We have been examining polymorphisms in genes involved in the leptin signaling pathway, to identify gene variants impacting on body composition. We are currently studying a variant MC3R that is associated with adiposity in children and which appears to have functional significance for MC3R signal transduction. Children who were homozygous variant for both the polymorphisms (Thr6Lys and Val81Ile) had significantly greater BMI-SD score, fat mass, body circumference measurements, and higher plasma levels of insulin and leptin compared with wild type or heterozygous children. In vitro studies subsequently found that expression was significantly lower for the double mutant MC3R. Ongoing studies attempt to understand the mechanisms by which these sequence alterations may impact body weight. Recent studies suggest energy intake is increased in those having these polymorphisms. Knock-in mice expressing the human wild type and human double-mutant MC3R have recently been developed in collaboration with Dr. Westphal, and will be studied during the next two years. We have also recently investigated the BDNF-TrkB pathway in regards to body mass in children. We found serum BDNF was significantly lower in overweight children (p=0.03);We next explored if some obese individuals with low serum BDNF have mutations that alter BDNF function. In collaboration with Drs. Farooqi and ORahilly, we studied a one child with functional haploinsufficiency for BDNF with increased ad libitum food intake and severe early onset obesity. We then assessed the role of BDNF haploinsufficiency as a cause of obesity in patients with syndromes that are due to deletions in the vicinity of 11p14.1, where the human BDNF gene is found. Using a comparative genomic hybridization approach, we examined genotype-phenotype relationships in patients with the WAGR (Wilms Tumor, Aniridia, Genitourinary, and Renal abnormalities) syndrome. In 33 subjects with heterozygous 11p deletions ranging in size from 1.0-26.5 Mb, 19 had regions of deletion that involved the BDNF gene (BDNF+/-). Compared to those with intact BDNF (BDNF+/+), BDNF+/- had significantly greater body mass during childhood, starting at age 2y. 100% of BDNF+/- were overweight by age 10y vs. only 20% of BDNF+/+ (P<0.0001). Parent-completed hyperphagia questionnaires also suggested significantly greater hyperphagic behavior, drive, and severity for BDNF+/- than BDNF+/+. Mean serum BDNF was approximately 50% lower among BDNF+/- (P=0.001). Analysis of the telomeric deletion boundaries indicated the presence of a critical region for pediatric-onset overweight within 80 kb of BDNF exon 1. These findings provide strong evidence for the role of BDNF in human energy homeostasis. A full characterization of the energy intake and expenditure of subjects with WAGR syndrome and other 11p deletion syndromes is underway. Other studies are directed at understanding other genetic, physiological, psychological, and metabolic factors that place children at-risk for undue weight gain. We have found that leptin is an important predictor of weight gain in children: those with high leptin gain even more weight when followed longitudinally. We have validated prior associations between body weight and variation in the FTO gene and found SNPs in FTO to be associated with behavioral loss of control over eating and are actively studying SNPs in histaminergic receptors for their associations with body weight and energy intake. Recent investigations concentrating on binge eating behaviors in children suggest that such behaviors are also associated with adiposity in children, predict future weight gain in children at-risk for overweight, and predict both greater energy consumption during meals and decreased satiety after eating. The ability to consume large quantities of palatable foods, especially when coupled with decreased subsequent satiety, may play a role in the greater weight gain found in binge eating children. These data also suggest that interventions targeting disordered eating behaviors may potentially be useful in preventing excessive fat gain in children prone to obesity. An ongoing protocol examines efficacy of interpersonal therapy as a weight gain preventive strategy. Given the rapid increase in the prevalence of obesity, the development of treatments for obesity in childhood is urgently needed. In three clinical protocols, we have studied pharmacotherapeutic approaches to the control of body weight, We recently completed a placebo-controlled randomized trial, studying whether the weight loss of African American and Caucasian children and adolescents who have obesity-related comorbidities was improved by the use of orlistat 120 mg TID. 200 adolescents, 61% African American, mean BMI 41.70.6 kg/m2 were studied. Adolescents treated with orlistat lost more weight (orlistat -2.90.7 vs. placebo -0.60.7 kg, P=0.011), BMI units (-1.720.24 vs. -0.700.24 kg/m2, P=0.002), and fat mass (-3.90.8 vs. -1.40.8 kg, P=0.029). We concluded that orlistat added to a behavioral program significantly improved weight loss over a 6-month interval, but had little impact on obesity-related co-morbid conditions in overweight adolescents. A second recently-completed study examined the mechanism by which metformin may affect the body weight of younger children who have hyperinsulinemia, and are therefore at risk for later development of type 2 diabetes. We conducted a single-center, 6-month, randomized, double-blind, placebo-controlled trial of the effects of metformin, 1000 mg BID administered with meals, in 100 severely overweight children (6-12y) who manifested hyperinsulinemia and insulin resistance. Subjects participated in a monthly weight reduction program. Compared to placebo-treated children, those randomized to metformin decreased BMI (metformin -0.910.3 vs. placebo +0.230.3 kg/m2, P=0.006), BMI-Z score (-0.110.02 vs. -0.040.02, P=0.02), and body fat mass (-1.40.7 vs. +2.10.7 kg, P<0.001) to a significantly greater extent. Serum glucose (-2.40.9 vs. +1.61.2 mg/dL, P=0.018), HOMA-IR (-0.190.4 vs. +0.950.4, P=0.05), and total cholesterol (-9.92.7 vs. +1.14.8, P=0.04) also decreased more in metformin-treated compared to placebo-treated children. We concluded that metformin, added to a monthly behavioral program, significantly improved weight loss, insulin resistance, and cholesterol over a 6-month interval in severely overweight, insulin-resistant children. An ongoing trial investigates modulation of histaminergic tone for its impact on energy intake. We expect to initiate additional translational trials in the coming year related to modulation of the leptin signaling pathway.
Showing the most recent 10 out of 130 publications
