BBS10 encodes a vertebrate-specific chaperonin-like protein and is a major BBS locus

Intrinsic protein-protein interaction-mediated and chaperonin-assisted sequential assembly of stable bardet-biedl syndrome protein complex, the BBSome

The pleiotropic features of obesity, retinal degeneration, polydactyly, kidney abnormalities, cognitive impairment, hypertension, and diabetes found in Bardet-Biedl syndrome (BBS) make this disorder an important model disorder for identifying molecular mechanisms involved in common human diseases. To date, 16 BBS genes have been reported, seven of which (BBS1, 2, 4, 5, 7, 8, and 9) code for proteins that form a complex known as the BBSome. The function of the BBSome involves ciliary membrane biogenesis. Three additional BBS genes (BBS6, BBS10, and BBS12) have homology to type II chaperonins and interact with CCT/TRiC proteins and BBS7 to form a complex termed the BBS-chaperonin complex. This complex is required for BBSome assembly. Little is known about the process and the regulation of BBSome formation. We utilized point mutations and null alleles of BBS proteins to disrupt assembly of the BBSome leading to the accumulation of BBSome assembly intermediates. By characterizing BBSome assembly intermediates, we show that the BBS-chaperonin complex plays a role in BBS7 stability. BBS7 interacts with BBS2 and becomes part of a BBS7-BBS2-BBS9 assembly intermediate referred to as the BBSome core complex because it forms the core of the BBSome. BBS1, BBS5, BBS8, and finally BBS4 are added to the BBSome core to form the complete BBSome.

A case series of Bardet-Biedl syndrome in a large Turkish family and review of the literature

BACKGROUND AND AIM

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive genetic disorder. We aimed to report a case series of Bardet-Biedl syndrome in a Turkish family and review the literature.

PATIENTS AND METHODS

This family had 3 females and 4 males, totally 7 alive; 2 children (1 female and 1 male) had died. Parents were consanguineous. The first was a birth of twins and female sibling of these twins had congenital anal atresia and died when she was three-month old. Third (30 yr), 4th (28 yr) and 9th (19 yr) alive siblings were obese, blind and diabetic. We detected truncal obesity, postaxial polydactyly, cognitive impairment and hypogonadism. Rod-cone dystrophy was detected in ophthalmic examination. With these typical clinical findings, BBS was diagnosed. There was also a male member of the family which shared the same features of his affected brothers but he had died while he was eight months old. Marked glycosuria was determined and urine density was 1021 g/cm3. There was not any further endocrinological abnormality. Fasting blood glucose levels were changing between 290 and 452 mg/dl and the last glycated hemoglobine levels (A1c) were 9.3%, 11.2% and 12.8%, respectively. Diabetes mellitus and obesity were treated with diet, exercise, multiple daily insulin injections and metformine at the dose of 2000 mg/d.

CONCLUSIONS

Although it is an infrequent condition due to autosomal recessive transmission, consanguineous marriage may increase the risk of emergence of BBS. Genetic counseling is a very important issue in the family of patients with BBS, in order to prevent new cases.

In search of triallelism in Bardet-Biedl syndrome

Bardet-Biedl syndrome (BBS) is a model disease for ciliopathy in humans. The remarkable genetic heterogeneity that characterizes this disease is consistent with accumulating data on the interaction between the proteins encoded by the 14 BBS genes identified to date. Previous reports suggested that such interaction may also extend to instances of oligogenic inheritance in the form of triallelism which defies the long held view of BBS as an autosomal recessive disease. In order to investigate the magnitude of triallelism in BBS, we conducted a comprehensive analysis of all 14 BBS genes as well as the CCDC28B-modifier gene in a cohort of 29 BBS families, most of which are multiplex. Two in trans mutations in a BBS gene were identified in each of these families for a total of 20 mutations including 12 that are novel. In no instance did we observe two mutations in unaffected members of a given family, or observe the presence of a third allele that convincingly acted as a modifier of penetrance and supported the triallelic model of BBS. In addition to presenting a comprehensive genotype/phenotype overview of a large set of BBS mutations, including the occurrence of nonsyndromic retinitis pigmentosa in a family with a novel BBS9 mutation, our study argues in favor of straightforward autosomal recessive BBS in most cases.

Direct role of Bardet-Biedl syndrome proteins in transcriptional regulation

Primary cilia are conserved organelles that play crucial roles as mechano- and chemosensors, as well as transducing signaling cascades. Consequently, ciliary dysfunction results in a broad range of phenotypes: the ciliopathies. Bardet-Biedl syndrome (BBS), a model ciliopathy, is caused by mutations in 16 known genes. However, the biochemical functions of the BBS proteins are not fully understood. Here we show that the BBS7 protein (localized in the centrosomes, basal bodies and cilia) probably has a nuclear role by virtue of the presence of a biologically confirmed nuclear export signal. Consistent with this observation, we show that BBS7 interacts physically with the polycomb group (PcG) member RNF2 and regulate its protein levels, probably through a proteasome-mediated mechanism. In addition, our data supports a similar role for other BBS proteins. Importantly, the interaction with this PcG member is biologically relevant because loss of BBS proteins leads to the aberrant expression of endogenous RNF2 targets in vivo, including several genes that are crucial for development and for cellular and tissue homeostasis. Our data indicate a hitherto unappreciated, direct role for the BBS proteins in transcriptional regulation and potentially expand the mechanistic spectrum that underpins the development of ciliary phenotypes in patients.

BBS proteins interact genetically with the IFT pathway to influence SHH-related phenotypes

There are numerous genes for which loss-of-function mutations do not produce apparent phenotypes even though statistically significant quantitative changes to biological pathways are observed. To evaluate the biological meaning of small effects is challenging. Bardet–Biedl syndrome (BBS) is a heterogeneous autosomal recessive disorder characterized by obesity, retinopathy, polydactyly, renal malformations, learning disabilities and hypogenitalism, as well as secondary phenotypes including diabetes and hypertension. BBS knockout mice recapitulate most human phenotypes including obesity, retinal degeneration and male infertility. However, BBS knockout mice do not develop polydacyly. Here we showed that the loss of BBS genes in mice result in accumulation of Smoothened and Patched 1 in cilia and have a decreased Shh response. Knockout of Bbs7 combined with a hypomorphic Ift88 allele (orpk as a model for Shh dysfuction) results in embryonic lethality with e12.5 embryos having exencephaly, pericardial edema, cleft palate and abnormal limb development, phenotypes not observed in Bbs7^−/− mice. Our results indicate that BBS genes modulate Shh pathway activity and interact genetically with the intraflagellar transport (IFT) pathway to play a role in mammalian development. This study illustrates an effective approach to appreciate the biological significance of a small effect.

Phenotypic variability of Bardet-Biedl syndrome: focusing on the kidney

Bardet-Biedl syndrome (BBS) is a multisystemic developmental disorder diagnosed on the basis of the presence of obesity, retinal defects, polydactyly, hypogonadism, renal dysfunction, and learning disabilities. The syndrome is genetically heterogeneous with 14 BBS genes identified to date. Since the cloning of the first gene in 2000, a combination of genetic, in vitro, and in vivo studies have highlighted ciliary dysfunction as a primary cause of BBS pathology. Pleiotropy of ciliopathy phenotypes and complex genetic interactions between causal and modifying alleles of ciliary genes contribute to phenotypic variability. In particular, kidney disease in BBS is clinically heterogeneous, but is now recognized as a cardinal feature and a major cause of mortality in BBS.

Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model reveals common BBS-associated phenotypes and Bbs3 unique phenotypes

Bardet-Biedl syndrome (BBS) is a heterogeneous disorder characterized by obesity, retinopathy, polydactyly, and congenital anomalies. The incidence of hypertension and diabetes are also increased in BBS patients. Mutation of 16 genes independently causes BBS, and seven BBS proteins form the BBSome that promotes ciliary membrane elongation. BBS3 (ARL6), an ADP ribosylation factor-like small GTPase, is not part of the BBSome complex. The in vivo function of BBS3 is largely unknown. Here we developed a Bbs3 knockout model and demonstrate that Bbs3(-/-) mice develop BBS-associated phenotypes, including retinal degeneration, male infertility, and increased body fat. Interestingly, Bbs3(-/-) mice develop some unique phenotypes not seen in other BBS knockout models: no overt obesity, severe hydrocephalus, and elevated blood pressure (shared by some but not all BBS gene knockout mice). We found that endogenous BBS3 and the BBSome physically interact and depend on each other for their ciliary localization. This finding explains the phenotypic similarity between Bbs3(-/-) mice and BBSome subunit knockout mice. Loss of Bbs3 does not affect BBSome formation but disrupts normal localization of melanin concentrating hormone receptor 1 to ciliary membranes and affects retrograde transport of Smoothened inside cilia. We also show that the endogenous BBSome and BBS3 associate with membranes and the membrane association of the BBSome and BBS3 are not interdependent. Differences between BBS mouse models suggest nonoverlapping functions to individual BBS protein.

Differentiating Alström from Bardet-Biedl syndrome (BBS) using systematic ciliopathy genes sequencing

INTRODUCTION

Early onset retinal degeneration associated with obesity can present a diagnostic challenge in paediatric ophthalmology practice. Clinical overlap between Bardet-Biedl syndrome (BBS) and Alström syndrome has been described, although the two entities are genetically distinct. To date, 16 genes are known to be associated with BBS (BBS1-16) and only one gene has been identified for Alström syndrome (ALMS1).

MATERIALS AND METHODS

In collaboration with the French National Center for Sequencing (CNS, Evry), all coding exons and flanking introns were sequenced for 27 ciliopathy genes (BBS1-12, MGC1203, TTC21b, AHI1, NPHP2-8 (NPHP6=BBS14), MKS1(BBS13), MKS3, C2ORF86, SDCCAG8, ALMS1) in 96 patients referred with a clinical diagnosis of BBS. ALMS1 gene analysis included sequencing of all coding exons.

RESULTS

BBS known gene mutations were found in 44 patients (36 with two mutations and 8 heterozygous). ALMS1 mutations were found in four cases. The rate of ALMS1 mutations among patients suspected of having BBS was 4.2%.

DISCUSSION

Clinically, all four patients presented early-onset severe retinal degeneration with congenital nystagmus associated with obesity. The difficult early differential diagnosis between the two syndromes is outlined. One mutation had already been reported (c.11310delAGAG/p.R3770fsX) and three were novel (c.2293C > T/p.Q765X, c.6823insA/p.R2275fsX, c.9046delA/p.N3016fsX).

CONCLUSIONS

Ciliopathy genes sequencing can be very helpful in providing a timely diagnosis in this group of patients, hence appropriate genetic counselling for families and adequate medical follow-up for affected children.

Mutations in SDCCAG8/NPHP10 Cause Bardet-Biedl Syndrome and Are Associated with Penetrant Renal Disease and Absent Polydactyly

The ciliopathies are an expanding group of disorders caused by mutations in genes implicated in the biogenesis and function of primary cilia. Bardet-Biedl syndrome (BBS) is a model ciliopathy characterized by progressive retinal degeneration, obesity, polydactyly, cognitive impairment, kidney anomalies and hypogonadism. Mutations in SDCCAG8(NPHP10) were described recently in patients with nephronophthisis and retinal degeneration (Senior-Loken syndrome; SLS). Given the phenotypic and genetic overlap between known ciliopathy genes, we hypothesized that mutations in SDCCAG8 might also contribute alleles to more severe, multisystemic ciliopathies. We performed genetic and phenotypic analyses of 2 independent BBS cohorts. Subsequent to mutation screening, we made a detailed phenotypic analysis of 5 families mutated for SDCCAG8 (3 homozygous and 2 compound heterozygous mutations) and conducted statistical analyses across both cohorts to examine possible phenotype-genotype correlations with mutations at this locus. All patients with mutations in SDCCAG8 fulfilled the diagnostic criteria for BBS (retinal degeneration, obesity, cognitive defects, renal failure, hypogonadism). Interestingly, none of the patients with primary SDCCAG8 mutations had polydactyly, a frequent but not obligatory BBS feature. In contrast, the same patients displayed early-onset renal failure, obesity, as well as recurrent pulmonary and ENT infections. Comparison of the phenotypes of these families with our entire BBS cohort indicated that renal impairment and absent polydactyly correlated significantly with causal SDCCAG8 mutations. Thus, SDCCAG8 mutations are sufficient to cause BBS in 1-2% of our combined cohorts, and define this gene as the sixteenth BBS locus (BBS16). The absence of polydactyly and the concomitant, apparently fully penetrant association with early kidney failure represents the first significant genotype-phenotype correlation in BBS that potentially represents an indicator for phenotype-driven priority screening and informs specific patient management.

Potential amelioration of morbidity in patients with chromosomal anomalies: relevance to Bardet-Biedl syndrome

Given the genetic basis of their disease, children with major chromosomal abnormalities including Bardet-Biedl syndrome (BBS) are generally considered to have a guarded prognosis with persistence or progression of disease manifestations. Although various therapeutic interventions are commonly used to control signs and symptoms of illness, parents of BBS children are usually cautioned against hoping for sustained improvement. A case of a 21-month-old girl, diagnosed with BBS, manifesting signs of worsening visual impairment, obesity, irascible and disordered behaviour, as well as developmental delay, is presented. After initial evaluation suggested specific biochemical deficiencies, nutritional status correction was undertaken and the patient's signs and symptoms subsequently resolved over the course of several months. To the authors' knowledge, this is the first case report of sustained resolution of all disease manifestations in the face of previously deteriorating health in a young child with this major chromosomal abnormality. It appears that biochemical imbalances and insufficiencies resulting from abnormal metabolism and excretion are potentially amenable to extraordinary dietary supplementation, with partial or complete resolution of clinical abnormalities. It is recommended that all children with chromosomal abnormalities have biochemical and nutritional status evaluation with correction of disordered biochemistry as is possible.

