Unique Genetic Features of Lean NAFLD: A Review of Mechanisms and Clinical Implications

Non-alcoholic fatty liver disease (NAFLD) affects 25% of the global population. About 20% have a normal body mass index (BMI) and a variant known as lean NAFLD. Unlike typical NAFLD cases associated with obesity and diabetes, lean NAFLD causes liver disease by mechanisms not related to excess weight or insulin resistance. Genetic disorders are among the major factors in developing lean NAFLD, and genome-wide association studies have identified several genes associated with the condition. This review aims to increase awareness by describing the genetic markers linked to NAFLD and the defects involved in developing lean NAFLD.

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.

[Correlation between PNPLA3 rs738409 and TM6SF2 rs58542926 gene polymorphism and primary liver cancer in the Han Population of China's Northeast region]

Objective: To investigate the correlation between patatin-like phospholipase domain-containing 3 (PNPLA3) rs738409 and transmembrane 6 superfamily member 2 (TM6SF2) rs58542926 gene polymorphisms and the incidence of primary liver cancer in the Han population of China's Northeast region. Methods: A case-control study was used to enroll 521 patients with primary liver cancer as the case group and 164 healthy people as the control group. The case group was divided into groups with and without liver cirrhosis according to etiology. The polymerase chain reaction (PCR) method was used to detect the genetic polymorphisms of PNPLA3 rs738409 and TM6SF2 rs58542926, respectively. Results: Compared with the control group, the frequency distribution of PNPLA3 rs738409 G allele in the case group was significantly different (OR = 1.583, P = 0.001). Further grouping showed that there was no statistically significant difference between the control and hepatitis C-related liver cancer group (P = 0.161), but there were significant differences in other groups (P < 0.05). Compared with the control group, the frequency of TM6SF2 rs58542926 T allele in the case group was significantly higher than that in the control group (OR = 1.759, P = 0.048). After grouping, the frequency of CT/TT genotype in the liver cancer group combined with liver cirrhosis and the T allele frequency in the alcohol-related liver cancer group had statistically significant difference (P = 0.045 and 0.032, respectively) when compared with control group. The patients were divided into CG/GG group and CC group, and CT/TT group and CC group according to whether they carried PNPLA3 rs738409 G allele, and TM6SF2 rs58542926 T allele. Results showed that there were no statistically significant differences in liver enzyme indexes, albumin (ALB), total bilirubin (TBIL), alpha-fetoprotein (AFP) and fasting blood glucose between CG/GG group and CC group, and CT/TT group and CC group. The patients with liver cirrhosis in the case group were divided into≥7 groups and < 7 groups according to the Child-Pugh score. Results showed that there were no statistically significant difference in the Child-Pugh score between PNPLA3 rs738409 CG/GG group and CC group patients and TM6SF2 rs58542926 CT/TT group and CC group patients (P > 0.05). Conclusion: PNPLA3 rs738409 and TM6SF2 rs58542926 gene polymorphisms are correlated with the occurrence of primary liver cancer in the Han population of China's Northeast region. PNPLA3 rs738409 and TM6SF2 rs58542926 gene polymorphisms have no effect on indexes' such as liver enzymes, ALB, TBIL, AFP and FBS in primary liver cancer..

[Effects of PNPLA3, TM6SF2 gene polymorphisms and its interactions with smoking and alcohol drinking on hepatitis B virus-associated hepatocellular carcinoma]

