Mutations and allelic loss of p53 in primary tumor DNA from potentially cured patients with colorectal carcinoma

Complete reduction of p53 expression by RNA interference following heterozygous knockout in porcine fibroblasts

Tumor suppressor p53 plays a critical role in the regulation of cell cycle and apoptosis in mammals. Mutations of p53 often cause various cancers. Murine models have improved our understanding on tumorigenesis associated with p53 mutations. However, mice and humans are different in many ways. For example, the short lifespans of mice limit the clinical application of the data obtained from this species. Porcine model could be an alternative as pigs share many anatomical and physiological similarities with humans. Here, we modified the expression levels of p53 messenger RNA (mRNA) and protein in porcine fetal fibroblasts using a combination of gene targeting and RNA interference. First, we disrupted the p53 gene to produce p53 knockout (KO) cells. Second, the p53 shRNA expression vector was introduced into fibroblasts to isolate p53 knockdown (KD) cells. We obtained p53 KO, KD, and KO + KD fibroblasts which involve p53 KO and KD either separately or simultaneously. The mRNA expression of p53 in p53 KO fibroblasts was similar to that in the wild-type control. However, the mRNA expression levels of p53 in KD and KO + KD cells were significantly decreased. The p53 protein level significant reduced in p53 KD. Interestingly, no p53 protein was detected in KO + KD, suggesting a complete reduction of the protein by synergistic effect of KO and KD. This study demonstrated that various expression levels of p53 in porcine fibroblasts could be achieved by gene targeting and RNA interference. Moreover, complete abolishment of protein expression is feasible using a combination of gene targeting and RNA interference.

Co-evolution of somatic variation in primary and metastatic colorectal cancer may expand biopsy indications in the molecular era

Introduction

Metastasis is thought to be a clonal event whereby a single cell initiates the development of a new tumor at a distant site. However the degree to which primary and metastatic tumors differ on a molecular level remains unclear. To further evaluate these concepts, we used next generation sequencing (NGS) to assess the molecular composition of paired primary and metastatic colorectal cancer tissue specimens.

Methods

468 colorectal tumor samples from a large personalized medicine initiative were assessed by targeted gene sequencing of 1,321 individual genes. Eighteen patients produced genomic profiles for 17 paired primary:metastatic (and 2 metastatic:metastatic) specimens.

Results

An average of 33.3 mutations/tumor were concordant (shared) between matched samples, including common well-known genes (APC, KRAS, TP53). An average of 2.3 mutations/tumor were discordant (unshared) among paired sites. KRAS mutational status was always concordant. The overall concordance rate for mutations was 93.5%; however, nearly all (18/19 (94.7%)) paired tumors showed at least one mutational discordance. Mutations were seen in: TTN, the largest gene (5 discordant pairs), ADAMTS20, APC, MACF1, RASA1, TP53, and WNT2 (2 discordant pairs), SMAD2, SMAD3, SMAD4, FBXW7, and 66 others (1 discordant pair).

Conclusions

Whereas primary and metastatic tumors displayed little variance overall, co-evolution produced incremental mutations in both. These results suggest that while biopsy of the primary tumor alone is likely sufficient in the chemotherapy-naïve patient, additional biopsies of primary or metastatic disease may be necessary to precisely tailor therapy following chemotherapy resistance or insensitivity in order to adequately account for tumor evolution.

The prognostic value of c-Kit, K-ras codon 12, and p53 codon 72 mutations in Egyptian patients with stage II colorectal cancer

BACKGROUND

The prognosis for patients with colorectal cancer (CRC) depends mainly on standard clinicopathologic factors. However, patients with similar disease characteristics exhibit various outcomes, especially in stage II. Therefore, the identification of molecular prognostic markers is needed to predict patient outcomes.

METHODS

The authors assessed the prognostic value of c-Kit (also called cluster of differentiation 117 [CD117] or KIT), cyclooxygenase-2 (COX-2), tumor protein 53 (p53), and Kirsten rat sarcoma viral oncogene homolog (K-ras) aberrations in 90 patients with stage II CRC using immunohistochemistry and molecular techniques. The results were correlated with standard clinicopathologic prognostic factors, overall survival (OS), and disease-free survival (DFS).

RESULTS

COX2 and c-Kit overexpression was detected in 54.6% and 59.3% of patients, respectively. Overexpression of p53 was detected in 47 patients, including 29 who had mutations, and a unique mutation pattern was detected with 3 hotspots at codons 72, 245, and 273. Overexpression of ras was detected in 44 patients, including 37 who had mutations. On multivariate analysis, c-Kit overexpression, p53 codon 72 mutations, perforation, and performance status were independent prognostic factors for DFS (P=.054, P=.015, P<.0001, and P=.043, respectively); whereas codon 12 K-ras mutation, performance status, and perforation were independent prognostic factors for OS (P=.033, P=.006, and P<.0001, respectively).

