CRISPR/Cas9 Epigenome Editing Potential for Rare Imprinting Diseases: A Review

Imprinting diseases (IDs) are rare congenital disorders caused by aberrant dosages of imprinted genes. Rare IDs are comprised by a group of several distinct disorders that share a great deal of homology in terms of genetic etiologies and symptoms. Disruption of genetic or epigenetic mechanisms can cause issues with regulating the expression of imprinted genes, thus leading to disease. Genetic mutations affect the imprinted genes, duplications, deletions, and uniparental disomy (UPD) are reoccurring phenomena causing imprinting diseases. Epigenetic alterations on methylation marks in imprinting control centers (ICRs) also alters the expression patterns and the majority of patients with rare IDs carries intact but either silenced or overexpressed imprinted genes. Canonical CRISPR/Cas9 editing relying on double-stranded DNA break repair has little to offer in terms of therapeutics for rare IDs. Instead CRISPR/Cas9 can be used in a more sophisticated way by targeting the epigenome. Catalytically dead Cas9 (dCas9) tethered with effector enzymes such as DNA de- and methyltransferases and histone code editors in addition to systems such as CRISPRa and CRISPRi have been shown to have high epigenome editing efficiency in eukaryotic cells. This new era of CRISPR epigenome editors could arguably be a game-changer for curing and treating rare IDs by refined activation and silencing of disturbed imprinted gene expression. This review describes major CRISPR-based epigenome editors and points out their potential use in research and therapy of rare imprinting diseases.

Update on Diabetes Mellitus and Glucose Metabolism Alterations in Prader-Willi Syndrome

PURPOSE OF REVIEW

This review summarizes our current knowledge on type 2 diabetes mellitus (T2DM) and glucose metabolism alterations in Prader-Willi syndrome (PWS), the most common syndromic cause of obesity, and serves as a guide for future research and current best practice.

RECENT FINDINGS

Diabetes occurs in 10-25% of PWS patients, usually in adulthood. Severe obesity is a significant risk factor for developing of T2DM in PWS. Paradoxically, despite severe obesity, a relative hypoinsulinemia, without the expected insulin resistance, is frequently observed in PWS. The majority of PWS subjects with T2DM are asymptomatic and diabetes-related complications are infrequent. Long-term growth hormone therapy does not adversely influence glucose homeostasis in all ages, if weight gain does not occur. Early intervention to prevent obesity and the regular monitoring of glucose levels are recommended in PWS subjects. However, further studies are required to better understand the physiopathological mechanisms of T2DM in these patients.

Prader Willi syndrome: endocrine updates and new medical therapies

PURPOSE OF REVIEW

Prader Willi syndrome is characterized not only by hyperphagia frequently resulting in obesity, but also by endocrine dysfunction across a variety of axes. This article reviews the most recent literature regarding possible causes of hyperphagia and the nature of endocrinopathies seen in Prader Willi syndrome, as well as current research into possible therapies.

RECENT FINDINGS

Investigation into neurologic, metabolic and hormonal drivers of hyperphagia and obesity has revealed new insights and clarified underlying pathophysiology. Additional studies continue to elucidate the hormonal deficiencies seen in the syndrome, allowing for improvements in clinical care.

SUMMARY

The underlying causes of the hyperphagia and progressive obesity frequently seen in Prader Willi Syndrome are largely unknown and likely multifactorial. Understanding the hormonal and metabolic drivers at work in PWS, as well as the nature of other hormonal dysfunction seen in the syndrome is necessary to guide current management and future research directions.

Age Distribution, Comorbidities and Risk Factors for Thrombosis in Prader-Willi Syndrome

