Inactivation of the DNA repair gene O⁶‐methylguanine‐DNA methyltransferase by promoter hypermethylation and its relationship to aflatoxin B₁‐DNA adducts and p53 mutation in hepatocellular carcinoma

Abstract

Hepatocellular carcinoma (HCC) is one of the most common human malignant neoplasms with almost 500,000 estimated new cases enumerated worldwide during the most recent year surveyed. 1 Its etiology is not completely clear, but strong associations exist with hepatitis B and C virus infection and several dietary or environmental factors, including aflatoxin B 1 (AFB 1 ). The molecular pathogenesis of HCC appears to involve multiple genetic aberrations in the molecular control of hepatocyte proliferation, differentiation and death and the maintenance of genomic integrity. This process is influenced by the cumulative activation and inactivation of oncogenes, tumor suppressor genes and other genes. 0 6 -methylguanine-DNA methyltransferase (MGMT) is a 22,000 kd protein that is critical for the rapid reversal of methylation of guanine. 2,3 Alkylation of DNA at the O 6 position of guanine is a potentially mutagenic lesion mainly due to the tendency of O 6 -methylguanine to pair with thymine during replication, resulting in the conversion of a guanine-cytosine to an adenine-thymine base pair. 4 The level of MGMT activity varies greatly between species and cell types. 5,6 In humans, liver contains the highest level of MGMT activity. 5 The amounts of MGMT protein are decreased in some tumors and tumor cell lines. 7,8 Loss of expression is rarely due to deletion, mutation or rearrangement of the MGMT gene, but methylation of a discrete region of the CpG island of MGMT has been associated with the silencing of the gene in cell lines. 9 -11 Hypermethylation of normally unmethylated CpG islands in the promoter regions of many genes correlates with loss of transcription. 12 Early MGMT inactivation has been linked with gene mutation in some critical growth regulatory genes; K-ras mutations, especially G to A, are more common in colon cancer after MGMT inactivation. 13 Inactivation of the MGMT gene by promoter hypermethylation has been found in brain tumors, colon cancer, lung cancer and other cancers. 2 Recently, it was reported that MGMT promoter hypermethylation was present significantly more often in tumors with a G to A mutation in p53 than in tumors with other types of p53 mutation or in tumors with wild-type p53. 14,15 In our study, we explored the possible role of inactivation of MGMT in the development of HCC. We had previously analyzed these samples for p53 gene and protein alterations as well as measured DNA damage levels caused by the dietary carcinogen AFB 1 . 16 More recently, we reported a high frequency of promoter hypermethylation of the human ras association domain family 1A (RASSF1A) gene in HCC. 17 Thus, we also investigated the relationship between these biomarkers and methylation status.

Xeroderma pigmentosum group C gene expression is predominantly regulated by promoter hypermethylation and contributes to p53 mutation in lung cancers

“…Mutation in p53 was shifted to a G-A mutation by MGMT promoter hypermethylation in colorectal cancer (Esteller et al, 2000), and similar findings were later found for NSCLC (Wolf et al, 2001) and hepatocellular cancer (Zhang et al, 2003). These mutations have been attributed to endogenous deamination of methylated cytosine to thymine.…”

“…MGMT inactivation through promoter hypermethylation was the first example of a repair gene whose expression correlated with a p53 mutation pattern (Esteller et al, 2000;Wolf et al, 2001;Zhang et al, 2003). Mutation in p53 was shifted to a G-A mutation by MGMT promoter hypermethylation in colorectal cancer (Esteller et al, 2000), and similar findings were later found for NSCLC (Wolf et al, 2001) and hepatocellular cancer (Zhang et al, 2003).…”

“…Hypermethylation of O 6 -methylguanine-DNA methyltransferase (MGMT) has been shown to correlate with a G:C to A:T mutation pattern in p53 (Esteller et al, 2000;Wolf et al, 2001;Zhang et al, 2003). We thus determined whether p53 mutation patterns likewise shifted due to XPC hypermethylation; all 59 cases with p53 mutations were examined in this regard.…”

Quantitative promoter methylation analysis of hepatocellular carcinoma, cirrhotic and normal liver

“…Transcriptional silencing of the MGMT gene can be caused by promoter methylation but also by exposure to certain carcinogens. 33 Our findings indicate that MGMT promoter methylation is probably not involved in HCC development and is not a useful molecular marker. This is consistent with previous results by Esteller et al 6 By contrast, Zhang et al found MGMT gene methylation in 39% of HCC and a correlation with aflatoxin B1-exposure.…”

“…This is consistent with previous results by Esteller et al 6 By contrast, Zhang et al found MGMT gene methylation in 39% of HCC and a correlation with aflatoxin B1-exposure. 33 These discrepant results might be explained by the influence of aflatoxin B1-exposure in the study population examined. Furthermore, for MGMT gene-promoter methylation analysis, it is important to define the CpG islands to be analyzed as demonstrated by Matsukura et al 34 In contrast to MGMT, APC and DAP-K, the methylation status of the promoter of p16 INK4a and GSTP1, which encodes for the gluthatione S-transferase, show a strong correlation with hepatocarcinogenesis.…”