Figure 4. Electrochemical assay for the detection of methyltransferase activity.

A self-assembled monolayer of DNA tethered to methylene blue on a gold electrode can be used to measure the methyltransferase activity of assayed solutions. The electrode is first treated with either purified methyltransferases, such as human Dnmt1, or lysates from tumors. If the DNA is not fully methylated, then a restriction enzyme can cut the DNA at a restriction site, highlighted in blue, within the DNA. After washing, the electrochemical signal is turned off, leading to a low percent of signal remaining compared to the signal before treatment with the restriction enzyme. If the DNA is methylated, corresponding to high methyltransferase activity, then the DNA is protected from restriction and the electrochemical signal is unperturbed, corresponding to a high percent of signal remaining. This assay has been used to detect the methyltransferase activity from tumor lysates (right). For tumor lysates tested with this device, the percent of signal remaining after treatment with restriction enzymes is high owing to increased methyltransferase activity. Lysates from adjacent tissue, however, have low percent signal remaining. Thus, lysates from tumors and normal tissue can be distinguished using this assay.