Prader–Willi syndrome (PWS) is an imprinting disorder caused by lack of expression of the paternally inherited 15q11.2–q13 chromosome region. The risk of death from obesity-related complications can worsen with age, but survival trends are improving. Comorbidities and their complications such as thrombosis or blood clots and venous thromboembolism (VTE) are uncommon but reported in PWS. Two phases of analyses were conducted in our study: unadjusted and adjusted frequency with odds ratios and a regression analysis of risk factors. Individuals with PWS or non-PWS controls with exogenous obesity were identified by specific International Classification of Diseases (ICD)-9 diagnostic codes reported on more than one occasion to confirm the diagnosis of PWS or exogenous obesity in available national health claims insurance datasets. The overall average age or average age per age interval (0–17 year, 18–64 year, and 65 year+) and gender distribution in each population were similar in 3136 patients with PWS and 3945 non-PWS controls for comparison purposes, with exogenous obesity identified from two insurance health claims dataset sources (i.e., commercial and Medicare advantage or Medicaid). For example, 65.1% of the 3136 patients with PWS were less than 18 years old (subadults), 33.2% were 18–64 years old (adults), and 1.7% were 65 years or older. After adjusting for comorbidities that were identified with diagnostic codes, we found that commercially insured PWS individuals across all age cohorts were 2.55 times more likely to experience pulmonary embolism (PE) or deep vein thrombosis (DVT) than for obese controls (p-value: 0.013; confidence interval (CI): 1.22–5.32). Medicaid-insured individuals across all age cohorts with PWS were 0.85 times more likely to experience PE or DVT than obese controls (p-value: 0.60; CI: 0.46–1.56), with no indicated age difference. Age and gender were statistically significant predictors of VTEs, and they were independent of insurance coverage. There was an increase in occurrence of thrombotic events across all age cohorts within the PWS patient population when compared with their obese counterparts, regardless of insurance type.

mTOR and autophagy pathways are dysregulated in murine and human models of Schaaf-Yang syndrome

MAGEL2 is a maternally imprinted, paternally expressed gene, located in the Prader-Willi region of human chromosome 15. Pathogenic variants in the paternal copy of MAGEL2 cause Schaaf-Yang syndrome (SHFYNG), a neurodevelopmental disorder related to Prader-Willi syndrome (PWS). Patients with SHFYNG, like PWS, manifest neonatal hypotonia, feeding difficulties, hypogonadism, intellectual disability and sleep apnea. However, individuals with SHFYNG have joint contractures, greater cognitive impairment, and higher prevalence of autism than seen in PWS. Additionally, SHFYNG is associated with a lower prevalence of hyperphagia and obesity than PWS. Previous studies have shown that truncating variants in MAGEL2 lead to SHFYNG. However, the molecular pathways involved in manifestation of the SHFYNG disease phenotype are still unknown. Here we show that a Magel2 null mouse model and fibroblast cell lines from individuals with SHFYNG exhibit increased expression of mammalian target of rapamycin (mTOR) and decreased autophagy. Additionally, we show that SHFYNG induced pluripotent stem cell (iPSC)-derived neurons exhibit impaired dendrite formation. Alterations in SHFYNG patient fibroblast lines and iPSC-derived neurons are rescued by treatment with the mTOR inhibitor rapamycin. Collectively, our findings identify mTOR as a potential target for the development of pharmacological treatments for SHFYNG.

Uniparental disomy and pretreatment IGF-1 may predict elevated IGF-1 levels in Prader-Willi patients on GH treatment

Pediatric patients with Prader-Willi syndrome (PWS) can be treated with recombinant human GH (rhGH). These patients are highly sensitive to rhGH and the standard doses suggested by the international guidelines often result in IGF-1 above the normal range. We aimed to evaluate 1 the proper rhGH dose to optimize auxological outcomes and to avoid potential overtreatment, and 2 which patients are more sensitive to rhGH. In this multicenter real-life study, we recruited 215 patients with PWS older than 1 year, on rhGH at least for 6 months, from Italian Centers for PWS care. We collected auxological parameters, rhGH dose, IGF-1 at recruitment and (when available) at start of treatment. The rhGH dose was 4.3 (0.7/8.4) mg/m²/week. At recruitment, IGF-1 was normal in 72.1% and elevated in 27.9% of the patients. In the group of 115 patients with IGF-1 available at start of rhGH, normal pretreatment IGF-1 and uniparental disomy were associated with elevated IGF-1 during the therapy. No difference in height and growth velocity was found between patients treated with the highest and the lowest range dose. The rhGH dose prescribed in Italy seems lower than the recommended one. Normal pretreatment IGF-1 and uniparental disomy are risk factors for elevated IGF-1. The latter seems to be associated with higher sensitivity to GH. In case of these risk factors, we recommend a more accurate titration of the dose to avoid overtreatment and its potential side effects.

