There are two ways to fix things: the quick way and the careful one. In cancer, cells with BRCA1 mutations tend to choose the quick fix, leaving them vulnerable to medicines like PARP inhibitors.
PARP inhibitors cause DNA to break. When broken strands and sketchy repairs build up, the cancer cell will die. One of the biggest challenges with PARP inhibitors, however, is that cancers often learn to resist them.
New research from the lab of Dipanjan Chowdhury, PhD, co-director of the Center for BRCA and Related Genes at Dana-Farber has discovered one reason why. When the BRCA1 protein malfunctions, cells tip toward performing quick repairs, creating that vulnerability to PARP inhibitors. But when another system called CST also malfunctions, the double negative ends up being a positive for cancer cells. The cells begin to perform more careful repairs and are no longer as vulnerable to PARP inhibitors.
The findings, published in the journal Science, are important because mutations that disable BRCA1 and CST can be detected in patients and could help guide the decision to use or not use PARP inhibitors.
“The scope of the use of PARP inhibitors has expanded a lot recently, moving from ovarian cancer to breast and now prostate and pancreatic cancer, so new strategies to address resistance are becoming increasingly important,” says Chowdhury. “Understanding these mechanisms helps us know what to look for in patients and could help us devise better, more personalized treatment strategies.”
Quick or careful
It has been known for a long time that when both the BRCA1 protein and a large system called 53BP1 are disabled in a cell, resistance to PARP inhibitors occurs. Exactly how the two were related, however, was not fully understood.
To uncover the details, Chowdhury and Michelle Swift, PhD, a postdoctoral fellow in his lab, partnered with Patrick Sung, PhD, and his laboratory at the University of Texas Health Science Center. Their work focused on a sub-system inside 53BP1 called CST, a complex of three proteins called CTC, STN1, and TEN1 that along with BRCA1 governs the repair of breaks in double stranded DNA.
The team in Texas used their biochemistry expertise to purify and observe how CST proteins, BRCA1, and other proteins behave together in a test tube. Swift employed her cell biology expertise to see if similar interactions occurred between the proteins in cells. The work was funded by a joint Program Project Grant from the National Institutes of Health.
“It’s helpful to be able to combine biochemistry with cell biology,” says Swift. “We can map exactly where proteins interact and then learn more by studying them under different conditions in cells.”
The team learned that CST and BRCA1 grapple with one another physically to guide DNA repair tactics in a cell. BRCA1 is known to help direct cells to perform homologous DNA repair, which carefully reconnects strands by overlapping and reconnecting single stranded ends that have matching DNA sequences.
Swift and colleagues found that BRCA1 also plays a previously unrecognized role as a blocker of CST. CST, when BRCA1 is not present, blocks proteins that perform the first steps required to carry out these careful repairs. Specifically, it blocks enzymes called EXO1 and BLM-DNA2 from trimming back one side of a broken double strand so it can neatly overlap and reconnect with a match.
“CST directly blocks the players involved in carrying out homologous repair of DNA,” says Swift.
This interference tips the cell toward performing quick repairs. These quick repairs simply glue the loose ends together, a process called non-homologous end joining that could potentially introduce mutations and make cells more vulnerable to PARP inhibitors.
Translation to patients
With this new understanding, Chowdhury is now working with clinical investigators at Dana-Farber to work out a plan for screening for mutations in the genes that play a role in these mechanisms in patients. This information could help the team determine if any of these gene mutations would be valuable information for guiding treatment with PARP inhibitors.
“As the director of the Center for BRCA and Related Genes, this is what we are talking about. These are the related genes,” says Chowdhury. “Knowing what they are helps us do more directed screening panels in patients and will give us a better handle on resistance.”
Written by: Beth Dougherty