This article was edited to specify the MC4R pathway, rather than the POMC pathway, in the final quote.
NEW YORK – Methylation patterns on the proopiomelanocortin (POMC) gene that arise during early embryonic development raise the risk of obesity, according to research from Berlin's Charité Medical University.
The study, published in Science Translational Medicine, showed that POMC methylation impairs the melanocortin 4 receptor (MC4R) signaling pathway in a manner similar to certain obesity-associated POMC genetic mutations.
POMC, according to Peter Kühnen, director of Charité's Center for Pediatric Endocrinology and the study's senior author, is a good gene to examine when studying obesity because it encodes a protein that induces satiety, or the sense of fullness needed to prevent overeating. The POMC protein triggers the generation and secretion of melanocyte-stimulating hormones (MSH), which activate cellular MC4R signaling to prevent overeating.
"It's one of the signals for our brain to feel full," Kühnen said, but added that genetics alone do not explain the whole picture, as people may exhibit the same symptoms of POMC-related genetic obesity without having a pathological variant.
This is why Kühnen's team turned their attention to the epigenetic regulation of body weight. "The idea was that if there are modifications on the epigenetic level, this might interfere with the pathway for the regulation of satiety and this might be associated with increased risk [of obesity]," he said.
As part of the study, Kühnen and his colleagues administered Rhythm Pharmaceuticals' MC4R agonist Imcivree (setmelanotide) to five individuals with nongenetic obesity but with POMC hypermethylation and observed an average 4.66-pound reduction in weight as well as a decrease in associated hunger scores over a 38-week period.
While promising, Kühnen, who also conducted the first investigator-initiated study of Imcivree but is not employed by Rhythm, cautioned against drawing conclusions about clinical utility from this study alone.
"A large clinical trial will be necessary to evaluate the impact of [Imcivree] on POMC hypermethylated patients," he said. "Without such a trial it will not be appropriate to state that this MC4R agonist would be a treatment option for these patients or even raise any hopes that [Imcivree] may be a treatment option in the future."
David Meeker, president and CEO of Rhythm Pharmaceuticals, concurred with Kühnen's assessment but also expressed cautious optimism about the study's results.
"This study builds on our early but expanding understanding of the various drivers for obesity," Meeker said. For example, he noted that the study shows the impact of epigenetics on POMC-expressing neurons and, consequently, MC4R signaling. Additionally, data from the small number of patients treated provided further evidence of Imcivree's efficacy in those with impaired MC4R pathway signaling.
Kühnen and his team had previously identified hypermethylation at a CpG island between two exons of the POMC gene in obese versus non-obese people. They looked further into that mechanism in the current genome-wide association study (GWAS) of 1,128 individuals from the longitudinal Heinz-Nixdorf Recall study for whom there was also Illumina's Infinium HumanMethylation450K BeadChip methylation data.
In these individuals, they identified seven exonic methylation sites whose methylation patterns did not appear to be genetically determined. Further analysis revealed that women were more likely to have obesity-related hypermethylated sites than men. For the time being, the reason for this sex-specific effect remains unknown.
To get a sense of when POMC methylation might occur in development, Kühnen and his colleagues analyzed data from 32 monozygotic twins and 38 dizygotic twins. They observed that monozygotic twins almost always shared POMC methylation patterns, while no correlation appeared between dizygotic twins, suggesting that methylation occurred early on, prior to embryo cleavage.
The team further confirmed this finding in a human embryonic stem cell model.
As nutrition has been seen to alter DNA methylation in mouse models and that maternal access to carbon-1 (C1) metabolites such as betaine, choline, folate, and methionine affects POMC methylation in offspring, Kühnen's team differentiated human stem cells in the presence of C1 metabolites to get a rough idea of the role such metabolites might play in DNA methylation. "You need these metabolites for the methyltransferase enzyme," Kühnen said, referring to the enzyme responsible for modifying DNA by adding methyl groups to cytosines.
Changing C1 metabolite concentrations during early growth phases led to gene expression changes for multiple C1 metabolizing enzymes and resulted in altered POMC methylation in fully differentiated cells. Although Kühnen said that humans obtain most of those metabolites from food, the degree to which a mother's diet actually influences DNA methylation in utero remains an open question.
"This is an in vitro system," Kühnen cautioned. "It's very important not to overinterpret the results."
The evidence gathered from in vitro observations encouraged Kühnen and his team to test the effects of Imcivree treatment in humans, leading to the aforementioned proof-of-concept trial in Science Translational Medicine.
"This is an elegant study on human POMC expression level controlled by DNA methylation related to obesity development," commented Qingchun Tong, professor of molecular medicine at the University of Texas Health Science Center, who was not involved in the study. "The results nicely demonstrate the point that obesity development can be influenced by environments through modulating DNA methylation levels, in this case, of the POMC gene."
Kühnen is now planning a larger study that expands the scope to the level of the whole methylome, in an effort to identify other disease risk-associated regions.
Tong added that another important avenue of research may be in investigating the functional consequences of POMC hypomethylation. One of his own previous studies, for instance, showed that while the loss of POMC function led to obesity in mice, the reverse had no obvious impact on body weight.
"It would be interesting to examine whether human populations with gain of function of POMC neurons, [for example with] low DNA methylation, exhibit any difference in body weight," he said, adding that that would "reveal whether the observed biased role of POMC neurons in mice is also applicable to humans."
Rhythm's Meeker added that while this study's results are too early to affect the company's plans with respect to Imcivree, "the more research that is done on the [MC4R] pathway and the factors which may influence its function, the better it is for patients, the field in general, and Rhythm as an interested partner in all of this."