Genetics and Epigenetics

Brain-derived neurotrophic factor in human subjects with function-altering melanocortin-4 receptor variants

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Abstract

Background:

In rodents, hypothalamic brain-derived neurotrophic factor (BDNF) expression appears to be regulated by melanocortin-4 receptor (MC4R) activity. The impact of MC4R genetic variation on circulating BDNF in humans is unknown.

Objective:

The objective of this study is to compare BDNF concentrations of subjects with loss-of-function (LOF) and gain-of-function (GOF) MC4R variants with those of controls with common sequence MC4R.

Methods:

Circulating BDNF was measured in two cohorts with known MC4R sequence: 148 subjects of Pima Indian heritage ((mean±s.d.): age, 15.7±6.5 years; body mass index z-scores (BMI-Z), 1.63±1.03) and 69 subjects of Hispanic heritage (10.8±3.6 years; BMI-Z, 1.57±1.07). MC4R variants were characterized in vitro by cell surface expression, receptor binding and cyclic AMP response after agonist administration. BDNF single-nucleotide polymorphisms (SNPs) rs12291186, rs6265 and rs7124442 were also genotyped.

Results:

In the Pima cohort, no significant differences in serum BDNF was observed for 43 LOF subjects versus 65 LOF-matched controls (age, sex and BMI matched; P=0.29) or 20 GOF subjects versus 20 GOF-matched controls (P=0.40). Serum BDNF was significantly associated with genotype for BDNF rs12291186 (P=0.006) and rs6265 (P=0.009), but not rs7124442 (P=0.99); BDNF SNPs did not interact with MC4R status to predict serum BDNF. In the Hispanic cohort, plasma BDNF was not significantly different among 21 LOF subjects, 20 GOF subjects and 28 controls (P=0.79); plasma BDNF was not predicted by BDNF genotype or BDNF-x-MC4R genotype interaction.

Conclusions:

Circulating BDNF concentrations were not significantly associated with MC4R functional status, suggesting that peripheral BDNF does not directly reflect hypothalamic BDNF secretion and/or that MC4R signaling is not a significant regulator of the bulk of BDNF expression in humans.

Introduction

Brain-derived neurotrophic factor (BDNF) is believed to function downstream of the leptin-proopiomelanocortin signaling pathway to have a key role in the regulation of energy balance.1, 2, 3, 4 Bdnf heterozygous knockout mice display hyperphagia and obesity,5, 6 as do mice where depletion of hypothalamic BDNF has been produced.7 Human BDNF haploinsufficiency, either due to heterozygous deletion in patients with WAGR/11p deletion syndrome8 or disruption of BDNF expression in a child with interstitial 11p inversion,9 is associated with decreased serum BDNF concentrations, hyperphagia and obesity. Patients with Prader–Willi syndrome, a disorder caused by lack of expression of paternally derived genes on chromosome 15q11–13 and characterized by hyperphagia and obesity, have decreased serum and plasma BDNF concentrations compared with body mass index (BMI)-matched controls.10 These findings suggest the possibility that alterations in BDNF may be a mechanism through which other disorders of energy homeostasis affect food intake and body weight.

The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor that is highly expressed in the hypothalamic paraventricular nucleus and dorsal motor nucleus of the vagus.11 In animal studies, MC4R appears to serve an intermediary role within the leptin pathway, acting downstream of the leptin receptor and upstream of BDNF signaling.1, 2, 3, 4 Leptin-receptor-deficient db/db mice have decreased hypothalamic Bdnf expression;12 their obesity and impaired glucose metabolism are ameliorated by intracerebroventricular administration of BDNF.13 In rodents, both proopiomelanocortin- and Agouti-related peptide-expressing neurons in the arcuate nucleus project to the ventromedial hypothalamus, where BDNF is highly expressed.1, 14 MC4R activation induces Bdnf expression in cultured rat astrocytes,4 and Mc4r homozygous knockout mice are hyperphagic, obese and have decreased hypothalamic Bdnf expression.1 The anorexic effects of MC4R activation can be blocked by administering an anti-BDNF antibody in the third ventricle2 and the orexigenic effects of MC4R antagonism are abrogated by BDNF co-administration in the fourth ventricle.3 These data suggest an important role for BDNF downstream of MC4R within the central nervous system.

