Bardet-Biedl Syndrome Phenotype Research Articles

Bbs1 gene is required for wound healing (216.5)

Bardet‐Biedl syndrome (BBS) is a pleiotropic autosomal recessive disorder associated with several features including obesity, polydactyly, retinal degeneration, hypogenitalism, leaning difficulty and renal dysfunction. Mice bearing an M390R mutation in Bbs1 gene (which a common mutation in human BBS) recapitulates many BBS phenotypes including obesity and hypogenitalism. However, the role and contribution of Bbs1 gene to cell migration and tissue repair remains unclear. Using wound healing assays, we found that cultured mouse embryonic fibroblasts (MEFs) derived from Bbs1M390R knock‐in mice have severely compromised migratory ability compared to the MEF derived from wild type mice. Activation of AKT by platelet‐derived growth factor‐AA (PDGF‐AA) was also markedly reduced in Bbs1M390R MEF, suggesting that decreased PDGFRα signaling contributes to the migratory defects. Immunofluorescence microscopy analysis with acetylated α‐tubulin showed that both wt and BBS1‐Ki MEFs form primary cilia. The primary cilia were oriented towards the leading edge of the wound in wt MEFs. In marked contrast, primary cilia of BBs‐KI MEFs appeared randomly positioned and lacked an axis of orientation towards the edge of the wound. Furthermore, in vivo wound healing assays showed defects in Bbs1M390R knock‐in mouse compared to control littermates. These findings demonstrate that the Bbs1 gene is required for migration and tissue repairs.Grant Funding Source: NIH and AHA

The Centriolar Satellite Protein AZI1 Interacts with BBS4 and Regulates Ciliary Trafficking of the BBSome

Bardet-Biedl syndrome (BBS) is a well-known ciliopathy with mutations reported in 18 different genes. Most of the protein products of the BBS genes localize at or near the primary cilium and the centrosome. Near the centrosome, BBS proteins interact with centriolar satellite proteins, and the BBSome (a complex of seven BBS proteins) is believed to play a role in transporting ciliary membrane proteins. However, the precise mechanism by which BBSome ciliary trafficking activity is regulated is not fully understood. Here, we show that a centriolar satellite protein, AZI1 (also known as CEP131), interacts with the BBSome and regulates BBSome ciliary trafficking activity. Furthermore, we show that AZI1 interacts with the BBSome through BBS4. AZI1 is not involved in BBSome assembly, but accumulation of the BBSome in cilia is enhanced upon AZI1 depletion. Under conditions in which the BBSome does not normally enter cilia, such as in BBS3 or BBS5 depleted cells, knock down of AZI1 with siRNA restores BBSome trafficking to cilia. Finally, we show that azi1 knockdown in zebrafish embryos results in typical BBS phenotypes including Kupffer's vesicle abnormalities and melanosome transport delay. These findings associate AZI1 with the BBS pathway. Our findings provide further insight into the regulation of BBSome ciliary trafficking and identify AZI1 as a novel BBS candidate gene.

A case of Bardet-Biedl syndrome complicated with intracranial hypertension in a Japanese child

Bardet-Biedl syndrome (BBS) is a rare heterogeneous autosomal recessive disorder characterized by rod-cone dystrophy, postaxial polydactyly, truncal obesity, hypogonadism, learning disability, and renal anomaly that are caused by ciliary dysfunction. 16 genes have been associated with the BBS phenotype. Although recent pathophysiological studies using animal models have shown that ciliary dysfunction may induce hydrocephalus, there have been no reports of BBS with intracranial hypertension. We here describe a 9-year-old Japanese girl who was diagnosed as having BBS and later received renal transplantation due to chronic renal failure. She also exhibited intracranial hypertension, including papilledema and increased intrathecal pressure (260–300mmH2O), but her brain magnetic resonance imaging was normal. No genetic abnormalities were detected by DNA chip analysis or exome sequencing. Her papilledema improved following administration of acetazolamide. This is the first report of a case of BBS complicated with intracranial hypertension and its treatment.