Objective: To explore the SNP effects of patatin-like phospholipase domain which containing 3 (PNPLA3), transmembrane 6 superfamily member 2 (TM6SF2) gene, environmental effects of smoking, alcohol drinking and interaction between gene-gene, gene-environment and drinking-smoking on hepatitis B virus-associated hepatocellular carcinoma (HBV-HCC). Methods: We collected anticoagulant peripheral blood from patients of HBV-HCC, chronic hepatitis B (CHB), liver cirrhosis (LC) and from healthy controls to detect the single nucleotide polymorphism (SNP) of patatin-like phospholipase domain containing 3 (PNPLA3) gene loci rs738409 and transmembrane 6 superfamily member 2 (TM6SF2) gene loci rs58542926, using the flight mass spectrometry method. The optimal assignment value of gene polymorphisms was defined by using the online SNP stats. Hardy-Weinberg (H-W) balance was tested for SNP. Effects of the genetic and environmental factors to HBV-HCC were analyzed by using the multiple classification logistic regression method. The gene-gene, gene-smoking and alcohol drinking interaction effects were investigated by Fork-Life analysis and binary logistic regression methods. Results: The frequency distribution of CHB group rs738409 loci seemed not in conformity with the H-W balance (χ(2)=11.980, P<0.005). Two loci frequency distributions in the other groups were all in accordandce with the H-W balance. After adjusting for influences on age and sex and comparing to the healthy group, the rs58542926 mutation appeared as OR=1.659, 95%CI: 1.026-2.684, P=0.039, in the HBV-HCC group. When comparing to CHB group, the HBV-HCC group presented that drinking as OR=1.680, 95%CI: 1.121-2.519, P=0.012. When comparing to the LC group, the ORs of drinking and smoking were 1.539 (1.071-2.213) and 1.453 (1.005-2.099) respectively, in the HBV-HCC group. When comparing to the CHB+LC group, interactions between the HBV-HCC group were found rs738409 and rs58542926 on additive model OR=1.548 (U=1.885, P=0.029) and OR=1.658 (P=0.024) on logistic regression model while drinking was rs738409 on interaction additive model with OR=1.811(U=1.965, P=0.024). As for drinking and mutation of rs738409, the multiplication model of logistic regression showed no statistically significant differences. Interaction between smoking and drinking appeared as OR=1.756 (P<0.001) in the logistics regression multiplication model. Conclusions: Factors as mutation of TM6SF2, smoking and drinking all appeared as risk factors for HBV-HCC. Mutations of both PNPLA3 and TM6SF2, together with smoking and drinking all served as risk factors for HBV-HCC. However, the mutation of single PNPLA3 appeared as a protective factor on HBV-HCC.

Promoting genetics in non-alcoholic fatty liver disease: Combined risk score through polymorphisms and clinical variables

Non-alcoholic fatty liver disease (NAFLD) has a prevalence of approximately 30% in western countries, and is emerging as the first cause of liver cirrhosis and hepatocellular carcinoma (HCC). Therefore, risk stratification emerges as fundamental in order to optimize human and economic resources, and genetics displays intrinsic characteristics suitable to fulfill this task. According to the available data, heritability estimates for hepatic fat content range from 20% to 70%, and an almost 80% of shared heritability has been found between hepatic fat content and fibrosis. The rs738409 single nucleotide polymorphism (SNP) in patatin-like phospholipase domain-containing protein 3 gene and the rs58542926 SNP in transmembrane 6 superfamily member 2 gene have been robustly associated with NAFLD and with its progression, but promising results have been obtained with many other SNPs. Moreover, there has been proof of the additive role of the different SNPs in determining liver damage, and there have been preliminary experiences in which risk scores created through a few genetic variants, alone or in combination with clinical variables, were associated with a strongly potentiated risk of NAFLD, non-alcoholic steatohepatitis (NASH), NASH fibrosis or NAFLD-HCC. However, to date, clinical translation of genetics in the field of NAFLD has been poor or absent. Fortunately, the research we have done seems to have placed us on the right path: We should rely on longitudinal rather than on cross-sectional studies; we should focus on relevant outcomes rather than on simple liver fat accumulation; and we should put together the genetic and clinical information. The hope is that combined genetic/clinical scores, derived from longitudinal studies and built on a few strong genetic variants and relevant clinical variables, will reach a significant predictive power, such as to have clinical utility for risk stratification at the single patient level and even to esteem the impact of intervention on the risk of disease-related outcomes. Well-structured future studies would demonstrate if this vision can become a reality.

Impaired hepatic lipid synthesis from polyunsaturated fatty acids in TM6SF2 E167K variant carriers with NAFLD

BACKGROUND

Carriers of the transmembrane 6 superfamily member 2 E167K gene variant (TM6SF2^EK/KK) have decreased expression of the TM6SF2 gene and increased risk of NAFLD and NASH. Unlike common 'obese/metabolic' NAFLD, these subjects lack hypertriglyceridemia and have lower risk of cardiovascular disease. In animals, phosphatidylcholine (PC) deficiency results in a similar phenotype. PCs surround the core of VLDL consisting of triglycerides (TGs) and cholesteryl-esters (CEs). We determined the effect of the TM6SF2 E167K on these lipids in the human liver and serum and on hepatic gene expression and studied the effect of TM6SF2 knockdown on hepatocyte handling of these lipids.