Brain tissue- and region-specific abnormalities on volumetric MRI scans in 21 patients with Bardet-Biedl syndrome (BBS)

Background

Bardet-Biedl syndrome (BBS) is a heterogeneous human disorder inherited in an autosomal recessive pattern, and characterized by the primary findings of obesity, polydactyly, hypogonadism, and learning and behavioural problems. BBS mouse models have a neuroanatomical phenotype consisting of third and lateral ventriculomegaly, thinning of the cerebral cortex, and reduction in the size of the corpus striatum and hippocampus. These abnormalities raise the question of whether humans with BBS have a characteristic morphologic brain phenotype. Further, although behavioral, developmental, neurological and motor defects have been noted in patients with BBS, to date, there are limited reports of brain findings in BBS. The present study represents the largest systematic evaluation for the presence of structural brain malformations and/or progressive changes, which may contribute to these functional problems.

Methods

A case-control study of 21 patients, most aged 13-35 years, except for 2 patients aged 4 and 8 years, who were diagnosed with BBS by clinical criteria and genetic analysis of known BBS genes, and were evaluated by qualitative and volumetric brain MRI scans. Healthy controls were matched 3:1 by age, sex and race. Statistical analysis was performed using SAS language with SAS STAT procedures.

Results

All 21 patients with BBS were found to have statistically significant region- and tissue-specific patterns of brain abnormalities. There was 1) normal intracranial volume; 2) reduced white matter in all regions of the brain, but most in the occipital region; 3) preserved gray matter volume, with increased cerebral cortex volume in only the occipital lobe; 4) reduced gray matter in the subcortical regions of the brain, including the caudate, putamen and thalamus, but not in the cerebellum; and 5) increased cerebrospinal fluid volume.

Conclusions

There are distinct and characteristic abnormalities in tissue- and region- specific volumes of the brain in patients with BBS, which parallel the findings, described in BBS mutant mouse models. Some of these brain abnormalities may be progressive and associated with the reported neurological and behavioral problems. Further future correlation of these MRI scan findings with detailed neurologic and neuropsychological exams together with genotype data will provide better understanding of the pathophysiology of BBS.

Molecular analysis of Bardet-Biedl syndrome families: report of 21 novel mutations in 10 genes

PURPOSE

Bardet-Biedl syndrome (BBS) is genetically heterogeneous with 15 BBS genes currently identified, accounting for approximately 70% of cases. The aim of our study was to define further the spectrum of BBS mutations in a cohort of 44 European-derived American, 8 Tunisian, 1 Arabic, and 2 Pakistani families (55 families in total) with BBS.

METHODS

A total of 142 exons of the first 12 BBS-causing genes were screened by dideoxy sequencing. Cases in which no mutations were found were then screened for BBS13, BBS14, BBS15, RPGRIP1L, CC2D2A, NPHP3, TMEM67, and INPP5E.

RESULTS

Forty-three mutations, including 8 frameshift mutations, 10 nonsense mutations, 4 splice site mutations, 1 deletion, and 20 potentially or probably pathogenic missense variations, were identified in 46 of the 55 families studied (84%). Of these, 21 (2 frameshift mutations, 4 nonsense mutations, 4 splice site mutations, 1 deletion, and 10 missense variations) were novel. The molecular genetic findings raised the possibility of triallelic inheritance in 7 Caucasian families, 1 Arabian family, and 1 Tunisian patient. No mutations were detected for BBS4, BBS11, BBS13, BBS14, BBS15, RPGRIP1L, CC2D2A, NPHP3, TMEM67, or INPP5E.

CONCLUSIONS

This mutational analysis extends the spectrum of known BBS mutations. Identification of 21 novel mutations highlights the genetic heterogeneity of this disorder. Differences in European and Tunisian patients, including the high frequency of the M390R mutation in Europeans, emphasize the population specificity of BBS mutations with potential diagnostic implications. The existence of some BBS cases without mutations in any currently identified BBS genes suggests further genetic heterogeneity.

Ciliopathies: an expanding disease spectrum

Ciliopathies comprise a group of disorders associated with genetic mutations encoding defective proteins, which result in either abnormal formation or function of cilia. As cilia are a component of almost all vertebrate cells, cilia dysfunction can manifest as a constellation of features that include characteristically, retinal degeneration, renal disease and cerebral anomalies. Additional manifestations include congenital fibrocystic diseases of the liver, diabetes, obesity and skeletal dysplasias. Ciliopathic features have been associated with mutations in over 40 genes to date. However, with over 1,000 polypeptides currently identified within the ciliary proteome, several other disorders associated with this constellation of clinical features will likely be ascribed to mutations in other ciliary genes. The mechanisms underlying many of the disease phenotypes associated with ciliary dysfunction have yet to be fully elucidated. Several elegant studies have crucially demonstrated the dynamic ciliary localisation of components of the Hedgehog and Wnt signalling pathways during signal transduction. Given the critical role of the cilium in transducing "outside-in" signals, it is not surprising therefore, that the disease phenotypes consequent to ciliary dysfunction are a manifestation of aberrant signal transduction. Further investigation is now needed to explore the developmental and physiological roles of aberrant signal transduction in the manifestation of ciliopathy phenotypes. Utilisation of conditional and inducible murine models to delete or overexpress individual ciliary genes in a spatiotemporal and organ/cell-specific manner should help clarify some of the functional roles of ciliary proteins in the manifestation of phenotypic features.

Cilia in the nervous system: linking cilia function and neurodevelopmental disorders

PURPOSE OF REVIEW

Ciliopathies are genetic disorders caused by defects of primary ciliary structure and/or function and are characterized by pleiotropic clinical features. The ciliopathies include several partially overlapping syndromes such as Joubert syndrome, Bardet-Biedl syndrome and Meckel-Gruber syndrome, all of which have pronounced neurodevelopmental features. Here we focus on potential roles of cilia in central nervous system function, to explore how impairments may cause brain malformation and neurodevelopmental disease.

RECENT FINDINGS

Cilia have long been considered as 'sensory cellular antennae', responding as chemo-sensors, mechano-sensors and thermo-sensors, although their roles in development were not well understood until recently. The surprising finding that disparate syndromes are all due to defects of the primary cilia, along with the recent advances in genetics, has helped elucidate further roles of primary cilia beyond sensory functions. Several molecules that are associated with key signaling pathways have been discovered in primary cilia. These include sonic hedgehog, wingless, planar cell polarity and fibroblast growth factor, which are essential for many cellular processes. Additionally, mutations in 'ciliome' genes have largely shown developmental defects such as abnormal body axis and brain malformation, implying disrupted cilia-related signaling pathways. Accordingly, the emerging theme is that primary cilia may play roles as modulators of signal transduction to help shape cellular responses within the environmental context during both development and homeostasis.

SUMMARY

The link between cilia and signal pathways has become a framework for understanding the pathogenesis of ciliopathies. Despite recent progress in ciliary biology, fundamental questions remain about how cilia regulate neuronal function in the central nervous system. Therefore, investigation of ciliary function in the nervous system may reveal cilia-modulating mechanisms in neurodevelopmental processes, as well as suggest new treatments for disease.

Patients with Bardet-Biedl syndrome have hyperleptinemia suggestive of leptin resistance

OBJECTIVE

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous disorder of the primary cilium associated with obesity. In BBS mouse models, ciliary dysfunction leads to impaired leptin signaling and hyperleptinemia before obesity onset. To study the pathophysiology of obesity in BBS, we compared patients with BBS and body mass index Z-score (BMI-Z)-matched controls.

