Impaired photoreceptor protein transport and synaptic transmission in a mouse model of Bardet-Biedl syndrome.

Bardet-Biedl syndrome (BBS) is an oligogenic syndrome whose manifestations include retinal degeneration, renal abnormalities, obesity and polydactylia. Evidence suggests that the main etiopathophysiology of this syndrome is impaired intraflagellar transport (IFT). In this study, we study the Bbs4-null mouse and investigate photoreceptor structure and function after loss of this gene. We find that Bbs4-null mice have defects in the transport of phototransduction proteins from the inner segments to the outer segments, before signs of cell death. Additionally, we show defects in synaptic transmission from the photoreceptors to secondary neurons of the visual system, demonstrating multiple functions for BBS4 in photoreceptors.

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

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

Disruption of Bardet-Biedl syndrome ciliary proteins perturbs planar cell polarity in vertebrates.

The evolutionarily conserved planar cell polarity (PCP) pathway (or noncanonical Wnt pathway) drives several important cellular processes, including epithelial cell polarization, cell migration and mitotic spindle orientation. In vertebrates, PCP genes have a vital role in polarized convergent extension movements during gastrulation and neurulation. Here we show that mice with mutations in genes involved in Bardet-Biedl syndrome (BBS), a disorder associated with ciliary dysfunction, share phenotypes with PCP mutants including open eyelids, neural tube defects and disrupted cochlear stereociliary bundles. Furthermore, we identify genetic interactions between BBS genes and a PCP gene in both mouse (Ltap, also called Vangl2) and zebrafish (vangl2). In zebrafish, the augmented phenotype results from enhanced defective convergent extension movements. We also show that Vangl2 localizes to the basal body and axoneme of ciliated cells, a pattern reminiscent of that of the BBS proteins. These data suggest that cilia are intrinsically involved in PCP processes.

Loss of BBS proteins causes anosmia in humans and defects in olfactory cilia structure and function in the mouse.

Defects in cilia are associated with several human disorders, including Kartagener syndrome, polycystic kidney disease, nephronophthisis and hydrocephalus. We proposed that the pleiotropic phenotype of Bardet-Biedl syndrome (BBS), which encompasses retinal degeneration, truncal obesity, renal and limb malformations and developmental delay, is due to dysfunction of basal bodies and cilia. Here we show that individuals with BBS have partial or complete anosmia. To test whether this phenotype is caused by ciliary defects of olfactory sensory neurons, we examined mice with deletions of Bbs1 or Bbs4. Loss of function of either BBS protein affected the olfactory, but not the respiratory, epithelium, causing severe reduction of the ciliated border, disorganization of the dendritic microtubule network and trapping of olfactory ciliary proteins in dendrites and cell bodies. Our data indicate that BBS proteins have a role in the microtubule organization of mammalian ciliated cells and that anosmia might be a useful determinant of other pleiotropic disorders with a suspected ciliary involvement.

Triallelic inheritance: a bridge between Mendelian and multifactorial traits.

The increasing identification of disease genes is revealing a growing number of traits that fail to conform to traditional Mendelian paradigms, thereby creating new challenges to both genetic investigators and clinicians. Bardet-Biedl syndrome (BBS) is one such disorder that has helped to define 'oligogenic' inheritance, a term that implies that some diseases are not inherited as simple single-gene Mendelian disorders and yet are not classic complex traits, but rather fit a model in which mutations in a small number of genes may interact genetically to manifest the phenotype. BBS is a pleiotropic disorder characterized by postnatal obesity, post-axial polydactyly, and progressive retinal dystrophy. Eight BBS loci have been identified to date and six of these genes have been cloned. Mutation analysis of these BBS genes in a cohort of patients has led to the description of the novel phenomenon of 'triallelic inheritance', wherein families were identified in which three mutations from genes at two different BBS loci segregate with expression of the disease. Modeling the cooperative ability of alleles of different genes at distinct loci to give rise to a particular phenotype will facilitate the understanding of complex multifactorial and polygenic traits.