In humans, inactivation of MC4R is the most common monogenic cause of severe, early-onset obesity,15 with more pronounced weight gain in childhood compared with that in adulthood.16 In addition, two MC4R polymorphisms, V103I and 1251L, which cause decreased sensitivity to Agouti-related peptide inhibition and increased MC4R activity, respectively,17 appear to confer protection against obesity.18 Despite the strong animal evidence for MC4R’s role as a regulator of BDNF, there have been no previous studies examining BDNF concentrations in patients with loss-of-function (LOF) or gain-of-function (GOF) MC4R variants. The brain is believed to be the primary source of BDNF and circulating BDNF concentrations are thought to reflect cerebral output of BDNF.19 Therefore, examination of peripheral BDNF in subjects with MC4R variants could yield insights for the role of human MC4R signaling as a regulator of BDNF secretion. We hypothesized that BDNF would be higher in subjects with GOF MC4R variants and lower in subjects with LOF MC4R variants compared with BDNF in subjects with common sequence or non-function-altering MC4R variants. We secondarily hypothesized that the effects of MC4R variants on BDNF concentrations within individuals would be more pronounced in childhood than in adulthood. Finally, because BDNF single-nucleotide polymorphisms (SNPs) have been associated with obesity in genome-wide association studies,20 we hypothesized that BDNF SNPs would be associated with altered BDNF concentrations and would modify the effects of MC4R functional variants.

Materials and methods

Subjects

Two cohorts that had previously undergone MC4R genotyping were studied.

Pima cohort

Children and adults, age 5 years, of Pima Indian heritage were participants in an NIH longitudinal health study of members of the Gila River Indian Community in Arizona, as previously described.21 Sequencing of the MC4R coding region was performed in 6760 subjects, as previously described.16 Non-diabetic subjects were included in the current study if baseline height, weight and stored fasting serum (collected during 1970–2006) were available. All adults provided written informed consent; a parent or guardian provided informed consent for children. This study was approved by the institutional review board of the National Institutes of Diabetes and Digestive and Kidney Diseases. Forty subjects with heterozygous LOF MC4R variants and 3 subjects with homozygous LOF MC4R variants were individually matched with common sequence MC4R control subjects (LOF-C) by sample storage time (±2 years), sex, age (±1 year) and BMI (±1 kg m−2). Attempts were made to match 2 LOF-C for each LOF, but due to limitations in serum availability from children with high BMI z-scores (BMI-Z), some LOF subjects had only 1 LOF-C match for a total of 65 LOF-C. Of these individuals, 23 LOF and 43 LOF-C had measurements both in childhood and adulthood. Twenty subjects with GOF MC4R variants were individually matched 1:1 with 20 common sequence MC4R control subjects (GOF-C).

Hispanic cohort

Children and adolescents, aged 4–19 years, of Hispanic heritage were participants in the USDA/ARS Children’s Nutrition Research Center/Baylor College of Medicine sponsored Viva La Familia longitudinal study in Texas, as previously described.22 Sequencing of the entire MC4R gene plus 3′- and 5′-flanking regions was performed in 376 parents, and the 25 identified SNPs, including 7 non-synonymous SNPs in the coding region of MC4R, were genotyped in 1016 children and 613 additional parents, as previously described.23 Subjects were included in the current cross-sectional study if baseline height, weight and stored fasting plasma (collected during 2000–2004) were available. Subjects with diabetes at baseline visit and subjects with incomplete genotyping results for the seven non-synonymous coding region SNPs or a mixture of LOF and GOF MC4R variants were excluded from the current study. All children and their parents gave written informed consent or assent. The protocol was approved by the Institutional Review Board for Human Subject Research for Baylor College of Medicine and Affiliated Hospitals and Texas Biomedical Research Institute (formerly, Southwest Foundation for Biomedical Research). Subjects with the common MC4R sequence (n=25) or non-function-altering MC4R variants (n=3) were categorized together as controls for comparison with subjects who had LOF (n=21) and GOF (n=20) variants. Control subjects were selected to be similar in age, sex and BMI-Z with LOF and GOF subjects but were not individually matched. Body composition was determined by dual-energy X-ray absorptiometry with a Delphi-A whole-body scanner (Hologic Inc., Waltham, MA, USA). Total body and regional estimates of fat mass and fat-free mass were obtained by using the manufacturer’s software (version 11.2).

