Microsatellite instability and survival in rectal cancer

Microsatellite instability and promoter hypermethylation in colorectal cancer in India

Microsatellite instability (MSI) is an important factor in tumor development and is a hypermutable phenotype caused by the loss of DNA mismatch repair activity. It is important to identify tumors with microsatellite instability as the patients have a better prognosis and differ with response to chemotherapy. Limited data are available on the incidence of MSI in Indian colorectal cancers (CRCs). The objectives of this study were to identify the extent of MSI in Indian CRC patients below 50 years and to determine promoter methylation status of hMLH1 and hMSH2 in relation to MSI. A total of 450 patients were diagnosed with CRC, out of which 91 individuals were recruited as per Bethesda guidelines and were tested for instability by the NCI-recommended Bethesda panel (BAT25, BAT26, D2S123, D5S346, and D17S2720) using labeled primers. The fragments were separated and analyzed on a Beckman GeXP sequencer. Promoter methylation status was determined by restriction enzyme digestion and PCR. MSI (high and low) was seen in 48.4% (44/91) of CRC patients, out of which microsatellite instability-high (MSI-H) was detected in 13.2% (12/91) and microsatellite instability-low (MSI-L) in 35.2% (32/91) and the rest were microsatellite stable (MSS), 51.6% (47/91). Majority of the MSI-H tumors were adenocarcinomas (10/12), in the rectum (8/12), and moderately or poorly differentiated (12/12). Promoter hypermethylation was seen in 75% of the MSI-H, 56.24% of MSI-L, and only 23.4% of MSS individuals. MSI (high and low) was associated with 48.4% of CRC patients, and a significantly higher proportion of promoter hypermethylation of hMLH1 and hMSH2 genes was associated with instable tumors.

Prognostic value of CpG island methylator phenotype among colorectal cancer patients: a systematic review and meta-analysis

BACKGROUND

Divergent findings regarding the prognostic value of CpG island methylator phenotype (CIMP) in colorectal cancer (CRC) patients exist in current literature. We aim to review data from published studies in order to examine the association between CIMP and CRC prognosis.

MATERIALS AND METHODS

A comprehensive search for studies reporting disease-free survival (DFS), overall survival (OS), or cancer-specific mortality of CRC patients stratified by CIMP is carried out. Study findings are summarized descriptively and quantitatively, using adjusted hazard ratios (HRs) as summary statistics.

RESULTS

Thirty-three studies reporting survival in 10 635 patients are included for review. Nineteen studies provide data suitable for meta-analysis. The definition of CIMP regarding gene panel, marker threshold, and laboratory method varies across studies. Pooled analysis shows that CIMP is significantly associated with shorter DFS (pooled HR estimate 1.45; 95% confidence interval (CI) 1.07-1.97, Q = 3.95, I(2) = 0%) and OS (pooled HR estimate 1.43; 95% CI 1.18-1.73, Q = 4.03, I(2) = 0%) among CRC patients irrespective of microsatellite instability (MSI) status. Subgroup analysis of microsatellite stable (MSS) CRC patients also shows significant association between shorter OS (pooled HR estimate 1.37; 95% CI 1.12-1.68, Q = 4.45, I(2) = 33%) and CIMP. Seven studies have explored CIMP's value as a predictive factor on stage II and III CRC patient's DFS after receiving adjuvant 5-fluorouracil (5-FU) therapy: of these, four studies showed that adjuvant chemotherapy conferred a DFS benefit among CIMP(+) patients, one concluded to the contrary, and two found no significant correlation. Insufficient data was present for statistical synthesis of CIMP's predictive value among CRC patients receiving adjuvant 5-FU therapy.

CONCLUSION

CIMP is independently associated with significantly worse prognosis in CRC patients. However, CIMP's value as a predictive factor in assessing whether adjuvant 5-FU therapy will confer additional survival benefit to CRC patients remained to be determined through future prospective randomized studies.

Molecular characteristics of mismatch repair genes in sporadic colorectal tumors in Czech patients

BACKGROUND

Mismatch repair (MMR) genes are known to be frequently altered in colorectal cancer (CRC). Both genetics and epigenetics modifications seems to be relevant in this phenomenon, however it is still not clear how these two aspects are interconnected. The present study aimed at characterizing of epigenetic and gene expression profiles of MMR genes in sporadic CRC patients from the Czech Republic, a country with one of the highest incidences of this cancer all over Europe.

