parp1

PARP1 (Poly [ADP-ribose] polymerase 1)

PARP1 is the founding and dominant member of the PARP enzyme family. It is a nuclear NAD+-consuming enzyme activated by DNA strand breaks; it synthesizes poly(ADP-ribose) (PAR) chains on target proteins to orchestrate the DNA damage response. Its aging relevance is dual: (1) as a high-fidelity genome guardian, PARP1 activity positively correlates with species lifespan across mammals; (2) under conditions of chronic DNA damage, hyperactivated PARP1 depletes cellular NAD+, suppressing SIRT1 and SIRT3, linking genomic stress directly to deregulated nutrient sensing and metabolic decline.

Identity

• UniProt: P09874 (PARP1_HUMAN)
• NCBI Gene: 142
• HGNC symbol: PARP1 (HGNC:270)
• Ensembl: ENSG00000143799
• GenAge ID: 60
• Mouse ortholog: Parp1 (one-to-one)
• Length: 1,014 amino acids (canonical isoform)

Domain architecture

PARP1’s modular structure couples DNA damage sensing to catalytic activity:

Domain	Residues (approx.)	Function
Zn-finger 1 (ZF1)	1–97	Binds single-strand DNA breaks
Zn-finger 2 (ZF2)	106–209	Binds double-strand DNA breaks
Zn-binding PADR1	224–356	Structural; supports ZF3 / inter-domain communication
BRCT domain	387–484	DNA-binding; mediates intrastrand transfer
WGR domain	524–633	Nucleosome bridging; allosteric relay to catalytic domain
PARP alpha-helical	662–787	Autoinhibitory fold; unwinds on activation
Catalytic (ART) domain	788–1014	Transfers ADP-ribose from NAD+ onto substrate

Activation requires simultaneous engagement of the ZF1/ZF2 DNA-sensing domains and allosteric communication through WGR to relieve autoinhibition — a conformational mechanism that ensures tight coupling of DNA damage to catalytic output.

PARylation chemistry and NAD+ consumption

PARP1 cleaves NAD+ at the nicotinamide-ribose bond, covalently attaches the first ADP-ribose unit onto acidic (Glu, Asp) or hydroxyl (Ser, with HPF1 as co-factor) residues on target proteins, and then extends the chain into branched PAR polymers up to ~200 units in length. Key substrates include histone H1, H2A, H2B, and PARP1 itself (auto-PARylation). The PAR signal:

1. Recruits repair factors — xrcc1 BRCT1 makes direct protein-protein contact with auto-poly(ADP-ribosyl)ated PARP1, preferential for the auto-PARylated form (Masson 1998 — verified R32a on [[xrcc1]] page); the original “BRCT1 reads PAR chains as a polymer-binding module” framing is an oversimplification. The MRN complex accumulation at PAR-modified break sites is documented separately and is not the load-bearing XRCC1-recruitment mechanism.
2. Relaxes chromatin — PARylation of histones reduces their affinity for DNA, transiently opening the break site.
3. Auto-PARylation terminates signaling — electrostatic repulsion of the heavily charged PAR chains dissociates PARP1 from DNA, freeing the break for downstream repair machinery.

Stoichiometric NAD+ cost. Each PAR chain consumes multiple NAD+ molecules. Under mild, transient damage, the local NAD+ pool recovers rapidly via NAMPT-driven salvage (nampt). Under chronic or severe damage, PARP1 hyperactivation becomes a net drain: cellular NAD+ falls, reducing substrate availability for the NAD+-dependent deacetylases SIRT1 and SIRT3 ¹.

Dimension	Status
Pathway conserved in humans?	yes
Phenotype conserved in humans?	yes (NAD+ decline with age is documented in human blood and muscle)
Replicated in humans?	partial (NAD+ depletion documented; causal attribution to PARP1 hyperactivation specifically is in-progress)

Aging relevance

1. Cross-species correlation: PARP1 activity and lifespan

Bürkle and colleagues measured poly(ADP-ribosyl)ation capacity in permeabilised mononuclear blood cells from 13 mammalian species spanning a ~30-fold lifespan range ². PARP1 activity positively correlated with species maximum lifespan (r ≈ 0.8, P < 0.01). Long-lived species (human, naked mole rat) showed substantially higher PARylation capacity than short-lived species (mouse, rat), even when normalized for body mass. needs-replication — this correlation has not been replicated in an independent multi-species dataset; causal direction is not established from correlational data alone.

