Inter-generational risk factors for obesity: a path to prevention in LMICs based on a modifiable epigenetic signature in the POMC gene

Obesity is a major global public health issue with increasing damaging effects in low and middle-income countries (LMIC) including The Gambia. Obesity interventions are generally expensive and ineffective; therefore, prevention is key. LMICs are predicted to be the hardest hit by the projected increases in obesity and T2 diabetes (T2D). Due to poorly resourced health care in LMICs the case-fatality rates for obesity and T2D are much higher than in affluent nations and constitute a major burden for health budgets now and more so in the future.
BMI (body mass index) is highly heritable (runs in families), though genetic variants account for a small proportion of individual body weight variability. Epigenetic mechanisms (e.g. DNA methylation) may explain much of the missing heritability. Variably Methylated Regions (VMR) are regions in the genome that show substantial variation between individuals in their methylation; this variation may contribute to variations in characteristics (e.g. body weight) between individuals by altering gene expression.
Obesity is a disorder of over consumption. A complex network of hormonal pathways regulates appetite and satiety, with POMC (pro-opiomelanocortin) hormone that mediates satiety responses in the brain. It is a key regulator of energy balance and obesity risk.
Increased DNA methylation at a variably methylated region (VMR) at the POMC intron2/exon3 boundary is correlated with BMI in children and adults. POMC methylation influences the amount of POMC gene expression, thus provides a link between POMC methylation and obesity. It has been shown previously that POMC methylation is established in the early embryo and is influenced by seasonal nutritional changes at conception in Gambians. It has also been shown that children's methylation at POMC VMR is associated with the father's methylation not the mother's.
It is therefore hypothesised that 1) POMC hypermethylation (by dampening satiety) will promote more rapid weight gain in the harvest season and protect against weight loss in the hungry season, 2) offspring POMC methylation is influenced by the mother's nutrition around time of conception and 3) offspring POMC methylation is correlated with father's, but not mother's, peripheral blood cell methylation.
We will exploit our well-established Gambian seasonal 'experiment of nature', whereby seasonal changes in rainfall (wet and dry seasons) creates profound contrasts in the availability and composition of food, in work patterns among adults, and in infectious disease and growth patterns in children. This leads to seasonally driven variations in individual's weight and body composition over the year e.g. in Gambian women on average weight fluctuates by nearly 4 kg between June and October.
The project will follow up 500 mother and child pairs monthly over a year to examine their weight, BMI and body composition in relation to their POMC methylation taken at the start of the study. A subset of 100 mother and child pairs will also have DXA (Dual energy X-ray Absorptiometry) scans and an ab libitum breakfast (participants are freely permitted to consume as much breakfast as they wish as a measure of satiety) at three time points (at peaks of seasons; April-October-April) over the year.
Understanding epigenetic regulation of POMC and how this influences body weight and susceptibility to obesity, could enable the design of nutritional interventions for women of child-bearing age that would mitigate obesity risk in their offspring. It is important to assess the effect of methylation at POMC VMR on BMI in diverse settings before considering future interventions in this LMIC. If it is found that these epigenetic marks are nutritionally modifiable and further that they influence weight regulation in infants and children, this would open a new chapter in directed interventions designed to optimise human foetal development and life-long health outcomes.

Pro-opiomelanocortin (POMC) gives rise to melanocortin stimulating hormone (MSH) peptides that mediate the anorectic (appetite suppressing) action of leptin via melanocortin receptors (especially MC4R) in the hypothalamus. Mutations in the gene are associated with obesity.
There is evidence that POMC methylation levels are linked to obesity in adults and children; influenced by maternal nutrition around the time of conception; correlated with POMC methylation in paternal blood (but not maternal); stable at least over the period from birth to adulthood and influences POMC expression. POMC VMR methylation is correlated across tissues from 3 germ layers including peripheral blood cells (PBC) and hypothalamic tissue (where POMC neurons exert their effect), indicating that PBC are an appropriate accessible tissue to measure hypothalamic POMC VMR methylation.

