🧬 Aging Wiki

fanci

Properties1
aliasesFANCI, KIAA1794, Fanconi anemia group I protein, FA complementation group I

11 min read

⚠️ Auto-extracted by Claude on 2026-05-12 — synthesized from UniProt Q9NVI1, NCBI Gene 55215, and widely-cited FA-pathway literature. The FANCI–RAD50 axis (Chen 2025) is cited from the abstract only due to closed-access PDF (#gap/no-fulltext-access). Numerics may be approximate; verify before relying on quantitative claims.

FANCI (Fanconi anemia complementation group I)

FANCI is a 1,328-amino-acid nuclear DNA-repair protein that heterodimerizes with fancd2 to form the ID2 complex — the central effector node of the Fanconi anemia (FA) pathway. Monoubiquitination of the ID2 complex activates interstrand crosslink (ICL) repair and scaffolds downstream homologous-recombination factors. A 2025 Science paper (Chen et al.) identifies FANCI as a partner of rad50 at general DNA double-strand breaks (DSBs), with the interaction enhanced by chromatin-retained cgas — extending FANCI’s functional reach beyond its canonical ICL context.

Identity

  • UniProt: Q9NVI1 (FANCI_HUMAN) — Swiss-Prot (manually reviewed)
  • NCBI Gene: 55215
  • HGNC: 25568
  • Ensembl: ENSG00000140525
  • Chromosomal location: 15q26.1
  • Mouse ortholog: Fanci (NCBI Gene 208836); one-to-one ortholog
  • Length: 1,328 amino acids; 4 isoforms via alternative splicing
  • GenAge: not listed (FANCI absent from GenAge Build 21 human gene set)
  • OMIM: MIM 609053 (Fanconi anemia complementation group I)

Key functional domains and PTMs

FANCI adopts a solenoid fold comprising 65 helices, 17 beta strands, and 10 turns, enabling it to cradle and scan double-stranded DNA as part of the ID2 heterodimer 1.

Key post-translational modifications:

  • Monoubiquitination at Lys-523 — the activation mark deposited by the FA core complex; required for nuclear focus formation and HR-factor recruitment 1
  • Phosphorylation at Ser-407, Ser-556, Ser-730, Thr-952, Ser-1121 — ATR-dependent phosphorylation at multiple sites modulates DNA-binding and complex dynamics
  • Deubiquitination by USP1–UAF1 — removes the Lys-523 mark to terminate the repair signal; UAF1 binds FANCI directly 2

In the Fanconi anemia pathway (ID2 complex, monoubiquitination, ICL repair)

FANCI forms a constitutive heterodimer with fancd2, together designated the ID2 complex. Upon detection of an interstrand crosslink (ICL) — a covalent lesion that locks both DNA strands and blocks replication forks — the upstream FA core complex (comprising FANCA, FANCB, FANCC, FANCE, FANCF, FANCG, FANCL, FANCM, FAAP24, FAAP100) monoubiquitinates both FANCI at Lys-523 and FANCD2 at Lys-561 in a cooperative, interdependent manner 3. Note (per 2026-05-13 verifier pass on fancd2): the relationship between FANCI and FANCD2 monoubiquitination in cells is interdependent (Lemonidis 2023 model: FANCI-ub maintains FANCD2-ub by inhibiting USP1–UAF1-mediated deubiquitination), but Longerich et al. 2014 (NAR) showed FANCD2 monoubiquitination is largely independent of FANCI ubiquitination in the original kinetic experiments — i.e., the FANCD2 mono-ub event does not require prior FANCI ub. The mature consensus integrates both: FANCI ub is dispensable for the initial FANCD2 ub event but required for sustaining/maintaining it through repair cycles.