Prader- Willi syndrome: An uptodate on endocrine and metabolic complications

Prader-Willi syndrome (PWS) is a genetic disorder characterized by short stature, low lean body mass, muscular hypotonia, mental retardation, behavioral abnormalities, dysmorphic features, and excessive appetite with progressive obesity. It is caused by lack of expression of genes on the paternally inherited chromosome 15q11.2-q13. This genetic disorder has an estimated prevalence that ranges between 1/10,000-1/30,000. Hypothalamic dysfunction is a common finding in PWS and it has been implicated in several manifestations of this syndrome such as hyperphagia, temperature instability, high pain threshold, sleep disordered breathing, and multiple endocrine abnormalities. These include growth hormone deficiency, central adrenal insufficiency, hypogonadism, hypothyroidism, and obesity often complicated by type 2 diabetes. The aim of this manuscript is to overview the current literature on metabolic and endocrine complications of PWS, focusing on human studies and providing insights on the physio pathological mechanisms. A careful management of metabolic and endocrine complications can contribute to improve quality of life, prevent complications, and prolong life expectancy of PW patients.

Hormonal and metabolic effects of carbohydrate restriction in children with Prader-Willi syndrome

OBJECTIVE

Macronutrient regulation of hyperphagia and adiposity in Prader-Willi syndrome (PWS) is poorly understood. We compared fasting and postprandial concentrations of hormones and metabolites in eight PWS children (age 9-18 years) fed, in random order, low carbohydrate, high-fat (LC, 15% carb; 65% fat; 20% protein) and low-fat, high carbohydrate (LF, 65% carb, 15% fat, 20% protein) diets matched for calories and protein.

METHODS

Participants were randomized to consume either the LC or LF diet during a first hospital admission and the second diet during a subsequent admission. Blood samples were obtained after overnight fasting and 1 hour after a mixed meal.

RESULTS

Relative to subjects consuming the LF diet, subjects consuming the LC diet had: lower postprandial insulin concentrations (P = 0.02); higher fasting GLP-1 AND GIP concentrations and increased postprandial GLP-1 (P < 0.02); reduced ratio of fasting ghrelin to GLP-1 (P = 0.0078); increased FFA and fatty acid oxidation, as assessed by concentrations of even-chain acylcarnitines (P < 0.001); lower fasting TG and TG/HDL ratio (P < 0.01); and higher concentrations of branch chain amino acids (P < 0.01). There were no changes in glucose, PYY, or adiponectin. CRP, AST and ALT were all higher (P < 0.01) on the LC diet.

CONCLUSIONS

Increases in GLP-1 with low carbohydrate feeding and reductions in the ratio of ghrelin to GLP-1 might limit food intake and improve glycaemic control in PWS. Other potential benefits of carbohydrate restriction may include fat mobilization and oxidation and reductions in the TG/HDL ratio, a marker of insulin resistance. However, increases in CRP, AST and ALT necessitate longer-term studies of low carbohydrate efficacy and safety.

Analysis of Circulating Mediators of Bone Remodeling in Prader-Willi Syndrome

We tested the hypothesis that the levels of bone remodeling mediators may be altered in Prader-Willi syndrome (PWS). We assessed RANKL, OPG, sclerostin, DKK-1 serum levels, and bone metabolism markers in 12 PWS children (7.8 ± 4.3 years), 14 PWS adults (29.5 ± 7.2 years), and 31 healthy controls matched for sex and age. Instrumental parameters of bone mineral density (BMD) were also evaluated. Lumbar spine BMD Z-scores were reduced in PWS children (P < 0.01), reaching osteopenic levels in PWS adults. PWS patients showed lower 25(OH)-vitamin D serum levels than controls (P < 0.001). Osteocalcin was increased in PWS children but reduced in adults respect to controls (P < 0.005 and P < 0.01, respectively). RANKL levels were higher in both pediatric and PWS adults than controls (P < 0.004), while OPG levels were significantly reduced (P < 0.004 and P < 0.006, respectively). Sclerostin levels were increased in children (P < 0.04) but reduced in adults compared to controls (P < 0.01). DKK-1 levels did not show significant difference between patients and controls. In PWS patients, RANKL, OPG, and sclerostin significantly correlated with metabolic and bone instrumental parameters. Consistently, with adjustment for age, multiple linear regression analysis showed that BMD and osteocalcin were the most important predictors for RANKL, OPG, and sclerostin in children, and GH and sex steroid replacement treatment in PWS adults. We demonstrated the involvement of RANKL, OPG, and sclerostin in the altered bone turnover of PWS subjects suggesting these molecules as markers of bone disease and new potential pharmacological targets to improve bone health in PWS.

