BRAF mutation is associated with DNA methylation in serrated polyps and cancers of the colorectum.
OBJECTIVE
Mutations in BRAF have been linked with colorectal cancers (CRC) showing high level microsatellite instability (MSI-H). However, the distribution of BRAF mutations in MSI-H cancers remains to be clarified with respect to precursor lesions and the CpG island methylator phenotype (CIMP).


METHODS
Forty three hyperplastic polyps (HP), nine mixed polyps (MP), five serrated adenomas (SA), 28 conventional adenomas (AD), 18 hereditary non-polyposis colorectal cancers (HNPCC), and 127 sporadic CRC (46 MSI-H and 81 non-MSI-H) were collected from patients undergoing colectomy for either CRC or hyperplastic polyposis. Twenty five of 57 serrated lesions were derived from four patients with hyperplastic polyposis. HP were further subdivided according to recently documented morphological criteria into 27 classical HP and 16 variant lesions described as "sessile serrated adenoma" (SSA). All tumours were screened for BRAF activating mutations.


RESULTS
The BRAF mutation was more frequent in SSA (75%) and MP (89%) than in classical HP (19%), SA (20%), and AD (0%) (p<0.0001), and also in sporadic MSI-H cancers (76%) compared with HNPCC (0%) and sporadic non-MSI-H cancers (9%) (p<0.0001). The BRAF mutation was identified more often in CIMP-high serrated polyps (72%) and CIMP-high CRC (77%) than in CIMP-low (30%) and CIMP-negative (13%) polyps (p = 0.002) as well as CIMP-low (18%) and CIMP-negative (0%) CRC (p<0.0001).


CONCLUSIONS
The BRAF mutation was frequently seen in SSA and in sporadic MSI-H CRC, both of which were associated with DNA methylation. Sporadic MSI-H cancers may originate in SSA and not adenomas, and BRAF mutation and DNA methylation are early events in this "serrated" pathway.
Kambara
T
T
Conjoint Gastroenterology Laboratory, Royal Brisbane and Women's Hospital Research Foundation and Queensland Institute of Medical Research, Brisbane 4029, Australia.
Simms
L A
LA
Whitehall
V L J
VL
Spring
K J
KJ
Wynter
C V A
CV
Walsh
M D
MD
Barker
M A
MA
Arnold
S
S
McGivern
A
A
Matsubara
N
N
Tanaka
N
N
Higuchi
T
T
Young
J
J
Jass
J R
JR
Leggett
B A
BA
D000236:Adenoma / Q000235:genetics / Q000473:pathology; D000368:Aged; D003111:Colonic Polyps / Q000235:genetics / Q000473:pathology; D015179:Colorectal Neoplasms / Q000235:genetics* / Q000473:pathology; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000235:genetics / Q000473:pathology; D018899:CpG Islands / Q000235:genetics; D019175:DNA Methylation; D005260:Female; D011905:Genes, ras / Q000235:genetics; D006801:Humans; D044483:Intestinal Polyposis / Q000235:genetics* / Q000473:pathology; D008297:Male; D018895:Microsatellite Repeats / Q000235:genetics; D008875:Middle Aged; D009154:Mutation; D010641:Phenotype; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics*
C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf