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(American Journal of Pathology. 2005;166:1069-1075.)
© 2005 American Society for Investigative Pathology

BRAF Mutations in Aberrant Crypt Foci and Hyperplastic Polyposis

Robyn Beach^*, Annie On-On Chan, Tsung-Teh Wu^*, Jill A. White^*, Jeffrey S. Morris, Simon Lunagomez, Russell R. Broaddus^*, Jean-Pierre J. Issa, Stanley R. Hamilton^* and Asif Rashid^*

From the Departments of Pathology,^* Biostatistics, and Leukemia, The University of Texas M.D. Anderson Cancer Center, Houston, Texas; and the Department of Medicine, University of Hong Kong, Hong Kong, People’s Republic of China

	Abstract

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Patients with hyperplastic polyposis have multiple hyperplastic polyps (HPs) and increased risk of colorectal carcinomas. Aberrant crypt foci (ACF) are postulated to be the earliest precursor lesions in colorectal carcinogenesis. We evaluated BRAF mutations by DNA sequencing in 53 ACF from patients with sporadic colorectal carcinomas and familial adenomatous polyposis, in 18 sporadic HPs from patients with resected colorectal cancer, and in 70 HPs, 4 serrated adenomas, 3 admixed hyperplastic-adenomatous polyps, 10 tubular adenomas, and 6 carcinomas from 17 patients with multiple/large HPs and/or hyperplastic polyposis. BRAF mutation status was compared with clinicopathological features and other genetic alterations by marginal logistic regression. BRAF mutation was present in only 2% of ACF and 6% of sporadic HPs. In contrast, BRAF mutation was present in 43% of HPs (P = 0.01 versus sporadic HPs), 75% of serrated adenomas, 33% of admixed hyperplastic-adenomatous polyps, 30% of tubular adenomas, and 33% of carcinomas from patients with multiple/large HPs and/or hyperplastic polyposis. BRAF mutation status in patients with multiple/large HPs and/or hyperplastic polyposis correlated with HPs from the same patient (odds ratio, 5.8; P = 0.0002) but associated with younger age (odds ratio, 0.83; P = 0.006 compared to older age), with a large HP (odds ratio, 22.5; P = 0.01 compared with patients with multiple HPs), with location of HPs in the right colon (odds ratio, 3.0; P = 0.03), and with methylation of the p16 gene and the MINT31 locus [odds ratio, 12.2 (P = 0.0001) and 4.4 (P = 0.02), respectively]. Our study shows that BRAF mutation status is heterogeneous among patients with multiple/large HPs and/or hyperplastic polyposis, suggesting differences in pathogenesis of HPs that indicate subsets within this phenotype.

Sporadic HPs are usually present in the left colon, small in size, and considered to be benign in nature. However, adenocarcinoma arising in the setting of colorectal hyperplastic polyps (HPs) or serrated adenomas (SAs, polyps with serrated architecture and dysplasia)¹¹ especially in patients with hyperplastic polyposis have been described.^12-16 In addition, an alternative pathway of colorectal carcinogenesis with a hyperplastic polyp-serrated adenoma-adenocarcinoma sequence has been recently proposed.^17-19

Patients with hyperplastic polyposis, characterized phenotypically by the presence of numerous HPs and/or large HPs, have increased risk of colorectal cancer.^15,16,20-27 It has been proposed that a majority of sporadic HPs from the right colon and HPs from patients with hyperplastic polyposis are morphologically distinct.²⁸ Previous studies have shown that genetic and epigenetic alterations frequent in colorectal carcinoma are present in sporadic HPs, and in HPs, SAs, admixed hyperplastic-adenomatous polyps (AHAP, polyps with admixed hyperplastic and adenomatous foci),¹¹ tubular adenomas, and carcinomas of patients with hyperplastic polyposis, these alterations include KRAS mutations, chromosome 1p loss, MSI, CpG island methylation of p16 gene and other loci, and CpG island methylator phenotype (CIMP) with concordant methylation of CpG islands.^{4,15,16,29-33}

