BBS10 — Bardet-Biedl syndrome 10

The BBS10 gene provides the body with instructions for making a protein that is essential for the normal function of cells. Specifically, this protein acts like a molecular helper, or "chaperone," that assists in building a very important structure called the BBSome. The BBSome is a complex of proteins that works inside a tiny, hair-like projection on the surface of many cells, known as the primary cilium. In the eyes, these cilia are crucial for the light-sensing cells (photoreceptors) to work properly and survive. The cilia act like microscopic highways, transporting vital materials back and forth within the cell. When a person has mutations (harmful changes) in both of their copies of the BBS10 gene, their body cannot make a functional BBS10 protein. Without it, the BBSome cannot be assembled correctly, and the cellular "highways" in the cilia break down. In the retina, this leads to the gradual death of the light-sensing cells, causing a condition called progressive retinal dystrophy. This usually starts with difficulty seeing in the dark (night blindness) during childhood and slowly worsens over time, often leading to severe vision loss or legal blindness by adulthood. Because primary cilia are found on cells all over the body, mutations in the BBS10 gene cause a condition called Bardet-Biedl syndrome (BBS), which affects many different organs. In addition to vision loss, individuals with BBS often experience weight gain and obesity starting in early childhood, extra fingers or toes (polydactyly), learning difficulties, and kidney problems. BBS is an inherited condition that follows an "autosomal recessive" pattern. This means that for a child to have the condition, both parents must carry one copy of the mutated gene and pass it on. The parents themselves usually do not show any symptoms of the disease.

Gene description: BBS10 encodes a chaperonin-like protein involved in the assembly and stability of the BBSome complex, essential for ciliary function.

Patient and family guide: The BBS10 gene provides the body with instructions for making a protein that is essential for the normal function of cells. Specifically, this protein acts like a molecular helper, or "chaperone," that assists in building a very important structure called the BBSome. The BBSome is a complex of proteins that works inside a tiny, hair-like projection on the surface of many cells, known as the primary cilium. In the eyes, these cilia are crucial for the light-sensing cells (photoreceptors) to work properly and survive. The cilia act like microscopic highways, transporting vital materials back and forth within the cell. When a person has mutations (harmful changes) in both of their copies of the BBS10 gene, their body cannot make a functional BBS10 protein. Without it, the BBSome cannot be assembled correctly, and the cellular "highways" in the cilia break down. In the retina, this leads to the gradual death of the light-sensing cells, causing a condition called progressive retinal dystrophy. This usually starts with difficulty seeing in the dark (night blindness) during childhood and slowly worsens over time, often leading to severe vision loss or legal blindness by adulthood. Because primary cilia are found on cells all over the body, mutations in the BBS10 gene cause a condition called Bardet-Biedl syndrome (BBS), which affects many different organs. In addition to vision loss, individuals with BBS often experience weight gain and obesity starting in early childhood, extra fingers or toes (polydactyly), learning difficulties, and kidney problems. BBS is an inherited condition that follows an "autosomal recessive" pattern. This means that for a child to have the condition, both parents must carry one copy of the mutated gene and pass it on. The parents themselves usually do not show any symptoms of the disease.

Gene function: BBS10 is vital for the proper assembly and stability of the BBSome, which is critical for protein transport within photoreceptor cilia. Its function ensures the delivery of phototransduction components to the outer segments and the removal of waste products, maintaining photoreceptor health. Mutations disrupt this process, leading to severe rod-cone dystrophy and vision impairment.

Protein structure: The BBS10 gene encodes a protein consisting of 723 amino acids. Structurally, the BBS10 protein is characterized by its homology to the Type II chaperonin family, which includes proteins like CCT/TRiC that are involved in folding cellular proteins. The protein contains distinct structural domains typical of chaperonins, including an equatorial domain that houses the ATP-binding site, an intermediate domain, and an apical domain that is thought to be involved in substrate binding. Unlike some other BBS-associated chaperonin-like proteins (such as BBS6), BBS10 retains a functional ATP-binding motif, which is essential for its activity. The intermediate and equatorial domains are particularly crucial for its structural integrity and function. In terms of assembly, BBS10 does not function alone; it forms a transient, intermediate complex with two other chaperonin-like proteins, BBS6 and BBS12. This tripartite complex is necessary to stabilize BBS7 and facilitate the subsequent, stepwise assembly of the entire octameric BBSome complex, which is vital for ciliary transport.

