Affinage

TTC21B

Tetratricopeptide repeat protein 21B · UniProt Q7Z4L5

Length
1316 aa
Mass
150.9 kDa
Annotated
2026-06-10
31 papers in source corpus 11 papers cited in narrative 12 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

TTC21B encodes IFT139, a tetratricopeptide-repeat protein that localizes to primary cilia and is required for retrograde intraflagellar transport; its loss produces bulb-like accumulations of IFT proteins at ciliary tips and impairs retrograde IFT velocity, the hallmark of retrograde transport failure (PMID:18327258). Through this transport function, IFT139 acts as a negative modulator of Sonic Hedgehog signaling: in its absence Smoothened and full-length GLI accumulate in cilia and the pathway is overactivated, with excess SHH driving forebrain patterning defects that are rescued by reducing Shh ligand dosage (PMID:18327258, PMID:19732765). This SHH role is context-dependent, since cell-type-specific ablation in cerebellar Bergmann glia instead reduces Hedgehog signaling, indicating IFT139 can augment or restrain the pathway depending on cellular context (PMID:29615573). Beyond the cilium, IFT139 relocates along extended microtubule networks in non-ciliated differentiated podocytes, where it governs ciliogenesis, cytoskeletal organization, and cell migration (PMID:24876116). Mutations in TTC21B cause a broad ciliopathy spectrum—isolated nephronophthisis, syndromic Jeune asphyxiating thoracic dystrophy, and FSGS-associated podocyte disease—and contribute modifier alleles across this spectrum, with disease-associated variants behaving as hypomorphs that only partially rescue ciliogenesis defects (PMID:21258341, PMID:24876116, PMID:41378128). TTC21B is also required for neural progenitor proliferation and differentiation in the developing forebrain, where its loss causes microcephaly with altered mitotic spindle orientation (PMID:41384431).

Mechanistic history

Synthesis pass · year-by-year structured walk · 11 steps
  1. 2008 High

    Established the core molecular function of TTC21B by showing it is a ciliary TPR protein required for retrograde IFT, answering what the gene product physically does in the cilium.

    Evidence ENU mutant mouse (aln), RNAi knockdown in IMCD cells, and live imaging of IFT88-EYFP retrograde velocity

    PMID:18327258

    Open questions at the time
    • Does not define the IFT-A subunit architecture or direct binding partners of IFT139
    • Mechanism by which the TPR domains engage the retrograde motor not resolved
  2. 2008 High

    Linked the transport defect to signaling by showing retrograde IFT failure causes ciliary accumulation of Smoothened/GLI and SHH overactivation, defining TTC21B as a negative modulator of Hedgehog.

    Evidence Immunofluorescence of Smoothened and GLI in aln mutant cilia plus pathway reporter assays

    PMID:18327258

    Open questions at the time
    • Does not establish whether GLI accumulation is a direct consequence of failed retrograde export or downstream of broader ciliary disruption
  3. 2009 High

    Tested causality of the signaling model in vivo by showing forebrain patterning defects depend on excess SHH, confirming the phenotype is driven by ligand-dependent overactivation.

    Evidence Genetic epistasis reducing Shh dosage in Ttc21b mutant mice with in situ hybridization and immunohistochemistry

    PMID:19732765

    Open questions at the time
    • Does not address SHH-independent contributions to the brain phenotype
  4. 2011 High

    Connected the cilial mechanism to human disease, establishing TTC21B/IFT139 as both a causal ciliopathy gene and a modifier across the ciliopathy spectrum.

    Evidence Resequencing of a large ciliopathy cohort with zebrafish in vivo complementation and in vitro variant assays

    PMID:21258341

    Open questions at the time
    • Genotype-phenotype determinants distinguishing nephronophthisis from Jeune syndrome not resolved
  5. 2014 High

    Revealed a non-ciliary, cytoskeletal function by showing IFT139 relocates to microtubule networks in differentiated podocytes and controls migration and cytoskeleton, broadening its role beyond IFT.

