Molecular changes in the Ki-ras and APC genes in colorectal adenomas and carcinomas arising in the same patient

Adenoma and carcinoma components in colonic tumors show discordance for KRAS mutation

Activating mutations in KRAS are common events in the pathogenesis of colorectal carcinoma and predict response to treatment with anti-EGFR antibodies. Molecular pathology testing for KRAS mutations has become the standard of practice for patients with metastatic colorectal carcinoma. Despite the known histologic and molecular differences between adenomas and carcinomas, the concordance of KRAS mutation between adenomas and carcinomas has not been established leaving some open questions regarding the appropriate choice of tissue for KRAS mutation analysis and correct interpretation of the test results. To address these questions, we analyzed the concordance of KRAS mutation in 70 tumors that contained both adenoma and carcinoma components (2 cases of intramucosal carcinoma, 66 cases with invasion of the submucosa, and 2 invading the muscularis propria). For each case, DNA was separately isolated from the adenoma and the carcinoma component and analyzed for KRAS mutation using direct sequencing. Overall, 30 (43%) of the adenoma cases and 36 (51%) of the carcinoma cases were positive for KRAS mutation. Of the 70 cases, 16 (23%) showed discordant results. Interestingly, the fraction of discordant cases went down as the depth of carcinoma invasion increased. In summary, we identified significant KRAS mutation discordance between the adenoma and carcinoma component of the lesion. Our results suggest that effort should be made to analyze only the invasive component of the lesion and that caution should be taken when interpreting a result based on DNA extracted from noninvasive elements.

Ki-ras gene mutations are invariably present in low-grade mucinous tumors of the vermiform appendix

OBJECTIVE

Low-grade mucinous tumors of the appendix appear to have a simple histological structure. Paradoxically, reports have suggested a greater frequency of Ki-ras gene mutation in these lesions than in more complex lesions such as benign colonic adenomas and carcinomas. We assessed several molecular genetic changes, including Ki-ras gene mutations, in a large series of low-grade mucinous tumors of the appendix.

MATERIAL AND METHODS

We retrospectively ascertained low-grade mucinous tumors of the appendix from computerized pathology records. Extracted DNA was analyzed for APC and DCC gene loss of heterozygosity, microsatellite instability and for the presence of Ki-ras gene mutation using standard molecular techniques. Controls consisted of normal appendices, other appendiceal neoplasms, and ovarian mucinous cystadenomas.

RESULTS

A total of 31 low-grade appendiceal mucinous tumors were identified. All were microsatellite stable and none demonstrated loss of heterozygosity for the APC or DCC genes. By contrast, all 31 lesions contained a Ki-ras gene mutation.

CONCLUSIONS

The presence of a Ki-ras gene mutation in all lesions, with no other molecular changes identified, strongly suggests a possible etiological role of the Ki-ras mutation in the development of this particular lesion of the appendix. Based on other work regarding intestinal bacteria, we hypothesize a relationship between chronic inflammation of the appendix from bacterial overgrowth and Ki-ras gene mutation.

Adequacy of Colonoscopic Biopsy Specimens for Molecular Analysis: A Comparative Study With Colectomy Tissue

Molecular analyses of tumors are increasingly useful for prognosis and for guiding therapy. Colonoscopic biopsy provides the first source of tissue for most cases of colorectal carcinoma and therefore might become an important source for molecular analyses. We have addressed the question whether molecular analyses of colonoscopic biopsy yield results similar to the findings from the surgical specimen. Further, we analyzed 2 separate areas of the colectomy specimen to assess tumor heterogeneity. We evaluated 3 samples from each of 67 patients for point mutations in the KRAS gene, loss of heterozygosity (LOH) at the Adenomatous Polyposis Coli (APC) and Deleted in Colon Cancer (DCC) genes and for microsatellite instability (MSI) using polymerase chain reaction based techniques. The average time interval between biopsy and surgery was 2.2+/-0.15 weeks. Lesions were from all colon segments and all surgical stages. The degree of agreement between the biopsy and surgical sites was high for APC LOH, MSI, and KRAS mutations (kappa=0.85, 1.00, and 0.93, respectively) but less so for DCC LOH (kappa=0.62). Colonoscopic biopsies are an acceptable source of neoplastic DNA for studies of KRAS, APC LOH, and MSI, but less so for DCC LOH, primarily resulting from technical considerations.

Clinical and genetic findings in an Ashkenazi Jewish population with colorectal neoplasms

BACKGROUND

The authors evaluated the frequency of the carrier status of three ancestral colorectal neoplasm-associated mutations (APC:I1307K, BLM(Ash), and MSH2*1906G>C) found in the Jewish population among a case series with documented colorectal neoplasms. They further compared family and personal histories plus environmental exposures of the carriers and noncarriers of the I1307K mutation and examined clinical differences with regard to the colorectal neoplasms and the specific molecular genetic changes in these lesions.