The RAS-RAF-MEK (mitogen-activated protein/extracellular signal-regulated kinase kinase)-ERK (extracellular signal-regulated kinase)-MAP (mitogen-activated protein) kinase pathway mediates cellular responses to growth signals. BRAF mutations have been found in a variety of human cancers including colorectal carcinomas and melanomas.^34-39 Mutations in BRAF occur in two regions of the BRAF kinase domain, ie, the activation segment that protects the substrate binding site, and less commonly, the G loop that mediates binding of ATP.⁴⁰ BRAF mutations have also been reported in sporadic HPs and in SAs, including a few from patients with hyperplastic polyposis.^41-43 In this study, we evaluated BRAF mutations in ACF from patients with FAP and sporadic colorectal cancers, in sporadic HPs, and in HPs, SAs, AHAPs, tubular adenomas, and colorectal carcinomas from the patients with multiple/large HPs and/or hyperplastic polyposis. We compared the BRAF mutation status with polyp and patient characteristics, including correlation among multiple HPs from the same patient.

	Materials and Methods

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Characteristics of Patients and Specimens

All patients had given informed consent for the collection of specimens according to institutional guidelines. ACF were isolated from the grossly normal mucosa in 10 colectomy specimens from patients with sporadic colorectal cancers and from the nonpolypoid mucosa in two colectomy specimens from FAP patients with numerous polyps but no cancer. These ACF have been characterized previously.¹⁰ Thirty ACF were from patients with sporadic colorectal cancers and 23 ACF from FAP patients. The ACF were classified as dysplastic, heteroplastic, or mixed (features of both dysplastic and heteroplastic ACF).¹⁰

Eighteen sporadic HPs from 15 patients undergoing resection of colorectal cancer at The University of Texas MD Anderson Cancer Center, Houston, TX, and the patients and specimens from patients with multiple/large HPs and/or hyperplastic polyposis have been reported previously (Figure 1) .^16,31 The patients were classified into three groups based on the number and size of HPs: large HPs (patients with HP greater than 1 cm), hyperplastic polyposis (patients with more than 20 HPs), and multiple HPs (patients with 5 to 10 HPs), as described previously.¹⁶ Predominance of HPs in the right colon and predominance of HPs in the left colorectum were defined by the location of the majority of HPs in the right colon or in the left colon and rectum, respectively.³¹ We evaluated 70 HPs, 4 SAs, 3 AHAPs, 10 tubular adenomas, and 6 carcinomas from 17 patients with multiple/large HPs and/or hyperplastic polyposis.

View larger version (36K): [in this window] [in a new window]   Figure 1. Clinicopathological features of patients, BRAF mutation status, other genetic alterations, and CIMP status of HPs, SAs, AHAPs, tubular adenomas, and carcinomas from patients with large/multiple HPs, and/or hyperplastic polyposis.

Exons 11 and 15 of the BRAF gene were amplified and sequenced as previously described.³⁵ Exons 11 and 15 were amplified by genomic polymerase chain reaction using intronic primers and a commercial DNA sequencing kit according to the manufacturer’s instructions (BigDye Terminator version 1.1 cycle sequencing kit; Applied Biosystems, Foster City, CA). The polymerase chain reaction products were analyzed with an Applied Biosystems 3730 automated sequencer using forward and reverse primers. All mutations were confirmed by an independent polymerase chain reaction amplification and sequencing. All BRAF mutations identified were a missense mutation at codon 599, exon 15 replacing GTG (valine) to GAG (glutamic acid). No mutations were identified in exon 11 or other codons of exon 15. Germline mutations were excluded by sequencing nonlesional DNA from these patients.

KRAS Mutations, Loss of Heterozygosity of Chromosome 1p, MSI-High, CIMP Status

KRAS mutation status of ACF, and KRAS mutations, loss of heterozygosity of chromosome 1p, MSI and CIMP status of sporadic HPs, and of HPs, SAs, AHAPs, tubular adenomas, and carcinomas from patients with multiple/large HPS and/or hyperplastic polyposis have been reported previously.^10,16,31 MSI-high was defined by presence of allelic shift in comparison with control DNA in at least 30% of evaluated markers. Methylation was assessed at the p16 gene and loci methylated in tumor (MINT): MINT1, MINT2, and MINT31. MINT1 is an island associated with a cDNA transcript of unknown function. MINT2 corresponds to a CpG island that is in the 5' region of a cDNA with an open reading frame that has no protein homology. MINT31 is 2 kb upstream of the CACNAIG, a T-type calcium channel gene (J.P. Issa, unpublished data). HPs, SAs, adenomas, and carcinomas were classified as CIMP-high if two or more (50%) of the p16 gene or MINT loci were methylated, CIMP-low if one (25%) marker was methylated, and CIMP-negative if no marker was methylated.