METHODS

Expression levels and CpG promoter methylation status of all MMR genes were evaluated in DNA from tumor and adjacent mucosal samples of 53 incident CRC patients.

RESULTS

We have found significantly increased transcription levels in EXO1 gene in tumor tissues (P = 0.05) and significant over-expression of MSH3 gene in colon tumors when compared to adjacent mucosal tissues (P = 0.02). Interestingly, almost all MMR genes were differently expressed when localization of tumors was compared. In particular, colon tumors showed an up-regulation of EXO1, MSH2, MSH3, MSH6, and PMS2 genes in comparison to rectal tumors (P = 0.02). Expression levels of all MMR genes positively correlated between each other. The promoter methylation of MLH1 gene was observed in 9% of CRC tissues only.

CONCLUSIONS

In our study, we have observed different pattern of MMR genes expression according to tumor localization. However, a lack of association between methylation in MMR genes and their corresponding expressions was noticed in this study, the relationship between these two aspects is worthy to be analyzed in larger population studies and in pre-malignant stages.

Differences in survival between colon and rectal cancer from SEER data

Background

Little is known about colorectal cancer or colon and rectal cancer. Are they the same disease or different diseases?

Objectives

The aim of this epidemiology study was to compare the features of colon and rectal cancer by using recent national cancer surveillance data.

Design and setting

Data included colorectal cancer (1995–2008) from the Surveillance, Epidemiology, and End Results Program (SEER) database. Only adenocarcinoma was included for analysis.

Patients

A total of 372,130 patients with a median follow-up of 32 months were analyzed.

Main outcome measures

Mean survival of patients with the same stage of colon and rectal cancer was evaluated.

Results

Around 35% of patients had stage information. Among them, colon cancer patients had better survival than those with rectal cancer, by a margin of 4 months in stage IIB. In stage IIIC and stage IV, rectal cancer patients had better survival than colon cancer patients, by about 3 months. Stage IIB colorectal cancer patients had a poorer prognosis than those with stage IIIA and IIIB colorectal cancer. After adjustment of age, sex and race, colon cancer patients had better survival than rectal cancer of stage IIB, but in stage IIIC and IV, rectal cancer patients had better survival than colon cancer.

Limitations

The study is limited by its retrospective nature.

Conclusion

This was a population-based study. The prognosis of rectal cancer was not worse than that of colon cancer. Local advanced colorectal cancer had a poorer prognosis than local regional lymph node metastasis. Stage IIB might require more aggressive chemotherapy, and no less than that for stage III.

Clinical utility of KRAS and BRAF mutations in a cohort of patients with colorectal neoplasms submitted for microsatellite instability testing

BACKGROUND

Molecular analysis has become important in colorectal carcinoma (CRC) evaluation. Alterations in KRAS, BRAF, or mismatch repair (MMR) genes may determine therapeutic response or define a hereditary cancer syndrome. Correlation of DNA studies with clinical findings will further clarify the clinical utility of these markers.

PATIENTS AND METHODS

A retrospective study was performed on 111 paraffin-embedded tumor specimens submitted for microsatellite instability (MSI) testing based on clinical history or histologic examination, or both. DNA samples were screened for 7 KRAS mutations and the BRAF p.V600E mutation using fluorescent allele-specific polymerase-chain reaction (PCR) and capillary electrophoresis. Clinical data were collected through chart review.

RESULTS

Fifty-eight male and 53 female patients were studied. The incidence of KRAS and BRAF mutations was 49.5% and 7.2%, respectively. Dideoxy sequencing verified KRAS mutation status in 46 of 49 specimens tested. There was a trend toward significance of individual KRAS mutations on survival (P = .003). Dually positive KRAS and MSI tumors exclusively demonstrated p.G12D and p.G13D mutations (G>A transitions). BRAF-mutated tumors were predominantly right-sided and associated with a borderline worse prognosis. Forty-eight percent of tumors with MSI were present in the left colon or rectum.

CONCLUSION

Allele-specific PCR is an accurate and convenient method to assess KRAS and BRAF mutations and may detect mutations not identified by dideoxy sequencing. KRAS mutation status, in conjunction with morphologic or clinical parameters, may be useful in determining whether a tumor should be tested for MSI. MSI testing should not be considered exclusively in right-sided lesions. BRAF analysis may not be useful in rectal adenocarcinomas and should be evaluated in larger studies.