METHODS

Liver biopsies were taken from subjects characterized with respect to the TM6SF2 genotype, serum and liver lipidome, gene expression and histology. In vitro, after TM6SF2 knockdown in HuH-7 cells, we compared incorporation of different fatty acids into TGs, CEs, and PCs.

RESULTS

The TM6SF2^EK/KK and TM6SF2^EE groups had similar age, gender, BMI and HOMA-IR. Liver TGs and CEs were higher and liver PCs lower in the TM6SF2^EK/KK than the TM6SF2^EE group (p<0.05). Polyunsaturated fatty acids (PUFA) were deficient in liver and serum TGs and liver PCs but hepatic free fatty acids were relatively enriched in PUFA (p<0.05). Incorporation of PUFA into TGs and PCs in TM6SF2 knockdown hepatocytes was decreased (p<0.05). Hepatic expression of TM6SF2 was decreased in variant carriers, and was co-expressed with genes regulated by PUFAs.

CONCLUSIONS

Hepatic lipid synthesis from PUFAs is impaired and could contribute to deficiency in PCs and increased intrahepatic TG in TM6SF2 E167K variant carriers.

Relationships between Genetic Variations of PNPLA3, TM6SF2 and Histological Features of Nonalcoholic Fatty Liver Disease in Japan

Background/Aims

It is important to determine the noninvasive parameters of histological features in nonalcoholic fatty liver disease (NAFLD). The aim of this study was to investigate the value of genetic variations as surrogate markers of histological features.

Methods

The parameters that affected the histological features of NAFLD were investigated in 211 Japanese patients with biopsy-proven NAFLD. The relationships between genetic variations in PNPLA3 rs738409 or TM6SF2 rs58542926 and histological features were analyzed. Furthermore, the impact of genetic variations that affected the pathological criteria for the diagnosis of nonalcoholic steatohepatitis (NASH) (Matteoni classification and NAFLD activity score) was evaluated.

Results

The fibrosis stage of PNPLA3 GG was significantly more progressive than that of CG by multiple comparisons. Multivariate analysis identified PNPLA3 genotypes as predictors of fibrosis of stage 2 or more, but the impact tended to decrease at stage 3 or greater. There were no significant differences among the histological features of the three genotypes of TM6SF2. PNPLA3 genotypes partly affected the definition of NASH by the NAFLD activity score, but TM6SF2 genotypes did not affect the definition of NASH.

Conclusions

In Japanese patients with biopsy-proven NAFLD, PNPLA3 genotypes may partly affect histological features, including stage of fibrosis, but the TM6SF2 genotype does not affect histological features.

Genetic background in nonalcoholic fatty liver disease: A comprehensive review

In the Western world, nonalcoholic fatty liver disease (NAFLD) is considered as one of the most significant liver diseases of the twenty-first century. Its development is certainly driven by environmental factors, but it is also regulated by genetic background. The role of heritability has been widely demonstrated by several epidemiological, familial, and twin studies and case series, and likely reflects the wide inter-individual and inter-ethnic genetic variability in systemic metabolism and wound healing response processes. Consistent with this idea, genome-wide association studies have clearly identified Patatin-like phosholipase domain-containing 3 gene variant I148M as a major player in the development and progression of NAFLD. More recently, the transmembrane 6 superfamily member 2 E167K variant emerged as a relevant contributor in both NAFLD pathogenesis and cardiovascular outcomes. Furthermore, numerous case-control studies have been performed to elucidate the potential role of candidate genes in the pathogenesis and progression of fatty liver, although findings are sometimes contradictory. Accordingly, we performed a comprehensive literature search and review on the role of genetics in NAFLD. We emphasize the strengths and weaknesses of the available literature and outline the putative role of each genetic variant in influencing susceptibility and/or progression of the disease.