Uses Cryo-electron Microscopy to Advance Breast and Ovarian Cancer Research

Posted by Jerry Carter on September 2nd, 2021

\"BRCA1-BARD1 is important for DNA repair. It is directly related to cancer, because hundreds of BRCA1 and BARD1 gene mutations have been found in cancer patients,\" Dr. Georges Mer, lead author of the paper and structural biologist and biochemist at the Mayo Clinic, said. \"But no one knows whether these mutations or variants of unknown significance are cancer susceptible, because we don\'t know whether these mutations are located in the BRCA1-BARD1 region that is important for function.” Now, because we can see how BRCA1-BARD1 works, so we have a good understanding of which areas of BRCA1-BARD1 are important for function.

In the cell, the complex of DNA and histones is compounded into so-called chromatin and packed into bundles called nucleosomes. DNA damage response proteins need to enter chromatin to repair damaged DNA. BRCA1-BARD1 helps repair broken DNA strands, which helps the maintenance and survival of cells. But if this is a strategy for cancer cells to survive chemotherapy, this function may also be blocked or inactivated.

\"We used two techniques, cryo-electron microscopy and nuclear magnetic resonance spectroscopy, to understand how BRCA1-BARD1 associates with nucleosomes (repetitive units of chromatin) at near atomic resolution and how BRCA1-BARD1 modifies chromatin,\" Dr. Mer explained.

Under a cryo-electron microscope, the purified BRCA1-BARD1 binds to nucleosomes, together called macromolecules, which are quickly frozen and then imaged with an electron microscope. Macromolecules are located in different ways in the sample, so the computer program evaluates all the positioning data to create a 3D structure. Dr. Mer and his team also used nuclear magnetic resonance spectroscopy to detect the BRCA1-BARD1 nucleosome complex, which uses strong magnets to detect the relative positions of atoms in macromolecules. Using these imaging tools, scientists can see the activity of BRCA1-BARD1 and discover new functions of the complex.

“We show how BRCA1-BARD1 attaches ubiquitin to nucleosomes, but we also determine that BRCA1-BARD1 can recognize ubiquitin that has been attached to nucleosomes and break DNA signals,\" Dr. Mer said. “We found an unexpected crosstalk through which ubiquitin recognition by BRCA1-BARD1 enhanced its ubiquitin attachment activity, which helped us to better understand how BRCA1-BARD1 performs its function.”

Dr. Mer and his team hope that the high-resolution images of BRCA1-BARD1 can help guide patient care and future cancer treatment in two ways: classify variants of unknown significance and guide drug development more accurately.

\"With these 3D structures, we should be able to convert some unknown mutations into mutations that may induce cancer,\" Dr. Meyer said. \"This work is also expected to have a long-term impact on drug development, because the 3D structure of BRCA1-BARD1 and our nucleosome complex may help design small molecules, for example, to inactivate BRCA1-BARD1.\"

In addition to Dr. Mer, the other authors of the paper are Dr. Qi Hu; Dr. Maria Victoria Botuyan; Dr. Debiao Zhao; Dr. Gao Feng Cui; and Ellie. This study was funded by the National Institutes of Health, the Mayo Clinic Cancer Center, the Mayo Clinic Biomedical Discovery Center, and the Ovarian Cancer Research Alliance.

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Jerry Carter

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Jerry Carter
Joined: November 1st, 2019
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