CONCLUSIONS

The current results provide evidence for the prognostic value of c-Kit overexpression in patients with stage II CRC. The high p53 mutation rate and the unique hotspot in codon 72 have not been reported previously in CRC. This may be related to environmental or racial features that are unique to the studied population.

Recent advances in the molecular diagnosis and prognosis of colorectal cancer

Colon cancer remains a leading cause of mortality worldwide despite the well-characterized molecular events in the adenoma-to-carcinoma sequence. There has been a strong emphasis on early detection of colon cancer, and fecal DNA-based methods have been developed to assist with early screening. Tissue-based assays have been utilized for many years to assess tumor aggressiveness and to determine prognosis and response to chemotherapeutic interventions. The most widely used serum marker for colon cancer (carcinoembryonic antigen) remains a useful modality to assess for occult disease following curative resection. Identification of tumor mutations in circulating tumor cells and microarray analysis holds a great deal of promise in the diagnosis and prognosis of patients with colorectal cancer. The inhibitors of apoptosis may be important markers to determine resistance to radiation cytotoxicity in rectal cancer. This report presents a summary of the current status of the molecular markers of colorectal cancer to establish a diagnosis, determine prognosis and chemoradiotherapeutic interventions, and assess relapse following curative surgery.

Different p53 mutation patterns in colorectal tumors from smokers and nonsmokers

Epidemiological studies consistently find associations between colorectal cancer and cigarette smoking; however, there are little molecular data supporting the association. To examine the relationship between cigarette smoking and colorectal cancer, we compared p53 mutation patterns in colorectal tumors from smokers and nonsmokers. In this study, 153 tumor tissues from colorectal cancer patients, including 63 smokers and 90 nonsmokers, were examined for p53 mutation and p53 protein expression by direct sequencing and immunohistochemistry (IHC), respectively. p53 mutations were detected in 77 of 153 (50.3%) colorectal tumors, and no difference was observed in the p53 mutation frequencies in tumors from smokers and nonsmokers (33 of 63, 50.8% for smokers vs. 44 of 90, 48.9% for nonsmokers, P = 0.743). IHC showed that p53-immunoreactive tumors were positively correlated with p53-mutated tumors (P < 0.0001). G:C-->A:T transition and G:C-->T:A transversion were the predominant types of mutations detected in the tumor p53 genes. G:C-->A:T mutation was relatively more common in nonsmokers than in smokers (93.5% for nonsmokers vs. 77.3% for smokers), although this difference was not significant. The frequency of deletion mutation in smoker tumors, however, was significantly higher than that in nonsmoker tumors (7 of 33, 21.2% for smokers vs. 1 of 44, 2.3% for nonsmokers, P = 0.01). Although there were only a few cases of p53 deletion mutation in this study, the observation of a higher frequency of p53 deletion mutation in smoker tumors supports the association between cigarette smoking and the development of colorectal cancer.

Colon cancer and apoptosis

BACKGROUND

The implementation of new therapeutic options for the management of metastatic colon cancer mandates a revisit to apoptosis and its role in colon cancer tumorigenesis with an emphasis on the mechanisms leading to chemotherapeutic resistance and immune system evasion of colon cancer cells.

DATA SOURCES

Literature regarding molecular apoptosis mechanisms and the role of apoptosis in colon cancer progression are reviewed by this article.

CONCLUSION

Programmed cell death has rapidly emerged as a potential target for cancer treatment at various stages of tumor progression. Chemoprevention, immuno-regulation, and metastasis are prospective targets by which apoptotic mechanisms could be utilized in the prevention and management of tumorigenesis. Understanding how defects in the death receptor pathway of apoptosis permit colon cancer cells to escape the immune system would allow for treatment options whereby the body's immune system could again recognize and eliminate unwanted cells.

Selective loss of codon 72 proline p53 and frequent mutational inactivation of the retained arginine allele in colorectal cancer

According to recent reports, some cancer types exhibit nonrandom allele loss at codon 72 in exon 4 of the p53 gene [coding for proline (72Pro) or arginine (72Arg)]. To clarify this phenomenon for colorectal cancer and to find out if this preferential loss might have any functional significance, p53 loss of heterozygosity (LOH) and p53 mutations were investigated in a group of 61 colorectal cancers and 28 liver metastases, and were correlated with clinicopathologic factors. A comparison of a patient's blood codon 72 status with a healthy control group did not reveal an enhanced risk of developing colorectal tumors for one of the two isoforms. p53-LOH and p53 mutations were found in 62.2% and 39.4% of primary tumors, respectively, and in 57.9% and 25% of hepatic metastases, respectively. In 14 heterozygous cases showing exon 4-LOH, only the 72Pro allele was lost and the retained 72Arg was preferentially mutated. In general, p53 mutations were significantly associated with the 72Arg tumor status (P < .001). Distal tumors showed allelic losses of the p53 gene more commonly than proximal tumors (P = .054). The prevalence of 72Arg increased in frequency with higher Dukes stage (P = .056). We suggest that either the preferential loss of 72Pro or the mutation of the 72Arg in colorectal cancer and hepatic metastases is associated with malignant potential and might reflect carcinogenic exposure, particularly in the distal part of the large intestines.

