Each cell in our body experiences up to tens of thousands of DNA-damaging events per day, primarily from routine cellular maintenance or exposure to toxins in our environment. Despite the high frequency of wear-and-tear, cells have efficient repair mechanisms that work tirelessly, allowing our bodies to maintain our DNA in each one of our cells throughout our lifetimes.
The breakdown in this ongoing dance of DNA damage and repair is one of the root causes of cancer. Sarah Hengel, an assistant professor of biology at Tufts, Kara Bernstein, a professor of biochemistry at the University of Pennsylvania School of Medicine, and their collaborators have identified a set of DNA repair support proteins that may become a target for the treatment of female reproductive cancers. Their research was recently published in the journal Nature Communications.
Unlike breast cancer, endometrial/uterine and ovarian cancers have few effective treatment options outside of surgery, and it's not uncommon for these tumors to return after first-line chemotherapeutics. Often patients with these cancers are prescribed chemotherapeutics, but sometimes this strategy stops working and scientists don't really know why.
"For these patients, there aren't any good therapies, and so my whole focus is using my background in drug discovery to target proteins we think are involved for female reproductive cancers," Hengel said.
Her research team is focused on two DNA repair proteins -- SWSAP1 and SWS1, known together as the Shu complex. Using a million-dollar advanced research instrument called a C-trap, which uses lasers as optical tweezers and allows her team to watch proteins interact with DNA in real time, Hengel made a surprising discovery about how these proteins work.
The tool makes it possible for Hengel to express cancer variants or mutations in the protein that come from a patient's DNA. "We make the proteins directly from cancer cells, extract them, and literally look at how those mutant proteins function," she said. "It may be that some patients have mutations that function differently from those that don't have mutations. This will ultimately be able to determine which variants are bad and susceptible to specific treatments in the future."
Potential drug target
The Shu complex, it turns out, may act as a backup system for BRCA2, a well-known protein that when mutated may lead to breast and endometrial/uterine cancer. SWSAP1-SWS1 helps repair broken DNA by bringing a suite of repair proteins to the site of damage and keeping them attached and moving along the DNA to conduct their repairs. BRCA2 removes coatings on DNA strands to ensure that repair proteins can reach their targets.
Both BRCA2 and the Shu complex were found to also collaborate with similar proteins. One important player is the "molecular matchmaker" RAD51, which attaches itself to broken DNA strands and helps them find their way to their sibling strand, which then serves as a template to guide the repair of its twin.
Another protein, RPA, acts like a first responder in DNA repair, latching onto and protecting the "split ends" of single strand DNA. When the Shu complex is present, it helps guide RPA to move along the DNA strand, allowing for more efficient repair.
"We knew that RAD51 and RPA were related to the Shu complex, but no one had really done a thorough biochemical or cellular characterization," said Hengel. "That's where this whole work began."
While these repair processes are highly effective, they're not perfect. Over time, accumulated DNA damage caused from environmental toxins can contribute to aging and diseases like cancer. Shu complex defects, for example, have been identified in breast, uterine/endometrial, and prostate cancers.
Cancer cells experience more DNA damage than healthy cells due to their rapid division, making them especially reliant on repair mechanisms. When these repair systems are compromised -- either through natural mutations or targeted treatments -- tumors become more vulnerable to destruction.
Knowing this, Hengel and her team wanted to see what happens when tumors have their Shu complex knocked out of commission. They found that certain cancers rely heavily on these proteins to survive, revealing an exploitable weakness.
For example, cell lines without functioning Shu complexes were more sensitive to the chemotherapeutic drug Olaparib and other molecules that can disrupt aspects of the DNA repair process. This observation led the researchers to conclude that the Shu complex could be a potential drug target for cancers with defective versions of these molecules.
Future research will explore new compounds that could work in tandem with existing courses of chemotherapy, as well as how these proteins may play a role in fertility and reproductive health. Hengel and her team at Tufts recently published a review on this topic in NAR Molecular Medicine.