Polymorphisms in the amino acid transporter solute carrier family 6 (neurotransmitter transporter) member 14 gene contribute to polygenic obesity in French Caucasians

Immunogenetic Aspects of Sarcopenic Obesity

Sarcopenic obesity (SO) is a combination of obesity and sarcopenia, with diagnostic criteria defined as impaired skeletal muscle function and altered body composition (e.g., increased fat mass and reduced muscle mass). The mechanism of SO is not yet perfectly understood; however, the pathogenesis includes aging and its complications, chronic inflammation, insulin resistance (IR), and hormonal changes. Genetic background is apparent in the pathogenesis of isolated obesity, which is most often polygenic and is characterized by the additive effect of various genetic factors. The genetic etiology has not been strictly established in SO. Still, many data confirm the existence of pathogenic gene variants, e.g., Fat Mass and Obesity Associated Gene (FTO), beta-2-adrenergic receptor (ADRB2) gene, melanocortin-4 receptor (MC4R) and others with obesity. The literature on the role of these genes is scarce, and their role has not yet been thoroughly established. On the other hand, the involvement of systemic inflammation due to increased adipose tissue in SO plays a significant role in its pathophysiology through the synthesis of various cytokines such as monocyte chemoattractant protein-1 (MCP-1), IL-1Ra, IL-15, adiponectin or CRP. The lack of anti-inflammatory cytokine (e.g., IL-15) can increase SO risk, but further studies are needed to evaluate the exact mechanisms of implications of various cytokines in SO individuals. This manuscript analyses various immunogenetic and non-genetic factors and summarizes the recent findings on immunogenetics potentially impacting SO development.

Factors Predisposing the Response to Lumacaftor/Ivacaftor in People with Cystic Fibrosis

Lumacaftor/ivacaftor (LUMA-IVA) therapy is prescribed to people with cystic fibrosis (pwCF) homozygous for the Phe508del-CFTR variant to restore CFTR protein function. There is, however, large inter-individual variability in treatment response. Here, we seek to identify clinical and/or genetic factors that may modulate the response to this CFTR modulator therapy. A total of 765 pwCF older than 12 years under LUMA-IVA therapy and with lung function and nutritional measurements available before and after treatment initiation were included. Response to treatment was determined by the change in lung function and nutritional status, from baseline and over the first two years after initiation, and it was assessed by weighted generalized estimating equation models. Gains in lung function and nutritional status were observed after 6 months of treatment (on average 2.11 ± 7.81% for percent predicted FEV₁ and 0.44 ± 0.77 kg/m² for BMI) and sustained over the 2 years. We observed that the more severe patients gained the most in lung function and nutritional status. While females started with a nutritional status more impaired than males, they had a larger response and regained BMI Z-score values similar to men after 2 years of treatment. We observed no association between variants in solute carrier (SLC) genes and the respiratory function response to LUMA-IVA, but the SLC6A14 rs12839137 variant was associated with the nutritional response. Further investigations, including other genomic regions, will be needed to fully explore the inter-individual variability of the response to LUMA-IVA.

Unconventional Functions of Amino Acid Transporters: Role in Macropinocytosis (SLC38A5/SLC38A3) and Diet-Induced Obesity/Metabolic Syndrome (SLC6A19/SLC6A14/SLC6A6)

Amino acid transporters are expressed in mammalian cells not only in the plasma membrane but also in intracellular membranes. The conventional function of these transporters is to transfer their amino acid substrates across the lipid bilayer; the direction of the transfer is dictated by the combined gradients for the amino acid substrates and the co-transported ions (Na⁺, H⁺, K⁺ or Cl⁻) across the membrane. In cases of electrogenic transporters, the membrane potential also contributes to the direction of the amino acid transfer. In addition to this expected traditional function, several unconventional functions are known for some of these amino acid transporters. This includes their role in intracellular signaling, regulation of acid–base balance, and entry of viruses into cells. Such functions expand the biological roles of these transporters beyond the logical amino acid homeostasis. In recent years, two additional unconventional biochemical/metabolic processes regulated by certain amino acid transporters have come to be recognized: macropinocytosis and obesity. This adds to the repertoire of biological processes that are controlled and regulated by amino acid transporters in health and disease. In the present review, we highlight the unusual involvement of selective amino acid transporters in macropinocytosis (SLC38A5/SLC38A3) and diet-induced obesity/metabolic syndrome (SLC6A19/SLC6A14/SLC6A6).

Orexin, serotonin, and energy balance

The lateral hypothalamus is critical for the control of ingestive behavior and spontaneous physical activity (SPA), as lesion or stimulation of this region alters these behaviors. Evidence points to lateral hypothalamic orexin neurons as modulators of feeding and SPA. These neurons affect a broad range of systems, and project to multiple brain regions such as the dorsal raphe nucleus, which contains serotoninergic neurons (DRN) important to energy homeostasis. Physical activity is comprised of intentional exercise and SPA. These are opposite ends of a continuum of physical activity intensity and structure. Non‐goal‐oriented behaviors, such as fidgeting, standing, and ambulating, constitute SPA in humans, and reflect a propensity for activity separate from intentional activity, such as high‐intensity voluntary exercise. In animals, SPA is activity not influenced by rewards such as food or a running wheel. Spontaneous physical activity in humans and animals burns calories and could theoretically be manipulated pharmacologically to expend calories and protect against obesity. The DRN neurons receive orexin inputs, and project heavily onto cortical and subcortical areas involved in movement, feeding and energy expenditure (EE). This review discusses the function of hypothalamic orexin in energy‐homeostasis, the interaction with DRN serotonin neurons, and the role of this orexin‐serotonin axis in regulating food intake, SPA, and EE. In addition, we discuss possible brain areas involved in orexin–serotonin cross‐talk; the role of serotonin receptors, transporters and uptake‐inhibitors in the pathogenesis and treatment of obesity; animal models of obesity with impaired serotonin‐function; single‐nucleotide polymorphisms in the serotonin system and obesity; and future directions in the orexin–serotonin field.

This article is categorized under:

Metabolic Diseases > Molecular and Cellular Physiology

Potential Biological Targets Prediction, ADME Profiling, & Molecular Docking studies of Novel Steroidal Products from Cunninghamella Blakesleana

BACKGROUND

New potential biological targets prediction through inverse molecular docking technique is an another smart strategy to forecast the possibility of compounds being biologically active against various target receptors.

OBJECTIVES

In this case of designed study, we screened our recently obtained novel acetylinic steroidal biotransformed products [(1) 8-β-methyl-14-α-hydroxy∆4tibolone (2) 9-α-Hydroxy∆4 tibolone (3) 8-β-methyl-11-β-hydroxy∆4tibolone (4) 6-β-hydroxy∆4tibolone, (5) 6-β-9-α-dihydroxy∆4tibolone (6) 7-β-hydroxy∆4tibolone) ] from fungi Cunninghemella Blakesleana to predict their possible biological targets and profiling of ADME properties.

METHOD

The prediction of pharmacokinetics properties membrane permeability as well as bioavailability radar properties were carried out by using Swiss target prediction, and Swiss ADME tools, respectively these metabolites were also subjected to predict the possible mechanism of action along with associated biological network pathways by using Reactome data-base.

