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Genomic instability and pre-cancerous lesions: the role of R-loops gradually revealed

Claudia Legris
09/04/2020
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In the cellular construction process, DNA transcription and replication mechanisms can sometimes collide. These collisions lead to errors in DNA replication in the cell, and resulting defective cells can then lead to cancer.
Chen - R-loops

When poorly copied, the cells have defects that could subsequently lead to the occurrence of cancer. RNA:DNA hybrids structures whose role may be positive or negative. At the heart of this process are R-loops: In an article published in Nature Communications on August 7th, 2020, researchers from Institut Curie and the Institute of Human Genetics shed some light on their role.

R-loops have already been studied in detail. They are found at both the beginning and at the end of the gene

Explains Chun-Long Chen, Head of Institut Curie’s Replication and Genome Instability program team.

Researchers have identified several areas where they play a role, such as the regulation of gene transcription,  transcriptional termination, and even cell division. “But R-loops can also cause negative and toxic reactions related to genome instability.  What we don’t yet know, however, is why some R-loops are toxic and others are not.  That is what we’re trying to find out.”

Identifying the role of Topoisomerase I

The research team focused its efforts on cells depleted of Topoisomerase I (TopI), an enzyme that relaxes DNA supercoils and prevents the formation the three-stranded nucleic structures known as R-loops. DNA generally takes the form of a pair of strands coiled in a double helix, but during replication, this double helix separates and forms a fork, to which new nucleotide sequences attach themselves. When the transcription and replication processes collide, they cause the fork to stall and the gene replication to stop.

Our results show that we can avoid frontal conflicts between replication and transcription when the replication fork stop contains R-loops at the end of the highly-expressed genes (terminators or TTS).  This helps maintain the genome’s integrity.

Continues Chun-Long Chen. In this way, Topoisomerase I prevents replication stress in highly-expressed genes.

In-depth breast cancer research

The researchers are also collaborating in this research with the French National Cancer Institute (INCa), where they are focusing their work on breast cancer.

A recent study showed that there was an accumulation of R-loops on the genes that cause breast cancer,” explains Chen.  “What we want to do now is to use cellular models to determine whether or not Topoisomerase I inhibitors affect both the accumulation of R-loops and the resolution of the transcription-replication collision.

Their research is of vital importance, and may pave the way for new therapeutic and preventive options for breast cancer treatment in the future.

R-loops - Chen

An example image showing R-loop foci detected by Immunofluorescence with S9.6 antibody in normal HeLa cells (Ctrl) and in Telomerase I (Top1) depletion cells (shTop1). Right: Quantification. Images, provided by P. Pasero lab (IGH)

This study was published in Nature Communications on Friday August 7th, 2020. Read more.