Different patterns of DNA methylation of the two distinct O6-methylguanine-DNA methyltransferase (O6-MGMT) promoter regions in colorectal cancer

Abstract

Colorectal cancer (CRC) is the third most common cancer worldwide. Colorectal cancer incidence differs widely among different geographic regions. In addition to mutational changes, epigenetic mechanisms also play important roles in the pathogenesis of CRCs. O6-methylguanine-DNA methyltransferase (O(6)-MGMT) is a DNA repair protein and in the absence of MGMT activity, G-to-A transition may accumulate in the specific genes such as K-ras and p53. To identify which CpG sites are critical for its downregulation, we analyzed the methylation status of the MGMT gene promoter in two sites in CRC patients. Then we compared the frequency of their methylation changes with the results of our previously reported K-ras gene mutation, APC2 and p16 methylation. MGMT methylation was examined in 92 tumor samples. A methylation specific PCR (MSP) method was performed for two loci of MGMT gene which described as MGMT-A and MGMT-B. The prevalence of MGMT-A, and MGMT-B methylation was 49/91 (53.8%), and 83/92 (90.2%), respectively. We detected high frequency of MGMT-B but not MGMT-A methylation in tumor tissues with APC2 methylation. Our results showed that MGMT-B methylation is significantly associated with K-ras gene mutation rather than MGMT-A (p = 0.04). Simultaneously, an inverse correlation was found between p16 and MGMT-B methylation simultaneously (p = 0.02). Our study indicated that hypermethylation of the specific locus near the MGMT start codon is critical for cancer progression. MGMT-B assessment that is associated with K-ras mutation can have a prognostic value in patients with CRC.

MGMT-B Gene Promoter Hypermethylation in Patients with Inflammatory Bowel Disease - A Novel Finding

“…Therefore, DNA integrity and DNA repair enzymes are crucial especially in IBD patients. MGMT removes alkyl adducts from the DNA especially O6-methylguanine, which is read as an adenine by the DNA polymerase (Mokarram, et al, 2013). Our promising results in recent publication showed that MGMT-B methylation is significantly associated with Kras gene mutation rather than MGMT-A (Mokarram, et al, 2013).…”

“…MGMT removes alkyl adducts from the DNA especially O6-methylguanine, which is read as an adenine by the DNA polymerase (Mokarram, et al, 2013). Our promising results in recent publication showed that MGMT-B methylation is significantly associated with Kras gene mutation rather than MGMT-A (Mokarram, et al, 2013). Defect in MGMT leads to G to A transition mutation which shown in CRC patients with K-ras mutation (Mokarram, et al, 2013).…”

“…Since, methylation silencing of MGMT-B rather than MGMT-A was significantly associated with Kras gene mutation in CRC patients but not in normal tissues (Mokarram, et al, 2013), we hypothesized that methylation of MGMT-B as caretaker gene may be responsible for dysregulation of signaling through Kras mutation in IBD patients.…”

“…DNA Mismatch repair elements and O6-methylguanine-DNA methyltransferase (O6-MGMT) are DNA repair proteins. MGMT loss of activity associated with creating a permissive state for G>A mutations in protooncogenes such as Kras and p53 (Mokarram et al, 2013). Kras mutations occur in about 60% of CRCs and are an early event in UC carcinogenesis (Aust, et al, 2005;Lao and Grady, 2011).…”

“…MGMT promoter has two loci (Lin, et al, 2011;Mokarram, et al, 2013) which we described as MGMT-A DOI:http://dx.doi.org/10.7314/APJCP.2015.16.5.1945 …”