Genetic abnormalities in Prader-Willi syndrome and lessons from mouse models

Genomic imprinting and its effects on postnatal growth and adult metabolism

Imprinted genes display parent-of-origin-specific expression with this epigenetic system of regulation found exclusively in therian mammals. Historically, defined imprinted gene functions were almost solely focused on pregnancy and the influence on the growth parameters of the developing embryo and placenta. More recently, a number of postnatal functions have been identified which converge on resource allocation, both for animals in the nest and in adults. While many of the prenatal functions of imprinted genes that have so far been described adhere to the "parental conflict" hypothesis, no clear picture has yet emerged on the functional role of imprints on postnatal metabolism. As these roles are uncovered, interest in the potential for these genes to influence postnatal metabolism and associated adult-onset disease outcomes when dysregulated has gathered pace. Here, we review the published data on imprinted genes and their influence on postnatal metabolism, starting in the nest, and then progressing through to adulthood. When observing the functional effects of these genes on adult metabolism, we must always be careful to acknowledge the influence both of direct expression in the relevant metabolic tissue, but also indirect metabolic programming effects caused by their modulation of both in utero and postnatal growth trajectories.

Effects of recombinant human growth hormone treatment on growth, body composition, and safety in infants or toddlers with Prader-Willi syndrome: a randomized, active-controlled trial

Background

Prader-Willi syndrome (PWS) is a rare complex genetic disorder and is characterized by short stature, muscular hypotonia, abnormal body composition, psychomotor retardation, and hyperphagia. Recombinant human growth hormone (rhGH) treatment improves the symptoms in children with PWS, and early treatment results in more favorable outcomes. However, systematic studies in infants and toddlers under 2 years of age are lacking. This multicenter, randomized, active-controlled, parallel-group, open-label, Phase III study aimed to evaluate the safety of rhGH (Eutropin, LG Chem, Ltd.) and its efficacy on growth, body composition, and motor and cognitive development in infants and toddlers with PWS compared with a comparator treatment (Genotropin, Pfizer, Inc.). Eligible Korean infants or toddlers with PWS were randomly assigned to receive Eutropin or comparator (both 0.24 mg/kg/week, 6 times/week) for 1 year. Height standard deviation score (SDS), body composition, and motor and cognitive development were measured.

Results

Thirty-four subjects (less than 24 months old) were randomized into either the Eutropin (N = 17) group or the comparator (N = 17) group. After 52 weeks of rhGH treatment, height SDS and lean body mass increased significantly from baseline in both groups: the mean height SDS change (SD) was 0.75 (0.59) in the Eutropin group and 0.95 (0.66) in the comparator group, and the mean lean body mass change (SD) was 2377.79 (536.25) g in the Eutropin group and 2607.10 (641.36) g in the comparator group. In addition, percent body fat decreased significantly: the mean (SD) change from baseline was − 8.12% (9.86%) in the Eutropin group and − 7.48% (10.26%) in the comparator group. Motor and cognitive developments were also improved in both groups after the 1-year treatment. The incidence of adverse events was similar between the groups.

Conclusions

rhGH treatment for 52 weeks in infants and toddlers with PWS improved growth, body composition, and motor and cognitive development, and efficacy and safety outcomes of Eutropin were comparable to those of Genotropin. Hence, Eutropin is expected to provide safe and clinically meaningful improvements in pediatric patients with PWS.

Trial registration

The study was registered at ClinicalTrials.gov (identifier: NCT02204163) on July 30, 2014.

URL: https://clinicaltrials.gov/ct2/show/NCT02204163?term=NCT02204163&rank=1

Neuronatin deletion causes postnatal growth restriction and adult obesity in 129S2/Sv mice

OBJECTIVE

Imprinted genes are crucial for the growth and development of fetal and juvenile mammals. Altered imprinted gene dosage causes a variety of human disorders, with growth and development during these crucial early stages strongly linked with future metabolic health in adulthood. Neuronatin (Nnat) is a paternally expressed imprinted gene found in neuroendocrine systems and white adipose tissue and is regulated by the diet and leptin. Neuronatin expression is downregulated in obese children and has been associated with stochastic obesity in C57BL/6 mice. However, our recent studies of Nnat null mice on this genetic background failed to display any body weight or feeding phenotypes but revealed a defect in glucose-stimulated insulin secretion due to the ability of neuronatin to potentiate signal peptidase cleavage of preproinsulin. Nnat deficiency in beta cells therefore caused a lack of appropriate storage and secretion of mature insulin.