    Evidence Immunofluorescence in fetal tissue and cultured podocytes, siRNA knockdown, migration assays, and partial rescue by p.P209L mutant

    PMID:24876116

    Open questions at the time
    • Molecular basis of the cilium-to-microtubule relocalization not defined
    • Whether the cytoskeletal role is IFT-A-dependent unresolved
  6. 2017 Medium

    Demonstrated context dependence of the SHH role by showing Bergmann-glia ablation reduces rather than elevates Hedgehog signaling, complicating the simple negative-modulator model.

    Evidence Cell-type-specific conditional knockout in Bergmann glia and Purkinje cells with cerebellar histology and immunohistochemistry

    PMID:29615573

    Open questions at the time
    • Mechanism producing opposite SHH directionality in different cell types unknown
    • Single lab, two cell-type conditions
  7. 2019 Medium

    Identified a ciliary GPCR modifier of the TTC21B neural phenotype, implicating ciliary signaling receptors in TTC21B-dependent development.

    Evidence QTL mapping, congenic strains, CRISPR alleles, ciliary localization assays, and double-mutant genetic interaction

    PMID:31730647

    Open questions at the time
    • Direct biochemical interaction between GPR63 and TTC21B not shown
    • Downstream signaling axis of the interaction unresolved
  8. 2021 Medium

    Placed TTC21B in a paralog and Hedgehog network during skeletogenesis via genetic interaction with the THM2 paralog and Gli2.

    Evidence Compound mouse mutants, micro-CT, osteoblast cultures, and Gli2 genetic epistasis

    PMID:33683377

    Open questions at the time
    • Findings center on Thm2/Thm1 interaction rather than direct TTC21B mechanism
    • Biochemical relationship between paralogs not established
  9. 2022 Medium

    Showed a developmental-stage-specific requirement for TTC21B in pathological cilia maintenance, with adult deletion attenuating ADPKD cystogenesis.

    Evidence Conditional and global Ttc21b deletion in juvenile vs. adult Pkd1 ADPKD mice with cilia length, O-GlcNAc, and inflammation readouts

    PMID:35644283

    Open questions at the time
    • Mechanism linking cilia maintenance to cystogenesis severity not defined
    • Basis of the juvenile/adult difference unknown
  10. 2025 Medium

    Confirmed hypomorphic loss-of-function for specific patient alleles by quantitative ciliogenesis rescue, strengthening variant interpretation in podocyte disease.

    Evidence Podocyte knockdown plus wild-type vs. mutant overexpression rescue with ciliogenesis assays and structural bioinformatics

    PMID:41378128

    Open questions at the time
    • Structural consequences predicted only by bioinformatics
    • Single lab in vitro system
  11. 2025 Medium

    Defined a cellular mechanism for TTC21B microcephaly through impaired neural progenitor proliferation and altered mitotic spindle orientation.

    Evidence Ttc21b null mouse with progenitor marker immunohistochemistry, BrdU/EdU kinetics, and spindle angle measurement

    PMID:41384431

    Open questions at the time
    • Whether spindle defects are cilium-dependent or reflect the cytoskeletal role unresolved
    • SHH contribution to the proliferation defect not separated

Open questions

Synthesis pass · forward-looking unresolved questions
  • How TTC21B/IFT139 mechanistically switches between restraining and augmenting SHH signaling across cell types, and how its ciliary IFT-A role relates to its microtubule/cytoskeletal functions, remain unresolved.
  • No unified model reconciling context-dependent SHH directionality
  • Direct IFT-A subunit interactions and motor coupling for IFT139 not biochemically defined
  • Relationship between ciliary and non-ciliary microtubule functions unclear

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0008092 cytoskeletal protein binding 1
Localization
GO:0005929 cilium 3 GO:0005856 cytoskeleton 1
Pathway
R-HSA-1266738 Developmental Biology 2 R-HSA-162582 Signal Transduction 2 R-HSA-1852241 Organelle biogenesis and maintenance 1
Complex memberships
IFT-A