Signature of microsatellite instability, KRAS and BRAF gene mutations in German patients with locally advanced rectal adenocarcinoma before and after neoadjuvant 5-FU radiochemotherapy

BACKGROUND

Multiple activating mutations of the signal- and repair pathway, such as BRAF-, KRAS-mutations and microsatellite instabilities are involved in colorectal cancer pathogenesis. Molecular characterization of specifically locally advanced rectal cancers is scarce. Therefore the retrospective study addresses the intratumoral status of KRAS, BRAF and microsatellites loci with respect to tumor response and patients' antecedent including nicotine abusus, familial history, and health care to further molecularly identify rectal cancer patients.

METHODS

The study assesses the molecular status of 50 rectal cancer samples (25 before and 25 after neoadjuvant 5-FU radiochemotherapy). KRAS and BRAF mutations were examined through two independent analytical methods (sequencing and SNaPshot) to ensure efficient mutation detection. The microsatellite analysis was conducted using a fluorescent multiplex PCR-based method.

RESULTS

KRAS mutations were found in 9 of 25 (36%) rectal cancer patients and were not significantly associated with the response to therapy (P=0.577), age (P=0.249) or sex of the patient (P=0.566). No link exists between KRAS mutation status and nodal (P=0.371) or metastatic stage (P=0.216). For two patients, KRAS mutation status changed after application of neoadjuvant 5-FU radiochemotherapy. All tumor samples were diagnosed BRAF-negative. Two rectal cancer patients exhibited a MSI-H phenotype and showed no tumor response.

CONCLUSIONS

So one can conclude that (I) KRAS mutations status may change after neoadjuvant 5-FU radiochemotherapy relevant for further therapeutic decisions; (II) MSI-H patients do not respond to neoadjuvant 5-FU radiochemotherapy. Further prospective studies are needed to validate these results.

The CpG island methylator phenotype (CIMP) in colorectal cancer

It is clear that colorectal cancer (CRC) develops through multiple genetic and epigenetic pathways. These pathways may be determined on the basis of three molecular features: (i) mutations in DNA mismatch repair genes, leading to a DNA microsatellite instability (MSI) phenotype, (ii) mutations in APC and other genes that activate Wnt pathway, characterized by chromosomal instability (CIN) phenotype, and (iii) global genome hypermethylation, resulting in switch off of tumor suppressor genes, indicated as CpG island methylator phenotype (CIMP). Each of these pathways is characterized by specific pathological features, mechanisms of carcinogenesis and process of tumor development. The molecular aspects of these pathways have been used clinically in the diagnosis, screening and management of patients with colorectal cancer. In this review we especially describe various aspects of CIMP, one of the important and rather recently discovered pathways that lead to colorectal cancer.

Specific mutations in KRAS codons 12 and 13, and patient prognosis in 1075 BRAF wild-type colorectal cancers

PURPOSE

To assess prognostic roles of various KRAS oncogene mutations in colorectal cancer, BRAF mutation status must be controlled for because BRAF mutation is associated with poor prognosis, and almost all BRAF mutants are present among KRAS wild-type tumors. Taking into account experimental data supporting a greater oncogenic effect of codon 12 mutations compared with codon 13 mutations, we hypothesized that KRAS codon 12-mutated colorectal cancers might behave more aggressively than KRAS wild-type tumors and codon 13 mutants.

EXPERIMENTAL DESIGN

Using molecular pathological epidemiology database of 1,261 rectal and colon cancers, we examined clinical outcome and tumor biomarkers of KRAS codon 12 and 13 mutations in 1,075 BRAF wild-type cancers (i.e., controlling for BRAF status). Cox proportional hazards model was used to compute mortality HR, adjusting for potential confounders, including stage, PIK3CA mutations, microsatellite instability, CpG island methylator phenotype, and LINE-1 methylation.

RESULTS

Compared with patients with KRAS wild-type/BRAF wild-type cancers (N = 635), those with KRAS codon 12 mutations (N = 332) experienced significantly higher colorectal cancer-specific mortality [log-rank P = 0.0001; multivariate HR, 1.30; 95% confidence interval (CI), 1.02-1.67; P = 0.037], whereas KRAS codon 13-mutated cases (N = 108) were not significantly associated with prognosis. Among the seven most common KRAS mutations, c.35G>T (p.G12V; N = 93) was associated with significantly higher colorectal cancer-specific mortality (log-rank P = 0.0007; multivariate HR, 2.00; 95% CI, 1.38-2.90, P = 0.0003) compared with KRAS wild-type/BRAF wild-type cases.