The monoubiquitinated ID2 complex undergoes a conformational change: cryo-EM structures demonstrate that ubiquitinated FANCD2–FANCI adopts a closed “clamp” conformation enclosing double-stranded DNA, with ubiquitin acting as a molecular pin that stabilizes the closed form 1. Structural note: Alcón et al. 2020 (NSMB) reconstituted the structure using Gallus gallus (chicken) FANCD2–FANCI orthologs — chicken FANCI K525 corresponds to human FANCI K523 — and in that reconstituted system FANCD2 was monoubiquitinated by the chicken FA core complex; FANCI itself was not monoubiquitinated in the structural preparation. The closed-clamp conformation is therefore established for FANCD2-ub-only ID2, not strictly ID2-(ub)2. This chromatin-clamped complex then scaffolds the downstream “ID2-interactome” of HR and nuclease factors required to unhook and repair the ICL:

  • BRCA2 / FANCD1 — Rad51 loader; delivers RAD51 filament to the resected ssDNA
  • PALB2 / FANCN — bridging factor between BRCA1 and BRCA2 at the break
  • RAD51 — strand invasion mediator for HR template copying
  • FAN1 nuclease — ICL unhooking endonuclease
  • SLX4 / FANCP — scaffold for SLX1, MUS81, XPF endonucleases; coordinates dual incisions flanking the crosslink

FANCI ubiquitination maintains FANCD2 ubiquitination by preventing excessive deubiquitination and enabling re-ubiquitination within the DNA-bound complex — an interdependent positive-feedback loop that sustains repair at the lesion site until completion 3.

Repair is terminated when USP1–UAF1 removes the ubiquitin marks. UAF1 binds FANCI directly, providing substrate specificity for the deubiquitinase complex 2.

DimensionStatusNotes
Pathway conserved in humans?yesCanonical FA pathway; all components are human genes
Phenotype conserved in humans?yesBiallelic mutations cause Fanconi anemia in humans
Replicated in humans?yesFanconi anemia is the direct human loss-of-function phenotype

In general DSB repair — the Chen 2025 FANCI–RAD50 axis

A 2025 Science paper using naked mole-rat biology as a model system (Chen et al., doi:10.1126/science.adp5056) identified a novel regulatory circuit in which chromatin-retained cgas — when prolonged by the distinct NMR chromatin-affinity mechanism — enhances the interaction between FANCI and rad50, promoting RAD50 recruitment to DNA damage sites and potentiating homologous recombination 4.

This extends FANCI’s canonical role: whereas its FA-pathway activity is specific to ICLs, the Chen 2025 data suggest FANCI participates at general DSB repair foci through a RAD50-dependent mechanism. rad50 is the ATPase subunit of the MRN complex (MRE11–RAD50–NBS1), which is the initial DSB sensor; its recruitment facilitates end resection and ATM activation.

Key mechanistic ambiguities — no-fulltext-access:

  1. ID2 complex vs monomeric FANCI. The abstract reports an “interaction between repair factors FANCI and RAD50.” It is unclear whether the relevant FANCI species is (a) the intact monoubiquitinated ID2 complex (FANCI + FANCD2), (b) free FANCI monomer, or (c) FANCI in a distinct sub-complex at DSBs. Resolving this would determine whether FANCD2 is co-recruited and whether monoubiquitination is required for the RAD50 interaction.
  2. Direct vs scaffolded interaction. Whether cGAS retention enhances FANCI–RAD50 interaction through direct protein–protein contact, through cGAS acting as a structural scaffold, or via a chromatin-accessibility mechanism is not resolvable from the abstract alone.
  3. Species specificity. The Chen 2025 study uses naked mole-rat biology. Whether the magnitude or mechanism of cGAS-enhanced FANCI–RAD50 interaction is conserved in humans at physiological cGAS-chromatin retention levels requires independent verification. needs-human-replication

In aging

Fanconi anemia as a segmental progeroid syndrome

Biallelic loss-of-function mutations in any FA pathway gene — including FANCI — cause Fanconi anemia (FA), characterized by progressive bone marrow failure, cancer predisposition (leukemia, squamous cell carcinomas), skeletal abnormalities, and growth retardation. FA is classified as a segmental progeroid syndrome — it reproduces selected features of accelerated aging (genomic instability, bone marrow failure, cancer predisposition) without recapitulating all aging phenotypes 5.

This progeroid overlap arises because the FA pathway is critical in long-lived stem cell populations, particularly hematopoietic stem cells (HSCs), whose function depends on genome fidelity over decades. Loss of ICL-repair capacity in HSCs causes progressive genotoxic attrition resembling the clonal HSC dynamics of normal aging at accelerated tempo 6.

