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Molecular Biology, Pathobiology, and Genetics |
1 Department of Pathology, University of Utah Health Sciences Center; 2 Department of Family and Preventive Medicine, Health Research Center, Salt Lake City, Utah and 3 Kaiser Permanente Medical Center, Walnut Creek, California
Requests for reprints: Wade S. Samowitz, Department of Pathology, University of Utah Health Sciences Center, Salt Lake City, UT 84132. Phone: 801-585-2556; Fax: 801-585-3831; E-mail: wade.samowitz{at}hsc.utah.edu.
| Abstract |
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| Introduction |
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We have previously used a large population-based series of colon cancers to show numerous relationships between CIMP and clinicopathologic variables, especially the V600E BRAF mutation, in multivariate analyses.4 We now evaluate the relationship between the V600E BRAF mutation and/or CIMP and survival in colon cancer as well as the relationship of this mutation to other clinicopathologic variables.
| Materials and Methods |
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Information on age at time of diagnosis, sex, tumor site, and tumor stage were available from the Northern California Tumor Registry, Sacramento Tumor Registry, and Utah Cancer Registry. These registries are members of the Surveillance, Epidemiology, and End Results program. Proximal tumors were defined as cecum through transverse colon; tumors in the splenic flexure, descending, and sigmoid colon were defined as distal. Tumors were staged by AJCC criteria (11). Vital status, date of death, primary cause of death, and two contributing causes of death were obtained from local tumor registries using death certificate information. Active follow-up of people diagnosed with cancer is done through the cancer registries on a continuous basis. Vital status as of December 30, 1998 was obtained for all study participants. For individuals whose vital status or cause of death could not be determined through local tumor registries, National Death Index tapes were used. Months of survival were calculated by subtracting the date of last contact or death from the date of diagnosis. Deaths from any cause as well as deaths attributed to colon cancer were assessed. Family history of cancer information was collected as part of an interviewer-administered questionnaire. Study participants were asked to list first names of all first-degree blood relatives, including parents, siblings, and children. For each family member enumerated, it was determined if that person ever had been diagnosed with cancer, age at diagnosis, and type of cancer. All aspects of this study were approved by the University of Utah and KPMCP institutional review boards.
BRAF V600E mutation detection. The BRAF V600E mutations have been determined in a previous study.4 Briefly, exon 15 of BRAF was amplified from DNA previously extracted from microdissected tumors (4) using the forward primer 5'-TCATAATGCTTGCTCTGATAGGA-3' and the reverse primer 5'-CTTTCTAGTAACTCAGCAGC-3'. Amplifications were carried out using AmpliTaq Gold and a PCR profile consisting of a 9-minute initial denaturation at 95°C followed by 35 cycles of 20 seconds at 95°C, 20 seconds at 60°C, and 30 seconds at 72°C with a 5-minute final extension at 72°C. Mutations were verified by sequencing in both directions. Sufficient DNA was available for analysis of 919 tumors. Mutations other than the V600E mutation were identified in 4 tumors but were not used in any of the statistical analyses.
CpG island methylator phenotype. CIMP had been determined in a previous study.4 Briefly, sodium bisulfate modification was done on DNA extracted from tumors microdissected for previous studies (4). Methylation-specific PCR was then done as described previously for the following CpG islands: MINT 1, MINT 2, MINT 31, p16, and hMLH1 (12). This panel was being used at the time our study began by the group that originally described CIMP and its importance in colorectal cancer, and their criterion for CIMP high was methylation of two or more of these CpG islands (12, 13). CIMP low was defined as less than two of five markers methylated. Sufficient DNA for CIMP determination was present for 838 tumors with BRAF V600E results and 26 tumors without BRAF V600E results.
