Evidence for a genetic influence

Twin studies suggest a heritability of fat mass (fraction of the age adjusted phenotypic variance accounted for by genetic factors) of between 40% and 70% with a concordance of 0.7–0.9 between monozygotic twins compared to 0.35–0.45 between dizygotic twins.5 ,6 While these associations may in part be explained by sharing the same childhood environment, a number of studies have described a closer relation between the weights of adoptees and their biological parents rather than their adoptive parents.7 These genetic influences are not confined to the extremes of obesity, but exert their effect across the whole range of body weight and are consistent with a polygenic inheritance of fat mass.

Leptin and the leptin receptor

The severely obese ob/ob mouse, first described by Ingalls et al in 1950,13 inherits its early onset obesity in an autosomal recessive pattern and weighs three times more than normal mice by maturity. A decrease in thermogenesis is the earliest demonstrable defect; hyperphagia is seen after weaning, followed by the development of severe obesity caused by the preferential deposition of fat. The hyperinsulinaemia, fasting hyperglycaemia, infertility, thyroid dysfunction, stunted linear growth, and low sympathetic tone ofob/ob mice are shared by another strain of severely obese mice, db/db. In 1994, Friedman and colleagues cloned and characterised the ob gene which is expressed predominantly in white adipose tissue and encodes the 167 amino acid secreted protein, leptin.14 The ob transcript is mutant in both strains of ob/ob mice which are leptin deficient as a result. Administration of recombinant leptin completely reverses their phenotypical abnormalities,15-17 but has no effect indb/db mice which harbour a mutation in the cytoplasmic domain of the leptin receptor,18 abundantly expressed in the hypothalamus.

Human congenital leptin deficiency

In man, serum leptin concentrations have been found, in general, to correlate positively with indices of obesity.19 We have previously identified two cousins within a highly consanguineous family of Pakistani origin with severe obesity of early onset: an 8 year old girl weighing 86 kg and a 2 year old boy weighing 29 kg.20These children were noted to be severely hyperphagic, constantly demanding food, with an intense drive to eat which was never satisfied. They did not exhibit any of the clinical features suggestive of the recognised childhood obesity syndromes, had a normal karyotype, thyroid, and adrenal function, but were hyperinsulinaemic and had an advanced bone age. They were found to have undetectable concentrations of serum leptin and were homozygous for a deletion of a single guanine nucleotide at codon 133 which results in a truncated protein. Both sets of parents were heterozygous for this mutation in keeping with an autosomal recessive pattern of inheritance. There was no evidence of substantial impairment in basal or total energy expenditure and body temperature was consistently normal, indicating that leptin may be less central to the regulation of energy expenditure in humans than in mice, although subtle defects of energy expenditure are difficult to measure.

Subsequently Strobel and colleagues have described a Turkish family in which three severely obese siblings were found to be leptin deficient because of a missense mutation in the leptin gene.21Additionally, Clement and colleagues identified three morbidly obese sisters with very high serum leptin concentrations who were found to be homozygous for a mutation in a splice donor site of the leptin receptor, which results in loss of the transmembrane and cytoplasmic domains because of exon skipping.22 There were a number of similarities with the leptin deficient subjects. These sisters were also born of normal birth weight, and exhibited rapid weight gain in the first few months of life, with severe hyperphagia and aggressive behaviour when denied food. Basal temperature and resting metabolic rate were normal. In contrast, they had mild growth retardation in early childhood with impaired basal and stimulated growth hormone secretion and decreased insulin like growth factor 1 (IGF-1) and IGF-BP3 concentrations. There was evidence of hypothalamic hypothyroidism, although as in the leptin deficient subjects, cortisol concentrations were normal and they were normoglycaemic with mildly raised plasma insulins.

Of note, the two Turkish adults with leptin deficiency and the sisters with mutations in the leptin receptor did not undergo pubertal development with biochemical features of hypogonadotropic hypogonadism. In the older of our two patients (a 9 year old girl) it is difficult to confirm hypogonadotropic hypogonadism as she is clinically prepubertal, but a child with a bone age of 12.5 years would usually have started to develop some secondary sexual characteristics. While bone age is frequently advanced in obese children, an advance in excess of three years has rarely been reported.23 In this instance, advanced bone age could not be attributed to excessive and/or premature secretion of adrenal or ovarian sex steroids as their plasma concentrations were low and the physiological basis for this is not clear.

Morbid obesity was inherited in an autosomal recessive pattern in both leptin and leptin receptor families and the absence of morbid obesity and normal sexual maturation in the heterozygote parents indicates that the lack of one normal allele does not greatly affect weight regulation or neuroendocrine function.