METHODS

To further explore the potential role of Nnat in the regulation of body weight and adiposity, we studied classical imprinting-related phenotypes such as placental, fetal, and postnatal growth trajectory patterns that may impact upon subsequent adult metabolic phenotypes.

RESULTS

Here we find that, in contrast to the lack of any body weight or feeding phenotypes on the C57BL/6J background, deletion of Nnat in mice on 129S2/Sv background causes a postnatal growth restriction with reduced adipose tissue accumulation, followed by catch up growth after weaning. This was in the absence of any effect on fetal growth or placental development. In adult 129S2/Sv mice, Nnat deletion was associated with hyperphagia, reduced energy expenditure, and partial leptin resistance. Lack of neuronatin also potentiated obesity caused by either aging or high fat diet feeding.

CONCLUSIONS

The imprinted gene Nnat plays a key role in postnatal growth, adult energy homeostasis, and the pathogenesis of obesity via catch up growth effects, but this role is dependent upon genetic background.

Somatropin therapy for children with prader-willi syndrome : guidelines for use

Prader-Willi syndrome is a neurogenetic disorder that occurs due to the lack of a paternally expressed gene or genes on chromosome 15q11-q13. Many of the symptoms present in Prader-Willi syndrome are due to a hypothalamic-pituitary dysfunction. The main characteristics are muscular hypotonia, delayed psychomotor development, insatiable appetite resulting in overweight if a diet is not maintained, compromised growth and puberty resulting in a short final height and incomplete sexual development, respiratory disturbances, and dysmorphic features. Individuals with Prader-Willi syndrome have compromised growth and abnormal body composition with increased fat mass, decreased lean body mass, and low bone density, resembling a growth hormone-deficient status. Somatropin treatment has a beneficial effect on growth with increased final height and an improvement in and maintenance of body composition, as well as a beneficial effect on respiratory functions. Before initiating somatropin therapy, weight should be kept at an appropriate level, and polysomnography, as well as an otorhinolaryngologic examination should be performed. During somatropin therapy, carbohydrate metabolism and the development of scoliosis should be monitored, as well as bodyweight.A comprehensive team to manage the various components of medical, psychologic, and sociologic care is required for individuals with Prader-Willi syndrome.

The role of genomic imprinting in biology and disease: an expanding view

Genomic imprinting is an epigenetic phenomenon that results in monoallelic gene expression according to parental origin. It has long been established that imprinted genes have major effects on development and placental biology before birth. More recently, it has become evident that imprinted genes also have important roles after birth. In this Review, I bring together studies of the effects of imprinted genes from the prenatal period onwards. Recent work on postnatal stages shows that imprinted genes influence an extraordinarily wide-ranging array of biological processes, the effects of which extend into adulthood, and play important parts in common diseases that range from obesity to psychiatric disorders.

Pleiotropic genetic syndromes with developmental abnormalities associated with obesity

Childhood obesity is a common and complex problem that may persist in adulthood. It may present as a component of genetic syndromes associated with dysmorphic features, developmental abnormalities, mental retardation and/or learning disabilities and often neuroendocrine dysfunction. Although the chromosomal abnormalities of these rare syndromes are already known, the specific genetic and pathophysiological mechanisms leading to the distinct phenotypes and obesity still remain unclarified. New exciting genetic pathways contributing to syndrome phenotype and leading to obesity have recently been identified. Prader-Willi syndrome is caused by loss of expression of the C/D box HBII-84 cluster of snoRNAs. Dysfunction of the primary cilium, thought to have important signalling functions, may contribute to disease phenotype and obesity in Bardet-Biedl, Alstrom and Carpenter syndromes. In this mini-review current knowledge of clinical and genetic characteristics is summarized as well as the pathogenesis of these syndromes with special emphasis on the pathogenesis of obesity.

Early diagnosis and multidisciplinary care reduce the hospitalization time and duration of tube feeding and prevent early obesity in PWS infants

BACKGROUND/AIMS

To describe and evaluate the impact of very early diagnosis and multidisciplinary care on the evolution and care of infants presenting with Prader-Willi syndrome (PWS).

METHODS

19 infants diagnosed with PWS before the second month of life were followed by a multidisciplinary team. Median age at the time of analysis was 3.1 years [range 0.4-6.5]. The data were compared with data collected in 1997 from 113 questionnaires filled out by members of the French PWS Association. The patients from this latter data set were 12.0 years [range 4 months to 41 years] at the time of analysis, with a median age of 36 months at diagnosis.

