Genetics of obesity in humans and animal models

TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline) inhibits the consumption of “snacks” in mice

There is considerable evidence that alpha2-adrenergic receptor activity exerts a pivotal role in initiation of feeding behavior. The appetite suppressant and monoamine release effects of TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline), a putative selective alpha2-adrenergic compound, were compared to those of fenfluramine, a reference drug that produces an anorectic effect via presynaptic release and reuptake inhibition of serotonin. The drugs were administered to two groups of mice that had learned to consume either sweet milk or chocolate pellets (i.e. "snacks") during the low-activity/reduced-feeding "light" portion of their light/dark cycle. The selectivity of the drugs to suppress the consumption of snacks was determined by comparing doses of each drug that inhibited the animals' consumption of snacks to doses of each drug that have been shown, or were shown, to impact the motor (i.e. locomotor, rotarod, and inclined-screen side effect-like tests) or conditioned taste aversion (CTA) behavior of mice. An evaluation of TDIQ as a releaser of monoamines was determined in rodent brain synaptosomes. The administration of TDIQ or fenfluramine inhibited the consumption of the snacks, and a comparison of their ED50 doses indicated that TDIQ is about 3 times more potent than fenfluramine (1.3 mg/kg vs. 4.2 mg/kg, respectively) in the sweet milk test and almost equipotent to fenfluramine (19.4 mg/kg vs. 18.4 mg/kg, respectively) in the chocolate pellet assay. The selectivity of the appetite suppressant effect of TDIQ was differentiated from that of fenfluramine on the basis that TDIQ exhibited a wide separation between its dose-response effects that suppressed snack consumption and its minimal effects in tests that measured behavioral impairment. Moreover, TDIQ was distinguished from fenfluramine in that it displayed very low potencies as a presynaptic releaser of monoamines. Finally, TDIQ (0.3 mg/kg-30.0 mg/kg) did not induce a conditioned taste aversion. TDIQ may represent a novel chemical entity that exhibits a significantly favorable therapeutic-like (i.e. appetite suppressant) effect to side effect-like ratio.

Genetic and genomic analysis of a fat mass trait with complex inheritance reveals marked sex specificity

The integration of expression profiling with linkage analysis has increasingly been used to identify genes underlying complex phenotypes. The effects of gender on the regulation of many physiological traits are well documented; however, “genetical genomic” analyses have not yet addressed the degree to which their conclusions are affected by sex. We constructed and densely genotyped a large F2 intercross derived from the inbred mouse strains C57BL/6J and C3H/HeJ on an apolipoprotein E null (ApoE^−/−) background. This BXH.ApoE^−/− population recapitulates several “metabolic syndrome” phenotypes. The cross consists of 334 animals of both sexes, allowing us to specifically test for the dependence of linkage on sex. We detected several thousand liver gene expression quantitative trait loci, a significant proportion of which are sex-biased. We used these analyses to dissect the genetics of gonadal fat mass, a complex trait with sex-specific regulation. We present evidence for a remarkably high degree of sex-dependence on both the cis and trans regulation of gene expression. We demonstrate how these analyses can be applied to the study of the genetics underlying gonadal fat mass, a complex trait showing significantly female-biased heritability. These data have implications on the potential effects of sex on the genetic regulation of other complex traits.

Although their genomes are nearly identical, the males and females of a species exhibit striking differences in many traits, including complex traits such as obesity. This study combines genetic and genomic tools to identify in parallel quantitative trait loci (QTLs) for a measure of gonadal fat mass and for expression of transcripts in the liver. The results are used to explore the relationship between genetic variation, sexual differentiation, and obesity in the mouse model. Using over 300 intercross progeny of two inbred mouse strains, five loci in the genome were found to be highly correlated with abdominal fat mass. Four of the five loci exhibited opposite effects on obesity in the two sexes, a phenomenon known as sexual antagonism. To identify candidate genes that may be involved in obesity through their expression in the liver, global gene expression analysis was employed using microarrays. Many of these expression QTLs also show sex-specific effects on transcription. A hotspot for trans-acting QTLs regulating the expression of transcripts whose abundance is correlated with gonadal fat mass was identified on Chromosome 19. This region of the genome colocalizes with a clinical QTL for gonadal fat mass, suggesting that it harbors a good candidate gene for obesity.

The future of genetic research on appetitive behavior

The risk for developing obesity has a significant genetic component. Several quantitative trait loci and candidate genes have been identified using current methodological approaches however the information gained thus far is insufficient to adequately explain the genetics underlying human obesity. The completion of a draft of the human genome sequence, the potential benefit of single nucleotide polymorphisms association studies for identifying risk conferring alleles, and developing functional genomics technologies promise to accelerate obesity gene discovery. These advances, used with current evaluative tools (murine molecular genetic techniques), may increase our understanding of human obesity, and ultimately provide better approaches to diagnosis and treatment.

Genetic contributions to body weight in mice: relationship of exploratory behavior to weight

OBJECTIVE

The A/J and C57BL/6J mouse strains differ markedly in their exploratory behavior and their weight gain on a high-fat diet. We examined the genetic contributions of exploratory behavior to body weight and tested for shared, pleiotropic loci influencing energy homeostasis.

