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IQCB1
IQ motif containing B1
IQCB1 is located on the long (q) arm of chromosome 3, at band 3q13.33. Arm ratio per GRCh38 - banding schematic.
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Overview
IQCB1 (IQ motif containing B1) is located on chromosome 3 at band 3q13.33 and encodes a 598-amino-acid protein essential for the structure and function of primary cilia. Cilia are antenna-like organelles that project from cell surfaces and serve critical roles in sensing environmental signals and coordinating cellular responses. The IQCB1 protein localises to the transition zone of cilia, a region that controls the movement of proteins in and out of the ciliary compartment.
Pathogenic variants in IQCB1 follow an autosomal recessive inheritance pattern, meaning that affected individuals typically inherit one altered copy from each parent. Disruption of IQCB1 function particularly affects tissues where cilia play essential roles, notably the photoreceptor cells of the retina and the epithelial cells of kidney tubules. The gene is included on UK carrier screening panels due to its association with serious early-onset disease.
What the gene does
The IQCB1 protein functions as a structural component of the ciliary transition zone, the base region of cilia that acts as a selective barrier controlling which proteins can enter the ciliary compartment. This protein interacts with several other ciliary proteins, including members of the Bardet-Biedl syndrome (BBS) protein complex and CEP290, forming a network that maintains ciliary architecture and regulates protein trafficking.
Within photoreceptor cells of the retina, IQCB1 helps transport proteins required for phototransduction into the outer segment, the specialised ciliary structure where light detection occurs. In kidney tubular cells, the protein supports ciliary signalling pathways that regulate cellular differentiation and tissue organisation. The multiple IQ motifs within IQCB1 are thought to mediate calcium-dependent protein interactions, allowing the protein to respond to changing cellular conditions and coordinate the assembly of multi-protein complexes at the ciliary base.
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Chromosome location
IQCB1 is located on the long arm of chromosome 3 at cytogenetic band 3q13.33. The gene spans approximately 35 kilobases of genomic DNA and contains 17 exons. This chromosomal region has been associated with ciliopathy-related phenotypes through linkage studies in families with retinal and renal disease, leading to the identification of IQCB1 as the causative gene.
Protein structure
The IQCB1 protein comprises 598 amino acids organised into several functional regions. The N-terminal region (amino acids 1-157) mediates interaction with BBS1, BBS8 and BBS9, components of the BBSome complex involved in ciliary trafficking. A larger interaction region spanning amino acids 287-598 binds CEP290 as well as multiple BBS proteins (BBS1, BBS2, BBS4, BBS5, BBS7, BBS8 and BBS9).
The protein contains four IQ motifs: IQ 1 (amino acids 294-317), IQ 2 (amino acids 318-338), IQ 3 (amino acids 387-416), and IQ 4 (amino acids 417-437). These IQ domains are calmodulin-binding motifs that typically mediate calcium-sensitive protein interactions. A coiled coil region (amino acids 336-373) likely facilitates protein dimerisation or contributes to structural organisation. The C-terminal region (amino acids 530-598) provides an additional interaction surface for BBS proteins, reinforcing the protein's role in coordinating ciliary protein networks.
Key variants
Pathogenic variants in IQCB1 are distributed throughout the gene, with both missense variants affecting specific functional domains and truncating variants predicted to result in loss of protein function. The majority of reported pathogenic variants are private to individual families, though some variants show recurrent occurrence in specific populations. Carrier frequency for pathogenic IQCB1 variants is low in most populations, though certain communities may have elevated carrier rates due to founder effects.
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.100G>T | p.Glu34Ter | Pathogenic/Likely pathogenic | ★★☆☆ | IQCB1-related disorder |
c.1332G>A | p.Trp444Ter | Pathogenic | ★★☆☆ | IQCB1-related disorder |
c.1333C>T | p.Arg445Ter | Pathogenic | ★★☆☆ | Nephronophthisis |
c.1532_1536dup | p.Gln513Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Nephronophthisis |
c.488-1G>A | - | Pathogenic/Likely pathogenic | ★★☆☆ | Nephronophthisis |
c.493C>T | p.Gln165Ter | Pathogenic | ★★☆☆ | Nephronophthisis |
c.757del | p.Cys253fs | Pathogenic/Likely pathogenic | ★★☆☆ | Nephronophthisis |
c.758del | p.Cys253fs | Pathogenic/Likely pathogenic | ★★☆☆ | IQCB1-related disorder |
c.812del | p.Ser271fs | Pathogenic | ★★☆☆ | Leber congenital amaurosis |
c.862G>T | p.Glu288Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Senior-Loken syndrome 5 |
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
Pathogenic variants in IQCB1 cause Senior-Loken syndrome, a ciliopathy characterised by the combination of retinal degeneration and kidney disease. The condition typically presents in childhood with progressive vision loss due to retinal dystrophy, alongside nephronophthisis, a form of chronic kidney disease that often progresses to kidney failure during childhood or adolescence.
Senior-Loken syndrome follows autosomal recessive inheritance, requiring pathogenic variants in both copies of IQCB1. The severity and age of onset can vary between individuals, though most affected children develop significant visual impairment and kidney dysfunction before reaching adulthood. IQCB1-related Senior-Loken syndrome represents one genetic subtype of this condition, which can also result from variants in other ciliary genes.
- Senior-Loken syndrome Dedicated page coming soon
- Senior-Løken syndrome (IQCB1) Dedicated page coming soon
Inheritance pattern
Conditions caused by pathogenic IQCB1 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
IQCB1 appears on several NHS Genomic Medicine Service gene panels with green (high-evidence) classifications. The gene is included on the Fetal anomalies panel (R21 release), Ophthalmological ciliopathies panel, Rare multisystem ciliopathy disorders panel, Renal ciliopathies panel, and Retinal disorders panel (R32 release). These panel memberships reflect the established role of IQCB1 variants in causing recognisable patterns of retinal and kidney disease that may be identified through prenatal imaging, paediatric ophthalmology assessment, or nephrology evaluation.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is Senior-Loken syndrome?
Senior-Loken syndrome is an autosomal recessive condition combining retinal degeneration with nephronophthisis, a form of kidney disease. Children with this syndrome typically develop progressive vision loss and kidney dysfunction, with many requiring kidney transplantation during childhood or adolescence.
How is IQCB1 inherited?
IQCB1-related conditions follow autosomal recessive inheritance. An affected individual has typically inherited one pathogenic variant from each parent, who are usually unaffected carriers with one working gene copy.
Why are cilia important in the retina and kidneys?
In retinal photoreceptor cells, cilia form the outer segment where light detection occurs, and proper ciliary function is essential for transporting proteins needed for vision. In kidney tubular cells, cilia sense fluid flow and chemical signals, helping regulate tissue development and maintenance. Disruption of ciliary function in these tissues leads to the retinal and kidney disease seen in Senior-Loken syndrome.