CONCLUSIONS

KRAS codon 12 mutations (in particular, c.35G>T), but not codon 13 mutations, are associated with inferior survival in BRAF wild-type colorectal cancer. Our data highlight the importance of accurate molecular characterization in colorectal cancer.

The differential impact of microsatellite instability as a marker of prognosis and tumour response between colon cancer and rectal cancer

BACKGROUND

Microsatellite instability (MSI) is a distinct molecular phenotype of colorectal cancer related to prognosis and tumour response to 5-fluorouracil (5-FU)-based chemotherapy. We investigated the differential impact of MSI between colon and rectal cancers as a marker of prognosis and chemotherapeutic response.

METHODS

PCR-based MSI assay was performed on 1125 patients. Six hundred and sixty patients (58.7%) had colon cancer and 465 patients (41.3%) had rectal cancer.

RESULTS

Among 1125 patients, 106 (9.4%) had high-frequency MSI (MSI-H) tumours. MSI-H colon cancers (13%) had distinct phenotypes including young age at diagnosis, family history of colorectal cancer, early Tumor, Node, Metastasis (TNM) stage, proximal location, poor differentiation, and high level of baseline carcinoembryonic antigen (CEA), while MSI-H rectal cancers (4.3%) showed similar clinicopathological characteristics to MSS/MSI-L tumours except for family history of colorectal cancer. MSI-H tumours were strongly correlated with longer disease free survival (DFS) (P=0.005) and overall survival (OS) (P=0.009) than MSS/MSI-L tumours in colon cancer, while these positive correlations were not observed in rectal cancers. The patients with MSS/MSI-L tumours receiving 5-FU-based chemotherapy showed good prognosis (P=0.013), but this positive association was not observed in MSI-H (P=0.104).

CONCLUSION

These results support the use of MSI status as a marker of prognosis and response to 5-FU-based chemotherapy in patients with colon cancers. Further study is mandatory to evaluate the precise role of MSI in patients with rectal cancers and the effect of 5-FU-based chemotherapy in MSI-H tumours.

CpG island methylator phenotype infers a poor disease-free survival in locally advanced rectal cancer

BACKGROUND

Locally advanced rectal cancers are treated with preoperative radiochemotherapy (RCT). However, subsets of patients have no benefit from preoperative treatment. Since epigenetic modifications, including DNA methylation, may influence response to neoadjuvant treatment we studied the CpG island methylator phenotype (CIMP) in patients who received a 5-fluouracil based RCT.

METHODS

One hundred fifty patients with locally advanced rectal cancer, treated within a phase III clinical trial (CAO/ARO/AIO-94 and -04), were included in this analysis. CIMP was assessed by methylation specific PCR (MSP) using RUNX3, SOCS1, NEUROG1, IGF2, and CACNA1G as a marker panel. Loss of mismatch repair gene (MMR) expression was assessed by immunohistochemistry for a subset of patients. KRAS and BRAF mutation status were assessed using Sanger sequencing.

RESULTS

The CIMP status could be established in all 150 patients. Fifteen (10%) revealed CIMP positivity (≥3 methylated promoters), whereas 135 patients (90%) where classified as CIMP negative. Analysis for MMR status did not reveal any microsatellite instability (MSI). A single mutation of the BRAF gene (D594G) was detected. The KRAS gene (exon 1, 2, and 3) was mutated in 65 tumors (43%) but was not correlated to a specific CIMP status. Three- and 5-year disease-free survival was notably worse in CIMP positive patients (56% and 0% vs 80% and 75%; P < .01) suggesting an increased likelihood of poor clinical outcome (HR 5.5; 95%CI: [2.1, 13.9]).

CONCLUSION

CIMP positivity, defined by methylation of at least 3 specific gene promoters, is an infrequent event in locally advanced rectal cancer. However, it increases the likelihood of distant metastases. Therefore, the CIMP status may be included as a molecular marker for the identification of high-risk patients and might contribute to individual treatment stratification.