Measurement of BDNF and leptin concentrations

In the Pima cohort, a morning fasting venous blood sample was obtained in a serum separator tube, permitted to clot at room temperature for 30 min and then centrifuged at 1500 g to separate serum, which was stored at −80 °C. In the Hispanic cohort, a morning fasting venous blood sample was obtained in a potassium EDTA tube and centrifuged at 1000 g to separate plasma, which was stored at −80 °C. BDNF concentrations were measured using a commercial enzyme-linked immunosorbent assay (ELISA) with intra- and interassay variabilities of 3.8% and 7.6%, respectively, and a minimum detection limit of 20 pg ml−1 (Human BDNF Quantikine ELISA, R&D Systems, Minneapolis, MN, USA). Samples were diluted 1:20 for serum and 1:5 for plasma before BDNF assay, and the average value of duplicate measurements was used for data analyses. In the Pima cohort, leptin concentrations were measured using a commercial ELISA kit with intra- and interassay variabilities of 3.0% and 4.2%, respectively, and a minimum detection limit of 7.8 pg ml−1 (Human Leptin Quantikine ELISA, R&D Systems). Serum samples were diluted 1:100 before leptin assay, and the average of duplicate measurements was used for data analyses. In the Hispanic cohort, leptin concentrations were measured by a commercial radioimmunoassay kit, as previously described.23

Functional characterization of MC4R variants

Several previously identified variants found in subjects of the present study had been described as LOF in prior studies:24 D37Stop,16 G55V,25, 26 R165G,16 R165Q,16, 17 I269N16, 26 and A303P.16 Two variants were described as GOF in a prior study: V103I17 and I251L.17 One variant, F202L,27, 28 was described as non-function-altering in prior studies. Independent confirmatory functional characterization was performed for G55V, R165G, R165Q, I269N, F202L and A303P, as well as for two previously undescribed MC4R variants, C172R and M208V. In vitro mutagenesis of MC4R, stable transfections, binding studies of α-melanocyte-stimulating hormone (α-MSH) and 4-norleucine, 7-D-phenylalanine-α-MSH (NDP-MSH, a superpotent agonist for the MC4R)29 in intact cells, stimulation of intracellular cyclic AMP production by α-MSH and NDP-MSH, and imaging of cells stably expressing MC4R by confocal microscopy were performed using procedures previously described.30

Genotyping of BDNF SNPs

SNPs in BDNF (rs12291186, rs6265 and rs7124442 located within an intron, within the protein-coding exon and in the 3′-untranslated region, respectively, and spanning the major linkage disequilibrium block of BDNF with high D0.93 and low R20.2) were genotyped to examine their potential modification of MC4R functional effect on BDNF expression. In the Pima cohort, these SNPs were genotyped using SNPlex Genotyping System 48-plex (Applied Biosystems, Foster City, CA, USA) on an automated DNA capillary sequencer (model 3730, Applied Biosystems). In the Hispanic cohort, rs6265 and rs7124442 were genotyped using the Illumina HumanOmni1-Quad v1.0 BeadChips (San Diego, CA, USA), as previously described,31 whereas rs12291186 was genotyped by TaqMan SNP Genotyping Assay (Applied Biosystems).

