Haploinsufficiency of the melanocortin-4 receptor: part of a thrifty ?

Roger D. Cone

J Clin Invest. 2000;106(2):185-187. https://doi.org/10.1172/JCI10628.

Commentary

Allelic variation in certain “thrifty” is proposed to contribute to human by altering the efficiency of energy storage in adipocytes without compromising survival or reproductive efficiency. The nature of these genes has been uncertain, but family studies and clinical analyses in two reports in this issue of the JCI (1, 2) show that in MC4R, encoding the melanocortin-4 receptor, may be responsible for more than 4% of severe obesity in some populations. Family studies and careful clinical analyses argue that MC4R has the properties of a that could contribute to a thrifty . Clinical studies as well as personal experience show that there is variability in the efficiency with which humans store calories in the form of fat. Extreme examples are found in the remarkable plights of the Pima people of central Arizona and the Nauru Islanders of the West Pacific. These two ethnic groups suffer from a polygenic syndrome of obesity and diabetes with an incidence as high as 50% in the context of access to high-fat Western diets and a tremendous reduction in physical activity. Studies of these groups led to the theory of the “thrifty genotype” — the notion that living on the edge of survival for a sufficiently long period had fixed in the gene pool a collection of that promote […]

Find the latest version: https://jci.me/10628/pdf Haploinsufficiency of the melanocortin-4 receptor: Commentary part of a thrifty genotype? See related articles, pages 235–243 and 271–279. Roger D. Cone

Vollum Institute, Oregon Health Sciences University, 3181 SW Sam Jackson Park Road, Portland, Oregon 97201, USA. Phone: (503) 494-4668; Fax: (503) 494-4534; E-mail: [email protected].

Allelic variation in certain “thrifty” genes and feeding behavior validated the than exclusively in the skin (6, 7). The is proposed to contribute to human obe- notion of an adipostat that responds agouti protein acts as a competitive sity by altering the efficiency of energy to changing energy stores, and it fur- antagonist, not only of the MC1-R, the storage in adipocytes without compro- ther suggested that single genes could melanocyte-stimulating hormone mising survival or reproductive efficien- alter the set point of adipose stores receptor, but also of the MC4-R (8). cy. The nature of these genes has been (4). However, the apparent role of lep- With the demonstration that the MC4- uncertain, but family studies and clinical tin as a gatekeeper of reproductive R is expressed in hypothalamic nuclei analyses in two reports in this issue of competence (5) and the resulting involved in the regulation of energy the JCI (1, 2) show that mutations in infertility in leptin-deficient mice homeostasis (9), this finding led to the MC4R, encoding the melanocortin-4 (and, presumably, humans with defec- hypothesis that the agouti obesity syn- receptor, may be responsible for more tive leptin signaling) argue that OB or drome results from chronic blockade of than 4% of severe obesity in some popu- any other genes acting broadly on the hypothalamic MC4-R signaling. This lations. Family studies and careful clini- leptin axis would not be candidate hypothesis is now largely accepted, cal analyses argue that MC4R has the thrifty genes. MC4R, conversely, acts based on the demonstration that a properties of a gene that could con- as an obesity gene without being cyclic peptide antagonist of the MC4-R tribute to a thrifty phenotype. required for reproduction. stimulates food intake and energy stor- Clinical studies as age (10), and that deletion of well as personal experi- the mouse MC4R gene ence show that there is recapitulates the agouti variability in the effi- Family studies and careful clinical obesity syndrome, ciency with which including the unique humans store calories analyses argue that MC4R has the neuroendocrine pheno- in the form of fat. properties of a gene that could type of increased linear Extreme examples are growth (11). These data found in the remark- contribute to a thrifty phenotype. argue that the central able plights of the Pima melanocortin circuitry people of central Ari- tonically inhibits energy zona and the Nauru Islanders of the Characterization of the storage. The curious finding that het- West Pacific. These two ethnic groups melanocortin system erozygous MC4R knock-out mice devel- suffer from a polygenic syndrome of The agouti gene was first shown to be op an adipose mass intermediate to that obesity and diabetes with an incidence responsible for one of the five mono- of wild-type and homozygous knock- as high as 50% in the context of access to genic obesity syndromes characterized outs suggests further that the circuit high-fat Western diets and a tremen- in the mouse. The agouti obesity syn- acts as a rheostat to regulate energy stor- dous reduction in physical activity. drome in the mouse resembles common age. Studies of these groups led to the theo- human obesity much more than syn- The high-affinity binding of agouti ry of the “thrifty genotype” — the notion dromes that result from leptin- or leptin to the MC4-R was neatly explained by that living on the edge of survival for a receptor–deficiency. Agouti mice are the discovery of an agouti homologue, sufficiently long period had fixed in the obese but fertile, and they exhibit few agouti-related protein (AGRP), which gene pool a collection of alleles that associated endocrine abnormalities, is expressed in the neuropeptide promote storage of fat in times of plen- outside of the hyperinsulinemia and Y–expressing neurons of the hypothal- ty (3). To be maintained in the gene incompletely penetrant diabetes nor- amus that regulate satiety. That AGRP pool, such alleles would have to pro- mally associated with that degree of adi- occurs in nerve terminals in most of mote survival in times of famine to a posity. Somatic growth of agouti mice is the same sites where hypothalamic greater extent than they might compro- unusually rapid, a feature shared with (POMC) termi- mise health and reproductive efficiency, humans with early-onset obesity. The nals are found adds a further degree of even if, as proposed originally, they were dominant of agouti from the obese regulatory complexity to the system to promote diabetes directly. AY agouti mouse results from a gene (12, 13). AGRP mRNA and protein lev- rearrangement, fusing the Raly promot- els are dramatically upregulated by Characterization of leptin er to the agouti coding sequences to fasting or the absence of leptin, and The identification of the hormone produce a chimeric gene that expresses this upregulation is blocked by leptin leptin as a regulator of metabolic level the agouti mRNA ubiquitously, rather administration (14, 15). The AGRP

