Ocular impairment as the first and only manifestation of Bardet-Biedl syndrome: A case report

Bardet-Biedl syndrome is a ciliopathy mainly associated with retinal dystrophy, renal dysfunction, post-axial polydactyly, obesity, cognitive deficit and hypogonadism. The symptoms associated with retinal dystrophy do not usually appear until the first decade of life, so the diagnosis is usually delayed. Ocular involvement may be the initial form of manifestation of this syndrome, it may even be the only one, so it should be taken into account in the differential diagnosis of amblyopia in a child who does not improve despite correct compliance with treatment. A case of low visual acuity in a pediatric patient is presented as an initial manifestation that leads to the diagnosis of Bardet-Biedl Syndrome, and which is also the only symptom that the patient presents to date, despite being a multisystem disease.

The characterization and comorbidities of heterozygous Bardet-Biedl syndrome carriers

Introduction: Bardet-Biedl syndrome (BBS) is a rare autosomal recessive disorder with clinical features of retinal dystrophy, obesity, postaxial polydactyly, renal anomalies, learning disabilities, hypogonadism, and genitourinary abnormalities. Nevertheless, previous studies on the phenotypic traits of BBS heterozygous carriers have generated inconclusive results. The aim of our study was to investigate the impact of BBS heterozygosity on carriers when compared to non-carriers within the Taiwanese population. Materials and Methods: This study follows a hospital-based case-control design. We employed the Taiwan Biobank version 2 (TWBv2) array to identify three specific loci associated with BBS (rs773862084, rs567573386, and rs199910690). In total, 716 patients were included in the case group, and they were compared to a control group of 2,864 patients who lacked BBS alleles. The control group was selected through gender and age matching at a ratio of 1:4. The association between BBS-related loci and comorbidity was assessed using logistic regression models. Results: We found that BBS heterozygous carriers exhibited a significant association with elevated BMI levels, especially the variant rs199910690 in MKS1 (p=0.0037). The prevalence of comorbidities in the carriers' group was not higher than that in the non-carriers' group. Besides, the average values of the biochemistry data showed no significant differences, except for creatinine level. Furthermore, we conducted a BMI-based analysis to identify specific risk factors for chronic kidney disease (CKD). Our findings revealed that individuals carrying the CA/AA genotype of the BBS2 rs773862084 variant or the CT/TT genotype of the MKS1 rs199910690 variant showed a reduced risk of developing CKD, irrespective of their BMI levels. When stratified by BMI level, obese males with the MKS1 rs199910690 variant and obese females with the BBS2 rs773862084 variant exhibited a negative association with CKD development. Conclusion: We found that aside from the association with overweight and obesity, heterozygous BBS mutations did not appear to increase the predisposition of individuals to comorbidities and metabolic diseases. To gain a more comprehensive understanding of the genetic susceptibility associated with Bardet-Biedl Syndrome (BBS), further research is warranted.

Reduced cone photoreceptor function and subtle systemic manifestations in two siblings with loss of SCLT1

BACKGROUND

The sodium channel and clathrin linker 1 gene (SCLT1) has been involved in the pathogenesis of various ciliopathy disorders such as Bardet-Biedl syndrome, orofaciodigital syndrome type IX, and Senior-Løken syndrome. Detailed exams are warranted to outline all clinical features. Here, we present a family with a milder phenotype of SCLT1-related disease.

MATERIAL AND METHODS

Comprehensive eye examination including fundus images, OCT, color vision, visual fields and electroretinography were performed. Affected individuals were assessed by a pediatrician and a medical geneticist for systemic features of ciliopathy. Investigations included echocardiography, abdominal ultrasonography, blood work-up for diabetes, liver and kidney function. Genetic testing included NGS retinal dystrophy panel, segregation analysis and transcriptome sequencing.

RESULTS

Two male children, age 10 and 8 years, were affected with attention deficit hyperactivity disorder (ADHD), obesity and mild photophobia. The ophthalmic exam revealed reduced best-corrected visual acuity (BCVA), strabismus, hyperopia, astigmatism and moderate red-green defects. Milder changes suggesting photoreceptors disease were found on retinal imaging. Electroretinogram confirmed cone photoreceptors dysfunction. Genetic testing revealed a homozygous likely pathogenic, splice-site variant in SCLT1 gene NM_144643.3: c.1439 + 1del in the proband and in the affected brother. The unaffected parents were heterozygous for the SCLT1 variant. Transcriptome sequencing showed retention of intron 16 in the proband.

CONCLUSIONS

In this report, we highlight the importance of further extensive diagnostics in patients with unexplained reduced vision, strabismus, refractive errors and ADHD spectrum disorders. SCLT1-related retinal degeneration is very rare and isolated reduced function of cone photoreceptors has not previously been observed.

Early development and adaptive functioning in children with Bardet-Biedl syndrome

This study had two aims. Aim one investigated achievement of 10 developmental milestones in children with Bardet-Biedl syndrome (BBS). Aim one data were derived from retrospective responses by caregivers of individuals with BBS who are enrolled in the Clinical Registry Investigating Bardet-Biedl syndrome (CRIBBS). CRIBBS is a natural history registry acquiring serial observations. Aim two investigated early adaptive skills using the Adaptive Behavior Assessment System (ABAS-II 0-5) completed by caregivers of children with BBS aged from 0 to 5. There were 652 individuals with milestone information (with some variability based on availability of information for specific milestones), and 101 individuals (including 95 among the 652) with ABAS-II information. Results revealed wide-ranging delays in adaptive skills, particularly in the domain of Self-Care. Expressive language appears to be the most frequently delayed developmental milestone. We found a difference by BBS genotype wherein individuals with BBS1 had higher adaptive/developmental scores than individuals with BBS10. Age also carried a significant association with adaptive skills diverging farther from a normative trajectory as children with BBS progress through early childhood.

Management of Monogenic and Syndromic Obesity

Similar to the general population, lifestyle interventions focused on nutrition and physical activity form the foundation for treating obesity caused by rare genetic disorders. Additional therapies, including metreleptin and setmelanotide, that target defects within the leptin signaling pathway can effectively synergize with lifestyle efforts to treat monogenic disorders of leptin, leptin receptor, proopiomelanocortin (POMC), and proprotein convertase subtilisin/kexin type 1 (PCSK1) and syndromic conditions, such as the ciliopathies Bardet-Biedl and Alström syndromes, whose pathophysiological mechanisms also converge on the leptin pathway. Investigational treatments for Prader-Willi syndrome target specific defects caused by reduced expression of paternally derived genes within the chromosome 15q region.

Review of the phenotypes and genotypes of Bardet-Biedl syndrome from China

Objective: To analyze the phenotypes, genotypes, and the relationship of phenotypes and genotypes for Chinese patients with Bardet-Biedl syndrome (BBS). Methods: The Chinese Wanfang and Weipu data, and PubMed were searched up to December 2022. Patients with detailed clinical feature data were involved in the analysis. Results: A total of 153 Chinese patients, including 87 males, 53 females, and 12 unknown, were enrolled. Their ages ranged from 1.2 to 44 years old with a mean of 16.70 ± 9.90 years old. Among these patients, 80 (52.29%) were reported by ophthalmologists, and only 24 (15.68%) reported by pediatricians. Most patients (132/137, 96.35%) had visual problems; 131/153 (85.62%) had polydactyly; 124/132 (93.93%) were overweight or obese; 63/114 (55.26%) had renal abnormalities; kidney dysfunction was found in 33 (21.57%); 83/104 (79.81%) had hypogonadism and/or genital hypoplasia; and 111/136 (81.62%) had mental retardation. In this series, genetic analysis was performed in 90 (58.82%) patients, including 22 BBS7 (24.71%), 20 BBS2 (22.73%), and 10 BBS10 (11.24%) patients. Moreover, 11 fetuses were diagnosed prenatally in the last 4 years except for one patient in 2004 year. It was noted that BBS7 had higher penetrance. BBS2 had higher hearing impairment and lower renal abnormality penetrance. BBS10 also had lower renal abnormality penetrance as well. Conclusion: Misdiagnosis or miss diagnosis of BBS may be common in China. In patients with polydactyly, visual impairment, obesity, renal abnormalities, hypogonadism, and mental retardation, or in fetuses with polydactyly and/or renal abnormalities, BBS should be considered in the differential diagnosis. Other deformities should be evaluated carefully and genetic analysis should be performed as early as possible.

