Scientists advance breast, ovarian cancer research with cryo-electron microscopy

Date:
July 28, 2021
Source:
Mayo Clinic
Summary:
Using advanced imaging technology, scientists have provided an
unprecedented understanding of the BRCA1-BARD1 protein complex,
which is often mutated in patients with breast or ovarian
cancer. Their paper identifies aspects of how BRCA1-BARD1 functions,
supporting future translational research, cancer prevention efforts
and drug development.



FULL STORY ========================================================================== Using advanced imaging technology, Mayo Clinic scientists have provided
an unprecedented understanding of the BRCA1-BARD1 protein complex, which
is often mutated in patients with breast or ovarian cancer. Their paper, published in Nature, identifies aspects of how BRCA1-BARD1 functions, supporting future translational research, cancer prevention efforts and
drug development.


========================================================================== "BRCA1-BARD1 is important for DNA repair. It has direct relevance to
cancer because hundreds of mutations in the BRCA1 and BARD1 genes have
been identified in cancer patients," says Georges Mer, Ph.D., a Mayo
Clinic structural biologist and biochemist who is the lead author
of the paper. "But no one knows if these mutations, or variants of
unknown significance, are cancer- predisposing or not because we do
not know whether the variants are located in a region of BRCA1-BARD1
that is important for function. Now because we can see how BRCA1-BARD1
works, we have a good idea of what regions of BRCA1-BARD1 are important
for function." In a cell, the complex of DNA and histone proteins are complexed into what's called chromatin, and packaged into bundles called nucleosomes. DNA damage response proteins need to access chromatin to
repair damaged DNA. BRCA1-BARD1 contributes to fixing broken DNA strands,
which helps in the maintenance and survival of cells. But it is also
a function that could possibly be blocked or inactivated if this is a
strategy a cancer cell uses to survive chemotherapy.

Cryo-electron microscopy and nuclear magnetic resonance spectroscopy
"We used two techniques -- cryo-electron microscopy and nuclear magnetic resonance spectroscopy -- to understand at near-atomic resolution how
BRCA1- BARD1 associates with the nucleosome, the repeating unit of
chromatin, and how BRCA1-BARD1 modifies chromatin," explains Dr. Mer.

In cryo-electron microscopy, purified BRCA1-BARD1 bound to nucleosomes, together referred to as macromolecules, are flash-frozen then imaged using
an electron microscope. The macromolecules are oriented in various ways
within the sample so a computer program evaluates all the orientation data
to create a 3D structure. Dr. Mer and his team also examined BRCA1-BARD1 nucleosome complexes with nuclear magnetic resonance spectroscopy,
which uses a strong magnet to probe the relative positions of atoms
within macromolecules. Using these imaging tools, the scientists could visualize BRCA1-BARD1 in action and uncover a new function of the complex.

"We showed how BRCA1-BARD1 attaches ubiquitin to the nucleosome,
but we also determined that BRCA1-BARD1 recognizes ubiquitin already
attached to the nucleosome, which serves as a signal for broken
DNA," says Dr. Mer. "We discovered an unexpected cross-talk by which
ubiquitin recognition by BRCA1- BARD1 enhances its ubiquitin attachment activity, and this helps us better understand how BRCA1-BARD1 performs
its function." The researchers created a video from the cryo-electron microscopy data to show where the protein complex interacts with the
nucleosome [see link below].

From discovery science to patient care Dr. Mer and his team expect that high-resolution images of BRCA1-BARD1 can help guide patient care and
future treatment of cancer in two ways: classifying variants of unknown significance and directing drug development with more accuracy.

"With these 3D structures, we should be able to convert several variants
of unknown significance to likely cancer-predisposing variants," says
Dr. Mer.

"This work is also expected to have an impact on drug development in the
long term because the 3D structures of BRCA1-BARD1 in complex with the nucleosome we generated may help in the design of small molecules that
could, for example, inactivate BRCA1-BARD1." In addition to Dr. Mer,
other authors on the paper are Qi Hu, Ph.D.; Maria Victoria Botuyan,
Ph.D.; Debiao Zhao, Ph.D.; Gaofeng Cui, Ph.D.; and Elie Mer.

This research was funded by the National Institutes of Health, Mayo
Clinic Cancer Center, Mayo Clinic Center for Biomedical Discovery,
and the Ovarian Cancer Research Alliance, and was made possible through cryo-electron microscopy and nuclear magnetic resonance instrumentation
at the Pacific Northwest Center for Cryo-EM and Mayo Clinic, respectively.

========================================================================== Story Source: Materials provided by Mayo_Clinic. Original written by
Sara Tiner. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* YouTube_video:_Cryo-Electron_Microscopy_helps_Mayo_Clinic_Scientists
Advance_Cancer_Research ========================================================================== Journal Reference:
1. Qi Hu, Maria Victoria Botuyan, Debiao Zhao, Gaofeng Cui, Elie Mer,
Georges Mer. Mechanisms of BRCA1-BARD1 nucleosome recognition and
ubiquitylation. Nature, 2021; DOI: 10.1038/s41586-021-03716-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210728154455.htm

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