[Bardet-Biedl syndrome: cilia and obesity - from genes to integrative approaches]

Genetic obesity: an update with emerging therapeutic approaches

Based on the genetic contribution, childhood obesity can be classified into 3 groups: common polygenic obesity, syndromic obesity, and monogenic obesity. More genetic causes of obesity are being identified along with the advances in the genetic testing. Genetic obesities including syndromic and monogenic obesity should be suspected and evaluated in children with early-onset morbid obesity and hyperphagia under 5 years of age. Patients with syndromic obesity have early-onset severe obesity associated specific genetic syndromes including Prader-Willi syndrome, Bardet-Biedle syndrome, and Alstrom syndrome. Syndromic obesity is often accompanied with neurodevelopmental delay or dysmorphic features. Nonsyndromic monogenic obesity is caused by variants in single gene which are usually involved in the regulation of hunger and satiety associated with the hypothalamic leptin-melanocortin pathway in central nervous system. Unlike syndromic obesity, patients with monogenic obesity usually show normal neurodevelopment. They would be presented with hyperphagia and early-onset severe obesity with additional clinical symptoms including short stature, red hair, adrenal insufficiency, hypothyroidism, hypogonadism, pituitary insufficiencies, diabetes insipidus, increased predisposition to infection or intractable recurrent diarrhea. Identifying patients with genetic obesity is critical as new innovative therapies including melanocortin 4 receptor agonist have become available. Early genetic evaluation enables to identify treatable obesity and provide timely intervention which may eventually achieve favorable outcome by establishing personalized management.

Rare genetic causes of obesity: diagnosis and management in clinical care

Rare genetic forms of obesity are linked to impaired energy balance (i.e., eating behaviour and energy expenditure) involving hypothalamic pathways. More than 60 genes coding for proteins located in the hypothalamic leptin/melanocortin pathway contribute to the development of these rare forms of obesity. The ambition of the French National Protocol for the Diagnosis and Care (PNDS) of Obesity of Rare Causes was to establish practical recommendations for assessment and management at all ages. This report is available on the website of the French Health Authority (HAS). In addition to severe obesity, patients often display obesity-related comorbidities and neuropsychological/psychiatric disorders. These complex conditions make clinical management particularly challenging. Early diagnosis is critical for the organization of coordinated specialized multidisciplinary care, with mandatory interaction between caregivers, social partners and families. Strategies to prevent aggravation of obesity consist in limiting access to food, establishing a reassuring daily eating environment, and the practice of sustained adapted supervised daily physical activity. The implementation of genetic diagnosis in clinical practice now enables a personalized medicine approach with access to new drug therapies, and improves the analysis of the risk/benefit ratio of bariatric surgery.

BBS4 regulates the expression and secretion of FSTL1, a protein that participates in ciliogenesis and the differentiation of 3T3-L1

Bardet-Biedl syndrome is a model ciliopathy. Although the characterization of BBS proteins has evidenced their involvement in cilia, extraciliary functions for some of these proteins are also being recognized. Importantly, understanding both cilia and cilia-independent functions of the BBS proteins is key to fully dissect the cellular basis of the syndrome. Here we characterize a functional interaction between BBS4 and the secreted protein FSTL1, a protein linked to adipogenesis and inflammation among other functions. We show that BBS4 and cilia regulate FSTL1 mRNA levels, but BBS4 also modulates FSTL1 secretion. Moreover, we show that FSTL1 is a novel regulator of ciliogenesis thus underscoring a regulatory loop between FSTL1 and cilia. Finally, our data indicate that BBS4, cilia and FSTL1 are coordinated during the differentiation of 3T3-L1 cells and that FSTL1 plays a role in this process, at least in part, by modulating ciliogenesis. Therefore, our findings are relevant to fully understand the development of BBS-associated phenotypes such as obesity.

The cilium: a cellular antenna with an influence on obesity risk

Primary cilia are organelles that are present on many different cell types, either transiently or permanently. They play a crucial role in receiving signals from the environment and passing these signals to other parts of the cell. In that way, they are involved in diverse processes such as adipocyte differentiation and olfactory sensation. Mutations in genes coding for ciliary proteins often have pleiotropic effects and lead to clinical conditions, ciliopathies, with multiple symptoms. In this study, we reviewed observations from ciliopathies with obesity as one of the symptoms. It shows that variation in cilia-related genes is itself not a major cause of obesity in the population but may be a part of the multifactorial aetiology of this complex condition. Both common polymorphisms and rare deleterious variants may contribute to the obesity risk. Genotype-phenotype relationships have been noticed. Among the ciliary genes, obesity differs with regard to severity and age of onset, which may relate to the influence of each gene on the balance between pro- and anti-adipogenic processes. Analysis of the function and location of the proteins encoded by these ciliary genes suggests that obesity is more linked to activities at the basal area of the cilium, including initiation of the intraflagellar transport, but less to the intraflagellar transport itself. Regarding the role of cilia, three possible mechanistic processes underlying obesity are described: adipogenesis, neuronal food intake regulation and food odour perception.

