Human PNPLA3-I148M variant increases hepatic retention of polyunsaturated fatty acids

The role of hepatic lipid composition in obesity-related metabolic disease

Obesity is a primary antecedent to non-alcoholic fatty liver disease whose cardinal feature is excessive hepatic lipid accumulation. Although total hepatic lipid content closely associates with hepatic and systemic metabolic dysfunction, accumulating evidence suggests that the composition of hepatic lipids may be more discriminatory. This review summarises cross-sectional human studies using liver biopsy/lipidomics and proton magnetic resonance spectroscopy to characterise hepatic lipid composition in people with obesity and related metabolic disease. A comprehensive literature search identified 26 relevant studies published up to 31st March 2021 which were included in the review. The available evidence provides a consistent picture showing that people with hepatic steatosis possess elevated saturated and/or monounsaturated hepatic lipids and a reduced proportion of polyunsaturated hepatic lipids. This altered hepatic lipid profile associates more directly with metabolic derangements, such as insulin resistance, and may be exacerbated in non-alcoholic steatohepatitis. Further evidence from lipidomic studies suggests that these deleterious changes may be related to defects in lipid desaturation and elongation, and an augmentation of the de novo lipogenic pathway. These observations are consistent with mechanistic studies implicating saturated fatty acids and associated bioactive lipid intermediates (ceramides, lysophosphatidylcholines and diacylglycerol) in the development of hepatic lipotoxicity and wider metabolic dysfunction, whilst monounsaturated fatty acids and polyunsaturated fatty acids may exhibit a protective role. Future studies are needed to prospectively determine the relevance of hepatic lipid composition for hepatic and non-hepatic morbidity and mortality; and to further evaluate the impact of therapeutic interventions such as pharmacotherapy and lifestyle interventions.

Serum lipids are associated with nonalcoholic fatty liver disease: a pilot case-control study in Mexico

Background

Nonalcoholic fatty liver disease (NAFLD) is a leading cause of chronic liver disease and cirrhosis. NAFLD is mediated by changes in lipid metabolism and known risk factors include obesity, metabolic syndrome, and diabetes. The aim of this study was to better understand differences in the lipid composition of individuals with NAFLD compared to controls, by performing direct infusion lipidomics on serum biospecimens from a cohort study of adults in Mexico.

Methods

A nested case-control study was conducted with a sample of 98 NAFLD cases and 100 healthy controls who are participating in an on-going, longitudinal study in Mexico. NAFLD cases were clinically confirmed using elevated liver enzyme tests and liver ultrasound or liver ultrasound elastography, after excluding alcohol abuse, and 100 controls were identified as having at least two consecutive normal alanine aminotransferase (ALT) and aspartate aminotransferase (AST) (< 40 U/L) results in a 6-month period, and a normal liver ultrasound elastography result in January 2018. Samples were analyzed on the Sciex Lipidyzer Platform and quantified with normalization to serum volume. As many as 1100 lipid species can be identified using the Lipidyzer targeted multiple-reaction monitoring list. The association between serum lipids and NAFLD was investigated using analysis of covariance, random forest analysis, and by generating receiver operator characteristic (ROC) curves.

Results

NAFLD cases had differences in total amounts of serum cholesterol esters, lysophosphatidylcholines, sphingomyelins, and triacylglycerols (TAGs), however, other lipid subclasses were similar to controls. Analysis of individual TAG species revealed increased incorporation of saturated fatty acyl tails in serum of NAFLD cases. After adjusting for age, sex, body mass index, and PNPLA3 genotype, a combined panel of ten lipids predicted case or control status better than an area under the ROC curve of 0.83.

Conclusions

These preliminary results indicate that the serum lipidome differs in patients with NAFLD, compared to healthy controls, and suggest that assessing the desaturation state of TAGs or a specific lipid panel may be useful clinical tools for the diagnosis of NAFLD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12944-021-01526-5.

Genetics Is of the Essence to Face NAFLD

Nonalcoholic fatty liver disease (NAFLD) is the commonest cause of chronic liver disease worldwide. It is closely related to obesity, insulin resistance (IR) and dyslipidemia so much so it is considered the hepatic manifestation of the Metabolic Syndrome. The NAFLD spectrum extends from simple steatosis to nonalcoholic steatohepatitis (NASH), a clinical condition which may progress up to fibrosis, cirrhosis and hepatocellular carcinoma (HCC). NAFLD is a complex disease whose pathogenesis is shaped by both environmental and genetic factors. In the last two decades, several heritable modifications in genes influencing hepatic lipid remodeling, and mitochondrial oxidative status have been emerged as predictors of progressive hepatic damage. Among them, the patatin-like phospholipase domain-containing 3 (PNPLA3) p.I148M, the Transmembrane 6 superfamily member 2 (TM6SF2) p.E167K and the rs641738 membrane bound-o-acyltransferase domain-containing 7 (MBOAT7) polymorphisms are considered the most robust modifiers of NAFLD. However, a forefront frontier in the study of NAFLD heritability is to postulate score-based strategy, building polygenic risk scores (PRS), which aggregate the most relevant genetic determinants of NAFLD and biochemical parameters, with the purpose to foresee patients with greater risk of severe NAFLD, guaranteeing the most highly predictive value, the best diagnostic accuracy and the more precise individualized therapy.

Circulating Fatty Acids Associated with Advanced Liver Fibrosis and Hepatocellular Carcinoma in South Texas Hispanics

BACKGROUND

Hispanics in South Texas have high rates of hepatocellular carcinoma (HCC) and nonalcoholic fatty liver disease (NAFLD). Liver fibrosis severity is the strongest predictive factor of NAFLD progression to HCC. We examined the association between free fatty acids (FA) and advanced liver fibrosis or HCC in this population.

METHODS

We quantified 45 FAs in plasma of 116 subjects of the Cameron County Hispanic Cohort, 15 Hispanics with HCC, and 56 first/second-degree relatives of Hispanics with HCC. Liver fibrosis was assessed by FibroScan.

RESULTS

Advanced liver fibrosis was significantly associated with low expression of very long chain (VLC) saturated FAs (SFA), odd chain SFAs, and VLC n-3 polyunsaturated FAs [PUFA; AOR; 95% confidence interval (CI), 10.4 (3.7-29.6); P < 0.001; 5.7 (2.2-15.2); P < 0.001; and 3.7 (1.5-9.3); P = 0.005]. VLC n3-PUFAs significantly improved the performance of the noninvasive markers for advanced fibrosis - APRI, FIB-4, and NFS. Plasma concentrations of VLC SFAs and VLC n-3 PUFAs were further reduced in patients with HCC. Low concentrations of these FAs were also observed in relatives of patients with HCC and in subjects with the PNPLA3 rs738409 homozygous genotype.

CONCLUSIONS

Low plasma concentrations of VLC n-3 PUFAs and VLC SFAs were strongly associated with advanced liver fibrosis and HCC in this population. Genetic factors were associated with low concentrations of these FAs as well.

IMPACT

These results have implications in identifying those at risk for liver fibrosis progression to HCC and in screening this population for advanced fibrosis. They also prompt the evaluation of VLC n-3 PUFA or VLC SFA supplementation to prevent cirrhosis and HCC.