Molecular genetic analysis of 103 sporadic colorectal tumours in Czech patients

The Czech Republic has one of the highest incidences of colorectal cancer (CRC) in Europe. To evaluate whether sporadic CRCs in Czech patients have specific mutational profiles we analysed somatic genetic changes in known CRC genes (APC, KRAS, TP53, CTNNB1, MUTYH and BRAF, loss of heterozygosity (LOH) at the APC locus, microsatellite instability (MSI), and methylation of the MLH1 promoter) in 103 tumours from 102 individuals. The most frequently mutated gene was APC (68.9% of tumours), followed by KRAS (31.1%), TP53 (27.2%), BRAF (8.7%) and CTNNB1 (1.9%). Heterozygous germline MUTYH mutations in 2 patients were unlikely to contribute to the development of their CRCs. LOH at the APC locus was found in 34.3% of tumours, MSI in 24.3% and MLH1 methylation in 12.7%. Seven tumours (6.9%) were without any changes in the genes tested. The analysis yielded several findings possibly specific for the Czech cohort. Somatic APC mutations did not cluster in the mutation cluster region (MCR). Tumours with MSI but no MLH1 methylation showed earlier onset and more severe mutational profiles compared to MSI tumours with MLH1 methylation. TP53 mutations were predominantly located outside the hot spots, and transitions were underrepresented. Our analysis supports the observation that germline MUTYH mutations are rare in Czech individuals with sporadic CRCs. Our findings suggest the influence of specific ethnic genetic factors and/or lifestyle and dietary habits typical for the Czech population on the development of these cancers.

Colorectal cancer prognosis depends on T-cell infiltration and molecular characteristics of the tumor

The aim of this study was to relate the density of tumor infiltrating T cells to cancer-specific survival in colorectal cancer, taking into consideration the CpG island methylator phenotype (CIMP) and microsatellite instability (MSI) screening status. The T-cell marker CD3 was stained by immunohistochemistry in 484 archival tumor tissue samples. T-cell density was semiquantitatively estimated and scored 1-4 in the tumor front and center (T cells in stroma), and intraepithelially (T cells infiltrating tumor cell nests). Total CD3 score was calculated as the sum of the three CD3 scores (range 3-12). MSI screening status was assessed by immunohistochemistry. CIMP status was determined by quantitative real-time PCR (MethyLight) using an eight-gene panel. We found that patients whose tumors were highly infiltrated by T cells (total CD3 score ≥7) had longer survival compared with patients with poorly infiltrated tumors (total CD3 score ≤4). This finding was statistically significant in multivariate analyses (multivariate hazard ratio, 0.57; 95% confidence interval, 0.31-1.00). Importantly, the finding was consistent in rectal cancer patients treated with preoperative radiotherapy. Although microsatellite unstable tumor patients are generally considered to have better prognosis, we found no difference in survival between microsatellite unstable and microsatellite stable (MSS) colorectal cancer patients with similar total CD3 scores. Patients with MSS tumors highly infiltrated by T cells had better prognosis compared with intermediately or poorly infiltrated microsatellite unstable tumors (log rank P=0.013). Regarding CIMP status, CIMP-low was associated with particularly poor prognosis in patients with poorly infiltrated tumors (multivariate hazard ratio for CIMP-low versus CIMP-negative, 3.07; 95% confidence interval, 1.53-6.15). However, some subset analyses suffered from low power and are in need of confirmation by independent studies. In conclusion, patients whose tumors are highly infiltrated by T cells have a beneficial prognosis, regardless of MSI, whereas the role of CIMP status in this context is less clear.

Analysis of differentially expressed genes in human rectal carcinoma using suppression subtractive hybridization

The existence and treatment of rectal cancer are important for the function of defecation and the quality of life. However, the precise mechanisms of rectal carcinogenesis remain unclear. To screen the overexpressed gene in rectal carcinoma, we performed suppressive subtractive hybridization (SSH) on rectal carcinoma cells and the corresponding normal rectal cells. A total of 64 recombinant clones were subjected to DNA sequencing analysis, and 9 known genes were found to overexpressed in the tumors compared with those of the normal tissues. The genes are ST3 beta-galactoside alpha-2,3-sialyltransferase (ST3GAL5), interferon-induced transmembrane protein 3 (IFITM3), platelet-derived growth factor A-associated protein 1 (PDAP1), AlkB alkylating repair homolog 3 (ALKBH3), nucleoside diphosphate linked moiety X (Nudix)-type motif 14 (NUDT14), calponin 2 (CNN2), mitogen-activated protein kinase 14 (MAPK14), aconitase 1 (ACO1), and selenophosphate synthetase 1 (SEPHS1). The expression profiles of the genes were further confirmed in rectal carcinoma cells and the corresponding normal rectal cells of 12 patients by quantitative real-time RT-PCR. Our results revealed that ST3GAL5, IFITM3, PDAP1, ALKBH3, NUDT14, CNN2, MAPK14, ACO1, and SEPHS1 may be involved in rectal carcinogenesis.