METHODS

Analyses were performed on tissue from stored paraffin-embedded blocks for the three germline mutations plus the KRAS mutation and APC loss of heterozygosity (LOH) and APC gene sequencing.

RESULTS

Fifty-four of the 429 individuals (12.6%) were found to carry the APC:I1307K mutation, whereas 4 (0.9%) were found to be heterozygous for the BLM(Ash) mutation and 3 (0.7%) were carriers of the MSH21906G>C* mutation. Carriers of the I1307K mutation did not appear to differ from noncarriers with regard to the number of neoplasms, patient age at detection, or tumor location within the colon. There was no significant difference noted between I1307K carriers and noncarriers with regard to the percentage of patients with first-degree relatives with colorectal carcinoma. A significant risk for APC LOH was found in lesions from carriers who smoked cigarettes compared with nonsmokers. The I1307K mutation was found to be clearly associated with a somatic additional adenine insertion in the region of codons 1306-1309, but other mutations in the region of codons 1277-1348 were found to be no more prevalent in carriers than in noncarriers.

CONCLUSIONS

In Jewish individuals previously diagnosed with a colorectal neoplasm, MSH2*1906G>C is uncommon but has been associated with carcinoma occurring at a young age. The BLM(Ash) mutation is uncommon and appears to be of little effect. The I1307K mutation is common among Jews who have had colorectal neoplasms, but overall it was found to have little effect clinically in the current study group. There may be a gene-environment interaction between the I1307K mutation and cigarette use.

Comparative molecular pathology of sporadic hyperplastic polyps and neoplastic lesions from the same individual

AIM

The biology of colorectal hyperplastic polyps is of considerable relevance, because recent evidence suggests that under certain circumstances hyperplastic polyps may be precursors of neoplasms. The aim of this study was to assess and compare the clinical and molecular characteristics of hyperplastic polyps and neoplastic lesions removed from patients without the hyperplastic polyposis syndrome.

METHODS

One hundred and twenty six patients were identified through a series of genetic epidemiological studies. Each patient had at least one neoplastic lesion and one hyperplastic polyp; there was a total of 147 hyperplastic polyps. All lesions were evaluated for K-ras mutations, loss of heterozygosity (LOH) of the adenomatous polyposis coli (APC) gene, and microsatellite instability.

RESULTS

K-ras mutation was detected in 15 (10%) hyperplastic polyps, all from the rectosigmoid colon. No hyperplastic polyp had APC LOH or microsatellite instability. Patients with adenomas or carcinomas showing K-ras mutations were not more likely to have hyperplastic polyps with K-ras mutations. The average number of adenomas did not differ between those patients with hyperplastic polyps with K-ras mutations and those without K-ras mutations. There was no association between the hyperplastic polyp and the adenoma regarding the colon segments from which the two lesions were removed.

CONCLUSIONS

The sporadic hyperplastic polyp is a lesion with limited molecular change and no relation to patients' neoplastic lesions.

Ki-ras gene mutations, LOH of the APC and DCC genes, and microsatellite instability in primary colorectal carcinoma are not associated with micrometastases in pericolonic lymph nodes or with patients' survival

AIMS

The primary aim of this study was to look for possible correlations between molecular genetic changes in primary colorectal cancer and the presence or absence of micrometastases in the accompanying pericolonic lymph nodes. The secondary aim was to correlate the data on these molecular genetic changes and micrometastases with survival.

METHODS

One hundred and twenty five Dukes's stage B colorectal cancers from 1989 to 1992 were analysed. The primary tumours were evaluated for Ki-ras mutation, adenomatous polyposis coli (APC) loss of heterozygosity (LOH), deleted in colon cancer (DCC) LOH, and microsatellite instability using standard molecular techniques. All available lymph nodes were immunohistochemically stained for micrometastases.

RESULTS

Micrometastases were present in 41% of patients. There were significantly more lymph nodes removed in the patients with micrometastases. Micrometastases were not associated with Ki-ras mutation, APC LOH, DCC LOH, or microsatellite instability, even when controlling for the number of lymph nodes removed. None of the molecular variables considered had a significant impact on either overall survival or on death with disease.

CONCLUSIONS

There are insufficient data to justify using molecular genetic changes in primary colorectal carcinomas as prognostic markers. Micrometastases do not provide prognostic information on survival. There is value in increasing the numbers of lymph nodes removed and analysed along with the primary tumour.

Influence of age on adenomatous polyposis coli and p53 mutation frequency in sporadic colorectal cancer?rarity of co-occurrence of mutations in APC, K-ras, and p53 genes

INTRODUCTION

Previous studies have investigated the possibility that specific mutations may be related to specific clinicopathological features. However, most previous investigations included only an average age (40-80 years) group of sporadic colorectal cancers and, moreover, studied only a single gene in isolation. Therefore, the influence of age on these mutation frequencies remains unclear, despite age being considered a risk factor for genetic mutation.

