Snord116 is critical in the regulation of food intake and body weight

Hypothalamic loss of Snord116 and Prader-Willi syndrome hyperphagia: the buck stops here?

Hyperphagia and obesity are the best-known manifestations of Prader-Willi syndrome (PWS) and are responsible for most of the overall morbidity and mortality associated with the disease. Yet these PWS symptoms remain poorly understood and without effective pharmacologic therapies. Mouse models attempting to recapitulate both the genetic alterations and marked hyperphagia plus obesity of PWS have been enigmatic, leading to skepticism about the use of mouse models to investigate PWS. In this issue of the JCI, Polex-Wolf and colleagues challenge the skeptics by successfully inducing hyperphagia following bilateral mediobasal hypothalamic deletion of the Snord116 gene from adult mice. Obesity also resulted, although only in a subset of mice. While this approach represents an exciting advance, highlighting a pathologic effect of loss of mediobasal hypothalamic Snord116 expression on the development of PWS's hallmark symptoms, the variability in the body-weight and body composition responses to this site-selective gene deletion raises several questions.

Obesity management in Prader-Willi syndrome: current perspectives

Prader-Willi syndrome (PWS) is a complex multisystem disorder due to the absent expression of the paternally active genes in the PWS critical region on chromosome 15 (15q11.2-q13). The syndrome is considered the most common genetic cause of obesity, occurring in 1:10,000-1:30,000 live births. Its main characteristics include neonatal hypotonia, poor feeding, and lack of appetite in infancy, followed by weight gain, lack of satiety, and uncontrolled appetite, frequently after the age of 2-3 years. The clinical picture includes short stature, multiple endocrine abnormalities (hypogonadism, growth hormone/insulin-like growth factor-I axis dysfunction, hypothyroidism, central adrenal insufficiency), dysmorphic features, scoliosis, osteoporosis, mental retardation, and behavioral and psychiatric problems. Subjects with PWS will become severely obese unless their food intake is strictly controlled. Constant and obsessive food seeking behavior can make life very difficult for both the family and caretakers. Prevention of obesity is mandatory in these patients from the first years of life, because once obesity develops it is difficult to maintain the control of food intake. In fact, PWS subjects die prematurely from complications conventionally related to obesity, including diabetes mellitus, metabolic syndrome, sleep apnea, respiratory insufficiency, and cardiovascular disease. The mechanisms underlying hyperphagia in PWS are not completely known, and to date no drugs have proven their efficacy in controlling appetite. Consequently, dietary restriction, physical activity, and behavior management are fundamental in the prevention and management of obesity in PWS. In spite of all available therapeutic tools, however, successful weight loss and maintenance are hardly accomplished. In this context, clinical trials with new drugs have been initiated in order to find new possibilities of a therapy for obesity in these patients. The preliminary results of these studies seem to be encouraging. On the other hand, until well-proven medical treatments are available, bariatric surgery can be taken into consideration, especially in PWS patients with life-threatening comorbidities.

Hypothalamus Specific Re-Introduction of SNORD116 into Otherwise Snord116 Deficient Mice Increased Energy Expenditure

The Snord116 gene cluster has been recognised as a critical contributor to the Prader-Willi syndrome (PWS), with mice lacking Snord116 displaying many classical PWS phenotypes, including low postnatal body weight, reduced bone mass and increased food intake. However, these mice do not develop obesity as a result of increased energy expenditure. To understand the physiological function of SNORD116 better and potentially rescue the altered metabolism of Snord116^-/- mice, we used an adeno-associated viral (AAV) approach to reintroduce the product of the Snord116 gene into the hypothalamus in Snord116^-/- mice at different ages. The results obtained show that mid-hypothalamic re-introduction of SNORD116 in 6-week-old Snord116^-/- mice leads to significantly reduced body weight and weight gain, which is associated with elevated energy expenditure. Importantly, when the intervention targets other areas such as the anterior region of the hypothalamus or the reintroduction occurs in older mice, the positive effects on energy expenditure are diminished. These data indicate that the metabolic symptoms of PWS develop gradually and the Snord116 gene plays a critical role during this process. Furthermore, when we investigated the consequences of SNORD116 re-introduction under conditions of thermoneutrality where the mild cold stress influences are avoided, we also observed a significant increase in energy expenditure. In conclusion, the rescue of mid-hypothalamic Snord116 deficiency in young Snord116 germline deletion mice increases energy expenditure, providing fundamental information contributing to potential virus-mediated genetic therapy in PWS.