Rare Genetic Forms of Obesity: Clinical Approach and Current Treatments in 2016

Obesity results from a synergistic relationship between genes and the environment. The phenotypic expression of genetic factors involved in obesity is variable, allowing to distinguish several clinical pictures of obesity. Monogenic obesity is described as rare and severe early-onset obesity with abnormal feeding behavior and endocrine disorders. This is mainly due to autosomal recessive mutations in genes of the leptin-melanocortin pathway which plays a key role in the hypothalamic control of food intake. Melanocortin 4 receptor(MC4R)-linked obesity is characterized by the variable severity of obesity and no notable additional phenotypes. Mutations in the MC4R gene are involved in 2-3% of obese children and adults; the majority of these are heterozygous. Syndromic obesity is associated with mental retardation, dysmorphic features, and organ-specific developmental abnormalities. Additional genes participating in the development of hypothalamus and central nervous system have been regularly identified. But to date, not all involved genes have been identified so far. New diagnostic tools, such as whole-exome sequencing, will probably help to identify other genes. Managing these patients is challenging. Indeed, specific treatments are available only for specific types of monogenic obesity, such as leptin deficiency. Data on bariatric surgery are limited and controversial. New molecules acting on the leptin-melanocortin pathway are currently being developed.

TMEM107 Is a Critical Regulator of Ciliary Protein Composition and Is Mutated in Orofaciodigital Syndrome

The proximate causes of multiple human genetic syndromes (ciliopathies) are disruptions in the formation or function of the cilium, an organelle required for a multitude of developmental processes. We previously identified Tmem107 as a critical regulator of cilia formation and embryonic organ development in the mouse. Here, we describe a patient with a mutation in TMEM107 that developed atypical Orofaciodigital syndrome (OFD), and show that the OFD patient shares several morphological features with the Tmem107 mutant mouse including polydactyly and reduced numbers of ciliated cells. We show that TMEM107 appears to function within cilia to regulate protein content, as key ciliary proteins do not localize normally in cilia derived from the Tmem107 mouse mutant and the human patient. These data indicate that TMEM107 plays a key, conserved role in regulating ciliary protein composition, and is a novel candidate for ciliopathies of unknown etiology.

Bardet-Biedl syndrome: Is it only cilia dysfunction?

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous, pleiotropic disorder, characterized by both congenital and late onset defects. From the analysis of the mutational burden in patients to the functional characterization of the BBS proteins, this syndrome has become a model for both understanding oligogenic patterns of inheritance and the biology of a particular cellular organelle: the primary cilium. Here we briefly review the genetics of BBS to then focus on the function of the BBS proteins, not only in the context of the cilium but also highlighting potential extra-ciliary roles that could be relevant to the etiology of the disorder. Finally, we provide an overview of how the study of this rare syndrome has contributed to the understanding of cilia biology and how this knowledge has informed on the cellular basis of different clinical manifestations that characterize BBS and the ciliopathies.

[Primary cilia control different steps of brain development]

The role of primary cilia in adult neurons remains elusive, however their developmental functions during brain morphogenesis have been recently highlighted thanks to mouse models. Unmistakably, they are needed for Hedgehog (Hh)-dependent patterning in the forebrain. Not only for Hh reception itself, but most importantly for a downstream event in the Hh transduction pathway, independent of Hh ligand: the Gli3 processing. Indeed, phenotypes due to cilia disruption in the developing brain, such as early patterning, olfactory bulb or corpus callosum formation, can be rescued by reintroducing Gli3-R (the short truncated form of Gli3 working as a transcriptional repressor of Hh target gene). In addition, primary cilia control the proliferation rate in different neural progenitors in the cortex, the hippocampus and the cerebellum; they are required for proper migration of interneurons. And cilia dysfunction is correlated with hydrocephaly, synaptogenesis defects and aberrant axonal tract projections. Most of these neurodevelopmental defects can be related to the various neurological features frequently observed across the ciliopathy spectrum. And thus, understanding the underlying mechanisms of these diverse functions of primary cilia in the brain is a new fundamental challenge.

[Non-ciliary functions of cilia proteins]

Cilia proteins have long been characterized for their role in cilia formation and function, and their implications in ciliopathies. However, several cellular defects induced by cilia proteins deregulation suggest that they could have non-ciliary roles. Indeed, several non-ciliary functions have been recently characterized for cilia proteins including roles in intra-cellular and in vesicular transport, in spindle orientation or in the maintenance of genomic stability. These observations thus raise the crucial question of the contribution of non-ciliary functions of cilia proteins to the pathological manifestations associated with ciliopathies such as polycystic kidney disease.

[Elongation of the axoneme and dynamics of intraflagellar transport]

Cilia and flagella are essential organelles in most eukaryotes including human beings. In this review, we will discuss the mode of assembly of these complex organelles that depends on a dynamic process called intraflagellar transport or IFT. IFT delivers structural elements at the distal end of the cilium where assembly takes place, thereby allowing the growth of the organelle. We next discuss the different models for control of cilium length and their alterations in ciliopathies, genetic diseases associated to ciliary defects.