A quick-and-dirty form of DNA repair may be favored over its more fastidious counterpart, provided a protein complex—the newly discovered shieldin—joins the scene. Shieldin attaches itself near the stumpy, blunt ends of broken DNA, forcing them to be stuck back together directly, via a mechanism called nonhomologous end joining (NHEJ).
Because this mechanism is targeted by platinum chemotherapies and PARP inhibitors, its protector, the aptly named shieldin, can help ensure the success of these anticancer therapies. Alternatively, if shieldin is scarce or defective, cancer cells may become resistant. Specifically, cancer cells may sidestep platinum chemotherapies and PARP inhibitors by resorting to an alternative form of DNA repair called homologous recombination.
These finding emerged from a study led by researchers based at the University of Toronto. "PARP inhibitors hold great promise for breast and ovarian cancer treatment, but we must understand why they sometimes don't work, or stop working altogether,” noted Daniel Durocher, Ph.D., a professor of molecular genetics at the University of Toronto. “Knowing more about how cancer evades PARP inhibition by studying basic DNA repair mechanisms brings us a big step closer to this objective, which will improve how we treat some of the most intractable cancers."
Durocher is the senior author of a study that appeared July 18 in the journal Nature, in an article entitled, “The shieldin complex mediates 53BP1-dependent DNA repair.” 53BP1 (p53-binding protein 1) is an important regulator of the cellular response to double-strand breaks. It promotes the NHEJ mechanism, which is induced in immune cells to help generate antibodies. It also participates in the fusion of deprotected telomeres.
To uncover the shieldin complex, Durocher and colleagues analyzed breast cancer cells and mice that had mutations in the gene BRCA1. They used CRISPR-Cas9 genetic manipulation technology to search for gene mutations that caused cells to become resistant to the PARP inhibitor drugs olaparib and talaoparib, as well as the platinum chemotherapy cisplatin.
Through painstaking experiments, the researchers were able to pick out key gene mutations that led to drug resistance, which proteins these had an effect on, and work out what these proteins did in cells.
“Here we identify a 53BP1 effector complex, shieldin, that includes C20orf196 (also known as SHLD1), FAM35A (SHLD2), CTC-534A2.2 (SHLD3), and REV7,” wrote the authors of the Nature article. “Shieldin localizes to double-strand-break sites in a 53BP1- and RIF1-dependent manner, and its SHLD2 subunit binds to single-stranded DNA via OB-fold domains that are analogous to those of RPA1 and POT1.”
When the researchers introduced mutations into the Shieldin complex—which stop it from forming and protecting broken DNA ends—cells are free to repair DNA via a different method, and this means PARP inhibitors are no longer effective.
“Loss of shieldin impairs non-homologous end-joining, leads to defective immunoglobulin class switching and causes hyper-resection,” the paper continued. “Mutations in genes that encode shieldin subunits also cause resistance to poly(ADP-ribose) polymerase inhibition in BRCA1-deficient cells and tumors, owing to restoration of homologous recombination.”
"We next need to show that these mutations actually occur in patients and are clinically important,” explained study co-author Chris Lord, Ph.D., a professor of cancer genomics at the Institute of Cancer Research, London. “If that's the case, we should be able to test for these mutations as a way of monitoring treatment and spotting early signs of resistance."