Researchers at Colorado State University have identified an alternate method to study changes during the DNA replication process in lab settings using genetically modified yeast. The new approach offers a clearer window than current drug methods used to understand cell cycle arrest – a fundamental mechanism that is key to treating cancers and genetic issues.
The findings were published in the Proceedings of the National Academy of Sciences and were led at CSU by Assistant Professor Grant Schauer in the Department of Biochemistry and Molecular Biology. The work focuses on hydroxyurea, a chemotherapy drug used in clinical settings to treat cancer, which is also frequently used in research settings to arrest cells in their development cycle for study. Doing so allows researchers to better explore the complex process of how a genome's DNA is accurately copied in cells before they divide.
This process happens frequently in the body. However, health issues arise if the DNA being copied becomes altered by harmful metabolic intermediates, UV light or chemotherapeutics. When cells undergoing replication encounter these problems at specific biologically managed checkpoints, the process is stopped to prevent further issues. Hydroxyurea is used to study how and when a cell halts the replication process by activating these stopping points for study of the intricate biological processes at play.
For decades, the drug was believed to work by halting production of the building blocks for DNA, but Schauer's team noticed that the drug was also creating damaging and reactive oxygen species in key parts of the cell at the same time. Schauer said those unwanted reactions clouded insight into the cell's "kill switch" mechanisms, which prevent DNA from being copied incorrectly in harsh oxidative environments.
"Our work shows that hydroxyurea is stopping this replication process in a less specific way than anyone had originally thought," he said. "We found that oxidation was inhibiting DNA polymerases – the enzymatic machines that directly copy the DNA – by targeting iron atoms in the enzymes and making them apart. Something that persisted even after the drug was removed from the process."
To address that problem, the CSU team developed a system that uses genetically engineered yeast cells known as RNR-deg. That system provides a less toxic and quickly reversible alternative to hydroxyurea to stop the process. Because hydroxyurea is widely used today, the new approach outlined in the paper could significantly change how research around cell arrest is accomplished.
Schauer said the team used flow cytometry to study the DNA content and processes of cells for the work. Research funding for the paper came from the National Institutes of Health and several undergraduate researchers contributed to the work, Schauer added.
Hannah Reitman, an undergraduate biochemistry student, served as an author on the paper after collecting and analyzing data for the project. She said work in the lab was intimidating at first but became a great learning experience.
"Through research on this project and in the lab, I have learned so many techniques and concepts that I would not have access to in a lecture," she said. "You gain so much independence in lab and learn really good problem-solving skills. That is something that I will continue to expand upon throughout my entire career and I am eternally grateful to have."
Schauer said the team will continue to work on this topic and plans to start transferring the technique from the yeast cells into human cells.
"RNR-deg yeast strain seems to be a very viable and potentially superior alternative," he said. "It does not have any of the negative effects of hydroxyurea that have likely been clouding our understanding to this point. This is an important finding, and I look forward to continuing our research towards use in human cells in the future."
