# Understanding DNA break repair pathway choice regulation by the cNHEJ inhibitor CYREN

> **NIH NIH R01** · SALK INSTITUTE FOR BIOLOGICAL STUDIES · 2020 · $440,115

## Abstract

Project Summary
DNA double stranded breaks (DSBs) are deleterious lesions that require rapid repair to avoid the loss of
genetic information, genomic instability, neoplastic transformation and cancer formation. In human systems,
the two predominant pathways for double stranded DNA break repair are canonical non-homologous end
joining (cNHEJ) and homologous recombination (HR). The cNHEJ machinery recognizes breaks,
indiscriminately joins them independent of sequence context and is therefore considered error prone and
potentially genotoxic. HR relies on resection of the 5’ strand with the generation of single stranded 3’
overhangs, which invade homologous sister chromatids to promote error-free break repair. Choice between
HR and cNHEJ depends primarily on the cell cycle stage and the nature of the break. During G1 of the cell
cycle HR is inhibited by RIF1 and 53BP1, which prevent the required BRCA1/2 complex assembly and end
resection for HR initiation. During S and G2, when sister chromatids are available as a template for HR, both
cNHEJ and HR pathways can be employed and compete to repair DSBs. End resection is activated by CtIP in
S and G2 phases and promotes HR, but it is unclear how the abundant and efficient cNHEJ machinery is
suppressed in S and G2 to allow resection at break sites and commencement of HR, thereby ensuring error-
free repair of lesions to preserve genome integrity. CYREN (Cell cYcle REgulator of NHEJ) was originally
identified in a screen for potential modulators of retroviral infection. Later, an alternatively spliced isoform of
CYREN, CYREN-2, was found as short open reading frame translated polypeptide and shown to interact with
the Ku70/80 heterodimer, pointing at a potential role in cNHEJ. Here it is proposed to investigate the discovery
that CYREN modulates the cell cycle dynamics of cNHEJ and that the small protein is a direct cell cycle
regulator of cNHEJ. In three specific aims it is proposed to investigate the mechanism of how CYREN inhibits
cNHEJ through the CYREN interaction with the Ku heterodimer complex (AIM 1), how CYREN is cell cycle
regulated and controls the cell cycle regulation of DSB repair pathway choice (AIM2) and finally what the
effects of CYREN deletion and overexpression are, how the deregulation of cell cycle control of cNHEJ
influences genome maintenance and genome instability and whether cells that lack CYREN are sensitive to
DNA damage causing agents (AIM 3). In summary, this grant proposal focuses on CYREN, a novel regulator
of DNA repair pathway choice, the mechanism of how CYREN is regulated and controls cNHEJ, whether lack
of CYREN causes genome instability and whether CYREN targeting can be exploited to sensitize cancer cells
to treatment with genotoxic agents.

## Key facts

- **NIH application ID:** 9928415
- **Project number:** 5R01CA227934-03
- **Recipient organization:** SALK INSTITUTE FOR BIOLOGICAL STUDIES
- **Principal Investigator:** Jan Karlseder
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $440,115
- **Award type:** 5
- **Project period:** 2018-06-06 → 2023-05-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9928415

## Citation

> US National Institutes of Health, RePORTER application 9928415, Understanding DNA break repair pathway choice regulation by the cNHEJ inhibitor CYREN (5R01CA227934-03). Retrieved via AI Analytics 2026-05-26 from https://api.ai-analytics.org/grant/nih/9928415. Licensed CC0.

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