Introduction

PWS is a relatively common contiguous gene syndrome (estimated prevalence 1/15 000) resulting from the lack of expression of paternal alleles within the chromosome region 15q11-q13 which includes SNURF-SNRPN and several small nucleolar RNAs (snoRNAs) genes ^{1, 2} and is shown in Figure 1. It is the best known example of a human disorder involving genomic imprinting³. The phenotype, firstly described by Prader, Labhart and Willi in 1956⁴ includes neonatal hypotonia, early onset of hyperphagia and development of morbid obesity, short stature, hypogonadism, learning disabilities, behavioural problems and psychiatric phenotypes with severe consequences and difficult management issues for patients, families and carers⁵.

Natural history of the syndrome

Recent progress in early diagnosis has shed light on the natural history of this syndrome and has revealed more complex nutritional phases than the 2 classically described emphasizing that excessive weight gain start before the increase of calorie intake^{6, 7}. In addition eating behavior in individuals with PWS have been well characterized including a permanent obsession with food and excessive food searching, food storage, foraging and hoarding and deficit in satiation (longer meal duration and higher food intake to fill full) and satiety (early return of hunger after meal)^{8, 9}. Their overeating leads to extremely morbid obesity with major consequences on morbidity and mortality.

Behavioural and psychiatric problems

It is now become apparent that eating behaviour troubles are part of general behavioural problems with learning difficulties, deficit of social cues and psychiatric disorders. Indeed individuals with PWS show pronounced emotional lability and a striking inability to control their emotions, which results in frequent temper outbursts. They also show poor social adjustment, with poor peer relationships, tendencies towards social withdrawal, and few attributions of feeling in social relationships compared with others with the same level of intellectual disabilities¹⁰.

Endocrine dysfunctions with a focus on growth hormone (GH)

PWS is a genetic paradigm for hypothalamic-pituitary endocrine dysfunction whose mechanism remains poorly understood. The prevalence of endocrine dysfunctions has been poorly studied and we reported it in a publication involving the first 142 children included in the French national data base¹¹. Indeed 87 % of the children presented with a low peak of GH and almost 100% have low IGF-1 values. The label for GH treatment in children with PWS was obtained in 2000 both in Europe and USA with different criteria i) treatment of growth retardation in USA and ii) optimizing body composition and/or growth retardation in Europe. Early diagnosis performed in most countries around 1 month of life has opened the opportunity to start treatment early in life. In our experience in France, infants are now starting GH treatment at 6 months of life. This early treatment has been shown to increase head circumference¹² and significantly ameliorated psychomotor development particularly in those infants with lower motor development¹³. We recently published that children with PWS displayed higher IGF-1 levels compared with children with GH deficiency suggesting a higher GH sensitivity¹⁴. In order to demonstrate this hypothesis, we implemented a prospective national clinical trial which ended last year and confirmed the same results (see figure 2). The pathophysiology of this observation is under investigation in our laboratory using cultured dermal fibroblasts. The implications of these data is that start of GH treatment must be done with escalating dose regarding the clinical tolerance (importance to search for snoring and ENT investigation prior to and on GH treatment) in order to avoid a rapid and excessive IGF-1 levels. In our review on the deaths observed in children with PWS treated or not-treated with GH, we proposed that the cluster of sudden deaths observed during the first months after the start of GH treatment may result from a too rapid and excessive increase of IGF-1 levels which may in turn stimulate adenoids enlargement thus worsening the possibility of obstructive apnoea¹⁵.

In adults GH treatment is not indicated in those patients re-evaluated at completion of growth and who are not GHD using the adolescent definition, a situation found in 50% of the patients. Interestingly we recently reported that the beneficial effect of GH treatment in childhood in body composition and metabolic modifications remains after the interruption of treatment in adults with PWS¹⁶ and is shown in Table 1.

Pathophysiology of PWS

Is there a unique gene responsible for the entire syndrome or complementary effects of some of the imprinted genes?

The chromosomal region involved in PWS is very complex encompassing imprinted and non-imprinted genes. Figure 1 shows a schematic representation of the region. To date there is no animal model that completely reproduces the human PWS phenotype and particularly the nutritional phases which start with deficit of suckling and failure to thrive and later on lead to early morbid obesity with hyperphagia and deficit of satiety. The hypothetical minimal paternal deletion region associated with PWS phenotype deduced from clinical cases with chromosomal translocations removes SNORD109A, the SNORD116 cluster (30 snoRNAs copies) and IPW¹⁷ and is shown in figure 1 by the dotted frame. Increasing evidence from animal and clinical data suggests that the loss of the paternally expressed snoRNA SNORD116 gene cluster plays a significant role in many of the features of the PWS phenotype¹⁸. Indeed, 3 human cases have been reported showing that the deletion of the SNORD116 gene cluster leads to PWS features including hyperphagia and obesity^19-21. We also reported a new case with the shortest microdeletion (manuscript submitted) confirming the role of the critical minimal region for PWS.