Dimension	Status
Pathway conserved in humans?	yes
Phenotype conserved in humans?	yes (human cells show highest PARylation capacity in the Bürkle dataset)
Replicated in humans?	no (single-lab cross-species study; causal directionality untested)

2. NAD+ depletion and sirtuin inhibition

PARP1 and sirt1 compete for the same NAD+ pool. In aged tissues, DNA damage burden increases (a consequence of accumulating genomic-instability), driving chronic low-level PARP1 activation. Mouchiroud et al. demonstrated in C. elegans that mutation or RNAi knockdown of pme-1 (the worm PARP-1 homolog, which is the dominant worm PARP activity) extended lifespan by ~20–29%, and that pharmacological PARP inhibition (AZD2281 or ABT-888) extended lifespan by ~15–23% via the worm sirtuin sir-2.1 ¹. These effects required sir-2.1 and involved induction of the mitochondrial unfolded protein response (UPR^mt) and FOXO transcription factor DAF-16. Conversely, supplementation with NAD+ precursors (NR or NAM, not NMN) mimicked PARP inhibition by restoring NAD+ and activating sir-2.1, extending worm lifespan. In mammalian hepatocytes, both PARP inhibition and NR treatment induced mitochondrial biogenesis in a SIRT1-dependent manner. Not yet replicated in humans. needs-human-replication

3. PARP1 as SASP amplifier

Senescent cells sustain elevated DNA damage signaling (the DNA segments with chromatin alterations in senescence, or DNA-SCARS) that constitutively activates PARP1. This maintains a low-level PAR signal that, via NF-κB — whose activation is partly PAR-dependent — amplifies SASP cytokine production. PARP inhibition reduces NF-κB-driven SASP in some senescence models. no-mechanism — the precise PAR → NF-κB node in senescent cells is not fully resolved; flagged as unsourced pending a primary source citation.

BRCA synthetic lethality and PARP inhibitor pharmacology

The synthetic lethal relationship between PARP1 inhibition and BRCA1/2 deficiency was established in two simultaneous 2005 Nature papers:

• Bryant et al. showed that BRCA2-deficient tumour cells are exquisitely sensitive (~1000-fold) to PARP inhibitors in vitro and in xenograft models, because BER-dependent SSB repair can no longer be backed up by homologous recombination ³.
• Farmer et al. independently demonstrated the same lethality in BRCA1/2-deficient cells and proposed a mechanistic model in which PARP trapping at stalled replication forks generates DSBs that only HR-proficient cells can resolve ⁴.

Olaparib clinical development

Fong et al. 2009 (NEJM) reported the first-in-human phase I trial of olaparib (AZD2281) in 60 patients with advanced solid tumours (not all BRCA mutation carriers; 22 were confirmed carriers of BRCA1 or BRCA2 and 1 had a strong family history but declined testing; the remainder were wild-type or unknown BRCA status) ⁵. Of the 19 evaluable BRCA mutation carriers with ovarian, breast, or prostate cancer:

• 9/19 (47%) had a partial or complete radiologic response by RECIST (objective response rate)
• 12/19 (63%) had clinical benefit (radiologic or tumor-marker response, or stable disease ≥4 months)
• In ovarian cancer specifically: 8 of 15 BRCA1- or BRCA2-mutated ovarian cancer patients had a partial or complete radiologic response by RECIST
• In breast cancer: 3 BRCA2-mutated breast cancer patients enrolled; 1 had a complete remission (>60 weeks) and 1 had stable disease
• The maximum tolerated dose was established as 400 mg twice daily (maximum administered dose 600 mg twice daily)
• Confirmed the synthetic-lethal concept in humans; no objective antitumor responses were observed in patients without known BRCA mutations

Olaparib (Lynparza, AstraZeneca) received FDA approval in December 2014 for BRCA-mutated advanced ovarian cancer; subsequently expanded to BRCA-mutated breast (2018), pancreatic (2019), and prostate (2020) cancers. Three additional PARP inhibitors are FDA-approved: rucaparib, niraparib, talazoparib. These are not aging interventions; they are listed here because the druggability-tier-1 status is driven by cancer approvals, and because PARP inhibitor pharmacology is directly relevant to any experimental NAD+ rescue strategy in aging.

NAD+ restoration as an aging intervention angle

PARP1 hyperactivation in aged tissues is a mechanistic node connecting genomic-instability (accumulating DNA damage) to deregulated-nutrient-sensing (NAD+ depletion → SIRT1 suppression). Two intervention strategies address this:

1. NAD+ precursor supplementation — NMN or NR bypass PARP1 to replenish NAD+ directly; see nampt for the salvage pathway context.
2. PARP inhibition in aging — using subtherapeutic PARP inhibitor doses to reduce chronic NAD+ drain. Preclinical evidence in aged mice shows partial NAD+ restoration and improved mitochondrial function. needs-human-replication — no randomized human trial of PARP inhibition for NAD+ restoration in non-cancer aging populations has been completed as of 2026-05-05.

