INTRODUCTION
Obesity is an important public health issue that is prevalent worldwide and is associated with early death and complications such as type 2 diabetes, dyslipidemia, hypertension, and cardiovascular diseases and certain cancers [1,2]. According to the genetic contribution, obesity can be classified into three categories. The majority cases of obesity are classified as “common polygenic obesity,” which result from the interaction of genetic susceptibilities and environmental factors [3]. “Syndromic obesity” is characterized by obesity combined with developmental delay or dysmorphism. Bardet-Biedle, Prader-Willi, and Alstrom syndromes are well known examples of syndromic obesity. “Monogenic obesity” is resulted from pathogenic variants in single genes, which are generally involved in the hypothalamic leptin-melanocortin pathway associated with regulation of hunger and satiety [3]. Monogenic obesity has a Mendelian inheritance pattern, low prevalence, and early-onset severe clinical manifestations, whereas common polygenic obesity has heritability similar to other multifactorial diseases and high prevalence [4]. Genetic variants in single genes lead to large effects and high penetrance in monogenic obesity. In contrast, various of variants in several genes interacting with environmental factors are responsible for polygenic obesity. However, the influence of genetics on obesity is on a continuous spectrum rather than the distinct phenotypic classification [4]. Although the majority cases of obesity are common polygenic obesity, many insights regarding the biology and molecular mechanisms have been discovered from monogenic obesity.
This review covers the genetics of monogenic and common polygenic obesity, including the difference in genetic approaches, known monogenic obesity disorders, therapeutic implications, and perspectives of genetic studies in obesity.
GENETIC APPROACHES FOR MONOGENIC OBESITY
Early studies for identification of causative genes in monogenic obesity were designed in a case-controlled format. Thus, patients with severe early-onset obesity and their affected or unaffected family members were investigated for identification of causative genes by Sanger sequencing analysis [4]. However, the advances in next-generation sequencing (NGS) technology led to expansion of candidate gene panel sequencing and whole exome or whole genome sequencing. NGS-based genetic investigations in groups of patients with severe early-onset obesity have identified causative genetic variations in monogenic obesity.
MONOGENIC OBESITY
Genetic studies have found that the leptin-melanocortin pathway plays a crucial role in regulation of appetite, hunger, satiety, and body mass index (BMI), and many cases of monogenic obesity result from defects in this pathway [5]. Leptin, a hormone secreted from adipose tissue, works through the leptin receptor located in the arcuate nucleus in the hypothalamus. It stimulates the production of proopiomelanocortin (POMC) in the POMC neurons. Then, proprotein convertase subtilisin/kexin type 1 (PCSK1) mediates the process of converting POMC to melanocyte-stimulating hormone (MSH) [5]. MSH interact with the melanocortin receptors, including the melanocortin 4 receptor (MC4R) in the brain. Activation of MC4R results in reduction of food intake and concomitant increase of energy comsumption [5]. In addition, leptin-mediated synaptic plasticity of neurons is modulated by brain-derived neurotrophic factor (BDNF) in the brain [3].
Clinical manifestations of individuals with monogenic obesity are distinct from those of common polygenic obesity. Patients with monogenic obesity gains body weight rapidly starting early infancy, and the velocity of increasing BMI reach to peak during childhood [3]. Therefore, clinicians should suspect monogenic obesity when patients have extreme obesity (BMI ≥ 120% of the 95th percentile or ≥ 35 kg/m2) during childhood, fast weight gain in the first several years after birth, with hyperphagia [6]. Patients with monogenic obesity can have additional clinical manifestations, such as short stature, red hair, susceptibility to infection, recurrent diarrhea, or endocrine defects, including pituitary insufficiencies, diabetes insipidus, adrenal insufficiency, hypothyroidism, or hypogonadism [3,6]. The clinical and genetic implications of monogenic obesity disorders are shown in Table 1.
