Endocrine phenotype of 6q16.1-q21 deletion involving SIM1 and Prader-Willi syndrome-like features.

Proximal interstitial 6q deletion involving Single-minded 1 (SIM1) gene causes a syndromic form of obesity mimicking Prader-Willi syndrome. In addition to obesity, Prader-Willi syndrome includes several other endocrinopathies, such as hypothyroidism, growth hormone deficiency, and hypogonadotropic hypogonadism. The endocrine phenotype of interstitial 6q deletion remains largely unknown, although clinical similarities between Prader-Willi syndrome and interstitial 6q deletion suggest endocrine abnormalities also may contribute to the interstitial 6q deletion phenotype. This report describes the endocrine phenotype in a propositus with the Prader-Willi-like syndrome associated with an interstitial 6q deletion including the SIM1 gene. Detailed endocrine evaluation of the propositus during childhood and adolescence revealed hypopituitarism, though initial endocrine evaluations during infancy were unremarkable. Our patient raises the possibility that hypopituitarism may be part of the phenotype, especially short stature, caused by interstitial 6q deletion. SIM1 plays an important role in the development of neuroendocrine lineage cells, implicating SIM1 haploinsufficiency in the pathophysiology of hypopituitarism seen in our propositus. Early identification of endocrine abnormalities can improve clinical outcome by allowing timely introduction of hormone replacement therapy. Hence, we suggest that detailed endocrine evaluation and longitudinal endocrine follow up be performed in individuals with proximal interstitial 6q deletion involving SIM1.

Ablation of Sim1 neurons causes obesity through hyperphagia and reduced energy expenditure.

Single-minded 1 (Sim1) is a transcription factor necessary for development of the paraventricular nucleus of the hypothalamus (PVH). This nucleus is a critical regulator of appetite, energy expenditure and body weight. Previously we showed that Sim1(+/-) mice and conditional postnatal Sim1(-/-) mice exhibit hyperphagia, obesity, increased linear growth and susceptibility to diet-induced obesity, but no decrease in energy expenditure. Bilateral ablation of the PVH causes obesity due to hyperphagia and reduced energy expenditure. It remains unknown whether Sim1 neurons regulate energy expenditure. In this study, Sim1cre mice were bred to homozygous inducible diphtheria toxin receptor (iDTR) mice to generate mice expressing the simian DTR in Sim1 cells. In these mice, Sim1 neuron ablation was performed by intracerebroventricular (ICV) injection of diphtheria toxin. Compared to controls, mice with Sim1 neuron ablation became obese (with increased fat mass) on a chow diet due to increased food intake and reduced energy expenditure. In post-injection mice, we observed a strong inverse correlation between the degree of obesity and hypothalamic Sim1 expression. The reduction in baseline energy expenditure observed in these mice was accompanied by a reduction in activity. This reduction in activity did not fully account for the reduced energy expenditure as these mice exhibited decreased resting energy expenditure, decreased body temperature, decreased brown adipose tissue temperature, and decreased UCP1 expression suggesting an impairment of thermogenesis. In injected mice, hypothalamic gene expression of Sim1, oxytocin (OXT) and thyrotropin releasing hormone (TRH) was reduced by about 50%. These results demonstrate that Sim1 neurons in adult mice regulate both food intake and energy expenditure. Based on the body of work in the field, feeding regulation by Sim1 neurons likely occurs in both the PVH and medial amygdala, in contrast to energy expenditure regulation by Sim1 neurons, which likely is localized to the PVH.

Postnatal Sim1 deficiency causes hyperphagic obesity and reduced Mc4r and oxytocin expression.

