Divergent role of Mitochondrial Amidoxime Reducing Component 1 (MARC1) in human and mouse.

Recent human genome-wide association studies have identified common missense variants in MARC1, p.Ala165Thr and p.Met187Lys, associated with lower hepatic fat, reduction in liver enzymes and protection from most causes of cirrhosis. Using an exome-wide association study we recapitulated earlier MARC1 p.Ala165Thr and p.Met187Lys findings in 540,000 individuals from five ancestry groups. We also discovered novel rare putative loss of function variants in MARC1 with a phenotype similar to MARC1 p.Ala165Thr/p.Met187Lys variants. In vitro studies of recombinant human MARC1 protein revealed Ala165Thr substitution causes protein instability and aberrant localization in hepatic cells, suggesting MARC1 inhibition or deletion may lead to hepatoprotection. Following this hypothesis, we generated Marc1 knockout mice and evaluated the effect of Marc1 deletion on liver phenotype. Unexpectedly, our study found that whole-body Marc1 deficiency in mouse is not protective against hepatic triglyceride accumulation, liver inflammation or fibrosis. In attempts to explain the lack of the observed phenotype, we discovered that Marc1 plays only a minor role in mouse liver while its paralogue Marc2 is the main Marc family enzyme in mice. Our findings highlight the major difference in MARC1 physiological function between human and mouse.

Germline Mutations in CIDEB and Protection against Liver Disease.

BACKGROUND: Exome sequencing in hundreds of thousands of persons may enable the identification of rare protein-coding genetic variants associated with protection from human diseases like liver cirrhosis, providing a strategy for the discovery of new therapeutic targets. METHODS: We performed a multistage exome sequencing and genetic association analysis to identify genes in which rare protein-coding variants were associated with liver phenotypes. We conducted in vitro experiments to further characterize associations. RESULTS: The multistage analysis involved 542,904 persons with available data on liver aminotransferase levels, 24,944 patients with various types of liver disease, and 490,636 controls without liver disease. We found that rare coding variants in APOB, ABCB4, SLC30A10, and TM6SF2 were associated with increased aminotransferase levels and an increased risk of liver disease. We also found that variants in CIDEB, which encodes a structural protein found in hepatic lipid droplets, had a protective effect. The burden of rare predicted loss-of-function variants plus missense variants in CIDEB (combined carrier frequency, 0.7%) was associated with decreased alanine aminotransferase levels (beta per allele, -1.24 U per liter; 95% confidence interval [CI], -1.66 to -0.83; P = 4.8×10-9) and with 33% lower odds of liver disease of any cause (odds ratio per allele, 0.67; 95% CI, 0.57 to 0.79; P = 9.9×10-7). Rare coding variants in CIDEB were associated with a decreased risk of liver disease across different underlying causes and different degrees of severity, including cirrhosis of any cause (odds ratio per allele, 0.50; 95% CI, 0.36 to 0.70). Among 3599 patients who had undergone bariatric surgery, rare coding variants in CIDEB were associated with a decreased nonalcoholic fatty liver disease activity score (beta per allele in score units, -0.98; 95% CI, -1.54 to -0.41 [scores range from 0 to 8, with higher scores indicating more severe disease]). In human hepatoma cell lines challenged with oleate, CIDEB small interfering RNA knockdown prevented the buildup of large lipid droplets. CONCLUSIONS: Rare germline mutations in CIDEB conferred substantial protection from liver disease. (Funded by Regeneron Pharmaceuticals.).

Accumulation of PNPLA3 on lipid droplets is the basis of associated hepatic steatosis.

Fatty liver disease (FLD) is a disorder in which accumulation of triglycerides (TGs) in the liver can lead to inflammation, fibrosis, and cirrhosis. Previously, we identified a variant (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) that is strongly associated with FLD, but the mechanistic basis for the association remains elusive. Although PNPLA3 has TG hydrolase activity in vitro, inactivation or overexpression of the WT protein in mice does not cause steatosis. In contrast, expression of two catalytically defective forms of PNPLA3 (I148M or S47A) in sucrose-fed mice causes accumulation of both PNPLA3 and TGs on hepatic lipid droplets (LDs). To determine if amassing PNPLA3 on LDs is a cause or consequence of steatosis, we engineered a synthetic isoform of PNPLA3 that uncouples protein accumulation from loss of enzymatic activity. Expression of a ubiquitylation-resistant form of PNPLA3 in mice caused accumulation of PNPLA3 on hepatic LDs and development of FLD. Lowering PNPLA3 levels by either shRNA knockdown or proteolysis-targeting chimera (PROTAC)-mediated degradation reduced liver TG content in mice overexpressing PNPLA3(148M). Taken together, our results show that the steatosis associated with PNPLA3(148M) is caused by accumulation of PNPLA3 on LDs.