Mutation profile of Bardet-Biedl syndrome patients from India: Implicative role of multiallelic rare variants and oligogenic inheritance pattern

Bardet-Biedl syndrome (BBS), a rare primary form of ciliopathy, with heterogeneous clinical and genetic presentation is characterized by rod cone dystrophy, obesity, polydactyly, urogenital abnormalities, and cognitive impairment. Here, we delineate the genetic profile in a cohort of 108 BBS patients from India by targeted gene sequencing-based approach for a panel of ciliopathy (including BBS) and other inherited retinal disease genes. We report here a higher frequency of BBS10 and BBS1 gene variations. A different spectrum of variations including a putatively novel gene TSPOAP1, for BBS was identified. Increased percentage frequency of digenic variants (36%) in the disease cohort, role of modifiers in familial cases are some of the salient observations in this work. This study appends the knowledge of BBS genetics pertaining to patients from India. We observed a different molecular epidemiology of BBS patients in this study cohort compared to other reports, which emphasizes the need for molecular testing in affected patients.

Crohn's Disease in a Patient With Bardet-Biedl Syndrome: Random Anomaly or Rare Phenotypic Trait?

Bardet-Biedl syndrome (BBS) is an autosomal recessive multisystem nonmotile ciliopathy. There are anecdotal reports of the co-occurrence of BBS and autoimmune diseases, including inflammatory bowel disease (IBD). We present the first case report of a child with BBS7 who developed Crohn disease, adding to the evidence on the association between BBS and IBD. A 13-year-old girl with BBS7 presented with abdominal pain and significant weight loss (-13%), but without other classical symptoms of IBD, such as diarrhea and blood loss. Fecal calprotectin was elevated, but on gastroscopy and colonoscopy, no macroscopic abnormalities were found. Ultrasound and MRI revealed an intestinal stenosis which was treated surgically. Histopathological examination confirmed the diagnosis Crohn disease. In conclusion, the reported co-occurrence of BSS and autoimmune diseases and the atypical presentation of IBD in this patient warrant a low threshold to perform diagnostic tests for IBD in patients with BBS and gastrointestinal symptoms.

Caregiver burden in Bardet-Biedl syndrome: findings from the CARE-BBS study

BACKGROUND

Bardet-Biedl syndrome (BBS) is a rare, genetically heterogeneous obesity syndrome associated with hyperphagia. Given the early onset of BBS symptoms in childhood and multifaceted complications, this study aimed to quantify the caregiver burden associated with BBS.

METHODS

A cross-sectional, multi-country survey of caregivers from the United States (US), United Kingdom (UK), Canada, and Germany was designed to quantify the extent of caregiver burden associated with obesity and hyperphagia symptoms (i.e., uncontrollable hunger) among patients with BBS.

RESULTS

A total of 242 caregivers across the four countries met the inclusion criteria and completed the survey. The mean (standard deviation [SD]) age of the caregivers was 41.9 (6.7) years, and the mean (SD) age of individuals with BBS in their care was 12.0 (3.7) years. Hyperphagia contributed to a BBS diagnosis in 230 of 242 individuals (95.0%). On average, caregivers used eight different weight management approaches for those in their care and expressed a strong desire for more effective weight management methods. Based on the Impacts of Hyperphagia: Caregiver version, patients' hyperphagia had a moderate-to-severe impact on caregiver mood (56.6%), sleep (46.6%), and relationships (48.0%). Caregivers reported experiencing a high level of personal strain (mean [SD], 17.1 [2.9]) and family impact (mean [SD] score, 26.0 [3.8]) due to BBS, as measured by the Revised Impact on Family Scale. Among caregivers in the workforce, there also was high impairment in total work productivity (mean [SD], 60.9% [21.4%]) due to caring for patients with BBS according to the Work Productivity and Activity Impairment. More than half (53%) of the caregivers reported spending over 5,000 out-of-pocket in local currency for medical expenses for the patient with BBS in their care.

CONCLUSIONS

Obesity and hyperphagia have negative impacts on the lives of caregivers of patients with BBS. The burden is demonstrated to be multifaceted, with various components that may interact with and confound each other, including intensive weight management efforts, productivity loses, impaired family dynamics and out-of-pocket medical expenses.

Bardet-Biedl syndrome caused by compound heterozygosity in BBS12 gene: a case report of one family with three affected members

Introduction

Bardet-Biedl syndrome (BBS) is a rare genetic syndrome caused by a mutation in one of 26 different genes responsible for normal structure and/or function of primary cilia. The syndrome is characterized by multiorgan involvement with gradual onset of occurrence of clinical signs and symptoms resulting in great phenotypic variability and what is more important, often difficulties with establishing the timely diagnosis.

Case report

We report a case of a one family with three members with BBS caused by a very rare mutation, a compound heterozygosity in BB12 gene. Even though all three patients have the same type of mutation, they express a significant diversity in clinical expression as well as renal impairment.

Conclusion

This is a case report of a rare clinical syndrome caused by a very rare genetic mutation and it emphasizes the importance of genetic analysis in the timely diagnosis of oligosymptomatic patients with BBS, in order to possibly prevent long-term complications.

Modelling renal defects in Bardet-Biedl syndrome patients using human iPS cells

Bardet-Biedl syndrome (BBS) is a ciliopathy with pleiotropic effects on multiple tissues, including the kidney. Here we have compared renal differentiation of iPS cells from healthy and BBS donors. High content image analysis of WT1-expressing kidney progenitors showed that cell proliferation, differentiation and cell shape were similar in healthy, BBS1, BBS2, and BBS10 mutant lines. We then examined three patient lines with BBS10 mutations in a 3D kidney organoid system. The line with the most deleterious mutation, with low BBS10 expression, expressed kidney marker genes but failed to generate 3D organoids. The other two patient lines expressed near normal levels of BBS10 mRNA and generated multiple kidney lineages within organoids when examined at day 20 of organoid differentiation. However, on prolonged culture (day 27) the proximal tubule compartment degenerated. Introducing wild type BBS10 into the most severely affected patient line restored organoid formation, whereas CRISPR-mediated generation of a truncating BBS10 mutation in a healthy line resulted in failure to generate organoids. Our findings provide a basis for further mechanistic studies of the role of BBS10 in the kidney.

Interview-Based Patient- and Caregiver-Reported Experiences of Hunger and Improved Quality of Life with Setmelanotide Treatment in Bardet-Biedl Syndrome

INTRODUCTION

Bardet-Biedl syndrome (BBS) is a rare genetic disease associated with hyperphagia, a pathologic insatiable hunger, due to impaired signaling in the melanocortin-4 receptor (MC4R) pathway. The impact of hyperphagia on the lives of patients with BBS and their families has not been fully characterized.

METHODS

Patients with BBS or their caregivers who participated in clinical trials of the MC4R agonist setmelanotide (NCT03013543 and NCT03746522) were included in this qualitative study. Telephone interviews were conducted using a semistructured interview guide to explore patient experience and caregiver observations of hyperphagia before and during setmelanotide treatment.

RESULTS

Nineteen interviews (8 patients, 11 caregivers) were conducted. The term "hunger" (rather than "hyperphagia") was used in interviews to ensure common terminology. Before setmelanotide treatment, all participants described their (or their child's) hunger as all-consuming, leading to an obsessive focus on food. Nine participants recalled intense, continuous hunger, and most participants (5 patients, 10 caregivers) reported lack of control with eating. Negative impacts on patients' lives included difficulties with concentration, emotional and physical manifestations, and impaired relationships. All participants experienced or observed improvements in hunger and health outcomes during treatment, the most meaningful of which included weight loss and decrease in obsessive focus on food and food-seeking behaviors. All participants reported improvements in either physical and/or emotional well-being and being satisfied with setmelanotide.

