Type 2 diabetes sex-specific effects associated with E167K coding variant in TM6SF2

Interplay between gut microbiome, host genetic and epigenetic modifications in MASLD and MASLD-related hepatocellular carcinoma

Metabolic dysfunction-associated steatotic liver disease (MASLD) encompasses a wide spectrum of liver injuries, ranging from hepatic steatosis, metabolic dysfunction-associated steatohepatitis (MASH), fibrosis, cirrhosis to MASLD-associated hepatocellular carcinoma (MASLD-HCC). Recent studies have highlighted the bidirectional impacts between host genetics/epigenetics and the gut microbial community. Host genetics influence the composition of gut microbiome, while the gut microbiota and their derived metabolites can induce host epigenetic modifications to affect the development of MASLD. The exploration of the intricate relationship between the gut microbiome and the genetic/epigenetic makeup of the host is anticipated to yield promising avenues for therapeutic interventions targeting MASLD and its associated conditions. In this review, we summarise the effects of gut microbiome, host genetics and epigenetic alterations in MASLD and MASLD-HCC. We further discuss research findings demonstrating the bidirectional impacts between gut microbiome and host genetics/epigenetics, emphasising the significance of this interconnection in MASLD prevention and treatment.

Basic and translational evidence supporting the role of TM6SF2 in VLDL metabolism

PURPOSE OF REVIEW

Transmembrane 6 superfamily member 2 ( TM6SF2 ) gene was identified through exome-wide studies in 2014. A genetic variant from glutamic acid to lysine substitution at amino acid position 167 (NM_001001524.3:c.499G> A) (p.Gln167Lys/p.E167K, rs58542926) was discovered (p.E167K) to be highly associated with increased hepatic fat content and reduced levels of plasma triglycerides and LDL cholesterol. In this review, we focus on the discovery of TM6SF2 and its role in VLDL secretion pathways. Human data suggest TM6SF2 is linked to hepatic steatosis and cardiovascular disease (CVD), hence understanding its metabolic pathways is of high scientific interest.

RECENT FINDINGS

Since its discovery, completed research studies in cell, rodent and human models have defined the role of TM6SF2 and its links to human disease. TM6SF2 resides in the endoplasmic reticulum (ER) and the ER-Golgi interface and helps with the lipidation of nascent VLDL, the main carrier of triglycerides from the liver to the periphery. Consistent results from cells and rodents indicated that the secretion of triglycerides is reduced in carriers of the p.E167K variant or when hepatic TM6SF2 is deleted. However, data for secretion of APOB, the main protein of VLDL particles responsible for triglycerides transport, are inconsistent.

SUMMARY

The identification of genetic variants that are highly associated with human disease presentation should be followed by the validation and investigation into the pathways that regulate disease mechanisms. In this review, we highlight the role of TM6SF2 and its role in processing of liver triglycerides.

Human Induced Pluripotent Stem Cell based Hepatic-Modeling of Lipid metabolism associated TM6SF2 E167K variant

BACKGROUND AND AIMS

TM6SF2 rs58542926 (E167K) is associated with an increase in the prevalence of Metabolic Disfunction-Associated Steatotic Liver Disease (MASLD). Despite all the investigation related to the role of this variant in lipid metabolism, conflicting results in mouse studies underscore the importance of creating a human model for understanding the TM6SF2 mechanism. Therefore, the aim of this study is to generate a reliable human in vitro model that mimic the effects of the TM6SF2 E167K mutation and can be used for future mechanism studies.

APPROACH AND RESULTS

We performed gene editing on human-induced pluripotent stem cells (iPSC) derived from a healthy individual to obtain the cells carrying the TM6SF2 E167K mutation. After hepatic differentiation, a decrease in TM6SF2 protein expression was observed in the mutated-induced hepatocyte. An increase in intracellular lipid droplets and a decrease in the efflux of cholesterol and ApoB100 were also observed. Transcriptomics analysis showed up-regulation of genes related to the transport, flux, and oxidation of lipids, fatty acids, and cholesterol in TM6SF2 E167K cells. Additionally, signs of cellular stress were observed in the ER and mitochondria.

CONCLUSIONS

Our findings indicate that induced hepatocytes generated from iPSC carrying the TM6SF2 E167K recapitulate the effects observed in human hepatocytes from individuals with the TM6SF2 mutation. This study characterizes an in vitro model that can be used as a platform to help in the identification of potential clinical targets and therapies and to understand the mechanism by which the TM6SF2 E167K variant leads to vulnerability to MASLD.