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2008 THM1/TTC21B encodes a tetratricopeptide repeat-containing protein that localizes to primary cilia and is required for retrograde intraflagellar transport (IFT). Loss of THM1 in mouse (alien mutant) causes bulb-like structures at cilia tips where IFT proteins (e.g., IFT88) are sequestered, characteristic of retrograde IFT failure. RNAi knockdown of Ttc21b in IMCD cells expressing IFT88-EYFP recapitulated the cilial phenotype, and live imaging confirmed impaired retrograde IFT velocity. ENU mutagenesis (aln mouse), RNAi knockdown, live fluorescence imaging of IFT88-EYFP, immunolocalization Nature genetics High 18327258
2008 THM1/TTC21B negatively modulates Sonic Hedgehog (SHH) signaling. In aln mutant cilia lacking functional THM1, Smoothened and full-length GLI proteins accumulate in cilia, and SHH pathway is overactivated. This uncouples anterograde IFT (required for GLI activation) from retrograde IFT (which modulates/restrains GLI activation), positioning TTC21B as a retrograde IFT-dependent negative modulator of SHH signaling. ENU mutagenesis mouse model, immunofluorescence localization of Smoothened and GLI proteins in cilia, pathway reporter assays Nature genetics High 18327258
2009 Loss of Ttc21b in mouse causes elevated SHH signaling in the rostral forebrain, including near the zona limitans intrathalamica. Reducing Shh ligand levels genetically ameliorated the forebrain patterning defects, establishing that the Ttc21b brain phenotype is caused by excessive SHH signaling downstream of impaired retrograde IFT. ENU mutant mouse, genetic epistasis (reducing Shh dosage), in situ hybridization, immunohistochemistry Developmental biology High 19732765
2011 TTC21B encodes the retrograde IFT protein IFT139. Mutations in TTC21B cause both isolated nephronophthisis and syndromic Jeune asphyxiating thoracic dystrophy. In vivo (zebrafish) and in vitro functional evaluations showed that a significant enrichment of pathogenic alleles from ciliopathy cases were functionally deleterious, establishing TTC21B as both a causal ciliopathy gene and a modifier allele contributor across the ciliopathy spectrum. Resequencing of a large ciliopathy cohort, zebrafish in vivo complementation assays, in vitro functional evaluation of variants Nature genetics High 21258341
2014 IFT139 (TTC21B protein) localizes to the base of the primary cilium in developing/undifferentiated podocytes, and relocates along extended microtubule networks in non-ciliated adult/differentiated podocytes. Knockdown of IFT139 in podocytes causes primary cilia defects, abnormal cell migration, and cytoskeleton alterations. The p.P209L mutation partially rescues these defects, demonstrating its hypomorphic nature. Immunofluorescence localization in human fetal tissue and cultured podocytes, siRNA knockdown, cell migration assay, rescue by overexpression of wild-type and mutant constructs Journal of the American Society of Nephrology : JASN High 24876116
2017 In the developing cerebellum, Ttc21b ablation in Bergmann glia (not just neurons) causes accumulation of ectopic granule cells and, unexpectedly, a reduction in SHH signaling—indicating that in some cellular contexts Ttc21b augments rather than attenuates SHH signaling. This context-dependent role was confirmed by cell-type-specific conditional ablation. Conditional (Cre-mediated) knockout in Bergmann glia and Purkinje cells, immunohistochemistry, cerebellar histology Journal of developmental biology Medium 29615573