CONCLUSIONS

Hyperphagia and resulting food-seeking behaviors have notable negative impacts on quality of life in patients with BBS and caregivers. Setmelanotide improved hyperphagia, reduced body weight and obsessive focus on food, and facilitated improvements in physical and emotional well-being for both patients and caregivers.

TRIAL REGISTRATION

NCT03013543 and NCT03746522.

Novel multi-allelic variants, two BBS2 and one PKD1 variant, of renal ciliopathies: A case report and literature review

BACKGROUND

Bardet-Biedl syndrome (BBS) and autosomal dominant polycystic kidney disease (ADPKD) are renal ciliopathies. BBS has 22 pathogenic genes, and ADPKD is mainly caused by PKD1 and PKD2 variants. Cases with tri-allelic variants of BBS and PKD1 are rare.

CASE PRESENTATION

The proband was an 11-year-old Chinese male with cysts in both kidneys, blurred vision, hyperopia, and short fingers and toes. The patient underwent a kidney transplant due to rapid deterioration of renal failure. During follow-up, a smaller field of vision, a slow increase in height, and a weight gain were observed. In addition, renal function and anemia were improved. High-throughput sequencing analysis showed two heterozygous variants in BBS2 (c.563delT (p.I188Tfs*13) inherited from the father and c.534+1G > t (splicing) from the mother) and one heterozygous variant in PKD1 (c.6223C > T (p.R2075C)) inherited from the mother.

CONCLUSION

This paper reported a ciliopathy patient with multi-allelic variants (two BBS2 variants and one PKD1 variant) that may lead to early symptoms and more rapid progression. An early genetic diagnosis may contribute to predicting disease progression and guiding management and follow-up.

Subretinal gene therapy delays vision loss in a Bardet-Biedl Syndrome type 10 mouse model

Blindness in Bardet-Biedl syndrome (BBS) is caused by dysfunction and loss of photoreceptor cells in the retina. BBS10, mutations of which account for approximately 21% of all BBS cases, encodes a chaperonin protein indispensable for the assembly of the BBSome, a cargo adaptor important for ciliary trafficking. The loss of BBSome function in the eye causes a reduced light sensitivity of photoreceptor cells, photoreceptor ciliary malformation, dysfunctional ciliary trafficking, and photoreceptor cell death. Cone photoreceptors lacking BBS10 have congenitally low electrical function in electroretinography. In this study, we performed gene augmentation therapy by injecting a viral construct subretinally to deliver the coding sequence of the mouse Bbs10 gene to treat retinal degeneration in a BBS10 mouse model. Long-term efficacy was assessed by measuring the electrical functions of the retina over time, imaging of the treated regions to visualize cell survival, conducting visually guided swim assays to measure functional vision, and performing retinal histology. We show that subretinal gene therapy slowed photoreceptor cell death and preserved retinal function in treated eyes. Notably, cone photoreceptors regained their electrical function after gene augmentation. Measurement of functional vision showed that subretinal gene therapy provided a significant benefit in delaying vision loss.

Lethal neonatal respiratory failure due to biallelic variants in BBS1 and monoallelic variant in TTC21B

BACKGROUND

Bardet-Biedl syndrome (BBS) is a rare, autosomal recessive ciliopathy characterized by early onset retinal dystrophy, renal anomalies, postaxial polydactyly, and cognitive impairment with considerable phenotypic heterogeneity. BBS results from biallelic pathogenic variants in over 20 genes that encode key proteins required for the assembly or primary ciliary functions of the BBSome, a heterooctameric protein complex critical for homeostasis of primary cilia. While variants in BBS1 are most frequently identified in affected individuals, the renal and pulmonary phenotypes associated with BBS1 variants are reportedly less severe than those seen in affected individuals with pathogenic variants in the other BBS-associated genes.

CASE-DIAGNOSIS

We report an infant with severe renal dysplasia and lethal pulmonary hypoplasia who was homozygous for the most common BBS1 pathogenic variant (c.1169 T > G; p.M390R) and also carried a predicted pathogenic variant in TTC21B (c.1846C > T; p.R616C), a genetic modifier of disease severity of ciliopathies associated with renal dysplasia and pulmonary hypoplasia.

CONCLUSIONS

This report expands the phenotypic spectrum of BBS with the first infant with lethal neonatal respiratory failure associated with biallelic, pathogenic variants in BBS1 and a monoallelic, predicted pathogenic variant in TTC21B. BBS should be considered among the ciliopathies in the differential diagnosis of neonates with renal dysplasia and severe respiratory failure.

Bardet-Biedl Syndrome: Current Perspectives and Clinical Outlook

The Bardet Biedl syndrome (BBS) is a rare inherited disorder considered a model of non-motile ciliopathy. It is in fact caused by mutations of genes encoding for proteins mainly localized to the base of the cilium. Clinical features of BBS patients are widely shared with patients suffering from other ciliopathies, especially autosomal recessive syndromic disorders; moreover, mutations in cilia-related genes can cause different clinical ciliopathy entities. Besides the best-known clinical features, as retinal degeneration, learning disabilities, polydactyly, obesity and renal defects, several additional clinical signs have been reported in BBS, expanding our understanding of the complexity of its clinical spectrum. The present review aims to describe the current knowledge of BBS i) pathophysiology, ii) clinical manifestations, highlighting both the most common and the less described features, iii) current and future perspective for treatment.

Ciliary signaling proteins are mislocalized in the brains of Bardet-Biedl syndrome 1-null mice

In the brain, primary cilia are found on most, if not all, central neurons. The importance of neuronal cilia is underscored by the fact that human diseases caused by primary cilia dysfunction, which are known as ciliopathies, are associated with neuropathologies, including neuropsychiatric disorders and learning and memory deficits. Neuronal cilia are enriched for certain G protein-coupled receptors and their downstream effectors, suggesting they sense and respond to neuromodulators in the extracellular milieu. GPCR ciliary localization is disrupted in neurons from mouse models of the ciliopathy Bardet-Biedl syndrome, with GPCRs failing to localize to cilia, indicating the Bardet-Biedl syndrome proteins are required for trafficking of G protein-coupled receptors into neuronal cilia. Yet, dopamine receptor 1 accumulates in cilia in the absence of Bardet-Biedl syndrome proteins, suggesting Bardet-Biedl syndrome proteins are required for normal ciliary import and export. To further explore the roles of the Bardet-Biedl syndrome proteins in neuronal cilia, we examined localization of ciliary signaling proteins in a new constitutive Bbs1 knockout mouse model. Interestingly, we find that two additional ciliary G protein-coupled receptors (Gpr161 and Gpr19) abnormally accumulate in cilia on Bardet-Biedl syndrome neurons. In addition, we find that the GPCR signaling protein β-arrestin accumulates in a subset of cilia in the brain, suggesting the presence of additional unidentified ciliary G protein-coupled receptors. These results confirm the importance of the Bardet-Biedl syndrome proteins in establishing ciliary GPCR pathways and indicate that loss of Bbs1 leads to complex changes in the localization of signaling proteins in the brain.

Rab-like small GTPases in the regulation of ciliary Bardet-Biedl syndrome (BBS) complex transport

Primary cilia, microtubule-based hair-like structures protruding from most cells, contain membranes enriched in signaling molecules and function as sensory and regulatory organelles critical for development and tissue homeostasis. Intraflagellar transport (IFT), cilia-specific bidirectional transport, is required for the assembly, maintenance, and function of cilia. BBSome, the coat complex, acts as the adaptor between the IFT complex and membrane proteins and is therefore essential for establishing the specific compartmentalization of signaling molecules in the cilia. Recent findings have revealed that three ciliary Rab-like small GTPases, IFT27, IFT22, and Rabl2, play critical regulatory roles in ciliary BBSome transport. In this review, we provide an overview of these three Rab-like small GTPases and their relationship with BBSome.

