**2.5. Melanocortin‐4 receptor deficiency (MC4R)**

Among all forms of monogenic obesity, the most common is caused by MC4‐R deficiency. Heterozygous mutations have been reported in many ethnic groups of obese patients and prevalence varies from 0.5 to 1.0% in obese adults, up to 6% in individuals with severe infantile onset obesity [45]. In 2014, a case of childhood obesity associated with compound hetero‐ zygosity for two mutations of *MC4R* gene (OMIM \*155541), mapped at 18q21.32, was descri‐ bed [46]. In the same year, another new inactivating homozygous mutation of the *MC4R* gene in a girl with the severe obesity and hyperphagia was reported [47].

Mutations of this gene are codominant with variable penetrance and expressivity in hetero‐ zygous carriers [48]. Both heterozygous and homozygous mutations in *MC4R* have been implicated in obesity, but extreme obesity is incompletely penetrant in heterozygous patients [3]. Also, in these patients, genetic and environmental factors influence the severity of obesity associated with mutations of *MC4‐R*.

The main clinical features include hyperphagia in early appearance (but not as severe as that seen in leptin deficiency) and an increase in fat mass, lean mass and bone mineral density [45]. These patients also have an accelerated growth that seems to be a consequence of hyperinsu‐ linemia which such patients present from the earliest periods of life. It is apparently not related to a dysfunction of the GH axis [3, 49]. Despite this early hyperinsulinemia, obese adult subjects who are heterozygous for mutations in the *MC4R* gene are not at increased risk of developing glucose intolerance and NIDDM compared to controls of similar age and adiposity [12, 45].

Currently, there are no specific therapies for the MC4‐R deficiency, but these individuals may benefit from surgical therapies, which could be taken into consideration in adults [12].

## **2.6. PCSK1 deficiency (OMIM #600955)**

Pro‐protein convertases (PCs) are a family of serine endoproteases that cleave inactive pro‐ peptides into biologically active peptides [50]. Two of these pro‐protein, proprotein convertase, subtilisin/kexin‐type 1 (PCSK1) and PCSK2 are selectively expressed in neuroendocrine tissues and cleave pro‐hormones such as POMC, thyrotropin‐releasing hormone (TRH), GnRH, proinsulin, proglucagon [12].

Patients with heterozygous or homozygous mutations in the *PCSK1* gene (OMIM \*162150), mapped at 5q15, present small bowel enteropathy, early‐onset obesity and complex neuroen‐ docrine effects due to a failure to process the pro‐hormones such as diabetes insipidus, glucocorticoid deficiency, hypogonadism, and altered glucose homeostasis [51, 52].

A typical characteristic of these patients is a history of severe intestinal malabsorption in the neonatal period, probably due to altered cleavage of intestinal peptides in the enteroendocrine cells [51].

Over the past few years, two meta‐analysis about *PCSK1* mutations have been published: the first in 2014 confirmed the association of *PCSK1* SNPs with obesity and provides the first evidence that the association between *PCSK1 rs6232* and obesity is stronger for childhood obesity than for adult obesity; the second meta‐analysis tried to study the association of *PCSK1* variants *rs6232* and *rs6234/rs6235* with quantitative BMI variation and common obesity risk in subjects from diverse ethnic groups. In this study, cohort age‐group significantly modulated the association between *rs6232*, *rs6234/rs6235* and obesity with the effect sizes for both SNPs being stronger in children/adolescents than in adults.

It is thought also that the most common PCSK1 variants predispose to obesity especially in an "obesogenic" environment with free access to high‐caloric food [53].

Currently, there are no specific therapies for the PCSK1 deficiency, but these individuals frequently required a prolonged course of parenteral nutrition therapy, particularly in the first year of life [54]. However, exogenous administration of several hormone may be necessary in relation to the hormonal deficiencies diagnosed [54].

**Figure 2.** Girl with 6q16.3 deletion involving *SIM1* gene. It is evident that the extreme increase of the BMI of the patient and the reduction after the interdisciplinary approach.

#### **2.7. SIM1 deficiency**

carrying the *Tyr221Cys* variant are hyperphagic and showed increased linear growth, features

Specific treatment was not available until January 2016, when the US Food and Drug Admin‐ istration awarded orphan drug status to the first α‐MSH‐based therapy for obesity. The α‐MSH analog RM‐493 [43, 44], also known as setmelanotide, was awarded orphan drug status for

Among all forms of monogenic obesity, the most common is caused by MC4‐R deficiency. Heterozygous mutations have been reported in many ethnic groups of obese patients and prevalence varies from 0.5 to 1.0% in obese adults, up to 6% in individuals with severe infantile onset obesity [45]. In 2014, a case of childhood obesity associated with compound hetero‐ zygosity for two mutations of *MC4R* gene (OMIM \*155541), mapped at 18q21.32, was descri‐ bed [46]. In the same year, another new inactivating homozygous mutation of the *MC4R* gene

Mutations of this gene are codominant with variable penetrance and expressivity in hetero‐ zygous carriers [48]. Both heterozygous and homozygous mutations in *MC4R* have been implicated in obesity, but extreme obesity is incompletely penetrant in heterozygous patients [3]. Also, in these patients, genetic and environmental factors influence the severity of obesity

The main clinical features include hyperphagia in early appearance (but not as severe as that seen in leptin deficiency) and an increase in fat mass, lean mass and bone mineral density [45]. These patients also have an accelerated growth that seems to be a consequence of hyperinsu‐ linemia which such patients present from the earliest periods of life. It is apparently not related to a dysfunction of the GH axis [3, 49]. Despite this early hyperinsulinemia, obese adult subjects who are heterozygous for mutations in the *MC4R* gene are not at increased risk of developing glucose intolerance and NIDDM compared to controls of similar age and adiposity [12, 45].