Nonalcoholic fatty liver disease stratification by liver lipidomics

Nonalcoholic fatty liver disease (NAFLD) is a common metabolic dysfunction leading to hepatic steatosis. However, NAFLD's global impact on the liver lipidome is poorly understood. Using high-resolution shotgun mass spectrometry, we quantified the molar abundance of 316 species from 22 major lipid classes in liver biopsies of 365 patients, including nonsteatotic patients with normal or excessive weight, patients diagnosed with NAFL (nonalcoholic fatty liver) or NASH (nonalcoholic steatohepatitis), and patients bearing common mutations of NAFLD-related protein factors. We confirmed the progressive accumulation of di- and triacylglycerols and cholesteryl esters in the liver of NAFL and NASH patients, while the bulk composition of glycerophospho- and sphingolipids remained unchanged. Further stratification by biclustering analysis identified sphingomyelin species comprising n24:2 fatty acid moieties as membrane lipid markers of NAFLD. Normalized relative abundance of sphingomyelins SM 43:3;2 and SM 43:1;2 containing n24:2 and n24:0 fatty acid moieties, respectively, showed opposite trends during NAFLD progression and distinguished NAFL and NASH lipidomes from the lipidome of nonsteatotic livers. Together with several glycerophospholipids containing a C22:6 fatty acid moiety, these lipids serve as markers of early and advanced stages of NAFL.

Metabolic-associated fatty liver disease and lipoprotein metabolism

BACKGROUND

Non-alcoholic fatty liver disease, or as recently proposed 'metabolic-associated fatty liver disease' (MAFLD), is characterized by pathological accumulation of triglycerides and other lipids in hepatocytes. This common disease can progress from simple steatosis to steatohepatitis, and eventually end-stage liver diseases. MAFLD is closely related to disturbances in systemic energy metabolism, including insulin resistance and atherogenic dyslipidemia.

SCOPE OF REVIEW

The liver is the central organ in lipid metabolism by secreting very low density lipoproteins (VLDL) and, on the other hand, by internalizing fatty acids and lipoproteins. This review article discusses recent research addressing hepatic lipid synthesis, VLDL production, and lipoprotein internalization as well as the lipid exchange between adipose tissue and the liver in the context of MAFLD.

MAJOR CONCLUSIONS

Liver steatosis in MAFLD is triggered by excessive hepatic triglyceride synthesis utilizing fatty acids derived from white adipose tissue (WAT), de novo lipogenesis (DNL) and endocytosed remnants of triglyceride-rich lipoproteins. In consequence of high hepatic lipid content, VLDL secretion is enhanced, which is the primary cause of complex dyslipidemia typical for subjects with MAFLD. Interventions reducing VLDL secretory capacity attenuate dyslipidemia while they exacerbate MAFLD, indicating that the balance of lipid storage versus secretion in hepatocytes is a critical parameter determining disease outcome. Proof of concept studies have shown that promoting lipid storage and energy combustion in adipose tissues reduces hepatic lipid load and thus ameliorates MAFLD. Moreover, hepatocellular triglyceride synthesis from DNL and WAT-derived fatty acids can be targeted to treat MAFLD. However, more research is needed to understand how individual transporters, enzymes, and their isoforms affect steatosis and dyslipidemia in vivo, and whether these two aspects of MAFLD can be selectively treated. Processing of cholesterol-enriched lipoproteins appears less important for steatosis. It may, however, modulate inflammation and consequently MAFLD progression.

Nonalcoholic fatty liver disease (NAFLD) from pathogenesis to treatment concepts in humans

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) comprises hepatic alterations with increased lipid accumulation (steatosis) without or with inflammation (nonalcoholic steatohepatitis, NASH) and/or fibrosis in the absence of other causes of liver disease. NAFLD is developing as a burgeoning health challenge, mainly due to the worldwide obesity and diabetes epidemics.

SCOPE OF REVIEW

This review summarizes the knowledge on the pathogenesis underlying NAFLD by focusing on studies in humans and on hypercaloric nutrition, including effects of saturated fat and fructose, as well as adipose tissue dysfunction, leading to hepatic lipotoxicity, abnormal mitochondrial function, and oxidative stress, and highlights intestinal dysbiosis. These mechanisms are discussed in the context of current treatments targeting metabolic pathways and the results of related clinical trials.

MAJOR CONCLUSIONS

Recent studies have provided evidence that certain conditions, for example, the severe insulin-resistant diabetes (SIRD) subgroup (cluster) and the presence of an increasing number of gene variants, seem to predispose for excessive risk of NAFLD and its accelerated progression. Recent clinical trials have been frequently unsuccessful in halting or preventing NAFLD progression, perhaps partly due to including unselected cohorts in later stages of NAFLD. On the basis of this literature review, this study proposed screening in individuals with the highest genetic or acquired risk of disease progression, for example, the SIRD subgroup, and developing treatment concepts targeting the earliest pathophysiolgical alterations, namely, adipocyte dysfunction and insulin resistance.

Metabolic drivers of non-alcoholic fatty liver disease

BACKGROUND

The incidence of non-alcoholic fatty liver disease (NAFLD) is rapidly increasing worldwide parallel to the global obesity epidemic. NAFLD encompasses a range of liver pathologies and most often originates from metabolically driven accumulation of fat in the liver, or non-alcoholic fatty liver (NAFL). In a subset of NAFL patients, the disease can progress to non-alcoholic steatohepatitis (NASH), which is a more severe form of liver disease characterized by hepatocyte injury, inflammation, and fibrosis. Significant progress has been made over the past decade in our understanding of NASH pathogenesis, but gaps remain in our mechanistic knowledge of the precise metabolic triggers for disease worsening.

SCOPE OF REVIEW

The transition from NAFL to NASH likely involves a complex constellation of multiple factors intrinsic and extrinsic to the liver. This review focuses on early metabolic events in the establishment of NAFL and initial stages of NASH. We discuss the association of NAFL with obesity as well as the role of adipose tissue in disease progression and highlight early metabolic drivers implicated in the pathological transition from hepatic fat accumulation to steatohepatitis.

MAJOR CONCLUSIONS

The close association of NAFL with features of metabolic syndrome highlight plausible mechanistic roles for adipose tissue health and the release of lipotoxic lipids, hepatic de novo lipogenesis (DNL), and disruption of the intestinal barrier in not only the initial establishment of hepatic steatosis, but also in mediating disease progression. Human genetic variants linked to NASH risk to date are heavily biased toward genes involved in the regulation of lipid metabolism, providing compelling support for the hypothesis that NASH is fundamentally a metabolic disease.

A genome-first approach to mortality and metabolic phenotypes in MTARC1 p.Ala165Thr (rs2642438) heterozygotes and homozygotes

BACKGROUND

A coding variant in MTARC1 (rs2642438; p.Ala165Thr) was recently associated with protection from cirrhosis in European individuals. However, its impact on overall and cause-specific mortality remained elusive.

METHODS

Using a genome-first approach, we explored a range of metabolic phenotypes and outcomes associated with MTARC1 p.Ala165Thr in the UKBiobank and the Penn-Medicine BioBank.

FINDINGS

MTARC1 p.Ala165Thr was significantly associated with higher triglycerides, lower total cholesterol, lower LDL-C, lower ApoB, lower HDL-C, lower ApoA-I and higher IGF-1. Per each minor allele, the risk of NAFLD was reduced by ~15%. The ALT-lowering and NAFLD-protective effect of MTARC1 p.Ala165Thr was amplified by obesity, diabetes mellitus and presence of PNPLA3 rs738409:G. In African-American and Black-British individuals, the allele frequency of MTARC1 p.Ala165Thr was lower, but carriers showed the same distinctive lipid phenotype. Importantly, MTARC1 p.Ala165Thr carriers did not show higher cardiovascular disease burden as evidenced by cardiac MRI and carotid ultrasound. In prospective analyses, the homozygous minor allele was associated with up to 39% lower rates of liver-related mortality, while no risk of increased overall or cardiovascular death could be observed. Strikingly, liver-related mortality was more than 50% reduced in diabetic participants or carriers of PNPLA3 rs738409:G.