Effects of TM6SF2 rs58542926 polymorphism on hepatocellular lipids and insulin resistance in early type 2 diabetes

BACKGROUND AND AIMS

Increased hepatocellular lipid content (HCL) is linked to insulin resistance, risk of type 2 diabetes and related complications. Conversely, a single-nucleotide polymorphism (TM6SF2EK; rs58542926) in the transmembrane 6 superfamily member 2-gene has been associated with nonalcoholic fatty liver disease (NAFLD), but lower cardiovascular risk. This case-control study tested the role of this polymorphism for tissue-specific insulin sensitivity during early course of diabetes.

METHODS AND RESULTS

Males with recent-onset type 2 diabetes with (TM6SF2EK: n = 16) or without (TM6SF2EE: n = 16) the heterozygous TM6SF2-polymorphism of similar age and body mass index, underwent Botnia-clamps with [6,6-2H2]glucose to measure whole-body-, hepatic- and adipose tissue-insulin sensitivity. HCL was assessed with 1H-magnetic-resonance-spectroscopy. A subset of both groups (n = 24) was re-evaluated after 5 years. Despite doubled HCL, TM6SF2EK had similar hepatic- and adipose tissue-insulin sensitivity and 27% higher whole-body-insulin sensitivity than TM6SF2EE. After 5 years, whole-body-insulin sensitivity, HCL were similar between groups, while adipose tissue-insulin sensitivity decreased by 87% and 55% within both groups and circulating triacylglycerol increased in TM6SF2EE only.

CONCLUSIONS

The TM6SF2-polymorphism rs58542926 dissociates HCL from insulin resistance in recent-onset type 2 diabetes, which is attenuated by disease duration. This suggests that diabetes-related metabolic alterations dominate over effects of the TM6SF2-polymorphism during early course of diabetes and NAFLD.

Sex-Specific Differences in Lipoprotein Production and Clearance

Therapeutic approaches to reduce atherogenic lipid and lipoprotein levels remain the most effective and assessable strategies to prevent and treat cardiovascular disease. The discovery of novel research targets linked to pathways associated with cardiovascular disease development has enhanced our ability to decrease disease burden; however, residual cardiovascular disease risks remain. Advancements in genetics and personalized medicine are essential to understand some of the factors driving residual risk. Biological sex is among the most relevant factors affecting plasma lipid and lipoprotein profiles, playing a pivotal role in the development of cardiovascular disease. This minireview summarizes the most recent preclinical and clinical studies covering the effect of sex on plasma lipid and lipoprotein levels. We highlight the recent advances in the mechanisms regulating hepatic lipoprotein production and clearance as potential drivers of disease presentation. We focus on using sex as a biological variable in studying circulating lipid and lipoprotein levels.

TM6SF2-rs58542926 Genetic Variant Modifies the Protective Effect of a "Prudent" Dietary Pattern on Serum Triglyceride Levels

The epidemic prevalence of non-alcoholic fatty liver disease (NAFLD), despite extensive research in the field, underlines the importance of focusing on personalized therapeutic approaches. However, nutrigenetic effects on NAFLD are poorly investigated. To this end, we aimed to explore potential gene-dietary pattern interactions in a NAFLD case-control study. The disease was diagnosed with liver ultrasound and blood collection was performed after an overnight fast. Adherence to four a posteriori, data-driven, dietary patterns was used to investigate interactions with PNPLA3-rs738409, TM6SF2-rs58542926, MBOAT7-rs641738, and GCKR-rs738409 in disease and related traits. IBM SPSS Statistics/v21.0 and Plink/v1.07 were used for statistical analyses. The sample consisted of 351 Caucasian individuals. PNPLA3-rs738409 was positively associated with disease odds (OR = 1.575, p = 0.012) and GCKR-rs738409 with lnC-reactive protein (CRP) (beta = 0.098, p = 0.003) and Fatty Liver Index (FLI) levels (beta = 5.011, p = 0.007). The protective effect of a "Prudent" dietary pattern on serum triglyceride (TG) levels in this sample was significantly modified by TM6SF2-rs58542926 (pinteraction = 0.007). TM6SF2-rs58542926 carriers may not benefit from a diet rich in unsaturated fatty acids and carbohydrates in regard to TG levels, a commonly elevated feature in NAFLD patients.

