**4. Imprinted genetic syndromes**

Prader‐Willi syndrome (PWS, OMIM #176270) is a disorder caused by errors in genomic imprinting, which generally occur during both male and female gametogenesis. In particular, there is the loss of expression of paternal genes normally active and located in the chromosome 15q11‐q13 region [104–108]. Conversely, a loss of expression of the preferentially maternally expressed *UBE3A* (OMIM \*601623) gene in this region leads to Angelman syndrome (AS; OMIM #105830), an entirely different clinical disorder that causes developmental disabilities and neurological problems, such as difficulty speaking, balancing and walking, and, in some cases, seizures [109, 110].

According to several studies, most individuals with PWS (about two‐thirds) have a de novo paternally inherited deletion of the chromosome 15q11‐q13 region; about 25% of cases have maternal disomy 15 (chromosome 15 is inherited from the mother) [111]; less than 3% of patients have defects in the genomic imprinting center due to microdeletions or epimutations [104, 106, 112, 113], while rearrangements of the 15q11‐q13 region or chromosomal transloca‐ tions are rare [104, 114].

However, this syndrome, whose prevalence is around of 1/10,000–1/30,000, is considered the most common cause of syndomic obesity [115].

The cardinal features of PWS include infantile hypotonia, feeding difficulties due to a poor suck and failure to thrive (FTT), followed in later infancy or early childhood by excessive appetite with gradual development of obesity, short stature and/or decreased growth velocity due to growth hormone (GH) deficiency, intellectual disabilities (average IQ of 65), behavioral problems (e.g., temper tantrums, outburst and skin picking) and particular facial appearance (e.g., a small upturned nose, narrow bifrontal diameter with almond‐shaped eyes, down‐ turned corners of the mouth with sticky salivary secretions and generally lighter skin, hair and eye color than other family members) [105, 106]. Hypothalamic dysfunction has been implicated in many manifestations of this syndrome including hyperphagia, temperature instability, high pain threshold, sleep‐disordered breathing and multiple endocrine abnor‐ malities [105, 107, 108].

Initially, two nutritional phases have been described in children with PWS:


To date, instead, seven different nutritional phases (five main phases and sub‐phases in phases 1 and 2) have been identified.

As following, focusing on nutrition, although in the early phases, the child has poor appetite, the latter increases in phase 2b and leads progressively to hyperphagia, evident in phase 3 (**Table 3**).


Adapted with permission from Cassidy et al. [107].

For management of the disease and to identify an accurate treatment, it is important for both the present of typical clinical features that an appropriate genetic diagnosis, which may be carried out by NGS techniques, thanks to its low cost compared with traditional polymerase

Prader‐Willi syndrome (PWS, OMIM #176270) is a disorder caused by errors in genomic imprinting, which generally occur during both male and female gametogenesis. In particular, there is the loss of expression of paternal genes normally active and located in the chromosome 15q11‐q13 region [104–108]. Conversely, a loss of expression of the preferentially maternally expressed *UBE3A* (OMIM \*601623) gene in this region leads to Angelman syndrome (AS; OMIM #105830), an entirely different clinical disorder that causes developmental disabilities and neurological problems, such as difficulty speaking, balancing and walking, and, in some

According to several studies, most individuals with PWS (about two‐thirds) have a de novo paternally inherited deletion of the chromosome 15q11‐q13 region; about 25% of cases have maternal disomy 15 (chromosome 15 is inherited from the mother) [111]; less than 3% of patients have defects in the genomic imprinting center due to microdeletions or epimutations [104, 106, 112, 113], while rearrangements of the 15q11‐q13 region or chromosomal transloca‐

However, this syndrome, whose prevalence is around of 1/10,000–1/30,000, is considered the

The cardinal features of PWS include infantile hypotonia, feeding difficulties due to a poor suck and failure to thrive (FTT), followed in later infancy or early childhood by excessive appetite with gradual development of obesity, short stature and/or decreased growth velocity due to growth hormone (GH) deficiency, intellectual disabilities (average IQ of 65), behavioral problems (e.g., temper tantrums, outburst and skin picking) and particular facial appearance (e.g., a small upturned nose, narrow bifrontal diameter with almond‐shaped eyes, down‐ turned corners of the mouth with sticky salivary secretions and generally lighter skin, hair and eye color than other family members) [105, 106]. Hypothalamic dysfunction has been implicated in many manifestations of this syndrome including hyperphagia, temperature instability, high pain threshold, sleep‐disordered breathing and multiple endocrine abnor‐

Initially, two nutritional phases have been described in children with PWS:

**•** phase 1: the individual often presents FTT; he exhibits hypotonia with difficult feeding; **•** phase 2: the individual is hyperphagic, and this condition will lead to obesity [105, 108].

