Analysis of Ki-ras gene mutations associated with DNA diploid, aneuploid, and multiploid colorectal carcinomas using a crypt isolation technique

Analysis of genetic alterations associated with DNA diploidy, aneuploidy and multiploidy in gastric cancers

OBJECTIVE

Recent studies have shown a close association between DNA ploidy status (diploidy, aneuploidy and multiploidy) identified by the crypt isolation technique and specific genetic alterations in colorectal carcinomas. However, such an association has not been elucidated for gastric tumors, even though they share common genetic features with colorectal carcinomas. In the present study, we established an association between DNA ploidy status and genetic alterations in gastric cancer.

METHOD

The DNA ploidy status of gastric tumors was classified as diploid, aneuploid or multiploid using the crypt isolation technique, which allows isolation of pure tumor crypt from tumor tissue. Crypt isolation combined with DNA cytometric sorting, polymerase chain reaction assay using 26 microsatellite markers and direct sequencing of the p53 gene were used to detect allelic imbalances [loss of heterozygosity (LOH) or allelic loss], microsatellite imbalance (MSI) and mutation of p53 in 54 gastric cancers (13 diploid, 12 aneuploid, 29 multiploid).

RESULT

Diploid tumors showed few genetic alterations, including allelic imbalances and p53 mutations. In contrast, aneuploid tumors and multiploid tumors (in particular, aneuploid populations of multiploid tumors) exhibited multiple genetic alterations, including allelic imbalances and p53 mutations. In addition, the frequencies of genetic alterations observed in the corresponding diploid fractions of multiploid tumors were relatively higher than in diploid tumors. MSI was commonly observed in diploid, aneuploid and multiploid carcinomas.

CONCLUSIONS

The present results indicate that in gastric carcinomas, diploid tumors are generally non-LOH and MSI, whereas aneuploid and multiploid tumors are associated with LOH and MSI, suggesting that the genetic profile of these carcinomas is dependent on the tumor's ploidy status.

Analysis of SMAD4/DPC4 gene alterations in multiploid colorectal carcinomas

BACKGROUND

Although recent animal studies have shown that SMAD4/DPC4 gene alterations are essential for late-stage intestinal tumorigenesis, the role of SMAD4/DPC4 gene alterations in primary human colorectal carcinomas is not fully understood. Therefore, we attempted to clarify the role of the SMAD4/DPC4 gene during tumor progression of colorectal carcinoma.

METHODS

Differences in allelic imbalance (AI) and mutations of the SMAD4/DPC4 gene between diploid and aneuploid populations were analyzed for 30 sporadic DNA multiploid colorectal carcinomas (used as a tumor progression model and defined as the coexistence of diploid and aneuploid cells within the same tumor). The crypt isolation technique was coupled with DNA cytometric sorting and a polymerase chain reaction assay. In addition, hypermethylation of the promoter region was examined to clarify whether inactivation of gene expression occurred.

RESULTS

Although a SMAD4/DPC4 gene AI was detected in only 5 of 27 informative diploid populations, 25 of 27 aneuploid populations had a SMAD4/DPC4 gene AI. Mutation of the SMAD4/DPC4 gene was detected in only one aneuploid population of multiploid colorectal carcinomas, but not in the corresponding diploid population. In total, 20 available multiploid carcinomas were selected for methylation analysis, and no evidence of hypermethylation of the promoter region was found.

CONCLUSIONS

We suggest that, although mutation of the SMAD4/DPC4 gene and hypermethylation of the promoter region are infrequent events in colorectal tumorigenesis, AI at the SMAD4/DPC4 gene locus may play a key role in the progression of colorectal carcinomas.

Adenoma, adenocarcinoma and mixed carcinoid-adenocarcinoma arising in a small lesion of the colon

A unique tumor measuring 8 x 8 x 5 mm and composed of adenoma, adenocarcinoma and mixed carcinoid-adenocarcinoma arising in the ascending colon is reported. The mixed carcinoid-adenocarcinoma, in which adenocarcinomatous and carcinoid components intermingled, originated in the mucosa, penetrated the muscularis mucosa and extended into the submucosa. Immunohistochemically, carcinoid cells were positive for neuroendocrine markers and adenocarcinoma cells were intracytoplasmicly positive for carcinoembryonic antigen. Ultrastructurally, membrane-bound electron dense granules varying in shape, size and electron density were detected in the cytoplasm of carcinoid cells. No mutations of p53 and k-ras genes were detected in adenomatous, adenocarcinomatous or mixed carcinoid-adenocarcinoma components. The morphological appearances of the present case strongly suggests the histogenesis of this tumor in an adenoma-adenocarcinoma-carcinoid tumor sequence.

Analysis of genetic alterations, classified according to their DNA ploidy pattern, in the progression of colorectal adenomas and early colorectal carcinomas