Behavioral Phenotyping of Bbs6 and Bbs8 Knockout Mice Reveals Major Alterations in Communication and Anxiety

The primary cilium is an organelle with a central role in cellular signal perception. Mutations in genes that encode cilia-associated proteins result in a collection of human syndromes collectively termed ciliopathies. Of these, the Bardet-Biedl syndrome (BBS) is considered one of the archetypical ciliopathies, as patients exhibit virtually all respective clinical phenotypes, such as pathological changes of the retina or the kidney. However, the behavioral phenotype associated with ciliary dysfunction has received little attention thus far. Here, we extensively characterized the behavior of two rodent models of BBS, Bbs6/Mkks, and Bbs8/Ttc8 knockout mice concerning social behavior, anxiety, and cognitive abilities. While learning tasks remained unaffected due to the genotype, we observed diminished social behavior and altered communication. Additionally, Bbs knockout mice displayed reduced anxiety. This was not due to altered adrenal gland function or corticosterone serum levels. However, hypothalamic expression of Lsamp, the limbic system associated protein, and Adam10, a protease acting on Lsamp, were reduced. This was accompanied by changes in characteristics of adult hypothalamic neurosphere cultures. In conclusion, we provide evidence that behavioral changes in Bbs knockout mice are mainly found in social and anxiety traits and might be based on an altered architecture of the hypothalamus.

Diagnosis and genetic analysis of a case with Bardet-Biedl syndrome caused by compound heterozygous mutations in the BBS12 gene

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive ciliopathy, which is caused by mutations mainly in genes encoding BBSome complex and IFT complex. Here, we reported a 21-year-old female with BBS characterized by three primary features including obesity, retinitis pigmentosa sine pigmento and bilateral renal cysts. She also had some secondary features such as diabetes mellitus, nonalcoholic fatty liver disease, subclinical hypothyroidism and mild conductive hearing damage. Whole exome sequencing revealed two compound heterozygous mutations in exon 2 of the BBS12 gene (c.188delC, p.T63fs and c.1993_1995del, p.665_665del) in this patient. Sanger sequencing showed that her father and mother carried c.188delC (p.T63fs) and c.1993_1995del (p.665_665del) variants, respectively, while her parents were free of BBS-related symptoms. In conclusion, this case reported two novel mutations (c.188delC, p.T63fs and c.1993_1995del, p.665_665del) of the BBS12 gene in a girl presented with BBS, which provides novel genetic resources for studies of the disease. Meanwhile, the BBS case shows the entire development progress from her birth to adulthood, which helps facilitate clinicians' understanding of BBS.

Interactions between C8orf37 and FAM161A, Two Ciliary Proteins Essential for Photoreceptor Survival

Mutations in C8orf37 cause Bardet-Biedl syndrome (BBS), retinitis pigmentosa (RP), and cone-rod dystrophy (CRD), all manifest in photoreceptor degeneration. Little is known about which proteins C8orf37 interacts with to contribute to photoreceptor survival. To determine the proteins that potentially interact with C8orf37, we carried out a yeast two-hybrid (Y2H) screen using C8orf37 as a bait. FAM161A, a microtubule-binding protein localized at the photoreceptor cilium required for photoreceptor survival, was identified as one of the preys. Double immunofluorescence staining and proximity ligation assay (PLA) of marmoset retinal sections showed that C8orf37 was enriched and was co-localized with FAM161A at the ciliary base of photoreceptors. Epitope-tagged C8orf37 and FAM161A, expressed in HEK293 cells, were also found to be co-localized by double immunofluorescence staining and PLA. Furthermore, interaction domain mapping assays identified that the N-terminal region of C8orf37 and amino acid residues 341-517 within the PFAM UPF0564 domain of FAM161A were critical for C8orf37-FAM161A interaction. These data suggest that the two photoreceptor survival proteins, C8orf37 and FAM161A, interact with each other which may contribute to photoreceptor health.

Progressive retinal degeneration of rods and cones in a Bardet-Biedl syndrome type 10 mouse model

Bardet-Biedl syndrome (BBS) is a multi-organ autosomal-recessive disorder caused by mutations in at least 22 different genes. A constant feature is early-onset retinal degeneration leading to blindness. Among the most common forms is BBS type 10 (BBS10), which is caused by mutations in a gene encoding a chaperonin-like protein. To aid in developing treatments, we phenotyped a Bbs10 knockout (Bbs10-/-) mouse model. Analysis by optical coherence tomography (OCT), electroretinography (ERG) and a visually guided swim assay (VGSA) revealed a progressive degeneration (from P19 to 8 months of age) of the outer nuclear layer that is visible by OCT and histology. Cone ERG was absent from at least P30, at which time rod ERG was reduced to 74.4% of control levels; at 8 months, rod ERG was 2.3% of that of controls. VGSA demonstrated loss of functional vision at 9 months. These phenotypes progressed more rapidly than retinal degeneration in the Bbs1M390R/M390R knock-in mouse. This study defines endpoints for preclinical trials that can be utilized to detect a treatment effect in the Bbs10-/- mouse and extrapolated to human clinical trials.

Two novel variants in a Bardet-Biedl syndrome type 5 patient with severe renal phenotype

Bardet-Biedl syndrome type 5 (BBS5) has never been reported in Chinese populations. The aim of this study is to report the first BBS5 case in China, explore the phenotype and genotype correlation. The case was male, Han nationality, born with polydactyly and gained weight after birth, accompanied by polydipsia, polyuria and nocturia. He was found having low vision at the age of 7 years, and having insufficient renal function at the age of 20 years. After hospitalization, he was found to have suffered from atrophy of the whole layer of macular retina, and end stage of kidney disease, presenting with shrinking and cyst-like changes of bilateral kidneys. Whole-exome sequencing was performed among the proband and his parents (Trios), further validated using Sanger sequencing and quantitative polymerase chain reaction. Two novel compound heterozygous variants of BBS5 gene [a missense variant NC_000002.12, NM_152384.3:c.1A>G(p.Met1?)& a large deletion c.(?_-60)_(386+1_387-1)del] were detected. BBS is rare, whereas BBS5 is rarer. Herein, we reported a Chinese BBS5 patient with severe renal phenotype and identified two novel BBS5 variants. This article is protected by copyright. All rights reserved.

Atypical phenotype of a patient with Bardet-Biedl syndrome type 4

Background

Bardet–Biedl syndrome (BBS) is a multisystemic disorder characterized by rod–cone dystrophy, truncal obesity, postaxial polydactyly, cognitive impairment, male hypogonadotropic hypogonadism, complex female genitourinary malformations, and renal abnormalities. There is a large clinical and also genetic heterogeneity in BBS. Here, we report a patient with polydactyly, hyperechogenic kidneys increased in size with normal corticomedullary differentiation, anal imperforation, and malformation of genitals with presence of a genital tubercle with ventral urethral meatus associated with two unfused lateral genital swelling and absent urethral folds, in the context of 46, XY karyotype.

Methods

Karyotype and solo exome sequencing were performed to look for a genetic etiology for the features described in our patient.

Results

We identified a homozygous in‐frame deletion of exons 4 to 6 in the BBS4 gene (NM‐033028 (BBS4‐i001): c.[(157‐?)_(405 +?)del] p.(Ala53‐Trp135del), which is classified as pathogenic variant. This analysis allowed the molecular diagnosis of BBS type 4 in this patient.

Conclusion

Complex genital malformations are only reported in female BBS6 patients yet, and genital abnormalities and anal imperforation are not reported in male BBS4 patients to date. We discuss the possible hypotheses for this phenotype, including the phenotypic overlap between ciliopathies.

Idiopathic intracranial hypertension in a child with Bardet-Biedl syndrome

BACKGROUND

Bardet-Biedl syndrome (BBS) is known to be associated with hydrocephalus, but not with idiopathic intracranial hypertension (IIH). Case presentation: We describe such a case and propose the pathogenesis. We also discuss the challenges of diagnosis, treatment, and monitoring outcomes in this population that is already at high risk of vision loss from retinal dystrophy.

CONCLUSION

IIH can result from a combination of risk factors in conjunction with the underlying dysfunctional cilia in BBS patients. Monitoring disease progression is difficult, and as such IIH may be underdiagnosed or missed. Management must be adjusted to account for BBS patients' impaired metabolic and renal physiology. It is important that clinicians be aware of these challenges in this vulnerable population, and regular monitoring should be done to avoid preventable vision loss.