RESULTS

The duration of their hospitalization time was significantly reduced from 30.0 [range 0-670] to 21 [range 0-90] days (p = 0.043). The duration of gastric tube feeding was significantly reduced from 30.5 [range 0-427] to 15 [range 0-60] days (p = 0.017). Growth hormone treatment was started at a mean age of 1.9 +/- 0.5 years in 10 infants and L-thyroxine in 6 infants. Only 1 infant became obese at 2.5 years.

CONCLUSION

Early diagnosis combined with multidisciplinary care decreases the hospitalization time, duration of gastric tube feeding and prevents early obesity in PWS infants.

[Therapeutical approach of obesity in Prader-Willi Syndrome]

Prader-Willi Syndrome (PWS) is a multisystemic genetic disease characterized by hypotonia, mental retardation, characteristic facial appearance, hyperphagia, and compulsive eating due to hypothalamic dysfunction. PWS is caused by loss of function of genes located in chromosome 15q11-q13, an area subject to genomic imprinting. Obesity is a major cause of increased morbidity and mortality among patients with PWS. The objective of this study was to analyze the therapeutic options available for the treatment of the obesity in PWS including pharmacological and surgical strategies.

Growth hormone improves mobility and body composition in infants and toddlers with Prader-Willi syndrome

OBJECTIVES

To determine the effect of growth hormone (GH) on body composition and motor development in infants and toddlers with Prader-Willi syndrome (PWS).

STUDY DESIGN

Twenty-nine subjects with PWS (4-37 months of age) were randomized to GH treatment (1mg/m 2 /day) or observation for 12 months. Percent body fat, lean body mass, and bone mineral density were measured by dual x-ray absorptiometry; energy expenditure was measured by deuterium dilution; and motor constructs of mobility (M) and stability (S) were assessed using the Toddler Infant Motor Evaluation (TIME).

RESULTS

GH-treated subjects, compared with controls, demonstrated decreased percent body fat (mean, 22.6% +/- 8.9% vs 28.5% +/- 7.9%; P < .001), increased lean body mass (mean, 9.82 +/- 1.9 kg vs 6.3 +/- 1.9 kg; P < .001), and increased height velocity Z scores (mean, 5. 0 +/- 1.8 vs 1.4 +/- 1.0; P < .001). Patients who began GH before 18 months of age showed higher mobility skill acquisition compared with controls within the same age range (mean increase in raw score, 284 +/- 105 vs 206 +/- 63; P < .05).

CONCLUSIONS

GH treatment of infants and toddlers with PWS for 12 months significantly improves body composition and when begun before 18 months of age increases mobility skill acquisition. These results suggest that GH therapy instituted early in life may lessen deterioration of body composition in PWS while also accelerating motor development.

Animal models of mental retardation: from gene to cognitive function

About 2-3% of all children are affected by mental retardation, and genetic conditions rank among the leading causes of mental retardation. Alterations in the information encoded by genes that regulate critical steps of brain development can disrupt the normal course of development, and have profound consequences on mental processes. Genetically modified mouse models have helped to elucidate the contribution of specific gene alterations and gene-environment interactions to the phenotype of several forms of mental retardation. Mouse models of several neurodevelopmental pathologies, such as Down and Rett syndromes and X-linked forms of mental retardation, have been developed. Because behavior is the ultimate output of brain, behavioral phenotyping of these models provides functional information that may not be detectable using molecular, cellular or histological evaluations. In particular, the study of ontogeny of behavior is recommended in mouse models of disorders having a developmental onset. Identifying the role of specific genes in neuropathologies provides a framework in which to understand key stages of human brain development, and provides a target for potential therapeutic intervention.

Mouse models for neurological disease

The mouse has many advantages over human beings for the study of genetics, including the unique property that genetic manipulation can be routinely carried out in the mouse genome. Most importantly, mice and human beings share the same mammalian genes, have many similar biochemical pathways, and have the same diseases. In the minority of cases where these features do not apply, we can still often gain new insights into mouse and human biology. In addition to existing mouse models, several major programmes have been set up to generate new mouse models of disease. Alongside these efforts are new initiatives for the clinical, behavioural, and physiological testing of mice. Molecular genetics has had a major influence on our understanding of the causes of neurological disorders in human beings, and much of this has come from work in mice.