RESULTS

All the six screened compounds possess excellent drug ability criteria, and exhibited exceptionally excellent non inhibitory potential against all five isozymes of CYP450 enzyme complex, including (CYP1A2, CYP2C19, CYP2C9, CYP2D6, and CYP3A4) respectively. All the screened compounds are lying within the acceptable pink zone of bioavailability radar and showing excellent descriptive properties. Compounds [1-4 & 6] are showing high BBB (Blood Brain Barrier) permeation, while compound 5 is exhibiting high HIA (Human Intestinal Absorption) property of (Egan Egg).

CONCLUSION

In conclusion, the results of this study smartly reveals that in-silico based studies are considered to provide robustness towards a rational drug designing and development approach, therefore in this way it helps to avoid the possibility of failure of drug candidates in the later experimental stages of drug development phases.

α-Methyl-l-tryptophan as a weight-loss agent in multiple models of obesity in mice

α-Methyl-L-tryptophan (α-MLT) is currently in use as a tracer in its 11C-labeled form to monitor the health of serotonergic neurons in humans. In the present study, we found this compound to function as an effective weight-loss agent at pharmacological doses in multiple models of obesity in mice. The drug was able to reduce the body weight when given orally in drinking water (1 mg/ml) in three different models of obesity: normal mice on high-fat diet, Slc6a14-null mice on high-fat diet, and ob/ob mice on normal diet. Only the l-enantiomer (α-MLT) was active while the d-enantiomer (α-MDT) had negligible activity. The weight-loss effect was freely reversible, with the weight gain resuming soon after the withdrawal of the drug. All three models of obesity were associated with hyperglycemia, insulin resistance, and hepatic steatosis; α-MLT reversed these features. There was a decrease in food intake in the treatment group. Mice on a high-fat diet showed decreased cholesterol and protein in the serum when treated with α-MLT; there was however no evidence of liver and kidney dysfunction. Plasma amino acid profile indicated a significant decrease in the levels of specific amino acids, including tryptophan; but the levels of arginine were increased. We conclude that α-MLT is an effective, reversible, and orally active drug for the treatment of obesity and metabolic syndrome.

SLC6A14 deficiency is linked to obesity, fatty liver, and metabolic syndrome but only under conditions of a high-fat diet

SLC6A14 is a Na⁺/Cl^--coupled transporter for neutral/cationic amino acids, expressed in ileum and colon. A single-nucleotide polymorphism (SNP), rs2011162 (-22,510C > G), in SLC6A14 coding for the 3'-untranslated region (3'-UTR) is associated with obesity in humans. But the impact of this polymorphism on the transporter expression and its connection to obesity are not known. Our objective was to address these issues. The impact of rs2011162 (-22,510C > G) on SLC6A14 expression was monitored using a luciferase reporter. The link between Slc6a14 and obesity was investigated in wild type and Slc6a14^-/- mice when fed a normal diet or a high-fat diet. The obesity-associated 3'-UTR polymorphism reduced SLC6A14 expression. With a high-fat diet, Slc6a14^-/- mice gained more weight than wild type mice. With normal diet, there was no difference between the two genotypes. The gain in body weight with the high-fat diet in Slc6a14^-/- mice was accompanied with metabolic syndrome. With the high-fat diet, Slc6a14^-/- mice showed increased food intake, developed fatty liver, and altered plasma amino acid profile. The high-fat diet-associated hepatic steatosis in Slc6a14^-/- mice showed male preponderance. We conclude that the 3'-UTR SNP in SLC6A14 associated with obesity decreases the expression of SLC6A14 and that the deficiency of SLC6A14 is linked to obesity. This is supported by the findings that Slc6a14^-/- mice develop obesity, fatty liver, and metabolic syndrome. This connection is evident only with a high-fat diet. Therefore, dietary/pharmacologic interventions that induce SLC6A14 expression in the intestinal tract might have potential for obesity prevention.¹.

Amino Acid Transporter SLC6A14 (ATB0,+) - A Target in Combined Anti-cancer Therapy

Cancer cells are characterized by quick growth and proliferation, demanding constant supply of various nutrients. Several plasma membrane transporters delivering such compounds are upregulated in cancer. Solute carrier family 6 member 14 (SLC6A14), known as amino acid transporter B^0,+ (ATB^0,+) transports all amino acids with exception of the acidic ones: aspartate and glutamate. Its malfunctioning is correlated with several pathological states and it is upregulated in solid tumors. The high expression of SLC6A14 is prognostic and unfavorable in pancreatic cancer, while in breast cancer it is expressed in estrogen receptor positive cells. As many plasma membrane transporters it resides in endoplasmic reticulum (ER) membrane after translation before further trafficking through Golgi to the cell surface. Transporter exit from ER is strictly controlled. The proper folding of SLC6A14 was shown to be controlled from the cytoplasmic side by heat shock proteins, further exit from ER and formation of coatomer II (COPII) coated vesicles depends on specific interaction with COPII cargo-recognizing subunit SEC24C, phosphorylated by kinase AKT. Inhibition of heat shock proteins, known to be upregulated in cancer, directs SLC6A14 to degradation. Targeting proteins regulating SLC6A14 trafficking is proposed as an additional pharmacological treatment of cancer.

Update on SLC6A14 in lung and gastrointestinal physiology and physiopathology: focus on cystic fibrosis

The solute carrier family 6 member 14 (SLC6A14) protein imports and concentrates all neutral amino acids as well as the two cationic acids lysine and arginine into the cytoplasm of different cell types. Primarily described as involved in several cancer and colonic diseases physiopathological mechanisms, the SLC6A14 gene has been more recently identified as a genetic modifier of cystic fibrosis (CF) disease severity. It was indeed shown to have a pleiotropic effect, modulating meconium ileus occurrence, lung disease severity, and precocity of P. aeruginosa airway infection. The biological mechanisms explaining the impact of SLC6A14 on intestinal and lung phenotypes of CF patients are starting to be elucidated. This review focuses on SLC6A14 in lung and gastrointestinal physiology and physiopathology, especially its involvement in the pathophysiology of CF disease.

Unraveling genomic associations with feed efficiency and body weight traits in chickens through an integrative approach

BACKGROUND

Feed efficiency and growth rate have been targets for selection to improve chicken production. The incorporation of genomic tools may help to accelerate selection. We genotyped 529 individuals using a high-density SNP chip (600 K, Affymetrix®) to estimate genomic heritability of performance traits and to identify genomic regions and their positional candidate genes associated with performance traits in a Brazilian F2 Chicken Resource population. Regions exhibiting selection signatures and a SNP dataset from resequencing were integrated with the genomic regions identified using the chip to refine the list of positional candidate genes and identify potential causative mutations.

RESULTS

Feed intake (FI), feed conversion ratio (FC), feed efficiency (FE) and weight gain (WG) exhibited low genomic heritability values (i.e. from 0.0002 to 0.13), while body weight at hatch (BW1), 35 days-of-age (BW35), and 41 days-of-age (BW41) exhibited high genomic heritability values (i.e. from 0.60 to 0.73) in this F2 population. Twenty unique 1-Mb genomic windows were associated with BW1, BW35 or BW41, located on GGA1-4, 6-7, 10, 14, 24, 27 and 28. Thirty-eight positional candidate genes were identified within these windows, and three of them overlapped with selection signature regions. Thirteen predicted deleterious and three high impact sequence SNPs in these QTL regions were annotated in 11 positional candidate genes related to osteogenesis, skeletal muscle development, growth, energy metabolism and lipid metabolism, which may be associated with body weight in chickens.