MGMT promoter methylation, loss of expression and prognosis in 855 colorectal cancers

OBJECTIVE

O⁶-methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme. MGMT promoter hypermethylation and epigenetic silencing often occur as early events in carcinogenesis. However, prognostic significance of MGMT alterations in colorectal cancer remains uncertain.

METHODS

Utilizing a database of 855 colon and rectal cancers in two prospective cohort studies (the Nurses' Health Study and the Health Professionals Follow-up Study), we detected MGMT promoter hypermethylation in 325 tumors (38%) by MethyLight and loss of MGMT expression in 37% (247/672) of tumors by immunohistochemistry. We assessed the CpG island methylator phenotype (CIMP) using eight methylation markers [CACNA1G, CDKN2A (p16), CRABP1, IGF2, MLH1, NEUROG1, RUNX3, and SOCS1], and LINE-1 (L1) hypomethylation, TP53 (p53), and microsatellite instability (MSI).

RESULTS

MGMT hypermethylation was not associated with colorectal cancer-specific mortality in univariate or multivariate Cox regression analysis [adjusted hazard ratio (HR) = 1.03; 95% confidence interval (CI), 0.79-1.36] that adjusted for clinical and tumor features, including CIMP, MSI, and BRAF mutation. Similarly, MGMT loss was not associated with patient survival. MGMT loss was associated with G>A mutations in KRAS (p = 0.019) and PIK3CA (p = 0.0031).

CONCLUSIONS

Despite a well-established role of MGMT aberrations in carcinogenesis, neither MGMT promoter methylation nor MGMT loss serves as a prognostic biomarker in colorectal cancer.

The mutator phenotype in cancer: molecular mechanisms and targeting strategies

Normal human cells replicate their DNA with exceptional accuracy. It has been estimated that approximately one error occurs during DNA replication for each 10(9) to 10(10) nucleotides polymerized. In contrast, malignant cells exhibit multiple chromosomal abnormalities and contain tens of thousands of alterations in the nucleotide sequence of nuclear DNA. To account for the disparity between the rarity of mutations in normal cells and the large numbers of mutations present in cancer, we have hypothesized that during tumor development, cancer cells exhibit a mutator phenotype. As a defining feature of cancer, the mutator phenotype remains an as-yet unexplored therapeutic target: by reducing the rate at which mutations accumulate it may be possible to significantly delay tumor development; conversely, the large number of mutations in cancer may make cancer cells more sensitive to cell killing by increasing the mutation rate. Here we summarize the evidence for the mutator phenotype hypothesis in cancer and explore how the increased frequency of random mutations during the evolution of human tumors provides new approaches for the design of cancer chemotherapy.

DNA sequence profiles of the colorectal cancer critical gene set KRAS-BRAF-PIK3CA-PTEN-TP53 related to age at disease onset

The incidence of colorectal cancer (CRC) increases with age and early onset indicates an increased likelihood for genetic predisposition for this disease. The somatic genetics of tumor development in relation to patient age remains mostly unknown. We have examined the mutation status of five known cancer critical genes in relation to age at diagnosis, and compared the genomic complexity of tumors from young patients without known CRC syndromes with those from elderly patients. Among 181 CRC patients, stratified by microsatellite instability status, DNA sequence changes were identified in KRAS (32%), BRAF (16%), PIK3CA (4%), PTEN (14%) and TP53 (51%). In patients younger than 50 years (n = 45), PIK3CA mutations were not observed and TP53 mutations were more frequent than in the older age groups. The total gene mutation index was lowest in tumors from the youngest patients. In contrast, the genome complexity, assessed as copy number aberrations, was highest in tumors from the youngest patients. A comparable number of tumors from young (<50 years) and old patients (>70 years) was quadruple negative for the four predictive gene markers (KRAS-BRAF-PIK3CA-PTEN); however, 16% of young versus only 1% of the old patients had tumor mutations in PTEN/PIK3CA exclusively. This implies that mutation testing for prediction of EGFR treatment response may be restricted to KRAS and BRAF in elderly (>70 years) patients. Distinct genetic differences found in tumors from young and elderly patients, whom are comparable for known clinical and pathological variables, indicate that young patients have a different genetic risk profile for CRC development than older patients.