The Journal of Clinical Investigation | July 2000 | Volume 106 | Number 2 185 protein has been demonstrated to obesity in two separate families (18, 19). French patients (Table 1). Of the seven function as a competitive antagonist These independent frameshift mutations missense mutations, five are demon- of the central MC4-R and MC3-R were associated with obesity across two to strated by transfection into heterologous receptors (16). four generations, but the data sets were cells to have altered affinity for binding Evidence that melanocortin obesity too small to generate either significant to α-MSH or altered efficacy of activa- syndrome could occur in humans came LOD scores or meaningful physiological tion by ligand (partial activity). One from the astute recognition of an agouti data. The two reports from the O’Rahilly , L250Q, is constitutively active mouse–like syndrome in two families, and Froguel laboratories in this issue (1, and exhibits increased affinity for ligand. resulting from null mutations in the 2), as well as recent additional reports Mutations segregated with obesity in POMC gene (17). These patients have a (20–22), provide a clearer picture of the three of four families studied, but age at rare syndrome that includes adrenocorti- frequency and diversity of MC4R muta- onset and severity of obesity were vari- cotropic hormone (ACTH) insufficiency, tions (Table 1); a better understanding of able. In one family, the disease allele was red hair, and obesity, presumably result- the of the association of MC4R a frameshift mutation occurring after ing from a lack of ACTH peptide in the mutations with obesity and potential codon 12. Although this allele seems cer- serum, a lack of α melanocyte-stimulat- mechanisms; the first detailed clinical tain to produce no functional protein, ing hormone (α-MSH) in the skin, and a phenotype, to our knowledge; and the the family had no history of obesity out- lack of α-MSH in the brain, respectively. first report, to our knowledge, of individ- side of the proband, demonstrating that These data demonstrated, for the first uals with homozygous mutant alleles (1). the penetrance of the mutation is time, that the central melanocortin cir- incomplete: haploinsufficiency does not cuitry subserves energy homeostasis in Analysis of MCR4 defects from always lead to obesity. This was strongly humans as it does in the mouse. Shortly human families suggested by an additional study which thereafter, the two laboratories reporting Froguel and colleagues (2) have found failed to document obesity due to hap- in this issue of the JCI published the first eight new receptor mutations in a popu- loinsufficiency of the MC4R gene in 27 reports of heterozygous mutations in lation of 209 probands randomly select- individuals with deletions of 18q (23). MC4R, associated with nonsyndromic ed from a large cohort of morbidly obese Vaisse et al. (2) suggest that the finding of silent carriers in the family with the Table 1 frameshift after codon 12 argues against Allelic variants of the human MC4-R MC4-R–associated obesity occurring due to haploinsufficiency. Rather, they favor Receptor variant Pharmacology Allele frequency Reference a dominant-negative mechanism for cau- (EC50/Kd; values are (obese/lean) sation. While this mechanism is theoret- relative to wild-type) ically possible, the haploinsufficiency CTCT∆, nt633 null 0.008 (19) model remains tenable. In general, G pro- 0.003/0 (21) GATT insertion, nt732 null 0.012 (18) tein–coupled receptors are thought to act Val103Ile 0.021/0.023 (27) as monomers, and G proteins are found 0.013/0.051 (21) in vast excess of receptors in the cells, wild-type 0.017/0.021 (20) 0.019/0.015 (2) providing little opportunity for domi- 0.006/0.009 (1) nant-negative mechanisms. One phar- Stop codon, Tyr35 null 0.007/0 (21) macological study of MC4-R fails to doc- Ser30Phe 0.003/0 (21) ument any dominant-negative activity of Asp37Val 0.007/0 (21) ∆ Pro78Leu 0.003/0 (21) the CTCT nt633 mutant or the GATT Thr112Met 0.007/0 (21) insertion nt732 mutant (24). Finally, in wild-type 0/0.011 (20) the majority of families (eight of ten) 0.002/0 (1) Arg165Trp 0.003/0 (21) studied to date with null alleles of MC4R Gly252Ser 0.003/0 (21) or missense mutations that alter MC4-R Ile317Thr 0.003/0 (21) pharmacology, nearly all of the carriers of Ile251Leu 0.013/0 (21) the mutation are obese. For example, in a wild-type 0.007/0.004 (2) 0.024/0.028 (1) previous family study, 18 of 19 individu- Silent change, C579T wild-type 0.013 (21) als containing different null alleles from Ile137Thr 14.8/7.5 0.003/0 (20) three unrelated families were obese (22). C593T, silent wild-type 0.002/0.004 (2) 47-48insG null 0.002/0 (2) The identification of a constitutively Thr11Ser 1/1 partially active 0.002/0 (2) active mutant also may be used as an Arg18Cys 1/1 partially active 0.002/0 (2) argument against haploinsufficiency. Thr150Ile 11.1/2.5 0.002/0 (2) Ile170Val 29/ partially active 0.002/0 (2) However, in the case of the Arg165Trp 19.9/ partially active 0.002/0 (2) receptor, one mutant receptor found Leu250Gln constitutively active/0.11 0.002/0 (2) associated with that Ile301Thr 4.4/ partially active 0.002/0 (2) was constitutively active in heterologous codon279insGT null 0.002/0 (1) C deletion, 28 bp 3′ of stop 0.002/0 (1) cells (25) appeared constitutively desen- Asn62Ser partially active 0.002/0 (1) sitized in transgenic mice (26), implying Arg165Gln 0.002/0 (1) that the mutation created a in Val253Ile 0.002/0 (1) Cys271Tyr no activity 0.002/0 (1) vivo. Particularly given the ability of hap- loinsufficiency to cause obesity in an

