FAM111A

FAM111A is a nuclear, PCNA-associated trypsin-like serine protease that safeguards DNA replication by removing protein obstacles from replication forks and that doubles as a viral restriction factor (PMID:32165630, PMID:23093934). At the fork, FAM111A binds PCNA through a PIP box and uses its protease and DNA-binding domains to clear DNA-protein crosslinks, including PARP1-DNA complexes trapped by PARP inhibitors, a function distinct from SPRTN (PMID:32165630). It also promotes efficient activation of replication origins under normal conditions, and unrestrained protease activity drives ssDNA accumulation, DNA damage, and cell death dependent on intact peptidase activity and S-phase entry but not on PCNA binding (PMID:37793778). Catalytic activity is governed by dimerization: FAM111A dimerizes via an N-terminal helix within the serine protease domain, triggering a disorder-to-order activation cascade that builds the oxyanion hole and confers chymotrypsin-like specificity through a narrow, recessed active site; dimerization is required for substrate cleavage and DPC bypass but is dispensable for autocleavage (PMID:38453899). In its antiviral role, FAM111A restricts SV40 and orthopoxvirus replication, localizing to nucleoli in a cell-cycle-dependent manner and relocalizing to SV40 replication centers in a manner dependent on viral DNA replication (PMID:23093934, PMID:30333173, PMID:28320935); against vaccinia virus it degrades the nuclear pore complex to translocate to the cytoplasm and then targets the viral DNA-binding protein I3 for autophagic degradation, an activity antagonized by the poxviral serpin SPI-1 which prevents nuclear export and directly inhibits FAM111A through covalent complex formation (PMID:37607234, PMID:39798873). Basal FAM111A expression is set by IRF2 and is repressed by the androgen receptor (PMID:28320935, PMID:40446667). Hypermorphic gain-of-function variants in the catalytic domain cause Kenny-Caffey syndrome and osteocraniostenosis, including biallelic recessive variants that mildly elevate protease activity without altering dimerization (PMID:39932783).

1. 2012 High

   Established FAM111A as a host factor whose removal phenocopies the SV40 large T antigen function, defining its first biological role as a viral restriction factor.

   Evidence AP-MS of SV40 LT C-terminus, siRNA depletion and viral replication rescue assays

   PMID:23093934

   Open questions at the time

   • Did not define the molecular activity of FAM111A
   • Mechanism of restriction unresolved
2. 2013 Medium

   Linked FAM111A to Kenny-Caffey syndrome and osteocraniostenosis and predicted a trypsin-like peptidase fold, framing pathogenesis as a question of catalysis versus interaction.

   Evidence Exome sequencing of patients with molecular modeling of mutation positions

   PMID:23684011

   Open questions at the time

   • No in vitro catalytic assay performed
   • Mutation effect on protease activity inferred only from modeling
3. 2017 High

   Extended FAM111A restriction to orthopoxvirus SPI-1 deletion mutants and identified IRF2 as a transcriptional regulator setting basal FAM111A levels.

   Evidence Genome-wide siRNA screen with secondary validation, microarray, qRT-PCR and immunoblotting

   PMID:28320935

   Open questions at the time

   • Mechanism by which FAM111A restricts poxvirus not defined
   • Direct FAM111A targets in poxvirus unknown
4. 2018 High

   Defined FAM111A's cell-cycle-dependent nucleolar localization and its replication-dependent relocalization to viral replication centers, tying restriction to DNA replication.

   Evidence Immunofluorescence, aphidicolin inhibition and replication-defective SV40 mutants with FAM111A depletion

   PMID:30333173

   Open questions at the time

   • Molecular substrate at viral replication centers not identified
   • Trigger for nucleolar release not defined
5. 2020 High

   Identified FAM111A as a PCNA-interacting protease that clears DNA-protein crosslinks at forks, distinguishing it from SPRTN and specifying PARP1-DNA complexes as obstacles.

   Evidence PCNA interaction assays, PIP-box and DNA-binding domain mutagenesis, PARP inhibitor and fork protection assays

   PMID:32165630

   Open questions at the time

   • Direct cleavage of PARP1 not biochemically reconstituted in this study
   • Substrate spectrum at forks incomplete
6. 2021 Medium

   Showed the IRF2-FAM111A axis restricts ZIKV and enhances RFC3-mediated restriction independently of type I IFN, broadening FAM111A's antiviral scope to an RNA virus.

