Robert Dent

Medical Sequencing at the Extremes of Human Body Mass

Body weight is a quantitative trait with significant heritability in humans. To identify potential genetic contributors to this phenotype, we resequenced the coding exons and splice junctions of 58 genes in 379 obese and 378 lean individuals. Our 96-Mb survey included 21 genes associated with monogenic forms of obesity in humans or mice, as well as 37 genes that function in body weight-related pathways. We found that the monogenic obesity-associated gene group was enriched for rare nonsynonymous variants unique to the obese population compared with the lean population. In addition, computational analysis predicted a greater fraction of deleterious variants within the obese cohort. Together, these data suggest that multiple rare alleles contribute to obesity in the population and provide a medical sequencing-based approach to detect them.

Association of functionally significant Melanocortin-4 but not Melanocortin-3 receptor mutations with severe adult obesity in a large North American case–control study

Functionally significant heterozygous mutations in the Melanocortin-4 receptor (MC4R) have been implicated in 2.5% of early onset obesity cases in European cohorts. The role of mutations in this gene in severely obese adults, particularly in smaller North American patient cohorts, has been less convincing. More recently, it has been proposed that mutations in a phylogenetically and physiologically related receptor, the Melanocortin-3 receptor (MC3R), could also be a cause of severe human obesity. The objectives of this study were to determine if mutations impairing the function of MC4R or MC3R were associated with severe obesity in North American adults. We studied MC4R and MC3R mutations detected in a total of 1821 adults (889 severely obese and 932 lean controls) from two cohorts. We systematically and comparatively evaluated the functional consequences of all mutations found in both MC4R and MC3R. The total prevalence of rare MC4R variants in severely obese North American adults was 2.25% (CI(95%): 1.44-3.47) compared with 0.64% (CI(95%): 0.26-1.43) in lean controls (P < 0.005). After classification of functional consequence, the prevalence of MC4R mutations with functional alterations was significantly greater when compared with controls (P < 0.005). In contrast, the prevalence of rare MC3R variants was not significantly increased in severely obese adults [0.67% (CI(95%): 0.27-1.50) versus 0.32% (CI(95%): 0.06-0.99)] (P = 0.332). Our results confirm that mutations in MC4R are a significant cause of severe obesity, extending this finding to North American adults. However, our data suggest that MC3R mutations are not associated with severe obesity in this population.

Lack of Support for the Association between GAD2 Polymorphisms and Severe Human Obesity

The demonstration of association between common genetic variants and chronic human diseases such as obesity could have profound implications for the prediction, prevention, and treatment of these conditions. Unequivocal proof of such an association, however, requires independent replication of initial positive findings. Recently, three (−243 A>G, +61450 C>A, and +83897 T>A) single nucleotide polymorphisms (SNPs) within glutamate decarboxylase 2 (GAD2) were found to be associated with class III obesity (body mass index > 40 kg/m2). The association was observed among 188 families (612 individuals) segregating the condition, and a case-control study of 575 cases and 646 lean controls. Functional data supporting a pathophysiological role for one of the SNPs (−243 A>G) were also presented. The gene GAD2 encodes the 65-kDa subunit of glutamic acid decarboxylase—GAD65. In the present study, we attempted to replicate this association in larger groups of individuals, and to extend the functional studies of the −243 A>G SNP. Among 2,359 individuals comprising 693 German nuclear families with severe, early-onset obesity, we found no evidence for a relationship between the three GAD2 SNPs and obesity, whether SNPs were studied individually or as haplotypes. In two independent case-control studies (a total of 680 class III obesity cases and 1,186 lean controls), there was no significant relationship between the −243 A>G SNP and obesity (OR = 0.99, 95% CI 0.83–1.18, p = 0.89) in the pooled sample. These negative findings were recapitulated in a meta-analysis, incorporating all published data for the association between the −243G allele and class III obesity, which yielded an OR of 1.11 (95% CI 0.90–1.36, p = 0.28) in a total sample of 1,252 class III obese cases and 1,800 lean controls. Moreover, analysis of common haplotypes encompassing the GAD2 locus revealed no association with severe obesity in families with the condition. We also obtained functional data for the −243 A>G SNP that does not support a pathophysiological role for this variant in obesity. Potential confounding variables in association studies involving common variants and complex diseases (low power to detect modest genetic effects, overinterpretation of marginal data, population stratification, and biological plausibility) are also discussed in the context of GAD2 and severe obesity.