186 The Journal of Clinical Investigation | July 2000 | Volume 106 | Number 2 inbred mouse (11), the most parsimo- tive feeding behavior phenotype and no ubiquitously expressed in mice carrying the lethal yellow (Ay) mutation. Genes Dev. nious interpretation of the data set at difference in mean heights in their group 7:454–467. this time is that a partial reduction in of obese adults (2). 8. Lu, D., et al. 1994. Agouti protein is an antago- MC4-R function resulting from haploin- The data from these two papers leave nist of the melanocyte-stimulating hormone receptor. Nature. 371:799–802. sufficiency can cause obesity with vari- little doubt that heterozygous muta- 9. Mountjoy, K.G., Mortrud, M.T., Low, M.J., able penetrance and . tions in MC4R predispose carriers to Simerly, R.B., and Cone, R.D. 1994. Localiza- In the paper from O’Rahilly’s group nonsyndromic obesity, most likely as a tion of the melanocortin-4 receptor (MC4-R) in (1), MC4R sequences were determined result of haploinsufficiency. These find- neuroendocrine and autonomic control circuits in the brain. Mol. Endocrinol. 8:1298–1308. in 243 unrelated United Kingdom ings are important for a host of reasons. 10. Fan, W., Boston, B.A., Kesterson, R.A., Hruby, probands, selected based on their devel- The mutations may be very useful tools V.J., and Cone, R.D. 1997. Role of opment of severe obesity before the age for understanding MC4-R function and melanocortinergic neurons in feeding and the agouti obesity syndrome. Nature. 385:165–168. of 10, and in 54 nonobese controls. the cellular and molecular basis of ener- 11. Huszar, D., et al. 1997. Targeted disruption of This group identified a new insertion gy homeostasis. Furthermore, numer- the melanocortin-4 receptor results in obesity and a new deletion mutant, as well as ous pharmaceutical and biotechnology in mice. Cell. 88:131–141. 12. Ollmann, M.M., et al. 1997. Antagonism of cen- five new obesity-associated missense companies are attempting to develop tral melanocortin receptors in vitro and in vivo mutations (Table 1). Two of these mis- drugs for the treatment of obesity that by agouti-related protein. Science. 278:135–137. sense mutations were characterized for act via the MC4-R and related neural 13. Graham, M., Shuttre, J.R., Sarmiento, U., Sarosi, I., and Stark, K.L. 1997. Overexpression of Agrt functional activity, using a cAMP circuits downstream of leptin. These leads to obesity in transgenic mice. Nat. Genet. response element–luciferase reporter drugs may act quite differently in 17:273–274. assay; the C271Y mutant lacked patients with MC4R mutations or other 14. Mizuno, T.M., and Mobbs, C.V. 1999. 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