Ki-ras, p53, and microsatellite instability. Codon 12 and 13 Ki-ras mutations, p53 mutations in exons 5 to 8, and MSI were determined in previous studies (4, 14, 15). These studies preceded the development of the Bethesda consensus panel; the MSI markers used were BAT-26 (a mononucleotide repeat, which by itself is a very good measure of generalized instability), TGFßRII (a coding mononucleotide repeat, which is unstable in most colorectal cancers with MSI), and a panel of 10 tetranucleotides repeats, which show a high correlation with the Bethesda consensus panel and BAT-26 (16). A hierarchical approach was then used for MSI determination; 824 tumors were classified (either stable or unstable) for BAT-26, 59 tumors (which did not show results for BAT-26) were classified using TGFßRII, and 3 tumors (which showed no results for either BAT-26 or TGFßRII) were classified using the panel of 10 tetranucleotide repeats; in that case, if
30% of the 10 tetranucleotide repeats were unstable, the tumor was classified as unstable, and if <30% were unstable, the tumor was classified as stable. MSI could not be determined for 25 tumors.
Statistical analysis. Univariate relationships between BRAF V600E mutation and age at diagnosis, tumor site, AJCC stage, gender, grade of differentiation, a histologic classification of mucinous, mucin-producing, or signet ring, family history of colorectal cancer (defined as colorectal cancer in a first-degree relative), mutations in Ki-ras and p53, and CIMP were evaluated using logistic regression to calculate odds ratios (OR). All ORs were unadjusted, except for family history of colorectal cancer, which was adjusted for age at diagnosis. Survival data were available for 930 individuals with CIMP and/or BRAF results; 905 of these had BRAF mutational data and 857 of these had CIMP data. Five-year survival was evaluated using Kaplan-Meier plots for mortality due to all causes. Associations and interactions between BRAF, CIMP, and survival were evaluated among microsatellite-stable tumors using Cox proportional hazards models adjusting for age at diagnosis, AJCC stage, and tumor site. Median follow-up time was 65 months. All data analyses were done using SAS version 8.2 (SAS Institute, Cary, NC).
| Results |
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Five-year survival. Univariate relationships between several clinicopathologic variables and percent 5-year survival are shown in Table 2. Significant relationships with poor survival were seen with age, proximal tumor site, higher AJCC stage, and poor differentiation, whereas MSI was associated with an improved prognosis. Both the BRAF V600E mutation and CIMP high were associated with a poorer 5-year survival in microsatellite-stable tumors (16.7% and 48.7%, respectively). Among CIMP high microsatellite-stable tumors, the V600E BRAF mutation was associated with a significantly poorer 5-year survival (14.6% versus 55.8%; P < 0.0001). Microsatellite-unstable tumors with or without the V600E BRAF mutation were associated with an excellent percent 5-year survival (76.2 and 75.0%, respectively).
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Other BRAF mutations. Four other BRAF mutations were identified (Table 6) but were not used in the above analyses. Three of the mutations occurred in codon 594 (D594N, D594G, and D594G) and were present in microsatellite-stable, CIMP low tumors. All of these individuals survived at least 5 years (although all were relatively low-stage tumors). The fourth mutation (G606R) occurred in a microsatellite-stable, CIMP high tumor and was associated with death due to colon cancer after 19 months.
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| Discussion |
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The evaluation of the V600E BRAF mutation in unstable tumors was hampered by the relatively small numbers of such cancers. Still, and in contrast with stable tumors, the V600E BRAF mutation did show the relationship to increased age typically exhibited by CIMP high colon cancer (18).4
The V600E BRAF mutation was present in 5% (40 of 803) of microsatellite-stable colon cancers and was associated with a significantly worse survival than microsatellite-stable tumors without this mutation. This was true in a univariate analysis after adjustment for age, stage, and tumor site, in stage-stratified analyses for AJCC stages 2 to 4, and in a Kaplan-Meier analysis for stages 2 to 4 (Tables 2, 3, and 4; Fig. 1). Because >90% of microsatellite-stable tumors with the V600E BRAF mutation are also CIMP high, it is important to see whether this poorer survival was specifically related to this mutation or whether it was simply a function of CIMP. Although CIMP high stable tumors were associated with a significantly worse 5-year survival than CIMP low stable tumors in univariate and age-adjusted analyses, the effect of CIMP high on survival was less than that seen with BRAF (Tables 2 and 3). In addition, and in contrast to the BRAF V600E mutation, no significant relationships were seen for CIMP in either an analysis adjusted for age, stage, and tumor site, in stage-stratified analyses, or in a Kaplan Meier analysis (Table 3; Fig. 2). The relatively minor effects of CIMP high on survival we observed are consistent with a previous study (18) and suggest that the effect of the V600E mutation on survival in stable tumors is not dependent on CIMP. In a direct comparison of CIMP high stable tumors with and without the V600E mutation, tumors with the V600E mutation had a significantly worse 5-year survival (14.6% versus 55.8%; P < 0.0001), and in multivariate analyses of CIMP and BRAF, only BRAF mutations had an effect on survival (Table 5).