CONCLUSIONS

The use of a high-density SNP array to identify QTL which were integrated with whole genome sequence signatures of selection allowed the identification of candidate genes and candidate causal variants. One novel QTL was detected providing additional information to understand the genetic architecture of body weight traits. We identified QTL for body weight traits, which were also associated with fatness in the same population. Our findings form a basis for further functional studies to elucidate the role of specific genes in regulating body weight and fat deposition in chickens, generating useful information for poultry breeding programs.

GutSelf: Interindividual Variability in the Processing of Dietary Compounds by the Human Gastrointestinal Tract

Nutritional research is currently entering the field of personalized nutrition, to a large extent driven by major technological breakthroughs in analytical sciences and biocomputing. An efficient launching of the personalized approach depends on the ability of researchers to comprehensively monitor and characterize interindividual variability in the activity of the human gastrointestinal tract. This information is currently not available in such a form. This review therefore aims at identifying and discussing published data, providing evidence on interindividual variability in the processing of the major nutrients, i.e., protein, fat, carbohydrates, vitamins, and minerals, along the gastrointestinal tract, including oral processing, intestinal digestion, and absorption. Although interindividual variability is not a primary endpoint of most studies identified, a significant number of publications provides a wealth of information on this topic for each category of nutrients. This knowledge remains fragmented, however, and understanding the clinical relevance of most of the interindividual responses to food ingestion described in this review remains unclear. In that regard, this review has identified a gap and sets the base for future research addressing the issue of the interindividual variability in the response of the human organism to the ingestion of foods.

Amino Acid Transporters as Disease Modifiers and Drug Targets

Amino acids perform a variety of functions in cells and organisms, particularly in the synthesis of proteins, as energy metabolites, neurotransmitters, and precursors for many other molecules. Amino acid transport plays a key role in all these functions. Inhibition of amino acid transport is pursued as a therapeutic strategy in several areas, such as diabetes and related metabolic disorders, neurological disorders, cancer, and stem cell biology. The role of amino acid transporters in these disorders and processes is well established, but the implementation of amino acid transporters as drug targets is still in its infancy. This is at least in part due to the underdeveloped pharmacology of this group of membrane proteins. Recent advances in structural biology, membrane protein expression, and inhibitor screening methodology will see an increased number of improved and selective inhibitors of amino acid transporters that can serve as tool compounds for further studies.

Emerging Roles for Serotonin in Regulating Metabolism: New Implications for an Ancient Molecule

Serotonin is a phylogenetically ancient biogenic amine that has played an integral role in maintaining energy homeostasis for billions of years. In mammals, serotonin produced within the central nervous system regulates behavior, suppresses appetite, and promotes energy expenditure by increasing sympathetic drive to brown adipose tissue. In addition to these central circuits, emerging evidence also suggests an important role for peripheral serotonin as a factor that enhances nutrient absorption and storage. Specifically, glucose and fatty acids stimulate the release of serotonin from the duodenum, promoting gut peristalsis and nutrient absorption. Serotonin also enters the bloodstream and interacts with multiple organs, priming the body for energy storage by promoting insulin secretion and de novo lipogenesis in the liver and white adipose tissue, while reducing lipolysis and the metabolic activity of brown and beige adipose tissue. Collectively, peripheral serotonin acts as an endocrine factor to promote the efficient storage of energy by upregulating lipid anabolism. Pharmacological inhibition of serotonin synthesis or signaling in key metabolic tissues are potential drug targets for obesity, type 2 diabetes, and nonalcoholic fatty liver disease (NAFLD).

Amino acid transporter SLC6A14 depends on heat shock protein HSP90 in trafficking to the cell surface

Plasma membrane transporter SLC6A14 transports all neutral and basic amino acids in a Na/Cl - dependent way and it is up-regulated in many types of cancer. Mass spectrometry analysis of overexpressed SLC6A14-associated proteins identified, among others, the presence of cytosolic heat shock proteins (HSPs) and co-chaperones. We detected co-localization of overexpressed and native SLC6A14 with HSP90-beta and HSP70 (HSPA14). Proximity ligation assay confirmed a direct interaction of overexpressed SLC6A14 with both HSPs. Treatment with radicicol and VER155008, specific inhibitors of HSP90 and HSP70, respectively, attenuated these interactions and strongly reduced transporter presence at the cell surface, what resulted from the diminished level of the total transporter protein. Distortion of SLC6A14 proper folding by both HSPs inhibitors directed the transporter towards endoplasmic reticulum-associated degradation pathway, a process reversed by the proteasome inhibitor - bortezomib. As demonstrated in an in vitro ATPase assay of recombinant purified HSP90-beta, the peptides corresponding to C-terminal amino acid sequence following the last transmembrane domain of SLC6A14 affected the HSP90-beta activity. These results indicate that a plasma membrane protein folding can be controlled not only by chaperones in the endoplasmic reticulum, but also those localized in the cytosol.

Established and emerging strategies to crack the genetic code of obesity

Tremendous progress has been made in the genetic elucidation of obesity over the past two decades, driven largely by technological, methodological and organizational innovations. Current strategies for identifying obesity-predisposing loci/genes, including cytogenetics, linkage analysis, homozygosity mapping, admixture mapping, candidate gene studies, genome-wide association studies, custom genotyping arrays, whole-exome sequencing and targeted exome sequencing, have achieved differing levels of success, and the identified loci in aggregate explain only a modest fraction of the estimated heritability of obesity. This review outlines the successes and limitations of these approaches and proposes novel strategies, including the use of exceptionally large sample sizes, the study of diverse ethnic groups and deep phenotypes and the application of innovative methods and study designs, to identify the remaining obesity-predisposing genes. The use of both established and emerging strategies has the potential to crack the genetic code of obesity in the not-too-distant future. The resulting knowledge is likely to yield improvements in obesity prediction, prevention and care.

Can uterine secretion of modified histones alter blastocyst implantation, embryo nutrition, and transgenerational phenotype?

Extracellular histones support rodent and human embryo development in at least two ways. First, these molecules in uterine secretions protect embryos from inflammation caused by pathogens that gain access to the reproductive tract. Also, histones in uterine secretions likely support penetration of the uterine epithelium by blastocysts during embryo implantation. Extracellular histones seem to preserve amino acid transport system B0,+ in blastocysts by inhibiting its activity. Preservation of system B0,+ is needed because, at the time of invasion of the uterine epithelium by motile trophoblasts, system B0,+ is likely reactivated to help remove tryptophan from the implantation chamber. If tryptophan is not removed, T-cells proliferate and reject the implanting blastocyst. Epigenetic modification of histones could alter their promotion of normal implantation through, say, incomplete tryptophan removal and, thus, allow partial T-cell rejection of the conceptus. Such partial rejection could impair placental development, embryonal/fetal nutrition, and weight gain prior to birth. Small-for-gestational-age offspring are predisposed to developing metabolic syndrome, obesity, and associated complications as adults. Shifting expression of these phenotypes might contribute to transgenerational variation and evolution. The spectrum of possible extracellular histone targets in early development warrant new research, especially since the effects of epigenetic histone modifications might be transgenerational.