Abstract 14: Central Nervous System Deletion of the Bbs1 Gene Causes Obesity and Hypertension in Mice

Bardet-Biedl syndrome (BBS) is a pleiotropic autosomal recessive disorder with several features including obesity and hypertension. Systemic knock-out mouse models lacking expression of Bbs2, Bbs4 and Bbs6 genes, and Bbs1 M390R knock-in recapitulated many of the BBS phenotypes including obesity. However, the role and contribution of different tissues to the various phenotypes associated with BBS including obesity and hypertension remains unclear. To address this, we generated a new conditional knockout mouse where exon 3 of the Bbs1 gene is floxed. Cre-mediated recombination causes a frame shift resulting in a premature stop. We assessed whether deletion of the Bbs1 gene in the central nervous system (CNS) affects body weight and arterial pressure. Breeding Bbs1 flox with nestin Cre mice created mice deficient in Bbs1 gene only in the CNS as indicated by the loss of Bbs1 gene expression (by RT-PCR) in the hypothalamus, hippocampus, cortex and brainstem, but not in peripheral tissues such as adipose tissue, liver, kidney and skeletal muscle. Importantly, Bbs1 flox /nestin Cre mice display an obesity phenotype as indicated by the increased (P<0.05) body weight (40±1 g vs. 31±1 g in controls) and fat mass measured by MRI (23±2 g vs. 9±1 g in controls) in 25 weeks old mice. We found that the obesity phenotype in Bbs1 flox /nestin Cre mice is due to both an increase (P<0.05) in food intake (4.0±0.2 g vs. 3.1±0.3 g in controls) and reduction in energy expenditure as indicated by the decreased (P<0.05) O 2 consumption (2.8±0.3 mL/100g/min vs. 3.2±0.2 mL/100g/min in controls) and heat production (8.3±0.8 kcal/kg/h vs. 9.4±0.7 kcal/kg/h in controls). These results indicate that hyperphagia and low metabolic rate explain the development of obesity in Bbs1 flox /nestin Cre mice. Finally, we assessed by radiotelemetry the consequence on arterial pressure of ablating the Bbs1 gene throughout the CNS. Interestingly, CNS deletion of the Bbs1 gene recapitulates the hypertension phenotype of BBS as indicated by the elevated mean arterial pressure in Bbs1 flox /nestin Cre (123±3 mmHg) relative to littermate controls (112±4 mmHg, P=0.02). These findings demonstrate that Bbs genes in the CNS are critical for energy homeostasis and arterial pressure regulation.

Ectopic expression of human BBS4 can rescue Bardet-Biedl syndrome phenotypes in Bbs4 null mice.

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous autosomal recessive disorder characterized by obesity, retinal degeneration, polydactyly, hypogenitalism and renal defects. Recent findings have associated the etiology of the disease with cilia, and BBS proteins have been implicated in trafficking various ciliary cargo proteins. To date, 17 different genes have been reported for BBS among which BBS1 is the most common cause of the disease followed by BBS10, and BBS4. A murine model of Bbs4 is known to phenocopy most of the human BBS phenotypes, and it is being used as a BBS disease model. To better understand the in vivo localization, cellular function, and interaction of BBS4 with other proteins, we generated a transgenic BBS4 mouse expressing the human BBS4 gene under control of the beta actin promoter. The transgene is expressed in various tissues including brain, eye, testis, heart, kidney, and adipose tissue. These mice were further bred to express the transgene in Bbs4 null mice, and their phenotype was characterized. Here we report that despite tissue specific variable expression of the transgene, human BBS4 was able to complement the deficiency of Bbs4 and rescue all the BBS phenotypes in the Bbs4 null mice. These results provide an encouraging prospective for gene therapy for BBS related phenotypes and potentially for other ciliopathies.

BBS7 is required for BBSome formation and its absence in mice results in Bardet-Biedl syndrome phenotypes and selective abnormalities in membrane protein trafficking

Bardet-Biedl Syndrome (BBS) is a pleiotropic and genetically heterozygous disorder caused independently by numerous genes (BBS1-BBS17). Seven highly conserved BBS proteins (BBS1, 2, 4, 5, 7, 8 and 9) form a complex known as the BBSome, which functions in ciliary membrane biogenesis. BBS7 is both a unique subunit of the BBSome and displays direct physical interaction with a second BBS complex, the BBS chaperonin complex. To examine the in vivo function of BBS7, we generated Bbs7 knockout mice. Bbs7(-/-) mice show similar phenotypes to other BBS gene mutant mice including retinal degeneration, obesity, ventriculomegaly and male infertility characterized by abnormal spermatozoa flagellar axonemes. Using tissues from Bbs7(-/-) mice, we show that BBS7 is required for BBSome formation, and that BBS7 and BBS2 depend on each other for protein stability. Although the BBSome serves as a coat complex for ciliary membrane proteins, BBS7 is not required for the localization of ciliary membrane proteins polycystin-1, polycystin-2, or bitter taste receptors, but absence of BBS7 leads to abnormal accumulation of the dopamine D1 receptor to the ciliary membrane, indicating that BBS7 is involved in specific membrane protein localization to cilia.