DESIGN AND METHODS

Fifty patients with BBS were matched 2:1 by age, sex, race, and BMI-Z with 100 controls. Patients with BBS and controls were compared for differences in body composition (dual-energy x-ray absorptiometry, abdominal magnetic resonance imaging), blood pressure Z-score (BP-Z; standardized for age, sex, and height), and fasting concentrations of leptin, lipids, insulin, and glucose. Patients with BBS were also compared by genotype.

RESULTS

Leptin, triglycerides, intraabdominal fat mass, and diastolic BP-Z were significantly greater in patients with BBS than in the controls. BBS1 (27%) and BBS10 (30%) mutations were the most prevalent. Patients with BBS10 mutations had significantly higher BMI-Z, greater visceral adiposity, and greater insulin resistance than those with BBS1 mutations.

CONCLUSIONS

Patients with BBS had higher leptin than expected for their degree of adiposity, consistent with the notion that ciliopathy-induced leptin signaling dysfunction is associated with leptin resistance. The preferential deposition of fat intraabdominally in patients with BBS may indicate a predisposition for metabolic complications, including hypertension and hypertriglyceridemia. The observation of disparate results in the BBS10 vs. BBS1 mutation groups is the first demonstration of physiological differences among patients with BBS caused by mutations in distinct genes. These results suggest that the obesity of BBS is distinct from nonsyndromic obesity.

Functional analysis of BBS3 A89V that results in non-syndromic retinal degeneration

Bardet–Biedl syndrome (BBS) is a syndromic form of retinal degeneration. Recently, homozygosity mapping with a consanguineous family with isolated retinitis pigmentosa identified a missense mutation in BBS3, a known BBS gene. The mutation in BBS3 encodes a single amino acid change at position 89 from alanine to valine. Since this amino acid is conserved in a wide range of vertebrates, we utilized the zebrafish model system to functionally characterize the BBS3 A89V mutation. Knockdown of bbs3 in zebrafish alters intracellular transport, a phenotype observed with knockdown of all BBS genes in the zebrafish, as well as visual impairment. Here, we find that BBS3 A89V is sufficient to rescue the transport delays induced by the loss of bbs3, indicating that this mutation does not affect the function of BBS3 as it relates to syndromic disease. BBS3L A89V, however, was unable to rescue vision impairment, highlighting a role for a specific amino acid within BBS3 that is necessary for visual function, but dispensable in other cell types. These data aid in our understanding of why patients with the BBS3 A89V missense mutation only present with isolated retinitis pigmentosa.

Bardet-Biedl syndrome highlights the major role of the primary cilium in efficient water reabsorption

Studies of the primary cilium, now known to be present in all cells, have undergone a revolution, in part, because mutation of many of its proteins causes a large number of diseases, including cystic kidney disease. Bardet-Biedl syndrome (BBS) is an inherited ciliopathy characterized, among other dysfunctions, by renal defects for which the precise role of the cilia in kidney function remains unclear. We studied a cohort of patients with BBS where we found that these patients had a urinary concentration defect even when kidney function was near normal and in the absence of major cyst formation. Subsequent in vitro analysis showed that renal cells in which a BBS gene was knocked down were unciliated, but did not exhibit cell cycle defects. As the vasopressin receptor 2 is located in the primary cilium, we studied BBS-derived unciliated renal epithelial cells and found that they were unable to respond to luminal arginine vasopressin treatment and activate their luminal aquaporin 2. The ability to reabsorb water was restored by treating these unciliated renal epithelial cells with forskolin, a receptor-independent adenylate cyclase activator, showing that the intracellular machinery for water absorption was present but not activated. These findings suggest that the luminal receptor located on the primary cilium may be important for efficient transepithelial water absorption.

Mutation analysis in Bardet-Biedl syndrome by DNA pooling and massively parallel resequencing in 105 individuals

Bardet-Biedl syndrome (BBS) is a rare, primarily autosomal-recessive ciliopathy. The phenotype of this pleiotropic disease includes retinitis pigmentosa, postaxial polydactyly, truncal obesity, learning disabilities, hypogonadism and renal anomalies, among others. To date, mutations in 15 genes (BBS1-BBS14, SDCCAG8) have been described to cause BBS. The broad genetic locus heterogeneity renders mutation screening time-consuming and expensive. We applied a strategy of DNA pooling and subsequent massively parallel resequencing (MPR) to screen individuals affected with BBS from 105 families for mutations in 12 known BBS genes. DNA was pooled in 5 pools of 21 individuals each. All 132 coding exons of BBS1-BBS12 were amplified by conventional PCR. Subsequent MPR was performed on an Illumina Genome Analyzer II™ platform. Following mutation identification, the mutation carrier was assigned by CEL I endonuclease heteroduplex screening and confirmed by Sanger sequencing. In 29 out of 105 individuals (28%), both mutated alleles were identified in 10 different BBS genes. A total of 35 different disease-causing mutations were confirmed, of which 18 mutations were novel. In 12 additional families, a total of 12 different single heterozygous changes of uncertain pathogenicity were found. Thus, DNA pooling combined with MPR offers a valuable strategy for mutation analysis of large patient cohorts, especially in genetically heterogeneous diseases such as BBS.

Familial Mediterranean fever in a large Lebanese family: multiple MEFV mutations and evidence for a Founder effect of the p.[M694I] mutation

Familial Mediterranean fever (FMF) is an autoinflammatory autosomal recessive disease characterized by recurrent fever crises and serous inflammation. The MEFV gene responsible for the disease was identified on chromosome 16, and 5 of the mutations discovered so far in the gene are most frequently encountered in FMF patients: p.[M694V], p.[V726A], p.[M680I] and p.[M694I] in exon 10, and p.[E148Q] in exon 2. The present work describes multiple MEFV mutations and the corresponding haplotypes for 31 FMF patients as well as 32 "healthy" individuals of a large consanguineous Lebanese family. The DNAs were screened for MEFV mutations, and determination of the corresponding haplotypes was performed for all individuals by genotyping 4 microsatellites surrounding the gene. Five different mutations were detected in this one family, which is unexpected in such a genetic isolate. A phenotypic variability was also observed. The haplotype carrying the p.[M694I] allele, detected in all the family branches, was well conserved and therefore seems to be the ancestral one.

Recurrence risks for Bardet-Biedl syndrome: Implications of locus heterogeneity

PURPOSE

Bardet-Biedl syndrome is a pleiotropic multiple anomaly syndrome inherited in an autosomal recessive pattern. It is now known that this disorder has locus heterogeneity, with causative mutations identified in as many as 14 genes. The aim of this study was to derive locus-specific recurrence risk estimates for family members of a proband affected with Bardet-Biedl syndrome.

METHODS

Mutation data from 187 probands affected with Bardet-Biedl syndrome were used. The authors counted the relative proportion of families with mutations at each of 10 loci and estimated locus-specific carrier rates for mutations using Hardy-Weinberg principles and an aggregate population frequency of 1/100,000 for the phenotype. Locus-specific recurrence risks were calculated for relatives of an affected proband.