Amino Acid Transport Across the Mammalian Intestine

The small intestine mediates the absorption of amino acids after ingestion of protein and sustains the supply of amino acids to all tissues. The small intestine is an important contributor to plasma amino acid homeostasis, while amino acid transport in the large intestine is more relevant for bacterial metabolites and fluid secretion. A number of rare inherited disorders have contributed to the identification of amino acid transporters in epithelial cells of the small intestine, in particular cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. These are most readily detected by analysis of urine amino acids, but typically also affect intestinal transport. The genes underlying these disorders have all been identified. The remaining transporters were identified through molecular cloning techniques to the extent that a comprehensive portrait of functional cooperation among transporters of intestinal epithelial cells is now available for both the basolateral and apical membranes. Mouse models of most intestinal transporters illustrate their contribution to amino acid homeostasis and systemic physiology. Intestinal amino acid transport activities can vary between species, but these can now be explained as differences of amino acid transporter distribution along the intestine. © 2019 American Physiological Society. Compr Physiol 9:343-373, 2019.

Integration of genome wide association studies and whole genome sequencing provides novel insights into fat deposition in chicken

Excessive fat deposition is a negative factor for poultry production because it reduces feed efficiency, increases the cost of meat production and is a health concern for consumers. We genotyped 497 birds from a Brazilian F₂ Chicken Resource Population, using a high-density SNP array (600 K), to estimate the genomic heritability of fat deposition related traits and to identify genomic regions and positional candidate genes (PCGs) associated with these traits. Selection signature regions, haplotype blocks and SNP data from a previous whole genome sequencing study in the founders of this chicken F2 population were used to refine the list of PCGs and to identify potential causative SNPs. We obtained high genomic heritabilities (0.43-0.56) and identified 22 unique QTLs for abdominal fat and carcass fat content traits. These QTLs harbored 26 PCGs involved in biological processes such as fat cell differentiation, insulin and triglyceride levels, and lipid biosynthetic process. Three of these 26 PCGs were located within haplotype blocks there were associated with fat traits, five overlapped with selection signature regions, and 12 contained predicted deleterious variants. The identified QTLs, PCGs and potentially causative SNPs provide new insights into the genetic control of fat deposition and can lead to improved accuracy of selection to reduce excessive fat deposition in chickens.

On the origin of obesity: identifying the biological, environmental and cultural drivers of genetic risk among human populations

Genetic predisposition to obesity presents a paradox: how do genetic variants with a detrimental impact on human health persist through evolutionary time? Numerous hypotheses, such as the thrifty genotype hypothesis, attempt to explain this phenomenon yet fail to provide a justification for the modern obesity epidemic. In this critical review, we appraise existing theories explaining the evolutionary origins of obesity and explore novel biological and sociocultural agents of evolutionary change to help explain the modern-day distribution of obesity-predisposing variants. Genetic drift, acting as a form of 'blind justice,' may randomly affect allele frequencies across generations while gene pleiotropy and adaptations to diverse environments may explain the rise and subsequent selection of obesity risk alleles. As an adaptive response, epigenetic regulation of gene expression may impact the manifestation of genetic predisposition to obesity. Finally, exposure to malnutrition and disease epidemics in the wake of oppressive social systems, culturally mediated notions of attractiveness and desirability, and diverse mating systems may play a role in shaping the human genome. As an important first step towards the identification of important drivers of obesity gene evolution, this review may inform empirical research focused on testing evolutionary theories by way of population genetics and mathematical modelling.

Molecular genetics of human obesity: A comprehensive review

Obesity and its related health complications is a major problem worldwide. Hypothalamus and their signalling molecules play a critical role in the intervening and coordination with energy balance and homeostasis. Genetic factors play a crucial role in determining an individual's predisposition to the weight gain and being obese. In the past few years, several genetic variants were identified as monogenic forms of human obesity having success over common polygenic forms. In the context of molecular genetics, genome-wide association studies (GWAS) approach and their findings signified a number of genetic variants predisposing to obesity. However, the last couple of years, it has also been noticed that alterations in the environmental and epigenetic factors are one of the key causes of obesity. Hence, this review might be helpful in the current scenario of molecular genetics of human obesity, obesity-related health complications (ORHC), and energy homeostasis. Future work based on the clinical discoveries may play a role in the molecular dissection of genetic approaches to find more obesity-susceptible gene loci.

SLC44A2 single nucleotide polymorphisms, isoforms, and expression: Association with severity of Meniere's disease?

SLC44A2 was discovered as the target of an antibody that causes hearing loss. Knockout mice develop age related hearing loss, loss of sensory cells and spiral ganglion neurons. SLC44A2 has polymorphic sites implicated in human disease. Transfusion related acute lung injury (TRALI) is linked to rs2288904 and genome wide association studies link rs2288904 and rs9797861 to venous thromboembolism (VTE), coronary artery disease and stroke. Here we report linkage disequilibrium of rs2288904 with rs3087969 and the association of these SLC44A2 SNPs with Meniere's disease severity. Tissue-specific isoform expression differences suggest that the N-terminal domain is linked to different functions in different cell types. Heterozygosity at rs2288904 CGA/CAA and rs3087969 GAT/GAC showed a trend for association with intractable Meniere's disease compared to less severe disease and to controls. The association of SLC44A2 SNPs with VTE suggests that thrombi affecting cochlear vessels could be a factor in Meniere's disease.

Glutamine transporters in mammalian cells and their functions in physiology and cancer

The SLC (solute carrier)-type transporters (~400 in number) in mammalian cells consist of 52 distinct gene families, grouped solely based on the amino acid sequence (primary structure) of the transporter proteins and not on their transport function. Among them are the transporters for amino acids. Fourteen of them, capable of transporting glutamine across the plasma membrane, are found in four families: SLC1, SLC6, SLC7, and SLC38. However, it is generally thought that the members of the SLC38 family are the principal transporters for glutamine. Some of the glutamine transporters are obligatory exchangers whereas some function as active transporters in one direction. While most glutamine transporters mediate the influx of the amino acid into cells, some actually mediate the efflux of the amino acid out of the cells. Glutamine transporters play important roles in a variety of tissues, including the liver, brain, kidney, and placenta, as clearly evident from the biological and biochemical phenotypes resulting from the deletion of specific glutamine transporters in mice. Owing to the obligatory role of glutamine in growth and proliferation of tumor cells, there is increasing attention on glutamine transporters in cancer biology as potential drug targets for cancer treatment. Selective blockers of certain glutamine transporters might be effective in preventing the entry of glutamine and other important amino acids into tumor cells, thus essentially starving these cells to death. This could represent the beginning of a new era in the discovery of novel anticancer drugs with a previously unexplored mode of action. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Study of Three Single Nucleotide Polymorphisms in the SLC6A14 Gene in Association with Male Infertility

Although several genetic causes of male infertility are known, the condition in around 60.0–75.0% of infertile male patients appears to be idiopathic. In some, genetic causes may be polygenic and require several low-penetrance genes to produce a phenotype outcome. In others, pleiotropy, when a gene can produce several phenotypic traits, may be involved. We have investigated whether single nucleotide polymorphisms (SNPs) in the SLC6A14 [solute carrier family 6 (amino acid transporter), member 14] gene are associated with male infertility. This gene has previously been linked with obesity and cystic fibrosis, which are associated with male infertility. It has a role in the transport of tryptophan and synthesis of serotonin that are important for normal spermatogenesis and testicular function. We have analyzed three SNPs (rs2312054, rs2071877 and rs2011162) in 370 infertile men and 241 fertile controls from two different populations (Macedonian and Slovenian). We found that the rs2011162(G) allele and rs2312054(A)-rs2071877(C)-rs2011162(G) haplotype are present at lower frequencies in the infertile rather than the fertile men (p = 0.044 and p = 0.0144, respectively).

