Transcriptional Inhibition of Manganese Superoxide Dismutase (SOD2) Gene Expression by DNA Methylation of the 5′ CpG Island

Manganese superoxide dismutase (MnSOD) enzyme activity and SOD2 gene expression have often been reported to decrease during the development of cancer. SOD2 has also been implicated as a candidate tumor suppressor gene for human malignant melanoma. Genomic DNA methylation patterns are also known to change during carcinogenesis and serve as a mechanism for tumor suppressor gene inactivation. We hypothesized that decreased SOD2 gene expression in some malignant cell populations may be due, at least in part, to methylation of upstream transcriptional regulatory sequences in the SOD2 gene. To test this hypothesis we transfected methylated and unmethylated SOD2-CAT promoter-reporter constructs in cells known to express the SOD2 gene. Our results indicate that methylation of specific cytosines in the SOD2 5′ flanking region is sufficient to repress transcriptional activity of the SOD2 promoter by at least 50%. Moreover, we show that this transcriptional repression was likely mediated by inhibition of AP-2 DNA binding and transactivation from a methylated AP-2 binding site in the SOD2 promoter. DNA methylation may provide a mechanism for transcriptional inactivation of the SOD2 gene during the development of some cancers.