Translating information on genetic risk for body weight regulation into molecular mechanisms can have a significant impact on intervention and therapies. We are seeking to identify genetic variation and their molecular mechanisms that influences obesity through direct effects on the hypothalamus as it is the brain hub that regulates energy homeostasis and there is now considerable evidence for genetic influence to impact this brain region. However, the majority of genetic loci associated with this common, chronic disorder in the general population are located in noncoding regions of the genome, suggesting their influence on energy homeostasis is manifested through changes to the regulome. Thus, pinpointing the causal human variants and connecting them to their downstream targets in brain presents challenges of tissue access for study because much epigenetic control is species-, tissue- and context-specific. To overcome the barrier of limited human tissue access, we have developed a robust protocol for generating human induced pluripotent stem cell (iPSC)- derived neuronal cultures that recapitulate many of the features of hypothalamic neurons from the arcuate nucleus, including by benchmarking this in vitro model to in vivo events that are pivotal in hypothalamic development. We will use this human model and state of the art high throughput assays to map the currently uncharted regulatory landscape of the human hypothalamic neurons across 3 stages in development (early, mid, and late) and under experimental obesogenic conditions. Next, in order to precisely pinpoint the functional variants in BMI GWAS loci that have influence on body weight regulation through hypothalamic epigenomic regulation, we will identify those that influence chromatin accessibility and/or target gene expression by assay in 100 iPSC-derived neuronal lines generated from subjects of the San Antonio Mexican American Family Studies. GWAS variants with both properties have high potential to be causal and manifest effects on body mass index through changes in chromatin structure. Causal determination will be made for a set of these variants using genome editing techniques such as CRISPR/Cas9 to generate isogenic human neuronal cell lines that differ by genotype only at the single locus. Changes in exon-specific target gene expression and chromatin status will be assessed across the 3 developmental stages and under each obesogenic condition. Discovery of epigenetic mechanisms connected to genetic liability will translate the genetic risk information and identify potential underlying factors behind both heritable and diet-induced obesity susceptibility.
Overweight and obesity among the world's population is staggering. According to the World Health Organization (WHO), in 2014, the mean body mass index (BMI) of the world's population rose to 24 kg/m2, and 39% of adults aged 18 years and over were overweight (BMI between 25 and 30) while 13% were obese (BMI ? 30). Increase in mortality rates correlate with increasing degrees of overweight, as measured by body mass index. In the United States, it has been estimated that more than one-third of adults and 17% of youth are obese. An increasing trend in body weight has also been observed among infants and toddlers from birth to 2 years. In 2011, 7.1% were at or above the 97.7th percentile of sex-specific WHO weight for recumbent length growth charts. This puts millions of individuals of all ages at risk for life threatening disorders such as coronary heart disease, type 2 diabetes, cancers, and stroke. The fundamental cause of overweight and obesity is an energy imbalance which can be the result of genetic susceptibility and environmental influence such as diet and lifestyle. A key organ system that regulates energy balance is the brain hub called the hypothalamus. Neurons within this brain region receive signals regarding energy status and direct the whole body response. Understanding the genetic control of these key hypothalamic neurons as well as any disturbances in function due to hereditary genetic variation may point to new pathways for treatment options and prevention during development.
