Exploring the physiological role of an obesity-linked gene variant

Abstract

An Arg457Gln missense variant in the CREBRF gene (encoding for Cyclic AMP Response Element Binding Protein 3 Regulatory Factor) has previously been identified to paradoxically drive both excess body weight and reduced diabetes risk in numerous Pacific/Oceanic populations. Despite its apparent critical role in whole-body metabolism, few published studies have investigated this CREBRF missense variant at the molecular level. This thesis describes the metabolic characterisation of a novel mouse model in which the CREBRF Arg458Gln variant was knocked in to replace the endogenous CREBRF. To address the whole-body phenotype, male and female mice were examined on a regular chow diet or an 8-week high-fat challenge, followed by analyses of the tissue transcriptome and in vitro signalling to determine possible variant impacts to molecular pathways. Assessment of body composition found that lean mass and naso-anal length were significantly increased by the CREBRF variant in male mice, without effect on total body weight or fat mass. Glucose tolerance and indirect calorimetry assessments were likewise unchanged by genotype. Male chow-fed variant carriers displayed reduced sensitivity to insulin administration, as well as exaggerated fasting-induced trends in nutrient homeostasis, including alterations to tissue glycogen and lipid as well as plasma NEFA elevation. Microarray analysis performed on liver and gastrocnemius muscle tissue from this cohort revealed genotype-dependent differential expression of genes contributing to protein synthesis, processing, and turnover as well as cellular respiration and nutrient metabolism. A follow-up cohort saw no significant changes to muscle function assessed by exercise endurance or grip strength performance. In vitro examination of insulin and glucagon signalling pathways using a primary hepatocyte model revealed significant dampening of PKB signalling in cells isolated from male mice carrying the variant. Overall, this novel mouse model appears to show a whole-body growth phenotype effect of the CREBRF variant in males, with speculative roles in regulating energy homeostasis during periods of nutrient deprivation without benefit to insulin sensitivity or glucose tolerance. The mild effects of the Arg458Gln mutation in this mouse model may invite reconsideration of the link between CREBRF function and the risks of obesity and diabetes in variant allele carriers.