Mutations in both KRAS and BRAF may contribute to the methylator phenotype in colon cancer.
OBJECTIVE
Colorectal cancers (CRCs) with the CpG island methylator phenotype (CIMP) often associate with epigenetic silencing of hMLH1 and an activating mutation in the BRAF gene. However, the current CIMP criteria are ambiguous and often result in an underestimation of CIMP frequencies in CRCs. Because BRAF and KRAS belong to same signaling pathway, we hypothesized that not only mutations in BRAF but mutant KRAS may also associate with CIMP in CRC.


METHODS
We determined the methylation status in a panel of 14 markers (7 canonical CIMP-related loci and 7 new loci), microsatellite instability status, and BRAF/KRAS mutations in a collection of 487 colorectal tissues that included both sporadic and Lynch syndrome patients.


RESULTS
Methylation analysis of 7 CIMP-related markers revealed that the mean number of methylated loci was highest in BRAF-mutated CRCs (3.6) vs KRAS-mutated (1.2, P < .0001) or BRAF/KRAS wild-type tumors (0.7, P < .0001). However, analyses with 7 additional markers showed that the mean number of methylated loci in BRAF mutant tumors (4.4) was the same as in KRAS mutant CRCs (4.3, P = .8610). Although sporadic microsatellite instability high tumors had the highest average number of methylated markers (8.4), surprisingly, Lynch syndrome CRCs also demonstrated frequent methylation (5.1).


CONCLUSIONS
CIMP in CRC may result from activating mutations in either BRAF or KRAS, and the inclusion of additional methylation markers that correlate with mutant KRAS may help clarify CIMP in future studies. Additionally, aberrant DNA methylation is a common event not only in sporadic CRC but also in Lynch syndrome CRCs.
Nagasaka
Takeshi
T
Division of Gastroenterology, Department of Internal Medicine, and Charles A. Sammons Cancer Center and Baylor Research Institute, Baylor University Medical Center, Dallas, Texas 75246, USA.
Koi
Minoru
M
Kloor
Matthias
M
Gebert
Johannes
J
Vilkin
Alex
A
Nishida
Naoshi
N
Shin
Sung Kwan
SK
Sasamoto
Hiromi
H
Tanaka
Noriaki
N
Matsubara
Nagahide
N
Boland
C Richard
CR
Goel
Ajay
A
D000368:Aged; D003106:Colon / Q000201:enzymology; D015179:Colorectal Neoplasms / Q000201:enzymology / Q000235:genetics* / Q000473:pathology; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000201:enzymology / Q000235:genetics; D018899:CpG Islands; D019175:DNA Methylation; D044127:Epigenesis, Genetic; D005260:Female; D015971:Gene Expression Regulation, Enzymologic; D015972:Gene Expression Regulation, Neoplastic; D005838:Genotype; D005858:Germany; D006801:Humans; D007564:Japan; D008297:Male; D053842:Microsatellite Instability; D008875:Middle Aged; D009154:Mutation; D009367:Neoplasm Staging; D010641:Phenotype; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics*; D016283:Proto-Oncogene Proteins p21(ras); D018631:ras Proteins / Q000235:genetics*
C117307:KRAS protein, human; D011518:Proto-Oncogene Proteins; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D016283:Proto-Oncogene Proteins p21(ras); D018631:ras Proteins