Basal body dysfunction is a likely cause of pleiotropic Bardet-Biedl syndrome.

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous disorder characterized primarily by retinal dystrophy, obesity, polydactyly, renal malformations and learning disabilities. Although five BBS genes have been cloned, the molecular basis of this syndrome remains elusive. Here we show that BBS is probably caused by a defect at the basal body of ciliated cells. We have cloned a new BBS gene, BBS8, which encodes a protein with a prokaryotic domain, pilF, involved in pilus formation and twitching mobility. In one family, a homozygous null BBS8 mutation leads to BBS with randomization of left-right body axis symmetry, a known defect of the nodal cilium. We have also found that BBS8 localizes specifically to ciliated structures, such as the connecting cilium of the retina and columnar epithelial cells in the lung. In cells, BBS8 localizes to centrosomes and basal bodies and interacts with PCM1, a protein probably involved in ciliogenesis. Finally, we demonstrate that all available Caenorhabditis elegans BBS homologues are expressed exclusively in ciliated neurons, and contain regulatory elements for RFX, a transcription factor that modulates the expression of genes associated with ciliogenesis and intraflagellar transport.

BBS4 is a minor contributor to Bardet-Biedl syndrome and may also participate in triallelic inheritance.

Bardet-Biedl syndrome (BBS) is an uncommon multisystemic disorder characterized primarily by retinal dystrophy, obesity, polydactyly, and renal dysfunction. BBS has been modeled historically as an autosomal recessive trait, under which premise six independent BBS loci (BBS1-BBS6) have been mapped in the human genome. However, extended mutational analyses of BBS2 and BBS6, the first two BBS genes cloned, suggest that BBS exhibits a more complex pattern of inheritance, in which three mutations at two loci simultaneously are necessary and sufficient in some families to manifest the phenotype. We evaluated the spectrum of mutations in the recently identified BBS4 gene with a combination of haplotype analysis and mutation screening on a multiethnic cohort of 177 families. Consistent with predictions from previous genetic analyses, our data suggest that mutations in BBS4 contribute to BBS in <3% of affected families. Furthermore, integrated mutational data from all three currently cloned BBS genes raise the possibility that BBS4 may participate in triallelic inheritance with BBS2 and BBS1, but not the other known loci. Establishment of the loci pairing in triallelism is likely to be important for the elucidation of the functional relationships among the different BBS proteins.

Newfoundland rod-cone dystrophy, an early-onset retinal dystrophy, is caused by splice-junction mutations in RLBP1.

Some isolated populations exhibit an increased prevalence of rare recessive diseases. The island of Newfoundland is a characteristic geographic isolate, settled by a small number of families primarily during the late 1700s and early 1800s. During our studies of this population, we identified a group of families exhibiting a retinal dystrophy reminiscent of retinitis punctata albescens but with a substantially lower age at onset and more-rapid and distinctive progression, a disorder that we termed "Newfoundland rod-cone dystrophy" (NFRCD). The size of one of these families was sufficient to allow us to perform a genomewide screen to map the NFRCD locus. We detected significant linkage to markers on the long arm of chromosome 15, in a region encompassing RLBP1, the gene encoding the cellular retinaldehyde-binding protein. Previously, mutations in RLBP1 have been associated with other retinal dystrophies, leading us to hypothesize that RLBP1 mutations might also cause NFRCD. To test this hypothesis, we sequenced all coding exons and splice junctions of RLBP1. We detected two sequence alterations, each of which is likely to be pathogenic, since each segregates with the disease and is predicted to interfere with mRNA splicing. In contrast to some previously reported RLBP1 mutations, which yield a protein that may retain some residual activity, each NFRCD mutation is likely to give rise to a null allele. This difference may account for the severe phenotype in these families and exemplifies the molecular continuum that underlies clinically distinct but genetically related entities.