Previous studies of the frequency of functional MC4R variants have been performed in groups selected for obesity (24
), but in a more general population study, these variants were not associated with higher BMI (26
). For nonfunctional variants, meta-analysis of a V103I MC4R polymorphism in predominantly Caucasian subjects concluded that the isoleucine allele was modestly associated with protection from obesity (9
). This locus is monomorphic for the valine allele in the 426 Pima Indians in whom the gene was sequenced. A recent meta-analysis in a large group of Europeans found that a range of functional MC4R variants were more common in obese than lean individuals (28
). In a German population study, R165Q was rare (minor allele frequency 1.2 × 10−4
) and the individual was not obese (26
). In our general population study of full-heritage Pima Indians, the percentage of individuals carrying either the R165Q or the NT100 is 1.8%, and their association with higher BMI was clear. This was even more pronounced in the analysis excluding subjects with type 2 diabetes, presumably because of the influence of diabetes itself or associated medications on weight. Furthermore, in a subset of Pima Indians who also underwent metabolic studies, we found that energy expenditure tended to be lower (although not significantly so) among individuals heterozygous for one of these two variants. The R165Q variant has known functional consequences (3
). Although not proven, we propose that the NT100 also has functional consequences because it predicts a truncated receptor lacking critical domains. Therefore, we combined these rare variants to obtain additional statistical power and, in so doing, we have shown for the first time in humans that energy expenditure, adjusted for age, sex, FFM, and fat mass is lower in individuals heterozygous for either variant. The lower energy expenditure (of ~110–140 kcal/day) was apparent in analyzing 24 h and sleep energy expenditure (both measured in our respiratory chamber) and replicated by measurements of RMR, using a ventilated hood, on a separate day.
In mice, the MC4R is critical in the role of regulating energy homeostasis and is abundant in the hypothalamic paraventricular nucleus. MC4R knockout mice have early-onset obesity, hyperphagia, hyperinsulinemia, and hyperglycemia, with heterozygotes having an intermediate obese phenotype (4
). The importance of MC4R variants in humans was documented with discovery of a frameshift mutation associated with autosomal dominantly inherited obesity (29
). The clinical phenotype of these individuals (including both homozygous and heterozygous subjects) included early-onset obesity and excessive hunger during childhood with food-seeking behavior (2
). Children with MC4R variants ate nearly three times as many calories as their unaffected siblings in studies evaluating ad libitum food intake (3
), although this hyperphagia decreased toward adulthood. In these same studies, RMR, measured by indirect calorimetry, was not different based on comparisons with age- and sex-specific equations corrected for FFM (2
Our data indicate that reduced energy expenditure (by ~110–140 kcal/day) is part of the clinical phenotype of adults heterozygous for rare obesity-associated MC4R variants. Mice with MC4R knockout mutations have reduced energy expenditure (5
), but this is not due to reduced core body temperature or an inability to thermoregulate. MC4R knockout mice also have lower energy expenditure compared with leptin knockout (OB
) mice (27
). Although data on the association between MC4R variants and measured energy expenditure is limited in humans, the Val103Ile MC4R polymorphism, which has been negatively associated with obesity (27
), was also associated with higher RMRs in a separate study (32
). Although RMR was higher in this prior report, BMI was not different between individuals with the Val versus the 103lle allele, and individuals with the 103Ile allele tended to gain more weight over time (32
). Furthermore, unlike our studies, RMR was not performed after an inpatient period of weight stabilization. Therefore, increased food intake could account for the increased RMR. Despite recent evidence in mice for a role of MC4R in lipid metabolism (33
), we did not find differences in respiratory quotient or lipid oxidation in MC4r heterozygotes.
Evidence for a direct effect of MC4R on energy expenditure comes from rodent studies. In particular, MC4R is important in activating the sympathetic nervous system (34
). MC4R mediates the central response to leptin-induced expression of uncoupling protein 1 (UCP1) in brown adipose tissue of rats (35
), and sympathetic nerve activity in rats increased when an MC4R agonist was given (34
). Activation of the sympathetic nervous system increases basal energy metabolism (36
); therefore, loss of function in MC4R with resulting lower sympathetic tone would explain the lower energy expenditure in these individuals. It is not clear, however, if the lower energy expenditure in these adults precedes (and is therefore causative of their increased weight or whether this lower energy expenditure represents a failure to adapt to increased body size (thus making it difficult for these individuals to lose weight). Leptin concentrations and sympathetic nervous system activity increase with body size, thereby increasing energy expenditure and braking further weight gain (37
Although this analysis concentrates on the differences noted in energy expenditure, increased food intake (hyperphagia) may still account for a portion (perhaps even most) of the weight gain and adiposity among individuals with these variants. Our study did not include measurement of ad libitum food intake, and more careful under- or overfeeding studies would be needed to elucidate the relative contribution of each factor. In addition, while the energy-expenditure differences were relatively large, the EE measurements were performed on individuals on weight-maintaining diets in an inpatient setting, and were confirmed using measurements done at separate times on different individuals (only one individual with a variant was in the group with both 24-h energy expenditure and RMR), given the small numbers, these differences should be interpreted with caution. The adjusted mean difference in energy expenditure between heterozygotes and those with the common genotype was 110–140 kcal/day. If this deficit was maintained over a year, it would amount to ~40,150–51,000 kcal difference per year.
Our data indicate that full-heritage Pima Indians heterozygous for either a frameshift or a functional missense MC4R variant have higher BMI values and lower 24-h, sleeping, and resting energy expenditure. This is one of the clearest examples in humans of a genotype resulting in reduced energy expenditure and provides a link between the melanocortin system and energy expenditure. Further understanding of how this system affects energy expenditure may provide important clues for advances in obesity treatment and prevention.