New understanding of how chemotherapy drug interacts with DNA

A chemotherapy drug has been found to halt the replication of cancer cells via an unexpected mechanism.

Researchers from Cornell University, New York, have investigated the effect of a molecule used as a chemotherapy drug called etoposide on an enzyme that can affect the overall shape of DNA strands in the cell. They found disruption of this enzyme could halt replication of cancer cells and promote cell death, as the DNA was unable to iron out coils that prevent transcription and translation from occurring.

'We normally ask: What is the best way to study molecular machineries that take place on DNA? To understand how those enzymes work, we want to mimic what might be happening in the cell. Motor proteins pull on the DNA or apply a force on the DNA. So we said, ok, we can apply a force and see what happens.' said Michelle Wang, professor of physical sciences at Cornell University.

Typically, during transcription and replication, DNA becomes tangled, also known as supercoiling. The enzyme, topoisomerase II, is there to untangle DNA when coiling builds up. To do so, topoisomerase II breaks both DNA strands, untangles the knot and repairs the break. However, the chemotherapy drug investigated was found to stabilise DNA breaks, increasing the number of broken strands, which leads to cell death.

Publishing their results in Nature Chemical Biology, the scientists used three different techniques to monitor the effect of the chemotherapy drug on three topoisomerase II: yeast topoisomerase II, human topoisomerase II alpha and human topoisomerase II beta. One method used light to stretch DNA and form DNA loops. An increase in the drug increased the number of stable loops occurring in the DNA, changing the structure of it.

Another method used light to unzip DNA and observe how proteins interacted with DNA. In the presence of etoposide, topoisomerase II was converted into a roadblock and could not be removed. And removing topoisomerase II led to double stranded breaks in the DNA.

Researchers then used magnets to unzip the DNA. They found topoisomerase II bound to DNA can relax the DNA, preventing coiling. Studying the effect of the drug showed topoisomerase II was trapped in a stable state when it was present.

By investigating the drug's effects on topoisomerase II, scientists can better understand how other topoisomerase drugs behave.

'We anticipate that the techniques used here will be beneficial in the study of a broad range of topo catalytic inhibitors and poisons, serving as sensitive screening tools that can provide insights into defining drug mechanisms.' added Professor Wang.