   Evidence siRNA knockdown, overexpression, RT-qPCR, western blot and viral replication assays

   PMID:34930359

   Open questions at the time

   • Mechanistic link between FAM111A and RFC3 unresolved
   • Single lab, ZIKV restriction mechanism not defined
7. 2022 High

   Demonstrated that FAM111A loss alone does not recapitulate Kenny-Caffey electrolyte or skeletal phenotypes in mice, arguing against simple loss-of-function pathogenesis.

   Evidence Fam111a knockout mouse with serum/urine electrolyte, gene expression and histology readouts

   PMID:35715480

   Open questions at the time

   • Does not test gain-of-function disease alleles
   • Possible compensation in mouse not excluded
8. 2023 High

   Placed FAM111A in normal origin activation and showed that excess protease activity is toxic through ssDNA, DNA damage and S-phase-dependent death, decoupling toxicity from PCNA binding.

   Evidence siRNA depletion, WT and patient-mutant overexpression, ssDNA, survival and DNA damage marker assays

   PMID:37793778

   Open questions at the time

   • Substrates driving origin activation not identified
   • Link between toxicity and disease phenotype indirect
9. 2023 High

   Revealed an antiviral effector mechanism in which FAM111A degrades the nuclear pore complex to exit the nucleus and then drives autophagic degradation of vaccinia I3, with SPI-1 blocking nuclear export.

   Evidence Domain mutants, Co-IP, viral replication and DNA quantification, NPC degradation, autophagy assays and confocal microscopy

   PMID:37607234

   Open questions at the time

   • NPC components cleaved not enumerated
   • Mechanism of SPI-1 export block not resolved at this stage
10. 2024 High

    Solved the structural basis of FAM111A activation, showing dimerization via an N-terminal helix drives a disorder-to-order cascade to build the active site with chymotrypsin-like specificity, required for cleavage but not autocleavage.

    Evidence X-ray crystallography, mutagenesis, in vitro protease assays and cellular DPC bypass assays

    PMID:38453899

    Open questions at the time

    • Physiological dimerization trigger not defined
    • Full substrate repertoire not mapped
11. 2024 Medium

    Showed disruption of the FAM111A protease domain does not alter calcium homeostasis in mice, further dissociating the protease from the renal/skeletal disease phenotype.

    Evidence CRISPR/Cas9 knock-in mice with serum/urine Ca2+/PTH, intestinal absorption and micro-CT analysis

    PMID:38697929

    Open questions at the time

    • Only one allele showed a sex-specific bone effect
    • Human gain-of-function not modeled directly
12. 2025 High

    Reconstituted direct inhibition of FAM111A by the poxvirus serpin SPI-1 through covalent complex formation, defining a molecular mechanism for poxviral antagonism.

    Evidence In vitro protease inhibition and covalent complex assays, SPI-1 RCL mutagenesis and viral replication rescue

    PMID:39798873

    Open questions at the time

    • Cellular stoichiometry of inhibition not quantified
    • Single lab
13. 2025 Medium

    Connected FAM111A to androgen receptor signaling, showing AR represses FAM111A and that depletion sensitizes prostate cancer cells to PARP inhibitors, defining a co-regulatory loop with therapeutic relevance.

    Evidence AR ChIP, knockdown/overexpression, xenografts, subcellular fractionation/IF, PARP inhibitor and RT-qPCR assays

    PMID:40446667

    Open questions at the time

    • Direct AR binding consequence on FAM111A protease not tested
    • Single lab
14. 2025 Medium

    Demonstrated that biallelic hypermorphic variants causing recessive KCS/OCS act by quantitatively elevating protease activity without altering dimerization, establishing gain-of-function as the disease mechanism.

    Evidence Exome sequencing, recombinant protein production, in vitro protease and dimerization assays

    PMID:39932783

    Open questions at the time

    • Only one variant functionally characterized
    • Tissue-specific consequence not modeled

• How FAM111A protease activity is physiologically licensed and restrained at forks, and how its gain-of-function in disease tissues produces skeletal/endocrine pathology not seen in mouse loss models, remains unresolved.
• Endogenous dimerization/activation trigger unknown
• Complete fork substrate spectrum unmapped
• Mechanistic basis of tissue-specific disease phenotype undefined