Bardet-Biedl syndrome: The pleiotropic role of the chaperonin-like BBS6, 10, and 12 proteins

Bardet–Biedl syndrome (BBS) is a rare pleiotropic disorder known as a ciliopathy. Despite significant genetic heterogeneity, BBS1 and BBS10 are responsible for major diagnosis in western countries. It is well established that eight BBS proteins, namely BBS1, 2, 4, 5, 7, 8, 9, and 18, form the BBSome, a multiprotein complex serving as a regulator of ciliary membrane protein composition. Less information is available for BBS6, BBS10, and BBS12, three proteins showing sequence homology with the CCT/TRiC family of group II chaperonins. Even though their chaperonin function is debated, scientific evidence demonstrated that they are required for initial BBSome assembly in vitro. Recent studies suggest that genotype may partially predict clinical outcomes. Indeed, patients carrying truncating mutations in any gene show the most severe phenotype; moreover, mutations in chaperonin‐like BBS proteins correlated with severe kidney impairment. This study is a critical review of the literature on genetics, expression level, cellular localization and function of BBS proteins, focusing primarily on the chaperonin‐like BBS proteins, and aiming to provide some clues to understand the pathomechanisms of disease in this setting.

BBSome: a New Player in Hypertension and Other Cardiovascular Risks

The BBSome is an octameric protein complex involved in Bardet-Biedl syndrome (BBS), a human pleiotropic, autosomal recessive condition. Patients with BBS display various clinical features including obesity, hypertension, and renal abnormalities. Association studies have also linked the BBS genes to hypertension and other cardiovascular risks in the general population. The BBSome was originally associated with the function of cilia, a highly specialized organelle that extend from the cell membrane of most vertebrate cells. However, subsequent studies have implicated the BBSome in the control of a myriad of other cellular processes not related to cilia including cell membrane localization of receptors and gene expression. The development of animal models of BBS such as mouse lines lacking various components of the BBSome and associated proteins has facilitated studying their role in the control of cardiovascular function and deciphering the pathophysiological mechanisms responsible for the cardiovascular aberrations associated with BBS. These studies revealed the importance of the neuronal, renal, vascular, and cardiac BBSome in the regulation of blood pressure, renal function, vascular reactivity, and cardiac development. The BBSome has also emerged as a critical regulator of key systems involved in cardiovascular control including the renin-angiotensin system. Better understanding of the influence of the BBSome on the molecular and physiological processes relevant to cardiovascular health and disease has the potential of identifying novel mechanisms underlying hypertension and other cardiovascular risks.

Next-generation sequencing identified novel truncating mutations in BBS9 causing Bardet Biedl syndrome in two Iranian consanguineous families

OBJECTIVES

Bardet-Biedl syndrome (BBS) is an autosomal recessive pleiotropic ciliopathy, which includes multi-organ clinical manifestations. The known genes involved in the development of the disease account for the causality in about 80% of the examined cases.

MATERIALS & METHODS

We investigated two Iranian unrelated clinically diagnosed BBS patients, using a targeted next-generation sequencing panel consisting of 18 known BBS genes. The detected variants were investigated in the pedigree and studied using in silico tools for their pathogenicity. Patients' phenotypes were also assessed.

RESULTS

Novel homozygous variants were detected in BBS9 gene in each patient, c.2014C>T, p.Gln672Ter and c.673_674insAA, p.Gln225GlnfsX10. The variants were segregated in the corresponding pedigree and were authenticated to obtain enough evidence to be categorized as pathogenic variants.

CONCLUSION

Patients with truncating mutations in the same gene seem to show similar phenotypic features. Detection of novel and family-specific mutations is typically expected in the genetic hereditary diseases in Iran, which can finally lead to prevent the recurrence of the disease in the consanguineous marriages.

Bardet-Biedl syndrome: The longer we miss, the worse is the outcome

BACKGROUND

Bardet-Biedl syndrome (BBS) is characterized by obesity, cognitive abnormalities, rod-cone dystrophy, skeletal abnormalities, and many other secondary features.

CASE PRESENTATION

We describe a 28-year-old man presented with postaxial polydactyly, retinitis pigmentosa, obesity, hypogonadism and learning difficulties. Renal insufficiency in form of acute kidney injury was the presenting feature and this explain the worse outcome. The diagnosis was delayed despite being classic. This delay in diagnosis leads to a lot of complications that worsen the patient's condition.

CONCLUSION

The characteristics of BBS should be noted by doctors because an early diagnosis will result in a better outcome. The case was prone to numerous consequences due to the delay in diagnosis, which could have been avoided if an early diagnosis had been established.

Association of circulating miRNAS in patients with Alstrőm and Bardet-Biedl syndromes with clinical course parameters

BACKGROUND

Patients with the rare syndromic forms of monogenic diabetes: Alström syndrome (ALMS) and Bardet-Biedl syndrome (BBS) have multiple metabolic abnormalities, including early-onset obesity, insulin resistance, lipid disorders and type 2 diabetes mellitus. The aim of this study was to determine if the expression of circulating miRNAs in patients with ALMS and BBS differs from that in healthy and obese individuals and determine if miRNA levels correlate with metabolic tests, BMI-SDS and patient age.

METHODS

We quantified miRNA expression (Qiagen, Germany) in four groups of patients: with ALMS (n=13), with BBS (n=7), patients with obesity (n=19) and controls (n=23). Clinical parameters including lipids profile, serum creatinine, cystatin C, fasting glucose, insulin and C-peptide levels, HbA1c values and insulin resistance (HOMA-IR) were assessed in patients with ALMS and BBS.

RESULTS

We observed multiple up- or downregulated miRNAs in both ALMS and BBS patients compared to obese patients and controls, but only 1 miRNA (miR-301a-3p) differed significantly and in the same direction in ALMS and BBS relative to the other groups. Similarly, 1 miRNA (miR-92b-3p) was dysregulated in the opposite directions in ALMS and BBS patients, but diverged from 2 other groups. We found eight miRNAs (miR-30a-5p, miR-92b-3p, miR-99a-5p, miR-122-5p, miR-192-5p, miR-193a-5p, miR-199a-3p and miR-205-5p) that significantly correlated with at least of the analyzed clinical variables representing an association with the course of the diseases.

CONCLUSIONS

Our results show for the first time that serum miRNAs can be used as available indicators of disease course in patients with ALMS and BBS syndromes.

A case of Bardet-Biedl syndrome caused by a recurrent variant in BBS12: A case report

Bardet-Biedl syndrome (BBS) is a clinically and genetically heterogenous disorder that manifests as a result of primary cilia impairment. Cilia are present on most cell types, thus BBS is a multisystemic condition involving the majority of organ systems. The core features of the syndrome include retinal degeneration, obesity, polydactyly, cognitive impairment, renal anomalies and urogenital malformations. To date, pathogenic variants in 26 genes have been shown to be involved in the molecular basis of this rare ciliopathy. Of these causal loci, BBS12 accounts for ~8% of all cases. In this case report, an individual with BBS caused by a rare recurrent variant in BBS12 (NM_152618.3: c.1063C>T; p.Arg355^*) is described and compared with others with the same DNA variant, placing this finding in the context of the current literature.

Multiallelic Rare Variants in BBS Genes Support an Oligogenic Ciliopathy in a Non-obese Juvenile-Onset Syndromic Diabetic Patient: A Case Report

Juvenile-onset diabetes may occur in the context of a rare syndromic presentation, suggesting a monogenic etiology rather than a common multifactorial diabetes. In the present study, we report the case of a young diabetic Tunisian patient presenting learning problems, speech deficits, short stature, brachydactyly, and a normal weight. Whole exome sequencing analysis revealed five heterozygous genetic variants in BBS1, BBS4, BBS8, MKS1, and CEP290. These genes are involved in the regulation of cilium biogenesis and function. We analyzed variant combinations pathogenicity using the recently developed ORVAL tool, and we hypothesized that cumulative synergetic effects of these variants could explain the syndromic phenotype observed in our patient. Therefore, our investigation suggested a genetic diagnosis of Bardet-Biedl syndrome with an oligogenic inheritance pattern rather than a monogenic diabetes. Although there is no curative therapy for this ciliopathy at the moment, a genetic diagnosis may offer other supportive care options, including the prevention of other possible clinical manifestations of this syndrome, mainly renal abnormalities, obesity, liver fibrosis, and hypertension, as well as the genetic counseling for family members.