RESEARCH METHODS AND PROCEDURES

Segregating (AxB6)F2 intercross (n = 514) and (B6AF1xA/J)N2 backcross (N = 223) populations were studied, phenotyping for weight and exploratory behaviors. Relationships among traits were analyzed by correlations. Weight traits were dissected with a genome-wide scan.

RESULTS

Modest correlations were found between exploratory behaviors and weight, explaining 2% to 14% of the variance. Quantitative trait loci (QTL) for body weight at 8 weeks (wgt8), 10 weeks (wgt10), and 2-week weight gain (difference between weeks 8 and 10) on a 6% fat diet were mapped. Two QTL on chromosome 1 (peaks at 66 cM and 100 cM; Bw8q1) affected wgt8 [likelihood of the odds ratio (Lod), 3.0 and 4.4] and wgt10 (Lod, 2.2 and 3.4), respectively. In the backcross, a significant QTL on chromosome 4 (peak at 66 cM; Bw8q2) affected wgt 8 (Lod, 3.3) and wgt10 (Lod, 3.1). For 2-week weight gain, suggestive QTL were mapped on chromosomes 4 and 6. The chromosome 6 QTL region overlaps a human 7q locus for obesity. A search for between-strain sequence polymorphisms in the leptin and NPY genes was unrevealing.

DISCUSSION

In mice, loci influencing exploratory activity play a modest role in body-weight regulation. Some forms of obesity may emerge from loci regulating normal body weight.

Pharmacological approaches for the treatment of obesity

The high incidence of obesity, its multifactorial nature, the complexity and lack of knowledge of the bodyweight control system, and the scarcity of adequate therapeutics have fuelled anti-obesity drug development during a considerable number of years. Irrespective of the efforts invested by researchers and companies, few products have reached a minimum level of effectiveness, and even fewer are available in medical practice. As a consequence of anti-obesity research, our knowledge of the bodyweight control system increased but, despite this, the pharmacological approaches to the treatment of obesity have not resulted yet in effective drugs. This review provides a panoramic of the multiple different approaches developed to obtain workable drugs. These approaches, however, rely in only four main lines of action: control of energy intake, mainly through modification of appetite;control of energy expenditure, essentially through the increase of thermogenesis;control of the availability of substrates to cells and tissues through hormonal and other metabolic factors controlling the fate of the available energy substrates; andcontrol of fat reserves through modulation of lipogenesis and lipolysis in white adipose tissue. A large proportion of current research is centred on neuropeptidic control of appetite, followed by the development of drugs controlling thermogenic mechanisms and analysis of the factors controlling adipocyte growth and fat storage. The adipocyte is also a fundamental source of metabolic signals, signals that can be intercepted, modulated and used to force the brain to adjust the mass of fat with the physiological means available. The large variety of different approaches used in the search for effective anti-obesity drugs show both the deep involvement of researchers on this field and the large amount of resources devoted to this problem by pharmaceutical companies. Future trends in anti-obesity drug research follow closely the approaches outlined; however, the increasing mass of information on the molecular basis of bodyweight control and obesity will in the end prevail in our search for effective and harmless anti-obesity drugs.

Genetics of body-weight regulation

The role of genetics in obesity is twofold. Studying rare mutations in humans and model organisms provides fundamental insight into a complex physiological process, and complements population-based studies that seek to reveal primary causes. Remarkable progress has been made on both fronts, and the pace of advance is likely to accelerate as functional genomics and the human genome project expand and mature. Approaches based on mendelian and quantitative genetics may well converge, and lead ultimately to more rational and selective therapies.

Hypertension and nephropathy in diabetes mellitus: what is inherited and what is acquired?

Prolonged duration of diabetes mellitus, poor long term glycemic control and raised blood pressure have all been clearly related to the development of diabetic nephropathy. Evidence exists to suggest that a subset of individuals with diabetes have a genetic predisposition to diabetic nephropathy. Cases of diabetic nephropathy cluster in families and a parental history of hypertension is more common in patients with diabetic nephropathy. Current evidence suggests an important role for hypertension in the genetic susceptibility to diabetic nephropathy but the extent of this is unknown. While cellular and animal studies have generated a plethora of data regarding mechanisms involved in the role of hypertension and diabetic nephropathy, these are not helpful for drawing conclusions in humans. In the following review, we examine the available clinical, epidemiologic and family studies to assess the relationship between the development of hypertension and diabetic nephropathy in IDDM and NIDDM. We will demonstrate the differences in the epidemiology of hypertension in diabetes depending on the type of diabetes and thus, move the emphasis of nephropathy susceptibility away from hypertension per se. We hope to emphasize instead the homogeneity of nephropathy risk in both IDDM and NIDDM and also the idea that a common genetic susceptibility exists for all types of diabetes and is conditional on cumulative exposure to hyperglycemia. Regarding the interaction of hypertension and nephropathy in diabetes mellitus, any conclusions at this time about what is inherited and what is acquired must be regarded as speculative. However we will discuss some potential mechanisms of hypertension in the evolution of nephropathy and we will allude to the role for novel genetic studies in the search for nephropathy susceptibility gene(s).