Affinage

POLR3B

DNA-directed RNA polymerase III subunit RPC2 · UniProt Q9NW08

Length
1133 aa
Mass
127.8 kDa
Annotated
2026-06-10
38 papers in source corpus 11 papers cited in narrative 11 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

POLR3B (RPC2) encodes the second-largest catalytic subunit of RNA Polymerase III, which together with POLR3A forms the active center of the enzyme that transcribes tRNAs and other small non-coding RNAs (PMID:22036171). Its structural integrity is required to recruit and retain the POLR3K/Rpc11 subunit within the polymerase; a deletion disrupting the Polr3b–Polr3k interface reduces Rpc11 in the assembled complex, and the resulting phenotype is rescued by restoring Rpc11 dosage (PMID:18044988). Pol III biogenesis proceeds through defined assembly stages assisted by the PAQosome/HSP90 co-chaperone machinery, and biallelic loss-of-function or severe hypomorphic POLR3B variants (e.g. R103H, Δ10) block proper complex assembly, whereas dominant de novo missense variants instead cause aberrant association of individual subunits without globally destabilizing the enzyme — establishing two distinct pathogenic mechanisms (PMID:31221184, PMID:33417887, PMID:36451185, PMID:37635302). POLR3B function is essential in vivo: homozygous R103H is embryonically lethal in mice (PMID:31221184), intestinal-specific hypomorphism causes crypt proliferation failure, Wnt-signaling loss and apoptosis (PMID:28090567), and oligodendrocyte-specific expression of assembly-defective POLR3B impairs oligodendrocyte precursor proliferation and differentiation, producing severe hypomyelination (PMID:37635302). POLR3B variants cause hypomyelinating leukodystrophy through these mechanisms (PMID:37635302, PMID:35225888). Reduced Pol III output also reshapes downstream small ncRNA profiles via a functional excess of La protein (PMID:38410490).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps
  1. 2007 High

    Established that POLR3B's structural integrity is required to retain the Rpc11/POLR3K subunit in the Pol III complex, linking a specific protein–protein interface to enzyme function and phenotype.

    Evidence Zebrafish slim jim mutant characterization, comparable rpc2 deletion reconstituted in S. pombe with complex purification, and rescue by polr3k overexpression

    PMID:18044988

    Open questions at the time
    • Does not define the atomic-resolution interface between POLR3B and POLR3K
    • Does not establish whether reduced Rpc11 affects RNA cleavage versus recycling in human cells
  2. 2011 Medium

    Defined POLR3B as the second-largest catalytic subunit contributing to the Pol III active center and established loss-of-function as a disease mechanism at the transcript level.

    Evidence Whole-exome sequencing of patients, RT-PCR of a splice-site product, and demonstration of nonsense-mediated decay

    PMID:22036171

    Open questions at the time
    • Does not measure catalytic activity loss directly
    • Does not connect transcript loss to a specific cellular tRNA/ncRNA deficit
  3. 2016 High

    Showed POLR3B is required in a specific tissue context — intestinal crypt maintenance — connecting Pol III activity to proliferation, Wnt signaling and survival.

    Evidence VillinCre conditional hypomorphic mouse, histology, lineage tracing, enteroid culture

    PMID:28090567

    Open questions at the time
    • Does not identify which Pol III transcripts mediate the crypt phenotype
    • Mechanism linking Pol III loss to Wnt signaling not defined
  4. 2019 High

    Demonstrated that the R103H missense allele acts by impairing Pol III complex assembly and is essential for embryonic viability, distinguishing assembly disruption as a quantifiable pathogenic readout.

    Evidence Proteomics in human cells expressing R103H plus mouse embryonic lethality genetics

    PMID:31221184

    Open questions at the time
    • Does not define which assembly step is blocked
    • Does not link assembly defect to a specific transcriptional output
  5. 2021 Medium

    Distinguished dominant de novo missense variants — which cause aberrant subunit associations — from biallelic loss-of-function variants that disrupt overall assembly, defining two pathogenic mechanisms.