ICL accumulation and FA-pathway demand with age

ICLs are not merely exogenous (chemotherapy-induced) lesions. Multiple endogenous sources of ICL-forming agents accumulate with age:

  • Endogenous aldehydes (formaldehyde, acetaldehyde, acrolein, 3-aminopropanal) — byproducts of metabolic one-carbon metabolism and polyamine catabolism. Deficiency of aldehyde-detoxifying enzymes (ADH5, ALDH3-family, ALDH9A1) causes ICL accumulation requiring FA pathway repair 7.
  • Lipid aldehydes — produced by lipid peroxidation under oxidative stress; cross-link DNA directly 7.

The FA genomic instability signature has been identified in cancer samples from patients with FA pathway gene mutations, confirming that endogenous aldehyde exposure over a lifetime imposes a substantial ICL burden 8. This implies that partial reduction of FA-pathway efficiency (e.g., from aging-associated epigenetic silencing, DNA methylation of FANCI or partner genes, or accumulation of somatic FANCI mutations in aging HSCs) could accelerate ICL-driven genome instability even in people without inherited FA.

Sex-specific aging of the FA pathway

A 2025 study in human lymphocytes (Rall-Scharpf et al., Nucleic Acids Research) found that FA pathway activity declines specifically in older women, with a shift toward translesion synthesis as an alternative ICL-bypass route, while older men upregulate fork-remodeling and PARP-dependent repair 9. This sex-specific divergence in FA-pathway aging is a novel finding with potential implications for sex differences in FA-associated cancer risk at age. needs-replication — single study; lymphocyte-specific; mechanism unknown.

FA pathway in hematopoietic stem cell aging

Functional FANCD2 (the ID2 complex partner) is required in HSCs from fetal development through adult life. NIPA, a Fanconi anemia-associated protein that regulates FANCD2 nuclear localization, is essential for HSC function under replication stress; NIPA knockout mice develop bone marrow failure and a premature aging phenotype 10. These data reinforce that ID2-complex-dependent ICL repair is continuously required to maintain the HSC pool over a lifespan — and that partial attrition of this capacity phenocopies accelerated hematopoietic aging.

Knowledge gaps

  1. FANCI–RAD50 binding mode at DSBs. The structural interface between FANCI and RAD50 has not been defined. Does FANCI contact the RAD50 coiled-coil, the ATPase head, or MRE11? Is the interaction direct or bridged (via FANCD2, or via cGAS itself)? no-fulltext-access needs-replication
  2. Monoubiquitination requirement for RAD50 recruitment. Does the Chen 2025 cGAS→FANCI–RAD50 axis require prior monoubiquitination of FANCI at Lys-523, or does it operate via unmodified FANCI? This determines whether FA-core-complex activation is a prerequisite for the cGAS-enhanced DSB repair mechanism.
  3. Human conservation of the cGAS–FANCI–RAD50 axis. Chen 2025 was demonstrated in naked mole-rat cells; the extent to which human cGAS chromatin affinity (notably lower than NMR cGAS) produces a similar enhancement of FANCI–RAD50 interaction is unknown. needs-human-replication
  4. Age-related decline in FANCI expression / activity in human tissues. No GTEx aging-correlation data have been curated for FANCI (gtex-aging-correlation: null); whether FANCI protein levels or ubiquitination efficiency decline in aged human HSCs or other tissues is not established. unsourced
  5. Somatic FANCI mutations in aging. Whether FANCI accumulates somatic loss-of-function mutations in aged tissues (analogous to DNMT3A / TET2 in CHIP) is not documented. ICL-repair pathway genes are not established CHIP drivers, but the question has not been systematically studied.
  6. Druggability. FANCI has no clinical drug, probe, or clinical-stage inhibitor (Open Targets tier 4). No aging-context modulator exists. Potential strategies — FA-pathway enhancers to boost ICL repair in aging, or FANCI-specific ubiquitination stimulators — are entirely hypothetical.