Fetal ciliopathies: a retrospective observational single-center study

Purpose

Report on the diagnosis of prenatally suspected multisystem ciliopathies in a single center between 2002 and 2020.

Methods

Retrospective observational single-center study including pregnancies with prenatal ultrasound features of multisystem ciliopathies, such as hyperechogenic kidneys together with polydactyly and/or other skeletal and extraskeletal findings. Cases were compared according to their prenatal findings and outcomes.

Results

36 cases of multisystem ciliopathies were diagnosed. Meckel-Gruber syndrome (MKS) was the most common ciliopathy (n = 19/36, 52.8%), followed by disorders that belong to the group of short-rib thoracic dysplasia (SRTD, n = 10/36, 27.8%) McKusick–Kaufmann syndrome (MKKS, n = 4/36, 11.1%), Bardet–Biedl syndrome (BBS, n = 2/36, 5.5%) and Joubert syndrome (n = 1/36, 2.8%). All cases showed abnormalities of the kidneys, most often hyperechogenic parenchyma (n = 26/36, 72.2%), cystic dysplasia (n = 24/36, 66.7%), and/or bilateral kidney enlargement (n = 22/36, 61.1%). Oligohydramnios was mainly present in fetuses with MKS. Polydactyly (n = 18/36), abnormalities of the CNS (n = 25/36), and heart defects (n = 10/36) were associated in 50%, 69.4%, and 27.8%, respectively.

Conclusion

Prenatal detection of renal abnormalities associated with skeletal or brain abnormalities should raise the suspicion for multisystem ciliopathies. Prenatal ultrasound can help to differentiate between different diseases and pave the way for subsequent targeted genetic testing.

Gene therapy rescues olfactory perception in a clinically relevant ciliopathy model of Bardet-Biedl syndrome

Bardet-Biedl syndrome (BBS) is a hereditary genetic disorder that results in numerous clinical manifestations including olfactory dysfunction. Of at least 21 BBS-related genes that can carry multiple mutations, a pathogenic mutation, BBS1M390R, is the single most common mutation of clinically diagnosed BBS outcomes. While the deletion of BBS-related genes in mice can cause variable penetrance in different organ systems, the impact of the Bbs1M390R mutation in the olfactory system remains unclear. Using a clinically relevant knock-in mouse model homozygous for Bbs1M390R, we investigated the impact of the mutation on the olfactory system and tested the potential of viral-mediated, wildtype gene replacement therapy to rescue smell loss. The cilia of olfactory sensory neurons (OSNs) in Bbs1^M390R/M390R mice were significantly shorter and fewer than those of wild-type mice. Also, both peripheral cellular odor detection and synaptic-dependent activity in the olfactory bulb were significantly decreased in the mutant mice. Furthermore, to gain insight into the degree to which perceptual features are impaired in the mutant mice, we used whole-body plethysmography to quantitatively measure odor-evoked sniffing. The Bbs1^M390R/M390R mice showed significantly higher odor detection thresholds (reduced odor sensitivity) compared to wild-type mice; however, their odor discrimination acuity was still well maintained. Importantly, adenoviral expression of Bbs1 in OSNs restored cilia length and re-established both peripheral odorant detection and odor perception. Together, our findings further expand our understanding for the development of gene therapeutic treatment for congenital ciliopathies in the olfactory system.

Nephroplex: a kidney-focused NGS panel highlights the challenges of PKD1 sequencing and identifies a founder BBS4 mutation

Background

Genetic testing of patients with inherited kidney diseases has emerged as a tool of clinical utility by improving the patients’ diagnosis, prognosis, surveillance and therapy.

Methods

The present study applied a Next Generation Sequencing (NGS)-based panel, named NephroPlex, testing 115 genes causing renal diseases, to 119 individuals, including 107 probands and 12 relatives. Thirty-five (poly)cystic and 72 non (poly)cystic individuals were enrolled. The latter subgroup of patients included Bardet-Biedl syndrome (BBS) patients, as major components.

Results

Disease-causing mutations were identified in 51.5 and 40% of polycystic and non-polycystic individuals, respectively. Autosomal dominant polycystic kidney disease (ADPKD) patients with truncating PKD1 variants showed a trend towards a greater slope of the age-estimated glomerular filtration rate (eGFR) regression line than patients with (i) missense variants, (ii) any PKD2 mutations and (iii) no detected mutations, according to previous findings. The analysis of BBS individuals showed a similar frequency of BBS4,9,10 and 12 mutations. Of note, all BBS4-mutated patients harbored the novel c.332+1G>GTT variant, which was absent in public databases, however, in our internal database, an additional heterozygote carrier was found. All BBS4-mutated individuals originated from the same geographical area encompassing the coastal provinces of Naples.

Discussion

In conclusion, these findings indicate the potential for a genetic panel to provide useful information at both clinical and epidemiological levels.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s40620-021-01048-4.

Third case of Bardet-Biedl syndrome caused by a biallelic variant predicted to affect splicing of IFT74

Bardet-Biedl syndrome (BBS) is a rare ciliopathy characterized by rod-cone dystrophy, postaxial polydactyly, truncal obesity and renal anomalies with autosomal recessive inheritance. We describe a 6-year-old male with early onset retinal dystrophy, postaxial polydactyly, truncal obesity and motor delays. Exome sequencing revealed a homozygous variant predicted to affect splicing of the IFT74 gene, c.1685-1G > T. This is the third patient with BBS due to variants predicting loss of function in IFT74. All three patients have had retinal dystrophy, polydactyly, obesity, developmental differences, and a notable lack of renal anomalies. We recommend that IFT74 is added to gene panels for the diagnosis of BBS.

Ciliopathies: Coloring outside of the lines

Ciliopathy syndromes are a diverse spectrum of disease characterized by a combination of cystic kidney disease, hepatobiliary disease, retinopathy, skeletal dysplasia, developmental delay, and brain malformations. Though generally divided into distinct disease categories based on the pattern of system involvement, ciliopathy syndromes are known to display certain phenotypic overlap. We performed next-generation sequencing panel testing, clinical exome sequencing, and research-based exome sequencing reanalysis on patients with suspected ciliopathy syndromes with additional features. We identified biallelic pathogenic variants in BBS1 in a child with features of cranioectodermal dysplasia, and biallelic variants in BBS12 in a child with the clinical stigmata of Bardet-Biedl syndrome, but also with anal atresia. We additionally identified biallelic pathogenic variants in WDR35 and DYNC2H1 in children with predominant liver disease and ductal plate malformation without skeletal dysplasia. Our study highlights the phenotypic and genetic diversity of ciliopathy syndromes, the importance of considering ciliopathy syndromes as a disease-spectrum and screening for all associated complications in all patients, and describes exclusive extra-skeletal manifestations in two classical skeletal dysplasia syndromes.

Bardet-Biedl syndrome: Weight patterns and genetics in a rare obesity syndrome

BACKGROUND

Bardet-Biedl syndrome (BBS) is a rare genetic disorder that severely inhibits primary cilia function. BBS is typified by obesity in adulthood, but pediatric weight patterns, and thus optimal periods of intervention, are poorly understood.

OBJECTIVES

To examine body mass differences by age, gender, and genotype in children and adolescents with BBS.

METHODS

We utilized the largest international registry of BBS phenotypes. Anthropometric and genetic data were obtained from medical records or participant/family interviews. Participants were stratified by age and sex categories. Genotype and obesity phenotype were investigated in a subset of participants with available data.