    Evidence Protein modeling and proteomics of patient-derived cells with de novo variants

    PMID:33417887

    Open questions at the time
    • Functional consequence of aberrant subunit association on transcription not measured
    • Single-lab proteomic dataset
  6. 2022 High

    Resolved Pol III assembly into staged intermediates, implicated the PAQosome/HSP90 chaperone machinery in biogenesis, and showed pharmacological correction of an assembly defect.

    Evidence Mass spectrometry-based assembly assay, PAQosome interaction analysis, riluzole treatment in cells

    PMID:36451185

    Open questions at the time
    • Mechanism of riluzole correction not defined
    • PAQosome dependence not tested in vivo
  7. 2022 Medium

    Proposed an alternative, localization-based mechanism in which the R550X nonsense product mislocalizes to lysosomes and depresses mTOR signaling to block oligodendroglial differentiation.

    Evidence Overexpression of WT vs R550X in FBD-102b oligodendroglial precursors, localization, mTOR assay, differentiation assay, ibuprofen rescue

    PMID:35225888

    Open questions at the time
    • Overexpression-based, no in vivo confirmation
    • Relationship between lysosomal aggregation and loss of nuclear Pol III function not resolved
    • Single lab
  8. 2023 High

    Defined the cellular basis of hypomyelination by showing assembly-defective POLR3B impairs oligodendrocyte precursor proliferation and differentiation in vivo.

    Evidence AP-MS for assembly defect, subcellular western blot, conditional oligodendrocyte mouse model with lineage tracing and imaging

    PMID:37635302

    Open questions at the time
    • Which Pol III transcripts limit OPC maturation not identified
    • Does not connect assembly defect to a molecular trigger of differentiation arrest
  9. 2024 Medium

    Showed that reduced Pol III output reshapes the small ncRNA landscape through a functional excess of La protein, linking subunit deficiency to altered tRNA fragment and miRNA profiles.

    Evidence CRISPR-edited HEK293 POLR3B cells, RT-PCR, western blot, transcription assays, La knockdown, small-RNA sequencing, patient fibroblasts (preprint)

    PMID:38410490

    Open questions at the time
    • Preprint, not yet peer-reviewed
    • Causal contribution of altered ncRNA profiles to disease phenotype untested
  10. 2024 Medium

    Tested whether reduced Pol III dosage produces systemic or lifespan effects, revealing sexually dimorphic, organ-specific consequences without lifespan extension.

    Evidence Polr3b+/- mouse longitudinal phenotyping of bone, gut barrier and metabolic parameters

    PMID:38465473

    Open questions at the time
    • No molecular mechanism defined beyond reduced dosage
    • Basis of sexual dimorphism unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • It remains unresolved which specific Pol III transcripts mediate the tissue-selective phenotypes (oligodendrocyte, crypt) and how assembly-stage defects translate into defined transcriptional deficits.
  • No causal transcript identified for hypomyelination or crypt failure
  • No structural model of human POLR3B–POLR3K interface
  • Mechanistic reconciliation of nuclear assembly defect versus lysosomal mislocalization model lacking

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140098 catalytic activity, acting on RNA 2 GO:0003677 DNA binding 1
Localization
GO:0005634 nucleus 2 GO:0005764 lysosome 1 GO:0005829 cytosol 1
Pathway
R-HSA-392499 Metabolism of proteins 2 R-HSA-74160 Gene expression (Transcription) 2 R-HSA-8953854 Metabolism of RNA 1
Partners
POLR3APOLR3K
Complex memberships
RNA Polymerase III

