Novel O⁶-methylguanine-DNA methyltransferase SNPs: A frequency comparison of patients with familial melanoma and healthy individuals in Sweden

Abstract

The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) is involved in the cellular defense against alkylating agents. Genetic alterations in the MGMT gene may impair the protein's ability to remove alkyl groups from the O6-position of guanine, thereby raising the mutation rate and increasing the risk of cancer. We assessed polymorphisms in the promoter region and the 5 exons of the MGMT gene by PCR/SSCP and nucleotide sequence analysis of DNA extracted from blood samples. The population studied consisted of 89 melanoma patients, each belonging to a different family with a hereditary predisposition for melanoma, and 76 healthy individuals (blood donors). A total of 11 single nucleotide polymorphisms (SNPs) were detected, five in the promoter region, one in exon 1, two in exon 3 and three in exon 5. Six of the alterations were novel polymorphisms, of which five were located in the promoter region and one in exon 5. When the distribution of specific SNPs in cases and controls with only one variant was calculated; C575A was present only in melanoma patients (p=0.072). Moreover, while 20% of the healthy individuals had no SNPs this was the case in only 12.4% of the melanoma patients. However, no statistically significant differences were seen between cases and controls for any of the 11 SNPs.

Generating mutations but providing chemosensitivity: the role of O6-methylguanine DNA methyltransferase in human cancer

“…However, there are many reports of individual differences in MGMT activity in tumors. Several germline variants affecting the MGMT gene have been described (Imai et al, 1995;Rusin et al, 1999;Inoue et al, 2000;Egyhazi et al, 2002), but few studies have addressed the functional relevance of these nucleotides changes (Imai et al, 1995;Edara et al, 1996). No extensive analysis of the MGMT genotype and haplotypes distribution in healthy and cancer population exists.…”

O6-methylguanine-DNA-methyltransferase expression and gene polymorphisms in relation to chemotherapeutic response in metastatic melanoma

“…In a previous study (Egyházi et al, 2002), we investigated SNPs in the promoter and coding regions of the MGMT gene in blood from patients with familial melanoma and healthy Swedish individuals. In total, 11 SNPs of the MGMT gene were identified in that study, including variants at codons 53 and 84 in exon 3, and at codons 143, 178 and 197 in exon 5.…”

Differential inactivation of polymorphic variants of human O6-alkylguanine-DNA alkyltransferase

“…In contrast, the I143V/K178R variant is quite common with a frequency of c. 24% (11−28%) in various studies [17][18][19][20][21][22][23][24]27,[29][30][31], and it is active in protecting cells from alkylation damage. Even in individuals with one allele, the very strong selection pressure that is provided under conditions involving treatment with temozolomide or BCNU plus a hAGT inhibitor would select for cells in which a hAGT form resistant to an inhibitor was present.…”

“…The SNPS leading to the I143V and K178R changes are in almost perfect disequilibrium [17,18,24,30] and it is highly likely that both changes occur in the protein derived from this gene. In order to examine the extent to which two linked alterations contribute to the resistance to inhibitors observed, N-terminal His 6 -tagged hAGT proteins with the individual mutations were produced separately and tested (Table 3).…”

“…Variants W65C [15,16], L84F [15][16][17][18][19][20][21][22][23][24][25][26][27][28], I143V/ K178R [17][18][19][20][21][22][23][24]27,[29][30][31] and G160R [29,32] have been reported by multiple laboratories. These reports raise the possibility that there may be individual variation in response to hAGT inactivators.…”