This study describes a detailed investigation of the prevalence of MC4R coding mutations, assessment of their functionality, and analysis of life course growth patterns in a longitudinal study of health with subjects of predominantly Pima Indian heritage. In this founder population, 10 missense/nonsense mutations were found, of which 6 cause loss of function in vitro. Three of these mutations, D37stop, R165G, and A303P, have not been described in others and may be private to this population. Although each mutation individually was rare, 2.4% of the population was heterozygous for MC4R deficiency, providing support for the hypothesis that accumulation of multiple rare mutations in a population may have a role in susceptibility for a common disease. It is well established that mutations in MC4R that result in decreased cAMP production lead to an increased risk for obesity in both rodents and humans (2
), which seems to be attributed to hyperphagia as well as to decreased energy expenditure (4
), likely as a result of a reduction in sympathetic nervous system activity (25
). Our results show that the phenotypic effects of MC4R deficiency are more pronounced during childhood.
Previous investigators have anecdotally noted that the phenotype of people with MC4R deficiency may differ between children and adults (5
). For example, the hyperphagia seen in childhood has been reported to be less pronounced in adults, abating sometime in the late teenage years (5
). Our work adds to this previous literature by using data from a large, longitudinal population study to confirm in a more statistically robust way the differential effects of a defective MC4R. Pima children carrying a loss-of-function MC4R mutation had greater rates of weight gain, leading to higher BMIs. After completion of childhood growth, rates of BMI gain stabilized and were no longer different from the rest of the population. Thus, the BMI difference between those with and without MC4R deficiency did not increase further during adulthood but stayed stable at ~5 kg/m2
. As this was a population-based study, there was no selection bias for obese individuals. There is, however, a high prevalence of obesity in the Pima population during both childhood and adulthood (11
). Previous reports conflict on the effect of carrying a defective MC4R on the rate of BMI gain in adults, with one study (10
) reporting no increased BMI gain per year and another (6
) demonstrating an increased penetrance of obesity at age 40 years compared with self-reported BMI at age 20 years. However, no other study has reported prospective standardized weight and height measurements to investigate separate childhood versus adult effects of MC4R deficiency.
The reasons for accelerated weight gain only during childhood, and not adulthood, are not clear. Hyperresponsiveness of the pathways that frame the appetitive response to low peripheral energy stores during childhood may have had greater survival benefit in times of food deprivation. There also may be pathways that interact with the melanocortin system that are less active after the completion of linear growth. In support of the first possibility, a recent study showed that carrying a greater number of SNPs associated with adult BMI decreased the risk of failure to thrive during early infancy (27
). In support of the latter possibility, MC4R has been shown to be important in the regulation of the growth hormone axis (28
), which is known to decrease in physiologic importance with progression to adulthood. A common SNP near MC4R
also has been shown to have a stronger association with BMI before the age of 20 years, which weakened during adulthood (29
MC4R deficiency has been reported to lead to an early increase in stature (7
), although not every study has confirmed this finding (2
). Our study found an increase in childhood height z
score in individuals with MC4R deficiency as well as a taller final height by ~1.5 cm. The effect on linear growth may be related to the severity of MC4R deficiency, which would explain why our study, with partial loss-of-function mutations, found a relatively small effect and another recent study demonstrated a larger effect on final adult height of 5–7 cm in individuals heterozygous for complete loss-of-function mutations (28
). Pima Indian children have a different pattern of linear growth than the general U.S. population and exhibit increased early growth such that, on average, they are taller than the overall U.S. population between the ages of 4 and 14 years. However, final adult height in the Pima population is similar to the general population (16
Individuals with the I251L genotype had a significantly lower average BMI than the group with a normal MC4R genotype. We did not find any difference in in vitro signaling between this variant and the wild-type MC4R. Previous studies have found that the I251L polymorphism may be protective against obesity (31
). Our association results seem to indicate a protective effect as well; however, it should be noted that the group of individuals with this variant included no full-heritage Pima Indians and had an overall lower percentage of Pima heritage than the general population, which also may account, in part, for their lower mean BMI.
The lower blood pressure reported by others in adults (25
) was only observed during childhood in our study. Our exclusion of visits after the development of type 2 diabetes or the inclusion of all subjects regardless of obesity status may account for the difference. Our childhood results, as well as the results of others (25
), indicate that MC4R deficiency dissociates the link connecting hypertension with obesity and raises the possibility that the increased sympathetic nervous system activity seen with excess adiposity may be mediated through MC4R. The delayed menarche observed in this study may be related to a similar phenomenon, as the sympathetic nervous system is implicated in increasing gonadotropin-releasing hormone secretion prior to pubertal onset, and women with familial dysautonomia also have been reported to have delayed menarche (33
). The delayed menarche was an unexpected finding because it is contrary to what is usually observed with increased adiposity; however, it is interesting that, whereas other common obesity variants associated with early menarche in a genome-wide association study meta-analysis, a SNP near MC4R
did not (34
Consistent with higher BMIs, children with MC4R deficiency had higher glucose and insulin concentrations during an OGTT. However, these differences did not persist after adjustment for BMI. This is in contrast to the classic description of MC4R deficiency, which describes hyperinsulinemia out of proportion to BMI that improves with age (7
), although this has not always been confirmed by others (9
). We found an increased risk for type 2 diabetes with MC4R deficiency. Before the age of 20 years, this risk was only partially attributable to the increased BMI at a young age. Others have not found an increased prevalence of type 2 diabetes among adults with MC4R deficiency (5
), but we did find a modest risk of developing type 2 diabetes during adulthood that was fully attributable to the increased BMI. The increased risk for type 2 diabetes independent of BMI during childhood was possibly attributable, in part, to the rapidity of the weight gain. Potentially, the elevated risk is only evident in populations with high susceptibility for developing type 2 diabetes. It is unclear why MC4R deficiency would impart additional risk for diabetes beyond that conferred by an increased BMI, although recent studies have implicated MC4R in potentiating downstream insulin signaling (35
). It also is possible that BMI did not fully represent the effect of excess adiposity in developing insulin resistance and type 2 diabetes in the model.
In conclusion, in this population-based study, the impact of MC4R deficiency on the rate of body mass accumulation was greatest during childhood and became similar to the rest of the population after completion of childhood growth. We also found an increased risk for type 2 diabetes in individuals with an MC4R defect, which was more pronounced during childhood and independent from BMI during childhood. In addition, carrying a partial loss-of-function MC4R mutation was associated with a taller adult height, lower childhood systolic blood pressure, and later menarche. Understanding how monogenic changes cause differing phenotypic expression during childhood and adulthood may help to further insights into common obesity and emphasize the need for prevention efforts during childhood.