Evidence

Reading pass · 11 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2007 A splice-site mutation in zebrafish polr3b (slim jim mutant) causes deletion of 41 conserved amino acids from the Polr3b protein, impairing its interaction with Polr3k (Rpc11 ortholog). Reconstitution in S. pombe showed that the comparable rpc2 deletion leads to markedly reduced levels of Rpc11p in the recovered Pol III complex. Overexpression of zebrafish polr3k rescued exocrine defects, establishing that the phenotype is due to Rpc11 deficiency caused by the Polr3b structural disruption. Genetic mutant characterization (zebrafish), S. pombe engineering with comparable deletion, Pol III complex purification, rescue by polr3k cDNA overexpression PLoS biology High 18044988
2011 POLR3B (RPC2) encodes the second largest subunit of RNA Polymerase III; together with POLR3A (RPC1) it forms the active center of the polymerase and contributes to catalytic activity. A splice-site mutation in POLR3B was shown by RT-PCR to cause deletion of exon 18 from the mRNA, and a nonsense mutation underwent nonsense-mediated mRNA decay, confirming loss-of-function as the disease mechanism. Whole-exome sequencing, RT-PCR and sequencing of splice-site mutation product, NMD demonstration American journal of human genetics Medium 22036171
2019 The POLR3B R103H missense mutation severely impairs assembly of the Pol III complex, as demonstrated by proteomics in human cells. Homozygosity for this mutation is embryonically lethal in mice (no homozygotes detected at E9.5), indicating that R103H is a severe hypomorphic allele that disrupts Pol III biogenesis. Proteomics (mass spectrometry) in human cell line expressing R103H; mouse genetics (embryonic lethality) Molecular brain High 31221184
2021 De novo heterozygous missense variants in POLR3B cause aberrant association of individual Pol III enzyme subunits rather than affecting overall enzyme assembly or stability, a distinct pathogenic mechanism from the biallelic loss-of-function variants that disrupt overall complex assembly. Protein modeling and proteomic analysis of patient-derived cells with de novo POLR3B variants American journal of human genetics Medium 33417887
2022 A mass spectrometry-based assay characterized Pol III assembly stages and demonstrated that the R103H substitution in POLR3B causes specific assembly defects. The PAQosome (HSP90 co-chaperone complex) participates in Pol III biogenesis. Riluzole partially corrects the R103H-driven assembly defects. Mass spectrometry-based Pol III assembly assay, PAQosome interaction analysis, drug (riluzole) treatment in cells Molecular brain High 36451185
2023 The POLR3B Δ10 mutation (deletion of 10 amino acids) causes a Pol III assembly defect, as shown by affinity purification-mass spectrometry, and reduces POLR3BΔ10 protein levels in both cytoplasm and nucleus (western blot). In mice with postnatal oligodendrocyte-specific expression of the Δ10 mutant, defective oligodendrocyte precursor proliferation and differentiation results in failure to produce adequate mature oligodendrocytes during postnatal myelinogenesis, causing severe hypomyelination. Affinity purification–mass spectrometry (proteomics), western blot (subcellular fractionation), conditional mouse model with lineage tracing, immunofluorescence, immunohistochemistry, spectral confocal reflectance microscopy Brain : a journal of neurology High 37635302