MC4R deficiency
MC4R deficiency is the most common monogenic obesity disorder [3,7]. Pathogenic variants in the MC4R gene cause approximately 2% to 5% of the cases of extreme early-onset obesity [5,8]. Clinical manifestations of MC4R deficiency include hyperphagia, severe early-onset obesity, hyperinsulinemia accompanied with lean body mass increase, and rapid linear growth [9]. In contrast, individuals who have gain-offunction mutations in MC4R gene have decreased food intake, high energy expenditure, and low BMI [10]. The variants in MC4R gene can be inherited either in an autosomal dominant or recessive pattern. Clinical symptoms in patients carrying homozygous or compound heterozygous variants are more severe than those in patients with heterozygous variants.
Leptin deficiency
Leptin deficiency is an extremely rare monogenic obesity disorder and under 100 cases have been published to date [4,11]. Leptin deficiency is caused by the pathogenic variants in the LEP gene. The clinical symptoms include severe hyperphagia and early-onset obesity. If patients have biallelic variants in LEP, they can have additional clinical manifestation, including frequent infections and endocrine defects, such as hypogonadotropic hypogonadism and hypothyroidism [12]. Patients with leptin deficiency have nondetectable serum leptin levels [13].
Leptin receptor deficiency
Leptin receptor deficiency is a rare autosomal recessive disease caused by biallelic pathogenic variants in the LEPR gene [14]. The phenotype is similar to leptin deficiency except increase of serum leptin levels [14]. Individuals having leptin receptor deficiency show rapid weight gain and severe childhood obesity resulted from prominent food-seeking behaviors from childhood, hyperphagia, and insufficient satiety [15]. Accompanied endocrine defects are comparable to the those of leptin deficiency, but recurrent infections are nearly observed [14,15].
POMC deficiency
POMC deficiency is an autosomal recessive monogenic obesity disorder. Clinical symptoms include hyperphagia resulting in severe early-onset obesity, pigmentary abnormalities including red hair or pale skin color, and adrenal insufficiency. A pituitary preproprotein encoded by POMC is a precursor of several neuroendocrine peptides. α-MSH, one of the neuroendocrine peptides derived from POMC by the action of PCSK1, works for suppression of appetite and eating through the interaction with MC4R [16].
PCSK1 deficiency
Patients with PCSK1 deficiency presents with early-onset severe obesity accompanied by diarrhea in the infancy and postprandial hypoglycemia [17,18]. Additional clinical manifestations include neuroendocrine defects, such as diabetes insipidus, hypogonadotropic hypogonadism, adrenal insufficiency, and hypothyroidism [17,18]. Biallelic pathogenic variants in the PCSK1 gene cause impairment in converting prohormones to active hormones, gastric peptides, and proinsulin [3,17,18].
GENETIC APPROACHES FOR POLYGENIC OBESITY
Genetic variations attribute to common polygenic obesity have been identified through population-based large association studies [4]. Over the past 2 decades, the advances in genome-wide association studies (GWAS) accompanied by high-throughput genome-wide genotyping technics have enabled the identification of genetic variants throughout the whole genome [4]. In 2007, the GWAS for obesity phenotype demonstrated that several common variants in the intron of FTO was associated with BMI in the large populations [19,20]. Numerous following GWAS identified more than 1,000 loci associated with the obesity trait to date [4,21]. Following that, GWAS for refined traits of obesity have been conducted, such as percentage of body fat, lean body mass, imaging-derived adipose tissue, serum leptin and leptin receptor levels, and persistent healthy thinness [4,22-29]. These detailed phenotypes represent more accurate aspect of appetite and body weight regulation, and the identified loci are associated with more relevant biological pathways underlying obesity [4].
Although GWAS have concerned with common sequence variants with > 5% minor allele frequency, there are studies demonstrating the impact of copy number variations (CNVs) in common polygenic obesity. For example, the deletion of 1p31.1 near NEGR1 [30], 16p12.3 deletion upstream of GPRC5B [31], 10q11.2 CNV in PPTR1 [32], and 1p21.1 multi-allele CNV encompassing AMY1A [33] are reported as CNVs associated with obesity traits.
