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The Molecular Genetics of a Human Obesity Syndrome

Research Summary

Val Sheffield is interested in identifying genes, protein complexes, and disease mechanisms involved in Mendelian and complex human genetic disorders, including hereditary blindness, obesity, hypertension, and cardiovascular and neurological disorders.

The overall goal of my research is to understand the pathophysiology and improve management of specific human genetic diseases. A major focus of my laboratory is the study of hereditary eye diseases. Recent progress has resulted in the identification of disease genes involved in hereditary blindness, as well as identification of genes for disorders unrelated to vision.

An effective approach to the molecular dissection of complex diseases is to investigate Mendelian disorders that have phenotypic overlap with complex disease. An outstanding example of such a disorder and a major focus of the laboratory is the heterogeneous, autosomal recessive Bardet-Biedl syndrome (BBS). The pleiotropic features of BBS including retinal degeneration, obesity, polydactyly, congenital heart defects, renal abnormalities, hypogenitalism, neurological impairment, hydrocephalus, hypertension and diabetes mellitus make BBS an important model disorder for identifying molecular mechanisms and pathways involved in common human diseases (Figure 1). My laboratory was the first to map BBS genes and demonstrate the genetic heterogeneity of this disorder. We and others have now shown that there are at least 17 BBS genes.

My laboratory has played a key role in elucidating the function of BBS proteins, as well as the identification and characterization of BBS protein complexes. We have traditionally used genetic approaches including genetic linkage mapping, positional cloning, and more recently, high throughput DNA sequencing to identify genes associated with human diseases. Recently, we have used tandem affinity purification and mass spectrometry-based proteomics as a technology to identify protein complexes. Our use of biochemical methods to identify novel protein complexes and proteins that interact with these complexes have aided in the identification of pathways involved in specific clinical features of BBS including obesity and hydrocephalus. The fact that the mutation of multiple different genes can independently cause BBS has now been largely explained by the discovery of two BBS protein complexes: the BBSome (consisting of seven BBS proteins), which plays a role in intraflagellar transport; and a chaperone complex (consisting of three BBS proteins), which is required for BBSome assembly (Figure 2).

We have developed zebrafish and mouse models and have used them to demonstrate specific genetic interactions among BBS genes, to identify additional clinical features of BBS, and to provide insight into the pathophysiology of individual phenotypic components of BBS. Studies of animal models have shown that proteins that are affected by mutations causing BBS are components of cilia or function in intraflagellar or intracellular transport. Through our study of BBS, my laboratory has become interested in the study of signaling via the primary cilium. We are also interested in connecting the functions of BBS proteins and complexes to common human diseases. For example, BBS mouse studies have greatly contributed to our understanding of obesity in BBS. We hypothesized that central neurogenic mechanisms play a major pathophysiological role in obesity associated with deletion/mutation of Bbs genes in mice. We demonstrated that obesity in BBS is due to leptin resistance likely caused by mistrafficking of the leptin receptor (LepR) in the hypothalamus. We determined that obesity in BBS mutant mice (Bbs1M390R/M390R, Bbs2-/-, Bbs4-/- and Bbs6-/-) is associated with hyperphagia, decreased energy expenditure and leptin resistance. We studied the mechanism of leptin resistance in BBS mice by normalizing their body weights and circulating leptin levels using calorie-restriction. Despite attaining normal serum leptin levels, direct central administration of leptin fails to reduce food intake or activate hypothalamic LepR signaling (e.g. STAT3). These results indicate that leptin resistance in BBS mice is not secondary to obesity. Rather, these results indicate that leptin resistance is intrinsic and the cause of obesity in BBS mice. In addition, using biochemical methods, we demonstrated a direct interaction between BBS1 (a component of the BBSome) and LepR.

In addition to our work on obesity, we have demonstrated hypertension in some BBS knockout mice. We found that Bbs4- and Bbs6-, but not Bbs2-deficient mice, develop hypertension, and that central neurogenic mechanisms play a role in the hypertension associated with BBS. Collectively, our studies show that BBS mice have a novel mechanism of obesity and hypertension resulting from selective leptin resistance. The selective role of BBS proteins in blood pressure regulation is an important question for future work, as is the specific mechanism of selective (rather than general) leptin resistance.

Our work goes beyond the study of BBS and BBS associated phenotypes. For example, in keeping with our overall goal of understanding the pathophysiology and improving treatment of important human diseases, we have also developed a genetic mouse model of primary open-angle glaucoma, the second leading cause of irreversible blindness in humans. We have utilized this mouse model to identify novel pathways involved in glaucoma and notably, have intervened in the pathology of this disorder using drug therapy.

The overarching theme of on-going work is the improved understanding of complex inheritance mechanisms as it relates to ameliorating disease symptoms and development of novel disease treatments. The work implicating BBS proteins in cilia has increased general interest in cilia function and dysfunction, and greatly broadened understanding of the many types of disorders in which cilia are involved. Despite this progress, current knowledge is insufficient to fully explain the phenotypes observed in cilia-related disorders including BBS. A major goal of our current work is to clarify the molecular mechanisms involved in BBS and BBS-associated phenotypes. We are pursuing a better understanding of novel aspects of BBS phenotypes revealed or highlighted by the study of mouse models including hydrocephalus, hypertension, and glucose intolerance.

This work was supported in part by grants from the National Institutes of Health.

As of November 30, 2012

Scientist Profile

Investigator
University of Iowa
Genetics, Molecular Biology