RESULTS

Locus-specific carrier frequencies range from 1/250 to 1/2200, and the risks for an offspring of the sibling of an affected individual range from 1/1,500 to 1/13,000. The estimate of this risk derived under a locus homogeneity model is 1/960.

CONCLUSION

Variation of recurrence risks of this magnitude may have implications for genetic counseling of families with affected individuals, in particular about prenatal testing and other reproductive options. Similar analyses to determine locus-specific carrier frequencies for other phenotypes with significant locus heterogeneity may yield similarly relevant results.

Pentapeptide sharing between Corynebacterium diphtheria toxin and the human neural protein network

We describe the pentapeptides shared between the Corynebacterium diphtheria toxin and the human proteins associated with fundamental neural functions. We report that diphtheria toxin pentapeptides are spread among human antigens such as tuberous sclerosis proteins 1 and 2, reelin, contactin-4, neuroligins, semaphorin-5A, sodium channel protein type 1 subunit α, Williams-Beuren syndrome chromosomal region 1 protein, Williams-Beuren syndrome chromosomal region 20A protein. Williams-Beuren syndrome chromosomal region 8 protein, Bardet-Biedl syndrome 9 protein, Bardet-Biedl syndrome 10 protein, oligodendrocyte-myelin glycoprotein, neurofibromin-2, and periaxin. The data are discussed in relation to the bacterial immune escape phenomenon, and in the context of potential cross-reactions in diagnostic tests and immune therapies.

Disruption of the chaperonin containing TCP-1 function affects protein networks essential for rod outer segment morphogenesis and survival

Type II Chaperonin Containing TCP-1 (CCT, also known as TCP-1 Ring Complex, TRiC) is a multi-subunit molecular machine thought to assist in the folding of ∼ 10% of newly translated cytosolic proteins in eukaryotes. A number of proteins folded by CCT have been identified in yeast and cultured mammalian cells, however, the function of this chaperonin in vivo has never been addressed. Here we demonstrate that suppressing the CCT activity in mouse photoreceptors by transgenic expression of a dominant-negative mutant of the CCT cofactor, phosducin-like protein (PhLP), results in the malformation of the outer segment, a cellular compartment responsible for light detection, and triggers rapid retinal degeneration. Investigation of the underlying causes by quantitative proteomics identified distinct protein networks, encompassing ∼ 200 proteins, which were significantly affected by the chaperonin deficiency. Notably among those were several essential proteins crucially engaged in structural support and visual signaling of the outer segment such as peripherin 2, Rom1, rhodopsin, transducin, and PDE6. These data for the first time demonstrate that normal CCT function is ultimately required for the morphogenesis and survival of sensory neurons of the retina, and suggest the chaperonin CCT deficiency as a potential, yet unexplored, cause of neurodegenerative diseases.

Molecular diagnosis of known recessive ataxias by homozygosity mapping with SNP arrays

The diagnosis of rare inherited diseases is becoming more and more complex as an increasing number of clinical conditions appear to be genetically heterogeneous. Multigenic inheritance also applies to the autosomal recessive progressive cerebellar ataxias (ARCAs), for which 14 genes have been identified and more are expected to be discovered. We used homozygosity mapping as a guide for identification of the defective locus in patients with ARCA born from consanguineous parents. Patients from 97 families were analyzed with GeneChip Mapping 10K or 50K SNP Affymetrix microarrays. We identified six families homozygous for regions containing the autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) gene, two families homozygous for the ataxia-telangiectasia gene (ATM), two families homozygous for the ataxia with oculomotor apraxia type 1 (AOA1) gene, and one family homozygous for the AOA type 2 (AOA2) gene. Upon direct gene testing, we were able to identify a disease-related mutation in all families but one of the two kindred homozygous at the ATM locus. Although linkage analyses pointed to a single locus on chromosome 11q22.1-q23.1 for this family, clinical features, normal levels of serum alpha-foetoprotein as well as absence of mutations in the ATM gene rather suggest the existence of an additional ARCA-related gene in that interval. While the use of homozygosity mapping was very effective at pointing to the correct gene, it also suggests that the majority of patients harbor mutations either in the genes of the rare forms of ARCA or in genes yet to be identified.

Functional analyses of variants reveal a significant role for dominant negative and common alleles in oligogenic Bardet-Biedl syndrome

Technological advances hold the promise of rapidly catalyzing the discovery of pathogenic variants for genetic disease. However, this possibility is tempered by limitations in interpreting the functional consequences of genetic variation at candidate loci. Here, we present a systematic approach, grounded on physiologically relevant assays, to evaluate the mutational content (125 alleles) of the 14 genes associated with Bardet-Biedl syndrome (BBS). A combination of in vivo assays with subsequent in vitro validation suggests that a significant fraction of BBS-associated mutations have a dominant-negative mode of action. Moreover, we find that a subset of common alleles, previously considered to be benign, are, in fact, detrimental to protein function and can interact with strong rare alleles to modulate disease presentation. These data represent a comprehensive evaluation of genetic load in a multilocus disease. Importantly, superimposition of these results to human genetics data suggests a previously underappreciated complexity in disease architecture that might be shared among diverse clinical phenotypes.

The primary cilium: a signalling centre during vertebrate development

The primary cilium has recently stepped into the spotlight, as a flood of data show that this organelle has crucial roles in vertebrate development and human genetic diseases. Cilia are required for the response to developmental signals, and evidence is accumulating that the primary cilium is specialized for hedgehog signal transduction. The formation of cilia, in turn, is regulated by other signalling pathways, possibly including the planar cell polarity pathway. The cilium therefore represents a nexus for signalling pathways during development. The connections between cilia and developmental signalling have begun to clarify the basis of human diseases associated with ciliary dysfunction.

Genetic kidney diseases

Knowledge of the primary cause of a disease is essential for elucidation of its mechanisms, and for adequate classification, prognosis, and treatment. Recently, the causes of many kidney diseases have been shown to be single-gene defects-eg, steroid-resistant nephrotic syndrome, which is caused by podocin mutations in about 25% of children and nearly 15% of adults with the disease. Knowledge of a disease-causing mutation in a single-gene disorder represents one of the most robust diagnostic examples of personalised medicine because the mutation conveys an almost 100% risk of developing the disease by a defined age. Whereas single-gene diseases are rare disorders, polygenic risk alleles arise in common adult-onset diseases. In this Review, I will discuss prominent renal single-gene kidney disorders, and polygenic risk alleles of common disorders. I delineate how emerging techniques of total exome capture and large-scale sequencing will assist molecular genetic diagnosis, prognosis, and specific treatment, and lead to an improved elucidation of disease mechanisms, thus enabling development of new targeted drugs.

Identification of 11 novel mutations in eight BBS genes by high-resolution homozygosity mapping

BACKGROUND

Bardet-Biedl syndrome (BBS) is primarily an autosomal recessive disorder characterised by the five cardinal features retinitis pigmentosa, postaxial polydactyly, mental retardation, obesity and hypogenitalism. In addition, renal cysts and other anomalies of the kidney and urinary tract can be present. To date, mutations in 12 BBS genes as well as in MKS1 and CEP290 have been identified as causing BBS. The vast genetic heterogeneity of BBS renders molecular genetic diagnosis difficult in terms of the time and cost required to screen all 204 coding exons.