Knockout mice with deletion of only SNORD116 genes showed the phenotype of hyperphagia without the occurrence of obesity on regular and high-fat diets¹⁷. However these data do not argue against a role for the SNORD115 which may act in a complementary manner to the SNORD116 in the pathophysiology of PWS.

SnoRNAs are a group of small RNAs ranging from 80 to 300 nucleotides in length that are located primarily in nucleoli and are required for the chemical modification and maturation of rRNAs. Animal studies have recently shown that the SNORD116 cluster is expressed in the hypothalamus²². In addition, very recently, it has been shown that the enhanced expression of SNORD116 in hypothalamic nuclei was observed at weaning and young adult stages, but was less obvious postnatally when expression was significantly more widespread suggesting that the expression of the SNORD116 is regulated developmentally and may play a regulatory role on hypothalamic development and function.

What are the role of oxytocin and ghrelin in the pathophysiology?

A significant decrease in the number and volume of OT-producing neurons of the hypothalamic paraventricular nucleus (PVN) was reported in PW patients²³. Similarly, an alteration in the OT system was described in 2 PWS mouse models (Necdin KO and Magel2 KO)^{24, 25}. Besides its well-known effects on uterine contractions and milk secretion, OT is known as an anorexigenic hormone which effects on feeding control and regulation of AgRP^26-28 have been recently reported. Moreover individuals with PWS, unlike those with other known causes of obesity, have high circulating levels of the orexigenic hormone ghrelin, which may explain obesity^29-31.Ghrelin acts on various feeding circuits such as hypothalamic centers driving energy intake and hunger, on reward/dopaminergic circuits³² and in the hind brain. We showed that hyperghrelinemia is present very early in life in infants with Prader-Willi syndrome and precedes obesity³¹. Of note, ghrelin is now considered as a major signal mediating perinatally acquired predispositions to adult diseases³³.

Interestingly OT requires intact ghrelin to regulate satiety and conversely ghrelin modulates OT neurons, this crosstalk between these two hormones may be a key feature in the pathophysiology of this neurodevelopmental disease.

Very recently, we demonstrated in Magel 2 KO pups that a single OT injection before the first five hours of life completely rescued the newborn mice from early death by the recovery of normal suckling²⁵. For all these reasons we performed a proof-of-concept study with OT administration to PWS babies to see if it is well tolerated and may change early phenotype. Preliminary data are currently analysed.

OT was recently identified as a key neuropeptide in the social interactions of numerous species by enhancing peer recognition and bonding behaviour. In humans, nasal OT administration improves emotion recognition and face processing in healthy and autistic individuals. We therefore hypothesised that the possible dysfunction in OT may also explain, at least in part, the poor social adjustment of patients with PWS and their inability to control emotions, which in turn might explain their unpredictable disruptive behaviours and frequent temper outbursts. We were able to demonstrate in a double-blind, randomised, placebo-controlled study of 24 adult patients with PWS, that the group that received a single intranasal administration of 24 UI of OT displayed significantly increased trust in others and decreased sadness tendencies, with less disruptive behaviour and a trend towards decreased food intake, than did patients who received placebo³⁴. We also demonstrated in a brain imaging study using PET scan that the anterior cingulum, a brain region involved in the control of emotion, anticipation of movement, empathy, theory of mind and in feeding behaviour, is relatively poorly perfused in PWS, suggesting that OT pathways may be impaired³⁵. It would be then of great interest to see if the administration of OT may change the brain perfusion pathway in PWS.

The mechanism of the abnormal OT pathway is not known in PWS. In the MAGEL2 KO PWS mouse model a restricted production of bioactive oxytocin was detected specifically in the hypothalamus of the mutant new-borns, although the prohormone was normally produced²⁵.

Further animal and humans studies bench to bed studies are need to answer these questions. We are going to start two clinical trials to try to get more insights in the mechanism of actions of OT.

Conclusions

PWS is clearly a paradigm of endocrine dysfunction from a complex hypothalamic origin. It is a neurodevelopmental disorder whose pathophysiology is still unknown. The role of the SNORD116 gene seems to be crucial in the development of the hypothalamus and the early oxytocin-ghrelin dysfunction may be important to understand and possibly treat this very complex disease.

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