RESULTS

Height and weight measurements were available for 552 unique individuals with BBS. The majority of birth weights were in the normal range, but rates of overweight or obesity rapidly increased in early childhood, exceeding 90% after age 5. Weight z-scores in groups >2 years were above 2.0, while height z-scores approached 1.0, but were close to 0.0 in adolescents. Relative to those with the BBS10 genotype, the BBS1 cohort had a lower BMI z-score in the 2-5 and 6-11 age groups, with similar BMI z-scores thereafter. Children with biallelic loss of function (LOF) genetic variants had significantly higher BMI z-scores compared to missense variants.

CONCLUSION

Despite normal birth weight, most individuals with BBS experience rapid weight gain in early childhood, with high rates of overweight/obesity sustained through adolescence. Children with LOF variants are disproportionally affected. Our findings support the need for earlier recognition and initiation of weight management therapies in BBS.

Fetal hydrometrocolpos with pre-axial mirror polydactyly as a new variant of McKusick-Kaufman syndrome

This report describes a variant of McKusick-Kaufman syndrome presenting with a large fetal abdominal cyst of hydrometrocolpos at 37 weeks of gestation. The diagnosis was based on the ultrasound findings of a large homogeneous hypoechoic cyst (diameter of >10 cm) with incomplete septum, thickened wall, superiorly connecting to the dilated uterus, consistent with hydrometrocolpos. Additionally, pre-axial mirror polydactyly of the left foot was suspected. Postnatal examination/work-up confirmed the prenatal findings. This is the first report of prenatal diagnosis of hydrometrocolpos with complex polydactyly of mirror image pre-axial duplications containing nine toes instead of six-toe postaxial polydactyly.

A BBS1 SVA F retrotransposon insertion is a frequent cause of Bardet-Biedl syndrome

Bardet-Biedl syndrome (BBS) is a ciliopathy characterized by retinitis pigmentosa, obesity, polydactyly, cognitive impairment and renal failure. Pathogenic variants in 24 genes account for the molecular basis of >80% of cases. Toward saturated discovery of the mutational basis of the disorder, we carefully explored our cohorts and identified a hominid-specific SINE-R/VNTR/Alu type F (SVA-F) insertion in exon 13 of BBS1 in eight families. In six families, the repeat insertion was found in trans with c.1169 T > G, p.Met390Arg and in two families the insertion was found in addition to other recessive BBS loci. Whole genome sequencing, de novo assembly and SNP array analysis were performed to characterize the genomic event. This insertion is extremely rare in the general population (found in 8 alleles of 8 BBS cases but not in >10 800 control individuals from gnomAD-SV) and due to a founder effect. Its 2435 bp sequence contains hallmarks of LINE1 mediated retrotransposition. Functional studies with patient-derived cell lines confirmed that the BBS1 SVA-F is deleterious as evidenced by a significant depletion of both mRNA and protein levels. Such findings highlight the importance of dedicated bioinformatics pipelines to identify all types of variation.

Novel compound heterozygous pathogenic BBS5 variants in Filipino siblings with Bardet-Biedl syndrome (BBS)

PURPOSE

Bardet-Biedl syndrome (BBS) is rare in the Philippines and only limited information on the prevalent subtypes is available as yet. The purpose of this study is to present the clinical characteristics of two Filipino siblings presenting with mutations in BBS5.

PATIENTS AND METHODS

The Filipino female siblings, aged 11 and 14 years underwent comprehensive ophthalmologic evaluation. Fundus photography, macular optical coherence tomography (OCT) and electroretinography (ERG) were also obtained. Systemic workup was performed including radiographic imaging of limb defects, renal ultrasound, blood chemistry, and transvaginal ultrasound. Targeted Bardet-Biedl sequence analysis and deletion/duplication analysis were performed to determine potential pathogenic mutations.

RESULTS

Both children had poor visual acuity with a myopic refraction. There was a pigmentary retinopathy with retinal pigment epithelium changes and attenuation of vessels without waxy disc pallor. Generalized macular thinning and undetectable ERG responses were recorded. Physical examination revealed obesity, facial anomalies, brachydactyly, postaxial polydactyly, and clinodactyly of fifth digits. Both patients displayed cognitive developmental delay and hypogonadism. Molecular analysis revealed novel compound heterozygous mutations in BBS5 with c.143-1 G > A (splice acceptor) and c.925_931del (p.Gln309ilefs*14), each inherited from one asymptomatic parent.

CONCLUSION

These are probably the first reported BBS5 mutations causing Bardet-Biedl syndrome in the Philippines. Patients were managed by a multi-disciplinary team and the parents were counseled regarding the prognosis and additional complications associated with the syndrome.

The BBSome assembly is spatially controlled by BBS1 and BBS4 in human cells

Bardet-Biedl syndrome (BBS) is a pleiotropic ciliopathy caused by dysfunction of primary cilia. More than half of BBS patients carry mutations in one of eight genes encoding for subunits of a protein complex, the BBSome, which mediates trafficking of ciliary cargoes. In this study, we elucidated the mechanisms of the BBSome assembly in living cells and how this process is spatially regulated. We generated a large library of human cell lines deficient in a particular BBSome subunit and expressing another subunit tagged with a fluorescent protein. We analyzed these cell lines utilizing biochemical assays, conventional and expansion microscopy, and quantitative fluorescence microscopy techniques: fluorescence recovery after photobleaching and fluorescence correlation spectroscopy. Our data revealed that the BBSome formation is a sequential process. We show that the pre-BBSome is nucleated by BBS4 and assembled at pericentriolar satellites, followed by the translocation of the BBSome into the ciliary base mediated by BBS1. Our results provide a framework for elucidating how BBS-causative mutations interfere with the biogenesis of the BBSome.

Bardet-Biedl syndrome: Antenatal presentation of forty-five fetuses with biallelic pathogenic variants in known Bardet-Biedl syndrome genes

Bardet-Biedl syndrome (BBS) is an emblematic ciliopathy associated with retinal dystrophy, obesity, postaxial polydactyly, learning disabilities, hypogonadism and renal dysfunction. Before birth, enlarged/cystic kidneys as well as polydactyly are the hallmark signs of BBS to consider in absence of familial history. However, these findings are not specific to BBS, raising the problem of differential diagnoses and prognosis. Molecular diagnosis during pregnancies remains a timely challenge for this heterogeneous disease (22 known genes). We report here the largest cohort of BBS fetuses to better characterize the antenatal presentation. Prenatal ultrasound (US) and/or autopsy data from 74 fetuses with putative BBS diagnosis were collected out of which molecular diagnosis was established in 51 cases, mainly in BBS genes (45 cases) following the classical gene distribution, but also in other ciliopathy genes (6 cases). Based on this, an updated diagnostic decision tree is proposed. No genotype/phenotype correlation could be established but postaxial polydactyly (82%) and renal cysts (78%) were the most prevalent symptoms. However, autopsy revealed polydactyly that was missed by prenatal US in 55% of the cases. Polydactyly must be carefully looked for in pregnancies with apparently isolated renal anomalies in fetuses.

Cilia signaling and obesity

An emerging number of rare genetic disorders termed ciliopathies are associated with pediatric obesity. It is becoming clear that the mechanisms associated with cilia dysfunction and obesity in these syndromes are complex. In addition to ciliopathic syndromic forms of obesity, several cilia-associated signaling gene mutations also lead to morbid obesity. While cilia have critical and diverse functions in energy homeostasis including their roles in centrally mediated food intake as well as in peripheral tissues, many questions remain. Here, we briefly discuss the syndromic ciliopathies and monoallelic cilia signaling gene mutations associated with obesity. We also describe potential ways cilia may be involved in common obesity. We discuss how neuronal cilia impact food intake potentially through leptin signaling and changes in ciliary G protein-coupled receptor (GPCR) signaling. We highlight several recent studies that have implicated the potential for cilia in peripheral tissues such as adipose and the pancreas to contribute to metabolic dysfunction. Then we discuss the potential for cilia to impact energy homeostasis through their roles in both development and adult tissue homeostasis. The studies discussed in this review highlight how a comprehensive understanding of the requirement of cilia for the regulation of diverse biological functions will contribute to our understanding of common forms of obesity.