METHOD

Here, the use of genome-wide homozygosity mapping as a tool to identify homozygous segments at known BBS loci, in BBS individuals from inbred and outbred background, is reported.

RESULTS

In a worldwide cohort of 45 families, causative homozygous mutations in 20 families were identified via direct exon sequencing. Eleven of these mutations were novel, thereby increasing the number of known BBS mutations by 5% (11/218).

CONCLUSIONS

Thus, in the presence of extreme genetic locus heterogeneity, homozygosity mapping provides a valuable approach to the molecular genetic diagnosis of BBS and will facilitate the discovery of novel pathogenic mutations.

The molecular basis of human retinal and vitreoretinal diseases

During the last two to three decades, a large body of work has revealed the molecular basis of many human disorders, including retinal and vitreoretinal degenerations and dysfunctions. Although belonging to the group of orphan diseases, they affect probably more than two million people worldwide. Most excitingly, treatment of a particular form of congenital retinal degeneration is now possible. A major advantage for treatment is the unique structure and accessibility of the eye and its different components, including the vitreous and retina. Knowledge of the many different eye diseases affecting retinal structure and function (night and colour blindness, retinitis pigmentosa, cone and cone rod dystrophies, photoreceptor dysfunctions, as well as vitreoretinal traits) is critical for future therapeutic development. We have attempted to present a comprehensive picture of these disorders, including biological, clinical, genetic and molecular information. The structural organization of the review leads the reader through non-syndromic and syndromic forms of (i) rod dominated diseases, (ii) cone dominated diseases, (iii) generalized retinal degenerations and (iv) vitreoretinal disorders, caused by mutations in more than 165 genes. Clinical variability and genetic heterogeneity have an important impact on genetic testing and counselling of affected families. As phenotypes do not always correlate with the respective genotypes, it is of utmost importance that clinicians, geneticists, counsellors, diagnostic laboratories and basic researchers understand the relationships between phenotypic manifestations and specific genes, as well as mutations and pathophysiologic mechanisms. We discuss future perspectives.

Identification and functional analysis of the vision-specific BBS3 (ARL6) long isoform

Bardet-Biedl Syndrome (BBS) is a heterogeneous syndromic form of retinal degeneration. We have identified a novel transcript of a known BBS gene, BBS3 (ARL6), which includes an additional exon. This transcript, BBS3L, is evolutionally conserved and is expressed predominantly in the eye, suggesting a specialized role in vision. Using antisense oligonucleotide knockdown in zebrafish, we previously demonstrated that bbs3 knockdown results in the cardinal features of BBS in zebrafish, including defects to the ciliated Kupffer's Vesicle and delayed retrograde melanosome transport. Unlike bbs3, knockdown of bbs3L does not result in Kupffer's Vesicle or melanosome transport defects, rather its knockdown leads to impaired visual function and mislocalization of the photopigment green cone opsin. Moreover, BBS3L RNA, but not BBS3 RNA, is sufficient to rescue both the vision defect as well as green opsin localization in the zebrafish retina. In order to demonstrate a role for Bbs3L function in the mammalian eye, we generated a Bbs3L-null mouse that presents with disruption of the normal photoreceptor architecture. Bbs3L-null mice lack key features of previously published Bbs-null mice, including obesity. These data demonstrate that the BBS3L transcript is required for proper retinal function and organization.

Retinitis pigmentosa (RP), a disorder of retinal degeneration resulting in blindness, occurs due to mutations in dozens of different genes encoding proteins with highly diverse functions. To date, there are no effective therapies to delay or arrest retinal degeneration. RP places a large burden on affected families and on society as a whole. We have studied a syndromic form of RP known as Bardet-Biedl Syndrome (BBS), which leads to degeneration of the photoreceptor cells and is associated with non-vision abnormalities including obesity, hypertension, diabetes, and congenital abnormalities of the kidney, heart, and limbs. In this study we utilized two model systems, the zebrafish and mouse, to evaluate the function of a specific form of BBS (BBS3). We have identified a novel protein product of the BBS3 gene and demonstrated that functional and structural abnormalities of the eye occur when this form of BBS3 is absent. This finding is of significance because it indicates that BBS3 mutations can lead to non-syndromic blindness, as well as blindness associated with other clinical features. This work also indicates that treatment of BBS3 blindness will require replacement of a specific form of the BBS3 gene.

Functional modules, mutational load and human genetic disease

The ability to generate a massive amount of sequencing and genotyping data is transforming the study of human genetic disorders. Driven by such innovation, it is likely that whole exome and whole-genome resequencing will replace regionally focused approaches for gene discovery and clinical testing in the next few years. However, this opportunity brings a significant interpretative challenge to assigning function and phenotypic variance to common and rare alleles. Understanding the effect of individual mutations in the context of the remaining genomic variation represents a major challenge to our interpretation of disease. Here, we discuss the challenges of assigning mutation functionality and, drawing from the examples of ciliopathies as well as cohesinopathies and channelopathies, discuss possibilities for the functional modularization of the human genome. Functional modularization in addition to the development of physiologically relevant assays to test allele functionality will accelerate our understanding of disease architecture and enable the use of genome-wide sequence data for disease diagnosis and phenotypic prediction in individuals.

Chaperonin genes on the rise: new divergent classes and intense duplication in human and other vertebrate genomes

Background

Chaperonin proteins are well known for the critical role they play in protein folding and in disease. However, the recent identification of three diverged chaperonin paralogs associated with the human Bardet-Biedl and McKusick-Kaufman Syndromes (BBS and MKKS, respectively) indicates that the eukaryotic chaperonin-gene family is larger and more differentiated than previously thought. The availability of complete genome sequences makes possible a definitive characterization of the complete set of chaperonin sequences in human and other species.

Results

We identified fifty-four chaperonin-like sequences in the human genome and similar numbers in the genomes of the model organisms mouse and rat. In mammal genomes we identified, besides the well-known CCT chaperonin genes and the three genes associated with the MKKS and BBS pathological conditions, a newly-defined class of chaperonin genes named CCT8L, represented in human by the two sequences CCT8L1 and CCT8L2. Comparative analyses from several vertebrate genomes established the monophyletic origin of chaperonin-like MKKS and BBS genes from the CCT8 lineage. The CCT8L gene originated from a later duplication also in the CCT8 lineage at the onset of mammal evolution and duplicated in primate genomes. The functionality of CCT8L genes in different species was confirmed by evolutionary analyses and in human by expression data. Detailed sequence analysis and structural predictions of MKKS, BBS and CCT8L proteins strongly suggested that they conserve a typical chaperonin-like core structure but that they are unlikely to form a CCT-like oligomeric complex. The characterization of many newly-discovered chaperonin pseudogenes uncovered the intense duplication activity of eukaryotic chaperonin genes.

Conclusions

In vertebrates, chaperonin genes, driven by intense duplication processes, have diversified into multiple classes and functionalities that extend beyond their well-known protein-folding role as part of the typical oligomeric chaperonin complex, emphasizing previous observations on the involvement of individual CCT monomers in microtubule elongation. The functional characterization of newly identified chaperonin genes will be a challenge for future experimental analyses.