BBS1 Mutations in a Wide Spectrum of Phenotypes Ranging From Nonsyndromic Retinitis Pigmentosa to Bardet-Biedl Syndrome

To investigate the involvement of the Bardet-Biedl syndrome (BBS) gene BBS1 p.M390R variant in nonsyndromic autosomal recessive retinitis pigmentosa (RP). Homozygosity mapping of a patient with isolated RP was followed by BBS1 sequence analysis. We performed restriction fragment length polymorphism analysis of the p.M390R allele in 2007 patients with isolated RP or autosomal recessive RP and in 1824 ethnically matched controls. Patients with 2 BBS1 variants underwent extensive clinical and ophthalmologic assessment. In an RP proband who did not fulfill the clinical criteria for BBS, we identified a large homozygous region encompassing the BBS1 gene, which carried the p.M390R variant. In addition, this variant was detected homozygously in 10 RP patients and 1 control, compound heterozygously in 3 patients, and heterozygously in 5 patients and 6 controls. The 14 patients with 2 BBS1 variants showed the entire clinical spectrum, from nonsyndromic RP to full-blown BBS. In 8 of 14 patients, visual acuity was significantly reduced. In patients with electroretinographic responses, a rod-cone pattern of photoreceptor degeneration was observed. Variants in BBS1 are significantly associated with nonsyndromic autosomal recessive RP and relatively mild forms of BBS. As exemplified in this study by the identification of a homozygous p.M390R variant in a control individual and in unaffected parents of BBS patients in other studies, cis - or trans -acting modifiers may influence the disease phenotype. It is important to monitor patients with an early diagnosis of mild BBS phenotypes for possible life-threatening conditions.

Molecular diagnosis reveals genetic heterogeneity for the overlapping MKKS and BBS phenotypes

Hydrometrocolpos and polydactyly diagnosed in the prenatal period or early childhood may raise diagnostic dilemmas especially in distinguishing McKusick–Kaufman syndrome (MKKS) and the Bardet–Biedl syndrome (BBS). These two conditions can initially overlap. With time, the additional features of BBS appearing in childhood, such as retinitis pigmentosa, obesity, learning disabilities and progressive renal dysfunction allow clear differentiation between BBS and MKKS. Genotype overlap also exists, as mutations in the MKKS-BBS6 gene are found in both syndromes. We report 7 patients diagnosed in the neonatal period with hydrometrocolpos and polydactyly who carry mutations in various BBS genes ( BBS6, BBS2, BBS10, BBS8 and BBS12), stressing the importance of wide BBS genotyping in patients with this clinical association for diagnosis, prognosis and genetic counselling.

BBS6, BBS10, and BBS12 form a complex with CCT/TRiC family chaperonins and mediate BBSome assembly

Bardet-Biedl syndrome (BBS) is a human genetic disorder resulting in obesity, retinal degeneration, polydactyly, and nephropathy. Recent studies indicate that trafficking defects to the ciliary membrane are involved in this syndrome. Here, we show that a novel complex composed of three chaperonin-like BBS proteins (BBS6, BBS10, and BBS12) and CCT/TRiC family chaperonins mediates BBSome assembly, which transports vesicles to the cilia. Chaperonin-like BBS proteins interact with a subset of BBSome subunits and promote their association with CCT chaperonins. CCT activity is essential for BBSome assembly, and knockdown of CCT chaperonins in zebrafish results in BBS phenotypes. Many disease-causing mutations found in BBS6, BBS10, and BBS12 disrupt interactions among these BBS proteins. Our data demonstrate that BBS6, BBS10, and BBS12 are necessary for BBSome assembly, and that impaired BBSome assembly contributes to the etiology of BBS phenotypes associated with the loss of function of these three BBS genes.

Clinical and molecular characterisation of Bardet–Biedl syndrome in consanguineous populations: the power of homozygosity mapping

Bardet–Biedl syndrome (BBS) is a ciliopathy with pleiotropic effect that manifests primarily as renal insufficiency, polydactyly, retinal dystrophy and obesity. The current phenotype–genotype correlation is insufficient to predict the likely...

