Molecular characterization in human neurons of genes associated with the control of bodyweight and feeding behaviour

Abstract

Obesity is a major risk factor for many common diseases and has a significant heritable component. It is well documented that a powerful genetic component underlies the large variation in human body size in response to the modern day ‘obesogenic’ environment. Therefore, genetic approaches can be leveraged to help characterise the underpinning physiological and molecular mechanisms controlling food intake and body weight. Those genetic approaches can be furthered by increasingly abundant transcriptomic studies of murine and human hypothalamus – a brain area known to be a key regulator of food intake. The overarching aim of the project was to shed light on new appetitive control pathways and uncover new potential therapeutic targets to affect obesity. AgRP and POMC neurons in the hypothalamic arcuate nucleus show reciprocal response to feeding and fasting cues, so it can be speculated that reciprocally regulated genes in these neurons could have a role in regulating food intake and body-weight. Thus, in the first part of my thesis, we have identified novel genes downstream of POMC/AgRP that play a role in the control of energy homeostasis and compared expression profiles of candidate genes between mice and humans to assess relevance for human obesity. The five candidate genes uncovered include VGF, SCG2, PTPRN, RGS4 and RAB3B. We then turned to leveraging genetic information to uncover new genes with a role in energy homeostasis. Saeed et.al. have used the consanguineous Pakistani population to identify mutations in several novel obesity genes, including homozygous mutations in ROCK1 and KSR2. In parallel, whole exome-sequence analyses for adult BMI in up to 587,027 individuals revealed rare loss of function variants in the BSN gene that lead to severe obesity in humans. We anticipated involvement of the novel genes of interest in brain control of food intake. We have developed a human neuronal cell model method for characterizing novel obesity-linked genes of interest. The top affected pathways emerged from the transcriptomic studies of BSN+/- effects on human neurons include ‘Neuroactive ligand-receptor interaction’ and ‘negative regulation of neurogenesis’, and also ‘Respiratory chain complex I (gamma subunit) mitochondrial’. Therefore we propose that rare mutations in BSN might confer their effects on BMI through widespread dysregulation of neurodevelopment, neurogenesis, and altered neuronal oxidative phosphorylation in neurons within the central feeding circuitry. In summary, we have uncovered five candidate food intake control genes: (VGF, SCG2, PTPRN, RGS4 and RAB3B) through murine and human hypothalamic transcriptomic studies; developed a human neuronal cell model for functional characterization of relevant genes and uncovered a link between mutations in novel obesity-linked gene BSN and neuronal development.