POMC and the melanocortin pathway

The behavioural and neuroendocrine effects of leptin are thought to be mediated through its actions at hypothalamic leptin receptors. The complex neurochemical systems downstream of leptin that regulate appetite and energy expenditure and integrate nutritional and circadian information into endocrine responses, are being dissected. Pro-opiomelanocortin (POMC) is produced by hypothalamic neurones of the arcuate nucleus and is sequentially cleaved by prohormone convertases to yield peptides (including α melanocyte stimulating hormone, αMSH) that have been shown to play a role in feeding behaviour. Forty per cent of POMC neurones in the arcuate nucleus express the mRNA for the long form of the leptin receptor, and POMC expression is regulated positively by leptin.24 There is clear evidence in rodents that αMSH, a melanocortin peptide produced from POMC, acts as a suppressor of feeding behaviour. The recent description of two unrelated German subjects with mutations in POMC associated with isolated adrenocorticotropic hormone (ACTH) deficiency, red hair pigmentation, hyperphagia, and severe, early onset obesity confirms the importance of POMC derived peptides in the regulation of body weight in humans.25 The hair, skin, and adrenal phenotype of these subjects is readily understandable on the basis of the known effects of POMC derived peptides in these tissues. Given the importance of prohormone convertase 1 (PC1) in the proteolytic processing of POMC, it is notable that we have reported a single patient with severe early onset obesity to have compound heterozygote mutations in the gene encoding PC1.26 In addition to obesity, this patient had primary amenorrhoea caused by hypogonadotropic hypogonadism, massive hyperproinsulinaemia, and impaired adrenal function.27

How does the lack of POMC derived ligands lead to obesity? Interestingly, one form of melanocortin receptor (MC4R) is highly expressed in areas of the hypothalamus known to be involved in feeding. Mice in which the MC4 receptor has been disrupted by gene targeting are hyperphagic, becoming severely obese, hyperinsulinaemic, and exhibiting increased linear growth, but show no evidence of reproductive failure or corticosterone excess.28 We have recently described a father and son with hyperphagia, severe early onset obesity and tall stature, with mutations in MC4R.29 Both were heterozygous for a four base pair deletion in the region encoding the fifth of seven transmembrane domains, resulting in a truncated protein. Interestingly, both subjects were tall, in keeping with the increased linear growth seen in MC4R knockout mice, but had no other endocrine abnormalities. A French family described by Vaisse and colleagues with obesity associated with a mutation in MC4R have no obvious defect in resting energy expenditure.30 MC4R, therefore, represents the first locus at which mutations are associated with dominantly inherited morbid obesity in man, a finding that is consistent with the observation that mice heterozygous for a null MC4R allele exhibit weight gain intermediate to that seen in wild type and homozygous mutant littermates. An increasing number of obese families with MC4R mutations are now being described (Gu et al,31 Hinney et al,32 and our unpublished observations) making it the commonest known genetic cause of human obesity.

Conclusions

In the past three years, five single gene disorders resulting in early onset obesity have been characterised. Two of these, leptin receptor and PC1, have been restricted to single families. Three families with mutations in POMC or leptin have been described. Mutations in the MC4R appear to be a more common cause of obesity, perhaps because heterozygotes are clinically affected and therefore only one mutational event is required. Additionally, mutations in leptin, the leptin receptor, and PCI adversely affect reproduction and therefore would be expected to be rapidly bred out. Similarly POMC deficiency is fatal if unrecognised owing to profound cortisol deficiency. In contrast MCR4 deficiency appears to cause a relatively “pure” obesity syndrome with no impairment of early viability or reproductive function.

The discovery of these genetic defects has biological and clinical implications which are greater than the rarity of the individual diseases might suggest. Firstly, at least one disorder, namely leptin deficiency, is amenable to treatment. Such treatment represents the first, rationally based, hormone replacement therapy for any form of human obesity. Secondly, these disorders establish for the first time that human obesity can result from a simple fault in the wiring of the circuits concerned with energy homoeostasis without the need to implicate complex social and environmental factors. We hope that this may help to “destigmatise” obesity and validate it as a medical condition deserving of sympathetic handling and worthy of scientific study. Thirdly, they represent powerful confirmation that the growing body of data on the molecular circuitry controlling appetite and energy balance in rodents is directly relevant to humans, and provides strong support for the benefits of animal research. Fourthly, they provide validation of certain key molecules as potential targets for pharmacological manipulation. Finally, as the loci encompassing these genes have been found to be significantly associated with obesity related phenotypes in genome wide scans, they provide important candidate genes for examination in the more common forms of human obesity.33 ,34

Postscript

The authors welcome enquires regarding biochemical and genetic studies of patients with severe early onset obesity. The criteria for inclusion in these studies is a BMI >4 SD above the age related mean3 and an onset of obesity before 10 years of age.