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 few 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) and have 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. We are also exploring other syndromes associated with obesity that may be associated with dysregulation of leptin signaling, including Bardet Biedl syndrome (in collaboration with Dr. Biesecker) and Alstrom Syndrome. 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. Other investigations have examined how heat is dissipated in obese individuals and tested the hypothesis that alterations in heat management may contribute to the development of obesity. 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 is urgently needed. In four clinical protocols, we have studied pharmacotherapeutic approaches to the control of body weight. Two placebo-controlled randomized trials studied the effects on weight loss and on obesity-related comorbidities in children and adolescents. Orlistat 120 mg TID was studied in 200 adolescents, 61% African American, mean BMI 41.70.6 kg/m2. 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), but had little impact on obesity-related co-morbid conditions in overweight adolescents. Metformin 1000 mg BID was studied 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. These children are engaged in an ongoing 3-year open-label extension protocol to examine the long-term impact of metformin therapy on weight. Two other recently-completed trials investigated the effects of calcium on weight over two years and the impact of modulation of histaminergic tone on energy intake. We expect to initiate additional translational trials in the coming year related to modulation of the leptin signaling pathway.
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