Changes in Body Weight and Psychotropic Drugs: A Systematic Synthesis of the Literature

Introduction Psychotropic medication use is associated with weight gain. While there are studies and reviews comparing weight gain for psychotropics within some classes, clinicians frequently use drugs from different classes to treat psychiatric disorders. Objective To undertake a systematic review of all classes of psychotropics to provide an all encompassing evidence-based tool that would allow clinicians to determine the risks of weight gain in making both intra-class and interclass choices of psychotropics. Methodology and Results We developed a novel hierarchical search strategy that made use of systematic reviews that were already available. When such evidence was not available we went on to evaluate randomly controlled trials, followed by cohort and other clinical trials, narrative reviews, and, where necessary, clinical opinion and anecdotal evidence. The data from the publication with the highest level of evidence based on our hierarchical classification was presented. Recommendations from an expert panel supplemented the evidence used to rank these drugs within their respective classes. Approximately 9500 articles were identified in our literature search of which 666 citations were retrieved. We were able to rank most of the psychotropics based on the available evidence and recommendations from subject matter experts. There were few discrepancies between published evidence and the expert panel in ranking these drugs. Conclusion Potential for weight gain is an important consideration in choice of any psychotropic. This tool will help clinicians select psychotropics on a case-by-case basis in order to minimize the impact of weight gain when making both intra-class and interclass choices.

Distinct skeletal muscle fiber characteristics and gene expression in diet-sensitive versus diet-resistant obesity

Inter-individual variability in weight gain and loss under energy surfeit and deficit conditions, respectively, are well recognized but poorly understood phenomena. We documented weight loss variability in an intensively supervised clinical weight loss program and assessed skeletal muscle gene expression and phenotypic characteristics related to variable response to a 900 kcal regimen. Matched pairs of healthy, diet-compliant, obese diet-sensitive (ODS) and diet-resistant (ODR) subjects were defined as those in the highest and lowest quintiles for weight loss rate. Physical activity energy expenditure was minimal and comparable. Following program completion and weight stabilization, skeletal muscle biopsies were obtained. Gene expression analysis of rectus femoris and vastus lateralis indicated upregulation of genes and gene sets involved in oxidative phosphorylation and glucose and fatty acid metabolism in ODS compared with ODR. In vastus lateralis, there was a higher proportion of oxidative (type I) fibers in ODS compared with ODR women and lean controls, fiber hypertrophy in ODS compared with ODR women and lean controls, and lower succinate dehydrogenase in oxidative and oxidative-glycolytic fibers in all obese compared with lean subjects. Intramuscular lipid content was generally higher in obese versus lean, and specifically higher in ODS vs. lean women. Altogether, our findings demonstrate differences in muscle gene expression and fiber composition related to clinical weight loss success.

Effects of Weight Loss on Airway Responsiveness in Obese Adults With Asthma: Does Weight Loss Lead to Reversibility of Asthma?

BACKGROUND The growing epidemics of obesity and asthma are major public health concerns. Although asthma-obesity links are widely studied, the effects of weight loss on asthma severity measured by airway hyperresponsiveness (AHR) have received limited attention. The main study objective was to examine whether weight reduction reduces asthma severity in obese adults with asthma. METHODS In a prospective, controlled, parallel-group study, we followed 22 obese participants with asthma aged 18 to 75 years with a BMI ≥ 32.5 kg/m2 and AHR (provocative concentration of methacholine causing a 20% fall in FEV1 [PC20] < 16 mg/mL). Sixteen participants followed a behavioral weight reduction program for 3 months, and six served as control subjects. The primary outcome was change in AHR over 3 months. Changes in lung function, asthma control, and quality of life were secondary outcomes. RESULTS At study entry, participant mean ± SD age was 44 ± 9 years, 95% were women, and mean BMI was 45.7 ± 9.2 kg/m2. After 3 months, mean weight loss was 16.5 ± 9.9 kg in the intervention group, and the control group had a mean weight gain of 0.6 ± 2.6 kg. There were significant improvements in PC20 (P = .009), FEV1 (P = .009), FVC (P = .010), asthma control (P < .001), and asthma quality of life (P = .003) in the intervention group, but these parameters remained unchanged in the control group. Physical activity levels also increased significantly in the intervention group but not in the control group. CONCLUSIONS Weight loss in obese adults with asthma can improve asthma severity, AHR, asthma control, lung function, and quality of life. These findings support the need to actively pursue healthy weight-loss measures in this population.