Statistical analyses

The primary outcome measure was BDNF concentration. We calculated that a sample size of 20 subjects per MC4R functional status group would have >80% power to detect a difference in BDNF concentration of 7 ng ml−1 (1 s.d.10) at significance of P<0.05. In vitro MC4R characterization data were analyzed using Prism software (GraphPad Software, Inc., San Diego, CA, USA) to determine half-maximal inhibitory concentration (IC50) for the binding studies and half-maximal effective concentration (EC50) and maximal response (Rmax) for the stimulation studies. Maximal binding (Bmax), expressed as a percentage of the maximal amount of binding to common sequence MC4R, was calculated based on binding of 125I-NDP-MSH to variant MC4R in the absence of an unlabeled competitor. Human subject data were analyzed using SPSS Statistics software, v17.0 (IBM, Armonk, NY, USA). BMI s.d. scores (BMI-Z) were calculated based on normative standards for age and sex32 using the modified LMS method.33 Normative data for age 20 years were used to calculate BMI-Z from the measured height and weight of subjects older than 20 years. Subject characteristics were compared using independent samples t-tests, Mann–Whitney U-tests, analyses of variance with post-hoc least significant difference or Kruskal–Wallis tests with post-hoc U-tests. Longitudinal changes were assessed by paired t-tests or Wilcoxon signed-rank test and repeated-measures analysis of covariance adjusting for change in BMI-Z. Percentages were compared by Fisher’s exact and χ2-tests. Spearman’s and partial Pearson’s correlations assessed association between sample storage time and analyte concentrations. For analyses of covariance and linear regressions, BDNF and leptin were normalized by log transformation, and percent Indian heritage and percent body fat transformed by arcsine (square root). Covariates included age, sex, BMI-Z, sample storage time and BDNF genotypes, and in the Pima cohort, degree of Indian heritage based on self-reported ethnicities to the nearest eighth. Means±s.d. or medians (25–75th percentile) are shown for subject characteristics. Adjusted means±s.e.m. or back-transformed adjusted means±95% confidence intervals are shown for analyses of covariance. Genetic linkage disequilibrium analyses were performed using CubeX.34

Results

MC4R variant functional characterization

The cell surface expression, ligand binding and signaling properties of common and variant MC4Rs in response to agonist stimulation are shown in Table 1 and Supplementary Figures 1–4. The two MC4R variants with unknown function, C172R and M208V, were categorized as LOF because they had decreased ligand binding and lower basal and stimulated cyclic AMP generation compared with common sequence MC4R (Table 1 and Supplementary Figures 3–4). The finding from the other six variants we examined (G55V, R165G, R165Q, I269N, F202L and A303P) confirmed all prior reported categorizations (non-function-altering for F202L and LOF for the other five variants). Categorization of all subjects within the Pima and Hispanic cohorts are shown in Table 2.

Table 1 The ligand binding and signaling properties of common and variant MC4Rs in response to NDP-MSH or α-MSH stimulation
Table 2 Summary of functional properties of variant MC4Rs and their distribution in the cohorts studied

Pima cohort

Subject characteristics

A total of 148 subjects of Pima Indian heritage (age (mean±s.d.) 15.7±6.5 years; 50% female; BMI-Z, 1.63±1.03) were included in baseline cross-sectional analyses. Sixty-six of these subjects (23 LOF and 43 LOF-C) were also included in longitudinal analyses (baseline age: 13.8±1.7 years; follow-up age: 23.3±3.4 years; interval: 9.5±4.3 years; characteristics shown in Supplementary Table 1). Characteristics of Pima subjects grouped by MC4R functional status are shown in Table 3. LOF and GOF subjects were well matched with the control groups for age, sex, BMI and length of serum sample storage time (all P’s>0.3, Table 3). Higher BMI-Z was observed for subjects with LOF variants compared with those with GOF variants (P=0.004, Table 3), even after accounting for the higher individual mean percentage Indian heritage among LOF compared with GOF subjects (Figure 1a). Using percent Indian versus percent Pima heritage as covariates yielded similar results for all analyses.