Genome organization, function, and imprinting in Prader-Willi and Angelman syndromes

The chromosomal region, 15q11-q13, involved in Prader-Willi and Angelman syndromes (PWS and AS) represents a paradigm for understanding the relationships between genome structure, epigenetics, evolution, and function. The PWS/AS region is conserved in organization and function with the homologous mouse chromosome 7C region. However, the primate 4 Mb PWS/AS region is bounded by duplicons derived from an ancestral HERC2 gene and other sequences that may predispose to chromosome rearrangements. Within a 2 Mb imprinted domain, gene function depends on parental origin. Genetic evidence suggests that PWS arises from functional loss of several paternally expressed genes, including those that function as RNAs, and that AS results from loss of maternal UBE3A brain-specific expression. Imprinted expression is coordinately controlled in cis by an imprinting center (IC), a genetic element functional in germline and/or early postzygotic development that regulates the establishment of parental specific allelic differences in replication timing, DNA methylation, and chromatin structure.

Endocrine dysfunction in Prader-Willi syndrome: a review with special reference to GH

Prader-Willi syndrome is a genetic disorder occurring in 1 in 10,000-16,000 live-born infants. In the general population, approximately 60 people in every 1,000,000 are affected. The condition is characterized by short stature, low lean body mass, muscular hypotonia, mental retardation, behavioral abnormalities, dysmorphic features, and excessive appetite with progressive obesity. Furthermore, morbidity and mortality are high, probably as a result of gross obesity. Most patients have reduced GH secretory capacity and hypogonadotropic hypogonadism, suggesting hypothalamic-pituitary dysfunction. Replacement of GH and/or sex hormones may therefore be beneficial in Prader-Willi syndrome, and several clinical trials have now evaluated GH replacement therapy in affected children. Results of GH treatment have been encouraging: improved growth, increased lean body mass, and reduced fat mass. There was also some evidence of improvements in respiratory function and physical activity. The long-term benefits of GH treatment are, however, still to be established. Similarly, the role of sex hormone replacement therapy needs to be clarified as few data exist on its efficacy and potential benefits. In summary, Prader-Willi syndrome is a disabling condition associated with GH deficiency and hypogonadism. More active treatment of these endocrine disorders is likely to benefit affected individuals.

Hyperphagic short stature and Prader--Willi syndrome: a comparison of behavioural phenotypes, genotypes and indices of stress

BACKGROUND

The clinical features of hyperphagic short stature (HSS) include short stature secondary to growth hormone insufficiency, excessive appetite (hyperphagia) and mild learning disabilities. Affected children characteristically live in conditions of high psychosocial stress. Symptoms resolve when the child is removed from the stressful environment. Family studies indicate a genetic predisposition.

AIMS

To compare the behavioural and stress profiles of HSS with those of Prader--Willi syndrome (PWS), and to test the hypothesis that the genetic locus that predisposes to HSS co-inherits with the PWS locus at 15q11--13.

METHOD

Twenty-five children with HSS, mean age 9.1 (s.d. 3.8) years, 28% female, were compared with 30 children with PWS, mean age 8.8 (s.d. 2.8) years, 33% female.

RESULTS

The clinical profiles were largely similar across the conditions, but no evidence was found in HSS of co-inheritance of the PWS critical region.

CONCLUSIONS

Hyperphagic short stature is one of the very few behavioural diseases associated with a pathognomonic physiological abnormality. Investigations of the suggested genetic dysregulation, which is so sensitive to environmental influences, may well be of importance in a broader context.

Prader-Willi Syndrome: Clinical and Genetic Findings

Since the initial medical description by Prader, Labhart and Willi in 1956 of individuals with overlapping features, the Prader-Willi syndrome has become recognized as a classical but sporadic genetic syndrome. Prader-Willi syndrome is the most common genetic cause of life-threatening obesity in humans. It is estimated that there are 350,000-400,000 people with this syndrome worldwide. Prader-Willi Syndrome Association USA knows of more than 3,400 persons with Prader-Willi syndrome in the USA out of an approximate 17,000-22,000. Prader-Willi syndrome with an incidence of 1 in 10,000 to 25,000 individuals and Angelman syndrome, an entirely different clinical condition, were the first examples in humans of genetic imprinting. Genetic imprinting or the differential expression of genetic information depending on the parent of origin plays a significant role in other conditions including malignancies.