Mechanistic insights into Bardet-Biedl syndrome, a model ciliopathy.

Bardet-Biedl syndrome (BBS) is a multisystemic disorder typified by developmental and progressive degenerative defects. A combination of genetic, in vitro, and in vivo studies have highlighted ciliary dysfunction as a primary cause of BBS pathology, which has in turn contributed to the improved understanding of the functions of the primary cilium in humans and other vertebrates. Here we discuss the evidence linking the clinical BBS phenotype to ciliary defects, highlight how the genetic and cellular characteristics of BBS overlap with and inform other ciliary disorders, and explore the possible mechanistic underpinnings of ciliary dysfunction.

A BBSome Subunit Links Ciliogenesis, Microtubule Stability, and Acetylation

Primary cilium dysfunction affects the development and homeostasis of many organs in Bardet-Biedl syndrome (BBS). We recently showed that seven highly conserved BBS proteins form a stable complex, the BBSome, that functions in membrane trafficking to and inside the primary cilium. We have now discovered a BBSome subunit that we named BBIP10. Similar to other BBSome subunits, BBIP10 localizes to the primary cilium, BBIP10 is present exclusively in ciliated organisms, and depletion of BBIP10 yields characteristic BBS phenotypes in zebrafish. Unexpectedly, BBIP10 is required for cytoplasmic microtubule polymerization and acetylation, two functions not shared with any other BBSome subunits. Strikingly, inhibition of the tubulin deacetylase HDAC6 restores microtubule acetylation in BBIP10-depleted cells, and BBIP10 physically interacts with HDAC6. BBSome-bound BBIP10 may therefore function to couple acetylation of axonemal microtubules and ciliary membrane growth.

Bardet–Biedl syndrome proteins are required for the localization of G protein-coupled receptors to primary cilia

Primary cilia are ubiquitous cellular appendages that provide important yet not well understood sensory and signaling functions. Ciliary dysfunction underlies numerous human genetic disorders. However, the precise defects in cilia function and the basis of disease pathophysiology remain unclear. Here, we report that the proteins disrupted in the human ciliary disorder Bardet-Biedl syndrome (BBS) are required for the localization of G protein-coupled receptors to primary cilia on central neurons. We demonstrate a lack of ciliary localization of somatostatin receptor type 3 (Sstr3) and melanin-concentrating hormone receptor 1 (Mchr1) in neurons from mice lacking the Bbs2 or Bbs4 gene. Because Mchr1 is involved in the regulation of feeding behavior and BBS is associated with hyperphagia-induced obesity, our results suggest that altered signaling caused by mislocalization of ciliary signaling proteins underlies the BBS phenotypes. Our results also provide a potential molecular mechanism to link cilia defects with obesity.

A Core Complex of BBS Proteins Cooperates with the GTPase Rab8 to Promote Ciliary Membrane Biogenesis

Primary cilium dysfunction underlies the pathogenesis of Bardet-Biedl syndrome (BBS), a genetic disorder whose symptoms include obesity, retinal degeneration, and nephropathy. However, despite the identification of 12 BBS genes, the molecular basis of BBS remains elusive. Here we identify a complex composed of seven highly conserved BBS proteins. This complex, the BBSome, localizes to nonmembranous centriolar satellites in the cytoplasm but also to the membrane of the cilium. Interestingly, the BBSome is required for ciliogenesis but is dispensable for centriolar satellite function. This ciliogenic function is mediated in part by the Rab8 GDP/GTP exchange factor, which localizes to the basal body and contacts the BBSome. Strikingly, Rab8(GTP) enters the primary cilium and promotes extension of the ciliary membrane. Conversely, preventing Rab8(GTP) production blocks ciliation in cells and yields characteristic BBS phenotypes in zebrafish. Our data reveal that BBS may be caused by defects in vesicular transport to the cilium.

