Epigenetic downregulation of the DNA repair gene MED1/MBD4 in colorectal and ovarian cancer.
MED1 is a base excision repair enzyme that interacts with the mismatch repair protein MLH1 and maintains genomic integrity by binding methylated DNA and repairing spontaneous deamination events. MED1 mutations have been associated with microsatellite instability and accelerated colorectal cancer (CRC) tumorigenesis. We propose that promoter methylation may serve as an alternative epigenetic mechanism for MED1 gene suppression during sporadic CRC tumorigenesis. Methylation status of the MED1 promoter was investigated in a panel of ovarian and colorectal cancer cell lines. The MED1 promoter region was sequenced following bisulfite treatment and sequence analysis identified a CpG island within the MED1 promoter which is frequently and preferentially methylated (> or =50%) in ovarian and colorectal cancer cell lines with low/reduced MED1 expression. In vitro reversal of methylation restored MED1 expression. In colorectal cancer patients, when MED1 methylation was present, both tumor and matched mucosa were affected equally (mean frequency of methylation 24%) and there was no correlation between methylation and tumor stage. Patients without history of CRC showed significantly lower frequency of methylation (mean 14%, p < 0.05). Decreased MED1 transcript levels were observed in matched normal mucosa when compared to controls (median fold difference 8.0). Additional decreased expression was seen between mucosa and matched tumor (median fold decrease 4.4). Thus, MED1 promoter methylation and gene silencing occur in sporadic CRC patients and represent an early event in CRC tumorigenesis. Detection of MED1 methylation and gene suppression in normal colon mucosa may contribute to identifying patients at higher risk of developing CRC during screening procedures.
Howard
J Harrison
JH
Department of Surgery at the University of Alabama at Birmingham, USA.
Frolov
Andrey
A
Tzeng
Ching-Wei D
CW
Stewart
Ashley
A
Midzak
Andrew
A
Majmundar
Amar
A
Godwin
Andrew
A
Heslin
Martin
M
Bellacosa
Alfonso
A
Arnoletti
J Pablo
JP
D020137:Base Pair Mismatch; D015179:Colorectal Neoplasms / Q000235:genetics*; D019175:DNA Methylation; D004260:DNA Repair / Q000235:genetics*; D004273:DNA, Neoplasm / Q000235:genetics; D015536:Down-Regulation; D004706:Endodeoxyribonucleases / Q000235:genetics*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D056913:Mediator Complex Subunit 1; D010051:Ovarian Neoplasms / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics*
D004273:DNA, Neoplasm; C538855:MED1 protein, human; D056913:Mediator Complex Subunit 1; D014157:Transcription Factors; D004706:Endodeoxyribonucleases; C118712:MBD4 protein, human