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Ciliopathies are a group of heterogeneous disorders associated with ciliary dysfunction. Diseases in this group display considerable phenotypic variation within individual syndromes and overlapping phenotypes among clinically distinct disorders. Particularly, mutations in CEP290 cause phenotypically diverse ciliopathies ranging from isolated retinal degeneration, nephronophthisis and Joubert syndrome, to the neonatal lethal Meckel-Gruber syndrome. However, the underlying mechanisms of the variable expressivity in ciliopathies are not well understood. Here, we show that components of the BBSome, a protein complex composed of seven Bardet-Biedl syndrome (BBS) proteins, physically and genetically interact with CEP290 and modulate the expression of disease phenotypes caused by CEP290 mutations. The BBSome binds to the N-terminal region of CEP290 through BBS4 and co-localizes with CEP290 to the transition zone (TZ) of primary cilia and centriolar satellites in ciliated cells, as well as to the connecting cilium in photoreceptor cells. Although CEP290 still localizes to the TZ and connecting cilium in BBSome-depleted cells, its localization to centriolar satellites is disrupted and CEP290 appears to disperse throughout the cytoplasm in BBSome-depleted cells. Genetic interactions were tested using Cep290(rd16)- and Bbs4-null mutant mouse lines. Additional loss of Bbs4 alleles in Cep290(rd16/rd16) mice results in increased body weight and accelerated photoreceptor degeneration compared with mice without Bbs4 mutations. Furthermore, double-heterozygous mice (Cep290(+/rd16);Bbs4(+/-)) have increased body weight compared with single-heterozygous animals. Our data indicate that genetic interactions between BBSome components and CEP290 could underlie the variable expression and overlapping phenotypes of ciliopathies caused by CEP290 mutations.

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.

Obesity is a major public health problem in most developed countries and a major risk factor for diabetes and cardiovascular disease. Emerging evidence indicates that ciliary dysfunction can contribute to human obesity but the underlying molecular and cellular mechanisms are unknown. Bardet-Biedl syndrome (BBS) is a genetically heterogeneous human obesity syndrome associated with ciliary dysfunction. BBS proteins are thought to play a role in cilia function and intracellular protein/vesicle trafficking. Here, we show that BBS proteins are required for leptin receptor (LepR) signaling in the hypothalamus. We found that Bbs2(-/-), Bbs4(-/-) and Bbs6(-/-) mice are resistant to the action of leptin to reduce body weight and food intake regardless of serum leptin levels and obesity. In addition, activation of hypothalamic STAT3 by leptin is significantly decreased in Bbs2(-/-), Bbs4(-/-) and Bbs6(-/-) mice. In contrast, downstream melanocortin receptor signaling is unaffected, indicating that LepR signaling is specifically impaired in Bbs2(-/-), Bbs4(-/-) and Bbs6(-/-) mice. Impaired LepR signaling in BBS mice was associated with decreased Pomc gene expression. Furthermore, we found that BBS1 protein physically interacts with the LepR and that loss of BBS proteins perturbs LepR trafficking. Our data indicate that BBS proteins mediate LepR trafficking and that impaired LepR signaling underlies energy imbalance in BBS. These findings represent a novel mechanism for leptin resistance and obesity.

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous autosomal recessive disorder with the primary clinical features of obesity, pigmented retinopathy, polydactyly, hypogenitalism, mental retardation and renal anomalies. Associated features of the disorder include diabetes mellitus, hypertension and congenital heart disease. There are six known BBS loci, mapping to chromosomes 2, 3, 11, 15, 16 and 20. The BBS2 locus was initially mapped to an 18 cM interval on chromosome 16q21 with a large inbred Bedouin kindred. Further analysis of the Bedouin population allowed for the fine mapping of this locus to a 2 cM region distal to marker D16S408. Physical mapping and sequence analysis of this region resulted in the identification of a number of known genes and expressed sequence tag clusters. Mutation screening of a novel gene (BBS2) with a wide pattern of tissue expression revealed homozygous mutations in two inbred pedigrees, including the large Bedouin kindred used to initially identify the BBS2 locus. In addition, mutations were found in three of 18 unrelated BBS probands from small nuclear families.