| Summary: | <p>As prevalence of obesity and metabolic disorders increase, it is necessary to elucidate mechanisms driving body mass, body composition and energy homeostasis. Elevated waist-hip ratio (WHR) increases type 2 diabetes and cardiovascular disease risk. The TBX15-WARS2 locus contains several independent association signals for WHR adjusted for BMI (WHRadjBMI). However, whether these risk variants are functional remains to be determined. The aim of my DPhil was to characterise two potentially functional TBX15-coding polymorphisms, rs10494217 (p.H156N) and rs61730011 (p.M566R), at a cellular and organismal level to investigate their impact on TBX15 function in adipose tissue biology.</p>
<p>Firstly, I prioritised the mechanism-of-action of these two polymorphisms at either a DNA-regulatory or protein-coding level in silico. I showed the most likely mode-of-action was at the protein-coding level with p.H156N predicted to alter a loop structure in the T-box of TBX15, further implicated in altered protein-protein dimerisation. However, this was not recapitulated in vitro (within assay sensitivity limits).</p>
<p>TBX15-WARS2 signals were associated with differential TBX15 and WARS2 expression, and several Tbx15 and Wars2 mouse models had altered body composition phenotypes. I metabolically characterised heterozygous knock-out (Tbx15+/-) mice and mice carrying orthologous human polymorphisms p.H156N (Tbx15H156N) or p.M566R (Tbx15M566R) and showed all three alleles conferred resistance to fat mass gain.</p>
<p>Finally, to evaluate the effect of TBX15 on gene expression, I generated a TBX15 regulatory network using ChIP-sequencing of preadipocyte cell lines, and RNA-sequencing of adipose tissues from homozygous knock-out (Tbx15-/-) and Tbx15H156N mice. Functional enrichment of differentially expressed genes implicated TBX15 in regulating lineage determination, adipogenesis of committed preadipocyte cells and immune response. In summary, these results suggest TBX15 regulates adipose tissue biology and body composition and shows a novel functional role for two human WHRadjBMI risk variants. Further work is needed to elucidate underlying mechanisms of action that can be targeted therapeutically.</p>
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