We concluded that the SLC6A14 gene may be a population-specific, low-penetrance locus which confers susceptibility to male infertility/subfertility. Additional follow-up studies of a large number of infertile men of different ethnic backgrounds are needed to confirm such a susceptibility.

Defining the blanks--pharmacochaperoning of SLC6 transporters and ABC transporters

SLC6 family members and ABC transporters represent two extremes: SLC6 transporters are confined to the membrane proper and only expose small segments to the hydrophilic milieu. In ABC transporters the hydrophobic core is connected to a large intracellular (eponymous) ATP binding domain that is comprised of two discontiguous repeats. Accordingly, their folding problem is fundamentally different. This can be gauged from mutations that impair the folding of the encoded protein and give rise to clinically relevant disease phenotypes: in SLC6 transporters, these cluster at the protein–lipid interface on the membrane exposed surface. Mutations in ABC-transporters map to the interface between nucleotide binding domains and the coupling helices, which provide the connection to the hydrophobic core. Folding of these mutated ABC-transporters can be corrected with ligands/substrates that bind to the hydrophobic core. This highlights a pivotal role of the coupling helices in the folding trajectory. In contrast, insights into pharmacochaperoning of SLC6 transporters are limited to monoamine transporters – in particular the serotonin transporter (SERT) – because of their rich pharmacology. Only ligands that stabilize the inward facing conformation act as effective pharmacochaperones. This indicates that the folding trajectory of SERT proceeds via the inward facing conformation. Mutations that impair folding of SLC6 family members can be transmitted as dominant or recessive alleles. The dominant phenotype of the mutation can be rationalized, because SLC6 transporters are exported in oligomeric form from the endoplasmic reticulum (ER). Recessive transmission requires shielding of the unaffected gene product from the mutated transporter in the ER. This can be accounted for by a chaperone-COPII (coatomer protein II) exchange model, where proteinaceous ER-resident chaperones engage various intermediates prior to formation of the oligomeric state and subsequent export from the ER. It is likely that the action of pharmacochaperones is contingent on and modulated by these chaperones.

No evidence of association between the serotonin 2A receptor −1438G/A promoter polymorphism and childhood obesity in a Spanish population: A case-parent study and a matched case-control study

Serotonin has been related to feeding behaviour and body weight control through its suppressive effect on appetite. Conflicting results have been published in the literature regarding the association between the - 1438 G/A promoter polymorphism of the 5HT2A gene with obesity-related variables. The aim of this study was to assess the association between the--1438 G/A polymorphism of the 5HT2A gene with childhood obesity in a Spanish population. A total of 136 cases aged 6-16 years with BMI above the 97th percentile of the Spanish BMI reference data for age and gender were matched by gender and age (+/- 6 months) with 136 controls. Additionally, 43 obese children and their parents were selected for a family-based association study (case-parent study). Genotyping was carried out by polymerase chain reaction and restriction enzyme analysis. Conditional logistic regression and transmission/disequilibrium test were used to assess genotype-obesity association. In the matched case-control study, the crude and adjusted odds ratios for the association between 5HT2A--1438 G/A genotypes were nonsignificant. Likewise, no association is suggested by the case-parent study. In conclusion, it is unlikely that the--1438 G/A polymorphism of 5HT2A gene may influence obesity in a Spanish children population.

Involvement of the neutral amino acid transporter SLC6A15 and leucine in obesity-related phenotypes

Brain pathways, including those in hypothalamus and nucleus of the solitary tract, influence food intake, nutrient preferences, metabolism and development of obesity in ways that often differ between males and females. Branched chain amino acids, including leucine, can suppress food intake, alter metabolism and change vulnerability to obesity. The SLC6A15 (v7-3) gene encodes a sodium-dependent transporter of leucine and other branched chain amino acids that is expressed by neurons in hypothalamus and nucleus of the solitary tract. We now report that SLC6A15 knockout attenuates leucine's abilities to reduce both: a) intake of normal chow and b) weight gain produced by access to a high fat diet in gender-selective fashions. We identify SNPs in the human SLC6A15 that are associated with body mass index and insulin resistance in males. These observations in mice and humans support a novel, gender-selective role for brain amino acid compartmentalization mediated by SLC6A15 in diet and obesity-associated phenotypes.

Characterization of the transporterB0AT3 (Slc6a17) in the rodent central nervous system

BACKGROUND

The vesicular B0AT3 transporter (SLC6A17), one of the members of the SLC6 family, is a transporter for neutral amino acids and is exclusively expressed in brain. Here we provide a comprehensive expression profile of B0AT3 in mouse brain using in situ hybridization and immunohistochemistry.

RESULTS

We confirmed previous expression data from rat brain and used a novel custom made antibody to obtain detailed co-labelling with several cell type specific markers. B0AT3 was highly expressed in both inhibitory and excitatory neurons. The B0AT3 expression was highly overlapping with those of vesicular glutamate transporter 2 (VGLUT2) and vesicular glutamate transporter 1 (VGLUT1). We also show here that Slc6a17mRNA is up-regulated in animals subjected to short term food deprivation as well as animals treated with the serotonin reuptake inhibitor fluoxetine and the dopamine/noradrenaline reuptake inhibitor bupropion.

CONCLUSIONS

This suggests that the B0AT3 transporter have a role in regulation of monoaminergic as well as glutamatergic synapses.

The genetics of human obesity

It has long been known that there is a genetic component to obesity, and that characterizing this underlying factor would likely offer the possibility of better intervention in the future. Monogenic obesity has proved to be relatively straightforward, with a combination of linkage analysis and mouse models facilitating the identification of multiple genes. In contrast, genome-wide association studies have successfully revealed a variety of genetic loci associated with the more common form of obesity, allowing for very strong consensus on the underlying genetic architecture of the phenotype for the first time. Although a number of significant findings have been made, it appears that very little of the apparent heritability of body mass index has actually been explained to date. New approaches for data analyses and advances in technology will be required to uncover the elusive missing heritability, and to aid in the identification of the key causative genetic underpinnings of obesity.

The Human Obesity Gene Map: The 2004 Update

This paper presents the eleventh update of the human obesity gene map, which incorporates published results up to the end of October 2004. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTLs) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2004, 173 human obesity cases due to single-gene mutations in 10 different genes have been reported, and 49 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 166 genes which, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 221. The number of human obesity QTLs derived from genome scans continues to grow, and we have now 204 QTLs for obesity-related phenotypes from 50 genome-wide scans. A total of 38 genomic regions harbor QTLs replicated among two to four studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably with 358 findings of positive associations with 113 candidate genes. Among them, 18 genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, >600 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful publications and genomic and other relevant sites can be found at http://obesitygene.pbrc.edu.

