The BRCA1 protein helps to mend double-strand DNA breaks by promoting homologous recombination. Without it, cells can amass broken, jumbled, and fused chromosomes, which may cause them to stop growing or die. Although cells lacking BRCA1 seem like they should be vulnerable, loss of the protein instead seems to boost abnormal growth.
A JCB study determined how BRCA1-deficient cells protect themselves from genomic instability. The cells can accumulate fused and broken chromosomes (see image) because they can’t use homologous recombination to repair their DNA. But the cells can resume proliferation by cranking up their output of cathepsin L, which destroys 53BP1, a protein that prods the cells to use alternative, sloppier repair mechanisms.
Credit: Susana Gonzalo
Recent studies have shown that cells lacking BRCA1 compensate by cutting back on 53BP1. This protein helps orchestrate a different DNA repair mechanism, nonhomologous end joining (NHEJ), and it thwarts a key step in homologous recombination. Researchers think that, in cells without BRCA1, 53BP1 spurs excessive NHEJ that can cause fatal chromosomal chaos. But with 53BP1 out of the way, the cells are able to resume homologous recombination. That might explain why cells that lack BRCA1 and eliminate 53BP1 can withstand traditional chemotherapy compounds and PARP inhibitors, a new generation of anti-cancer drugs that are in clinical trials. But how do cancer cells turn down 53BP1?
Researchers previously found that certain mutant fibroblasts increase production of cathepsin L, a protease that destroys 53BP1. BRCA1-deficient cancer cells take advantage of the same mechanism, according to a team of researchers led by Susana Gonzalo from the Washington University School of Medicine. When they cultured breast cancer cells that were missing BRCA1, the cells stopped growing. After two weeks of lethargy, however, some cells, which the researchers dubbed BOGA cells (BRCA1-deficient cells that overcome growth arrest), began to divide again. These cells showed increased levels of cathepsin L and reduced amounts of 53BP1. Eliminating cathepsin L from BOGA cells or dosing them with vitamin D, a cathepsin L inhibitor, prevented the decline in 53BP1 abundance.
To find out whether boosting cathepsin L levels enabled the cancer cells to restart homologous recombination, the researchers monitored sites of DNA damage tagged by RAD51, a protein that helps promote homologous recombination. The cells that had stopped growing did not display RAD51 foci, but these foci were prevalent in BOGA cells with reduced 53BP1. Removing cathepsin L from BOGA cells increased 53BP1 levels and diminished the number of RAD51 foci.
If cells can't perform homologous recombination, they turn to repair mechanisms such as NHEJ that can lead to jumbled chromosomes. However, after DNA-breaking doses of radiation, BOGA cells exhibited few chromosome defects. The number of these flaws climbed after the researchers stabilized 53BP1 levels by inhibiting cathepsin L or trimming its abundance.
The team then analyzed tumor samples from breast cancer patients. Researchers suspect that cathepsin L attacks 53BP1 by entering the nucleus. Samples from patients with BRCA1 mutations or with triple-negative breast cancer—an aggressive form of the disease—showed high levels of nuclear cathepsin L and reduced quantities of 53BP1. That suggests tumors in these patients hike the amounts of cathepsin L in the nucleus to break down 53BP1 and restore homologous recombination.
"It's a new pathway that explains how breast cancer cells lose 53BP1," says Gonzalo. How cancer cells boost nuclear cathepsin L levels is unclear, she notes.
Triple-negative breast cancers are currently identified by their lack of Her2 and the estrogen and progesterone receptors. The work suggests that another trio of measurements—the amounts of 53BP1, cathepsin L, and vitamin D receptor in the nucleus—might help identify patients that are resistant to current breast cancer treatments. These people might respond to cathepsin inhibitors, some of which are undergoing animal testing. These compounds might steer the cells away from homologous recombination and leave them vulnerable to other therapies.
Grotsky, D.A., et al. 2013. J. Cell Biol. doi:10.1083/jcb.201204053.
About The Journal of Cell Biology (JCB)JCB is published by The Rockefeller University Press. All editorial decisions on manuscripts submitted are made by active scientists in conjunction with our in-house scientific editors. JCB content is posted to PubMed Central, where it is available to the public for free six months after publication. Authors retain copyright of their published works, and third parties may reuse the content for non-commercial purposes under a creative commons license. For more information, please visit www.jcb.org.
Rita Sullivan King | EurekAlert!
Rainbow colors reveal cell history: Uncovering β-cell heterogeneity
22.09.2017 | DFG-Forschungszentrum für Regenerative Therapien TU Dresden
The pyrenoid is a carbon-fixing liquid droplet
22.09.2017 | Max-Planck-Institut für Biochemie
Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.
A warming planet
Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.
The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...
Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...
Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!
When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...
For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.
Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...
19.09.2017 | Event News
12.09.2017 | Event News
06.09.2017 | Event News
22.09.2017 | Life Sciences
22.09.2017 | Medical Engineering
22.09.2017 | Physics and Astronomy