Women who carry a mutated copy of the BRCA1 gene have an elevated risk of developing breast and ovarian cancer. However, when a cell does not have fully functional BRCA1 along the way to becoming cancerous, it runs the risk of acquiring so many subsequent genetic mutations that it might kill itself before it can evolve into a tumor.
That challenge was the focus of a group of researchers between St. Louis and Spain who examined lab-generated cells with various knockouts as well as cells extracted from the tumors of cancer patients. Publishing their results in The Journal of Cell Biology, the researchers found that these cancer cells could overcome an imbalance in the DNA repair mechanism, caused by BRCA1’s absence, by eliminating BRCA1’s competitor protein, 53BP1.
In a non-cancerous cell, BRCA1 and 53BP1 usually compete with one another to bind to breaks in the double-stranded DNA, which can arise from outside sources like UV ray exposure. When BRCA1 binds to the break, it recruits a whole set of proteins involved in one repair mechanism called homologous recombination. This allows the break to be repaired by correctly adjoining the ends within the proper chromosome. In contrast, 53BP1 leads to the use of nonhomologous end joining, a process that will repair a break by sticking together two ends of DNA that may not result in the correct sequence.
According to Susana Gonzalo, an assistant professor in the Department of Biochemistry and Molecular Biology at the St. Louis University School of Medicine, both processes are essential and have to be in check.
“To have only nonhomologous end joining or homologous recombination is bad. You need to have a balance,” Gonzalo said. “That balance is established by these two proteins.”
Gonzalo explained that mice generated with a full knockout of BRCA1—where both copies of the gene are not functional—should not survive past their embryonic stage. However, previous researchers have shown that mice with a knockout of both BRCA1 and 53BP1 can not only develop beyond the embryonic stage and into full adult mice, they also have lower rates of tumor development. Although the lack of BRCA1 and 53BP1 means that the repair mechanisms have a little more difficulty in finding a double strand break, the key is that neither one is dominating.
“When you lose both of them there is still a balance. That balance is still there because that break is open for the machinery,” Gonzalo said.
In the lab, Gonzalo’s group worked with a BRCA1-deficient cell culture that did not proliferate normally. After two weeks, some cells were able to start growing again, finding a way to beat down their levels of 53BP1. They also observed a similar reduction in 53BP1 in samples from cancer patients.
“This strategy of getting rid of both proteins is what tumor cells use to survive. A cell that does not have BRCA1 will be very, very sick,” Gonzalo said. “How would you explain how a tumor grows in a BRCA1 deficient patient? What we are saying is that the tumors are finding a way to get rid of 53BP1 and that is the degradation of the protein.”
The degradation is the handiwork of cathepsin L, an enzyme that specializes in finding 53BP1 and breaking it down to amino acid. The cells that were able to grow again in Gonzalo’s lab managed to bring cathepsin L into the nucleus of cell and bring balance to the repair mechanisms.
“The cells are very clever and they get rid of the problem, in this case 53BP1,” Gonzalo said.
So where does this lead oncologists? Gonzalo and her team were also able to target these cells specifically by focusing on cathepsin L, dosing the cells with either cathepsin inhibitors or vitamin D. The end result was disruption of the repair mechanism balance, a stop to proliferation and increase in genetic instability among the BRCA1 deficient cells.
“Ideally, we could increase levels of 53BP1 and the cells will die. That’s what we tried to do,” Gonzalo said.