Life science
Medical & Healthcare, Drug discovery

Elucidation of the mechanism of pathway choice for therepair of DNA double strand breakDepartment of Biological Sciences, Graduate School of Science
Assistant Professor Shin-Ya Isobe

Researchmap https://researchmap.jp/IsobeSYreserchmap

Professor Chikashi Obuse

Researchmap https://researchmap.jp/obuse

Life science

Abstract
The daily bombardment of chemicals, radical oxygen and radiation can damage the DNA molecules. If left unrepaired, the damage could lead to genomic instability. Thankfully, evolution has created in the cell innate repair mechanisms to fix any damaged DNA.
For example, BRCA1, a protein that is well-known for its mutation inhereditary breast cancer, is an important part of the repairing double-strand DNA breaks. We have uncovered new mechanisms inwhich cells protect these broken DNA ends to make sure that they
are repaired correctly.

▍ Background & Results
The two mechanisms in the cell are non-homologous end joining(NHEJ) and homologous recombination (HR) for repairing DNA
double-strand break. While NHEJ and HR both function to repair
damaged DNA, they respond to different situations; types of damage, presence or absence of homologous template or cell cycle
stages, etc. What has continued to elude us is how the cell knows
which system to call.
The decision on which of these two pathways is carefully regulated by the cell in several different ways. One method involves aprotein called RIF1 binding to broken DNA ends through 53BP1,
where it prevents a nuclease from further resection of the breaksite to repair by HR. A protein called Shieldin can bind to this single-stranded DNA tail through RIF1 to prevent further resection, but

we suspected that other factors may also play a role in this process. Using proteomic approach, we found that PP1 binds specifically to RIF1 at the broken DNA ends, and that the physical interaction between these two proteins is necessary to block the nucleasefrom binding at the break sites. Importantly, the interaction between
PP1 and RIF1 helps to keep double-strand DNA breaks from developing a single-stranded "tail," which is what Shieldin binds to. Thismeans that PP1 acts earlier in the process than Shieldin. We also
uncovered that, following this stage, newly identified SCAI, binds to53BP1 to promote the recruitment of HR proteins including BRCA1at damage sites. Interestingly, RIF1 initially accumulated at sites ofDNA damage but was gradually replaced by SCAI. We concluded
that RIF1 and SCAI competed to bind to 53BP1 and this temporaltransition from RIF1 to SCAI after damage determined the choiceof DNA repair pathway.

▍ Significance of the research and Future perspective
Our findings reveal a novel mechanism of the pathway choice
for a double-strand DNA break repair. Given that problems withdouble-strand break repair are a crucial feature of many cancers,understanding more about how the cell decides which pathway to
use to fix these damaged sites could provide important insight into
cancer development. The results from our study could therefore
help develop new options for treating hereditary breast and ovariancancer in the future.

P a t e n t
Isobe SY. et al. Protein phosphatase 1 acts as a RIF1 effector to suppress DSB resection prior to Shieldin action. Cell Reports. 2021; 36(2): 109383. doi: 10.1016/j.celrep.2021.109383
T r e a t i s e Isobe SY. et al. Inhibition of RIF1 by SCAI Allows BRCA1-Mediated Repair. Cell Reports. 2017; 20(2): 297-307. doi: 10.1016/j.celrep.2017.06.056
Isono M. et al. BRCA1 Directs the Repair Pathway to Homologous Recombination by Promoting 53BP1 Dephosphorylation. Cell Reports. 2017; 18(2): 520-532. doi: 10.1016/j.celrep.2016.12.042
https://resou.osaka-u.ac.jp/ja/research/2017/20170712_1
https://resou.osaka-u.ac.jp/ja/research/2021/20210714_1
URL
https://www.eurekalert.org/news-releases/478166
https://www.eurekalert.org/news-releases/514955
K e y w o r d cancer, DNA damage, molecular biology, BRCA1