Obstructive sleep apnea syndrome: from phenotype to genetic basis

Obstructive sleep apnea syndrome (OSAS) is a complex chronic clinical syndrome, characterized by snoring, periodic apnea, hypoxemia during sleep, and daytime hypersomnolence. It affects 4-5% of the general population. Racial studies and chromosomal mapping, familial studies and twin studies have provided evidence for the possible link between the OSAS and genetic factors and also most of the risk factors involved in the pathogenesis of OSAS are largely genetically determined. A percentage of 35-40% of its variance can be attributed to genetic factors. It is likely that genetic factors associated with craniofacial structure, body fat distribution and neural control of the upper airway muscles interact to produce the OSAS phenotype. Although the role of specific genes that influence the development of OSAS has not yet been identified, current researches, especially in animal model, suggest that several genetic systems may be important. In this chapter, we will first define the OSAS phenotype, the pathogenesis and the risk factors involved in the OSAS that may be inherited, then, we will review the current progress in the genetics of OSAS and suggest a few future perspectives in the development of therapeutic agents for this complex disease entity.

The role of amino acid transporters in inherited and acquired diseases

Amino acids are essential building blocks of all mammalian cells. In addition to their role in protein synthesis, amino acids play an important role as energy fuels, precursors for a variety of metabolites and as signalling molecules. Disorders associated with the malfunction of amino acid transporters reflect the variety of roles that they fulfil in human physiology. Mutations of brain amino acid transporters affect neuronal excitability. Mutations of renal and intestinal amino acid transporters affect whole-body homoeostasis, resulting in malabsorption and renal problems. Amino acid transporters that are integral parts of metabolic pathways reduce the function of these pathways. Finally, amino acid uptake is essential for cell growth, thereby explaining their role in tumour progression. The present review summarizes the involvement of amino acid transporters in these roles as illustrated by diseases resulting from transporter malfunction.

Isolation and molecular characterization of the porcine SLC6A14 gene excludes it as a candidate gene for fat deposition and growth

The gene encoding solute carrier family 6 member 14 (SLC6A14) has been considered as a candidate gene affecting human obesity. In this study, full-length cDNA (2237 bp) and DNA sequence (24 541 bp) of the porcine SLC6A14 gene were isolated. The porcine SLC6A14 cDNA contains a 5&rsquo;-untranslated region of 57 bp, a 3&rsquo;-untranslated region of 254 bp, and an open reading frame of 1926 bp, encoding a deduced protein of 642 amino acids with a molecular mass of 72. 475 kDa and an isoelectric point of 7.82. The genomic structure of the porcine SLC6A14 gene is similar to mammalian orthologs, particularly in terms of exon size and exon/intron boundaries. It comprises 14 exons and 13 introns. A semi-quantitative RT-PCR showed that the porcine SLC6A14 mRNA expression was tissue-specific. Four SLC6A14 single-nucleotide polymorphisms (SNPs) were identified, and 3 informative SNPs were chosen for genotyping in a White Duroc &times; Erhualian resource population with phenotype data of growth and fatness traits. The association analysis showed that the c.1438 G>A nonsynonymous polymorphism was associated with birth weight and 21-day body weight (P < 0.05), while g.7944 A>T was associated with 46-day body weight. Linkage and radiation hybrid mapping assigned SLC6A14 to a region around SW1522 on SSCXp13, which did not fall in the confidence interval of the quantitative trait locus (QTL) for growth and fatness traits on SSCX in the resource population. These results indicate that SLC6A14 is not a positional candidate gene for the QTL affecting fatness and growth traits in pigs.

Obesity-related polymorphisms and their associations with the ability to regulate fat oxidation in obese Europeans: the NUGENOB study

Both obesity and insulin resistance have been related to low fat oxidation rates, which may be genetically determined. The association between variation in fat oxidation rates among obese subjects and genotype was studied for 42 common single-nucleotide polymorphisms (SNPs) in 26 candidate genes for fat oxidation, insulin resistance, and obesity, including FTO. Energy expenditure (EE) and fat oxidation were measured with indirect calorimetry during fasting and 3 h after a high fat load containing 95 energy% of fat (60% saturated fat, energy content 50% of estimated resting EE) in 722 obese subjects (541 women, 181 men) from 8 European centers. After adjustment for center and gender, -178 A>C CD36 (rs2232169) (P = 0.02), -22510 C>G SLC6A14 (women, rs2011162) (P = 0.03), and T690S C>G PCSK1 (rs6235) (P = 0.02) were related to a reduced fat oxidation, whereas 17 C>G SREBF1 (17 C>G) (P = 0.01) was related to increased fat oxidation in the fasting state. The ability to increase fat oxidation after a high fat load was increased in subjects with -174 G>C IL6 (rs1800795) (P = 0.01). Effect sizes range from 1.1 to 3.1% differences in fat oxidation (expressed as % of EE). FTO rs9939609 was not related to fat oxidation. At the same time, the results are not adjusted for multiple testing, thus none of the associations can be considered statistically significant. The results should therefore only be considered as leads to new hypotheses about effects of specific genetic polymorphisms on fasting and postprandial fat oxidation.

Interpreting metabolomic profiles using unbiased pathway models

Human disease is heterogeneous, with similar disease phenotypes resulting from distinct combinations of genetic and environmental factors. Small-molecule profiling can address disease heterogeneity by evaluating the underlying biologic state of individuals through non-invasive interrogation of plasma metabolite levels. We analyzed metabolite profiles from an oral glucose tolerance test (OGTT) in 50 individuals, 25 with normal (NGT) and 25 with impaired glucose tolerance (IGT). Our focus was to elucidate underlying biologic processes. Although we initially found little overlap between changed metabolites and preconceived definitions of metabolic pathways, the use of unbiased network approaches identified significant concerted changes. Specifically, we derived a metabolic network with edges drawn between reactant and product nodes in individual reactions and between all substrates of individual enzymes and transporters. We searched for "active modules"--regions of the metabolic network enriched for changes in metabolite levels. Active modules identified relationships among changed metabolites and highlighted the importance of specific solute carriers in metabolite profiles. Furthermore, hierarchical clustering and principal component analysis demonstrated that changed metabolites in OGTT naturally grouped according to the activities of the System A and L amino acid transporters, the osmolyte carrier SLC6A12, and the mitochondrial aspartate-glutamate transporter SLC25A13. Comparison between NGT and IGT groups supported blunted glucose- and/or insulin-stimulated activities in the IGT group. Using unbiased pathway models, we offer evidence supporting the important role of solute carriers in the physiologic response to glucose challenge and conclude that carrier activities are reflected in individual metabolite profiles of perturbation experiments. Given the involvement of transporters in human disease, metabolite profiling may contribute to improved disease classification via the interrogation of specific transporter activities.

Different alterations in rat intestinal glutamine transport during the progression of CLP- and LPS-induced sepsis

BACKGROUND

A marked deficiency of glutamine in clinical critical illness is correlated with mortality in the intensive care unit. Though intestinal glutamine transport was reported to be impaired in late sepsis, we hypothesized that there might be a different alteration in the early stage, with differential effects on the Na(+)-dependent glutamine transporters B(0)AT1, ATB(0,+), and ATA2.

MATERIALS AND METHODS

Sepsis was induced by cecal ligation and puncture or lipopolysaccharide intraperitoneal injection in Sprague Dawley rats, and the samples were collected at 0, 2, 6, 12, 24h. Small intestinal brush border glutamine transport was studied by a rapid filtration technique. The relative contributions of the three main transporter, B(0)AT1, ATB(0,+), and ATA2, were determined by competitive inhibition. The mRNA level of each transporter was analyzed by RT-PCR, and an extra immunohistochemistry analysis was performed to detect the localization of ATA2 protein in small intestine. Serum TNF-α and IL-10 concentrations were quantitated by ELISA.