The INSIG2 rs7566605 genetic variant does not play a major role in obesity in a sample of 24,722 individuals from four cohorts

BackgroundIn a genome-wide association study performed in the Framingham Offspring Cohort, individuals homozygous for the rs7566605 C allele located upstream of insulin-induced gene 2 (INSIG2) were reported to incur an increased risk of obesity. This finding was later replicated in four out of five populations examined. The goal of the study reported here was to assess the role of the INSIG2 single nucleotide polymorphism (SNP) in susceptibility to obesity in the prospective longitudinal Atherosclerosis Risk in Communities (ARIC) study (n = 14,566) and in three other cohorts: the Coronary Artery Risk Development in Young Adults (CARDIA) study (n = 3,888), the Genetic Epidemiology Network of Arteriopathy (GENOA) study (n = 4,766), and extremely obese and lean individuals ascertained at the University of Ottawa (n = 1,502). The combined study sample is comprised of 24,722 white, African-American, and Mexican-American participants.MethodsDifferences in mean body mass index (BMI) and other anthropometric measures including weight, waist circumference, and waist-to-hip ratio were assessed by a general linear model in individuals categorized by INSIG2 rs7566605 genotype. Multivariable logistic regression was used to predict the risk of obesity (BMI ≥ 30 kg/m2).ResultsThere was no discernable variation in the frequencies of the three INSIG2 SNP genotypes observed between white, Hispanic, and African-American obese individuals and non-obese study subjects. When the relationship between rs7566605 and BMI considered either as a categorical variable or a continuous variable was examined, no significant association with obesity was found for participants in any of the four study populations or in a combined analysis (p = 0.38) under a recessive genetic model. There was also no association between the INSIG2 polymorphism and the obesity-related quantitative traits except for a reduced waist-to-hip ratio in white ARIC study participants homozygous for the C allele, and an increased waist-to-hip ratio in African-Americans in the ARIC cohort with the same genotype (p = 0.04 and p = 0.01, respectively). An association with waist-to-hip ratio was not seen when the combined study sample was analyzed (p = 0.74).ConclusionThese results suggest that the INSIG2 rs7566605 variant does not play a major role in determining obesity risk in a racially and ethnically diverse sample of 24,722 individuals from four cohorts.

A 680 kb duplication at the FTO locus in a kindred with obesity and a distinct body fat distribution.

Common intronic SNPs in the human fat mass and obesity associated (FTO) gene are strongly associated with body mass index (BMI). In mouse models, inactivation of the Fto gene results in a lean phenotype, whereas overexpression of Fto leads to increased food intake and obesity. The latter finding suggests that copy number variants at the FTO locus might be associated with extremes of adiposity. To address this question, we searched for rare, private or de novo copy number variation in a cohort of 985 obese and 869 lean subjects of European ancestry drawn from the extremes of the BMI distribution, genotyped on Affymetrix 6.0 arrays. A ∼680 kb duplication, confirmed by real-time PCR and G-to-FISH analyses, was observed between ∼rs11859825 and rs9932411 in a 68-year-old male with severe obesity. The duplicated region on chromosome 16 spans the entire genome-wide association studies risk locus for obesity, and further encompasses RBL2, AKTIP, RPGRIP1L and all but the last exon of the FTO gene. Affected family members exhibit a unique obesity phenotype, characterized by increased fat distribution in the shoulders and neck with a significantly increased neck circumference. This phenotype was accompanied by increased peripheral blood expression of RBL2 with no alteration in expression of FTO or other genes in the region. No other duplications or deletions in this region were identified in the cohort of obese and lean individuals or in a further survey of 4778 individuals, suggesting that large rare copy number variants surrounding the FTO gene are not a frequent cause of obesity.

Is Type 2 Diabetes Associated with Impaired Capacity for Weight Loss

a Department of Biochemistry, Microbiology, and Immunology, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada b Ruddy Canadian Cardiovascular Genetics Centre, Ottawa, Ontario, Canada c Atherogenomics Laboratory, and Division of Cardiology, Department of Medicine, University of Ottawa Heart Institute, Ottawa, Ontario, Canada d Ottawa Hospital Weight Management Clinic, Ottawa, Ontario, Canada

Impaired awareness of hypoglycemia in a man with type 1 diabetes

A 32-year-old man with a 22-year history of type 1 diabetes presented to the diabetes clinic following a loss of consciousness at home the previous evening. The incident, which occurred when the patient was awake, was witnessed by his wife. She administered intramuscular glucagon within minutes.