Lack of red hair phenotype in a North-African obese child homozygous for a novel POMC null mutation: nonsense-mediated decay RNA evaluation and hair pigment chemical analysis

Body Fat Distribution Contributes to Defining the Relationship between Insulin Resistance and Obesity in Human Diseases

The risk for metabolic and cardiovascular complications of obesity is defined by body fat distribution rather than global adiposity. Unlike subcutaneous fat, visceral fat (including hepatic steatosis) reflects insulin resistance and predicts type 2 diabetes and cardiovascular disease. In humans, available evidence indicates that the ability to store triglycerides in the subcutaneous adipose tissue reflects enhanced insulin sensitivity. Prospective studies document an association between larger subcutaneous fat mass at baseline and reduced incidence of impaired glucose tolerance. Case-control studies reveal an association between genetic predisposition to insulin resistance and a lower amount of subcutaneous adipose tissue. Human peroxisome proliferator-activated receptorgamma (PPAR-γ) promotes subcutaneous adipocyte differentiation and subcutaneous fat deposition, improving insulin resistance and reducing visceral fat. Thiazolidinediones reproduce the effects of PPAR-γ activation and therefore increase the amount of subcutaneous fat while enhancing insulin sensitivity and reducing visceral fat. Partial or virtually complete lack of adipose tissue (lipodystrophy) is associated with insulin resistance and its clinical manifestations, including essential hypertension, hypertriglyceridemia, reduced HDL-c, type 2 diabetes, cardiovascular disease, and kidney disease. Patients with Prader Willi syndrome manifest severe subcutaneous obesity without insulin resistance. The impaired ability to accumulate fat in the subcutaneous adipose tissue may be due to deficient triglyceride synthesis, inadequate formation of lipid droplets, or defective adipocyte differentiation. Lean and obese humans develop insulin resistance when the capacity to store fat in the subcutaneous adipose tissue is exhausted and deposition of triglycerides is no longer attainable at that location. Existing adipocytes become large and reflect the presence of insulin resistance.

Study of LEP, MRAP2 and POMC genes as potential causes of severe obesity in Brazilian patients

PURPOSE

Monogenic forms of obesity are caused by single-gene variants which affect the energy homeostasis by increasing food intake and decreasing energy expenditure. Most of these variants result from disruption of the leptin-melanocortin signaling, which can cause severe early-onset obesity and hyperphagia. These mutation have been identified in genes encoding essential proteins to this pathway, including leptin (LEP), melanocortin 2 receptor accessory proteins 2 (MRAP2) and proopiomelanocortin (POMC). We aimed to investigate the prevalence of LEP, MRAP2 and POMC rare variants in severely obese adults, who developed obesity during childhood. To the best of our knowledge, this is the first study screening rare variants of these genes in patients from Brazil.

METHODS

A total of 122 Brazilian severely obese patients (BMI ≥ 35 kg/m²) were screened for the coding regions of LEP, MRAP2 and POMC by Sanger sequencing. All patients are candidates to the bariatric surgery. Clinical characteristics were described in patients with novel and/or potentially pathogenic variants.

RESULTS

Sixteen different variants were identified in these genes, of which two were novel. Among them, one previous variant with potentially deleterious effect in MRAP2 (p.Arg125Cys) was found. In addition, two heterozygous mutations in POMC (p.Phe87Leu and p.Arg90Leu) were predicted to impair protein function. We also observed a POMC homozygous 9 bp insertion (p.Gly99_Ala100insSerSerGly) in three patients. No pathogenic variant was observed in LEP.

CONCLUSION

Our study described for the first time the prevalence of rare potentially pathogenic MRAP2 and POMC variants in a cohort of Brazilian severely obese adults.

LEVEL OF EVIDENCE

Level V, cross-sectional descriptive study.

Proopiomelanocortin deficiency diagnosed in infancy in two boys and a review of the known cases

Proopiomelanocortin (POMC) deficiency is a rare monogenic disorder characterised by adrenocorticotropic hormone (ACTH) deficiency, red hair and hyperphagic obesity. Two unrelated cases presented with hypoglycaemia due to isolated ACTH deficiency in the neonatal period. POMC deficiency was suspected at age 2 years (c.133-2A>C) and at age 9 months (c.64del) due to infantile hyperphagic obesity. Neither patient had a convincing red hair phenotype at the time of diagnostic suspicion, illustrating the importance of suspecting POMC deficiency in isolated ACTH deficiency. Both patients have normal psychomotor development, whereas the only other reported case of c.64del had significant delay. This suggests, if ACTH deficiency is treated early in the neonatal period, that psychomotor retardation is not a part of the phenotype. We review 24 reported cases of POMC deficiency published to date. Although there is no current specific treatment for obesity in POMC deficiency, we anticipate that setmelanotide may be a useful future treatment option.