Colon cancer: genomics and apoptotic events

Colon cancer is the third most common cancer globally. The risk of developing colon cancer is influenced by a number of factors that include age and diet, but is primarily a genetic disease, resulting from oncogene over-expression and tumour suppressor gene inactivation. The induction and progression of the disease is briefly outlined, as are the cellular changes that occur in its progression. While colon cancer is uniformly amenable to surgery if detected at the early stages, advanced carcinomas are usually lethal, with metastases to the liver being the most common cause of death. Oncogenes and genetic mutations that occur in colon cancer are featured. The molecules and signals that act to eradicate or initiate the apoptosis cascade in cancer cells, are elucidated, and these include caspases, Fas, Bax, Bid, APC, antisense hTERT, PUMA, 15-LOX-1, ceramide, butyrate, tributyrin and PPARgamma, whereas the molecules which promote colon cancer cell survival are p53 mutants, Bcl-2, Neu3 and COX-2. Cancer therapies aimed at controlling colon cancer are reviewed briefly.

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.

Prognostic molecular markers for planning adjuvant chemotherapy trials in Dukes' B colorectal cancer patients: how much evidence is enough?

The benefit of postoperative adjuvant chemotherapy in patients with Dukes' B colorectal cancer is still uncertain and its routine use is not recommended. Prognostic biomarkers may be useful for identifying high-risk patients with resected, node-negative disease, and this stratification may represent an innovative strategy for designing adjuvant chemotherapy trials. Featured prognostic molecular markers can be divided into the following categories: cell proliferation indices (Ki-67, Mib-1, proliferating cell nuclear antigen); oncogenes/tumor suppressor genes [p53, K-ras, Deleted in Colorectal Cancer (DCC), Bcl-2, c-erbB2]; DNA repair (microsatellite instability); markers of angiogenesis (vascular count, vascular endothelial growth factor); markers of invasion/metastasis (plasminogen-related molecules, matrix metalloproteinases); and biochemical markers (thymidylate synthase). Studies that have investigated their prognostic role in Dukes' B colorectal cancer patients are reviewed here. Current data do not provide sufficient evidence for the incorporation of available prognostic biomarkers into clinical practice. However, a biomarker-based approach could be an effective strategy for improving results of postoperative adjuvant treatments in high-risk Dukes' B colorectal cancer patients. Markers of altered DCC function have shown promising prognostic role and sufficient prevalence in retrospective investigations and they deserve further assessment in prospective studies.

Prognostic value of tumoral thymidylate synthase and p53 in metastatic colorectal cancer patients receiving fluorouracil-based chemotherapy: phenotypic and genotypic analyses

PURPOSE

The aim of this multicenter prospective study was to evaluate the role of intratumoral parameters related to fluorouracil (FU) sensitivity in 103 metastatic colorectal cancer patients receiving FU-folinic acid.

PATIENTS AND METHODS

Liver metastatic biopsy specimens were obtained for all patients and primary tumor biopsy specimens for 54 patients. Thymidylate synthase (TS), folylpolyglutamate synthetase, and dihydropyrimidine dehydrogenase were measured by radioenzymatic assays; TS promoter polymorphism (2R/2R v 2R/3R v 3R/3R) was determined by polymerase chain reaction; and p53 protein and mutations were analyzed by immunoluminometric assay and denaturing gradient gel electrophoresis, respectively.

RESULTS

p53 mutations were observed in 56.7% of metastases. TS activity was significantly higher in 2R/3R tumors as compared with 2R/2R or 3R/3R. TS activity in metastasis was the only parameter linked to clinical responsiveness (responders exhibited the lower TS, P =.047). Univariate Cox analyses demonstrated that TS activity in primary tumor (the greater the TS, the poorer the survival; P =.040), TS promoter polymorphism in primary tumor (risk of death of 2R/3R v 2R/2R, 2.68; P =.035), and p53 stop mutation in metastasis (risk of death of stop mutations v wild type, 3.14; P =.018) were the only significant biologic predictors of specific survival. Stepwise analysis did not discriminate between TS activity and TS polymorphism.

CONCLUSION

Present results confirm the value of tumoral TS activity for predicting FU responsiveness, point out the importance of detailed p53 mutation analysis for predicting survival, and suggest that TS genotype in primary tumor carries a prognostic value similar to that of TS activity.