To date, instead, seven different nutritional phases (five main phases and sub‐phases in phases

chain reaction and direct Sanger sequencing [103].

**4. Imprinted genetic syndromes**

228 Adiposity - Omics and Molecular Understanding

cases, seizures [109, 110].

tions are rare [104, 114].

malities [105, 107, 108].

1 and 2) have been identified.

most common cause of syndomic obesity [115].

**Table 3.** Clinical characteristics of the nutritional phases seen in Prader‐Willi syndrome.

Analyzing the seven phases, we highlight the following:


As said previously, individuals with PWS present an appetite that gradually increases and leads to obesity. In recent years, some studies have been conducted to understand the mech‐ anisms controlling appetitive behavior, energy expenditure and body composition.

The central nervous system, in particular the hypothalamus that determines changes in energy balance, is involved in these processes.

One of the determining factors for the development of obesity in these patients is ghrelin, a 28 amino acid peptide produced in the stomach, that transmit satiety signal and whose level in obese PWS individuals is high [116, 117]. Circulating ghrelin levels are elevated in young children with PWS long before the onset of hyperphagia, especially during the early phase of poor appetite and feeding [118].

The literature reports that about 25% of the adults with PWS presents NIDDM (non‐insulin‐ dependent diabetes mellitus) [119]; however, some studies show that in PWS, children fasting insulin concentrations and homeostasis model assessment insulin resistance index are lower than in obese control [120].

This syndrome, as mentioned, represents an human disorder related to genomic imprinting.

Although the DNA sequence of the imprinted maternally and paternally inherited alleles is the same, multiple epigenetic factors (such as DNA methylation, histone modifications and chromatin conformation) ultimately will determine whether the imprinted allele is expressed or repressed [121, 122].

DNA methylation analysis is the most efficient way to start the genetic workup if PWS is suspected clinically, but it cannot distinguish the molecular class (i.e., deletion; uniparental disomy, UPD; or imprinting defect, ID). Therefore, once the diagnosis of PWS is established by DNA methylation analysis, determination of the molecular class is the next step.

There are different genetic testing used in PWS: CMA‐SNP array or FISH (fluorescence in situ hybridization) for deletion of 15q11.2‐q13, DNA polymorphism analysis for UPD or ID or testing with MS‐MLPA analysis for an IC deletion, important for the diagnosis of both of these individuals who do not have sufficient features because they are too young than of those who do not exhibit the typical phenotype [107].

#### **4.1. Cohen syndrome**

Cohen syndrome (CS) is an inherited disorder characterized by developmental delay, intel‐ lectual disability, microcephaly and hypotonia. Other features include progressive myopia, retinal dystrophy, hypermobility and distinctive facial features [6, 12]. Characteristic facial features include thick hair and eyebrows, long eyelashes, down‐slanting and wave‐shaped, a bulbous nasal tip, a smooth or shortened philtrum, and prominent upper central teeth [6, 12]. Children with CS tend to manifest failure to thrive in infancy and early childhood but subsequently become significantly overweight in the late childhood and adolescence. The obesity tends to be truncal in nature [6, 12]. In contrast to PWS, appetite and food intake are not increased during this time period, and activity is not noticeably decreased. Among individuals with CS, the prevalence of short stature is approximately 65% and delayed puberty 74%; clinical endocrinologic evaluations did not identify explanations for these findings [6, 12].

#### **4.2. 1p36 deletion syndrome**

1p36 deletion syndrome is a disorder characterized by severe intellectual disability, hypotonia, heart defects, hearing impairment and typical craniofacial features. In fact, patients with this syndrome show straight eyebrows, deeply set eyes, midface hypoplasia, broad and flat nasal root/bridge, long philtrum, pointed chin, large, late‐closing anterior fontanel, microbrachyce‐ phaly, epicanthal folds and posteriorly rotated, low‐set, abnormal ears. Other typical findings include brachy/camptodactyly and short feet. Developmental delay and intellectual disability of variable degree are present in all, and hypotonia in 95%. Seizures occur in 44–58% of affected individuals. Other findings include prenatal‐onset growth deficiency, structural brain abnor‐ malities, congenital heart defects, vision problems, deafness, skeletal anomalies, abnormalities of the external genitalia and renal abnormalities. Obesity, which occurs as the consequence of hyperphagia, is also frequently observed in patients with the 1p36 deletion syndrome [123]. In this recent report [124], 40% of patients had obesity and hypercholesterolemia, and 1 patient developed NIDDM. Some authors suggested candidate regions for hyperphagia and obesity, such as *PRKCZ*, that may be associated with obesity because this gene is involved in carbo‐ hydrate or lipid metabolism, or insulin signaling [123]. It is suggested that genetic or environ‐ mental factors more likely contribute to the development of obesity and DM. However, a subset of patients may become overweight and obese with hyperphagia and NIDDM [125]. Previous studies observed that obesity was found exclusively in female patients with 1p36 deletion who showed growth restriction during the fetal period [126]. Because patients with 1p36 deletion show hypotonia and hyperphagia with obesity and NIDDM, which are also characteristic features of patients with PWS, some patients with 1p36 deletion may be misdiagnosed as having PWS.

