# MECHANISMS OF DNA DOUBLE-STRAND BREAK END RESECTION AND REPAIR PATHWAY CHOICE..

> **NIH NIH R01** · UNIVERSITY OF TEXAS HLTH SCIENCE CENTER · 2024 · $20,158

## Abstract

ABSTRACT
DNA double strand breaks (DSBs) are induced by environmental and chemotherapeutic agents, and during
encounters of the DNA replication machinery with DNA damage. The two major, mechanistically distinct DSB
repair pathways are non-homologous DNA end joining (NHEJ) and homology-directed repair (HDR). NHEJ is
efficient but error-prone, while HDR is inherently accurate and represents the preferred repair tool for DNA
replication-associated DSBs. HDR commences with the resection of the 5’-terminated strand at break ends
to generate a DNA tail that serves as the template for assembly of the RAD51 recombinase filament. DSB
repair pathway choice is linked to cell cycle progression and is determined by whether or not a DSB undergoes
extensive resection. Long-range resection is principally mediated by the 5’-3’ exonuclease EXO1 or the BLM
helicase-DNA2 endonuclease. The chromatin reader 53BP1 nucleates the formation of a higher order
ensemble that harbors the RIF1 protein and the hetero-tetrameric Shieldin complex at DSB ends to block end
resection in the G1 phase of the cell cycle. The restrictive action of the 53BP1 axis is alleviated by BRCA1-
BARD1 in S and G2 phases via mechanisms that are poorly understood. Thus, BRCA1-deficient tumors, on
account of their HDR-deficiency, are particularly vulnerable to PARP inhibitors (PARPi) due to synthetic
lethality. However, dysfunction in the 53BP1 axis leads to HDR restoration and PARPi resistance. Importantly,
we now have compelling evidence that RIF1 and Shieldin strongly restrict the activity of the DNA end resection
enzymes. To elucidate the underpinnings of the DNA end resection restriction circuitry, we will employ a
combinatorial approach encompassing reconstitution biochemistry and cell biology to: (1) Delineate how RIF1
and Shieldin interfere with the activity of the 5’-3’ exonuclease EXO1 and of the helicase-endonuclease
complex BLM-DNA2 in DNA end resection and (2) Interrogate BRCA1-BARD1 for its ability to overcome the
restriction of DNA end resection imposed by RIF1 and Shieldin. Our studies will elucidate the intricate
regulatory networks that control DNA end resection onset and efficiency. Our endeavors will not only illuminate
the mechanistic principles of DSB repair pathway choice, but will also exert a major impact in our
understanding of how failure to properly process DSBs lead to neoplastic cell transformation and cancer, and
will provide actionable information to help guide the development of targeted cancer therapies to treat BRCA-
deficient cancers and circumvent drug resistance.

## Key facts

- **NIH application ID:** 11097602
- **Project number:** 3R01ES007061-30S1
- **Recipient organization:** UNIVERSITY OF TEXAS HLTH SCIENCE CENTER
- **Principal Investigator:** Patrick Sung
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $20,158
- **Award type:** 3
- **Project period:** 1995-01-01 → 2027-12-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/11097602

## Citation

> US National Institutes of Health, RePORTER application 11097602, MECHANISMS OF DNA DOUBLE-STRAND BREAK END RESECTION AND REPAIR PATHWAY CHOICE.. (3R01ES007061-30S1). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/11097602. Licensed CC0.

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