18q loss of heterozygosity in microsatellite stable colorectal cancer is correlated with CpG island methylator phenotype-negative (CIMP-0) and inversely with CIMP-low and CIMP-high

Background: The CpG island methylator phenotype ( CIMP) with widespread promoter methylation is a distinct epigenetic phenotype in colorectal cancer, associated with microsatellite instability-high ( MSI-high) and BRAF mutations. 18q loss of heterozygosity ( LOH) commonly present in colorectal cancer with chromosomal instability ( CIN) is associated with global hypomethylation in tumor cell. A recent study has shown an inverse correlation between CIN and CIMP ( determined by MINTs, p16, p14 and MLH1 methylation) in colorectal cancer. However, no study has examined 18q LOH in relation to CIMP-high, CIMP-low ( less extensive promoter methylation) and CIMP-0 ( CIMP-negative), determined by quantitative DNA methylation analysis. Methods: Utilizing MethyLight technology ( real-time PCR), we quantified DNA methylation in 8 CIMP-specific promoters {CACNA1G, CDKN2A ( p16), CRABP1, IGF2, MLH1, NEUROG1, RUNX3 and SOCS1} in 758 non-MSI-high colorectal cancers obtained from two large prospective cohorts. Using four 18q microsatellite markers ( D18S55, D18S56, D18S67 and D18S487) and stringent criteria for 18q LOH, we selected 374 tumors ( 236 LOH-positive tumors with = 2 markers showing LOH; and 138 LOH-negative tumors with = 3 informative markers and no LOH). Results: CIMP-0 ( 0/8 methylated promoters) was significantly more common in 18q LOH-positive tumors ( 59% = 139/236, p = 0.002) than 18q LOH-negative tumors ( 44% = 61/138), while CIMP-low/high ( 1/8 - 8/8 methylated promoters) was significantly more common ( 56%) in 18q LOH-negative tumors than 18q LOH-positive tumors ( 41%). These relations persisted after stratification by sex, location, or the status of MSI, p53 expression ( by immunohistochemistry), or KRAS/BRAF mutation. Conclusion: 18q LOH is correlated positively with CIMP-0 and inversely with CIMP-low and CIMP-high. Our findings provide supporting evidence for relationship between CIMP-0 and 18q LOH as well as a molecular difference between CIMP-0 and CIMP-low in colorectal cancer.