RESULTS

Intestinal glutamine transport showed a biphasic change with an early increase and a late decrease in both CLP and LPS group. The early increase of glutamine transport was mainly attributable to the increased contributions of ATA2 and ATB(0,+). The transport activities of B(0)AT1, ATB(0,+) altered mainly because of the number of transporters (mRNA level as an indicator), while turned to ATA2, the redistribution was also found to be involved. The plasma TNF-α and IL-10 levels, especially the former, showed similar changing profiles to glutamine transport and, thus, may have relevance to it.

CONCLUSION

Rat intestinal glutamine transport showed an early increase and a late decrease in sepsis, and may provide some information for sepsis treatment.

The genetic contribution to non-syndromic human obesity

The last few years have seen major advances in common non-syndromic obesity research, much of it the result of genetic studies. This Review outlines the competing hypotheses about the mechanisms underlying the genetic and physiological basis of obesity, and then examines the recent explosion of genetic association studies that have yielded insights into obesity, both at the candidate gene level and the genome-wide level. With obesity genetics now entering the post-genome-wide association scan era, the obvious question is how to improve the results obtained so far using single nucleotide polymorphism markers and how to move successfully into the other areas of genomic variation that may be associated with common obesity.

Common genetic variation near MC4R is associated with eating behaviour patterns in European populations

Both rs17782313 (near MC4R) and rs1421085 (FTO) polymorphisms have been consistently associated with increased risk of obesity and with body mass index (BMI) variation. An effect of both polymorphisms on satiety has recently been suggested. We genotyped rs17782313 and rs1421085 in 5764 relatives from 1109 French pedigrees with familial obesity, 1274 Swiss class III obese adults as well as in 4877 French adults and 5612 Finnish teenagers from two randomly selected population cohorts. In all subjects, eating behaviour traits were documented through questionnaires. We first assessed the association of both single nucleotide polymorphisms with BMI and then studied eating behaviour. Under an additive model, the rs17782313-C MC4R allele showed a trend towards higher percentages of snacking in both French obese children (P=0.01) and Swiss obese adults (P=0.04) as well as in adolescents from the Finnish general population (P=0.04). In French adults with familial obesity, this allele tended to be also associated with a higher Stunkard hunger score (P=0.02) and in obese children with a higher prevalence of eating large amounts of food (P=0.04). However, no consistent association of the FTO rs1421085-C allele and available eating behaviour trait was found in our studied populations. The rs17782313-C allele nearby MC4R may modulate eating behaviour-related phenotypes in European obese and randomly selected populations, in both children and adults, supporting a regulatory role of this genetic variant on eating behaviour, as previously shown for MC4R non-synonymous loss-of-function mutations. The potential effect of the obesity-associated FTO gene on eating behaviour deserves additional investigation.

Evidence of a quantitative trait locus for energy and macronutrient intakes on chromosome 3q27.3: the Quebec Family Study

BACKGROUND

Little is known about the genes influencing dietary energy and nutrient intakes, despite evidence that these intakes are influenced by genetic factors.

OBJECTIVE

We aimed to identify, by using a genome-wide linkage analysis, chromosomal regions harboring genes that affect energy and macronutrient intakes.

DESIGN

Energy, carbohydrate, lipid, and protein intakes were assessed in 836 subjects from 217 families by using a 3-d dietary record. A total of 443 markers were genotyped and tested for linkage; age- and sex-adjusted energy and macronutrient intakes were expressed in grams and as percentages of total energy intake. Regression-based (Haseman-Elston) and variance-component (MERLIN) methods were applied to test for linkage with dietary data. A maximum of 454 sibpairs from 217 nuclear families were available for analysis.

RESULTS

The genome scan provided suggestive evidence (P < or = 0.0023) for the presence of 6 quantitative trait linkages influencing total caloric and macronutrient intakes in the Québec Family Study. Of these, multiple linkages were found on chromosome 3q27.3, in a region harboring the adiponectin gene, at marker D3S1262 for energy [logarithm of odds (LOD): 2.24], carbohydrate (LOD: 2.00), and lipid (LOD: 1.65) intakes. The peak linkages for carbohydrate, lipid, and protein intakes were found on chromosomes 1p32.2 (LOD: 2.39), 1p35.2 (LOD: 2.41), and 10p15.3 (LOD: 2.72), respectively. The linkage results remained significant after adjustment for body mass index, which suggested that the genes underlying these quantitative trait linkages influence dietary intake independent of body size.

CONCLUSION

The linkage on chromosome 3q27.3 with energy, lipid, and carbohydrate intakes suggests that this region of the genome may harbor genes that influence energy and macronutrient intakes in humans.

Prevalence of melanocortin-4 receptor deficiency in Europeans and their age-dependent penetrance in multigenerational pedigrees

OBJECTIVE

Melanocortin-4 receptor (MC4R) deficiency is the most frequent genetic cause of obesity. However, there is uncertainty regarding the degree of penetrance of this condition, and the putative impact of the environment on the development of obesity in MC4R mutation carriers is unknown.

RESEARCH DESIGN AND METHODS

We determined the MC4R sequence in 2,257 obese individuals and 2,677 nonobese control subjects of European origin and established the likely functional impact of all variants detected. We then included relatives of probands carriers and studied 25 pedigrees, including 97 carriers and 94 noncarriers from three generations.

RESULTS

Of the MC4R nonsynonymous mutations found in obese subjects, 68% resulted in a loss of function in vitro. They were found in 1.72% of obese versus 0.15% of nonobesed subjects (P = 6.9 x 10(-10)). Among the families, abnormal eating behavior was more frequent in both MC4R-deficient children and adults than in noncarriers. Although BMI was inversely associated with educational status in noncarrier adults, no such relationship was seen in MC4R mutation carriers. We observed a generational effect, with a penetrance of 40% in MC4R-deficient adults aged >52 years, 60% in 18- to 52-year-old adults, and 79% in children. The longitudinal study of adult carriers showed an increasing age-dependent penetrance (37% at 20 years versus 60% at >40 years).

CONCLUSIONS

We have established a robust estimate of age-related penetrance for MC4R deficiency and demonstrated a generational effect on penetrance, which may relate to the development of an "obesogenic" environment. It remains to be seen whether appropriate manipulation of environmental factors may contribute to preventing the development of obesity even in those strongly genetically predisposed to it.

Bivariate genome linkage analysis suggests pleiotropic effects on chromosomes 20p and 3p for body fat mass and lean mass

Total body fat mass (TBFM) and total body lean mass (TBLM) are the major components of the human body. Although these highly correlated phenotypic traits are frequently used to characterize obesity, the specific shared genetic factors that influence both traits remain largely unknown. Our study was aimed at identifying common quantitative trait loci (QTLs) contributing to both TBFM and TBLM. We performed a whole genome-linkage scan study in a large sample of 3255 subjects from 420 Caucasian pedigrees. Bivariate linkage analysis was carried out in both the entire sample and gender-specific subsamples. Several potentially important genomic regions that may harbour QTLs important for TBFM and TBLM were identified. For example, 20p12-11 achieved a LOD score of 2.04 in the entire sample and, in the male subsample, two genomic regions, 20p12 (LOD=2.08) and 3p26-25 (LOD=1.92), showed suggestive linkage. In addition, two-point linkage analyses for chromosome X showed suggestive linkages on Xp22 in the entire sample (LOD=2.14) and significant linkage on Xp22 in the female subsample (LOD=3.05). Complete pleiotropy was suggested for 20p12 and 3p26-25 in males. Our results suggest that QTLs on chromosomes 20p12, 3p26-25 and Xp22 may jointly influence TBFM and TBLM. Further fine mapping and gene identification studies for these pleiotropic effects are needed.