Cardiovascular Regulation by the Neuronal BBSome

The BBSome, a complex of 8 BBS (Bardet-Biedl syndrome) proteins known for its role in the control of cilia function and other cellular processes, has been implicated in blood pressure control, but the underlying mechanisms are not fully understood. Here, we show that neuronal BBSome plays an important role in blood pressure regulation. Targeted inactivation of the BBSome in the nervous system through Bbs1 gene deletion causes sympathetically mediated increase in blood pressure in mice. This phenotype is reproduced by selective ablation of the Bbs1 gene from the LRb (leptin receptor)-expressing neurons. Strikingly, the well-known role of the BBSome in the regulation of cilia formation and function is unlikely to account for the prohypertensive effect of BBSome inactivation as disruption of the IFT (intraflagellar transport) machinery required for ciliogenesis by deleting the Ift88 gene in LRb neurons had no effect on arterial pressure and sympathetic nerve activity. Furthermore, we found that Bbs1 gene deletion from AgRP (agouti-related protein) neurons or POMC (proopiomelanocortin) neurons increased renal and splanchnic sympathetic nerve activity without altering blood pressure. This lack of blood pressure increase despite the sympathetic overdrive may be explained by vascular adrenergic desensitization as indicated by the reduced vascular contractile response evoked by phenylephrine and the decreased expression of adrenergic receptors. Our results identify the neuronal BBSome as a new player in hemodynamic, sympathetic, and vascular regulation, in a manner independent of cilia.

Rapidly Progressive Nephronophthisis in a 2-Year-Old Boy with a Homozygous SDCCAG8 Mutation

Nephronophthisis (NPHP) is an autosomal recessive cystic kidney disease that is characterized by primary ciliary dysfunction (ciliopathy) and progresses to end-stage kidney disease (ESKD) during the second decade of life (juvenile and adolescent NPHP) or before the age of 3 years (infantile NPHP). Here we describe the case of an infant with NPHP who carries a homozygous mutation in SDCCAG8 (also called NPHP10 or BBS16) that encodes SDCCAG8 (serologically defined colon cancer antigen 8). SDCCAG8 is localized at the centrioles of both renal epithelial cells and retinal photoreceptor cells. A mutation in SDCCAG8 is also associated with Bardet-Biedl syndrome (BBS), characterized by NPHP, obesity, polydactyly, and rod-cone dystrophy. A 2-year-old boy was referred to our hospital due to kidney dysfunction of unknown etiology; the patient presented with delayed development and opsoclonus but did not exhibit the clinical characteristics of BBS. Histological findings such as dilatation of tubules and irregular thickness of tubular basement membrane confirmed the diagnosis of NPHP. Four months after referral, the patient's renal function was rapidly deteriorated, and emergency peritoneal dialysis was initiated. Next-generation sequencing (NGS) was performed, showing that the patient carries a homozygous four-base-pair deletion in SDCCAG8 (c.849_852delTTTG, p.Cys283Ter). The patient's parents were also found to be heterozygous for this loss-of-function mutation. To the best of our knowledge, the present patient is the first case of biopsy-proven infantile NPHP with a homozygous SDCCAG8 mutation. We conclude that NGS is extremely useful in the identification of SDCCAG8-related NPHP as a cause of sudden-onset ESKD during infancy.

Bardet-Biedl Syndrome Caused by Skipping of SCLT1 Complicated by Microvesicular Steatohepatitis

We treated the case of a 22-year-old male patient with liver dysfunction. At 1 year of age, hepatic fibrosis was suspected. In addition, due to the presence of retinitis pigmentosa, renal failure, obesity, mental retardation, and hypogonadism, he was diagnosed with Bardet-Biedl syndrome (BBS). Skipping of exons 14 and 17 in the sodium channel and clathrin linker 1 (SCLT1) gene was observed. At 22 years of age, the liver enzyme levels were further elevated and a diagnosis of microvesicular steatohepatitis was made. Insulin resistance, a reduction of muscle mass, an impairment of the fatty acid metabolism, and hyperleptinemia in this syndrome may cause steatohepatitis.

Bardet-Biedl Syndrome in rhesus macaques: A nonhuman primate model of retinitis pigmentosa

The development of therapies for retinal disorders is hampered by a lack of appropriate animal models. Higher nonhuman primates are the only animals with retinal structure similar to humans, including the presence of a macula and fovea. However, few nonhuman primate models of genetic retinal disease are known. We identified a lineage of rhesus macaques with a frameshift mutation in exon 3 of the BBS7 gene c.160delG (p.Ala54fs) that is predicted to produce a non-functional protein. In humans, mutations in this and other BBS genes cause Bardet-Biedl syndrome, a ciliopathy and a syndromic form of retinitis pigmentosa generally occurring in conjunction with kidney dysfunction, polydactyly, obesity, and/or hypogonadism. Three full- or half-sibling monkeys homozygous for the BBS7 c.160delG variant, at ages 3.5, 4 and 6 years old, displayed a combination of severe photoreceptor degeneration and progressive kidney disease. In vivo retinal imaging revealed features of severe macular degeneration, including absence of photoreceptor layers, degeneration of the retinal pigment epithelium, and retinal vasculature atrophy. Electroretinography in the 3.5-year-old case demonstrated loss of scotopic and photopic a-waves and markedly reduced and delayed b-waves. Histological assessments in the 4- and 6-year-old cases confirmed profound loss of photoreceptors and inner retinal neurons across the posterior retina, with dramatic thinning and disorganization of all cell layers, abundant microglia, absent or displaced RPE cells, and significant gliosis in the subretinal space. Retinal structure, including presence of photoreceptors, was preserved only in the far periphery. Ultrasound imaging of the kidneys revealed deranged architecture, and renal histopathology identified distorted contours with depressed, fibrotic foci and firmly adhered renal capsules; renal failure occurred in the 6-year-old case. Magnetic resonance imaging obtained in one case revealed abnormally low total brain volume and unilateral ventricular enlargement. The one male had abnormally small testes at 4 years of age, but polydactyly and obesity were not observed. Thus, monkeys homozygous for the BBS7 c.160delG variant closely mirrored several key features of the human BBS syndrome. This finding represents the first identification of a naturally-occurring nonhuman primate model of BBS, and more broadly the first such model of retinitis pigmentosa and a ciliopathy with an associated genetic mutation. This important new preclinical model will provide the basis for better understanding of disease progression and for the testing of new therapeutic options, including gene and cell-based therapies, not only for BBS but also for multiple forms of photoreceptor degeneration.

Primary Cilium, An Unsung Hero in Maintaining Functional β-cell Population

A primary challenge in type 2 diabetes (T2D) is the preservation of a functional population of β-cells, which play a central role in regulating blood glucose levels. Two congenital disorders, Bardet-Biedl syndrome (BBS) and Alström syndrome (ALMS), can serve as useful models to understand how β-cells are normally produced and regenerated. Both are characterized by obesity, loss of β-cells, and defects in primary cilia - the sensory center of cells. Primary cilia are cellular protrusions present in almost every vertebrate cell. This antenna-like organelle plays a crucial role in regulating several signaling pathways that direct proper development, proliferation, and homeostasis. Mutations in genes expressing ciliary proteins or proteins present at or near the base of the cilium lead to disorders, collectively called ciliopathies. BBS and Alström syndrome are such disorders. Though both BBS and Alström patients are obese, their childhood diabetes rates are vastly different, suggesting distinct pathogenesis underlying these two ciliopathies. Clinical studies suggest that BBS patients are protected against early onset diabetes by sustained or enhanced β-cell function. In contrast, Alström patients are more prone to develop diabetes. They have hyperinsulinemia, yet their β-cells fail to sense glucose and to regulate insulin secretion accordingly. These data suggest a potential role for primary cilia in maintaining a functional β-cell population and that defects in cilia or in ciliary proteins impair development and function of β-cells. Identifying the respective roles of primary cilia and ciliary proteins, such as BBS and ALMS1 may shed light on β-cell biology and uncover potentially novel targets for diabetes therapy.