APC promoter hypermethylation contributes to the loss of APC expression in colorectal cancers with allelic loss on 5q.
BACKGROUND
Germ-line mutations of the APC gene are associated with familial adenomatous polyposis, and somatic mutations occur frequently in sporadic colorectal cancer. However, to abrogate APC function, both alleles must be inactivated. Recently, it has been demonstrated that epigenetic modification of the APC promoter influences APC silencing. Here we examined the influence of APC methylation on APC expression in tumors with and without LOH at the APC locus.


METHODS
137 sporadic colorectal cancer specimens were investigated for LOH at the 5q locus. The methylation status of the APC promoter was determined by methylation-specific PCR. APC expression was performed by immunohistochemistry.


RESULTS
Expression was reduced or lost in 110 of 137 (80%) tumors and LOH at 5q was found in 13 of 132 (10%) tumors. There was no difference in 5q LOH between tumors with or without intact APC expression. Vice versa, there was no difference in the APC expression in tumors with 5q LOH. Aberrant APC promoter methylation was detected in 33 of 118 (28%) tumors investigated. Of the tumors with 5q LOH for which methylation data were available, 4 of 11 (36%) were methylated versus 28 of 105 (27%) of those without LOH. No difference in methylation was observed in tumors without 5q LOH and normal APC expression and those without 5q LOH and reduced or missing APC expression. Importantly, none of the tumors with 5q LOH and normal APC staining were aberrantly methylated, whereas 50% of the cancers with LOH at 5q and reduced or absent staining were hypermethylated.


CONCLUSIONS
This report suggests that tumors with 5q LOH and reduced APC expression are more frequently hypermethylated at the APC promoter compared to those tumors with 5q LOH and normal APC expression. The association among APC promoter methylation status, 5q LOH, and reduced or lost APC expression suggests that de novo methylation plays an important role as a "second hit" in silencing APC expression in colorectal neoplasia.
Arnold
Christian N
CN
Department of Medicine and Comprehensive Cancer Center, University of California San Diego, La Jolla, California, USA.
Goel
Ajay
A
Niedzwiecki
Donna
D
Dowell
Jeannette M
JM
Wasserman
Linda
L
Compton
Carolyn
C
Mayer
Robert J
RJ
Bertagnolli
Monica M
MM
Boland
C Richard
CR
D025601:Adenomatous Polyposis Coli Protein / Q000235:genetics*; D002895:Chromosomes, Human, Pair 5 / Q000235:genetics*; D003106:Colon / Q000378:metabolism; D015179:Colorectal Neoplasms / Q000235:genetics* / Q000473:pathology; D019175:DNA Methylation; D004273:DNA, Neoplasm / Q000235:genetics; D015972:Gene Expression Regulation, Neoplastic; D020868:Gene Silencing; D006801:Humans; D019656:Loss of Heterozygosity; D011401:Promoter Regions, Genetic / Q000235:genetics*; D012007:Rectum / Q000378:metabolism
D025601:Adenomatous Polyposis Coli Protein; D004273:DNA, Neoplasm