Causal relationships between NAFLD, T2D and obesity have implications for disease subphenotyping

Abstract Background & Aims Non-alcoholic fatty liver disease (NAFLD), type 2 diabetes (T2D) and obesity are epidemiologically correlated with each other but the causal inter-relationships among them remains incompletely understood. We aim to explore the causal relationships among the three diseases. Methods Using both UK BioBank and the largest-to-date publicly available GWAS data, we performed a two-sample bidirectional Mendelian Randomization (MR) analysis to test the causal inter-relationships among NAFLD, T2D, and obesity. Transgenic mice expressing the human PNPLA3-I148M isoforms (TghPNPLA3-I148M) were used as an example to validate the causal effects and explore the underlying mechanisms. Results Genetically-instrumented NAFLD significantly increased the risk for T2D and central obesity but not insulin resistance or generalized obesity, while genetically-driven T2D, BMI and WHRadjBMI causally increase the NAFLD risk. The animal study focusing on PNPLA3 well-corroborated these causal effects: compared to the TghPNPLA3-I148I controls, the TghPNPLA3-I148M mice developed glucose intolerance and increased visceral fat, but normal insulin sensitivity, reduced body weight, and decreased circulating total cholesterol. Mechanistically, the TghPNPLA3-I148M mice demonstrated decreased pancreas insulin but increased glucagon secretion, which was associated with increased pancreatic inflammation. There were significantly suppressed transcription of hepatic cholesterol biosynthesis pathway genes while activation of thermogenic pathway genes in subcutaneous and brown adipose tissues but not in visceral fat of the TghPNPLA3-I148M mice compared to the TghPNPLA3-I148I mice. Conclusions Our study suggests that lifelong, genetically-driven NAFLD causally promotes T2D with a late onset Type-1 like diabetic subphenotype and central obesity; while genetically-driven T2D, obesity, and central obesity all causally increase the NAFLD risk. This causal relationship revealed new insights into the nature and nurture of these diseases and provided novel hypotheses for disease subphenotyping.