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The Molecular Genetics of a Human Obesity Syndrome
Summary: Val Sheffield is interested in identifying genes and disease mechanisms involved in Mendelian and complex human genetic disorders, including glaucoma, macular degeneration, autism, obesity, and cardiovascular disorders.
The identification of genes, sequence variations, and mechanisms involved in complex human disorders holds great promise for improving health care, but at the same time presents a difficult challenge to the scientific community. To better understand the genetics of complex human disorders, my laboratory has studied Mendelian (monogenic) disorders that share a phenotypic component with common complex disorders. We have used human populations to map dozens of disorders and have used positional cloning methods to identify numerous disease-causing genes. This work has provided insights into the types of genes, mutational events, and gene product interactions that are likely to contribute to common complex disorders, and has influenced our approach toward identifying genes involved in polygenic disorders, including obesity and autism. Recent work in our laboratory has focused on the study of Bardet-Biedl syndrome (BBS).
BBS has the primary features of obesity, retinal degeneration, polydactyly, hypogonadism, renal anomalies, and mental retardation. Secondary features of BBS include diabetes mellitus, hypertension, and congenital heart defects. A significant subset of patients have behavioral abnormalities, including autism. Nearly half of all BBS patients develop type II diabetes. The phenotypic features of BBS and the finding that carriers of the disorder may be predisposed to hypertension, diabetes mellitus, and obesity suggest that BBS genes and the biological systems they identify may contribute to common disorders.
Studies by us and others have now shown that there are at least 12 BBS loci, and our laboratory has used positional cloning methods, comparative genomic analysis, and mutation analysis of candidate genes to identify seven BBS genes (BBS1, BBS2, BBS3, BBS4, BBS6, BBS9, and BBS11). The use of population isolates and haplotype analysis made it possible to map the individual loci and narrow the disease intervals, even in the presence of extensive genetic heterogeneity. Our studies illustrate the value of haplotype comparisons of affected and unaffected individuals in population isolates, and the value of a large collection of unrelated probands from diverse populations for mutation analysis to confirm disease causation.
Mutation analysis of the BBS genes has revealed some interesting findings. For example, a recurrent 6-kb deletion mutation was identified in the BBS4 gene in unrelated families. The recurrence of this mutation is due to features of local genomic architecture. This illustrates that local sequence context can result in recurrence of similar mutations in unrelated individuals, a finding relevant to the study of common complex disorders. We are searching genome-wide for deletions and duplications (copy-number variation) associated with complex disorders such as autism and glaucoma. Recent data indicate the validity of this approach.
The extensive genetic heterogeneity of BBS syndrome raises the possibility of complex interactions between BBS genes. Sequence analysis of BBS1, the gene most commonly involved in this disorder, and the other known BBS genes in a large patient cohort indicates that BBS penetrance is explained by autosomal-recessive inheritance. The existence of inter- and intrafamilial clinical variation suggests, however, that there are genes that modify the BBS phenotype. These modifier genes may be the other BBS genes or genes that encode products that interact with BBS proteins. Identification of modifier genes may contribute to the understanding of common disorders associated with BBS, such as obesity and diabetes. We are studying complex interactions of BBS genes in zebrafish and mice. Our results show that simultaneous knockdown of expression of some combinations of BBS genes in zebrafish results in synergistic enhancement of observed phenotypes indicating genetic modification.
Despite initial expectations that the identification of the BBS genes would elucidate common mechanisms or pathways resulting in this pleiotropic phenotype, an obvious unifying disease mechanism was not readily apparent. Phylogenetic comparison of the BBS genes indicated, however, that these genes are conserved in the genomes of organisms that have cilia, but not in nonciliated organisms. This finding has two notable ramifications. First, it suggests that BBS genes are involved in cilia function, and second, it suggests a strategy for identifying additional BBS genes by comparing the genomes of multiple species.
The sequencing of known BBS genes in patients with the disorder results in the identification of disease-causing mutations in approximately 60 percent of all patients, a finding that indicates that numerous BBS genes remain to be discovered. A paucity of additional large families with BBS is an obstacle to the identification of additional BBS genes. Therefore, we are using a strategy that combines high-density single nucleotide polymorphism (SNP) mapping in isolated inbred cases and small families with comparative genomic analysis to identify the best BBS candidate genes within linked regions. Using this strategy, we compare known BBS proteins to the translated genomes of model organisms to identify a subset of organisms in which these proteins are conserved. By including multiple organisms that have relatively small genomes, we reduce the number of candidate genes, and the best candidate genes are revealed. We then use both DNA sequence analysis and functional assays to verify BBS genes.
In addition to the standard verification of disease causation with sequence and functional studies, we have used a large set of gene expression microarray data to search for networks of genes whose expression is highly correlated. Analysis of expression data from the eyes of 120 rats revealed that the expression of BBS genes is highly correlated. These data can be used to reveal genes whose expression is highly correlated with the known BBS genes, to identify additional BBS candidates, and to indicate that a specific candidate gene is a BBS gene. In the past several months, we used the above strategies to identify BBS9 and BBS11.
Our current efforts are also aimed at determining the pathophysiology of specific components of the BBS phenotype, including the function and interactions of the known BBS genes, and defining the components and specific interactions of a BBS protein complex. We have also generated knockout or knockin mouse models for Bbs1, Bbs2, Bbs3, Bbs4, Bbs6, and Bbs7. These mouse models are the first mammalian models to implicate cilia in the pathogenesis of BBS. We show that mice lacking Bbs gene expression have major components of the human phenotype, including obesity and hypertension. In addition, these mice have phenotypes associated with cilia dysfunction, including retinopathy, renal cysts, male infertility, and a deficit in olfaction. We demonstrate that BBS retinopathy involves apparently normal retina development, followed by apoptotic death of photoreceptors, the primary ciliated cells of the retina. Photoreceptor cell death is preceded by mislocalization of rhodopsin, indicating a defect in intracellular transport. Furthermore, we demonstrate that BBS mouse models have a defect in social function. The evaluation of knockout mice indicates additional phenotypes that should be evaluated in human patients. Finally, using zebrafish, we have demonstrated that knocking down the expression of BBS genes leads to abnormal intracellular transport, a finding that suggests a common mechanism involved in BBS pathogenesis.
The above work was supported in part by grants from the National Institutes of Health.
Last updated: March 29, 2007
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