Molecular function: The BBS10 gene encodes a protein that functions as a probable molecular chaperone, specifically belonging to the Type II chaperonin family. Its primary molecular role is to assist in the folding of proteins in an ATP-dependent manner. Unlike some other chaperonins, BBS10 retains its ATP-binding site, which is crucial for its function. The most critical known function of the BBS10 protein is its involvement in the initial steps of assembling the BBSome. The BBSome is a highly conserved octameric protein complex that is essential for the proper function of primary cilia. It acts as a coat complex that sorts and traffics membrane proteins into and out of the cilium. BBS10, along with BBS12 and BBS6 (MKKS), forms a transient chaperonin complex that mediates the interaction between BBS7 and other BBSome components, facilitating the stepwise assembly of the core BBSome. In photoreceptor cells, this complex is vital for the transport of phototransduction proteins across the connecting cilium to the outer segment. Disruption of BBS10 leads to a failure in BBSome assembly, resulting in defective ciliary transport, accumulation of proteins in the inner segment, and ultimately, photoreceptor cell death.

Expression pattern: The BBS10 gene is widely expressed across various tissues, reflecting its role in the ubiquitous primary cilium. High levels of expression are found in the retina, particularly within the photoreceptor cells (both rods and cones), where the protein is essential for the maintenance and function of the connecting cilium. This ciliary structure is critical for the massive daily transport of proteins between the inner and outer segments of photoreceptors. Beyond the retina, BBS10 is expressed in the brain (including the hippocampus, amygdala, and basal ganglia), kidney, adipose tissue, and developing limbs. This broad expression pattern correlates with the systemic manifestations of Bardet-Biedl syndrome, such as cognitive impairment, renal dysplasia, obesity, and polydactyly. The protein is expressed early in development, playing a crucial role in ciliogenesis and cellular differentiation pathways, including adipogenesis.

Mutation spectrum: The mutation spectrum of the BBS10 gene is diverse, encompassing missense, nonsense, frameshift, and splice-site variants. Frameshift and nonsense mutations, which lead to premature truncation of the protein and loss of function, are particularly common and are generally associated with a severe phenotype. The gene consists of only two exons, making it relatively straightforward to sequence. A significant feature of the BBS10 mutation spectrum is the presence of a major hotspot mutation: c.271dup (p.Cys91Leufs*5). This 1-base pair insertion in exon 2 is the most frequent pathogenic variant, accounting for up to 48% of all BBS10 mutant alleles in some populations, particularly those of European descent. It is considered a founder mutation. In total, dozens of distinct pathogenic variants have been identified in the BBS10 gene, contributing to its status as one of the most frequently mutated genes in Bardet-Biedl syndrome.

Pathogenic variants: 1. p.Cys91Leufs*5 (c.271dup) - The most common pathogenic variant, a frameshift mutation that leads to premature protein truncation. It is a major founder mutation in populations of European descent and is associated with a classic, severe BBS phenotype. 2. p.Glu679Lys (c.2035G>A) - A missense mutation that alters a conserved amino acid, likely disrupting the protein's chaperone function or its ability to interact with other BBSome components. 3. p.Arg272* (c.814C>T) - A nonsense mutation resulting in a truncated, non-functional protein, leading to typical BBS manifestations including severe retinal dystrophy. 4. p.Ser303Pro (c.907T>C) - A missense variant that has been identified in multiple families, affecting the structural integrity or functional domains of the BBS10 protein.

Clinical significance: Mutations in the BBS10 gene are a major cause of Bardet-Biedl syndrome (BBS), accounting for approximately 20% of all cases. Clinically, BBS10 mutations manifest as a severe, pleiotropic disorder characterized primarily by progressive retinal dystrophy, which typically begins with night blindness in childhood and progresses to severe visual impairment or legal blindness by early adulthood. The retinal phenotype is often described as a rod-cone dystrophy or atypical retinitis pigmentosa. In addition to the hallmark retinal degeneration, patients with BBS10 mutations frequently present with a spectrum of systemic features. These include early-onset truncal obesity, postaxial polydactyly (extra fingers or toes), cognitive impairment or learning disabilities, hypogonadism (particularly in males), and renal anomalies. Renal disease is a significant cause of morbidity and mortality in BBS patients. The severity and specific combination of these symptoms can vary widely even among individuals with the same mutation, highlighting the complex expressivity of the syndrome.

Inheritance: Autosomal Recessive

Chromosomal location: 12q21.2

Genotype-phenotype correlations: Genotype-phenotype correlations in BBS10-related Bardet-Biedl syndrome are complex and often challenging to establish definitively due to the high degree of clinical variability. However, some general trends have been observed. Patients with truncating mutations (such as nonsense or frameshift mutations, including the common c.271dup variant) generally present with a more severe phenotype compared to those with missense mutations. This severity is often reflected in an earlier onset of visual impairment, more pronounced obesity, and a higher incidence of renal abnormalities. Interestingly, while BBS10 mutations are among the most common causes of BBS, some studies suggest that patients with BBS10 mutations may have a slightly lower penetrance of certain features, such as renal anomalies, compared to patients with mutations in other major BBS genes like BBS1. However, the overall clinical picture remains severe, and the presence of modifying alleles in other BBS genes (oligogenic inheritance) can further influence the severity and specific manifestations of the disease in individual patients.

