weight (i.e. ∼0.01 mg/kg of lean body mass). The most dramatic example of leptin’s effects was with a 3‐year‐old boy, severely disabled by gross obesity (wt 42 kg), who now weighs 32 kg (75th centile for weight) after 48 months of leptin therapy (Fig. 4.1) [30].
The major effect of leptin was on appetite, with normalization of hyperphagia. Leptin therapy reduced energy intake during an 18 MJ ad libitum test meal by up to 84% (Fig. 4.2a). We were unable to demonstrate a major effect of leptin on basal metabolic rate or free‐living energy expenditure (Fig. 4.2b) but, as weight loss by other means is associated with a decrease in basal metabolic rate (BMR) [35], the fact that energy expenditure did not fall in our leptin‐deficient subjects is notable.
The administration of leptin permitted progression of appropriately timed pubertal development in the single child of appropriate age and did not cause the early onset of puberty in the younger children (Fig. 4.3) [30]. Free thyroxine and thyroid‐stimulating hormone (TSH) levels, although in the normal range before treatment, had consistently increased at the earliest post‐treatment time point and subsequently stabilized at this elevated level [30]. These findings are consistent with evidence from animal models that leptin influences thyrotropin‐releasing hormone (TRH) released from the hypothalamus [36] and from studies illustrating the effect of leptin deficiency on TSH pulsatility in humans [37].
Throughout the trial of leptin administration, weight loss continued in all subjects, albeit with refractory periods, which were overcome by increases in leptin dose. The families in the United Kingdom harbor a mutation that leads to a prematurely truncated form of leptin, and thus wild‐type leptin is a novel antigen to them. Thus, all subjects developed anti‐leptin antibodies after ∼6 weeks of leptin therapy, which interfered with an interpretation of serum leptin levels and, in some cases, were capable of neutralizing leptin in a bio‐assay. These antibodies are the likely cause of refractory periods occurring during therapy. The fluctuating nature of the antibodies probably reflects the complicating factor that leptin deficiency is itself an immunodeficient state and administration of leptin lead to a change from the secretion of predominantly Th2 to Th1 cytokines, which may directly influence antibody production. Thus far, we have been able to regain control of weight loss by increasing the dose of leptin.
Leptin receptor deficiency
Up to 3% of patients with severe obesity have been found to harbor mutations in the leptin receptor gene (LEPR) that are associated with a loss of function in vitro [38]. Whilst heterozygosity for LEP or LEPR mutations is associated with an increase in body weight, severe obesity requires the loss of two alleles due to homozygous or compound heterozygous mutations. Serum leptin levels are not disproportionately elevated in LEPR deficiency, although particular mutations located near the transmembrane domain can result in a truncated extracellular domain that may act as a false binding protein and result in abnormally elevated leptin levels [39, 40]. The clinical phenotype of congenital leptin receptor deficiency is similar to that of leptin deficiency with hyperphagia, severe early‐onset obesity, hypogonadism, and frequent infections.
POMC deficiency
Several unrelated children with obesity with homozygous or compound heterozygous mutations in POMC have been reported [41]. These children were hyperphagic, developing early‐onset obesity as a result of impaired melanocortin signaling in the hypothalamus. They presented in neonatal life with adrenal crisis due to isolated adrenocorticotropic hormone (ACTH) deficiency (POMC is a precursor of ACTH in the pituitary) and had pale skin and red hair due to the lack of MSH function at melanocortin 1 receptors in the skin, although hypopigmentation may be less obvious in children from different ethnic backgrounds. A number of missense mutations that affect POMC‐derived peptides have been described [42].
Prohormone convertase 1 deficiency
Further evidence for the role of the melanocortin system in the regulation of body weight in humans comes from the description of three patients with severe childhood obesity, abnormal glucose homeostasis, very low plasma insulin but elevated levels of proinsulin, hypogonadotropic hypogonadism and hypocortisolemia associated with elevated levels of POMC (Table 4.2). These subjects were found to be compound heterozygote/homozygous for mutations in prohormone convertase 1, which cleaves prohormones at pairs of basic amino acids, leaving C‐terminal basic residues, which are then excised by carboxypeptidase E (CPE) [43]. Although failure to cleave POMC is a likely mechanism for the obesity in these patients, prohormone convertase 1 (PC1) cleaves a number of other neuropeptides in the hypothalamus, such as glucagon‐like peptide 1, which may influence feeding behavior. Intriguingly, the second patient suffered from severe small intestinal absorptive dysfunction as well as the characteristic severe early‐onset obesity, impaired prohormone processing, and hypocortisolemia. We hypothesized that the small intestinal dysfunction seen in this patient, and to a lesser extent in the first patient we described, maybe the result of a failure of maturation of propeptides within the enteroendocrine cells and nerves that express PC1 throughout the gut.
Figure 4.1 Clinical response to leptin therapy in congenital leptin deficiency.
MC4R deficiency
Mutations in MC4R have been reported in up ∼6% of patients with severe early‐onset obesity, and are found at a frequency of approximately 1 in 300 in the general population, making this the most common monogenic form of obesity. While we found a 100% penetrance of early‐onset obesity in heterozygous probands, others have described carriers who were not obese. Given a large number of potential influences on body weight, it is perhaps not surprising that both genetic and environmental modifiers will have important effects on some pedigrees. Taking account of all these observations, co‐dominance, with modulation of expressivity and penetrance of the phenotype, is the most appropriate descriptor for the mode of inheritance.
Figure 4.2 Changes in energy intake and expenditure in two children with congenital leptin deficiency treated with recombinant leptin. (a) Change in ad libitum energy intake in a 3‐year‐old boy (Child B) with congenital leptin deficiency, before and one month after the initiation of leptin therapy. (b) Changes in energy expenditure in Child A (9‐year‐old girl) and Child B (3‐year‐old boy) in response to leptin. BMR, basal metabolic rate; TEE, total energy expenditure expressed per kg lean body weight (LBW).
(Source: Based on Rosenbaum et al. [35].)
Figure 4.3 Leptin therapy is associated with pulsatile gonadotropin secretion at an appropriate developmental age in child (a) (age 11 years) compared to child (b) (age 5 years).
(Source: Modified from Farooqi et al. [30].)
Detailed phenotypic