It’s estimated that over 281,000 women in the U.S. will be diagnosed with breast cancer this year, according to the National Cancer Institute. And about one in seven women will receive a breast cancer diagnosis in their lifetime.
For those with breast cancer gene 2 (BRCA2) mutations, the risk of developing breast cancer is much higher. Between 45 and 69 percent of those with this genetic mutation will develop breast cancer by 70 to 80 years of age.
Normally the BRCA2 gene acts as a tumor suppressor, promoting the successful repair of double-strand breaks in DNA. Harmful variants of this gene impair its DNA repair function, leading to the development of BRCA2-deficient tumors. PARP inhibitors, which are a type of drug that prevents cancerous cells from repairing themselves, exploit this vulnerability in BRCA2-deficient tumors by inducing a type of DNA damage that requires the BRCA2 protein for repair. The BRCA2 gene contains the genetic instructions to create the BRCA 2 protein.
This personalized treatment targets tumor cells and spares normal tissue. Currently, PARP inhibitors are efficiently employed to treat BRCA2-deficient tumors of the breast, ovary, pancreas, and prostate. But tumors develop resistance to PARP inhibitor treatments.
“Of course, the problem is, ‘What’s next?’” said Distinguished Professor of Microbiology and Molecular Genetics Wolf-Dietrich Heyer. “There are a number of ongoing efforts across the world in many laboratories to understand how one can enhance the efficacy of these PARP inhibitors.”
In a study that appeared recently in Nature Cell Biology, Heyer and an international team of collaborators explored the alternative molecular pathways BRCA2-deficient tumors employ to repair DNA damage and survive treatment. The research highlights genetic vulnerabilities in BRCA2-deficient tumors that could be exploited to stop the spread of cancerous cells.
“This paper is quite a breakthrough in trying to understand how repair pathway control operates at the double-strand break level,” said Heyer. “This is important because targeting these pathways is a new therapeutic route for treating cancer patients.”
Uncovering a temporal order
Prior to this study, scientists knew healthy cells employed a variety of proteins, like BRCA2, DNA polymerase theta (POLθ) and RAD52, to repair DNA. What wasn’t understood was how and when the cell decides which molecular pathway to utilize.
“One always asks, ‘Why are there multiple pathways? Are they working on top of each other?’” said Heyer. “No, the answer is there are multiple pathways because they act at different times.”
To uncover this biological insight, Heyer’s collaborator Professor Markus Löbrich, a renowned radiobiologist and expert cell biologist from the Technical University of Darmstadt (Germany) developed a way to study DNA repair throughout a cell’s lifecycle. A cell’s lifecycle consists of four phases, including growth (G1), DNA replication (S), growth (G2) and mitosis (M). Löbrich’s team discovered that a cell uses BRCA2 for DNA repair in the S and G2 phases, but POLθ-mediated DNA repair was only active during the M phase.
“In this publication, Markus and his team clarified that it’s a temporal control that ensures that the right pathway is used at the right time,” said Heyer. “We provided the underlying biochemical mechanisms that enforce this temporal order.”
Molecular underpinnings of tumor growth
Outfitted with this knowledge of how healthy cells function, the research team shifted their focus to BRCA2 mutants and RAD52, a DNA repair protein that’s integral to repairing double-strand breaks.
“BRCA2-deficient cells require RAD52 to survive,” said Heyer, noting that the rationale had previously been that RAD52 provides an alternative repair pathway.
Löbrich’s team instead showed that, unexpectedly, cells lacking both BRCA2 and RAD52 repaired DNA breaks better but did not survive. They also showed that POLθ was able to repair the breaks, though in a faulty way that led to the cell’s death during subsequent cell division. This led to the revelation that the role of RAD52 in BRCA2-deficient cells is not to promote repair but to delay repair by POLθ to M phase.
What keeps POLθ from working in normal cells prematurely? The results pointed to BRCA2 itself and RAD52. Heyer’s team showed this directly with purified proteins that both inhibit DNA synthesis by POLθ and by independent mechanisms. This required the contribution of purified POLθ by a third team led by Dr. Richard Wood of the MD Anderson Cancer Center, a world specialist on the function of POLθ.
Safer cancer treatment
According to Heyer, this foundational research could influence the design of therapeutics used to treat BRCA2-deficient tumors, specifically those that target the DNA repair mechanisms that allow a tumor’s spread.
“About one-quarter of breast cancers actually are deficient in DNA recombination,” said Heyer. “So the number of people who are potentially eligible for PARP inhibitors is that high.”
But treatment resistance is still a concern, which is why it’s imperative to develop additional approaches to enhance the efficacy of PARP inhibitors and outflank resistance mechanisms by targeting alternative repair mechanisms.
“Inhibitors targeting RAD52 and POLθ are already in development,” said Heyer. “What we are providing is the scientific underpinning of how the mechanisms work, which will impact their use and patient selection.”
Heyer added that, “The idea of personalized medicine based on synthetic lethality is that you genetically analyze a tumor and identify vulnerabilities that are specific to the tumor, meaning that you create conditions that not only knock out the tumor but really don’t affect normal cells. This lowers the toxicity of the treatment, makes it more effective and increases the quality of life for the patient.”
The Heyer and Löbrich teams will continue their collaboration exploring how these DNA repair pathways function in cancerous cells, with hopes of identifying and further defining how break repair is regulated by BRCA2 and RAD52.
POLθ-mediated end joining is restricted by RAD52 and BRCA2 until the onset of mitosis (Nature Cell Biology)