Similar to the discovery of genetic variations for other multifactorial diseases, the genetics of common obesity also have a substantial bias in study population [4]. As the majority of GWAS have been conducted in population from European ancestry, the result is not always generalizable to people of Asian, African, Hispanic, or other ancestries. Because obesity is a multifactorial disorder, some GWAS have considered environmental factors, including sex, age, physical activity, diet, or smoking into genetic analyses [34-37]. However, this approach is still challenging given that only 12 loci have been identified to date [4]. The impacts of environmental factors on obesity could attenuate or exacerbate the impacts of genetic variations. Nevertheless, the FTO locus and a healthy lifestyle have been replicated and reproduced firmly. A healthy diet or increased physical activity could attenuate the effect of the FTO locus on obesity risk by 30% to 40% [38,39]. As large-size population cohorts and biobanks, including UK biobank, All of Us, and Biobank Japan, are ongoing, more comprehensive meta-analysis could expand the insight regarding genetic-environmental interactions.
NEW PHARMACOLOGICAL THERAPEUTIC APPROACHES
Genetic insights have enabled development of new therapeutic drugs for several types of monogenic obesity in the context of precision medicine. The precise genetic diagnosis can allow the prescription of personalized treatment. Although standard genetic panel testing is not available so far, genetic testing in special patient groups who have early-onset extreme obesity are recommended for early diagnosis of “treatable” monogenic obesity and timely intervention [5]. The development of pharmacological agents for monogenic obesity is meaningful given that a therapeutic agent developed for one disorder could also benefit for other types of monogenic obesities because these diseases are caused by impairment of common leptin-melanocortin pathway [40]. Furthermore, these therapeutic approaches could be also effective in common polygenic obesity given that the genetic trait have an important role in common polygenic obesity, too.
Metreleptin
Subcutaneous injection of human recombinant leptin (metreleptin) is remarkably beneficial for patients with leptin deficiency resulting in rapid improvement in food-seeking behavior, a decrease of food intake, reduction of fat mass and consequential body weight loss [41]. It also lead to amelioration of metabolic and endocrine abnormalities, such as hyperinsulinemia, hyperlipidemia, fatty liver, and hypogonadotrophic hypogonadism [3,41]. Reported side effects of metreleptin include the production of neutralizing antibodies and an increased risk of lymphomas [3,42].
Setmelanotide
Setmelanotide is a selective MC4R agonist mimicking the POMC derivative α-MSH [43]. Daily subcutaneous injection of setmelanotide resulted in significant loss of body weight and reduction of hunger [43,44]. In 2020, the Food and Drug Administration (FDA) approved the use of setmelanotide for treatment of monogenic obesity due to POMC, PCSK1, or leptin receptor deficiency in adults and children aged 6 years and older [45]. The adverse events of setmelanotide were mild and included hyperpigmentation, nausea, vomiting, and injection site reactions [4]. The phase II or phase III clinical trials are ongoing to evaluate the efficacy of setmelanotide as a therapeutic option for other genetic disorders involving the MC4R pathway [3]. Efficacy of setmelanotide is also being investigated in patients with syndromic obesity, such as Bardet-Biedle syndrome and Alstrom syndrome as well as in the chromosomal rearrangement of the 16p11.2 [3,46,47].
CONCLUSION
It is being remarkably known that monogenic and polygenic obesities are not distinct categories, but are on a continuous spectrum and share biological pathways. As GWAS have discovered more obesity-associated loci, the majority of the loci identified in monogenic obesity involve the leptin-melanocortin pathway. Although the translation from variants discovered through GWAS to function is still challenging, in advent with high-throughput techniques, omics data, and advanced analytic tools could provide more opportunities to understand the genetics of obesity which could contribute to development of novel therapeutic approaches in context of precision medicine.
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