Eating behaviour in contrasting adiposity phenotypes: Monogenic obesity and congenital generalized lipodystrophy

Most known types of nonsyndromic monogenic obesity are caused by rare mutations in genes of the leptin-melanocortin pathway controlling appetite and adiposity. In contrast, congenital generalized lipodystrophy represents the most extreme form of leanness in humans caused by recessive mutations in four genes involved in phospholipid/triglyceride synthesis and lipid droplet/caveolae structure. In this disease, the inability to store triglyceride in adipocytes results in hypoleptinemia and ectopic hepatic and muscle fat accumulation leading to fatty liver, hypertriglyceridemia and severe insulin resistance. As a result of hypoleptinemia, patients with lipodystrophy show alterations in eating behaviour characterized by constant increased energy intake. As it occurs in obesity caused by genetic leptin deficiency, exogenous leptin rapidly reduces hunger scores in patients with congenital generalized lipodystrophy, with additional beneficial effects on glucose homeostasis and metabolic profile normalization. The melanocortin-4 receptor agonist setmelanotide has been used in the treatment of monogenic obesities. There is only one report on the effect of setmelanotide in a patient with partial lipodystrophy resulting in mild reductions in hunger scores, with no improvements in metabolic status. The assessment of contrasting phenotypes of obesity/leanness represents an adequate strategy to understand the pathophysiology and altered eating behaviour associated with adipose tissue excessive accumulation/paucity.

Two Cases With an Early Presented Proopiomelanocortin Deficiency-A Long-Term Follow-Up and Systematic Literature Review

Proopiomelanocortin (POMC) deficiency is an extremely rare inherited autosomal recessive disorder characterized by severe obesity, adrenal insufficiency, skin hypopigmentation, and red hair. It is caused by pathogenic variants in the POMC gene that codes the proopiomelanocortin polypeptide which is cleaved to several peptides; the most notable ones are adrenocorticotropic hormone (ACTH), alpha- and beta-melanocyte-stimulating hormones (α-MSH and β-MSH); the latter two are crucial in melanogenesis and the energy balance by regulating feeding behavior and energy homeostasis through melanocortin receptor 4 (MC4R). The lack of its regulation leads to polyphagia and early onset severe obesity. A novel MC4R agonist, setmelanotide, has shown promising results regarding weight loss in patients with POMC deficiency. A systematic review on previously published clinical and genetic characteristics of patients with POMC deficiency and additional data obtained from two unrelated patients in our care was performed. A 25-year-old male patient, partly previously reported, was remarkable for childhood developed type 1 diabetes (T1D), transient growth hormone deficiency, and delayed puberty. The second case is a girl with an unusual presentation with central hypothyroidism and normal pigmentation of skin and hair. Of all evaluated cases, only 50% of patients had characteristic red hair, fair skin, and eye phenotype. Central hypothyroidism was reported in 36% of patients; furthermore, scarce adolescent data indicate possible growth axis dysbalance and central hypogonadism. T1D was unexpectedly prevalent in POMC deficiency, reported in 14% of patients, which could be an underestimation. POMC deficiency reveals to be a syndrome with several endocrinological abnormalities, some of which may become apparent with time. Apart from timely diagnosis, careful clinical follow-up of patients through childhood and adolescence for possible additional disease manifestations is warranted.

Mammalian pigmentation is regulated by a distinct cAMP-dependent mechanism that controls melanosome pH

The production of melanin increases skin pigmentation and reduces the risk of skin cancer. Melanin production depends on the pH of melanosomes, which are more acidic in lighter-skinned than in darker-skinned people. We showed that inhibition of soluble adenylyl cyclase (sAC) controlled pigmentation by increasing the pH of melanosomes both in cells and in vivo. Distinct from the canonical melanocortin 1 receptor (MC1R)-dependent cAMP pathway that controls pigmentation by altering gene expression, we found that inhibition of sAC increased pigmentation by increasing the activity of tyrosinase, the rate-limiting enzyme in melanin synthesis, which is more active at basic pH. We demonstrated that the effect of sAC activity on pH and melanin production in human melanocytes depended on the skin color of the donor. Last, we identified sAC inhibitors as a new class of drugs that increase melanosome pH and pigmentation in vivo, suggesting that pharmacologic inhibition of this pathway may affect skin cancer risk or pigmentation conditions.