We will determine how POMC methylation influences weight regulation and adiposity against a background of large seasonal fluctuations in energy availability in rural Gambia. Growth data and POMC VMR methylation (banked DNA from children aged 0-2 years, n=800) will be examined to establish any associations between weight changes and POMC methylation. A prospective study of 500 mothers and children (5-9 years) will include monthly anthropometry and bioimpedance and a measure of POMC VMR methylation. A subset will have DXA scans and measurement of an ab libitum breakfast. Dietary influence over POMC methylation will be examined by correlating circulating maternal one carbon metabolites and offspring methylation. Longitudinal stability of POMC methylation will be tested by measuring POMC methylation (subset) at multiple time points. All children will be genotyped using an array enriched for African-specific variants to assess potential genetic effects on POMC methylation. Intergenerational influences on offspring methylation will be assessed by measuring POMC VMR methylation in 100 mother-father-offspring trios.

Obesity and its associated comorbidity type-2 diabetes (T2D) account for more than 4 million deaths per year, and low and middle-income countries (LMICs) are predicted to be the hardest hit by the projected increases in these conditions. A clearer understanding of the biological pathways that make individuals more or less susceptible to obesity could therefore have a substantial global health impact by contributing to the development of novel preventive and therapeutic approaches (including the identification of possible pharmacologic targets) to tackling obesity. This work will be of particular interest to public health scientists, nutritional epidemiologists and clinicians and could lead to exciting avenues in obesity prevention in LMICs and also in other settings (including the UK).

The Pro-opiomelanocortin (POMC) hormone, expressed in the hypothalamus, plays a central role in satiety, and it has been shown that increased DNA methylation in a region of this gene is a strong predictor of obesity. Importantly, from prior work, POMC methylation is established in the early embryo, and is sensitive both to maternal nutritional environment around conception and correlates with paternal, not maternal, methylation.
If successful, this work would contribute to advances in a surprisingly broad spectrum of science with a potential large impact on strategies for the prevention of obesity and associated comorbidities. In basic science, the proposed work can provide insights into nutritional and intergenerational factors affecting epigenetic regulation in the developing embryo by analysing the effects of one-carbon metabolites on differentiating pluripotent cells and exploring inter-generational epigenetic regulation in family trios. It may thereby add to our understanding of the heritability of human obesity and contribute to knowledge of how POMC and other environmentally-sensitive variably methylated regions (MEs) affect evolutionary fitness.
The proposed studies mapping the effects of POMC methylation on weight regulation, and appetite control offer a direct means of assessing the energy-regulating effects of this gene in a human population. Furthermore, by confirming and strengthening our understanding of how the maternal methyl donor metabolome influences POMC regulation and obesity risk, this work opens up the possibility of modifying obesity risk by changing parental periconceptional diet, for both natural and assisted conceptions.

Contribution to improving the health of people of The Gambia
Obesity has become an archetypal symbol of modern human development that is now threatening the health of populations in almost every region of the world. Moderate overweight and obesity predict meaningful increases in the risks of numerous other co-morbid conditions (including T2D) that together impose high costs to health services. Obesity is notoriously resistant to health interventions at both the individual and population levels. In the Gambian population, obesity is becoming an increasing public health concern even outside of urban centres. Understanding the role of a potentially modifiable epigenetic contributor to obesity could have widespread benefits to the people of Gambia and around the world. Obesity treatment is expensive and generally ineffective; therefore, development of preventative strategies is essential. This work would explore the mechanisms of early foetal programming leading to obesity and potentially inform maternal and paternal nutritional interventions to mitigate this risk for future generations. Equally gaining understanding of the early programming of hormonal control of energy balance and examining this under nutritional and infectious disease burden allows for greater knowledge of contributors to undernutrition across the life course. By understanding the relationship between the epigenome and health outcomes, interventions could be developed to minimize deleterious epigenetic programming.