MATERIALS AND METHODS

This study included 122 sporadic colorectal cancers from three different age groups and analyzed mutation frequencies of adenomatous polyposis coli (APC), K-ras, and p53 genes and microsatellite instability to determine their mutation frequencies and relationships with clinicopathological features.

RESULTS

Significantly lower p53 mutation frequencies were observed among young (32 years old or younger) and old (86 years old or older) patient groups compared with an average age (39-85 years old) patient group (14.3% and 19.2% versus 51.5%, P<0.001). APC mutation frequency (11.8%) was significantly lower in highly aggressive (Dukes' stage D) tumors ( P=0.003) than in the other stage tumors (Dukes' stage A, B, and C). Additionally, simultaneous occurrence of all three genetic alterations in an individual tumor was rare (below 5%). Statistical analysis further confirmed that mutation number in Dukes' D tumors occurred less frequently than expected in other stage tumors ( P=0.008).

CONCLUSIONS

Genetic alterations of sporadic colorectal cancers have different relationships with age or tumor stage. Additionally, most sporadic colorectal tumors do not necessarily require following the widely accepted genetic model, because the three key genetic mutations, APC, K-ras, and p53, rarely occur simultaneously.

The Adenomatous Polyposis Coli (APC) gene microsatellite marker D5S1385 is equally informative for loss of heterozygosity as the marker D5S346

Molecular analyses for loss of heterozygosity (LOH) at a gene locus are used to identify genomic imbalance in tumor tissue. A frequently utilized microsatellite marker for the Adenomatous Polyposis Coli (APC) gene is denoted D5S346. However, when an individual has two identical forms of this microsatellite, then a second microsatellite, also near the APC gene, is needed. We present data on an APC microsatellite marker designated D5S1385, located on the 5' end of the APC gene, and we compare these to data utilizing the marker D5S346. Polymerase chain reaction (PCR) was used, followed by gel electrophoresis. Homozygosity for the marker D5S1385 was present in 55 individuals, or 28%. Thirty-seven carcinomas and 32 adenomas were assessed for LOH from individuals informative with both microsatellite markers. D5S1385 detected LOH in 39 of 41 lesions, or 95%, for which D5S346 detected LOH. D5S1385 is moderately polymorphic and is informative at least as often as D5S346.

Distribution of genetic variants in preneoplastic areas of colorectal tumours

AIMS

Loss of heterozygosity (LOH) may vary almost randomly within a colorectal tumour due to the heterogeneous morphologic character of these tumours. Despite this, as a rule, single biopsies are the source of genetic material used in studies of markers important for prognosis, clinical behaviour of the disease, or susceptibility of specific tumours to different treatment modalities.

METHODS

To evaluate the importance of intratumoural variation for the results of analysis of LOH and point mutations in colorectal cancer and to determine the frequency of genetic alterations in different types of pre-neoplastic areas of the tumours, 36 consecutively operated patients with colorectal cancer were studied. After fixation, specimens were mounted on large slides containing the whole tumour. The specimens were sub classified into different areas defined as normal tissue, normal tissue closely adjacent the tumour mass, adenoma, dysplasia and invasive cancer cells. These areas were dissected and subjected to DNA extraction.

RESULTS

The extracted genomic DNA was studied for LOH at chromosome 5q, 17p, and 18q and for k-ras mutations. Overall, a correlation between the intratumoural degree of neoplastic progression and the frequency of LOH and k-ras mutations was seen. These correlations were significant (p<0.008) except for dysplasia/adenomatous tissue versus invasive cancer. Microsatellite instability was found in 9% of the tumours, all except one in invasive parts of the tumours.

CONCLUSIONS

This study demonstrates a statistical correlation between intratumoural differences in neoplastic degree of dedifferentiation and genetic instability in terms of LOH and point mutations of the k-ras gene in colorectal carcinoma. The importance of a careful dissection in order to localise the region with the highest probability of genetic aberrations and multiple biopsing must not be neglected. The observation that the prevalence of k-ras mutations and LOH are correlated to the degree of dedifferentiation within a colorectal tumour is in line with the concept that selected cell clones are responsible for the neoplastic progression of the tumour.

Molecular changes in the Ki-ras and APC genes in primary colorectal carcinoma and synchronous metastases compared with the findings in accompanying adenomas

AIMS

To compare the molecular genetic changes in the Ki-ras and adenomatous polyposis coli (APC) genes between colorectal carcinomas and synchronous metastases, and then to compare and contrast those changes with previously reported changes in the two genes between these carcinomas and accompanying adenomas. This expanded comparison would provide greater understanding of the progression of molecular changes in neoplastic tissue during the development of malignancy from a benign adenoma to carcinoma and then to metastatic spread of the malignancy.

