Nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes (T2D) often coincide but the physiological relationship is unclear. Important clues are provided by human genetics: variants associated with NAFLD-risk have divergent effects on T2D. At TM6SF2 the NAFLD-risk allele (rs58542926:T) predisposes to T2D whilst at GCKR (rs1260326:T), it protects. We hypothesize that these divergent effects result from the impact of NAFLD-raising alleles on hepatic lipid output (HLO) and hepatic glucose output (HGO) respectively.
To test this hypothesis, we applied structural equation modeling (SEM) to data from the IMI-DIRECT study, including 1350 European participants, 923 nondiabetic (C1) and 427 with recently-diagnosed T2D (C2). Insulin sensitivity (IS) was modeled from frequently sampled OGTT (C1) and mixed-meal tolerance tests (C2), and liver fat (LF) measured by MRI. SEM fit was assessed by comparing model χ2s with comparable null models: variables randomized to nodes in identical SEM definition giving comparable degrees of freedom and structure, 10,000 iterations.
The hypothesized model provided a better fit than null models (C1: χ2=242, P=0.005; C2: χ2=63, P=0.01). Decreasing HLO (via TM6SF2) was associated with an increase in LF (C1: ß=0.28 [SE 0.09], P=0.001; C2: ß=0.28 [0.12], P=0.02) and fasting plasma glucose (FG): these were mediated by LF and IS (C1: ß=0.09 [0.03], P=0.001; C2: ß=0.07 [0.03], P=0.027). Decreasing HGO (via GCKR) was not associated with LF in either cohort but was associated with a direct (non-mediated) decrease in FG (C1 only: ß=-0.08 [0.04], P=0.04).
The results support a model whereby increased genetic risk of NAFLD, arising from defects in two processes (HLO and HGO) results in opposing glycemic effects. This has important consequences for therapeutic strategies for combatting NAFLD: approaches to reduce NAFLD through targeting of HLO (as opposed to HGO) may also reduce risk of hyperglycaemia and T2D.
R.W. Koivula: None. P.W. Franks: Board Member; Self; Zoe Ltd. Research Support; Self; AstraZeneca, Boehringer Ingelheim International GmbH, Lilly Diabetes, Novo Nordisk A/S, Novo Nordisk Foundation, Sanofi, Servier. T.H. Hansen: None. M. Laakso: None. E. Pearson: None. F. Rutters: None. F. Karpe: None. J.W. Tomlinson: Advisory Panel; Self; Novo Nordisk Inc., Pfizer Inc., Poxel SA. A. Mahajan: None. M. McCarthy: Advisory Panel; Self; European Association for the Study of Diabetes, Pfizer Inc. Consultant; Self; Eli Lilly and Company, Merck & Co., Inc. Consultant; Spouse/Partner; Merck & Co., Inc. Research Support; Self; AbbVie Inc., Boehringer Ingelheim International GmbH. Research Support; Spouse/Partner; Diabetes UK. Research Support; Self; Janssen Pharmaceuticals, Inc., Merck & Co., Inc., National Institutes of Health. Research Support; Spouse/Partner; National Institutes of Health. Research Support; Self; Novo Nordisk A/S. Research Support; Spouse/Partner; Novo Nordisk A/S. Research Support; Self; Novo Nordisk Foundation, Roche Pharma, Sanofi-Aventis, Servier, Takeda Pharmaceutical Company Limited.
Novo Nordisk Foundation (NNF18OC0031650); Innovative Medicines Initiative Joint Undertaking (115317)