A Patient with Proopiomelanocortin Deficiency: An Increasingly Important Diagnosis to Make

Proopiomelanocortin (POMC) deficiency is a rare monogenic disorder with early-onset obesity. Investigation of this entity have increased our insight into the important role of the leptin-melanocortin pathway in energy balance. Here, we present a patient with POMC deficiency due to a homozygous c.206delC mutation in the POMC gene. We discuss the pathogenesis of this condition with emphasis on the crosstalk between hypothalamic and peripheral signals in the development of obesity and the POMC-melanocortin 4 receptors system as a target for therapeutic intervention.

Ethnic and population differences in the genetic predisposition to human obesity

Obesity rates have escalated to the point of a global pandemic with varying prevalence across ethnic groups. These differences are partially explained by lifestyle factors in addition to genetic predisposition to obesity. This review provides a comprehensive examination of the ethnic differences in the genetic architecture of obesity. Using examples from evolution, heritability, admixture, monogenic and polygenic studies of obesity, we provide explanations for ethnic differences in the prevalence of obesity. The debate over definitions of race and ethnicity, the advantages and limitations of multi-ethnic studies and future directions of research are also discussed. Multi-ethnic studies have great potential to provide a better understanding of ethnic differences in the prevalence of obesity that may result in more targeted and personalized obesity treatments.

Delayed diagnosis of proopiomelanocortin (POMC) deficiency with type 1 diabetes in a 9-year-old girl and her infant sibling

BACKGROUND

Proopiomelanocortin (POMC) protein, encoded by the POMC gene, is the precursor of adrenocorticotropic hormone (ACTH) that is released from the anterior pituitary gland. Homozygous mutations in the POMC gene is associated with hyperphagia, severe and early-onset obesity, adrenal insufficiency, hypopigmentation of the skin and red hair.

CASE PRESENTATION

A 9-year-old girl from a consanguineous family of Iraqi origin was diagnosed with type 1 diabetes. She also had a tall stature. Her laboratory assessment showed low cortisol and ACTH concentrations, normal renin and poor response to ACTH stimulation. Genetic testing revealed a novel biallelic mutation in the POMC gene. Her sibling who had severe obesity and central adrenal insufficiency was found to be a carrier of the same mutation. Both siblings had alabaster-colored skin and brown hair.

CONCLUSIONS

POMC deficiency results in significant morbidity due to obesity, and it is also a potentially life threatening disease because of adrenal insufficiency. Therefore any suggestive symptom or sign of POMC deficiency warrants detailed investigations.

Genome-wide association studies of human adiposity: Zooming in on synapses

The decade anniversary for genome-wide association studies (GWAS) is approaching, and this experimental approach has commenced a deeper understanding of the genetics underlying complex diseases. In obesity genetics the GIANT (Genetic Investigation of ANthropometric Traits) consortium has played a crucial role, recently with two comprehensive meta-analyses, one focusing on general obesity, analyzing body-mass index (BMI) and the other on fat distribution, focusing on waist-hip ratio adjusted for BMI. With the in silico methods applied in these two studies as the pivot, this review looks into some of the biological knowledge, beginning to emerge from the intricate genomic background behind the genetic determinants of human adiposity. These include synaptic dysfunction, where GWAS pinpoint potential new mechanisms in pathways already known to be linked with obesity.

Obesity genetics in mouse and human: back and forth, and back again

Obesity is a major public health concern. This condition results from a constant and complex interplay between predisposing genes and environmental stimuli. Current attempts to manage obesity have been moderately effective and a better understanding of the etiology of obesity is required for the development of more successful and personalized prevention and treatment options. To that effect, mouse models have been an essential tool in expanding our understanding of obesity, due to the availability of their complete genome sequence, genetically identified and defined strains, various tools for genetic manipulation and the accessibility of target tissues for obesity that are not easily attainable from humans. Our knowledge of monogenic obesity in humans greatly benefited from the mouse obesity genetics field. Genes underlying highly penetrant forms of monogenic obesity are part of the leptin-melanocortin pathway in the hypothalamus. Recently, hypothesis-generating genome-wide association studies for polygenic obesity traits in humans have led to the identification of 119 common gene variants with modest effect, most of them having an unknown function. These discoveries have led to novel animal models and have illuminated new biologic pathways. Integrated mouse-human genetic approaches have firmly established new obesity candidate genes. Innovative strategies recently developed by scientists are described in this review to accelerate the identification of causal genes and deepen our understanding of obesity etiology. An exhaustive dissection of the molecular roots of obesity may ultimately help to tackle the growing obesity epidemic worldwide.