METHODS

DNA was extracted from paraffin wax embedded tissue. This was followed by polymerase chain reaction and gel electrophoresis for mutations in the Ki-ras gene using single stranded conformational polymorphism analysis. Amplification of a CA repeat marker was used to assess loss of heterozygosity (LOH) at the APC gene.

RESULTS

The findings for the Ki-ras gene in 42 paired carcinomas and synchronous metastases were identical, regardless of whether or not the carcinoma and its companion adenoma had identical Ki-ras findings. The results of APC LOH for 39 paired carcinomas and synchronous metastases were also identical, whether or not the carcinoma and its companion adenoma had identical APC LOH findings. Results were uninformative for three pairs.

CONCLUSIONS

With respect to these two genes, a carcinoma may be discordant from its companion adenoma, but the metastasis remains consistent with the colonic carcinoma.

The characterization of somatic APC mutations in colonic adenomas and carcinomas in Ashkenazi Jews with the APC I1307K variant using linkage disequilibrium

Mutations of the adenomatous polyposis coli (APC) gene play a critical role in the development of colorectal neoplasms. A novel mechanism involves a germline variant, at codon 1307 of the APC gene. The mutation is thought to create an unstable segment of DNA, which facilitates the development of somatic mutations. I1307K has been shown to be ancestral in Middle Eastern populations. The aim of the present study was to confirm this observation for a Western population and to utilize this information in the characterization of the somatic changes in colorectal neoplasia in carriers of I1307K. DNA from 182 US Ashkenazim was screened for the I1307K variant, which was found in 22 (12.1%), and the ancestral nature of this variant was confirmed in this population by showing that all of those with the I1307K variant carried a specific allele at the D5S346 locus, while the majority shared a D5S1385 allele. Subsequently, 79 neoplasms were analysed from 15 I1307K carriers for loss of heterozygosity (LOH) at the D5S346 locus. LOH was detected in 18 neoplasms (23%). Of these 18, four neoplasms showed loss of the I1307K-associated allele, while 14 neoplasms had retained the I1307K-associated allele and, by implication, the I1307K variant. PCR products were also cloned into a plasmid vector to isolate individual APC alleles. For those neoplasms with LOH, 13 of the 18 neoplasms (72%) had a somatic mutation, of which 12 involved the I307K-bearing chromosome. This study is consistent with two previous studies in showing that additional somatic mutations close to codon 1307 are almost always on the I1307K-bearing chromosome, but either allele may demonstrate LOH. Further evidence for the interpretation of the action of I1307K as producing DNA instability is provided by analysing multiple neoplasms from the same person and by showing that these neoplasms have differing patterns of LOH and associated somatic mutations.

Targeted expression of oncogenic K-ras in intestinal epithelium causes spontaneous tumorigenesis in mice

BACKGROUND & AIMS

Ras oncoproteins are mutated in about 50% of human colorectal cancers, but their precise role in tumor initiation or progression is still unclear.

METHODS

This study presents transgenic mice that express K-ras(V12G), the most frequent oncogenic mutation in human tumors, under control of the murine villin promoter in epithelial cells of the large and small intestine.

RESULTS

More than 80% of the transgenic animals displayed single or multiple intestinal lesions, ranging from aberrant crypt foci (ACF) to invasive adenocarcinomas. Expression of K-ras(V12G) caused activation of the MAP kinase cascade, and the tumors were frequently characterized by deregulated cellular proliferation. Unexpectedly, we obtained no evidence of inactivating mutations of the tumor suppressor gene Apc, the "gatekeeper" in colonic epithelial proliferation. However, spontaneous mutation of the tumor-suppressor gene p53, a frequent feature in the human disease, was found in 3 of 7 tumors that were tested.

CONCLUSIONS

This animal model recapitulates the stages of tumor progression as well as a part of the genetic alterations found in human colorectal cancer. Furthermore, it indicates that activation of K-ras in concert with mutations in p53 may constitute a route to digestive tumor formation and growth, underlining the fact that the pathway to intestinal cancer is not necessarily a single road.

The future of colon cancer prevention

Chemoprevention science is in flux owing to rapid advances in postgenomic technology. We have witnessed enormous advances in the areas of early detection and molecular profiling of colorectal carcinogenesis; however, unique interpretive and technologic challenges persist. Neoplastic hallmarks must be iteratively tested and validated as markers of risk, targets for intervention, and/or markers of response in order to expedite the development of preventive interventions. In this review, we highlight several of the technologies that are revolutionizing our understanding of carcinogenesis and our approach to colorectal cancer prevention.