Bone mineral density in young adults with Prader-Willi syndrome: A randomized, placebo-controlled, crossover GH trial

CONTEXT

The prevalence of osteoporosis is increased in adults with Prader-Willi syndrome (PWS). In children with PWS, growth hormone (GH) treatment has beneficial effects on bone mineral density (BMD). BMD might deteriorate after cessation of GH at adult height (AH), while continuing GH might maintain BMD.

OBJECTIVE

To investigate the effects of GH vs placebo, and furthermore the effects of sex steroid replacement therapy (SSRT), on BMD in GH-treated young adults with PWS who had attained AH.

DESIGN

Two-year, randomized, double-blind, placebo-controlled, crossover GH study.

PATIENTS

Twenty-seven young adults with PWS were stratified for gender and BMI and then randomly and blindly assigned to receive GH (0.67 mg/m² /day) or placebo for 1 year, after which they crossed over to the alternative treatment for another year.

MEASUREMENTS

Bone mineral density of the total body (BMD_TB ) and lumbar spine (BMD_LS ) SDS were measured by dual-energy x-ray absorptiometry.

RESULTS

At AH, BMD_TB SDS was significantly lower compared to healthy peers (P < .01), while BMAD_LS SDS was similar. Both BMD_TB SDS and BMAD_LS SDS were similar during 1 year of GH vs 1 year of placebo. In hypogonadal young adults without SSRT, BMD_TB SDS and BMAD_LS SDS decreased during the 2-year study (P = .11 and P = .01), regardless of GH or placebo, while BMD_TB SDS increased in those with SSRT (P < .01).

CONCLUSIONS

Compared to GH treatment, 1 year of placebo after attainment of AH does not deteriorate BMD SDS in young adults with PWS. In addition, our data suggest that GH is not able to prevent the decline in BMD SDS in hypogonadal young adults with PWS, unless it is combined with SSRT.

Hypothalamic loss of Snord116 recapitulates the hyperphagia of Prader-Willi syndrome

Profound hyperphagia is a major disabling feature of Prader-Willi syndrome (PWS). Characterization of the mechanisms that underlie PWS-associated hyperphagia has been slowed by the paucity of animal models with increased food intake or obesity. Mice with a microdeletion encompassing the Snord116 cluster of noncoding RNAs encoded within the Prader-Willi minimal deletion critical region have previously been reported to show growth retardation and hyperphagia. Here, consistent with previous reports, we observed growth retardation in Snord116+/-P mice with a congenital paternal Snord116 deletion. However, these mice neither displayed increased food intake nor had reduced hypothalamic expression of the proprotein convertase 1 gene PCSK1 or its upstream regulator NHLH2, which have recently been suggested to be key mediators of PWS pathogenesis. Specifically, we disrupted Snord116 expression in the mediobasal hypothalamus in Snord116fl mice via bilateral stereotaxic injections of a Cre-expressing adeno-associated virus (AAV). While the Cre-injected mice had no change in measured energy expenditure, they became hyperphagic between 9 and 10 weeks after injection, with a subset of animals developing marked obesity. In conclusion, we show that selective disruption of Snord116 expression in the mediobasal hypothalamus models the hyperphagia of PWS.