Currently, there are no specific therapies for the MC4‐R deficiency, but these individuals may benefit from surgical therapies, which could be taken into consideration in adults [12].

Pro‐protein convertases (PCs) are a family of serine endoproteases that cleave inactive pro‐ peptides into biologically active peptides [50]. Two of these pro‐protein, proprotein convertase, subtilisin/kexin‐type 1 (PCSK1) and PCSK2 are selectively expressed in neuroendocrine tissues and cleave pro‐hormones such as POMC, thyrotropin‐releasing hormone (TRH), GnRH,

Patients with heterozygous or homozygous mutations in the *PCSK1* gene (OMIM \*162150), mapped at 5q15, present small bowel enteropathy, early‐onset obesity and complex neuroen‐ docrine effects due to a failure to process the pro‐hormones such as diabetes insipidus,

glucocorticoid deficiency, hypogonadism, and altered glucose homeostasis [51, 52].

of MC4R deficiency [42].

220 Adiposity - Omics and Molecular Understanding

POMC deficiency and Prader‐Willi syndrome [37].

**2.5. Melanocortin‐4 receptor deficiency (MC4R)**

associated with mutations of *MC4‐R*.

**2.6. PCSK1 deficiency (OMIM #600955)**

proinsulin, proglucagon [12].

in a girl with the severe obesity and hyperphagia was reported [47].

Single‐minded 1 (SIM1) is a transcription factor involved in the development of the supraoptic and paraventricular nuclei, acting downstream signal cascade of MC4‐R. Obesity and hyperphagia have been reported in a patient with a balanced translocation disrupting *SIM1* [55] and multiple heterozygous missense mutations (6q16.3; OMIM \*603128) [56]. However, some mutations of *SIM1* have incomplete penetrance and variable phenotype [57]. The similar phenotype between patients with SMI1 and MC4‐R deficiency suggests that some effects of SIM1 are mediated by altered melanocortin signaling. On the other hand, some children with *SIM1* mutations have neuro‐behavioral disorders including autism spectrum and "Prader‐ Willi‐like" phenotype (**Figure 2**) [3, 12].

In mice, hyperphagia associated with SIM1 deficit can be improved by the administration of oxytocin, a neurotransmitter involved in the modulation of emotion (impaired oxytocinergic signaling is also one possible mechanism implicated in the obesity) [58].

#### **2.8. Other types of non‐syndromic genetic obesity**

Mutations of the *BDNF* (*brain‐derived neurotrophic factor*, OMIM \*113505, mapped at 11p14.1) and its receptor TrKb (*tyrosin kinase B receptor*, OMIM \*600456, mapped at 9q21.33) are rare causes of monogenic obesity acting downstream signal cascade of MC4‐R and blocking translation [59].

BDNF's role in energy homeostasis emerged in the 1990s with the observation that intracere‐ broventricular BDNF administration suppresses appetite and induces weight loss in ro‐ dents, and *Bdnf* heterozygous knockout mice exhibit hyperphagia and obesity [60]. Complete lack of BDNF during embryologic development is perinatally lethal, but haploinsufficiency for BDNF or inactivating mutations of the BDNF receptor was associated with increased ad libitum food intake, severe early‐onset obesity, hyperactivity, and cognitive impairment [60, 61]. Multiple genome‐wide association studies of obesity in children and adults of different racial and ethnic populations have found associations for single‐nucleotide polymorphisms (SNPs) at the BDNF locus and BMI, in particular for *G196A* variant (*rs6265*), which leads to a valine to methionine substitution at the 66th amino acid position (*Val66Met*) of the N‐terminal prodomain of pro‐BDNF. Furthermore, modifying factors—particularly sex, lifestyle behav‐ iors, and psychotropic medication use—appear to be important confounders for the associa‐ tion between rs6265 and BMI [60–62]. In addition, the minor C allele of intronic *rs12291063* SNP was associated with lower BDNF expression and higher BMI [63].

*NTRK2* (*TrkB*) mutation (which interferes with receptor autophosphorylation) causes the same symptoms of BDNF deficiency such as hyperphagia, obesity, impaired nociception, and intellectual disability [64, 65]. Recently, a *de novo* mutations in *TrkB* was found in a boy with severe obesity and impairment in learning, memory and nociception, and in a girl with hyperphagia and severe obesity [66].

Another cause of non‐syndromic monogenic obesity is due to a gene mutation of *CART* (*cocaine‐ and amphetamine‐regulated transcript*, OMIM \*602606), mapped at 5q13.2. CART is an anorexigenic peptide produced by specific hypotalamic neurons in response to the stimulus of leptin. It would appear to mediate the termogenetic effects and energy expenditures characteristic of leptin. It has been shown that mutations in the *CART* gene are associated with reduced levels of the peptide encoded by it. Adolescents carrying a missense mutation in the *CART* gene exhibit severe obesity associated with anxiety and depression [11, 67, 68].

Other recent forms of monogenic obesity, still being defined, are associated with *MRAP2* (*melanocortin 2 receptor accessory protein 2*, OMIM \*615410, mapped at 6q14.2) mutation encod‐ ing a *MC4‐R* co‐receptor, and with *KSR2* (*Kinase suppressor of Ras 2*. OMIM \*610737, mapped at 12q24.22‐q24.23) mutation, a protein involved in intracellular signal with a role in energy homeostasis [69–72].