CONCLUSIONS

Together these data highlight MTARC1 as an important liver disease modifier that influences plasma lipids in an allele-dose-dependent manner without increasing cardiovascular outcomes. Our results point toward potential mechanisms and reveal a remarkable association with liver-related mortality calling for future studies exploring its therapeutic potential.

FUNDING

This study was funded by the German Research Foundation (DFG).

Advances in paediatric nonalcoholic fatty liver disease: Role of lipidomics

Due its close relationship with obesity, nonalcoholic fatty liver disease (NAFLD) has become a major worldwide health issue even in childhood. The most accepted pathophysiological hypothesis is represented by the “multiple hits” theory, in which both hepatic intracellular lipid accumulation and insulin resistance mainly contribute to liver injury through several factors. Among these, lipotoxicity has gained particular attention. In this view, the pathogenic role of different lipid classes in NAFLD (e.g., sphingolipids, fatty acids, ceramides, etc.) has been highlighted in recent lipidomics studies. Although there is some contrast between plasma and liver findings, lipidomic profile in the NAFLD context provides novel insights by expanding knowledge in the intricate field of NAFLD pathophysiology as well as by suggesting innovative therapeutic approaches in order to improve both NAFLD prevention and treatment strategies. Selective changes of distinct lipid species might be an attractive therapeutic target for treating NAFLD. Herein the most recent evidence in this attractive field has been summarized to provide a comprehensive overview of the lipidomic scenario in paediatric NAFLD.

Nonalcoholic fatty liver disease or metabolic dysfunction-associated fatty liver disease diagnoses and cardiovascular diseases: From epidemiology to drug approaches

BACKGROUND

A consensus of experts has proposed to replace the term nonalcoholic fatty liver disease (NAFLD), whose global prevalence is 25%, with metabolic dysfunction-associated fatty liver disease (MAFLD), to describe more appropriately the liver disease related to metabolic derangements. MAFLD is closely intertwined with type 2 diabetes, obesity, dyslipidaemia, all linked to a rise in the risk of cardiovascular disease (CVDs). Since controversy still stands on whether or not NAFLD/MAFLD raises the odds of CVD, the present review aims to evaluate the impact of NAFLD/MAFLD aetiologies on CV health and the potential correction by dietary and drug approaches.

RESULTS

Epidemiological studies indicate that NAFLD raises risk of fatal or non-fatal CVD events. NAFLD patients have a higher prevalence of arterial plaques and stiffness, coronary calcification, and endothelial dysfunction. Although genetic and environmental factors strongly contribute to NAFLD pathogenesis, a Mendelian randomization analysis indicated that the PNPLA3 genetic variant leading to NAFLD may not be causally associated with CVD risk. Among other genetic variants related to NAFLD, TM6SF2 appears to be protective, whereas MBOAT7 may favour venous thromboembolism.

CONCLUSIONS

NAFLD is correlated to a higher CVD risk which may be ameliorated by dietary interventions. This is not surprising, since new criteria defining MAFLD include other metabolic risk abnormalities fuelling development of serious adverse extrahepatic outcomes, for example CVD. The present lack of a targeted pharmacological approach makes the identification of patients with liver disease at higher CVD risk (eg diabetes, hypertension, obesity or high levels of C-reactive protein) of major clinical interest.

Mitochondrial Lipid Homeostasis at the Crossroads of Liver and Heart Diseases

The prevalence of NAFLD (non-alcoholic fatty liver disease) is a rapidly increasing problem, affecting a huge population around the globe. However, CVDs (cardiovascular diseases) are the most common cause of mortality in NAFLD patients. Atherogenic dyslipidemia, characterized by plasma hypertriglyceridemia, increased small dense LDL (low-density lipoprotein) particles, and decreased HDL-C (high-density lipoprotein cholesterol) levels, is often observed in NAFLD patients. In this review, we summarize recent genetic evidence, proving the diverse nature of metabolic pathways involved in NAFLD pathogenesis. Analysis of available genetic data suggests that the altered operation of fatty-acid β-oxidation in liver mitochondria is the key process, connecting NAFLD-mediated dyslipidemia and elevated CVD risk. In addition, we discuss several NAFLD-associated genes with documented anti-atherosclerotic or cardioprotective effects, and current pharmaceutical strategies focused on both NAFLD treatment and reduction of CVD risk.

Nutrigenomics and Nutrigenetics in Metabolic- (Dysfunction) Associated Fatty Liver Disease: Novel Insights and Future Perspectives

Metabolic- (dysfunction) associated fatty liver disease (MAFLD) represents the predominant hepatopathy and one of the most important systemic, metabolic-related disorders all over the world associated with severe medical and socio-economic repercussions due to its growing prevalence, clinical course (steatohepatitis and/or hepatocellular-carcinoma), and related extra-hepatic comorbidities. To date, no specific medications for the treatment of this condition exist, and the most valid recommendation for patients remains lifestyle change. MAFLD has been associated with metabolic syndrome; its development and progression are widely influenced by the interplay between genetic, environmental, and nutritional factors. Nutrigenetics and nutrigenomics findings suggest nutrition’s capability, by acting on the individual genetic background and modifying the specific epigenetic expression as well, to influence patients’ clinical outcome. Besides, immunity response is emerging as pivotal in this multifactorial scenario, suggesting the interaction between diet, genetics, and immunity as another tangled network that needs to be explored. The present review describes the genetic background contribution to MAFLD onset and worsening, its possibility to be influenced by nutritional habits, and the interplay between nutrients and immunity as one of the most promising research fields of the future in this context.

Non-Alcoholic Fatty Liver Disease: Metabolic, Genetic, Epigenetic and Environmental Risk Factors

Non-alcoholic fatty liver disease (NAFLD) is one of the most frequent causes of chronic liver disease in the Western world, probably due to the growing prevalence of obesity, metabolic diseases, and exposure to some environmental agents. In certain patients, simple hepatic steatosis can progress to non-alcoholic steatohepatitis (NASH), which can sometimes lead to liver cirrhosis and its complications including hepatocellular carcinoma. Understanding the mechanisms that cause the progression of NAFLD to NASH is crucial to be able to control the advancement of the disease. The main hypothesis considers that it is due to multiple factors that act together on genetically predisposed subjects to suffer from NAFLD including insulin resistance, nutritional factors, gut microbiota, and genetic and epigenetic factors. In this article, we will discuss the epidemiology of NAFLD, and we overview several topics that influence the development of the disease from simple steatosis to liver cirrhosis and its possible complications.

Impact of the Association Between PNPLA3 Genetic Variation and Dietary Intake on the Risk of Significant Fibrosis in Patients With NAFLD

INTRODUCTION

This study explored the relationship between patatin-like phospholipase domain-containing 3 gene (PNPLA3 rs738409), nutrient intake, and liver histology severity in patients with nonalcoholic fatty liver disease (NAFLD).

METHODS

PNPLA3-rs738409 variant was genotyped in 452 non-Hispanic whites with histologically confirmed NAFLD who completed Food Frequency Questionnaire within 6 months of their liver biopsy. The fibrosis severity on liver histology was the outcome of interest.