### **4.3. 16p11.2 deletion syndrome**

One of the determining factors for the development of obesity in these patients is ghrelin, a 28 amino acid peptide produced in the stomach, that transmit satiety signal and whose level in obese PWS individuals is high [116, 117]. Circulating ghrelin levels are elevated in young children with PWS long before the onset of hyperphagia, especially during the early phase of

The literature reports that about 25% of the adults with PWS presents NIDDM (non‐insulin‐ dependent diabetes mellitus) [119]; however, some studies show that in PWS, children fasting insulin concentrations and homeostasis model assessment insulin resistance index are lower

This syndrome, as mentioned, represents an human disorder related to genomic imprinting.

Although the DNA sequence of the imprinted maternally and paternally inherited alleles is the same, multiple epigenetic factors (such as DNA methylation, histone modifications and chromatin conformation) ultimately will determine whether the imprinted allele is expressed

DNA methylation analysis is the most efficient way to start the genetic workup if PWS is suspected clinically, but it cannot distinguish the molecular class (i.e., deletion; uniparental disomy, UPD; or imprinting defect, ID). Therefore, once the diagnosis of PWS is established

There are different genetic testing used in PWS: CMA‐SNP array or FISH (fluorescence in situ hybridization) for deletion of 15q11.2‐q13, DNA polymorphism analysis for UPD or ID or testing with MS‐MLPA analysis for an IC deletion, important for the diagnosis of both of these individuals who do not have sufficient features because they are too young than of those who

Cohen syndrome (CS) is an inherited disorder characterized by developmental delay, intel‐ lectual disability, microcephaly and hypotonia. Other features include progressive myopia, retinal dystrophy, hypermobility and distinctive facial features [6, 12]. Characteristic facial features include thick hair and eyebrows, long eyelashes, down‐slanting and wave‐shaped, a bulbous nasal tip, a smooth or shortened philtrum, and prominent upper central teeth [6, 12]. Children with CS tend to manifest failure to thrive in infancy and early childhood but subsequently become significantly overweight in the late childhood and adolescence. The obesity tends to be truncal in nature [6, 12]. In contrast to PWS, appetite and food intake are not increased during this time period, and activity is not noticeably decreased. Among individuals with CS, the prevalence of short stature is approximately 65% and delayed puberty 74%; clinical endocrinologic evaluations did not identify explanations for these findings [6, 12].

1p36 deletion syndrome is a disorder characterized by severe intellectual disability, hypotonia, heart defects, hearing impairment and typical craniofacial features. In fact, patients with this

by DNA methylation analysis, determination of the molecular class is the next step.

poor appetite and feeding [118].

230 Adiposity - Omics and Molecular Understanding

than in obese control [120].

or repressed [121, 122].

**4.1. Cohen syndrome**

**4.2. 1p36 deletion syndrome**

do not exhibit the typical phenotype [107].

16p11.2 microdeletion syndrome is a chromosomal anomaly characterized by developmental and language delays, intellectual disability, social impairments represented by autism spectrum disorders, variable dysmorphisms and predisposition to obesity. In fact, in a screening cohort of patients with extreme obesity, enriched for patients with birth defects and/ or neurocognitive deficiencies using method to detect copy number variations, recurrent, *de novo* deletions of 16p11.2 were identified in approximately 3% of cases. In these patients, durable weight loss has not been reported. So durable weight control is recommended although no data are available on the efficacy of early intervention in deletion carriers. However, impaired cognition may also result in abnormal eating behavior contributing to the obesity [127, 128]. Some data seem to hypothesize that this deletion may affect the neural circuitry involved in the energy balance. The early increase in head circumference seems to precede the onset of obesity [129]. The 16p11.2 deletion includes the *SH2B1* gene, an adaptor protein involved in leptin and insulin signaling which may be involved in the pathogenesis of the obesity and insulin resistance observed in this deletion [130].

Additionally, deficiencies of *SIM1* (single minded), *BDNF* (brain‐derived neurotrophic factor) and *NTRK2* (neurotrophic tyrosine receptor kinase encoding the TrK protein, the receptor for BDNF) genes are associated with syndromic conditions involved in the functioning of the hypothalamus downstream of MC4R‐expressing neurons and leading severe hyperphagic obesity. For example, haplodeficiency of *BDNF* has also been implicated in the obesity occur‐ ring in a subset of patients with WAGR (Wilms tumor, aniridia, genitourinary malformations and retardation) syndrome [62].