Table 3 Subject characteristics in the Pima cohort
Figure 1
figure1

Comparison of subjects in the Pima cohort by MC4R functional status. (a) BMI-Z (adjusted for age, sex and percent Indian heritage) was similar for LOF vs LOF-C and for GOF vs GOF-C. The LOF and LOF-C groups had higher adjusted BMI-Z than the GOF and GOF-C groups. Adjusted means±s.e.m. and P-values from post-hoc least significant difference pairwise comparisons are shown. (b) Serum leptin concentration (adjusted for age, sex, BMI-Z, percent Indian heritage and sample storage time) was not significantly different between groups. (c) Serum BDNF concentration (adjusted for age, sex, BMI-Z, percent Indian heritage and sample storage time) was not significantly different between groups. (d) Serum BDNF and leptin concentrations were positively correlated, but there was no significant difference in this relationship by MC4R functional status (P=0.08). For bd, log values were used for calculations. For b and c, back-transformed adjusted means±95% confidence intervals and overall P-value from analyses of covariance are shown.

Effect of sample storage time

As the collection of samples in the Pima cohort spanned several decades, we examined the effect of storage time. Serum leptin concentration was inversely correlated with storage time within the GOF (ρ=−0.70, P=0.001) and GOF-C (ρ=−0.64, P=0.002) groups, but not within the LOF (P=0.79) and LOF-C (P=0.25) groups, suggesting instability of leptin during long-term storage at lower concentrations, as the GOF and GOF-C groups had lower leptin than the LOF and LOF-C groups attributable to their lower BMIs (Table 3). Serum leptin remained significantly associated with storage time after adjusting for age, sex, BMI-Z, percent Indian heritage and MC4R functional status (r=−0.17, P=0.048). Serum BDNF concentrations were not significantly associated with storage time (P=0.24).

Leptin and BDNF associations

Leptin, which is expected to be proportional to adiposity, was positively associated with BMI-Z (r2=0.51, P<0.001, Supplementary Figure 5a), but there was no significant difference by MC4R functional status for unadjusted values (P=0.52), as well as after adjustment for age, sex, BMI-Z, percent Indian heritage and sample storage time (P=0.22, Figure 1b). Serum BDNF concentration was significantly but weakly associated with BMI-Z (r2=0.07, P=0.001, Supplementary Figure 5b). There was no difference by MC4R functional status for unadjusted BDNF values (P=0.54), as well as after adjustment for age, sex, BMI-Z, percent Indian heritage and sample storage time (P=0.58, Figure 1c). Of note, the three subjects who were homozygous LOF MC4R variants had serum BDNF concentrations that were all above the median value for LOF-C, GOF-C, GOF and the entire cohort combined. Serum BDNF and leptin concentrations were positively correlated in unadjusted analysis (r=0.31, P<0.0001, Figure 1d) and after adjustment for age, sex, BMI-Z, percent Indian heritage and sample storage time (r=0.24, P=0.003), but there was no significant difference in this relationship by MC4R functional status (P=0.08). Sex did not significantly interact with MC4R functional status to predict BDNF concentrations (P=0.07). BDNF concentrations were higher in childhood compared with that in adulthood (median (25–75th percentile): 22.8 (18.5–26.2) vs 18.7 (14.9–24.1) ng ml−1, P=0.002), including after adjustment for change in BMI-Z (P<0.001), but this difference was not significantly associated with MC4R status (P=0.09).

BDNF genotypes

Distribution of SNPs and linkage data are shown in Supplementary Table 1. Serum BDNF concentrations were significantly associated with BDNF rs12291186 (Supplementary Figure 6a, P=0.006) and rs6265 (Supplementary Figure 6b, P=0.009), but not rs7124442 (P=0.99) genotype. When both rs12291196 and rs6265 were included in the model, each SNP genotype remained independently associated with BDNF (P=0.01 and P=0.04, respectively). None of the BDNF SNPs, however, were associated with MC4R functional status and none interacted with MC4R in the linear models predicting serum BDNF concentrations (all P’s>0.3).