RESULTS

The distribution of PNPLA3 genotypes was CC: 28%, CG: 46%, and GG: 25%. High-carbohydrate (% of energy/d) intake was positively associated (adjusted [Adj] odds ratio [OR]: 1.03, P < 0.01), whereas higher n-3 polyunsaturated fatty acids (n-3 PUFAs) (g/d) (Adj. OR: 0.17, P < 0.01), isoflavones (mg/d) (Adj. OR: 0.74, P = 0.049), methionine (mg/d) (Adj. OR: 0.32, P < 0.01), and choline (mg/d) (Adj. OR: 0.32, P < 0.01) intakes were inversely associated with increased risk of significant fibrosis (stage of fibrosis ≥2). By using an additive model of inheritance, our moderation analysis showed that PNPLA3 rs738409 significantly modulates the relationship between carbohydrate (%), n-3 PUFAs, total isoflavones, methionine, and choline intakes and fibrosis severity in a dose-dependent, genotype manner. These dietary factors tended to have a larger and significant effect on fibrosis severity among rs738409 G-allele carriers. Associations between significant fibrosis and carbohydrates (Adj. OR: 1.04, P = 0.019), n-3 PUFAs (Adj. OR: 0.16, P < 0.01), isoflavones (Adj. OR: 0.65, P = 0.025), methionine (Adj. OR: 0.30, P < 0.01), and total choline (Adj. OR: 0.29, P < 0.01) intakes remained significant only among rs738409 G-allele carriers.

DISCUSSION

This gene-diet interaction study suggests that PNPLA3 rs738409 G-allele might modulate the effect of specific dietary nutrients on risk of fibrosis in patients with NAFLD.

Mitochondrial Mutations and Genetic Factors Determining NAFLD Risk

NAFLD (non-alcoholic fatty liver disease) is a widespread liver disease that is often linked with other life-threatening ailments (metabolic syndrome, insulin resistance, diabetes, cardiovascular disease, atherosclerosis, obesity, and others) and canprogress to more severe forms, such as NASH (non-alcoholic steatohepatitis), cirrhosis, and HCC (hepatocellular carcinoma). In this review, we summarized and analyzed data about single nucleotide polymorphism sites, identified in genes related to NAFLD development and progression. Additionally, the causative role of mitochondrial mutations and mitophagy malfunctions in NAFLD is discussed. The role of mitochondria-related metabolites of the urea cycle as a new non-invasive NAFLD biomarker is discussed. While mitochondria DNA mutations and SNPs (single nucleotide polymorphisms) canbe used as effective diagnostic markers and target for treatments, age and ethnic specificity should be taken into account.

Absence of Adiponutrin (PNPLA3) and Monoacylglycerol Lipase Synergistically Increases Weight Gain and Aggravates Steatohepatitis in Mice

Altered lipid metabolic pathways including hydrolysis of triglycerides are key players in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Whether adiponutrin (patatin-like phospholipase domain containing protein-3-PNPLA3) and monoacylglycerol lipase (MGL) synergistically contribute to disease progression remains unclear. We generated double knockout (DKO) mice lacking both Mgl and Pnpla3; DKO mice were compared to Mgl-/- after a challenge by high-fat diet (HFD) for 12 weeks to induce steatosis. Serum biochemistry, liver transaminases as well as histology were analyzed. Fatty acid (FA) profiling was assessed in liver and adipose tissue by gas chromatography. Markers of inflammation and lipid metabolism were analyzed. Bone marrow derived macrophages (BMDMs) were isolated and treated with oleic acid. Combined deficiency of Mgl and Pnpla3 resulted in weight gain on a chow diet; when challenged by HFD, DKO mice showed increased hepatic FA synthesis and diminished beta-oxidation compared to Mgl-/-.DKO mice exhibited more pronounced hepatic steatosis with inflammation and recruitment of immune cells to the liver associated with accumulation of saturated FAs. Primary BMDMs isolated from the DKO mice showed increased inflammatory activities, which could be reversed by oleic acid supplementation. Pnpla3 deficiency aggravates the effects of Mgl deletion on steatosis and inflammation in the liver under HFD challenge.

A Study of Blood Fatty Acids Profile in Hyperlipidemic and Normolipidemic Subjects in Association with Common PNPLA3 and ABCB1 Polymorphisms

Adiponutrin (patatin-like phospholipase domain-containing 3; PNPLA3), encoded in humans by the PNPLA3 gene, is a protein associated with lipid droplet and endoplasmic reticulum membranes, where it is apparently involved in fatty acid redistribution between triglycerides and phospholipids. A common polymorphism of PNPLA3 (I148M, rs738409), linked to increased PNPLA3 presence on lipid droplets, is a strong genetic determinant of non-alcoholic fatty liver disease (NAFLD) and of its progression. P-glycoprotein (Pgp, MDR1—multidrug resistance protein 1, ABCB1—ATP-binding cassette sub-family B member 1), encoded by the ABCB1 gene, is another membrane protein implicated in lipid homeostasis and steatosis. In the past, common ABCB1 polymorphisms have been associated with the distribution of serum lipids but not with fatty acids (FA) profiles. Similarly, data on the effect of PNPLA3 I148M polymorphism on blood FAs are scarce. In this study, a gas chromatography-flame ionization detection (GC-FID) method was optimized, allowing us to analyze twenty FAs (C14: 0, C15: 0, C15: 1, C16: 0, C16: 1, C17: 0, C17: 1, C18: 0, C18: 1cis, C18: 2cis, C20: 0, C20: 1n9, C20: 2, C20: 3n6, C20: 4n6, C20: 5, C23: 0, C24: 0, C24: 1 and C22: 6) in whole blood, based on the indirect determination of the fatty acids methyl esters (FAMES), in 62 hyperlipidemic patients and 42 normolipidemic controls. FA concentrations were then compared between the different genotypes of the rs738409 and rs2032582 (ABCB1 G2677T) polymorphisms, within and between the hyperlipidemic and normolipidemic groups. The rs738409 polymorphism appears to exert a significant effect on the distribution of blood fatty acids, in a lipidemic and fatty acid saturation state-depending manner. The effect of rs2032582 was less pronounced, but the polymorphism did appear to affect the relative distribution of blood fatty acids between hyperlipidemic patients and normolipidemic controls.

The PNPLA3-I148M Variant Confers an Antiatherogenic Lipid Profile in Insulin-resistant Patients

CONTEXT

The I148M (rs738409-G) variant in PNPLA3 increases liver fat content but may be protective against cardiovascular disease. Insulin resistance (IR) amplifies the effect of PNPLA3-I148M on liver fat.

OBJECTIVE

To study whether PNPLA3-I148M confers an antihyperlipidemic effect in insulin-resistant patients.

DESIGN

Cross-sectional study comparing the impact of PNPLA3-I148M on plasma lipids and lipoproteins in 2 cohorts, both divided into groups based on rs738409-G allele carrier status and median HOMA-IR.

SETTING

Tertiary referral center.

PATIENTS

A total of 298 obese patients who underwent a liver biopsy during bariatric surgery (bariatric cohort: age 49 ± 9 years, body mass index [BMI] 43.2 ± 6.8 kg/m2), and 345 less obese volunteers in whom liver fat was measured by proton magnetic resonance spectroscopy (nonbariatric cohort: age 45 ± 14 years, BMI 29.7 ± 5.7 kg/m2).

MAIN OUTCOME MEASURES

Nuclear magnetic resonance profiling of plasma lipids, lipoprotein particle subclasses and their composition.

RESULTS

In both cohorts, individuals carrying the PNPLA3-I148M variant had significantly higher liver fat content than noncarriers. In insulin-resistant and homozygous carriers, PNPLA3-I148M exerted a distinct antihyperlipidemic effect with decreased very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) particles and their constituents, and increased high-density lipoprotein particles and their constituents, compared with noncarriers. VLDL particles were smaller and LDL particles larger in PNPLA3-I148M carriers. These changes were geometrically opposite to those due to IR. PNPLA3-I148M did not have a measurable effect in patients with lower IR, and its effect was smaller albeit still significant in the less obese than in the obese cohort.