Relationship of EMAST and microsatellite instability among patients with rectal cancer

BACKGROUND

Elevated microsatellite instability at selected tetranucleotide repeats (EMAST) is a genetic signature identified in 60% of sporadic colon cancers and may be linked with heterogeneous expression of the DNA mismatch repair (MMR) protein hMSH3. Unlike microsatellite instability-high (MSI-H) in which hypermethylation of hMLH1 occurs followed by multiple susceptible gene mutations, EMAST may be associated with inflammation and subsequent relaxation of MMR function with the biological consequences not known. We evaluated the prevalence of EMAST and MSI in a population-based cohort of rectal cancers, as EMAST has not been previously determined in rectal cancers.

METHODS

We analyzed 147 sporadic cases of rectal cancer using five tetranucleotide microsatellite markers and National-Cancer-Institute-recommended MSI (mononucleotide and dinucleotide) markers. EMAST and MSI determinations were made on analysis of DNA sequences of the polymerase chain reaction products and determined positive if at least two loci were found to have frame-shifted repeats upon comparison between normal and cancer samples from the same patient. We correlated EMAST data with race, gender, and tumor stage and examined the samples for lymphocyte infiltration.

RESULTS

Among this cohort of patients with rectal cancer (mean age 62.2 ± 10.3 years, 36% female, 24% African American), 3/147 (2%) showed MSI (three males, two African American) and 49/147 (33%) demonstrated EMAST. Rectal tumors from African Americans were more likely to show EMAST than Caucasians (18/37, 49% vs. 27/104, 26%, p = 0.014) and were associated with advanced stage (18/29, 62% EMAST vs. 18/53, 37%, non-EMAST p = 0.02). There was no association between EMAST and gender. EMAST was more prevalent in rectal tumors that showed peri-tumoral infiltration compared to those without (30/49, 60% EMAST vs. 24/98, 25% non-EMAST, p = 0.0001).

CONCLUSIONS

EMAST in rectal cancer is common and MSI is rare. EMAST is associated with African-American race and may be more commonly seen with metastatic disease. The etiology and consequences of EMAST are under investigation, but its association with immune cell infiltration suggests that inflammation may play a role for its development.

Somatic alterations, metabolizing genes and smoking in rectal cancer

Cigarette smoking has been identified as a risk factor for rectal cancer. Our investigation evaluates associations between active and passive smoking and TP53, KRAS2, and BRAF V600E mutations, microsatellite instability (MSI), and CpG Island Methylator Phenotype (CIMP) in rectal tumors. We examine how genetic variants of GSTM1 and NAT2 alter these associations in a population-based, case-control study of 750 incident rectal cancer cases and 1,201 controls. Detailed tobacco exposure data were collected in an extensive questionnaire. DNA from blood was examined for GSTM1 and NAT2 variants. Tumor DNA was assessed to determine TP53 (exons 5-8), KRAS2 (codons 12-13) and BRAF mutations, MSI (BAT26 and TGFbetaRII analysis), and CIMP (methylation of CpG islands in CDKN2A, MLH1, MINT1, MINT2 and MINT31). Cigarette smoking (>20 pack-years, relative to nonsmokers) was associated with increased risk of TP53 mutations (OR = 1.4, 95% CI 1.02-2.0), BRAF mutations (OR = 4.2, 95% CI 1.3-14.2) and MSI (OR = 5.7, 95% CI 1.1-29.8) in rectal tumors. Long-term environmental tobacco smoke (ETS) exposure of >10 hr/wk was associated with increased risk of KRAS2 mutation (OR = 1.5, 95% CI 1.04-2.2). All smoking indicators were suggestive of increased risk in CIMP+ rectal cancer. GSTM1 and NAT2 were generally not associated with rectal tumor alterations; however, we observed an interaction of ETS and NAT2 in TP53-mutated tumors (p < 0.01). Our investigation shows active smoking is associated with increased risk of TP53, BRAF and MSI+ in rectal tumors and is suggestive of increased risk of CIMP+ tumors. ETS may increase risk of KRAS2 mutations; association with TP53 mutations and ETS may be influenced by NAT2.