Hispanic cohort

A total of 69 subjects of Hispanic heritage (age (mean±s.d.) 10.8±3.6 years; 45% female; BMI-Z, 1.57±1.07) were included in this study. Subject characteristics grouped by MC4R functional status are shown in Table 4. We observed no differences between LOF, GOF and controls for age (P=0.77), sex (P=0.95), BMI-Z (P=0.25), serum leptin concentration (P=0.50) or plasma BDNF concentration (P=0.45). As shown in Figure 2, after adjustment for age, sex and BMI-Z, no differences were observed for serum leptin (P=0.92) or plasma BDNF (P=0.79). Plasma BDNF and serum leptin concentrations were not correlated (r=0.08, P=0.52, Figure 2d) and were not associated with MC4R functional status (P=0.65). Sex did not significantly interact with MC4R functional status to predict BDNF concentrations (P=0.60). Distribution of SNPs and linkage data are shown in Supplementary Table 2. BDNF SNPs rs12291186, rs6265 and rs7124442 were not associated with plasma BDNF (P’s>0.3) and there were no significant interactions between BDNF SNPs with MC4R functional status in predicting plasma BDNF (all P’s>0.23). Removal of the three subjects with MC4R variants that were non-function-altering from the control cohort (leaving only subjects homozygous for the common genomic sequence for MC4R) did not significantly alter any findings. Examination of each variant separately did not reveal any significant differences in BDNF compared with the common sequence. The use of percent body fat or fat mass and fat-free mass instead of BMI-Z as covariates did not significantly alter any findings.

Table 4 Subject characteristics in the Hispanic cohort
Figure 2
figure2

Comparison of subjects in the Hispanic cohort by MC4R functional status. (a) BMI-Z (adjusted for age and sex) was not significantly different between LOF, controls and GOF groups. (b) Serum leptin concentration (adjusted for age, sex and BMI-Z) was not significantly different between groups. (c) Plasma BDNF concentration (adjusted for age, sex and BMI-Z) was not significantly different between groups. (d) Plasma BDNF and serum leptin concentrations were not correlated within the Hispanic cohort and there was no significant difference in this relationship by MC4R functional status (P=0.65). For bd, log values were used for calculations. For b and c, back-transformed adjusted means±95% confidence intervals and overall P-value from analyses of covariance are shown.

Discussion

Contrary to our primary hypothesis, we did not observe any significant differences in circulating BDNF concentrations in subjects with function-altering MC4R variants studied from two separate cohorts. The association of leptin with BMI-Z was similar among MC4R genotypes, suggesting that variant MC4R functioning does not alter leptin production or sensitivity independent of adiposity. Serum and plasma BDNF concentrations were not associated with MC4R functional status, suggesting that peripheral BDNF may not directly reflect hypothalamic BDNF expression or that MC4R signaling is not a significant regulator of the bulk of BDNF expression in humans. In fact, even within the hypothalamus, the subpopulation of BDNF-expressing neurons that are under the regulation of MC4R may be very few and region specific. Xu et al.1 reported that the caudal portion of the ventral medial hypothalamus is the region with the most severe reduction of BDNF expression in Ay mice that overexpress Agouti, an MC4R antagonist. Other regions of the hypothalamus, including the paraventricular, lateral and dorsomedial nuclei, as well as other regions of the brain, including the hippocampus and cerebral cortex, are not affected in the Ay mouse. Therefore, overall output of central BDNF may not be significantly affected by function-altering MC4R variants, which may exert their effects on energy balance via a small subpopulation of hypothalamic neurons through direct neuronal projections or local paracrine effects. In contrast, conditions that result in global decreases in BDNF expression due to BDNF haploinsufficiency are associated with lower peripheral BDNF concentrations as well as with hyperphagia and obesity.8, 9

We observed a positive correlation between serum BDNF and serum leptin, significant even after adjustment for BMI-Z; we also observed lack of interaction between MC4R genotype and this association. Together, these findings suggest that there may be associations between leptin and BDNF that are body mass- and MC4R independent. In our longitudinal analyses, we observed higher serum BDNF concentrations in adolescents compared with that in young adults, consistent with age-related differences in BDNF,35, 36 but these differences were not associated with MC4R functional status.