Statistical Analysis

Patients with more than one HP were represented multiple times in this data set. To model correctly the correlation among polyps coming from the same patient as well as simultaneously partition out the effects of the various factors considered, marginal logistic regression models for correlated binary data⁴⁴ were used to assess associations between BRAF mutations and the various polyp and patient characteristics. These associations were tested for association with BRAF mutations and were represented as odds ratios, in which an odds ratio of greater than one suggests positive correlation of BRAF mutations with patients or polyp characteristics, respectively. We used three models. The first model with no factors was used to estimate the correlation among the BRAF mutation status in polyps from the same patient, without adjusting for other covariates. A second model included various patient- and polyp-level factors, including the methylation status of the p16 gene and MINT1, MINT2, and MINT31 loci as potential predictors of BRAF mutation status. A third model was used with CIMP status (CIMP-high versus CIMP-low and CIMP-negative) substituted in place of the methylation status of the p16 gene and MINT1, MINT2, MINT31 loci, individually. The statistical analysis was performed using PROC GENMOD in SAS (SAS Institute, Cary, NC), using an assumption that all polyps within a patient were equally correlated. In all models, factors with P values less than 0.05 were considered statistically significant.

	Results

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ACF

Twenty-three ACF were from 2 FAP patients and 30 ACF from 10 patients with sporadic colorectal carcinomas. As previously reported,¹⁰ 91% (21 of 23) of ACF from FAP patients were dysplastic and 9% (2 of 23) were heteroplastic. In contrast, 87% (26 of 30) of ACF from patients with sporadic colorectal cancer were heteroplastic, only 10% (3 of 10) were dysplastic, and 3% (1 of 30) were mixed. BRAF mutation was present in 0% (0 of 23) of ACF from FAP patients and only 3% (1 of 30) of ACF from patients with sporadic colorectal cancers (Figure 2A) . In contrast, KRAS mutation was present in 4% (1 of 23) of ACF from FAP patients and 40% (12 of 30) of ACF from patients with sporadic colorectal cancers. BRAF mutation was present in a heteroplastic ACF and was the missense point mutation at codon 599 as described in the Materials and Methods.

View larger version (60K): [in this window] [in a new window]   Figure 2. A: Nucleotide sequencing of exon 15 of BRAF gene in ACF. B: Nucleotide sequencing of exon 15 of BRAF gene in HPs. The T to A missense point mutation at codon 599 with replacement of valine with glutamic acid is indicated by arrows. The wild-type and mutated nucleotide and amino acid sequences are shown on top.

The sporadic HPs were from 12 men and 3 women, with a mean age of 64 ± 11 years (range, 48 to 80 years). The mean size of the polyp in this group was 0.3 cm (range, 0.1 to 0.7 cm). There were 3 HPs from the right colon and 15 from the left colorectum. BRAF mutation was present in 6% (1 of 18) of sporadic HPs. BRAF mutation was again the missense point mutation at codon 599 described above. No KRAS mutation or methylation of the p16 gene or MINT loci were present in sporadic HPs.

Hyperplastic Polyposis

There were 11 men and 6 women with multiple/large HPs and/or hyperplastic polyposis. The mean age was 64 ± 12 years (range, 46 to 84 years). The demographic data and characteristics of each individual patient and the number of HPs, adenomas, and carcinomas in each individual are summarized in Figure 1 . BRAF mutations in patients are summarized in Figure 1 , and representative examples of sequencing are shown in Figure 2B . BRAF mutations were present in 43% (30 of 70) of HPs (P = 0.01 versus sporadic HPs), 75% (3 of 4) of SAs, 33% (1 of 3) of AHAPs, 30% (3 of 10) of tubular adenomas, and 33% (2 of 6) of carcinomas from patients with multiple/large HPs and/or hyperplastic polyposis. All BRAF mutations in HPs and other lesions from patients with multiple/large HPs and/or hyperplastic polyposis were the missense point mutation at codon 599 as described in the Materials and Methods.