Analysis of saliva samples in patients with Prader-Willi syndrome

Patients affected by Prader-Willi Syndrome (PWS) usually show orofacial dysfunction, poor oral hygiene, severe tooth wear, generalized caries and thick sticky saliva. The aim of this study was to evaluate molecular/ionic changings in PWS patients compared to controls, as well as unstimulated salivary flow rate (SFR); 7 patients with a mean age of 20.0±5.45 years were enrolled in the study group (PWS group) and 5 patients with a mean age of 22.6±3.05 years, in the control group. Results showed a greater Na+ (p=0.003), Cl+ (p=0.004) and P (p=0.001) concentration in saliva of PWS group as well as a greater concentration of secretory IgA (p=0.003) with a reduction of SFR (p=0.004) compared to controls. A Spearman’s analysis (based on the SFR of both groups) revealed an inverse correlation with Na (rho=-0.747), Cl (rho=-0.723), P (rho=-0.637) and sIgA (rho=-0.707) concentration and SFR, when linear regression model was performed only P and SFR were interdependent (ß=-0.748; p=0.005).

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.

C/D-box snoRNAs form methylating and non-methylating ribonucleoprotein complexes: Old dogs show new tricks

C/D box snoRNAs (SNORDs) are an abundantly expressed class of short, non-coding RNAs that have been long known to perform 2'-O-methylation of rRNAs. However, approximately half of human SNORDs have no predictable rRNA targets, and numerous SNORDs have been associated with diseases that show no defects in rRNAs, among them Prader-Willi syndrome, Duplication 15q syndrome and cancer. This apparent discrepancy has been addressed by recent studies showing that SNORDs can act to regulate pre-mRNA alternative splicing, mRNA abundance, activate enzymes, and be processed into shorter ncRNAs resembling miRNAs and piRNAs. Furthermore, recent biochemical studies have shown that a given SNORD can form both methylating and non-methylating ribonucleoprotein complexes, providing an indication of the likely physical basis for such diverse new functions. Thus, SNORDs are more structurally and functionally diverse than previously thought, and their role in gene expression is under-appreciated. The action of SNORDs in non-methylating complexes can be substituted with oligonucleotides, allowing devising therapies for diseases like Prader-Willi syndrome.

Growth Hormone Treatment in Children With Prader-Willi Syndrome: Three Years of Longitudinal Data in Prepubertal Children and Adult Height Data From the KIGS Database

CONTEXT

Longitudinal data of children with Prader-Willi syndrome (PWS) treated with genotropin were registered in the Pfizer International Growth Database (KIGS).

OBJECTIVE

To evaluate efficacy and safety of growth hormone (GH) treatment in a large group of children with PWS.

DESIGN

Data registered in KIGS from 1987 to 2012.

SETTING

Worldwide retrospective cohort study.

PATIENTS

Patients included 522 prepubertal children treated with GH for three years and 173 children who had reached adult height. Safety analysis included 2332 children. Intervention involved GH treatment.

MAIN OUTCOME MEASURE

Height standard deviation score (SDS), body mass index (BMI) SDS, occurrence of serious adverse events, and deaths reported in KIGS.

RESULTS

In prepubertal children, mean (standard deviation) height SDS improved to -0.31 (1.34) (P < 0.05) during three years of GH treatment. In the adolescent group, height SDS improved until the start of puberty to -0.22 (1.31) (P < 0.05) but had a loss of -0.77 (0.81) during puberty, resulting in a mean adult height SDS of -1.19 (1.37). Total height gain was 0.95 (1.32) SDS. BMI SDS increased in the prepubertal group from 1.11 (2.09) to 1.53 (1.43) (P < 0.05) and did not significantly change in the adolescent group, who had a BMI SDS at an adult height of 1.78 (1.26). KIGS contained 12 death reports.

CONCLUSIONS

GH treatment in children with PWS significantly improves linear growth. BMI remains on average below +2 SDS, in contrast to the natural course of increasing obesity in PWS. Safety should be closely monitored in children with PWS, with and without GH treatment.

The use of magnetic resonance imaging to characterize abnormal body composition phenotypes in youth with Prader-Willi syndrome

INTRODUCTION

Magnetic resonance imaging (MRI) provides detailed assessment of body composition compartments. No studies have employed state-of-the-art MRI methods to accurately examine abdominal adipose tissue (AT) and skeletal muscle in youth with Prader-Willi syndrome (PWS). Therefore, this study aimed to describe AT distribution and skeletal muscle in the abdominal region of youth with PWS using MRI.