Phenotypic characterization of Bbs4 null mice reveals age-dependent penetrance and variable expressivity

Bardet-Biedl syndrome (BBS) is a rare oligogenic disorder exhibiting both clinical and genetic heterogeneity. Although the BBS phenotype is variable both between and within families, the syndrome is characterized by the hallmarks of developmental and learning difficulties, post-axial polydactylia, obesity, hypogenitalism, renal abnormalities, retinal dystrophy, and several less frequently observed features. Eleven genes mutated in BBS patients have been identified, and more are expected to exist, since about 20-30% of all families cannot be explained by the known loci. To investigate the etiopathogenesis of BBS, we created a mouse null for one of the murine homologues, Bbs4, to assess the contribution of one gene to the pleiotropic murine Bbs phenotype. Bbs4 null mice, although initially runted compared to their littermates, ultimately become obese in a gender-dependent manner, females earlier and with more severity than males. Blood chemistry tests indicated abnormal lipid profiles, signs of liver dysfunction, and elevated insulin and leptin levels reminiscent of metabolic syndrome. As in patients with BBS, we found age-dependent retinal dystrophy. Behavioral assessment revealed that mutant mice displayed more anxiety-related responses and reduced social dominance. We noted the rare occurrence of birth defects, including neural tube defects and hydrometrocolpos, in the null mice. Evaluations of these null mice have uncovered phenotypic features with age-dependent penetrance and variable expressivity, partially recapitulating the human BBS phenotype.

Bardet-Biedl syndrome type 4 (BBS4)-null mice implicate Bbs4 in flagella formation but not global cilia assembly.

The functions of the proteins encoded by the Bardet-Biedl syndrome (BBS) genes are unknown. Mutations in these genes lead to the pleiotropic human disorder BBS, which is characterized by obesity, retinopathy, polydactyly, renal and cardiac malformations, learning disabilities, and hypogenitalism. Secondary features include diabetes mellitus and hypertension. Recently, it has been suggested that the BBS phenotypes are the result of a lack of cilia formation or function. In this study, we show that mice lacking the Bbs4 protein have major components of the human phenotype, including obesity and retinal degeneration. We show that Bbs4-null mice develop both motile and primary cilia, demonstrating that Bbs4 is not required for global cilia formation. Interestingly, male Bbs4-null mice do not form spermatozoa flagella, and BBS4 retinopathy involves apoptotic death of photoreceptors, the primary ciliated cells of the retina. These mutation data demonstrate a connection between the function of a BBS protein and cilia. To further evaluate an association between cilia and BBS, we performed homology comparisons of BBS proteins in model organisms and find that BBS proteins are specifically conserved in ciliated organisms.

The Bardet-Biedl protein BBS4 targets cargo to the pericentriolar region and is required for microtubule anchoring and cell cycle progression.

BBS4 is one of several proteins that cause Bardet-Biedl syndrome (BBS), a multisystemic disorder of genetic and clinical complexity. Here we show that BBS4 localizes to the centriolar satellites of centrosomes and basal bodies of primary cilia, where it functions as an adaptor of the p150(glued) subunit of the dynein transport machinery to recruit PCM1 (pericentriolar material 1 protein) and its associated cargo to the satellites. Silencing of BBS4 induces PCM1 mislocalization and concomitant deanchoring of centrosomal microtubules, arrest in cell division and apoptotic cell death. Expression of two truncated forms of BBS4 that are similar to those found in some individuals with BBS had a similar effect on PCM1 and microtubules. Our findings indicate that defective targeting or anchoring of pericentriolar proteins and microtubule disorganization contribute to the BBS phenotype and provide new insights into possible causes of familial obesity, diabetes and retinal degeneration.

Genetic and Mutational Analyses of a Large Multiethnic Bardet-Biedl Cohort Reveal a Minor Involvement of BBS6 and Delineate the Critical Intervals of Other Loci

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive disorder characterized primarily by obesity, polydactyly, retinal dystrophy, and renal disease. The significant genetic and clinical heterogeneity of this condition have substantially hindered efforts to positionally clone the numerous BBS genes, because the majority of available pedigrees are small and the disorder cannot be assigned to any of the six known BBS loci. Consequently, the delineation of critical BBS intervals, which would accelerate the discovery of the underlying genetic defect(s), becomes difficult, especially for loci with minor contributions to the syndrome. We have collected a cohort of 163 pedigrees from diverse ethnic backgrounds and have evaluated them for mutations in the recently discovered BBS6 gene (MKKS) on chromosome 20 and for potential assignment of the disorder to any of the other known BBS loci in the human genome. Using a combination of mutational and haplotype analysis, we describe the spectrum of BBS6 alterations that are likely to be pathogenic; propose substantially reduced critical intervals for BBS2, BBS3, and BBS5; and present evidence for the existence of at least one more BBS locus. Our data also suggest that BBS6 is a minor contributor to the syndrome and that some BBS6 alleles may act in conjunction with mutations at other BBS loci to cause or modify the BBS phenotype.