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TCTN1
tectonic family member 1
TCTN1 is located on the long (q) arm of chromosome 12, at band 12q24.11. Arm ratio per GRCh38 - banding schematic.
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Overview
TCTN1 (tectonic family member 1) is located at chromosome 12q24.11 and encodes a 587-amino-acid protein crucial for primary cilium assembly. Primary cilia are slender, antenna-like projections found on the surface of most human cells, serving as signalling hubs that detect extracellular cues and regulate developmental pathways. The TCTN1 protein localises to a structure called the transition zone at the base of the cilium, where it helps control which molecules enter and exit the ciliary compartment. Disruption of TCTN1 function impairs cilium formation and signalling, leading to a spectrum of developmental abnormalities that affect multiple organ systems.
What the gene does
The TCTN1 protein functions as a structural component of the transition zone, a gatekeeper region that separates the ciliary membrane from the rest of the cell membrane. This transition zone acts as a molecular sieve, ensuring that only appropriate signalling proteins and lipids enter the cilium whilst excluding others. TCTN1 works in concert with other transition-zone proteins to maintain the integrity of this diffusion barrier, which is essential for proper ciliary signalling. Loss of TCTN1 function disrupts signalling pathways that depend on primary cilia, including Sonic Hedgehog, Wnt, and planar cell polarity pathways. These pathways are critical during embryonic development, governing processes such as left-right patterning, kidney tubule formation, retinal architecture, and neural tube closure. In post-natal tissues, TCTN1 supports ongoing ciliary functions in the kidney, photoreceptor cells of the retina, and other organs where cilia coordinate sensory and homeostatic signalling.
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Chromosome location
TCTN1 is positioned on the long arm of chromosome 12 at band q24.11. The genomic locus spans multiple exons, though detailed exon count and transcript variant information vary across annotation databases. The chromosomal region 12q24 contains several other genes involved in development and cellular structure, but TCTN1 occupies a distinct segment within this band.
Protein structure
The TCTN1 protein comprises 587 amino acids. Structural analysis reveals a disordered region spanning amino acids 46 to 68, which may confer flexibility important for protein-protein interactions within the crowded environment of the transition zone. Domain architecture beyond this disordered segment has not been comprehensively characterised at the experimental level, though the protein is predicted to contain multiple transmembrane or membrane-associated segments that anchor it within the ciliary base.
Key variants
Pathogenic variants in TCTN1 are inherited in an autosomal recessive pattern, meaning that two altered copies (one from each parent) are required to cause disease. Reported variants include missense changes, frameshift deletions, and splice-site alterations that reduce or abolish protein function. The spectrum of variants reflects loss-of-function mechanisms, and individuals carrying a single pathogenic variant are typically asymptomatic carriers.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.1213_1216del | p.Asn405fs | Pathogenic/Likely pathogenic | ★★☆☆ | Joubert syndrome |
c.1418del | p.Pro473fs | Pathogenic/Likely pathogenic | ★★☆☆ | TCTN1-related disorder |
c.1465del | p.His489fs | Pathogenic/Likely pathogenic | ★★☆☆ | TCTN1-related disorder |
c.1775_1778del | p.Val592fs | Pathogenic/Likely pathogenic | ★★☆☆ | Meckel-Gruber syndrome |
c.26_29dup | p.Val11fs | Pathogenic | ★★☆☆ | Joubert syndrome |
c.342-2A>C | - | Pathogenic | ★★☆☆ | Joubert syndrome 13 |
c.411_412del | p.Arg138fs | Pathogenic/Likely pathogenic | ★★☆☆ | TCTN1-related disorder |
c.514G>T | p.Glu172Ter | Pathogenic/Likely pathogenic | ★★☆☆ | TCTN1-related disorder |
c.736A>T | p.Lys246Ter | Pathogenic/Likely pathogenic | ★★☆☆ | TCTN1-related disorder |
c.822+1G>A | - | Pathogenic/Likely pathogenic | ★★☆☆ | TCTN1-related disorder |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Biallelic pathogenic variants in TCTN1 are associated with a group of rare ciliopathies characterised by overlapping features affecting the kidneys, central nervous system, eyes, and skeleton. Clinical presentations can include cystic kidney disease, brain malformations such as cerebellar hypoplasia or encephalocele, retinal dystrophy, and polydactyly. The severity and combination of features vary among affected individuals, reflecting the broad role of primary cilia across developing and mature tissues. These conditions are collectively rare, with prevalence estimates difficult to establish due to phenotypic and genetic heterogeneity.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic TCTN1 variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
Within the NHS Genomic Medicine Service, TCTN1 holds green classification status across multiple PanelApp panels, reflecting strong evidence for its role in ciliopathy disorders. These panels include DDG2P, Fetal anomalies (green, version R21), Neurological ciliopathies, Ophthalmological ciliopathies, Rare multisystem ciliopathy disorders, and Renal ciliopathies. Green status indicates high confidence that variants in this gene cause the associated conditions, supporting its use in diagnostic and carrier screening within NHS laboratories.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What does it mean to be a carrier of a TCTN1 variant?
Carriers have one pathogenic variant and one normal copy of TCTN1. They typically show no symptoms because the normal copy produces sufficient protein. However, if both parents are carriers, each pregnancy has a 25% chance of inheriting two altered copies and developing a ciliopathy.
How are TCTN1-related conditions diagnosed?
Diagnosis usually involves clinical assessment of features such as kidney cysts, brain malformations, or eye abnormalities, followed by genetic testing through panels that include TCTN1. Imaging studies and specialist evaluation by nephrologists, neurologists, or ophthalmologists may be required.
Is TCTN1 included in carrier screening in the UK?
TCTN1 is part of carrier screening panels for autosomal recessive conditions, particularly those targeting populations or families with a history of ciliopathies. Testing is typically offered through accredited laboratories with genetic counsellor support to interpret results and discuss reproductive options.