METHODS

Anthropometric measures and whole-abdominal T1-weighted MRI were performed in sixteen (5 males and 11 females) youth diagnosed with PWS, and seventeen (10 males and 7 females) youth who did not have PWS (controls). Volume of subcutaneous, visceral, intermuscular, and total AT, and skeletal muscle in the abdominal region were quantified using a semiautomatic procedure. Results were summarized using median and interquartile range (IQR, 25th-75th), and ANCOVA test was used (with age and sex as covariates) to examine differences in body composition compartments between PWS and control group.

RESULTS

PWS group had similar age (10.5, 6.6-13.9 vs. 12.8, 10.0-14.4years; P=0.14) and BMI z-score (0.5, 0.2-1.3 vs. 0.2, -0.3 to 1.0; P=0.33) when compared with controls. Significant differences were observed in absolute volumes of total AT (PWS: 4.1, 2.0-6.6L; control: 2.9, 2.0-4.5L; P=0.01), subcutaneous AT (PWS: 2.8, 1.4-4.8L; control: 1.8, 1.1-3.2L; P=0.01), and intermuscular AT (PWS: 0.3, 0.1-0.4L; control: 0.3, 0.2-0.3L; P<0.005). Visceral AT/subcutaneous AT was lower in PWS (0.4, 0.3-0.5) compared to controls (0.5, 0.4-0.6), P=0.01. In addition, skeletal muscle volume was lower in PWS (1.5, 1.0-2.6L) compared to controls (3.1, 1.6-3.9L), P=0.03. Ratios of abdominal AT compartments to skeletal muscle were all higher in PWS compared to controls (all P<0.005).

CONCLUSIONS

PWS youth have greater abdominal adiposity, particularly subcutaneous AT and intermuscular AT, and lower volume of skeletal muscle compared to controls. The decreased ratio of visceral AT/subcutaneous AT in youth with PWS suggests an improved metabolic profile for the level of adiposity present; however, elevated ratios of AT to skeletal muscle suggest a sarcopenic obesity-like phenotype, which could lead to worse health outcomes.

When small RNAs become smaller: emerging functions of snoRNAs and their derivatives

Small nucleolar RNAs (snoRNAs) are molecules located in the cell nucleolus and in Cajal bodies. Many scientific reports show that snoRNAs are not only responsible for modifications of other RNAs but also fulfill multiple other functions such as metabolic stress regulation or modulation of alternative splicing. Full-length snoRNAs as well as small RNAs derived from snoRNAs have been implied in human diseases such as cancer or Prader-Willi Syndrome. In this review we describe emerging, non-canonical roles of snoRNAs and their derivatives with the emphasis on their role in human diseases.

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.

Deficiency in prohormone convertase PC1 impairs prohormone processing in Prader-Willi syndrome

Prader-Willi syndrome (PWS) is caused by a loss of paternally expressed genes in an imprinted region of chromosome 15q. Among the canonical PWS phenotypes are hyperphagic obesity, central hypogonadism, and low growth hormone (GH). Rare microdeletions in PWS patients define a 91-kb minimum critical deletion region encompassing 3 genes, including the noncoding RNA gene SNORD116. Here, we found that protein and transcript levels of nescient helix loop helix 2 (NHLH2) and the prohormone convertase PC1 (encoded by PCSK1) were reduced in PWS patient induced pluripotent stem cell-derived (iPSC-derived) neurons. Moreover, Nhlh2 and Pcsk1 expression were reduced in hypothalami of fasted Snord116 paternal knockout (Snord116p-/m+) mice. Hypothalamic Agrp and Npy remained elevated following refeeding in association with relative hyperphagia in Snord116p-/m+ mice. Nhlh2-deficient mice display growth deficiencies as adolescents and hypogonadism, hyperphagia, and obesity as adults. Nhlh2 has also been shown to promote Pcsk1 expression. Humans and mice deficient in PC1 display hyperphagic obesity, hypogonadism, decreased GH, and hypoinsulinemic diabetes due to impaired prohormone processing. Here, we found that Snord116p-/m+ mice displayed in vivo functional defects in prohormone processing of proinsulin, pro-GH-releasing hormone, and proghrelin in association with reductions in islet, hypothalamic, and stomach PC1 content. Our findings suggest that the major neuroendocrine features of PWS are due to PC1 deficiency.