2019 Gpr63, an orphan GPCR, is a genetic modifier of Ttc21b microcephaly. GPR63 can localize to primary cilia, and alleles affecting its ciliary localization genetically interact with Ttc21b null alleles to cause spina bifida aperta and earlier embryonic lethality, linking ciliary GPR63 function to TTC21B-dependent neural development. QTL mapping, congenic strain construction, CRISPR-Cas9 allele generation, in vitro ciliary localization assay, genetic interaction (double mutant phenotype analysis) PLoS genetics Medium 31730647
2021 THM2 (paralog of THM1/TTC21B) localizes to primary cilia and genetically interacts with Thm1: compound Thm2-/-; Thm1aln/+ mice exhibit impaired chondrocyte differentiation and increased Hedgehog signaling in the growth plate, with defective bone nodule formation in osteoblasts. Deletion of one Gli2 allele exacerbated the small phenotype, suggesting Thm2 and Thm1 interact in the Hedgehog pathway during skeletogenesis. Mouse genetics (double/triple mutant), micro-CT, radiography, immunohistochemistry, primary osteoblast cultures, genetic epistasis with Gli2 Cellular and molecular life sciences : CMLS Medium 33683377
2022 Deletion of Ttc21b in adult (but not juvenile) ADPKD mice markedly attenuates kidney cystogenesis and reduces cilia length, inflammation, and O-GlcNAc levels, revealing that the IFT-A component TTC21B is required for pathological cilia maintenance in mature kidney tubules and that its effect on ADPKD severity is development-stage-specific. Conditional/global Ttc21b deletion in juvenile and adult Pkd1 mutant ADPKD mouse models, kidney histology, cilia length measurement, O-GlcNAc immunoblot, inflammatory marker analysis Kidney international Medium 35644283
2025 TTC21B compound heterozygous mutations (p.C299R and p.C518R) disrupt ciliogenesis and shorten ciliary length in renal podocytes in vitro. Wild-type TTC21B fully rescues podocyte morphology defects caused by TTC21B knockdown, while each mutant only partially rescues, confirming their hypomorphic/loss-of-function effect on ciliogenesis. In vitro rescue experiments in podocytes (TTC21B knockdown + overexpression of wild-type or mutant), ciliogenesis assay (cilia number/length measurement), morphology quantification; bioinformatics structural analysis of mutants Frontiers in genetics Medium 41378128
2025 Ttc21b null mouse embryos exhibit microcephaly caused by disrupted neural progenitor proliferation and differentiation in the forebrain: enlarged ventricular zone, reduced cortical plate, fewer TBR2-positive intermediate progenitors, altered mitotic spindle angles (symmetric vs. asymmetric division defects), and shortened neuronal processes in the cortical plate. Early embryonic Ttc21b expression in neural precursors is required for perdurant TTC21B protein to sustain later progenitor proliferation. Ttc21b null mouse (alien allele), immunohistochemistry (TBR2, TBR1, mitotic markers), histology, progenitor proliferation kinetics (BrdU/EdU labeling), spindle angle measurement Disease models & mechanisms Medium 41384431
2025 SPECC1L and TTC21B/THM1 genetically interact: double or compound heterozygotes for Specc1l and Thm1 exhibit higher penetrance of cleft palate than Specc1l heterozygotes alone. Elevated F-actin caused by Specc1l loss negatively regulates primary cilia length, and reduced F-actin at the ciliary base in Specc1l mutant cells accompanies cilia shortening, suggesting cytoskeletal-ciliary crosstalk converges on Thm1-dependent IFT-A function. Double heterozygous mouse genetics, F-actin staining, cilia length measurement, F-actin depolymerization rescue experiment bioRxivpreprint Low 41278885

