PARP1’s job is genome surveillance: when it senses breaks or lesions in DNA, it adds a molecule called ADP-ribose to specific proteins, which act as a beacon to recruit other proteins that repair the break. When cancer cells can’t make BRCA proteins, they become dependent on repair pathways that PARP1 is involved in. So, when PARP1 is inhibited (which is the mechanism of several approved cancer drugs) cancer cells have no repair pathway available, and they die. In normal cells, genomic lesions occur naturally during DNA replication when a cell divides, and PARP1 play an important role in fixing these errors. But while healthy cells have other DNA repair pathways to fall back on, BRCA-deficient cancers (which include many breast and ovarian cancers) rely heavily on PARP1 because they lack BRCA proteins, which control the most effective form of DNA repair called homologous replication.
A new study led by University of Pittsburgh and UPMC Hillman Cancer Center researchers shows that PARP1 is involved in repair of telomeres, the lengths of DNA that protect the tips of chromosomes, and that impairing this process can lead to telomere shortening and genomic instability that can cause cancer. The new findings, published today in Nature Structural & Molecular Biology, are the first evidence that PARP1 also acts on telomeric DNA, opening up new avenues for understanding and improving PARP1-inhibiting cancer therapies. Although scientists discovered PARP1’s role in ADP-ribosylation of proteins about 60 years ago, professors O’Sullivan and Ahel in the Sir William Dunn School of Pathology at the University of Oxford and world-renowned experts in PARP1, had a hunch that there was more to learn about this enzyme and its role in cells.
No one thought that ADP-ribosylation at DNA was possible, but recent findings challenge this dogma. Poly-ADP-ribosylation has always beeen studied on proteins. Many enzymes and transcription factors in cell nucleus are known to be ADP-ribosylated, even the famous tumor suppressor p53. DNA-associated histones are also substrates for this modification, which consists in the creation of many units of ADP-ribose (PAR) linked each other like chainrings. This is called poly-ADP-ribosylation and is performed by Poly ADP-ribose Polymerases (PARPs), the most studied of all being PARP1. There is also mono-ADP-ribosylation which is performed by specific transferases (ADPRTs); preferential substrates, in this case, are proteins and enzymes in cytoplasm and mitochondria. PARP1 is one of the most important biomedical targets for cancer research, but it was thought that drugs targeting this enzyme only acted at proteins.
The information that PARP1 also modifies DNA, changes the game because scientists can target this aspect of PARP1 biology to improve cancer treatments. Scientists first compared normal human cells with those deficient in PARP1. Using special antibodies that bind to ADP-ribose and telomere-specific probes, they found that ADP-ribose attaches to telomeric DNA in normal cells but not in PARP1-deficient cells, showing that this enzyme is responsible for ADP-ribosylation of DNA. Next, they compared normal cells with those deficient in another enzyme called TARG1, which removes ADP-ribose. In absence of TARG1, ADP-ribose accumulated at telomeres, leading to disruption of telomere replication and premature telomere shortening. To show that these telomere defects were due to modification of telomeric DNA, researchrs took bacterial enzymes that function similarly to PARP1 and put them into human cells.
They used a guidance system to direct the enzymes to add ADP-ribose only at the telomeres and nowhere else in the genome; and found that if they load telomeres with ADP-ribose, their integrity is dramatically impaired, and it can kill the cell within days. Professor O’Sullivan hypothesizes that ADP-ribose affects telomere integrity by disrupting a protective structure called Shelterin that safeguards telomeres, but more research is needed to confirm this. In the meantime, a general thought about the issue is that would/could be better to use TARG inhibitors than PARP inhibitors to induce cancer cell death. With PARP inhibitors, in fact, there is no possibility to effectively repair DNA strand breaks and pharmacologic resistance is already reported by lab and clinical data. If one blocks poly ADP-ribose chain removal, on the contrary, the system “never shuts off” so to speak.
Gene expression (e.g. for cellular replication) is generally uncompatible with any DNA repair operational complex; in other words, they exclude each other. Same would go for ADP-ribosylation at telomeric DNA: once ADPR chains are attached, TARG inhibition would let them stay to artificially induce cell death by “programmed senescent death”, instead of programmed cell death or apoptosis, generally induced by PARP inhibitors.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
Scientific references
O’Sullivan J et al. Nucleic Acids Res. 2023; 51(20):11056-79.
Groslambert J, Prokhorova E et al. Cell Rep. 2023; 42(9):113113.
Prokhorova E et al. Mol Cell. 2021 Jun 17; 81(12):2640-2655.
Caron MC, Sharma AK et al. Nat Commun. 2019 Jul; 10(1):2954.