We first examined if the BRAF mutation status was correlated within HPs from the same patient. We used a model with no factors except the correlation. We found that the correlation was statistically significant (odds ratio, 5.8; P = 0.0002; Figure 1 and Table 1 ). The odds ratio of 5.8 means that given the presence of a BRAF mutation in a polyp from a given patient, the probability of another polyp in the same patient having the BRAF mutation is 5.8 times greater than if the first polyp did not have the BRAF mutation.

In the third model, CIMP status was substituted for the methylation statuses of the p16 gene, and MINT1, MINT2, and MINT31 in model two (Table 1) . HPs with CIMP-high were slightly more likely to have BRAF mutations, but this was not significant after adjusting for other factors. Other genetic alterations in HPs and other lesions have been previously reported.¹⁶ KRAS mutations were present in 9% (6 of 70) of HPs and 67% (2 of 3) of AHAPs but in none of 4 SAs, 10 tubular adenomas, or 6 carcinomas (Figure 1) . Chromosome 1p loss was present in 4% (3 of 70) of HPs, 33% (1 of 3) of AHAPs, 10% (1 of 10) of tubular adenoma, and 17% (1 of 6) of carcinomas, but in none of four SAs. MSI was present in 33% (1 of 30) of AHAPs, 10% (1 of 10) of tubular adenomas, and 17% (1 of 6) of carcinomas, but in none of four SAs. Except for one carcinoma with BRAF mutation and MSI, all other genetic alterations were inversely related to BRAF mutations in all other lesions.

	Discussion

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We studied BRAF mutations in ACF from patients with FAP and sporadic colorectal cancers, sporadic HPs, and HPs and other colorectal lesions from patients with multiple/large HPs and/or hyperplastic polyposis. We found BRAF mutation status was correlated in HPs from patients with multiple/large HPs, or hyperplastic polyposis. BRAF mutations have been reported in sporadic colorectal carcinomas,^34-40,42 adenomas,³⁵ and HPs.^35,41-43 However, BRAF mutations are uncommon in sporadic microsatellite-stable colorectal carcinomas but are more frequent in MSI-high carcinomas^36-39 because of methylation of hMLH1 gene.^36,39 All BRAF mutations in our study were T to A missense point mutation at codon 599 with replacement of valine with glutamic acid. No mutation of exon 11 or other codons of exon 15 were found. In contrast, previous studies have reported BRAF mutations involving other codons, although infrequently, in colorectal cancers and other colorectal lesions including HPs.^{34,35,37,39,41}

The reported frequency of BRAF mutations in HPs and other serrated lesions is variable. In our study BRAF mutations were present in 43% of HPs, 75% of SAs, and 33% of AHAPs from patients with multiple/large HPs and/or hyperplastic polyposis, but were infrequent in sporadic HPs that were predominately from the left colon and rectum. These results are corroborated by another study that reported BRAF mutations in 13% of sporadic serrated polyps (including HPs, SAs, and AHAP) but in 88% of serrated polyps from four patients with hyperplastic polyposis.⁴² However, two previous studies reported higher frequencies of BRAF mutations in serrated polyps from sporadic patients.^41,43 In one study BRAF mutations were present in 36% of HPs, 100% of SAs, and 20% of AHAP,⁴¹ and in the other 70% of HPs and 60% of SAs.⁴³ These differences could be because of the methodology used for the detection of BRAF mutations, or to heterogeneity and selection bias of the study populations among our and previous studies.

In our study, BRAF mutation status was correlated among multiple HPs from the same patient and was more frequent in younger patients, patients with a large HP and right-sided polyps. This is corroborated by reports of increased frequency of BRAF mutations in right-sided serrated polyps⁴² and right-sided colonic carcinomas.^36,39 We and others have previously reported more frequent CIMP-high in right-sided HPs,^31,45 and differences in topographic expression of p21^Waf1/Cip1 cyclin-dependent kinase inhibitor and Ki-67 proliferation marker in right- and left-sided HPs from these patients.¹⁶ In addition, the HPs from the right colon are morphologically different from the HPs in the left colorectum.²⁸ These data suggest that right-sided HPs are morphologically and genetically different from the left-sided HPs in patients with sporadic HPs and in those with multiple/large HPs and/or hyperplastic polyposis.