2016 Intestinal epithelium-specific hypomorphic mutation of Polr3b (via VillinCre) in mice causes reduced proliferation, abnormal epithelial architecture, loss of Wnt signaling, and dramatic increase in apoptotic cells in crypts, demonstrating that Polr3b is required for intestinal crypt maintenance during early postnatal development. Lineage tracing showed that Polr3b-deficient crypts are replaced by wild-type escapers. Conditional mouse knockout (VillinCre), histology, genetic lineage tracing (Rosa26-lox-stop-lox-YFP), enteroid culture, molecular analysis Cellular and molecular gastroenterology and hepatology High 28090567
2022 The HLD8-associated nonsense mutation R550X in POLR3B causes the mutant protein to localize as aggregates in lysosomes (not in the nucleus as wild-type), decreases lysosome-related mTOR signaling, and inhibits oligodendroglial precursor cell morphological differentiation. Ibuprofen (NSAID) reversed the differentiation defect and improved mTOR signaling. Overexpression of wild-type vs. R550X POLR3B in FBD-102b oligodendroglial precursor cell line; immunofluorescence localization, mTOR signaling assay, morphological differentiation assay, ibuprofen treatment Neurology international Medium 35225888
2012 INMAP, a truncated isoform of POLR3B, localizes to the nucleus in a punctate pattern dependent on its 209–290 amino acid region, and overexpression of INMAP inhibits the transcriptional activities of p53 and AP-1 in a dose-dependent manner. Deletion analysis with subcellular localization (immunofluorescence), luciferase transcriptional reporter assays for p53 and AP-1 activity Molecular and cellular biochemistry Medium 23124897
2024 A POLR3B:c.1625A>G;p.(Asn542Ser) disease variant causes mis-splicing of POLR3B mRNA. In genome-edited HEK293 cells, this leads to decreased levels of multiple Pol III subunits and TFIIIB, but auto-upregulation of POLR3E (the Pol III termination-reinitiation subunit). La protein was increased relative to its pre-tRNA ligands. Transcription assays showed greater deficiency for tRNA genes with 4T terminators versus ≥5T terminators. La knockdown decreased Pol III ncRNA expression independently of RNA stability. Small-RNA sequencing showed increased tRNA fragments from pre-tRNA 3′-trailers (tRF-1) and elevated miRNAs, indicating that decreased Pol III output creates a functional excess of La protein that reshapes small ncRNA profiles. CRISPR genome editing (POLR3B HEK293 cells), RT-PCR (mis-splicing), western blot (subunit levels), cellular transcription assays, La protein knockdown, small-RNA sequencing (tRF and miRNA profiling), patient fibroblast analysis bioRxiv (preprint)preprint Medium 38410490
2024 Polr3b heterozygous (Polr3b+/-) mice show sexually dimorphic, organ-specific effects: female Polr3b+/- mice have improved bone health but compromised gut barrier function and susceptibility to dermatitis; male Polr3b+/- mice are lighter with improved gut barrier function in old age. Neither sex showed extended lifespan, indicating reduced Pol III activity does not extend mammalian lifespan. Mouse genetics (Polr3b+/- C57BL/6N), longitudinal phenotypic analysis of bone, gut barrier, metabolic parameters Aging cell Medium 38465473