Non-alcoholic fatty liver disease and complications in type 1 and type 2 diabetes: A Mendelian randomization study

AIM

To investigate the potential causal relationship between non-alcoholic fatty liver disease (NAFLD) and complications in type 1 diabetes (T1D) and type 2 diabetes (T2D).

MATERIALS AND METHODS

Two-sample Mendelian randomization (MR) analysis was conducted to appraise after controlling for the confounding factors. Genetic instrument variables for NAFLD surrogated by chronically elevated serum alanine transferase were derived from a recent genome-wide association study. Diabetes-related complications, including diabetic ketoacidosis, nephropathy and retinopathy, were included as outcomes. Four complementary MR methods were used to test reliability.

RESULTS

Genetically instrumented NAFLD showed a suggestive causal association with ketoacidosis in T1D (odds ratio [OR]: 1.574; 95% confidence interval [CI]: 1.076, 2.302; P = .019; false discovery rate [FDR] = 0.096) and a significant causal association with early-stage kidney disease in T1D (OR: 1.249; 95% CI: 1.089, 1.432; P = 1.457 × 10^-3 , FDR = 0.015). Sensitivity analysis indicated low heterogeneity, low pleiotropy and high reliability of the causal estimates. However, the MR analyses failed to show a causal association between NAFLD and T1D retinopathy, T2D ketoacidosis, nephropathy and retinopathy.

CONCLUSIONS

This study supports a causal effect of genetically driven chronic serum alanine aminotransferase-associated NAFLD on early-stage kidney disease in T1D and a suggestive causal effect on ketoacidosis in T1D. However, MR studies did not provide enough evidence to suggest that NAFLD independently increases the risk of retinopathy in T1D and of ketoacidosis, nephropathy and retinopathy in T2D.

Genetic ablation of diabetes-associated gene Ccdc92 reduces obesity and insulin resistance in mice

Multiple genome-wide association studies (GWAS) have identified specific genetic variants in the coiled-coil domain containing 92 (CCDC92) locus that is associated with obesity and type 2 diabetes in humans. However, the biological function of CCDC92 in obesity and insulin resistance remains to be explored. Utilizing wild-type (WT) and Ccdc92 whole-body knockout (KO) mice, we found that Ccdc92 KO reduced obesity and increased insulin sensitivity under high-fat diet (HFD) conditions. Ccdc92 KO inhibited macrophage infiltration and fibrosis in white adipose tissue (WAT), suggesting Ccdc92 ablation protects against adipose tissue dysfunction. Ccdc92 deletion also increased energy expenditure and further attenuated hepatic steatosis in mice on an HFD. Ccdc92 KO significantly inhibited the inflammatory response and suppressed the NLR Family Pyrin Domain Containing 3 (NLRP3) inflammasome in WAT. Altogether, we demonstrated the critical role of CCDC92 in metabolism, constituting a potential target for treating obesity and insulin resistance.

Myeloid TM6SF2 Deficiency Inhibits Atherosclerosis

Genetic variants in transmembrane 6 superfamily member 2 (TM6SF2), such as E167K, are associated with atherosclerotic cardiovascular disease (ASCVD). Chronic inflammation and lipid-laden macrophage foam cell formation are the central pathogeneses in the development of atherosclerosis. This study was undertaken to illustrate the biological function of TM6SF2 in macrophages and its role during atherosclerosis development. We generated myeloid cell-specific Tm6sf2 knockout mice on ApoE-deficient background (LysM Cre+/Tm6sf2fl/fl/ApoE-/-, TM6 mKO) with littermate LysM Cre-/Tm6sf2fl/fl/ApoE-/- (Control) mice as controls. Mice were fed a Western diet for 12 weeks to induce atherosclerosis. Myeloid Tm6sf2 deficiency inhibited atherosclerosis and decreased foam cells in the plaques without changing the plasma lipid profile. RNA sequencing of bone marrow-derived macrophages (BMDMs) from TM6 mKO mice demonstrated the downregulation of genes associated with inflammation, cholesterol uptake, and endoplasmic reticulum (ER) stress. TM6SF2 was upregulated by oxidized low-density lipoprotein (oxLDL) in macrophages. Silencing TM6SF2 in THP-1-derived macrophages and Tm6sf2 deficiency in BMDMs reduced inflammatory responses and ER stress and attenuated cholesterol uptake and foam cell formation, while the overexpression of TM6SF2 showed opposite effects. In conclusion, myeloid TM6SF2 deficiency inhibits atherosclerosis development and is a potential therapeutic target for the treatment of atherogenesis.