Insulin controls food intake and energy balance via NPY neurons

OBJECTIVES

Insulin signaling in the brain has been implicated in the control of satiety, glucose homeostasis and energy balance. However, insulin signaling is dispensable in energy homeostasis controlling AgRP or POMC neurons and it is unclear which other neurons regulate these effects. Here we describe an ancient insulin/NPY neuronal network that governs energy homeostasis across phyla.

METHODS

To address the role of insulin action specifically in NPY neurons, we generated a variety of models by selectively removing insulin signaling in NPY neurons in flies and mice and testing the consequences on energy homeostasis.

RESULTS

By specifically targeting the insulin receptor in both fly and mouse NPY expressing neurons, we found NPY-specific insulin signaling controls food intake and energy expenditure, and lack of insulin signaling in NPY neurons leads to increased energy stores and an obese phenotype. Additionally, the lack of insulin signaling in NPY neurons leads to a dysregulation of GH/IGF-1 axis and to altered insulin sensitivity.

CONCLUSIONS

Taken together, these results suggest that insulin actions in NPY neurons is critical for maintaining energy balance and an impairment of this pathway may be causally linked to the development of metabolic diseases.

SNORD116 deletions cause Prader-Willi syndrome with a mild phenotype and macrocephaly

Prader-Willi syndrome is a complex condition caused by lack of expression of imprinted genes in the paternally derived region of chromosome 15 (15q11q13). A small number of patients with Prader-Willi phenotype have been discovered to have narrow deletions, not encompassing the whole critical region, but only the SNORD116 cluster, which includes genes codifying for small nucleolar RNAs. This kind of deletion usually is not detected by the classic DNA methylation analysis test. We present the case of a male patient with a mild Prader-Willi phenotype and a small deletion including SNORD116, diagnosed by methylation-sensitive multiplex ligation-dependent probe amplification (MLPA. The patient showed neonatal hypotonia, hyperphagia, obesity, central hypogonadism, hypothyroidism, strabismus. Stature and intellectual development are within the normal range. The presence of macrocephaly, observed in other cases of SNORD116 deletions as well, is uncommon for the classic phenotype of the syndrome.

Box C/D small nucleolar RNA genes and the Prader-Willi syndrome: a complex interplay

The nucleolus of mammalian cells contains hundreds of box C/D small nucleolar RNAs (SNORDs). Through their ability to base pair with ribosomal RNA precursors, most play important roles in the synthesis and/or activity of ribosomes, either by guiding sequence-specific 2'-O-methylations or by facilitating RNA folding and cleavages. A growing number of SNORD genes with elusive functions have been discovered recently. Intriguingly, the vast majority of them are located in two large, imprinted gene clusters at human chromosome region 15q11q13 (the SNURF-SNRPN domain) and at 14q32 (the DLK1-DIO3 domain) where they are expressed, respectively, only from the paternally and maternally inherited alleles. These placental mammal-specific SNORD genes have many features of the canonical SNORDs that guide 2'-O-methylations, yet they lack obvious complementarity with ribosomal RNAs and, surprisingly, they are processed from large, tandemly repeated genes expressed preferentially in the brain. This review summarizes our understanding of the biology of these peculiar SNORD genes, focusing particularly on SNORD115 and SNORD116 in the SNURF-SNRPN domain. It examines the growing evidence that altered levels of these SNORDs and/or their host-gene transcripts may be a primary cause of Prader-Willi syndrome (PWS; a rare disorder characterized by overeating and obesity) as well as abnormalities in signaling through the 5-HT2C serotonin receptor. Finally, the hypothesis that PWS may be a ribosomopathy (ribosomal disease) is also discussed. WIREs RNA 2017, 8:e1417. doi: 10.1002/wrna.1417 For further resources related to this article, please visit the WIREs website.

