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

> **NIH NIH R01** · UNIVERSITY OF PITTSBURGH AT PITTSBURGH · 2020 · $349,004

## Abstract

Abstract
Telomeres, the natural termini of chromosomes, are composed of 10-15kb of the TTAGGG sequence and are
critical regulators of healthy cellular physiology. These structures function as guardians of genome stability by
limiting unwanted DNA repair activity at chromosome ends, and by controlling the total number of times a cell
can divide thereby limiting the accumulation of genomic instability in actively proliferating cells. The sustained
growth of cells with inherently compromised telomeric structure and function can have catastrophic
consequences as it promotes the entanglement of chromosomes that may result in chromothripsis (Greek for
“chromosome shattering”) or breakage-fusion-bridge cycles, events that are strongly linked with cancer
initiation. To prevent this from occurring, shortening or spontaneous de-protection of telomeres activates cell
cycle checkpoint signaling that triggers senescence, an essential barrier to tumor formation. In order to survive,
proliferate and eventually infiltrate tissues and organs, cancer cells must bypass replicative senescence and
activate a telomere maintenance mechanism (TMM). Most cancer cells reactivate the catalytic subunit of
telomerase, hTERT, which is widely investigated. However, hTERT is suppressed in a number of cancers.
These cancers maintain telomere length by engaging the alternative lengthening of telomeres (ALT) pathway.
Recent data indicates that ALT is activated by defective histone dynamics during chromatin assembly that
results in perturbed replication fork progression through telomeres. Though many details of ALT are poorly
understood it is anticipated that the repair of these forks occurs via break-induced replication (BIR) and
homologous recombination. These processes are thought to occur within cellular structures termed ALT
associated PML bodies, or APBs, that are unique to ALT cancer cells. The apical involvement of replication
fork repair activities in sustaining the ALT pathway is underscored by recent observations where treatment of
ALT cells with generic replication inhibitors has been shown to prevent the assembly of APBs and ALT cancer
cells display enhanced sensitivity to ATR inhibitors. In following-up several hits from a proteomic purification of
telomeres from ALT+ cells we have identified that maintaining ADP-ribose equilibrium is a critical feature of the
ALT mechanism. Depletion of a unique enzyme, poly ADP-ribose glycohyrolase (PARG), which degrades poly
ADP-ribose (PAR), disrupts APB formation and negatively impacts ALT activity. PARG is an important
regulator of DNA repair that, until now, has not been associated with telomere regulation. This study
investigates the role of PARG in cancer cells that employ ALT and analyzes the effects of its inhibition on
cancer cell survival. In AIM 1 we will investigate telomere structure in cells with suppressed PARG, as well as
the spatiotemporal dynamics of telomeres. AIM 2 is designed as an extension of our preliminary ...

## Key facts

- **NIH application ID:** 9960438
- **Project number:** 5R01CA207209-05
- **Recipient organization:** UNIVERSITY OF PITTSBURGH AT PITTSBURGH
- **Principal Investigator:** Roderick O'Sullivan
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $349,004
- **Award type:** 5
- **Project period:** 2016-07-01 → 2021-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9960438

## Citation

> US National Institutes of Health, RePORTER application 9960438, Inhibition of the ALT pathway by interfering with Poly-ADP-Ribose metabolism (5R01CA207209-05). Retrieved via AI Analytics 2026-05-31 from https://api.ai-analytics.org/grant/nih/9960438. Licensed CC0.

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