A new addition to the DNA repair toolkit

Maintaining genomic integrity is crucial for cellular health and proper development. DNA double-strand breaks (DSB), one of the most detrimental forms of genetic damage, can arise from various sources including radiation, chemical exposure, and even the natural process of gene replication. Unrepaired DSB can lead to cell death, cancer, or premature aging.

Cells rely on two DNA repair pathways to counteract the detrimental effects of DSB. The non-homologous end joining (NHEJ) pathway directly rejoins broken DNA strands, while the homologous recombination (HR) one uses a homologous sister chromatid for more accurate repair.

Karim Mekhail in a blue shirt standing in front of a light brown brick wall

Karim Mekhail’s laboratory found a new DNA repair process in humans.

CREDIT: University of Toronto

In a new study, researchers from the University of Toronto, led by molecular biologist Karim Mekhail, discovered a new DNA repair mechanism. The team used live-cell imaging to study a commonly overlooked structure on the nuclear envelope and found that it helped fix DSB (1).

“We now think of the nuclear envelope in the context of DNA repair like a hospital,” said Mekhail. “This is because the DNA needs to access certain repair factors in the envelope.”

Mekhail and his team previously reported that kinesin motor proteins facilitate the transport of DSB within the nucleus in yeast to specialized repair centers where NHEJ and HR take place (2). “These motor proteins act as ambulances for damaged DNA,” said Mekhail. However, attempts to translate these findings into mammalian cells proved challenging. This discrepancy highlighted the potential existence of a distinct and previously unidentified mechanism for DSB trafficking in mammalian cells, necessitating further investigation.

Mekhail said, “We did not understand how damaged DNA gets to the nuclear envelope in humans or how the DNA associates with it during repair. We went about live-imaging the nucleus undergoing DNA damage in 3D using the latest technologies that we have.”

This approach revealed the formation of structures from the nuclear envelope that extended into the nucleoplasm. These structures, termed DSB-capturing nuclear envelope tubules (dsbNET), colocalized with DSB in a DNA damage-dependent manner. “Imagine that the hospital [nuclear envelope] is in a sci-fi world. The hospital building can be reshaped and extended to create a new wing. The hospital comes to you; you don't need to go to it,” said Mekhail.

I believe this could provide us with an entirely novel toolbox that we can use to go after many different cancers.
- Karim Mekhail, University of Toronto

To identify how dsbNET function, the group knocked down a protein involved in the NHEJ repair pathway, the kinesin family member 5B (KIF5B). Notably, KIF5B becomes significantly upregulated in various cancers, including breast cancers. Depleting KIF5B in breast cancer cells decreased tumor cell proliferation compared to controls, suggesting KIF5B is a positive regulator for dsbNET formation.

“I'm very familiar with Dr. Mekhail’s data and his previous research,” said Susan Gasser, a biochemist and director of the Foundation of the Swiss Institute for Experimental Cancer who was not part of this study. “I’m excited to see the implications of this new discovery,” she added.

Mekhail and his team are exploring how existing chemotherapies interact with dsbNET and DNA repair mechanisms. The group’s next steps include collaborating with clinicians to investigate dsbNET from patient tumor biopsies to better diagnose them.

Mekhail added, “I believe this could provide us with an entirely novel toolbox that we can use to go after many different cancers.”