Identification of 28 novel mutations in the Bardet-Biedl syndrome genes: the burden of private mutations in an extensively heterogeneous disease

Bardet-Biedl syndrome (BBS), an emblematic disease in the rapidly evolving field of ciliopathies, is characterized by pleiotropic clinical features and extensive genetic heterogeneity. To date, 14 BBS genes have been identified, 3 of which have been found mutated only in a single BBS family each (BBS11/TRIM32, BBS13/MKS1 and BBS14/MKS4/NPHP6). Previous reports of systematic mutation detection in large cohorts of BBS families (n > 90) have dealt only with a single gene, or at most small subsets of the known BBS genes. Here we report extensive analysis of a cohort of 174 BBS families for 12/14 genes, leading to the identification of 28 novel mutations. Two pathogenic mutations in a single gene have been found in 117 families, and a single heterozygous mutation in 17 families (of which 8 involve the BBS1 recurrent mutation, M390R). We confirm that BBS1 and BBS10 are the most frequently mutated genes, followed by BBS12. No mutations have been found in BBS11/TRIM32, the identification of which as a BBS gene only relies on a single missense mutation in a single consanguineous family. While a third variant allele has been observed in a few families, they are in most cases missenses of uncertain pathogenicity, contrasting with the type of mutations observed as two alleles in a single gene. We discuss the various strategies for diagnostic mutation detection, including homozygosity mapping and targeted arrays for the detection of previously reported mutations.

The Chlamydomonas reinhardtii BBSome is an IFT cargo required for export of specific signaling proteins from flagella

The Bardet-Biedl syndrome protein complex (BBSome) is a cargo adapter rather than an essential part of the intraflagellar transport (IFT) machinery.

In humans, seven evolutionarily conserved genes that cause the cilia-related disorder Bardet-Biedl syndrome (BBS) encode proteins that form a complex termed the BBSome. The function of the BBSome in the cilium is not well understood. We purified a BBSome-like complex from Chlamydomonas reinhardtii flagella and found that it contains at least BBS1, -4, -5, -7, and -8 and undergoes intraflagellar transport (IFT) in association with a subset of IFT particles. C. reinhardtii insertional mutants defective in BBS1, -4, and -7 assemble motile, full-length flagella but lack the ability to phototax. In the bbs4 mutant, the assembly and transport of IFT particles are unaffected, but the flagella abnormally accumulate several signaling proteins that may disrupt phototaxis. We conclude that the BBSome is carried by IFT but is an adapter rather than an integral component of the IFT machinery. C. reinhardtii BBS4 may be required for the export of signaling proteins from the flagellum via IFT.

Quality control of cytoskeletal proteins and human disease

Actins and tubulins are abundant cytoskeletal proteins that support diverse cellular processes. Owing to the unique properties of these filament-forming proteins, an intricate cellular machinery consisting minimally of the chaperonin CCT, prefoldin, phosducin-like proteins, and tubulin cofactors has evolved to facilitate their biogenesis. More recent evidence also suggests that regulated degradation pathways exist for actin (via TRIM32) and tubulin (via parkin or cofactor E-like). Collectively, these pathways maintain the quality control of cytoskeletal proteins ('proteostasis'), ensuring the appropriate function of microfilaments and microtubules. Here, we focus on the molecular mechanisms of the quality control of actin and tubulin, and discuss emerging links between cytoskeletal proteostasis and human diseases.

A Novel Familial BBS12 Mutation Associated with a Mild Phenotype: Implications for Clinical and Molecular Diagnostic Strategies

Bardet-Biedl syndrome (BBS) is an autosomal recessively inherited ciliopathy mainly characterized by rod-cone dystrophy, postaxial polydactyly, obesity, renal tract anomalies, and hypogonadism. To date, 14 BBS genes, BBS1 to BBS14, have been identified, accounting for over 75% of mutations in BBS families. In this study, we present a consanguineous family from Pakistan with postaxial polydactyly and late-onset retinal dysfunction. Adult affected individuals did not show any renal or genital anomalies, obesity, mental retardation or learning difficulties and did thus not fulfill the proposed clinical diagnostic criteria for BBS. We mapped the disease in this family to the BBS12 locus on chromosome 4q27 and identified the novel homozygous p.S701X nonsense mutation in BBS12 in all three affected individuals of this family. We conclude that BBS12 mutations might cause a very mild phenotype, which is clinically not diagnosed by the current diagnostic criteria for BBS. Consequently, we suggest the use of less strict diagnostic criteria in familial BBS families with mild phenotypic expression.

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.

The nonmotile ciliopathies

Over the last 5 years, disorders of nonmotile cilia have come of age and their study has contributed immeasurably to our understanding of cell biology and human genetics. This review summarizes the main features of the ciliopathies, their underlying genetics, and the functions of the proteins involved. We describe some of the key findings in the field, including new animal models, the role of ciliopathy proteins in signaling pathways and development, and the unusual genetics of these diseases. We also discuss the therapeutic potential for these diseases and finally, discuss important future work that will extend our understanding of this fascinating organelle and its associated pathologies.

Bardet-Biedl Syndrome in an African-American Patient: Should the Diagnostic Criteria Be Expanded to Include Hydrometrocolpos?

Bardet-Biedl Syndrome (BBS) is a multisystemic disorder diagnosed on the basis of a combination of primary and secondary clinical features that include retinal dystrophy, obesity, polydactyly, cognitive dysfunction, and renal malformations. We report a unique case of BBS in a 13-year old girl of African-American descent who presented with retinitis pigmentosa, obesity, polydactyly, learning disabilities, precocious puberty, hypertension, renal cysts, and Hirschprung disease. Further evaluation revealed a history of precocious puberty, which is antithetical to the common manifestations of BBS, while neuroimaging was suggestive of periventricular leukomalacia and neuro-electrophysiologic studies revealed diffuse cerebral disturbance, which may contribute to her neurological abnormalities. The patient was also diagnosed with hydrometrocolpos, a finding typical of McKusick-Kaufman Syndrome (MKKS) but infrequent in other disorders. This observation, together with recent findings in some mouse models of BBS, raises the question of whether hydrometrocolpos should be considered as an additional diagnostic criterion for BBS to be used in females in parallel to the criterion of hypogonadism in males, thereby improving diagnostic sensitivity.

Pleiotropic genetic syndromes with developmental abnormalities associated with obesity

Childhood obesity is a common and complex problem that may persist in adulthood. It may present as a component of genetic syndromes associated with dysmorphic features, developmental abnormalities, mental retardation and/or learning disabilities and often neuroendocrine dysfunction. Although the chromosomal abnormalities of these rare syndromes are already known, the specific genetic and pathophysiological mechanisms leading to the distinct phenotypes and obesity still remain unclarified. New exciting genetic pathways contributing to syndrome phenotype and leading to obesity have recently been identified. Prader-Willi syndrome is caused by loss of expression of the C/D box HBII-84 cluster of snoRNAs. Dysfunction of the primary cilium, thought to have important signalling functions, may contribute to disease phenotype and obesity in Bardet-Biedl, Alstrom and Carpenter syndromes. In this mini-review current knowledge of clinical and genetic characteristics is summarized as well as the pathogenesis of these syndromes with special emphasis on the pathogenesis of obesity.

