05). With the exception of one case, this was confirmed by the PCNA LI. The Stratification Index showed significantly different values in 20 cases, whereas the other morphometric indexes showed a less discriminatory result. Our findings objectively show that the degree of dysplasia in tubulo-villous adenomas should be analyzed in the villous sector. The existence of heterogeneous cellular populations has been confirmed both in the structural organization of cells and in some basic parameters such as the cell proliferation rate in colorectal adenomas. Our findings suggest that the occurrence of villous architectural growth is a secondary event in a tubular adenoma. Enhanced cellular proliferation of the villous area allows the progressive substitution of tubular structures.

K-RAS-2 gene mutations as predictors of metachronous colorectal adenomas

BACKGROUND

Mutations of K-RAS-2 gene and tumour suppressor genes have been found in both colorectal adenomas and carcinomas. The aim of this study was to investigate the prognostic value of K-RAS-2 gene mutations found in initial colorectal adenomas for predicting the risk of metachronous adenomas.

METHODS

Genomic DNA was extracted from formalin-fixed and paraffin-embedded adenomas larger than 5 mm in diameter removed at the initial total colonoscopy between 1980 and 1982. All patients underwent colonoscopic follow-up for at least 10 years. The sequence of exon 1 of the K-RAS-2 oncogene was amplified with the polymerase chain reaction technique and screened for mutation by single-strand conformation polymorphism analysis. All suspected mutations were confirmed by direct DNA sequencing. The predictive value of K-RAS-2 gene mutations for the risk of metachronous adenomas was assessed by chi-square testing and logistic regression analysis.

RESULTS

Of 54 patients 39 (72%) were male and 15 (28%) female. At the time the initial adenoma was removed, 31 (57%) patients were younger than 60, whereas 23 (43%) were 60 years or older. Point mutations of the K-RAS-2 oncogene were found in the index adenomas of 15 (27.7%) patients. Mutations were found more frequently in large (> or = 20 mm) adenomas and in adenomas with severe dysplasia (P = 0.0011 and P = 0.0310, respectively). There were no significant associations between K-RAS-2 mutations and anatomic location, histologic type, or number of synchronous initial lesions. Mutations were found predominantly at codon 12 with transversions from GGT to GTT (57%), from GGT to GAT (36%), and from GGT to TTT (one patient). The single mutation found at codon 13 showed a transversion from GGC to GAC. There were significant associations between size (> or = 20 mm) and K-RAS-2 mutation of the initial adenomas and the size (> 5 mm) of metachronous adenomas (P = 0.0259 and P = 0.0265, respectively). However, multivariate analysis showed that K-RAS-2 mutations did not provide a significant additional contribution to the prognostic value of the size of the initial adenoma (odds ratio, 7.62; 95% confidence interval (CI), 1.68-34.48) and the amount of villous structure (odds ratio, 0.22; 95% CI, 0.05-0.90) it contained.

CONCLUSIONS

Patients with large (> or = 20 mm) adenomas and adenomas with K-RAS-2 mutations found at the initial examination have a significantly higher risk of developing large (> 5 mm) metachronous adenomas during surveillance. Multivariate analysis of initial adenoma characteristics showed that the risk of metachronous colorectal adenomas can be adequately estimated by the size and the histologic type of the largest initial adenoma and that K-RAS-2 mutations are of secondary importance only. Further studies based on a larger series will have to identify the adenoma characteristics that will help to improve follow-up strategies.

Genetic heterogeneity in sporadic colorectal adenomas

The majority of colorectal cancers develop from adenomatous polyps under the influence of factors that are still poorly understood. Tumourigenesis is generally considered a multistep process in which multiple genetic alterations occur, eventually reflected in abnormalities of the cellular DNA content. Macroscopical features such as tumour size and tumour architecture (tubular, tubulovillous, or villous) are correlated wit the chance of malignancy in the lesion. Grade of dysplasia can be considered an indicator for the level of progression of the adenoma towards invasive carcinoma. These characteristics were correlated with the presence or absence of K-ras mutations and the DNA ploidy in a prospective study performed on 46 large sporadic colorectal adenomas resected by endoscopy. DNA ploidy and K-ras mutations were analysed in two samples taken at distant sites in the adenomas. Aneuploidy was present in 12 adenomas (26 per cent) and K-ras mutations occurred in 26 (57 per cent). A highly significant correlation was found between aneuploidy and adenoma size, architecture, and grade of dysplasia. The presence of K-ras mutations was significantly correlated only with the size of the adenomas. The proportion of adenomas with aneuploidy and/or a K-ras mutation increased when two samples were analysed instead of one. This observation suggests that the prevalence of genetic mutations and of aneuploidy is probably underestimated, as generally only one sample is investigated. No correlation was observed between K-ras mutations and ploidy. This study demonstrates the presence of genetic heterogeneity in colorectal adenomas and supports the notion that K-ras mutation is an early event, while aneuploidy is a late event in the adenoma-carcinoma sequence.