Single-minded 1 (SIM1) mutations are one of the few known causes of nonsyndromic monogenic obesity in both humans and mice. Although the role of Sim1 in the formation of the hypothalamus has been described, its postdevelopmental, physiological functions have not been well established. Here we demonstrate that postnatal CNS deficiency of Sim1 is sufficient to cause hyperphagic obesity. We conditionally deleted Sim1 after birth using CaMKII-Cre (alpha-calcium/calmodulin-dependent protein kinase II-Cre) lines to recombine a floxed Sim1 allele. Conditional Sim1 heterozygotes phenocopied germ line Sim1 heterozygotes, displaying hyperphagic obesity and increased length. We also generated viable conditional Sim1 homozygotes, demonstrating that adult Sim1 expression is not essential for mouse or neuron survival and revealing a dosage-dependent effect of Sim1 on obesity. Using stereological cell counting, we showed that the phenotype of both germ line heterozygotes and conditional Sim1 homozygotes was not attributable to global hypocellularity of the paraventricular nucleus (PVN) of the hypothalamus. We also used retrograde tract tracing to demonstrate that the PVN of germ line heterozygous mice projects normally to the dorsal vagal complex and the median eminence. Finally, we showed that conditional Sim1 homozygotes and germ line Sim1 heterozygotes exhibit a remarkable decrease in hypothalamic oxytocin (Oxt) and PVN melanocortin 4 receptor (Mc4r) mRNA. These results demonstrate that the role of Sim1 in feeding regulation is not limited to formation of the PVN or its projections and that the hyperphagic obesity in Sim1-deficient mice may be attributable to changes in the leptin-melanocortin-oxytocin pathway.

Screening for type 2 diabetes in obese youth.

OBJECTIVE: To assess available blood tests as potential screening tools for impaired glucose tolerance (IGT) and type 2 diabetes mellitus (T2DM). METHODS: We studied 468 obese (BMI mean: 34.4 kg/m(2)) children, including a subgroup with serum fasting insulin levels of >15 microIU/mL. Fasting laboratory tests included measurements of serum glucose and insulin, hemoglobin A1c (HbA1c), and 1,5-anhydroglucitol (insulin-resistant subgroup only) levels. An oral glucose-tolerance test was performed on each patient, and 2-hour postload serum glucose and insulin levels were obtained. Fasting blood glucose (BG), Homeostasis Model of Assessment for Insulin Resistance (HOMA-IR), HbA1c, and 1,5-anhydroglucitol values were used as predictors for exceeding various 2-hour BG cut-offs. Receiver operator characteristic curves were fitted to determine area-under-the-curve values as measures of screening efficacy. RESULTS: In the insulin-resistant subgroup, 3 (2%) patients had T2DM and 23 (12%) had IGT. Optimal sensitivity and specificity to detect T2DM were, respectively, 99% and 96% at HbA1c >or= 6.0%, and 96% and 88% at 1,5-anhydroglucitol < 17.0 microg/mL, with lower values for fasting BG and the HOMA-IR. In the entire study group, 9 (2%) patients had T2DM and 44 (9%) had IGT. Optimal sensitivity and specificity to detect T2DM were, respectively, 86% and 85% at HbA1c levels of 5.7%, 88%, and 93% at a fasting BG level of 104 mg/dL, and 62% and 70% at an HOMA-IR of 7.9. CONCLUSIONS: HbA1c, 1,5-anhydroglucitol, and fasting BG levels are good predictors of T2DM in obese children, whereas HOMA-IR values are not. HbA1c and 1,5-anhydroglucitol are excellent predictors of T2DM in insulin-resistant obese children.

Oxytocin deficiency mediates hyperphagic obesity of Sim1 haploinsufficient mice.

Single-minded 1 (Sim1) encodes a transcription factor essential for formation of the hypothalamic paraventricular nucleus (PVN). Sim1 haploinsufficiency is associated with hyperphagic obesity and increased linear growth in humans and mice, similar to the phenotype of melanocortin 4 receptor (Mc4r) mutations. PVN neurons in Sim1(+/-) mice are hyporesponsive to the melanocortin agonist melanotan II. PVN neuropeptides oxytocin (Oxt), TRH and CRH inhibit feeding when administered centrally. Consequently, we hypothesized that altered PVN neuropeptide expression mediates the hyperphagia of Sim1(+/-) mice. To test this hypothesis, we measured hypothalamic expression of PVN neuropeptides in Sim1(+/-) and wild-type mice. Oxt mRNA and peptide were decreased by 80% in Sim1(+/-) mice, whereas TRH, CRH, arginine vasopressin (Avp), and somatostatin mRNAs were decreased by 20-40%. Sim1(+/-) mice also showed abnormal regulation of Oxt but not CRH mRNA in response to feeding state. A selective Mc4r agonist activated PVN Oxt neurons in wild-type mice, supporting involvement of these neurons in melanocortin feeding circuits. To test whether Oxt itself regulates feeding, we measured the effects of central administration of an Oxt receptor antagonist or repeated doses of Oxt on food intake of Sim1(+/-) and wild-type mice. Sim1(+/-) mice were hypersensitive to the orexigenic effect of the Oxt receptor antagonist. Oxt decreased the food intake and weight gain of Sim1(+/-) mice at a dose that did not affect wild-type mice. Our results support the importance of Oxt neurons in feeding regulation and suggest that reduced Oxt neuropeptide is one mechanism mediating the hyperphagic obesity of Sim1(+/-) mice.

