CpG island methylator phenotype (CIMP) of colorectal cancer is best characterised by quantitative DNA methylation analysis and prospective cohort studies.
BACKGROUND
The concept of CpG island methylator phenotype (CIMP) is not universally accepted. Even if specific clinicopathological features have been associated with CIMP, investigators often failed to demonstrate a bimodal distribution of the number of methylated markers, which would suggest CIMP as a distinct subtype of colorectal cancer. Previous studies primarily used methylation specific polymerase chain reaction which might detect biologically insignificant low levels of methylation.


OBJECTIVE
To demonstrate a distinct genetic profile of CIMP colorectal cancer using quantitative DNA methylation analysis that can distinguish high from low levels of DNA methylation.


METHODS
We developed quantitative real time polymerase chain reaction (MethyLight) assays and measured DNA methylation (percentage of methylated reference) of five carefully selected loci (promoters of CACNA1G, CDKN2A (p16), CRABP1, MLH1, and NEUROG1) in 460 colorectal cancers from large prospective cohorts.


RESULTS
There was a clear bimodal distribution of 80 microsatellite instability-high (MSI-H) tumours according to the number of methylated promoters, with no tumours showing 3/5 methylated loci. Thus we defined CIMP as having >or=4/5 methylated loci, and 17% (78) of the 460 tumours were classified as CIMP. CIMP was significantly associated with female sex, MSI, BRAF mutations, and wild-type KRAS. Both CIMP MSI-H tumours and CIMP microsatellite stable (MSS) tumours showed much higher frequencies of BRAF mutations (63% and 54%) than non-CIMP counterparts (non-CIMP MSI-H (0%, p<10(-5)) and non-CIMP MSS tumours (6.6%, p<10(-4)), respectively).


CONCLUSIONS
CIMP is best characterised by quantitative DNA methylation analysis. CIMP is a distinct epigenotype of colorectal cancer and may be less frequent than previously reported.
Ogino
S
S
Department of Pathology, Brigham and Women's Hospital, Harvard Medical School, 75 Francis St, Boston, MA 02115, USA. shuji_ogino@dfci.harvard.edu
Cantor
M
M
Kawasaki
T
T
Brahmandam
M
M
Kirkner
G J
GJ
Weisenberger
D J
DJ
Campan
M
M
Laird
P W
PW
Loda
M
M
Fuchs
C S
CS
D015179:Colorectal Neoplasms / Q000235:genetics*; D018899:CpG Islands; D019175:DNA Methylation; D005819:Genetic Markers; D020022:Genetic Predisposition to Disease; D006306:Health Surveys; D006801:Humans; D018895:Microsatellite Repeats; D011446:Prospective Studies; D011518:Proto-Oncogene Proteins / Q000235:genetics; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics; D016283:Proto-Oncogene Proteins p21(ras); D020133:Reverse Transcriptase Polymerase Chain Reaction; D018631:ras Proteins
D005819:Genetic Markers; 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