Effect of Genotype and Previous GH Treatment on Adiposity in Adults With Prader-Willi Syndrome

CONTEXT

Adults with Prader-Willi syndrome (PWS) have an increased proportion of sc fat mass compared with body mass index (BMI)-matched controls, but whether the genotype influences body composition and metabolic profile remains controversial.

OBJECTIVE

To assess body composition and metabolic features in adults with PWS, according to genetic subtype. In addition, the effect of previous GH treatment was assessed. Main Outcomes and Measures: Body composition (Dual Energy X-ray Absorptiometry) and metabolic parameters were compared in PWS adults (mean age, 25.5 ± 8.9 y) with deletion (n = 47) or uniparental disomy (UPD) (n = 26), taking into account GH treatment in childhood and/or adolescence. In subgroups, adipocyte size, fasting total ghrelin levels, and resting energy expenditure were measured, and hyperphagia was assessed by the Dykens Hyperphagia Questionnaire.

MAIN OUTCOMES AND MEASURES

Body composition (Dual Energy X-ray Absorptiometry) and metabolic parameters were compared in PWS adults (mean age, 25.5 ± 8.9 y) with deletion (n = 47) or uniparental disomy (UPD) (n = 26), taking into account GH treatment in childhood and/or adolescence. In subgroups, adipocyte size, fasting total ghrelin levels, and resting energy expenditure were measured, and hyperphagia was assessed by the Dykens Hyperphagia Questionnaire.

RESULTS

In the whole sample, the deletion group had a higher BMI compared with UPD (40.9 ± 11.5 vs 34.6 ± 9.6 kg/m², P = .02), but there was no difference between groups in percent body fat, metabolic profile, adipocyte size, resting energy expenditure, hyperphagia score, or ghrelin levels. In subjects previously treated with GH, BMI was not different between UPD and deletion groups (33.0 ± 9.7 vs 33.5 ± 11.1 kg/m²). In addition, previous GH treatment was associated with decreased percent body fat and adipocyte volume only in the deletion group.

CONCLUSION

A deletion genotype in adults with PWS is associated with increased BMI. GH treatment in childhood and/or adolescence limits this deleterious phenotypic effect with improved adiposity markers. This study suggests relationships between the molecular phenotype of PWS and adipose tissue development as well as sensitivity to GH.

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.

Metabolic profiling in Prader-Willi syndrome and nonsyndromic obesity: sex differences and the role of growth hormone

OBJECTIVES

To identify metabolic factors controlling appetite and insulin sensitivity in PWS and assess effects of GH treatment.

METHODS

We compared amino acids, fatty acids and acylcarnitines in GH-treated and untreated PWS children and obese and lean controls to identify biomarkers associated with ghrelin, peptide YY and markers of insulin sensitivity (adiponectin and HOMA-IR).

RESULTS

Compared with obese controls (OC), children with PWS had fasting hyperghrelinaemia, hyperadiponectinaemia, hypoinsulinaemia and increased ghrelin/PYY. Hyperghrelinaemia, hyperadiponectinaemia and hypoinsulinaemia were more striking in PWS females than males, and decreases in BCAA were detected only in PWS females. GH-treated PWS subjects had lower leptin and higher IGF-1 and adiponectin than untreated subjects; fasting ghrelin, PYY and insulin levels were comparable. Ghrelin correlated inversely with BCAA in PWS but not OC. Adiponectin correlated negatively with BMIz and HOMA-IR in PWS; in contrast, adiponectin correlated more strongly with BCAA than BMIz or HOMA-IR in OC.

CONCLUSIONS

BCAA levels were lower in PWS females than OC females and correlated inversely with ghrelin. Low BCAA in PWS females may promote hyperghrelinaemia and hyperphagia, while hyperadiponectinaemia may maintain insulin sensitivity despite excess weight gain. GH treatment may reduce leptin and increase adiponectin, but does not affect fasting ghrelin or PYY.