Sim1 haploinsufficiency impairs melanocortin-mediated anorexia and activation of paraventricular nucleus neurons.

Single-minded 1 (SIM1) is one of only six genes implicated in human monogenic obesity. Haploinsufficiency of this hypothalamic transcription factor is associated with hyperphagic obesity and increased linear growth in both humans and mice. Additionally, Sim1 heterozygous mice show enhanced hyperphagia and obesity in response to a high-fat diet. Thus the phenotype of Sim1 haploinsufficiency is similar to that of agouti yellow (Ay), and melanocortin 4 receptor (Mc4r) knockout mice, both of which are defective in hypothalamic melanocortin signaling. Sim1 and Mc4r are both expressed in the paraventricular nucleus (PVN). Here we report that Sim1 heterozygous mice, which have normal energy expenditure, are hyperphagic despite having elevated hypothalamic proopiomelanocortin (Pomc) expression. In response to the melanocortin agonist melanotan-2 (MTII) they exhibit a blunted suppression of feeding yet increase their energy expenditure normally. They also fail to activate PVN neurons in response to the drug at a dose that induces robust c-Fos expression in a subset of Sim1 PVN neurons in wild-type mice. The resistance to melanocortin signaling in Sim1 heterozygotes is not due to a reduced number of Sim1 neurons in the PVN. Hypothalamic Sim1 gene expression is induced by leptin and MTII treatment. Our results demonstrate that Sim1 heterozygotes are resistant to hypothalamic melanocortin signaling and suggest that Sim1-expressing PVN neurons regulate feeding, but not energy expenditure, in response to melanocortin signaling.

SIM1 overexpression partially rescues agouti yellow and diet-induced obesity by normalizing food intake.

Single-minded 1 (SIM1) mutations are associated with obesity in mice and humans. Haploinsufficiency of mouse Sim1 causes hyperphagic obesity with increased linear growth and enhanced sensitivity to a high-fat diet, a phenotype similar to that of agouti yellow and melanocortin 4 receptor knockout mice. To investigate the effects of increased Sim1 dosage, we generated transgenic mice that overexpress human SIM1 and examined their phenotype. Compared with wild-type mice, SIM1 transgenic mice had no obvious phenotype on a low-fat chow diet but were resistant to diet-induced obesity on a high-fat diet due to reduced food intake with no change in energy expenditure. The SIM1 transgene also completely rescued the hyperphagia and partially rescued the obesity of agouti yellow mice, in which melanocortin signaling is abrogated. Our results indicate that the melanocortin 4 receptor signals through Sim1 or its transcriptional targets in controlling food intake but not energy expenditure.

Sim1 gene dosage modulates the homeostatic feeding response to increased dietary fat in mice.

Haploinsufficiency of the transcription factor gene Sim1 has been previously implicated in hyperphagic obesity in humans and mice. To investigate the relation between Sim1 dosage and hyperphagia, we generated sim1-knockout mice and studied their growth and feeding behavior. Heterozygous mice weaned on standard chow consumed 14% more food per day than controls and developed obesity, hyperinsulinemia, and hyperleptinemia. The sim1 heterozygous mice were also significantly longer than controls. Heterozygous animals had modestly increased feeding efficiency, suggesting reduced energy expenditure, but voluntary wheel-running activity did not differ significantly between the two groups. We studied the effect of dietary fat on the feeding behavior of heterozygous sim1 mutant mice. The tempo and severity of weight gain were much greater in animals weaned on a high-fat diet. When acutely challenged with increased dietary fat, sim1 heterozygous mice weaned on the chow diet markedly increased their food consumption and caloric intake, whereas control mice reduced the mass of food they consumed and maintained approximately isocaloric intake. In wild-type adult mice, we detected Sim1 expression in the paraventricular and supraoptic nuclei, as previously reported in neonates, as well as in the amygdala and lateral hypothalamus, all regions implicated in feeding behavior. Our results indicate that Sim1 gene dosage modulates the homeostatic feeding response to increased dietary fat and likely plays a physiological role in the regulation of energy balance.