The genetic alterations in sporadic HPs differ from the alterations in HPs from patients with multiple/large HPs, and/or hyperplastic polyposis. Sporadic HPs have more frequent KRAS mutations but less frequent BRAF mutations^41-43 or loss of chromosome 1p,^29,30,46 and lack CpG island methylation.^31,42,45 In contrast, the present study and previous studies have reported that the HPs from patients with multiple/large HPs and/or hyperplastic polyposis have frequent BRAF mutations and CpG island methylation, but infrequent KRAS mutations or loss of chromosome 1p.^16,31,42,45 Furthermore, KRAS mutation or loss of chromosome 1p was predominantly present in HPs from patients with predominance of HPs in left colorectum,^16,31 a set of patients that lacked BRAF mutations or CIMP-high HPs.

The data from our present study and previous studies^42,45 suggest that HPs and other lesions from patients with multiple/large HPs and/or hyperplastic polyposis have BRAF mutations and CIMP-high but lack MSI. In contrast, sporadic colon carcinomas with BRAF mutations frequently have MSI.^36-39 In our study, BRAF mutations were more frequent in tubular adenomas in patients with multiple/large HPs and/or hyperplastic polyposis compared to sporadic adenomas.³⁵ These data provide additional evidence that progression of colorectal carcinogenesis in patients with multiple/large HPs and/or hyperplastic polyposis is distinct from sporadic colorectal carcinomas. In some patients with multiple/large HPs and/or hyperplastic polyposis, HPs and other lesions have CIMP-high and BRAF mutations similar to sporadic CIMP-high colorectal carcinomas but lack MSI-high. Other patients have HPs and other lesions that lack BRAF mutation and CpG island methylation, as documented by four patients with loss of chromosome 1p or KRAS mutations in the majority of their HPs.^16,31

In our study BRAF mutations were infrequent in ACF from patients with sporadic colorectal cancers and FAP. Dysplastic ACF are characterized by abnormal epithelial proliferation in the upper aspects of the crypts, lack of KRAS mutations and methylation, and presence of APC mutations in dysplastic ACF from FAP patients but not patients with sporadic colorectal cancers.^8-10 The lack of BRAF mutations in dysplastic ACF or heteroplastic ACF from FAP patients is not surprising and is further corroborated by infrequent BRAF mutations in sporadic adenomas or adenomas from patients with FAP.³⁵ In contrast, heteroplastic ACF are characterized by lack of dysplasia, have proliferation mainly in the lower aspects of the crypts, have frequent KRAS mutations and methylation, and lack APC mutations.^4,8-10 Heteroplastic and mixed ACF resemble HPs and SAs histopathologically, respectively. These data suggest that either heteroplastic ACF are not precursors of HPs, SAs, and colorectal cancers with BRAF mutations, or BRAF mutation is a late event in a hyperplastic polyp-serrated adenoma-carcinoma sequence. Alternatively, these findings may be because of selection bias in our study with ACF from sporadic colorectal cancers and FAP, a patient-population that lacks BRAF mutations.

Recent studies have suggested a hyperplastic polyp-serrated adenoma-carcinoma pathway in colorectal carcinogenesis.^17-19 Right-sided sporadic colon carcinomas often have CpG island methylation and BRAF mutations. In this study we show that some patients with multiple/large HPs and/or hyperplastic polyposis have these molecular characteristics in multiple colonic lesions including HPs. As a consequence, it appears that subsets of patients whose lesions have different pathogenesis have similar phenotypes. Molecular characteristics are needed to identify those subsets.

	Footnotes

 
Address reprint requests to Asif Rashid M.D., Ph.D., Department of Pathology, Box 85, The University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX, 77030-4095. E-mail: arashid{at}mdanderson.org

Accepted for publication January 4, 2005.

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