Ambient temperature modulates the effects of the Prader-Willi syndrome candidate gene Snord116 on energy homeostasis

Germline deletion of the Prader-Willi syndrome (PWS) candidate gene Snord116 in mice leads to some classical symptoms of human PWS, notably reductions in body weight, linear growth and bone mass. However, Snord116 deficient mice (Snord116^-/-) do not develop an obese phenotype despite their increased food intake and the underlying mechanism for that is unknown. We tested the phenotypes of germline Snord116^-/- as well as neuropeptide Y (NPY) neuron specific Snord116^lox/lox/NPY^cre/+ mice at 30°C, the thermoneutral temperature of mice, and compared these to previous reports studies conducted at normal room temperature. Snord116^-/- mice at 30°C still weighed less than wild type but had increased body weight gain. Importantly, food intake and energy expenditure were no longer different at 30°C, and the reduced bone mass and nasal-anal length observed in Snord116^-/- mice at room temperature were also normalized. Mechanistically, the thermoneutral condition led to the correction of the mRNA expression of NPY and pro-opiomelanocortin (POMC), which were both previously observed to be significantly up-regulated at room temperature. Importantly, almost identical phenotypes and NPY/POMC mRNA expression alterations were also observed in Snord116^lox/lox/NPY^cre/+ mice, which lack the Snord116 gene only in NPY neurons. These data illustrate that mild cold stress is a critical factor preventing the development of obesity in Snord116^-/- mice via the NPY system. Our study highlights that the function of Snord116 in the hypothalamus may be to enhance energy expenditure, likely via the NPY system, and also indicates that Snord116 function in mice is strongly dependent on environmental conditions such as cold exposure.

Paradoxical leanness in the imprinting-centre deletion mouse model for Prader-Willi syndrome

Prader-Willi syndrome (PWS), a neurodevelopmental disorder caused by loss of paternal gene expression from 15q11-q13, is characterised by growth retardation, hyperphagia and obesity. However, as single gene mutation mouse models for this condition display an incomplete spectrum of the PWS phenotype, we have characterised the metabolic impairment in a mouse model for 'full' PWS, in which deletion of the imprinting centre (IC) abolishes paternal gene expression from the entire PWS cluster. We show that PWS-IC^del mice displayed postnatal growth retardation, with reduced body weight, hyperghrelinaemia and marked abdominal leanness; proportionate retroperitoneal, epididymal/omental and inguinal white adipose tissue (WAT) weights being reduced by 82%, 84% and 67%, respectively. PWS-IC^del mice also displayed a 48% reduction in proportionate interscapular brown adipose tissue (isBAT) weight with significant 'beiging' of abdominal WAT, and a 2°C increase in interscapular surface body temperature. Maintenance of PWS-IC^del mice under thermoneutral conditions (30°C) suppressed the thermogenic activity in PWS-IC^del males, but failed to elevate the abdominal WAT weight, possibly due to a normalisation of caloric intake. Interestingly, PWS-IC^del mice also showed exaggerated food hoarding behaviour with standard and high-fat diets, but despite becoming hyperphagic when switched to a high-fat diet, PWS-IC^del mice failed to gain weight. This evidence indicates that, unlike humans with PWS, loss of paternal gene expression from the PWS cluster in mice results in abdominal leanness. Although reduced subcutaneous insulation may lead to exaggerated heat loss and thermogenesis, abdominal leanness is likely to arise from a reduced lipid storage capacity rather than increased energy utilisation in BAT.

Impaired prohormone processing: a grand unified theory for features of Prader-Willi syndrome?

Prader-Willi syndrome (PWS) is a complex disorder that manifests with an array of phenotypes, such as hypotonia and difficulties in feeding during infancy and reduced energy expenditure, hyperphagia, and developmental delays later in life. While the genetic cause has long been known, it is still not clear how mutations at this locus produce this array of phenotypes. In this issue of the JCI, Burnett and colleagues used a comprehensive approach to gain insight into how PWS-associated mutations drive disease. Using neurons derived from PWS patient induced pluripotent stem cells (iPSCs) and mouse models, the authors provide evidence that neuroendocrine PWS-associated phenotypes may be linked to reduced expression of prohormone convertase 1 (PC1). While these compelling results support a critical role for PC1 deficiency in PWS, more work needs to be done to fully understand how and to what extent loss of this prohormone processing enzyme underlies disease manifestations in PWS patients.