Patterns of expression of Bardet-Biedl syndrome proteins in the mammalian cochlea suggest noncentrosomal functions

Bardet-Biedl syndrome is a heterogeneous disorder causing a spectrum of symptoms, including visual impairment, kidney disease, and hearing impairment. Evidence suggests that BBS gene mutations cause defective ciliogenesis and/or cilium dysfunction. Cochlear development is affected by BBS gene deletion, and adult Bbs6(-/-) and Bbs4(-/-) mice are hearing impaired. This study addresses BBS protein expression in the rodent cochlea, to gain a better understanding of its function in vivo. As predicted by in vitro studies, Bbs6 immunofluorescence was localized to the basal bodies of supporting cells and sensory hair cells prior to the onset of hearing. In adult tissue, Bbs6 expression persisted in afferent neurons, including within the dendrites that innervate hair cells, implicating Bbs6 in a sensory neuronal function. Bbs2, which interacts with Bbs6, was also localized to hair cell basal bodies and stereociliary bundles. Additionally, Bbs2 was expressed in supporting cells at their intercellular boundaries, in a spatiotemporal pattern mirroring the development of the microtubule network. Bbs4 localized to cilia and developing cytoplasmic microtubule arrays. Pcm-1, a microtubular protein that interacts with Bbs4 in vitro, showed a comparable expression. Depolymerization of microtubules in slice preparations of the living cochlea resulted in Bbs4 and Pcm-1 mislocalization. Pcm-1 was also mislocalized in Bbs4(-/-) mice. This suggests that Bbs4/Pcm-1 interactions may be important in microtubule-dependent cytoplasmic trafficking in vivo. In summary, our findings indicate that BBS proteins adopt a range of cellular distributions in vivo, not restricted to the centrosome or cilium, and so broaden the possible underlying pathomechanisms of the disease.

The dynamic cilium in human diseases

Cilia are specialized organelles protruding from the cell surface of almost all mammalian cells. They consist of a basal body, composed of two centrioles, and a protruding body, named the axoneme. Although the basic structure of all cilia is the same, numerous differences emerge in different cell types, suggesting diverse functions. In recent years many studies have elucidated the function of 9+0 primary cilia. The primary cilium acts as an antenna for the cell, and several important pathways such as Hedgehog, Wnt and planar cell polarity (PCP) are transduced through it. Many studies on animal models have revealed that during embryogenesis the primary cilium has an essential role in defining the correct patterning of the body. Cilia are composed of hundreds of proteins and the impairment or dysfunction of one protein alone can cause complete loss of cilia or the formation of abnormal cilia. Mutations in ciliary proteins cause ciliopathies which can affect many organs at different levels of severity and are characterized by a wide spectrum of phenotypes. Ciliary proteins can be mutated in more than one ciliopathy, suggesting an interaction between proteins. To date, little is known about the role of primary cilia in adult life and it is tempting to speculate about their role in the maintenance of adult organs. The state of the art in primary cilia studies reveals a very intricate role. Analysis of cilia-related pathways and of the different clinical phenotypes of ciliopathies helps to shed light on the function of these sophisticated organelles. The aim of this review is to evaluate the recent advances in cilia function and the molecular mechanisms at the basis of their activity.

Olfactory cilia: linking sensory cilia function and human disease

The olfactory system gives us an awareness of our immediate environment by allowing us to detect airborne stimuli. The components necessary for detection of these odorants are compartmentalized in the cilia of olfactory sensory neurons. Cilia are microtubule-based organelles, which can be found projecting from the surface of almost any mammalian cell, and are critical for proper olfactory function. Mislocalization of ciliary proteins and/or the loss of cilia cause impaired olfactory function, which is now recognized as a clinical manifestation of a broad class of human diseases, termed ciliopathies. Future work investigating the mechanisms of olfactory cilia function will provide us important new information regarding the pathogenesis of human sensory perception diseases.

Autosomal recessive Bardet-Biedl syndrome: first-degree relatives have no predisposition to metabolic and renal disorders

Bardet-Biedl Syndrome (BBS) is an autosomal recessive, multisystem, genetically heterogeneous, ciliopathic condition caused by mutations in multiple genes. Here we sought to determine if inheritance of a single BBS mutation increased the risks of frequent disorders of this syndrome such as obesity, hypertension, and diabetes. Various metabolic and renal diseases in a cohort of 46 patients with BBS, prospectively followed for up to 28 years, were compared to recent assessments of these factors in 96 relatives with a heterozygote mutation (carriers) and 37 relatives without a contributing mutation (non-carriers). Ten mutations in 6 genes causing this syndrome were identified in 21 families from whom DNA was obtained. The body mass index or the incidences of hypertension, diabetes, or stage 3 chronic kidney diseases were found to be similar between carriers and non-carriers but were all significantly less than those of family members with BBS. Similarly, the median age of onset of hypertension or diagnosis of stage 3 kidney disease, or the diagnosis of diabetes by age 70 were all significantly lower in those with BBS than in gene carriers or non-carriers. While our study shows that metabolic and renal events occurred frequently and at an early age in BBS, the heterozygous inheritance of any of the 10 described BBS mutations did not predispose family members to obesity, diabetes, hypertension, or renal impairment.

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.

Transient ciliogenesis involving Bardet-Biedl syndrome proteins is a fundamental characteristic of adipogenic differentiation

Bardet-Biedl syndrome (BBS) is an inherited ciliopathy generally associated with severe obesity, but the underlying mechanism remains hypothetical and is generally proposed to be of neuroendocrine origin. In this study, we show that while the proliferating preadipocytes or mature adipocytes are nonciliated in culture, a typical primary cilium is present in differentiating preadipocytes. This transient cilium carries receptors for Wnt and Hedgehog pathways, linking this organelle to previously described regulatory pathways of adipogenesis. We also show that the BBS10 and BBS12 proteins are located within the basal body of this primary cilium and inhibition of their expression impairs ciliogenesis, activates the glycogen synthase kinase 3 pathway, and induces peroxisome proliferator-activated receptor nuclear accumulation, hence favoring adipogenesis. Moreover, adipocytes derived from BBS-patients' dermal fibroblasts in culture exhibit higher propensity for fat accumulation when compared to controls. This strongly suggests that a peripheral primary dysfunction of adipogenesis participates to the pathogenesis of obesity in BBS.

Guidelines for the nomenclature of the human heat shock proteins

The expanding number of members in the various human heat shock protein (HSP) families and the inconsistencies in their nomenclature have often led to confusion. Here, we propose new guidelines for the nomenclature of the human HSP families, HSPH (HSP110), HSPC (HSP90), HSPA (HSP70), DNAJ (HSP40), and HSPB (small HSP) as well as for the human chaperonin families HSPD/E (HSP60/HSP10) and CCT (TRiC). The nomenclature is based largely on the more consistent nomenclature assigned by the HUGO Gene Nomenclature Committee and used in the National Center of Biotechnology Information Entrez Gene database for the heat shock genes. In addition to this nomenclature, we provide a list of the human Entrez Gene IDs and the corresponding Entrez Gene IDs for the mouse orthologs.