Source papers

Stage 0 corpus · 38 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2014 Clinical spectrum of 4H leukodystrophy caused by POLR3A and POLR3B mutations. Neurology 173 25339210
2011 Recessive mutations in POLR3B, encoding the second largest subunit of Pol III, cause a rare hypomyelinating leukodystrophy. American journal of human genetics 142 22036172
2011 Mutations in POLR3A and POLR3B encoding RNA Polymerase III subunits cause an autosomal-recessive hypomyelinating leukoencephalopathy. American journal of human genetics 137 22036171
2013 Mutations in POLR3A and POLR3B are a major cause of hypomyelinating leukodystrophies with or without dental abnormalities and/or hypogonadotropic hypogonadism. Journal of medical genetics 92 23355746
2010 The DC gate in Channelrhodopsin-2: crucial hydrogen bonding interaction between C128 and D156. Photochemical & photobiological sciences : Official journal of the European Photochemistry Association and the European Society for Photobiology 74 20126794
2007 Mutation of RNA Pol III subunit rpc2/polr3b Leads to Deficiency of Subunit Rpc11 and disrupts zebrafish digestive development. PLoS biology 61 18044988
2016 Phenotypic spectrum of POLR3B mutations: isolated hypogonadotropic hypogonadism without neurological or dental anomalies. Journal of medical genetics 41 27512013
2019 The leukodystrophy mutation Polr3b R103H causes homozygote mouse embryonic lethality and impairs RNA polymerase III biogenesis. Molecular brain 32 31221184
2016 Complete genome of Nitrosospira briensis C-128, an ammonia-oxidizing bacterium from agricultural soil. Standards in genomic sciences 24 27471578
2021 De novo variants in POLR3B cause ataxia, spasticity, and demyelinating neuropathy. American journal of human genetics 23 33417887
2013 Different patterns of cerebellar abnormality and hypomyelination between POLR3A and POLR3B mutations. Brain & development 23 23643445
2015 Large exonic deletions in POLR3B gene cause POLR3-related leukodystrophy. Orphanet journal of rare diseases 22 26045207
2015 POLR3A and POLR3B Mutations in Unclassified Hypomyelination. Neuropediatrics 15 26011300
2015 Recessive Mutations in POLR3B Encoding RNA Polymerase III Subunit Causing Diffuse Hypomyelination in Patients with 4H Leukodystrophy with Polymicrogyria and Cataracts. Clinical neuroradiology 14 26478204
2020 4H leukodystrophy caused by a homozygous POLR3B mutation: Further delineation of the phenotype. American journal of medical genetics. Part A 12 32319736
2016 The RNA polymerase III subunit Polr3b is required for the maintenance of small intestinal crypts in mice. Cellular and molecular gastroenterology and hepatology 12 28090567
2022 Novel de novo POLR3B mutations responsible for demyelinating Charcot-Marie-Tooth disease in Japan. Annals of clinical and translational neurology 10 35482004
2024 POLR3B is associated with a developmental and epileptic encephalopathy with myoclonic-atonic seizures and ataxia. Epilepsia 9 39348199
2023 Craniofacial features of POLR3-related leukodystrophy caused by biallelic variants in POLR3A, POLR3B and POLR1C. Journal of medical genetics 9 37197783
2023 Hypomyelination, hypodontia and craniofacial abnormalities in a Polr3b mouse model of leukodystrophy. Brain : a journal of neurology 9 37635302
2021 Whole-exome sequencing reveals POLR3B variants associated with progeria-related Wiedemann-Rautenstrauch syndrome. Italian journal of pediatrics 9 34289880
2022 Riluzole partially restores RNA polymerase III complex assembly in cells expressing the leukodystrophy-causative variant POLR3B R103H. Molecular brain 7 36451185
2019 POLR3B-associated leukodystrophy: clinical, neuroimaging and molecular-genetic analyses in four patients: clinical heterogeneity and novel mutations in POLR3B gene. Neurologia i neurochirurgia polska 6 31577365
2012 INMAP, a novel truncated version of POLR3B, represses AP-1 and p53 transcriptional activity. Molecular and cellular biochemistry 6 23124897
2023 Identification of POLR3B biallelic mutations-associated hypomyelinating leukodystrophy-8 in two siblings. Clinical genetics 4 36650939
2022 Intellectual disability associated with craniofacial dysmorphism due to POLR3B mutation and defect in spliceosomal machinery. BMC medical genomics 4 35436926
2022 Case report: Biallelic variants in POLR3B gene lead to 4H leukodystrophy from the study of brother and sister. Medicine 4 36042647
2024 Polr3b heterozygosity in mice induces both beneficial and deleterious effects on health during ageing with no effect on lifespan. Aging cell 3 38465473
2023 Uncertain significance mutation in the POLR3B gene in a Syrian boy with leukodystrophy: a case report. Annals of medicine and surgery (2012) 3 37554900
2023 A novel de novo variant in POLR3B gene associated with a primary axonal involvement of the largest nerve fibers. Journal of the peripheral nervous system : JPNS 3 37897416
2023 A New Case of Autosomal-Dominant POLR3B-Related Disorder: Widening Genotypic and Phenotypic Spectrum. Brain sciences 3 38002527
2022 The First Korean Siblings With Adult-Onset 4H Leukodystrophy Related to Nonsynonymous POLR3B Mutations. Neurology. Genetics 3 35434302
2024 POLR3B de novo variants are a rare cause of infantile myoclonic epilepsy. Seizure 2 39178560
2026 Genomic architecture of the resistance to Phytophthora cactorum 2 (RPc2) locus in strawberry (Fragaria × ananassa). The plant genome 1 41543137
2024 A POLR3B-variant reveals a Pol III transcriptome response dependent on La protein/SSB. bioRxiv : the preprint server for biology 1 38410490
2022 Hypomyelinating Leukodystrophy 8 (HLD8)-Associated Mutation of POLR3B Leads to Defective Oligodendroglial Morphological Differentiation Whose Effect Is Reversed by Ibuprofen. Neurology international 1 35225888
2026 [Hypogonadotropic hypogonadism due to pathogenic variants in the POLR3B gene]. Problemy endokrinologii 0 41640168
2025 POLR3B-Related Hypomyelinating Leukodystrophy Type 8 (4H Syndrome): A Case Series of Two Siblings. Cureus 0 40978896

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