Source papers

Stage 0 corpus · 31 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2011 TTC21B contributes both causal and modifying alleles across the ciliopathy spectrum. Nature genetics 289 21258341
2008 THM1 negatively modulates mouse sonic hedgehog signal transduction and affects retrograde intraflagellar transport in cilia. Nature genetics 273 18327258
2014 A homozygous missense mutation in the ciliary gene TTC21B causes familial FSGS. Journal of the American Society of Nephrology : JASN 83 24876116
2009 Ttc21b is required to restrict sonic hedgehog activity in the developing mouse forebrain. Developmental biology 64 19732765
2017 Contribution of the TTC21B gene to glomerular and cystic kidney diseases. Nephrology, dialysis, transplantation : official publication of the European Dialysis and Transplant Association - European Renal Association 39 26940125
2019 The Thm1 Zn(II)2 Cys6 transcription factor contributes to heat, membrane integrity and virulence in the insect pathogenic fungus Beauveria bassiana. Environmental microbiology 20 31211497
2022 Ttc21b deficiency attenuates autosomal dominant polycystic kidney disease in a kidney tubular- and maturation-dependent manner. Kidney international 17 35644283
2018 Mutations in TTC21B cause different phenotypes in two childhood cases in China. Nephrology (Carlton, Vic.) 17 28124483
2017 Ttc21b Is Required in Bergmann Glia for Proper Granule Cell Radial Migration. Journal of developmental biology 12 29615573
2019 A Compound Heterozygous Mutation in the Ciliary Gene TTC21B Causes Nephronophthisis Type 12. Journal of pediatric genetics 10 32714622
2022 Biallelic variants in TTC21B as a rare cause of early-onset arterial hypertension and tubuloglomerular kidney disease. American journal of medical genetics. Part C, Seminars in medical genetics 9 35289079
2022 A single heterozygous nonsense mutation in the TTC21B gene causes adult-onset nephronophthisis 12: A case report and review of literature. Molecular genetics & genomic medicine 8 36263627
2021 A novel heterotaxy gene: Expansion of the phenotype of TTC21B-spectrum disease. American journal of medical genetics. Part A 7 33547761
2021 Retinal dystrophy as part of TTC21B-associated ciliopathy. Ophthalmic genetics 7 33599192
2021 Thm2 interacts with paralog, Thm1, and sensitizes to Hedgehog signaling in postnatal skeletogenesis. Cellular and molecular life sciences : CMLS 7 33683377
2021 Case Report: Homozygous Pathogenic Variant P209L in the TTC21B Gene: A Rare Cause of End Stage Renal Disease and Biliary Cirrhosis Requiring Combined Liver-Kidney Transplantation. A Case Report and Literature Review. Frontiers in medicine 7 34957165
2020 Medullary Cystic Kidney Disease and Focal Segmental Glomerulosclerosis Caused by a Compound Heterozygous Mutation in TTC21B. Internal medicine (Tokyo, Japan) 7 32238723
2019 Gpr63 is a modifier of microcephaly in Ttc21b mouse mutants. PLoS genetics 7 31730647
2021 Tubuloglomerular Disease With Cone-Shaped Epiphyses Associated With Hypomorphic Variant and a Novel p.Cys14Arg in the TTC21B Gene: A Case Report. Frontiers in pediatrics 5 34805047
2019 [Clinical features and TTC21B genotype of a child with nephronophthisis type 12]. Zhongguo dang dai er ke za zhi = Chinese journal of contemporary pediatrics 5 31208513
2022 A case of proliferative glomerulosclerosis with compound heterozygous TTC21B mutations. Clinica chimica acta; international journal of clinical chemistry 4 35151619
2022 [Clinical phenotype analysis of 6 cases of TTC21B gene related nephronophthisis]. Zhonghua er ke za zhi = Chinese journal of pediatrics 4 35922195
2022 Biallelic mutations of TTC12 and TTC21B were identified in Chinese patients with multisystem ciliopathy syndromes. Human genomics 4 36273201
2025 Genetic interaction of Specc1l and Thm1 reveals cytoskeletal-ciliary crosstalk. bioRxiv : the preprint server for biology 2 41278885
2022 Genetic Interaction of Thm2 and Thm1 Shapes Postnatal Craniofacial Bone. Journal of developmental biology 2 35645293
2022 Lethal neonatal respiratory failure due to biallelic variants in BBS1 and monoallelic variant in TTC21B. Pediatric nephrology (Berlin, Germany) 1 35695966
2021 Complete Genome Sequence of Methanothermobacter sp. Strain THM-1, a Thermophilic and Hydrogenotrophic Methanogen Isolated from an Anaerobic Reactor in South Korea. Microbiology resource announcements 1 34553993
2026 Ttc21b is required for proper proliferation of neural progenitor cells. Disease models & mechanisms 0 41384431
2025 Perdurant TTC21B protein in the early mouse embryo is required for proper forebrain neural progenitor proliferation. bioRxiv : the preprint server for biology 0 39868177
2025 Molecular mechanisms of TTC21B gene mutations in nephronophthisis type 12 and genetic prevention through PGT. Frontiers in genetics 0 41378128
2024 Case report of visual quality in a patient with nephronophthisis 12- associated retinopathy secondary to TTC21B mutation. Documenta ophthalmologica. Advances in ophthalmology 0 39602043

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