Key interactors

Interactor	Role	Evidence type
xrcc1	Recruited to repair sites via PAR binding	experimental (biochemical)
nampt	Rate-limiting NAD+ biosynthesis; upstream of PARP1 substrate availability	genetic/pharmacological
sirt1	Competes for NAD+; suppressed by PARP1 hyperactivation	genetic/pharmacological
sirt3	Mitochondrial NAD+-dependent deacetylase; indirectly suppressed	genetic
brca1	HR pathway; BRCA1-deficient cells synthetically lethal with PARP inhibition	genetic
HPF1	Co-factor redirecting PARylation from Glu/Asp to Ser residues	structural/biochemical

Pathway membership

• base-excision-repair — primary BER scaffold; early response to SSBs (R19 batch)
• dna-damage-response — integrates with ATM/ATR signaling at DSBs
• homologous-recombination — synthetically lethal relationship; HR backs up stalled forks that PARP traps (R19 batch)
• nad-metabolism — major NAD+ consumer under damage conditions

Limitations and gaps

• NAD+ causality in human aging: The NAD+ depletion-PARP1 axis is mechanistically well-established in rodents; whether PARP1 hyperactivation is a quantitatively meaningful driver of human NAD+ decline (vs. declining biosynthesis, e.g., NAMPT reduction with age) remains unresolved. contradictory-evidence
• PARP inhibition as geroprotector: No clinical trial has tested a PARP inhibitor for healthspan or NAD+ restoration in non-cancer aging populations. needs-human-replication
• Bürkle cross-species correlation: Single-lab, non-interventional; confounders (body size, metabolic rate, DNA repair capacity) not fully disentangled. needs-replication
• SASP amplification mechanism: The PAR → NF-κB pathway in chronic senescence is inferred from cell-line data; tissue-level mechanistic validation is incomplete. no-mechanism
• Bürkle PDF unavailable locally: doi:10.1016/j.biocel.2004.10.006 is closed access (not_oa per a local paper archive). Quantitative claims (r≈0.8, P<0.01 for cross-species correlation) cannot be verified against the full text; retained with appropriate uncertainty. no-fulltext-access

Footnotes

Footnotes

1. mouchiroud-2013-nad-sirtuin-longevity · doi:10.1016/j.cell.2013.06.016 · n=multiple worm cohorts + mouse hepatocyte cell line (AML12) · in-vivo (C. elegans) + in-vitro (mammalian) · model: C. elegans pme-1 mutants (worm PARP-1 homolog) + pharmacological PARP inhibition (AZD2281, ABT-888); NAD+ precursors NR and NAM (not NMN); mouse hepatocytes for SIRT1 dependency · p<0.001 for lifespan extension; lifespan +15–29% · PDF downloaded and verified 2026-05-05 ↩ ↩²

2. doi:10.1016/j.biocel.2004.10.006 · review/correlational · model: 13 mammalian species (mononuclear blood cells) · r≈0.8 · no local PDF (closed access) no-fulltext-access ↩

3. bryant-2005-parp-brca2-synthetic-lethality · doi:10.1038/nature03443 · n=cell lines + xenografts (40 CD-1 nude mice for xenograft arm) · in-vitro/in-vivo · model: BRCA2-deficient V-C8 (Chinese hamster) and human breast cancer cell lines (MCF-7, MDA-MB-231 with siRNA); mouse xenograft · p<0.05 (γ-H2AX/RAD51 foci t-test); P<0.01, P<0.001 (clonogenic survival) · downloaded PDF available ↩

4. farmer-2005-parp-brca-synthetic-lethality · doi:10.1038/nature03445 · n=cell lines + xenografts (40 nude mice for in vivo arm) · in-vitro/in-vivo · model: BRCA1/2-deficient mouse embryonic stem cells (primary model); BRCA2-deficient CHO cells (>1,000-fold sensitivity in Supplementary); teratoma xenograft in BALB/c-nude mice · P<0.05 (siRNA viability reduction); P=0.03 and P=0.01 (in vivo tumour formation) · downloaded PDF available ↩

5. fong-2009-olaparib-brca-phase1 · doi:10.1056/NEJMoa0900212 · n=60 total (22 confirmed BRCA1/2 carriers; 19 evaluable BRCA carriers with ovarian/breast/prostate) · phase-1 · model: humans (advanced solid tumours; BRCA carriers and non-carriers enrolled) · 9/19 (47%) RECIST objective response rate in BRCA carriers; 12/19 (63%) clinical benefit rate (response + stable disease); MTD 400 mg BID · downloaded PDF available ↩