Epigenetic downregulation of the DNA repair gene MED1/MBD4 in colorectal and ovarian cancer

Abstract

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 (≥ 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.

Effect of UVB irradiation on microRNA expression in mouse epidermis

“…The base excision repair protein MED1 is a predicted target of mmu-miR-146a. MED1 interacts with the mismatch repair protein MLH1 and has a key role in the maintenance of genomic stability, with dual functions in DNA damage response and repair (10,11). MED1 acts as a thymine and uracil DNA N-glycosylase on T:G and U:G mismatches that occur at cytosine-phosphate-guanine (CpG) methylation sites due to the spontaneous deamination of 5-methylcytosine and cytosine, respectively.…”

Gene Expression, DNA Methylation and Prognostic Significance of DNA Repair Genes in Human Bladder Cancer

“…In contrast to reduced expression in HCC, MBD4 mRNA was overexpressed in glioblastoma [52]. In sporadic colon cancer, promoter methylation of MBD4 and decreased MBD4 mRNA expression was observed in an early stage of disease with a trend towards the higher stage and lower MBD4 expression [53]. In our study, MBD4 expression was decreased in 28% of tumors.…”

Silent beginning: Early silencing of the MED1/MBD4 gene in colorectal tumorigenesis

“…4,5 In the present issue of Cancer Biology & Therapy, Howard and colleagues show that the base excision repair gene MED1 is silenced by promoter hypermethylation in colorectal and ovarian cancers. 6 The MED1 gene, also known as MBD4 (methyl-CpG binding domain 4), encodes a protein containing an N-terminal methylCpG binding domain and a C-terminal catalytic domain, separated by a central region lacking a recognizable structure. 7 The MED1 protein has several functions.…”

“…In humans, MED1 is frequently mutated in mismatch-repair deficient tumors, especially colorectal cancers, displaying microsatellite instability (40%). 17,18 In particular, these tumors show a frameshift in two polyadenine ((A) 10 and (A) 6 ) tracts (at codons 310-313 and 247-248, respectively) leading to truncated MED1 protein lacking N-glycosylase activity. 19 It has been proposed that the truncated protein acts in a dominant negative way, inhibiting normal glycosylase activity of wild-type MED1.…”

“…20 Now Howard and colleagues show for the first time an additional mechanism of MED1 inactivation. 6 They report that loss of MED1 expression may be associated with hypermethylation of key CpG sites in its promoter region. The authors analysed the methylation status of MED1, by means of bisulfite sequencing, in a panel of ovarian and colorectal cancer cell lines, in primary colorectal adenomas and carcinomas, as well as in normal colorectal mucosa from patients and controls.…”