Several differences were observed when comparing findings in the Pima and Hispanic cohorts. In the Pima cohort, GOF subjects had significantly lower BMI-Z compared with LOF. However, in the Hispanic cohort, although GOF had mean BMI-Z that was 0.5 s.d. units lower than LOF (Table 4), this difference was not significant. Our sample sizes were powered based on the primary outcome measure, BDNF, rather than for BMI-Z, which would have required 60 subjects in each of the 3 MC4R groups within the Hispanic cohort to have 80% power to detect a significant difference; thus, the available sample size of individuals with GOF and LOF MC4R variants is a limitation of the study. The observation of relatively high BMI values among subjects with GOF MC4R variants (mean of 83rd percentile and 91st percentile in the Pima and Hispanic cohorts, respectively) is attributable to the very high prevalence of obesity in individuals of Pima descent (such that children with somewhat protective alleles have a high BMI compared with the US norms but below average BMI within their own population)16 and the selection criteria for the Hispanic cohort, which enrolled probands and their family members based on probands having BMI 95th percentile and fat mass 85th percentile.22

In the Pima group, we observed a positive correlation between serum BDNF and leptin, as well as associations of BDNF rs12291186 and rs6265 with serum BDNF concentration. Rs12291186 is located within an intron of BDNF, and the sequence change alters a putative binding site for the YY1 transcription factor (TRANSFAC 7.0 Public 2005). Rs6265 causes a Val66Met substitution in the N-terminal portion of the BDNF prohormone, which is thought to impair intracellular trafficking of BDNF, leading to decreased activity-dependent secretion of BDNF.37 However, plasma concentration in the Hispanic cohort was associated with neither serum leptin nor any of the BDNF SNPs, which may be attributable to the twofold higher coefficient of variability in plasma BDNF concentration. As BDNF in peripheral circulation is stored in and released from platelets,38 plasma BDNF concentrations are expected to be several fold lower than serum BDNF concentrations, and trace hemolysis can result in falsely elevated BDNF concentrations in plasma. Conversely, insufficient clotting of blood before separation of serum can lead to falsely diminished BDNF concentrations. A limitation of this study is the lack of platelet count values, which are often included as a covariate for analyses.39 Another limitation is the lack of body composition data for the Pima cohort. A further limitation of our study is the lack of samples from individuals during infancy and toddlerhood, ages at which genetic influences on energy homeostasis may be especially pronounced; however, comparison of longitudinal data during adolescence and adulthood in a subset of our Pima subjects showed no interaction between MC4R functional status and age in contributing to differences in BDNF.

We conclude that even though peripheral BDNF concentrations are, on average, lower in patients with disorders that cause global central nervous system BDNF insufficiency,8, 9 circulating BDNF may not be an adequate proxy measure for specific changes in MC4R-regulated hypothalamic BDNF secretion. Post-mortem cadaveric studies of human hypothalamic tissue from individuals with MC4R functional variants would be needed to address these questions more fully.

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Acknowledgements

Funding for this study was provided by the International Hyperphagia Conference Best Idea Grant from the Prader–Willi Syndrome Association (USA) (JCH and JAY), by the NIH Intramural Research Programs of NICHD and NIDDK, American Diabetes Association grant 1–12-BS212 (Y-XT), NIH (R01 DK59264 and R01 DK080457 (NFB), and C06 RR013556 and C06 RR017515) and USDA/ARS under Cooperative Agreement 58–6250–51000–037. JK and JAY are Commissioned Officers of the United States Public Health Service. Clinical Trials Registry: Subjects in the Pima cohort were enrolled in observational, non-interventional studies, registered as NCT00339482. Subjects in the Hispanic cohort were enrolled in observational, non-interventional studies.

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Correspondence to J C Han.

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Supplementary Information accompanies this paper on International Journal of Obesity website

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Hohenadel, M., Thearle, M., Grice, B. et al. Brain-derived neurotrophic factor in human subjects with function-altering melanocortin-4 receptor variants. Int J Obes 38, 1068–1074 (2014) doi:10.1038/ijo.2013.221

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Keywords

  • BDNF
  • MC4R
  • ELISA
  • SNP
  • leptin

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