Toronto researchers discover human DNA repair by nuclear metamorphosis

University of Toronto researchers have discovered a DNA repair mechanism that could help us understand how human cells stay healthy and could lead to new treatments for cancer and premature aging.

study published in the journal Structural and molecular biology of naturealso sheds light on the mechanism of action of some existing chemotherapy drugs.

“We believe this research solves the mystery of how DNA double-strand breaks occur and The nuclear envelope turns on for repair in human cells » said the professor Karim Mehailresearch associate researcher and professor laboratory medicine and pathabiology at U of T's Temerty School of Medicine.

“It also makes many discoveries previously published in other organisms applicable in the context of human DNA repair, helping science move even faster.”

DNA double-strand breaks occur when cells are exposed to radiation and chemicals and by internal processes such as DNA replication. They are among the most serious types of DNA damage because they can inhibit cell growth or contribute to aging and cancer.

A new discovery made in human cells in collaboration with Prof Razkallah HakemResearcher University health care network and Temerty Professor of Medicine, expands on previous research on DNA damage in yeast by Mehail and others.

In 2015, Mehail and Associates showed How motor proteins deep down the the nucleus of yeast cells transports double-strand breaks to “DNA hospital-like” protein complexes embedded in the nuclear envelope at the periphery of the nucleus.

Other studies have revealed related mechanisms during DNA repair in flies and other organisms. However, scientists studying similar mechanisms in human and other mammalian cells have reported little or no DNA mobility for most breaks.

“We knew that nuclear envelope proteins are important for DNA repair in many of these organisms, so we learned how to explain the limited mobility of damaged DNA in mammalian cells,” Mekhail says.

The answer is surprising and elegant.

When the DNA in the nucleus of a human cell is damaged, a specific network of microtubule filaments forms in the cytoplasm around the nucleus and push into the nuclear envelope. This induces the formation of small tubules or tubules that enter the nucleus and trap most of the double-strand breaks.

“It's like fingers pushing a ball,” Mekhail says. “When you squeeze the balloon, your fingers create tunnels in its structure, pushing some of the outer part of the balloon inside.”

Further research by the study authors detailed several aspects of this process. DNA damage response kinases and enzymes called tubulin acetyltransferase are key regulators of the process and promote tubule formation.

Enzymes leave a chemical imprint on a specific part of the microtubule filament, which causes them to attract small motor proteins and push the nuclear envelope. Therefore, repair-promoting protein complexes push the envelope deep into the nucleus and bridge the DNA break.

“This causes the nucleus to undergo a type of reversible metamorphosis, which allows the envelope DNA to temporarily enter the nucleus, capture and recombine the damaged DNA,” Mehail says.

The results have significant implications for the treatment of certain cancers.

Normal cells use nuclear envelope tubules to repair their DNA, but cancer cells need more of them. To investigate the potential impact of the mechanism, the team analyzed data from more than 8,500 patients with various cancers. This need has been observed in several cancers, including triple-negative breast cancer, which is highly aggressive.

“There is a huge effort to identify new therapeutic avenues for cancer patients, and this discovery is a big step forward,” he said. Judgeis a senior scientist at UHN's Queen Margaret Cancer Center and a professor in the Departments of Medical Biophysics and Laboratory Medicine and Pathology at U of T.

“Until now, scientists have been unclear about the relative effectiveness of the nuclear envelope in repairing damaged DNA in human cells. Our collaboration has shown that the use of factors that modulate the nuclear envelope to repair damaged DNA effectively inhibits the development of breast cancer,” says Hakem.

Aggressive triple-negative breast cancer has high levels of tubules, which may have more DNA damage than normal cells. When the researchers deleted the genes needed to control the tubules, the cancer cells were unable to form tumors.

One of the drugs used to treat triple negative breast cancer is a class of drugs called PARP inhibitors. PARP is an enzyme that binds to damaged DNA and helps repair it. PARP inhibitors block the repair enzyme by preventing the ends of DNA double-strand breaks in cancer cells from rejoining each other.

Cancer cells join together two broken ends that are not part of the same pair. As mismatches form, the resulting DNA structures become impossible for cells to copy and divide.

“Our study shows that it is the tubules that cause these discrepancies. When tubules are low, cancer cells become more resistant to PARP inhibitors,” says Hakem.

Collaboration between researchers from different fields has been essential to the discovery of cancer cells. The research highlights the importance of interdisciplinary collaboration, Mehail says.

“The brain power behind every project is very important. Every team member counts. Also, every right partner joining a research project is like getting another doctorate in a new specialty; it's powerful,” he says.

Mehail noted that the discovery also applies to premature aging conditions such as progeria. A rare genetic condition causes rapid aging in the first two decades of life, usually leading to early death.

Progeria is associated with a gene encoding lamin A. Mutations in this gene reduce the rigidity of the nuclear envelope. The team found that expression of mutant lamin A was sufficient to induce tubules, which were further strengthened by DNA-damaging agents. The team believes that even mild pressure on the nuclear envelope promotes tubule formation in prematurely senescent cells.

The results suggest that DNA repair in progeria may be disrupted by the presence of too many or poorly regulated tubules. The results of the study have implications for many other clinical conditions, Mekhail says.

“It's exciting to think about where these findings will take us next,” says Mehail. “We have great colleagues and great experience at Temerty Medicine and our partner hospitals. We are already following up on this discovery and using our work to develop new therapeutics.”

This research was supported by the Canadian Institutes of Health Research, the Royal Society of Canada, the University of Toronto, and Princess Margaret Hospital, University Health Network.

Read more about the results on the website of the department of laboratory medicine and pathobiology.