The V600E mutation did not have the same effect on survival in tumors with MSI, as unstable tumors with and without this mutation were associated with an excellent 5-year survival (76.2% and 75.0%, respectively). This emphasizes that it is not the V600E mutation per se that confers a poor prognosis but rather that the mutation has different effects depending on the genetic background in which it occurs and/or, perhaps, the oncogenic pathway that led to the development of the cancer. It should be stressed that this difference is not due to the presence or absence of CIMP, as >90% of both microsatellite-unstable and microsatellite-stable tumors with the V600E BRAF mutation are CIMP high (Table 1).
There has recently been a great deal of speculation that microsatellite-unstable colorectal cancers may develop from a subset of hyperplastic polyps (which often have BRAF mutations and are CIMP high) rather than from traditional adenomas (8). If this is true, then perhaps microsatellite-stable tumors with BRAF mutations develop from a different pathway (e.g., adenomas with BRAF mutations; ref. 2) or that both develop from the same serrated, hyperplastic polyp pathway but diverge with respect to clinical aggressiveness with the methylation of hMLH1 in a subset of tumors that then develop MSI. Future studies of relevant precursor lesions will be necessary to address this question.
Four other BRAF mutations were identified (Table 5). Three of them encode substitutions of uncharged for charged amino acids at codon 594. A similar mutation (D594V) has been shown to lead to impaired kinase activity of BRAF and loss of the ability to activate extracellular signal-regulated kinase (21). Interestingly, all three of these mutations occurred in CIMP low tumors and all three individuals survived at least 5 years (although all were relatively low stage tumors). The fourth mutation (G606R) was not analyzed by Wan et al., but it is similar to other activating mutations described in that study (including the V600E mutation) in that it occurred in the activation segment of the protein and was a substitution of a charged for an uncharged amino acid. This mutation occurred in a microsatellite-stable, CIMP high tumor and the individual died of colon cancer after 19 months, similar to the CIMP high context, increased stage, and poor survival we have seen with the more common V600E mutation.
Our findings with respect to the deleterious effect of BRAF mutations in microsatellite-stable cancers could have important clinical implications, especially if validated by other studies. Individuals with such tumors may be treated more aggressively, especially those with stage II cancers who typically do not receive chemotherapy at the present time. It may also be possible to develop specific therapies for these tumors. It is possible that these therapies would not involve any that are specifically directed against mutated BRAF, as these mutations do not have an effect on survival in microsatellite-unstable cancers. It is also possible, however, that in the right genetic background therapy directed against this oncogenic mutation could have beneficial effects.
In summary, this large population-based study showed that microsatellite-stable colon cancers with the V600E BRAF mutation differed from other CIMP high stable tumors by showing no relationship with increased age and by showing a strong relationship to a family history of colorectal cancer. This latter finding suggests that future exploration of the genetic and/or environmental factors which relate to this association, may be fruitful. In addition, microsatellite-stable tumors with the V600E BRAF mutation had a significantly worse overall survival than stable tumors without this mutation and specifically showed poorer survival in AJCC stages 2 to 4. This poor survival was not related to the CIMP high phenotype, and the V600E mutation did not affect the excellent prognosis of microsatellite-unstable carcinomas. Further research is needed to determine the mechanism through which this mutation is associated with poor survival in microsatellite-stable colon cancers and/or whether there is a novel carcinogenic pathway associated with these genetic changes in microsatellite-stable tumors which is particularly aggressive.
| Acknowledgments |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
We thank Dr. Bette Caan, Sandra Edwards, Karen Curtin, Leslie Palmer, and Judy Morse for data collection and management efforts of this study, Michael Hoffman for genotyping, and the Core Facility of the University of Utah Health Sciences Center for sequencing.
| Footnotes |
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Received 2/ 7/05. Revised 4/25/05. Accepted 5/ 6/05.
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