Research and therapeutic approaches: Currently, there are no FDA-approved treatments that can cure or halt the progression of the retinal degeneration caused by BBS10 mutations. Management is primarily supportive and multidisciplinary, focusing on treating the symptoms. This includes low-vision aids and educational support for visual impairment, strict diet and exercise regimens for obesity management, and monitoring and treatment for renal and endocrine complications. However, significant progress is being made in the pipeline for targeted therapies, particularly gene therapy. Proof-of-concept studies in animal models have shown promising results. For instance, subretinal delivery of an AAV (adeno-associated virus) vector carrying the wild-type Bbs10 gene in a Bbs10 knockout mouse model has been demonstrated to successfully delay vision loss and preserve photoreceptor structure. These preclinical successes are paving the way for potential future clinical trials. While no gene therapy for BBS10 is currently approved (unlike Luxturna for RPE65-related disease), the active research in AAV-mediated gene augmentation offers hope for a targeted treatment for the retinal dystrophy associated with this gene.

Diagnostic testing: Diagnosis of BBS10-related Bardet-Biedl syndrome typically involves a combination of clinical evaluation and molecular genetic testing. Given the genetic heterogeneity of BBS (with over 20 associated genes), multi-gene panel testing or whole exome sequencing (WES) is the preferred diagnostic approach. These methods can efficiently identify pathogenic variants in BBS10 alongside other BBS genes. Targeted testing may be used if a specific familial mutation is known. Genetic counseling is a critical component of the diagnostic process. Since BBS is inherited in an autosomal recessive manner, parents of an affected individual are obligate carriers and have a 25% chance of having another affected child in each pregnancy. Genetic counselors can help families understand the inheritance pattern, the variable expressivity of the disease, and the implications for family planning, including the availability of carrier screening for at-risk relatives and prenatal or preimplantation genetic testing options.

Animal models: The primary animal model used to study BBS10 is the Bbs10 knockout mouse (Bbs10-/-). These mice exhibit progressive retinal degeneration, which closely mimics the human phenotype. Studies have shown that in Bbs10-/- mice, cone electrical function is absent early on, even though cones are anatomically present, followed by progressive loss of both rods and cones. This model has been crucial for understanding the disease mechanism, demonstrating that the lack of BBS10 disrupts the assembly of the BBSome, leading to defective ciliary transport in photoreceptors. Additionally, these mice display other systemic features of Bardet-Biedl syndrome, such as obesity and renal abnormalities, making them a comprehensive model for the disease.

Population genetics: Bardet-Biedl syndrome is a rare disorder with an estimated overall prevalence of 1 in 140,000 to 1 in 160,000 in most populations of European descent. However, the prevalence is significantly higher in certain isolated or consanguineous populations, such as the Bedouin population in Kuwait (1 in 13,500) and populations in Newfoundland, Canada (1 in 17,500). Mutations in the BBS10 gene are one of the most common causes of BBS, accounting for approximately 20% of cases overall. The carrier frequency for BBS10 mutations in the general population is relatively low, but the specific c.271dup founder mutation is notably prevalent among individuals of European ancestry, significantly contributing to the overall disease burden in this demographic.

Selected references: 1. Stoetzel C, et al. BBS10 encodes a vertebrate-specific chaperonin-like protein and is a major BBS locus. Nat Genet. 2006;38(5):521-524. PMID: 16565715 2. Forsyth RL, et al. Bardet-Biedl Syndrome Overview. GeneReviews. 2023. PMID: 20301537 3. Hsu Y, et al. Subretinal gene therapy delays vision loss in a Bardet-Biedl syndrome type 10 mouse model. Mol Ther Nucleic Acids. 2023;32:100-114. PMID: 37025555 4. Alvarez-Satta M, et al. Bardet-Biedl Syndrome as a Chaperonopathy. Front Mol Biosci. 2017;4:55. PMID: 28824917 5. Dollfus H, et al. Bardet-Biedl syndrome improved diagnosis criteria and treatment updates. Eur J Hum Genet. 2024. PMID: 38383115 6. Mayer SK, et al. Knockout of Bbs10 results in lack of cone electrical function and progressive retinal degeneration. bioRxiv. 2022. (Preprint)