Promoter hypermethylation leads to decreased APC mRNA expression in familial polyposis and sporadic colorectal tumours, but does not substitute for truncating mutations.
Germline mutations in the tumour suppressor APC cause familial adenomatous polyposis (FAP), and somatic mutations are common in sporadic colorectal cancers (CRCs). Hypermethylation of APC promoter 1A has been reported in a substantial proportion of sporadic CRCs and may cause transcriptional silencing. Methylation has been proposed as an alternative to mutation or loss of heterozygosity as a mechanism of gene inactivation. However, the significance of APC methylation has remained unclear, because it has not previously been related to the presence of mono- or bi-allelic mutations at APC. We examined 103 FAP adenomas, 11 attenuated FAP adenomas, 31 sporadic CRCs and 30 CRC cell lines, all with known germline and/or somatic APC mutations. Overall, APC promoter 1A methylation was detected in 27-45% of colorectal tumours and cell lines, but generally not in histologically normal colorectum. In contrast to previous reports, methylation was detected in similar proportions of FAP/AFAP and sporadic CRCs. Importantly, methylation was independent of the presence, number and positions of APC mutations and was not associated with the CpG island methylator phenotype. Methylation resulted in a decrease or loss of 1A isoform mRNA and reduced total APC transcript levels, although expression was retained from promoter 1B. However, neither APC protein levels, nor transcription of a panel of Wnt target genes was associated with methylation status. Our data suggest that APC promoter 1A hypermethylation may influence APC expression levels in a subtle fashion, but methylation does not result in complete gene inactivation or act as a 'second hit'.
Segditsas
Stefania
S
Molecular and Population Genetics Laboratory, London Research Institute, Cancer Research UK, London WC2A 3PX, UK.
Sieber
Oliver M
OM
Rowan
Andrew
A
Setien
Fernando
F
Neale
Kay
K
Phillips
Robin K S
RK
Ward
Robyn
R
Esteller
Manel
M
Tomlinson
Ian P M
IP
D011125:Adenomatous Polyposis Coli / Q000235:genetics* / Q000378:metabolism; D025601:Adenomatous Polyposis Coli Protein / Q000235:genetics* / Q000378:metabolism; D045744:Cell Line, Tumor; D019175:DNA Methylation; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D019656:Loss of Heterozygosity / Q000235:genetics; D018716:Molecular Mimicry; D009154:Mutation / Q000235:genetics*; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism; D051153:Wnt Proteins / Q000378:metabolism
D025601:Adenomatous Polyposis Coli Protein; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D051153:Wnt Proteins