Human solute carrier SLC6A14 is the beta-alanine carrier

The beta-alanine carrier was characterized functionally in the 1960s to 1980s at the luminal surface of the ileal mucosal wall and is a Na(+)- and Cl(-)-dependent transporter of a number of essential and non-essential cationic and dipolar amino acids including lysine, arginine and leucine. beta-Alanine carrier-like function has not been demonstrated by any solute carrier transport system identified at the molecular level. A series of experiments were designed to determine whether solute carrier SLC6A14 is the molecular correlate of the intestinal beta-alanine carrier, perhaps the last of the classical intestinal amino acid transport systems to be identified at the molecular level. Following expression of the human SLC6A14 transporter in Xenopus laevis oocytes, the key functional characteristics of the beta-alanine carrier, identified previously in situ in ileum, were demonstrated for the first time. The transport system is both Na(+) and Cl(-) dependent, can transport non-alpha-amino acids such as beta-alanine with low affinity, and has a higher affinity for dipolar and cationic amino acids such as leucine and lysine. N-methylation of its substrates reduces the affinity for transport. These observations confirm the hypothesis that the SLC6A14 gene encodes the transport protein known as the beta-alanine carrier which, due to its broad substrate specificity, is likely to play an important role in absorption of essential nutrients and drugs in the distal regions of the human gastrointestinal tract.

Obesity and obstructive sleep apnea-hypopnea syndrome

Obstructive sleep apnea-hypopnea syndrome involves recurring episodes of total obstruction (apnea) or partial obstruction (hypopnea) of airways during sleep. Obstructive sleep apnea-hypopnea syndrome affects mainly obese individuals and it is defined by an apnea-hypopnea index of five or more episodes per hour associated with daytime somnolence. In addition to anatomical factors and neuromuscular and genetic factors, sleep disorders are also involved in the pathogenesis of sleep apnea. Obesity affects upper airway anatomy because of fat deposition and metabolic activity of adipose tissue. Obstructive sleep apnea-hypopnea syndrome and metabolic syndrome have several characteristics such as visceral obesity, hypertension and insulin resistance. Inflammatory cytokines might be related to the pathogenesis of sleep apnea and metabolic syndrome. Sleep apnea treatment includes obesity treatment, use of equipment such as continuous positive airway pressure, drug therapy and surgical procedures in selected patients. Currently, there is no specific drug therapy available with proven efficacy for the treatment of obstructive sleep apnea-hypopnea syndrome. Body-weight reduction results in improvement of sleep apnea, and obesity treatment must be emphasized, including lifestyle changes, anti-obesity drugs and bariatric surgery.

Fine mapping of mouse QTLs for fatness using SNP data

Quantitative trait loci (QTLs), as determined in crossbred studies, are a valuable resource to identify genes responsible for the corresponding phenotypic variances. Due to their broad chromosomal extension of some dozens of megabases, further steps are necessary to bring the number of candidate genes that underlie the detected effects to a reasonable order of magnitude. We use a set of 13,370 SNPs to identify informative haplotype blocks in 22 mouse QTLs for fatness. About half of the genes in a typical QTL overlap with haplotype blocks, which are different for the two base mouse lines, and which, thus, qualify for further analysis. For these genes we collect four more pieces of evidence for association with fat accumulation, namely (1) homology to genes identified in a Caenorhabditis elegans knock-out experiment as fat decreasing or fat increasing, (2) the overexpression of the genes in mouse fat, liver, muscle, or hypothalamus tissues, (3) the occurrence of a gene in several independently found QTLs, and (4) the information provided by gene ontology, to achieve a ranked list of 131 candidate genes. Ten genes fulfill three or four of the above sketched criteria and are discussed briefly, 121 further genes fulfilling two criteria are provided as on-line material. Viewing the genomic region of fatness-related QTLs under several different aspects is appropriate to assess the many thousands of genes that reside in such QTLs and to produce lists of more robust candidate genes.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Genes implicated in serotonergic and dopaminergic functioning predict BMI categories

OBJECTIVE

This study addressed the hypothesis that variation in genes associated with dopamine function (SLC6A3, DRD2, DRD4), serotonin function (SLC6A4, and regulation of monoamine levels (MAOA) may be predictive of BMI categories (obese and overweight + obese) in young adulthood and of changes in BMI as adolescents transition into young adulthood. Interactions with gender and race/ethnicity were also examined.

METHODS AND PROCEDURES

Participants were a subsample of individuals from the National Longitudinal Study of Adolescent Health (Add Health), a nationally representative sample of adolescents followed from 1995 to 2002. The sample analyzed included a subset of 1,584 unrelated individuals with genotype data. Multiple logistic regressions were conducted to evaluate the associations between genotypes and obesity (BMI > 29.9) or overweight + obese combined (BMI > or = 25) with normal weight (BMI = 18.5-24.9) as a referent. Linear regression models were used to examine change in BMI from adolescence to young adulthood.

RESULTS

Significant associations were found between SLC6A4 5HTTLPR and categories of BMI, and between MAOA promoter variable number tandem repeat (VNTR) among men and categories of BMI. Stratified analyses revealed that the association between these two genes and excess BMI was significant for men overall and for white and Hispanic men specifically. Linear regression models indicated a significant effect of SLC6A4 5HTTLPR on change in BMI from adolescence to young adulthood.

DISCUSSION

Our findings lend further support to the involvement of genes implicated in dopamine and serotonin regulation on energy balance.

The Gene-Diet Attica investigation on childhood obesity (GENDAI): overview of the study design

BACKGROUND

There is limited evidence on the role of genetic and environmental factors in the etiology of childhood obesity, a major health problem worldwide.

METHODS

The Gene-Diet Attica Investigation on childhood obesity (GENDAI) evaluates the contributions to and pivotal interactions of genetic, dietary and physical activity variables on children's weight. We describe the design, methodology, and present preliminary data. So far, 920 participants have been enrolled and the final projected sample is 1000 fifth- and sixth-grade students from selected elementary schools in Attica (10-14 years). In this school-based cross-sectional study, more than 400 variables describing anthropometric, dietary, clinical, genetic, sociodemographic and other lifestyle characteristics were collected from participating children and their families.

RESULTS

Increased body mass index was identified in 39.3% of subjects (30.5% overweight and 8.8% obese), with males presenting a more unfavorable metabolic profile, i.e., higher blood lipids, glucose, and insulin, compared to females. Normal-weight children had a significant advantage when compared to all children of increased weight in terms of lipid profile and insulin, as well as behaviors examined. Specifically, normal-weight children exhibited less skipping of meals and less sedentary activities.

CONCLUSIONS

The overall high prevalence of overweight and obesity in the current population is significant and underscores the need for environmental and genetic information that will shed light on the phenomenon of childhood obesity.