CONCLUSIONS

PNPLA3-I148M confers an antiatherogenic plasma lipid profile particularly in insulin-resistant individuals.

Oxidative Stress in NAFLD: Role of Nutrients and Food Contaminants

Non-alcoholic fatty liver disease (NAFLD) is often the hepatic expression of metabolic syndrome and its comorbidities that comprise, among others, obesity and insulin-resistance. NAFLD involves a large spectrum of clinical conditions. These range from steatosis, a benign liver disorder characterized by the accumulation of fat in hepatocytes, to non-alcoholic steatohepatitis (NASH), which is characterized by inflammation, hepatocyte damage, and liver fibrosis. NASH can further progress to cirrhosis and hepatocellular carcinoma. The etiology of NAFLD involves both genetic and environmental factors, including an unhealthy lifestyle. Of note, unhealthy eating is clearly associated with NAFLD development and progression to NASH. Both macronutrients (sugars, lipids, proteins) and micronutrients (vitamins, phytoingredients, antioxidants) affect NAFLD pathogenesis. Furthermore, some evidence indicates disruption of metabolic homeostasis by food contaminants, some of which are risk factor candidates in NAFLD. At the molecular level, several models have been proposed for the pathogenesis of NAFLD. Most importantly, oxidative stress and mitochondrial damage have been reported to be causative in NAFLD initiation and progression. The aim of this review is to provide an overview of the contribution of nutrients and food contaminants, especially pesticides, to oxidative stress and how they may influence NAFLD pathogenesis.

Insights into genetic variants associated with NASH-fibrosis from metabolite profiling

Several genetic discoveries robustly implicate five single-nucleotide variants in the progression of non-alcoholic fatty liver disease to non-alcoholic steatohepatitis and fibrosis (NASH-fibrosis), including a recently identified variant in MTARC1. To better understand these variants as potential therapeutic targets, we aimed to characterize their impact on metabolism using comprehensive metabolomics data from two population-based studies. A total of 9135 participants from the Fenland study and 9902 participants from the EPIC-Norfolk cohort were included in the study. We identified individuals with risk alleles associated with NASH-fibrosis: rs738409C>G in PNPLA3, rs58542926C>T in TM6SF2, rs641738C>T near MBOAT7, rs72613567TA>T in HSD17B13 and rs2642438A>G in MTARC1. Circulating levels of 1449 metabolites were measured using targeted and untargeted metabolomics. Associations between NASH-fibrosis variants and metabolites were assessed using linear regression. The specificity of variant-metabolite associations were compared to metabolite associations with ultrasound-defined steatosis, gene variants linked to liver fat (in GCKR, PPP1R3B and LYPLAL1) and gene variants linked to cirrhosis (in HFE and SERPINA1). Each NASH-fibrosis variant demonstrated a specific metabolite profile with little overlap (8/97 metabolites) comprising diverse aspects of lipid metabolism. Risk alleles in PNPLA3 and HSD17B13 were both associated with higher 3-methylglutarylcarnitine and three variants were associated with lower lysophosphatidylcholine C14:0. The risk allele in MTARC1 was associated with higher levels of sphingomyelins. There was no overlap with metabolites that associated with HFE or SERPINA1 variants. Our results suggest a link between the NASH-protective variant in MTARC1 to the metabolism of sphingomyelins and identify distinct molecular patterns associated with each of the NASH-fibrosis variants under investigation.

NAFLD, and cardiovascular and cardiac diseases: Factors influencing risk, prediction and treatment

BACKGROUND AND AIM

Non-alcoholic fatty liver disease (NAFLD), affecting up to around 30% of the world's adult population, causes considerable liver-related and extrahepatic morbidity and mortality. Strong evidence indicates that NAFLD (especially its more severe forms) is associated with a greater risk of all-cause mortality, and the predominant cause of mortality in this patient population is cardiovascular disease (CVD). This narrative review aims to discuss the strong association between NAFLD and increased risk of cardiovascular, cardiac and arrhythmic complications. Also discussed are the putative mechanisms linking NAFLD to CVD and other cardiac/arrhythmic complications, with a brief summary of CVD risk prediction/stratification and management of the increased CVD risk observed in patients with NAFLD.

RESULTS

NAFLD is associated with an increased risk of CVD events and other cardiac complications (left ventricular hypertrophy, valvular calcification, certain arrhythmias) independently of traditional CVD risk factors. The magnitude of risk of CVD and other cardiac/arrhythmic complications parallels the severity of NAFLD (especially liver fibrosis severity). There are most likely multiple underlying mechanisms through which NAFLD may increase risk of CVD and cardiac/arrhythmic complications. Indeed, NAFLD exacerbates hepatic and systemic insulin resistance, promotes atherogenic dyslipidaemia, induces hypertension, and triggers synthesis of proatherogenic, procoagulant and proinflammatory mediators that may contribute to the development of CVD and other cardiac/arrhythmic complications.

CONCLUSION

Careful assessment of CVD risk is mandatory in patients with NAFLD for primary prevention of CVD, together with pharmacological treatment for coexisting CVD risk factors.

Abdominal Fat and Metabolic Health Markers but Not PNPLA3 Genotype Predicts Liver Fat Accumulation in Response to Excess Intake of Energy and Saturated Fat in Healthy Individuals

Background: Saturated fat (SFA) has consistently been shown to increase liver fat, but the response appears variable at the individual level. Phenotypic and genotypic characteristics have been demonstrated to modify the hypercholesterolemic effect of SFA but it is unclear which characteristics that predict liver fat accumulation in response to a hypercaloric diet high in SFA.

Objective: To identify predictors of liver fat accumulation in response to an increased intake of SFA.

Design: We pooled our two previously conducted double-blind randomized trials (LIPOGAIN and LIPOGAIN-2, clinicaltrials.gov NCT01427140 and NCT02211612) and used data from the n = 49 metabolically healthy men (n = 32) and women (n = 17) randomized to a hypercaloric diet through addition of SFA-rich muffins for 7–8 weeks. Associations between clinical and metabolic variables at baseline and changes in liver fat during the intervention were analyzed using Spearman rank correlation. Linear regression was used to generate a prediction model.

Results: Liver fat increased by 33% (IQR 5.4–82.7%; P < 0.0001) in response to excess energy intake and this was not associated (r = 0.17, P = 0.23) with the increase in body weight (1.9 kg; IQR 1.1–2.9 kg). Liver fat accumulation was similar (P = 0.28) in carriers (33%, IQR 14–79%) and non-carriers (33%, IQR −11 to +87%) of the PNPLA3-I148M variant. Baseline visceral and liver fat content, as well as levels of the liver enzyme γ-glutamyl transferase (GT), were the strongest positive predictors of liver fat accumulation—in contrast, adiponectin and the fatty acid 17:0 in adipose tissue were the only negative predictors in univariate analyses. A regression model based on eight clinical and metabolic variables could explain 81% of the variation in liver fat accumulation.

Conclusion: Our results suggest there exists a highly inter-individual variation in the accumulation of liver fat in metabolically healthy men and women, in response to an increased energy intake from SFA and carbohydrates that occurs over circa 2 months. This marked variability in liver fat accumulation could largely be predicted by a set of clinical (e.g., GT and BMI) and metabolic (e.g., fatty acids, HOMA-IR, and adiponectin) variables assessed at baseline.

