# Inhibition of the ALT pathway by interfering with Poly-ADP-Ribose metabolism

> **NIH NIH R01** · UNIVERSITY OF PITTSBURGH AT PITTSBURGH · 2024 · $349,752

## Abstract

ABSTRACT
Cancer cells must activate a telomere elongation mechanism and acquire genomic alterations to survive. Many
of the most lethal cancers rely on the Alternative Lengthening of Telomeres (ALT) telomere elongation pathway.
ALT is strongly associated with recurrent mutations in genes encoding chromatin modifiers such as the ATRX-
DAXX chromatin remodeling/histone deposition complex. These disrupt the assembly of telomeric chromatin,
provoking replicative stress and double-strand breaks (DSBs) that stimulate specialized homology-directed DNA
repair (HDR) mechanisms to repair and extend telomeres. Thus, the ALT pathway represents a valuable target
for cancer therapy development. Despite significant advances in understanding these unique ALT-associated
HDR (ALT-HDR) mechanisms, the ALT pathway remains unexploited for cancer therapy development.
 The mono-ADP-ribosylation (MARylation) of chromatin has recently emerged as a key sensor and
initiator of the DNA damage response. MARylation predominantly occurs on serine residues and relies on HPF1,
which interacts with PARP1. MARylation of specific serines on histones seeds further PARylation that
destabilizes nucleosomes proximal to DNA double strand breaks (DSBs) and licenses the recruitment of the first
wave of DNA repair complexes. ARH3 is the only known enzyme capable of removing MAR synthesized by the
HPF1-PARP1 complex. Despite biochemical evidence supporting its fundamental role in genome maintenance,
the identity of additional cellular targets of serine MARylation, and effects of its deregulation, remain obscure. In
Aim 1 we will assess the impact of ARH3 and HPF1 disruption on ALT-HDR mechanisms and survival of ALT
cancer cells. In Aim 2, we will employ novel approach to track histones as they are being deposited at telomeres
and evaluate the impact that this has on transcription of TERRA and the recruitment of major mediators of ALT.
We will also employ innovative proteomics to identify telomeric targets of MARylation. The successful completion
of these aims will contribute to the understanding of this important chromatin mark during ALT-HDR and its
impact on cancer cell survival.

## Key facts

- **NIH application ID:** 10875564
- **Project number:** 5R01CA207209-08
- **Recipient organization:** UNIVERSITY OF PITTSBURGH AT PITTSBURGH
- **Principal Investigator:** Roderick O'Sullivan
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $349,752
- **Award type:** 5
- **Project period:** 2016-07-01 → 2027-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10875564

## Citation

> US National Institutes of Health, RePORTER application 10875564, Inhibition of the ALT pathway by interfering with Poly-ADP-Ribose metabolism (5R01CA207209-08). Retrieved via AI Analytics 2026-05-26 from https://api.ai-analytics.org/grant/nih/10875564. Licensed CC0.

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