DNA aneuploidy is a biological marker of the oncogenic potential of colorectal adenomas. The accumulation of genetic alterations of cancer-related genes is also essential for colorectal carcinogenesis. However, it is unclear whether there is any relationship between these genetic alterations and the DNA ploidy of colon tumour cells in the progression of colorectal adenomas and early colorectal carcinomas. Here we have studied the DNA ploidy state and genetic alterations occurring in colorectal tumours using the crypt isolation technique. Crypts isolated from a total of 106 colorectal tumors (adenoma, 93; early carcinoma, 13) were examined using a combination of flow cytometric analysis of DNA content, polymerase chain reaction-microsatellite assay, and single-strand conformation polymorphism assay for evidence of chromosomal allelic imbalance (AI; 17p; 5q; 18q) or p53 gene mutation. In addition, we examined microsatellite instability (MSI) with BAT 26 primer sets. DNA multiploidy was infrequently detected in colorectal adenomas (15.1%), in contrast to early carcinomas (46.2%). There was a significant difference in the incidence of AI of chromosome 18q between diploid adenomas and aneuploid populations of multiploid adenomas (18.1% vs 57.1%, p = 0.0043). Mutation of p53 was also found more frequently in aneuploid populations of early multiploid colorectal carcinomas than in early diploid colorectal carcinomas (66.7% vs 0%, p = 0.021). MSI was found in only 2 of 93 adenomas, with no MSI detected in early colorectal cancers. The two MSI-positive adenomas were diploid. We subdivided multiploid adenomas into two groups: those with a low or a high DNA index (DI). The incidence of genetic alterations of high-DI adenomas did not differ from those of low-DI adenomas. Allelic imbalance involving loci on chromosome 18q and mutations of p53 seems to be associated with the progression of diploidy to multiploidy in colorectal tumours. On the other hand, MSI may be associated with the development of some diploid tumours. In addition, the incidence of genetic alterations in the colorectal adenomas that we examined appears to be independent of the tumour's DNA index.

Evolution of DNA ploidy state and DNA index in colorectal adenomas and carcinomas using the crypt isolation technique: new hypothesis in colorectal tumorigenesis

The evolution of DNA diploid, aneuploid and multiploid (diploid and aneuploid) states that represent DNA types that are independent of genetic alterations in colorectal tumors were examined. Changes in the DNA index (DI) accompanying tumor development from adenoma to carcinoma were assessed. In colorectal adenomas and early cancers, the DNA was diploid or multiploid. A pure aneuploid state was observed in advanced carcinomas only, whereas the aneuploid DI values of adenomas were characterized by two distinct peaks. The DI values for the carcinomas were randomly distributed. However, in advanced carcinomas, aneuploid carcinomas tended to have lower DI whereas aneuploid populations within multiploid carcinomas tended to have higher DI. Early cancers were subdivided into two groups: a cancer region associated with an adenomatous region (group A tumors) and a cancer region that exhibited an absence of or a very limited adenomatous region (group B tumors). Group A tumor DI were lower than group B. It is suggested that low DI adenomas might transform into group A tumors, which consequently progress to advanced aneuploid carcinomas. In addition, group B tumors might derive predominantly from high DI adenomas or from group A tumors by high DI evolution, and might progress into advanced multiploid carcinomas. Therefore, the evolution of the DNA index might play an important role in the development of colorectal tumors.

Molecular validation of the modified Vienna classification of colorectal tumors

Although the Vienna classification has been introduced to resolve discrepancies in histological diagnoses of colorectal tumors between Western and Japanese pathologists, practical applications of this classification scheme have been problematic because invasion of the lamina propria of tumor cells is often difficult to recognize. Therefore, the following refinements of the classification criteria are needed: category 3, low-grade adenoma/dysplasia; category 4, intramucosal borderline neoplasia; 4-a, high-grade adenoma/dysplasia; 4-b, well-differentiated adenocarcinoma; category 5, definite carcinoma; 5-a, intramucosal moderately-differentiated adenocarcinoma; and 5-b, submucosal carcinoma. We attempted to test whether molecular genetic alterations are related to the modified classification scheme and whether they may help to further categorize the various intramucosal neoplasia grades of colorectal tumors. Two-hundred-thirty-two colorectal tumors were examined using flow cytometric analysis of DNA content, polymerase chain reaction microsatellite assays, and single-strand conformational polymorphism assays to detect abnormalities of DNA content, chromosomal allelic loss, and Ki-ras and p53 gene mutations. Microsatellite instability (MSI) was also examined. Frequencies of genetic alterations and DNA aneuploid states increased with an increase in the grade assigned according to the modified Vienna classification. MSI was a rare event in colorectal adenomas and their frequency of MSI did not correlate with tumor grade. The combined genetic and DNA ploidy data support the conclusion that analysis of genetic alterations and DNA aneuploid states may help in appropriate categorization of colorectal tumors according to the modified Vienna scheme. In addition, MSI-positive tumors may represent a specific subtype of colorectal adenomas.

Application of the crypt isolation technique to the assessment of genetic alterations of colorectal carcinomas

The crypt isolation technique (CIT) allows for the isolation of pure tumor crypts from colon tumor tissue. In a previous study we reported on the genetic alterations found in colorectal tumor crypts using the CIT; however, a direct comparison of the genetic alterations found in colorectal carcinomas using either conventional methods (CM) or the CIT has not previously been performed. Here, we analyzed the impact of this method on the genetic analysis of colon tumor cells by comparing the observed frequency of genetic alterations in colon tumors isolated using CM or the CIT. We used a combination of the CIT and the fluorescent polymerase chain reaction assay to accurately assess the incidence of allelic imbalances (AI) at a number of chromosomal loci (17p, 5q, 18q, 1p, 8p, 22q), microsatellite instability (MSI), and mutations of cancer-related genes (p53 and APC genes) in 48 sporadic colorectal carcinomas. In addition, genetic alterations seen in multiploid tumors (defined as tumors with both diploid and aneuploid cell populations) identified by the CIT were also examined. The incidence of AI at the chromosomal loci tested was more frequently detected in samples isolated from tumors using the CIT than in those isolated from the same tumors using CM. In contrast, we observed no differences in the frequency of MSI or cancer-related gene mutation between the two groups. Although there was no difference in the frequency of genetic alterations between tumors with evidence of multiploidy, sorting of diploid and aneuploid populations allowed detection of distinct genetic changes. The crypt isolation method thus appears to be useful in that it allows purification of tumor cells and the accurate assessment of their genetic alterations. In addition, it may also be of benefit in clarifying the genetic profile of multiploid tumor cell populations.