Prader-Willi syndrome: a review of clinical, genetic, and endocrine findings

INTRODUCTION

Prader-Willi syndrome (PWS) is a multisystemic complex genetic disorder caused by lack of expression of genes on the paternally inherited chromosome 15q11.2-q13 region. There are three main genetic subtypes in PWS: paternal 15q11-q13 deletion (65-75 % of cases), maternal uniparental disomy 15 (20-30 % of cases), and imprinting defect (1-3 %). DNA methylation analysis is the only technique that will diagnose PWS in all three molecular genetic classes and differentiate PWS from Angelman syndrome. Clinical manifestations change with age with hypotonia and a poor suck resulting in failure to thrive during infancy. As the individual ages, other features such as short stature, food seeking with excessive weight gain, developmental delay, cognitive disability and behavioral problems become evident. The phenotype is likely due to hypothalamic dysfunction, which is responsible for hyperphagia, temperature instability, high pain threshold, hypersomnia and multiple endocrine abnormalities including growth hormone and thyroid-stimulating hormone deficiencies, hypogonadism and central adrenal insufficiency. Obesity and its complications are the major causes of morbidity and mortality in PWS.

METHODS

An extensive review of the literature was performed and interpreted within the context of clinical practice and frequently asked questions from referring physicians and families to include the current status of the cause and diagnosis of the clinical, genetics and endocrine findings in PWS.

CONCLUSIONS

Updated information regarding the early diagnosis and management of individuals with Prader-Willi syndrome is important for all physicians and will be helpful in anticipating and managing or modifying complications associated with this rare obesity-related disorder.

Irisin and the Metabolic Phenotype of Adults with Prader-Willi Syndrome

CONTEXT

Hyperphagia, low resting energy expenditure, and abnormal body composition contribute to severe obesity in Prader Willi syndrome (PWS). Irisin, a circulating myokine, stimulates "browning" of white adipose tissue resulting in increased energy expenditure and improved insulin sensitivity. Irisin has not been previously studied in PWS.

OBJECTIVES

Compare plasma and salivary irisin in PWS adults and normal controls. Examine the relationship of irisin to insulin sensitivity and plasma lipids.

DESIGN AND STUDY PARTICIPANTS

A fasting blood sample for glucose, lipids, insulin, leptin, adinopectin, and irisin was obtained from 22 PWS adults and 54 healthy BMI-matched volunteers. Saliva was collected for irisin assay in PWS and controls.

RESULTS

Fasting glucose (77 ± 9 vs 83 ± 7 mg/dl, p = 0.004), insulin (4.1 ± 2.0 vs 7.9 ± 4.7 μU/ml, p<0.001), and triglycerides (74 ± 34 vs 109 ± 71 mg/dl, p = 0.007) were lower in PWS than in controls. Insulin resistance (HOMA-IR) was lower (0.79 ± 0.041 vs 1.63 ± 1.02, p<0.001) and insulin sensitivity (QUICKI) was higher (0.41 ± 0.04 vs 0.36 ± 0.03, p<0.001) in PWS. Plasma irisin was similar in both groups, but salivary irisin (64.5 ± 52.0 vs 33.0 ± 12.1ng/ml), plasma leptin (33.5 ± 24.2 vs 19.7 ± 19.3 ng/ml) and plasma adinopectin (13.0 ± 10.8 vs 7.6 ± 4.5μg/ml) were significantly greater in PWS (p<0.001). In PWS, plasma irisin showed positive Pearson correlations with total cholesterol (r = 0.58, p = 0.005), LDL-cholesterol (r = 0.59, p = 0.004), and leptin (r = 0.43, p = 0.045). Salivary irisin correlated negatively with HDL-cholesterol (r = -0.50, p = 0.043) and positively with LDL-cholesterol (r = 0.51, p = 0.037) and triglycerides (r = 0.50, p = 0.041).

CONCLUSIONS

Salivary irisin was markedly elevated in PWS although plasma irisin was similar to levels in controls. Significant associations with plasma lipids suggest that irisin may contribute to the metabolic phenotype of PWS.