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Understanding DNA Replication and G-Quadruplexes: Insights from Recent Findings
Every day, an astonishing number of cells in the human body undergo division, which is essential for replacing damaged or aged cells. In this intricate process, the genetic material—comprising over 3 billion base pairs of DNA—must be precisely duplicated from the existing cell to its progeny.
However, during this vital operation, cells can experience what researchers identify as “replication stress,” leading to an increased likelihood of errors. These inaccuracies can result in mutations that may contribute to cancer and various other health conditions.
One significant factor causing this replication stress is when the machineries responsible for copying DNA become physically impeded. One example of such an impediment is the structure called G-quadruplex (G4), which can form from certain DNA sequences rich in guanine bases (indicated by the letter G). This structure is compact and can hinder the processing and replication of DNA.
A research team at Memorial Sloan Kettering Cancer Center (MSK) utilized cryo-electron microscopy (cryo-EM) to investigate G4s, which have recently emerged as potential targets in cancer therapies, seeking to understand their effects on DNA replication. Remarkably, the team captured a groundbreaking image depicting how cellular replication machinery navigates along DNA in human cells.
Published in the prestigious journal Science on March 7, these findings reveal new aspects of how secondary DNA structures such as G4s can obstruct the replication process, shedding light on fundamental questions in human biology.
Examining G-Quadruplexes in Cancer
According to Dr. Sahil Batra, a co-first author of the study, “The DNA double helix is iconic in science; however, DNA can assume multiple configurations, including G-quadruplexes.” With ongoing research into drugs that target G4s in cancer cells, the underlying mechanisms of their detrimental effects remain unclear, prompting further exploration.
The researchers noted that G4s are connected to several well-known oncogenes, including MYC and KRAS, along with their potential role in enabling cancer cells to maintain their telomeres, the essential caps on chromosomes that affect cellular aging.
“The concept is that by targeting G4s in cancer cells, we can essentially immobilize them, disrupting the unwinding and copying of DNA, thus hindering cancer cell division and growth,” Dr. Dirk Remus explained. “While we have established a connection between G4s and genomic instability, our research provides a more detailed understanding of the mechanisms at play and their adverse impacts.”
Exploring the Dynamics of G4s
Structural biologists employ various methods to visualize how biological molecules shape and interact, offering unique insights not accessible through conventional techniques. This approach can identify potential targets to inhibit or enhance certain molecular activities.
The recent study confirms how these secondary structures can act as physical barriers for DNA replication machinery, while also raising new inquiries regarding how these challenges can be overcome to permit successful replication.
Dr. Richard Hite emphasized the importance of accurate DNA copying during cell division. Replisomes, large protein complexes, are responsible for unwinding the DNA double helix before synthesizing the new genetic material for daughter cells. The replication process unfolds as the double strands separate, allowing machinery to traverse along these single strands, much like a train on a track.
As graduate student Benjamin Allwein illustrated, “Cryo-EM provided visual evidence that G4 structures could become trapped within the CMG helicase—the engine facilitating the unwinding process—analogous to a blockage on a train track.” This understanding allows scientists to further explore the intricacies of DNA replication and the potential therapeutic implications surrounding G4s.
“If such obstacles led to complete and irreversible stalling, cellular division would be impossible,” Dr. Batra noted. “Thus, this research enhances our understanding of the mechanisms through which DNA undergoes repair and correction during replication. Failures in these mechanisms correlate with various diseases, including cancer and neurodegenerative disorders.”
A Surprising Finding on CMG Helicase Movement
Furthermore, the research uncovered an unanticipated aspect of how CMG helicase traverses DNA strands. “Proteins frequently navigate along DNA to read and process genetic information, but the molecular mechanics remain elusive,” Dr. Remus stated.
This study challenges existing knowledge derived from studies in simpler organisms, illustrating that the enzyme functions differently in complex organisms such as humans. The authors described its action as a “helical inchworm,” shifting between flat and spiral configurations, which propels its advance along the DNA strands.
Dr. Hite emphasized that this dynamic oscillation is crucial for unwinding the extensive 3 billion base pairs with each cell cycle.
Additional Authors, Funding, and Disclosures
Other contributors to this research include Charanya Kumar, Sujan Devbhandari, Jan-Gert Brüning, Soon Bahng, Chong Lee, and Kenneth Marians, all affiliated with MSK.
Funding for the study was provided by the National Institutes of Health (R35GM152094, R35GM126907), the National Cancer Institute (P30CA008748), a Basic Science Research Innovation Award from MSK, and a Pershing Square Sohn Cancer Prize. Additionally, Dr. Hite serves as a consultant for F. Hoffmann-LaRoche Ltd.
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