Modeling Non-Alcoholic Fatty Liver Disease (NAFLD) Using "Good-Fit" Genome-Editing Tools

Non-alcoholic fatty liver disease (NAFLD), which affects both adults and children, is the most common liver disorder worldwide. NAFLD is characterized by excess fat accumulation in the liver in the absence of significant alcohol use. NAFLD is strongly associated with obesity, insulin resistance, metabolic syndrome, as well as specific genetic polymorphisms. Severe NAFLD cases can further progress to cirrhosis, hepatocellular carcinoma (HCC), or cardiovascular complications. Here, we describe the pathophysiological features and critical genetic variants associated with NAFLD. Recent advances in genome-engineering technology have provided a new opportunity to generate in vitro and in vivo models that reflect the genetic abnormalities of NAFLD. We review the currently developed NAFLD models generated using clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) genome editing. We further discuss unique features of CRISPR/Cas9 and Cas9 variants, including base editors and prime editor, that are useful for replicating genetic features specific to NAFLD. We also compare advantages and limitations of currently available methods for delivering genome-editing tools necessary for optimal genome editing. This review should provide helpful guidance for selecting “good fit” genome-editing tools and appropriate gene-delivery methods for the successful development of NAFLD models and clinical therapeutics.

Hepatic lipid droplets: A balancing act between energy storage and metabolic dysfunction in NAFLD

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is defined by the abundance of lipid droplets (LDs) in hepatocytes. While historically considered simply depots for energy storage, LDs are increasingly recognized to impact a wide range of biological processes that influence cellular metabolism, signaling, and function. While progress has been made toward understanding the factors leading to LD accumulation (i.e. steatosis) and its progression to advanced stages of NAFLD and/or systemic metabolic dysfunction, much remains to be resolved.

SCOPE OF REVIEW

This review covers many facets of LD biology. We provide a brief overview of the major pathways of lipid accretion and degradation that contribute to steatosis and how they are altered in NAFLD. The major focus is on the relationship between LDs and cell function and the detailed mechanisms that couple or uncouple steatosis from the severity and progression of NAFLD and systemic comorbidities. The importance of specific lipids and proteins within or on LDs as key components that determine whether LD accumulation is linked to cellular and metabolic dysfunction is presented. We discuss emerging areas of LD biology and future research directions that are needed to advance our understanding of the role of LDs in NAFLD etiology.

MAJOR CONCLUSIONS

Impairments in LD breakdown appear to contribute to disease progression, but inefficient incorporation of fatty acids (FAs) into LD-containing triacylglycerol (TAG) and the consequential changes in FA partitioning also affect NAFLD etiology. Increased LD abundance in hepatocytes does not necessarily equate to cellular dysfunction. While LD accumulation is the prerequisite step for most NAFLD cases, the protein and lipid composition of LDs are critical factors in determining the progression from simple steatosis. Further defining the detailed molecular mechanisms linking LDs to metabolic dysfunction is important for designing effective therapeutic approaches targeting NAFLD and its comorbidities.

Identification of a Metabolic, Transcriptomic, and Molecular Signature of Patatin-Like Phospholipase Domain Containing 3-Mediated Acceleration of Steatohepatitis

BACKGROUND AND AIMS

The mechanisms by which the I148M mutant variant of the patatin-like phospholipase domain-containing 3 (PNPLA3^I148M ) drives development of nonalcoholic steatohepatitis (NASH) are not known. The aim of this study was to obtain insights on mechanisms underlying PNPLA3^I148M -induced acceleration of NASH.

APPROACH AND RESULTS

Hepatocyte-specific overexpression of empty vector (luciferase), human wild-type PNPLA3, or PNPLA3^I148M was achieved using adeno-associated virus 8 in a diet-induced mouse model of nonalcoholic fatty liver disease followed by chow diet or high-fat Western diet with ad libitum administration of sugar in drinking water (WDSW) for 8 weeks. Under WDSW, PNPLA3^I148M overexpression accelerated steatohepatitis with increased steatosis, inflammation ballooning, and fibrosis (P < 0.001 versus other groups for all). Silencing PNPLA3^I148M after its initial overexpression abrogated these findings. PNPLA3^I148M caused 22:6n3 docosahexanoic acid depletion and increased ceramides under WDSW in addition to increasing triglycerides and diglycerides, especially enriched with unsaturated fatty acids. It also increased oxidative stress and endoplasmic reticulum stress. Increased total ceramides was associated with signature of transducer and activator of transcription 3 (STAT3) activation with downstream activation of multiple immune-inflammatory pathways at a transcriptomic level by network analyses. Silencing PNPLA3^I148M reversed STAT3 activation. Conditioned media from HepG2 cells overexpressing PNPLA3^I148M increased procollagen mRNA expression in LX2 cells; this was abrogated by hepatocyte STAT3 inhibition.

CONCLUSIONS

Under WDSW, PNPLA3^I148M overexpression promotes steatosis and NASH by metabolic reprogramming characterized by increased triglycerides and diglycerides, n3 polyunsaturated fatty acid depletion, and increased ceramides with resultant STAT3 phosphorylation and downstream inflammatory pathway activation driving increased stellate cell fibrogenic activity.

Dysregulated lipid metabolism links NAFLD to cardiovascular disease

Background

Non-alcoholic fatty liver disease (NAFLD) is rapidly becoming a global health problem. Cardiovascular diseases (CVD) are the most common cause of mortality in NAFLD patients. NAFLD and CVD share several common risk factors including obesity, insulin resistance, and type 2 diabetes (T2D). Atherogenic dyslipidemia, characterized by plasma hypertriglyceridemia, increased small dense low-density lipoprotein (LDL) particles, and decreased high-density lipoprotein cholesterol (HDL-C) levels, is often observed in NAFLD patients.

Scope of review

In this review, we highlight recent epidemiological studies evaluating the link between NAFLD and CVD risk. We further focus on recent mechanistic insights into the links between NAFLD and altered lipoprotein metabolism. We also discuss current therapeutic strategies for NAFLD and their potential impact on NAFLD-associated CVD risk.

Major conclusions

Alterations in hepatic lipid and lipoprotein metabolism are major contributing factors to the increased CVD risk in NAFLD patients, and many promising NASH therapies in development also improve dyslipidemia in clinical trials.

The role of omics in the pathophysiology, diagnosis and treatment of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a multifaceted metabolic disorder, whose spectrum covers clinical, histological and pathophysiological developments ranging from simple steatosis to non-alcoholic steatohepatitis (NASH) and liver fibrosis, potentially evolving into cirrhosis, hepatocellular carcinoma and liver failure. Liver biopsy remains the gold standard for diagnosing NAFLD, while there are no specific treatments. An ever-increasing number of high-throughput Omics investigations on the molecular pathobiology of NAFLD at the cellular, tissue and system levels produce comprehensive biochemical patient snapshots. In the clinical setting, these applications are considerably enhancing our efforts towards obtaining a holistic insight on NAFLD pathophysiology. Omics are also generating non-invasive diagnostic modalities for the distinct stages of NAFLD, that remain though to be validated in multiple, large, heterogenous and independent cohorts, both cross-sectionally as well as prospectively. Finally, they aid in developing novel therapies. By tracing the flow of information from genomics to epigenomics, transcriptomics, proteomics, metabolomics, lipidomics and glycomics, the chief contributions of these techniques in understanding, diagnosing and treating NAFLD are summarized herein.

Perspectives on youth-onset nonalcoholic fatty liver disease

BACKGROUND

The prevalence and incidence of youth-onset nonalcoholic fatty liver disease (NAFLD) far exceeds other paediatric chronic liver diseases and represents a considerable public health issue globally.

METHODS

Here, we performed a narrative review of current knowledge regarding the epidemiology of paediatric NAFLD, selected concepts in pathogenesis, comorbidities, diagnosis, and management, and issues related to the transition to adulthood.