Pathway membership

  • dna-damage-response — ATR/ATM → FA-core-complex → ID2-monoubiquitination arm
  • homologous-recombination — ID2 complex scaffolds RAD51/BRCA2/PALB2 at ICLs; Chen 2025 RAD50 axis at general DSBs
  • Fanconi anemia pathway (implicit — no dedicated wiki page yet; closest canonical node is dna-damage-response) stub

Key interactors

PartnerInteraction typeNotes
fancd2Constitutive heterodimer (ID2 complex)Mutual stabilization; co-monoubiquitinated by FA core complex
FANCL / UBL5FA core complex subunitsDeposit monoubiquitin at Lys-523
USP1–UAF1DeubiquitinaseUAF1 contacts FANCI directly; removes Lys-523 mark 2
rad50Interaction at DSBsEnhanced by cGAS chromatin retention (Chen 2025) 4
brca2 / FANCD1Downstream HR factorScaffolded by monoubiquitinated ID2 at ICLs
palb2 / FANCNDownstream HR factorBridging BRCA1–BRCA2 at ICLs

Footnotes

Footnotes

  1. doi:10.1038/s41594-020-0380-1 · Alcón P et al. · Nat Struct Mol Biol 2020 · cryo-EM of FANCD2–FANCI closed clamp on DNA · in-vitro structural + biochemical · key result: monoubiquitination of FANCD2 induces closed-clamp conformation enclosing dsDNA; ubiquitin acts as molecular pin · archive: pending ↩ ↩2 ↩3

  2. doi:10.1038/s41594-021-00576-8 · Rennie ML et al. · Nat Struct Mol Biol 2021 · crystal structure of USP1–UAF1 bound to FANCI–ubiquitin · UAF1 establishes extensive FANCI interface · archive: pending ↩ ↩2 ↩3

  3. doi:10.15252/embj.2022111898 · Lemonidis K et al. · EMBO J 2022 · cryo-EM + biochemistry · interdependent FANCI–FANCD2 ubiquitination; FANCI ubiquitination prevents FANCD2 deubiquitination and enables re-ubiquitination · archive: pending ↩ ↩2

  4. chen-2025-nmr-cgas-hr-repair · Science 2025 · doi:10.1126/science.adp5056 · PMID 41066557 · cited_by: 17 · abstract-only (closed-access PDF) · #gap/no-fulltext-access — FANCI–RAD50 interaction enhanced by chromatin-retained cGAS; RAD50 recruitment to damage sites; HR potentiation; model: naked mole-rat cells ↩ ↩2

  5. doi:10.1002/humu.23367 · Mori T et al. · Hum Mutat 2018 · case series + review · ERCC4/XPF mutations in segmental progeroid syndromes; contextualizes FA progeroid overlap with other DNA-repair-deficiency disorders ↩

  6. doi:10.3724/abbs.2022053 · Li N et al. · Acta Biochim Biophys Sin 2022 · review · FA/BRCA pathway in hematopoietic system aging; ICL-repair capacity in long-lived HSCs ↩

  7. doi:10.1083/jcb.202407141 · Jung M et al. · J Cell Biol 2025 · in-vitro + cell-line · ALDH9A1 deficiency + FA pathway loss = synthetic lethality; polyamine-derived aldehydes (3-aminopropanal, acrolein) as endogenous ICL sources · archive: failed download ↩ ↩2

  8. doi:10.1038/s41586-022-05253-4 · Webster ALH et al. · Nature 2022 · n=1,000+ cancer genomes · observational (genomic) · FA pathway deficiency causes characteristic genomic rearrangement signature from endogenous aldehyde exposure · archive: pending ↩

  9. doi:10.1093/nar/gkaf498 · Rall-Scharpf M et al. · Nucleic Acids Research 2025 · in-vitro (human lymphocytes from aged donors) · observational · FA pathway activity declines in older women; shift to TLS; older men upregulate fork-remodeling · archive: completed (PDF available) ↩

  10. doi:10.1172/JCI126215 · Kreutmair S et al. · J Clin Invest 2020 · in-vivo (mouse KO) · NIPA loss causes FA-pathway dysfunction, HSC DNA damage accumulation, and premature aging + bone marrow failure phenotype · archive: pending ↩

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