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BBS4
Bardet-Biedl syndrome 4
BBS4 is located on the long (q) arm of chromosome 15, at band 15q24.1. Arm ratio per GRCh38 - banding schematic.
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Overview
BBS4 is located on chromosome 15 and encodes a 519-amino-acid protein integral to the structure and function of primary cilia. Cilia are antenna-like projections on the surface of most human cells that detect chemical and mechanical signals from the environment and coordinate cellular responses. The BBS4 protein is a core component of the BBSome, a multi-protein assembly that transports signalling receptors into and out of cilia.
When both copies of BBS4 carry pathogenic variants, individuals develop Bardet-Biedl syndrome, a ciliopathy affecting multiple organ systems. Because ciliary function is critical during development and throughout life, disruption of BBS4 leads to a broad spectrum of clinical features, including vision loss, renal impairment, and metabolic dysfunction. BBS4 follows an autosomal recessive inheritance pattern, meaning two altered copies are required for the condition to manifest.
What the gene does
The BBS4 protein participates in intraflagellar transport, the process by which molecular cargo moves along the microtubule scaffold inside cilia. As a member of the BBSome complex, BBS4 helps recognise and bind specific membrane proteins, particularly G-protein-coupled receptors, and escorts them to their correct positions within the cilium or retrieves them for recycling.
This trafficking function is essential for maintaining the cilium's signalling capacity. In photoreceptor cells of the retina, for example, proper localisation of visual signalling proteins depends on BBSome activity. In the kidney, cilia on tubular epithelial cells sense fluid flow and regulate cellular responses; disruption of this sensing mechanism contributes to cyst formation and renal dysfunction. The BBSome also influences hypothalamic pathways that regulate appetite and energy balance, which may explain the obesity phenotype observed when BBS4 function is lost.
BBS4 anchors to the centrosome and ciliary base through specific regions at its amino and carboxy termini, ensuring the BBSome is correctly positioned to carry out its transport functions. Loss of BBS4 activity destabilises the entire BBSome complex, impairing ciliary membrane composition and disrupting downstream signalling cascades.
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Chromosome location
BBS4 is located at chromosomal band 15q24.1 on the long arm of chromosome 15. The gene spans multiple exons encoding a 519-amino-acid protein. This genomic region has been mapped through linkage studies in families affected by Bardet-Biedl syndrome, and the gene was identified as one of several loci responsible for the condition.
Protein structure
The BBS4 protein is 519 amino acids in length and contains ten tetratricopeptide repeat (TPR) motifs arranged in tandem from amino acids 67 to 408. TPR motifs are structural modules that mediate protein-protein interactions, allowing BBS4 to bind other BBSome components and cargo proteins. The individual repeats are designated TPR 1 through TPR 10.
Two regions are required for localisation to centrosomes: amino acids 1-66 at the amino terminus and amino acids 338-519 at the carboxy terminus. A central region spanning amino acids 101-337, which includes TPR 2 through TPR 8, mediates interaction with PCM1, a protein involved in centriolar satellite assembly. Disordered regions are present at amino acids 1-25 and 440-519, likely providing structural flexibility for dynamic protein interactions during ciliary transport. The modular TPR architecture enables BBS4 to serve as a scaffold, coordinating interactions among multiple BBSome subunits and facilitating cargo recognition.
Key variants
Pathogenic variants in BBS4 are distributed across the gene and include missense, nonsense, and frameshift changes. Many pathogenic variants disrupt TPR motifs, impairing the protein's ability to interact with other BBSome components or cargo molecules. Loss-of-function variants that introduce premature stop codons or cause frameshift mutations typically abolish BBS4 protein expression, resulting in complete loss of BBSome-mediated transport.
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.1064_1106+97del | - | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.1140_1147del | p.Leu381fs | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome 4 |
c.1389dup | p.Ser464fs | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.276_277del | p.Ala94fs | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.31C>T | p.Gln11Ter | Pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.406G>T | p.Glu136Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.514dup | p.Ile172fs | Pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.55C>T | p.Gln19Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.642+2T>G | - | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
c.87del | p.Pro30fs | Pathogenic/Likely pathogenic | ★★☆☆ | Bardet-Biedl syndrome |
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 BBS4 cause Bardet-Biedl syndrome, a ciliopathy with highly variable clinical features. Affected individuals typically present with progressive retinal degeneration leading to vision loss, often beginning in childhood. Additional features may include postaxial polydactyly, renal structural abnormalities, obesity, hypogonadism, and learning difficulties. The severity and combination of features vary among individuals, even within the same family, reflecting modifier genes and environmental influences.
Bardet-Biedl syndrome is genetically heterogeneous, with pathogenic variants in more than 20 genes described. BBS4 accounts for a subset of cases, and individuals with BBS4-related disease show phenotypic overlap with other BBS subtypes, making molecular diagnosis essential for accurate genetic counselling.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic BBS4 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
BBS4 is included on multiple NHS Genomic Medicine Service gene panels with green classification, indicating strong evidence for its role in disease. These panels include Bardet Biedl syndrome (panel R107), Fetal anomalies (R21), Intellectual disability (R29), Retinal disorders (R32), Severe early-onset obesity (R149), Skeletal dysplasia (R104), and several ciliopathy-focused panels covering renal, ophthalmological, and multisystem presentations. BBS4 is also listed in the DDG2P (Developmental Disorders Genotype-Phenotype) database and the Limb disorders panel, reflecting the broad clinical spectrum associated with pathogenic variants in this gene.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What inheritance pattern does BBS4 follow?
BBS4 follows an autosomal recessive inheritance pattern. This means an individual must inherit a pathogenic variant from both parents to develop Bardet-Biedl syndrome. Carriers, who have one pathogenic variant, typically do not show symptoms.
How does BBS4 relate to cilia?
BBS4 encodes a core component of the BBSome, a protein complex that transports signalling receptors and other molecules into and out of cilia. Cilia are cellular structures essential for sensing environmental signals, and their dysfunction underlies the multisystem features of Bardet-Biedl syndrome.
Why are there multiple genes associated with Bardet-Biedl syndrome?
Bardet-Biedl syndrome is genetically heterogeneous, meaning pathogenic variants in any of more than 20 genes can cause the condition. These genes encode proteins involved in ciliary assembly, transport, and signalling, and disruption of these shared pathways produces overlapping clinical features.