RESULTS

Paediatric NAFLD has become increasingly more prevalent, especially in certain subgroups, such as children with obesity and certain races/ethnicities. The pathophysiology of paediatric NAFLD is complex and multifactorial, driven by an interaction of environmental and genetic factors. Once developed, NAFLD in childhood is associated with type 2 diabetes, hypertension, increased cardiovascular disease risk, and end-stage liver disease. This predicts an increased burden of morbidity and mortality in adolescents and young adults. Early screening and diagnosis are therefore crucial, and the development of noninvasive biomarkers remains an active area of investigation. Currently, treatment strategies are focused on lifestyle changes, but there is also research interest in pharmacological and surgical options. In the transition from paediatric to adult care, there are several potential challenges/barriers to treatment and research is needed to understand how best to support patients during this time.

CONCLUSIONS

Our understanding of the epidemiology and pathophysiology of paediatric NAFLD has increased considerably over recent decades, but several critical knowledge gaps remain and must be addressed in order to better mitigate the short-term and long-term risks of youth-onset NAFLD.

The Role of Metabolic Lipases in the Pathogenesis and Management of Liver Disease

Intracellular lipolysis is an enzymatic pathway responsible for the catabolism of triglycerides (TGs) that is complemented by lipophagy as the autophagic breakdown of lipid droplets. The hydrolytic cleavage of TGs generates free fatty acids (FFAs), which can serve as energy substrates, precursors for lipid synthesis, and mediators in cell signaling. Despite the fundamental and physiological importance of FFAs, an oversupply can trigger lipotoxicity with impaired membrane function, endoplasmic reticulum stress, mitochondrial dysfunction, cell death, and inflammation. Conversely, impaired release of FFAs and other lipid mediators can also disrupt key cellular signaling functions that regulate metabolism and inflammatory processes. This review will focus on specific functions of intracellular lipases in lipid partitioning, covering basic and translational findings in the context of liver disease. In addition, the clinical relevance of genetic mutations in human disease and potential therapeutic opportunities will be discussed.

Role of Patatin-Like Phospholipase Domain-Containing 3 Gene for Hepatic Lipid Content and Insulin Resistance in Diabetes

OBJECTIVE

The rs738409(G) single nucleotide polymorphism (SNP) in the patatin-like phospholipase domain-containing 3 (PNPLA3) gene associates with increased risk and progression of nonalcoholic fatty liver disease (NAFLD). As the recently described severe insulin-resistant diabetes (SIRD) cluster specifically relates to NAFLD, this study examined whether this SNP differently associates with hepatic lipid content (hepatocellular lipids [HCL]) and insulin sensitivity in recent-onset diabetes.

RESEARCH DESIGN AND METHODS

A total of 917 participants in the German Diabetes Study (GDS) underwent genotyping, hyperinsulinemic-euglycemic clamps with stable isotopic tracer dilution, and MRS.

RESULTS

The G allele associated positively with HCL (β = 0.36, P < 0.01), independent of age, sex, and BMI across the whole cohort, but not in the individual clusters. Those with SIRD exhibited lowest whole-body insulin sensitivity compared with those with severe insulin-deficient (SIDD), moderate obesity-related (MOD), moderate age-related (MARD), and severe autoimmune diabetes (SAID) clusters (all P < 0.001). Interestingly, the SIRD group presented with higher prevalence of the rs738409(G) SNP compared with other clusters and the glucose-tolerant control group (P < 0.05). HCL was higher in the SIRD group (median 13.6% [1st quartile 5.8; 3rd quartile 19.1] compared with the MOD (6.4 % [2.1; 12.4], P < 0.05), MARD (3.0% [1.0; 7.9], P < 0.001), SAID (0.4% [0.0; 1.5], P < 0.001), and glucose-tolerant (0.9% [0.4; 4.9), P < 0.001) group. Although the PNPLA3 polymorphism did not directly associate with whole-body insulin sensitivity in SIRD, the G-allele carriers had higher circulating free fatty acid concentrations and greater adipose tissue insulin resistance compared with noncarriers (both P < 0.001).

CONCLUSIONS

Members of the SIRD cluster are more frequently carriers of the rs738409(G) variant. The SNP-associated adipose tissue insulin resistance and excessive lipolysis may contribute to their NAFLD.

Genetic and metabolic factors: the perfect combination to treat metabolic associated fatty liver disease

The prevalence of nonalcoholic or more recently re-defined metabolic associated fatty liver disease (MAFLD) is rapidly growing worldwide. It is characterized by hepatic fat accumulation exceeding 5% of liver weight not attributable to alcohol consumption. MAFLD refers to an umbrella of conditions ranging from simple steatosis to nonalcoholic steatohepatitis which may finally progress to cirrhosis and hepatocellular carcinoma. MAFLD is closely related to components of the metabolic syndrome and to environmental factors. In addition to the latter, genetic predisposition plays a key role in MAFLD pathogenesis and strictly contributes to its progressive forms. The candidate genes which have been related to MAFLD hereditability are mainly involved in lipids remodeling, lipid droplets assembly, lipoprotein packaging and secretion, de novo lipogenesis, and mitochondrial redox status. In the recent years, it has emerged the opportunity to translate the genetics into clinics by aggregating the genetic variants mostly associated with MAFLD in polygenic risk scores. These scores might be used in combination with metabolic factors to identify those patients at higher risk to develop more severe liver disease and to schedule an individual therapeutic approach.

Genetic Pathways in Nonalcoholic Fatty Liver Disease: Insights From Systems Biology

Nonalcoholic fatty liver disease (NAFLD) represents a burgeoning worldwide epidemic whose etiology reflects multiple interactions between environmental and genetic factors. Here, we review the major pathways and dominant genetic modifiers known to be relevant players in human NAFLD and which may determine key components of the heritability of distinctive disease traits including steatosis and fibrosis. In addition, we have employed general assumptions which are based on known genetic factors in NAFLD to build a systems biology prediction model that includes functional enrichment. This prediction model highlights additional complementary pathways that represent plausible intersecting signaling networks that we define here as an NAFLD-Reactome. We review the evidence connecting variants in each of the major known genetic modifiers (variants in patatin-like phospholipase domain containing 3, transmembrane 6 superfamily member 2, membrane-bound O-acyltransferase domain containing 7, glucokinase regulator, and hydroxysteroid 17-beta dehydrogenase 13) to NAFLD and expand the associated underlying mechanisms using functional enrichment predictions, based on both preclinical and cell-based experimental findings. These major candidate gene variants function in distinct pathways, including substrate delivery for de novo lipogenesis; mitochondrial energy use; lipid droplet assembly, lipolytic catabolism, and fatty acid compartmentalization; and very low-density lipoprotein assembly and secretion. The NAFLD-Reactome model expands these pathways and allows for hypothesis testing, as well as serving as a discovery platform for druggable targets across multiple pathways that promote NAFLD development and influence several progressive outcomes. In conclusion, we summarize the strengths and weaknesses of studies implicating selected variants in the pathophysiology of NAFLD and highlight opportunities for future clinical research and pharmacologic intervention, as well as the implications for clinical practice.

Update on NAFLD genetics: From new variants to the clinic

Non-alcoholic fatty liver disease (NAFLD) is the leading cause of liver diseases in high-income countries and the burden of NAFLD is increasing at an alarming rate. The risk of developing NAFLD and related complications is highly variable among individuals and is determined by environmental and genetic factors. Genome-wide association studies have uncovered robust and reproducible associations between variations in genes such as PNPLA3, TM6SF2, MBOAT7, GCKR, HSD17B13 and the natural history of NAFLD. These findings have provided compelling new insights into the biology of NAFLD and highlighted potentially attractive pharmaceutical targets. More recently the development of polygenic risk scores, which have shown promising results for the clinical risk prediction of other complex traits (such as cardiovascular disease and breast cancer), have provided new impetus for the clinical validation of genetic variants in NAFLD risk stratification. Herein, we review current knowledge on the genetic architecture of NAFLD, including gene-environment interactions, and discuss the implications for disease pathobiology, drug discovery and risk prediction. We particularly focus on the potential clinical translation of recent genetic advances, discussing methodological hurdles that must be overcome before these discoveries can be implemented in everyday practice.