Working-for-Food Behaviors: A Preclinical Study in Prader-Willi Mutant Mice

Abnormal feeding behavior is one of the main symptoms of Prader-Willi syndrome (PWS). By studying a PWS mouse mutant line, which carries a paternally inherited deletion of the small nucleolar RNA 116 (Snord116), we observed significant changes in working-for-food behavioral responses at various timescales. In particular, we report that PWS mutant mice show a significant delay compared to wild-type littermate controls in responding to both hour-scale and seconds-to-minutes-scale time intervals. This timing shift in mutant mice is associated with better performance in the working-for-food task, and results in better decision making in these mutant mice. The results of our study reveal a novel aspect of the organization of feeding behavior, and advance the understanding of the interplay between the metabolic functions and cognitive mechanisms of PWS.

Prader-Willi syndrome and atypical submicroscopic 15q11-q13 deletions with or without imprinting defects

We report a 20 year follow up on a Caucasian female, now 26 years of age, with Prader-Willi syndrome (PWS) harboring an atypical 15q11-q13 submicroscopic deletion of 100-200 kb in size first detected in 1996 involving the imprinting center, SNRPN gene and surrounding region. PWS is a rare complex disorder caused by the loss of paternally expressed genes in the 15q11-q13 region. With high resolution chromosomal microarray and methylation - specific MLPA analysis, we updated the genetic findings on our patient and found a 209,819bp deletion including the SNURF-SNRPN gene complex which includes the imprinting center and the SNORD116 region. We compared with four other similarly reported individuals in the literature with atypical submicroscopic deletions within this region but without imprinting center involvement to better characterize the specific genetic lesions causing PWS clinical findings. Clinically, our patient met the diagnostic criteria of PWS including infantile hypotonia, a poor suck with feeding difficulties, global developmental delays and later food foraging, childhood obesity, small hands and skin picking. Small atypical deletions of comparable sizes were seen in the 15q11-q13 region in all five cases and similar behavioral/physical characteristics were found despite an imprinting defect in our patient. These results further support an overlapping critical deletion region involving the non-coding snoRNA SNORD116 in common in the five individuals playing a key role in contributing to the PWS phenotype.

Prader-Willi Critical Region, a Non-Translated, Imprinted Central Regulator of Bone Mass: Possible Role in Skeletal Abnormalities in Prader-Willi Syndrome

Prader-Willi Syndrome (PWS), a maternally imprinted disorder and leading cause of obesity, is characterised by insatiable appetite, poor muscle development, cognitive impairment, endocrine disturbance, short stature and osteoporosis. A number of causative loci have been located within the imprinted Prader-Willi Critical Region (PWCR), including a set of small non-translated nucleolar RNA's (snoRNA). Recently, micro-deletions in humans identified the snoRNA Snord116 as a critical contributor to the development of PWS exhibiting many of the classical symptoms of PWS. Here we show that loss of the PWCR which includes Snord116 in mice leads to a reduced bone mass phenotype, similar to that observed in humans. Consistent with reduced stature in PWS, PWCR KO mice showed delayed skeletal development, with shorter femurs and vertebrae, reduced bone size and mass in both sexes. The reduction in bone mass in PWCR KO mice was associated with deficiencies in cortical bone volume and cortical mineral apposition rate, with no change in cancellous bone. Importantly, while the length difference was corrected in aged mice, consistent with continued growth in rodents, reduced cortical bone formation was still evident, indicating continued osteoblastic suppression by loss of PWCR expression in skeletally mature mice. Interestingly, deletion of this region included deletion of the exclusively brain expressed Snord116 cluster and resulted in an upregulation in expression of both NPY and POMC mRNA in the arcuate nucleus. Importantly, the selective deletion of the PWCR only in NPY expressing neurons replicated the bone phenotype of PWCR KO mice. Taken together, PWCR deletion in mice, and specifically in NPY neurons, recapitulates the short stature and low BMD and aspects of the hormonal imbalance of PWS individuals. Moreover, it demonstrates for the first time, that a region encoding non-translated RNAs, expressed solely within the brain, can regulate bone mass in health and disease.