Review article: the emerging role of genetics in precision medicine for patients with non-alcoholic steatohepatitis

BACKGROUND

Non-alcoholic steatohepatitis (NASH) is a severe form of non-alcoholic fatty liver disease (NAFLD) characterised by liver fat accumulation, inflammation and progressive fibrosis. Emerging data indicate that genetic susceptibility increases risks of NAFLD, NASH and NASH-related cirrhosis.

AIMS

To review NASH genetics and discuss the potential for precision medicine approaches to treatment.

METHOD

PubMed search and inclusion of relevant literature.

RESULTS

Single-nucleotide polymorphisms in PNPLA3, TM6SF2, GCKR, MBOAT7 and HSD17B13 are clearly associated with NASH development or progression. These genetic variants are common and have moderate-to-large effect sizes for development of NAFLD, NASH and hepatocellular carcinoma (HCC). The genes play roles in lipid remodelling in lipid droplets, hepatic very low-density lipoprotein (VLDL) secretion and de novo lipogenesis. The PNPLA3 I148M variant (rs738409) has large effects, with approximately twofold increased odds of NAFLD and threefold increased odds of NASH and HCC per allele. Obesity interacts with PNPLA3 I148M to elevate liver fat content and increase rates of NASH. Although the isoleucine-to-methionine substitution at amino acid position 148 of the PNPLA3 enzyme inactivates its lipid remodelling activity, the effect of PNPLA3 I148M results from trans-repression of another lipase (ATGL/PNPLA2) by sequestration of a shared cofactor (CGI-58/ABHD5), leading to decreased hepatic lipolysis and VLDL secretion. In homozygous Pnpla3 I148M knock-in rodent models of NAFLD, targeted PNPLA3 mRNA knockdown reduces hepatic steatosis, inflammation and fibrosis.

CONCLUSION

The emerging genetic and molecular understanding of NASH paves the way for novel interventions, including precision medicines that can modulate the activity of specific genes associated with NASH.

Toward Genetic Prediction of Nonalcoholic Fatty Liver Disease Trajectories: PNPLA3 and Beyond

Nonalcoholic fatty liver disease (NAFLD) is on the verge of becoming the leading cause of liver disease. NAFLD develops at the interface between environmental factors and inherited predisposition. Genome-wide association studies, followed by exome-wide analyses, led to identification of genetic risk variants (eg, PNPLA3, TM6SF2, and SERPINA1) and key pathways involved in fatty liver disease pathobiology. Functional studies improved our understanding of these genetic factors and the molecular mechanisms underlying the trajectories from fat accumulation to fibrosis, cirrhosis, and cancer over time. Here, we summarize key NAFLD risk genes and illustrate their interactions in a 3-dimensional "risk space." Although NAFLD genomics sometimes appears to be "lost in translation," we envision clinical utility in trial design, outcome prediction, and NAFLD surveillance.

Effect of a ketogenic diet on hepatic steatosis and hepatic mitochondrial metabolism in nonalcoholic fatty liver disease

Weight loss by ketogenic diet (KD) has gained popularity in management of nonalcoholic fatty liver disease (NAFLD). KD rapidly reverses NAFLD and insulin resistance despite increasing circulating nonesterified fatty acids (NEFA), the main substrate for synthesis of intrahepatic triglycerides (IHTG). To explore the underlying mechanism, we quantified hepatic mitochondrial fluxes and their regulators in humans by using positional isotopomer NMR tracer analysis. Ten overweight/obese subjects received stable isotope infusions of: [D7]glucose, [13C4]β-hydroxybutyrate and [3-13C]lactate before and after a 6-d KD. IHTG was determined by proton magnetic resonance spectroscopy (1H-MRS). The KD diet decreased IHTG by 31% in the face of a 3% decrease in body weight and decreased hepatic insulin resistance (-58%) despite an increase in NEFA concentrations (+35%). These changes were attributed to increased net hydrolysis of IHTG and partitioning of the resulting fatty acids toward ketogenesis (+232%) due to reductions in serum insulin concentrations (-53%) and hepatic citrate synthase flux (-38%), respectively. The former was attributed to decreased hepatic insulin resistance and the latter to increased hepatic mitochondrial redox state (+167%) and decreased plasma leptin (-45%) and triiodothyronine (-21%) concentrations. These data demonstrate heretofore undescribed adaptations underlying the reversal of NAFLD by KD: That is, markedly altered hepatic mitochondrial fluxes and redox state to promote ketogenesis rather than synthesis of IHTG.

Leveraging Human Genetics to Identify Potential New Treatments for Fatty Liver Disease

Fatty liver disease (FLD), including its more severe pathologies, namely steatohepatitis, hepatocarcinoma, and cirrhosis, is the most common cause of chronic liver disease worldwide and is projected to become the leading cause of hepatocellular carcinoma and end-stage liver disease. FLD is heterogeneous with multiple etiologies and diverse histological phenotypes, so therapies will ultimately need to be individualized for relevant targets. Inherited factors contribute to FLD, and most of the genetic variation influencing liver disease development and progression is derived from genes involved in lipid biology, including PNPLA3, TM6SF2, GCKR, MBOAT7, and HSD17B13. From this point of view, we focus in this perspective on how human molecular genetics of FLD have highlighted defects in hepatic lipid handling as a major common mechanism of its pathology and how this insight could be leveraged to treat and prevent its more serious complications.

Fibroblast Growth Factor-21 to Adiponectin Ratio: A Potential Biomarker to Monitor Liver Fat in Children With Obesity

Background: There is a pressing need for effective and non-invasive biomarkers to track intrahepatic triglyceride (IHTG) in children at-risk for non-alcoholic fatty liver disease (NAFLD), as standard-of-care reference tools, liver biopsy and magnetic resonance imaging (MRI), are impractical to monitor the course disease. Objective: We aimed to examine the association between serum fibroblast growth factor (FGF)-21 to adiponectin ratio (FAR) and IHTG as assessed by MRI in children with obesity. Methods: Serum FGF21 and adiponectin levels and IHTG were measured at two time points (baseline, 6 months) in obese children enrolled in a clinical weight loss program. The association between percent change in FAR and IHTG at final visit was examined using a multiple linear regression model. Results: At baseline, FAR was higher in the subjects with NAFLD (n = 23, 35.8 ± 41.9 pg/ng) than without NAFLD (n = 35, 19.8 ± 13.7 pg/ng; p = 0.042). Forty-eight subjects completed both visits and were divided into IHTG loss (≥1% reduction than baseline), no change (within ±1% change), and gain (≥1% increase than baseline) groups. At 6 months, the percent change in FAR was different among the three groups (p = 0.005). Multiple linear regression showed a positive relationship between percent change in FAR and the final liver fat percent in sex and pubertal stage-similar subjects with NAFLD at baseline (slope coefficient 6.18, 95% CI 1.90-10.47, P = 0.007), but not in those without NAFLD. Conclusions: Higher value in percent increase in FAR is positively associated with higher level of IHTG percent value at 6 months in children with baseline NAFLD. FAR could be a potential biomarker to monitor the changes in IHTG in children with NAFLD.