Relationship between genetic variation at PPP1R3B and levels of liver glycogen and triglyceride.

Genetic variation at rs4240624 on chromosome 8 is associated with an attenuated signal on hepatic computerized tomography, which has been attributed to changes in hepatic fat. The closest coding gene to rs4240624, PPP1R3B, encodes a protein that promotes hepatic glycogen synthesis. Here, we performed studies to determine whether the x-ray attenuation associated with rs4240624 is due to differences in hepatic glycogen or hepatic triglyceride content (HTGC). A sequence variant in complete linkage disequilibrium with rs4240624, rs4841132, was genotyped in the Dallas Heart Study (DHS), the Dallas Liver Study, and the Copenhagen Cohort (n = 112,428) of whom 1,539 had nonviral liver disease. The minor A-allele of rs4841132 was associated with increased hepatic x-ray attenuation (n = 1,572; P = 4 × 10-5 ), but not with HTGC (n = 2,674; P = 0.58). Rs4841132-A was associated with modest, but significant, elevations in serum alanine aminotransferase (ALT) in the Copenhagen Cohort (P = 3 × 10-4 ) and the DHS (P = 0.004), and with odds ratios for liver disease of 1.13 (95% CI, 0.97-1.31) and 1.23 (1.01-1.51), respectively. Mice lacking protein phosphatase 1 regulatory subunit 3B (PPP1R3B) were deficient in hepatic glycogen, whereas HTGC was unchanged. Hepatic overexpression of PPP1R3B caused accumulation of hepatic glycogen and elevated plasma levels of ALT, but did not change HTGC. CONCLUSION: These observations are consistent with the notion that the minor allele of rs4841132 promotes a mild form of hepatic glycogenosis that is associated with hepatic injury. (Hepatology 2018;67:2182-2195).

The PNPLA3 variant associated with fatty liver disease (I148M) accumulates on lipid droplets by evading ubiquitylation.

A sequence variation (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) is strongly associated with fatty liver disease, but the underlying mechanism remains obscure. In this study, we used knock-in (KI) mice (Pnpla3148M/M ) to examine the mechanism responsible for accumulation of triglyceride (TG) and PNPLA3 in hepatic lipid droplets (LDs). No differences were found between Pnpla3148M/M and Pnpla3+/+ mice in hepatic TG synthesis, utilization, or secretion. These results are consistent with TG accumulation in the Pnpla3148M/M mice being caused by impaired TG mobilization from LDs. Sucrose feeding, which is required to elicit fatty liver in KI mice, led to a much larger and more persistent increase in PNPLA3 protein in the KI mice than in wild-type (WT) mice. Inhibition of the proteasome (bortezomib), but not macroautophagy (3-methyladenine), markedly increased PNPLA3 levels in WT mice, coincident with the appearance of ubiquitylated forms of the protein. Bortezomib did not increase PNPLA3 levels in Pnpla3148M/M mice, and only trace amounts of ubiquitylated PNPLA3 were seen in these animals. CONCLUSION: These results are consistent with the notion that the 148M variant disrupts ubiquitylation and proteasomal degradation of PNPLA3, resulting in accumulation of PNPLA3-148M and impaired mobilization of TG from LDs. (Hepatology 2017;66:1111-1124).

Inactivation of Tm6sf2, a Gene Defective in Fatty Liver Disease, Impairs Lipidation but Not Secretion of Very Low Density Lipoproteins.

A missense mutation (E167K) in TM6SF2 (transmembrane 6 superfamily member 2), a polytopic protein of unknown function, is associated with the full spectrum of fatty liver disease. To investigate the role of TM6SF2 in hepatic triglyceride (TG) metabolism, we inactivated the gene in mice. Chronic inactivation of Tm6sf2 in mice is associated with hepatic steatosis, hypocholesterolemia, and transaminitis, thus recapitulating the phenotype observed in humans. No dietary challenge was required to elicit the phenotype. Immunocytochemical and cell fractionation studies revealed that TM6SF2 was present in the endoplasmic reticulum and Golgi complex, whereas the excess neutral lipids in the Tm6sf2(-/-) mice were located in lipid droplets. Plasma VLDL-TG levels were reduced in the KO animals due to a 3-fold decrease in VLDL-TG secretion rate without any associated reduction in hepatic apoB secretion. Both VLDL particle size and plasma cholesterol levels were significantly reduced in KO mice. Despite levels of TM6SF2 protein being 10-fold higher in the small intestine than in the liver, dietary lipid absorption was only modestly reduced in the KO mice. Our data, taken together, reveal that TM6SF2 is required to mobilize neutral lipids for VLDL assembly but is not required for secretion of apoB-containing lipoproteins. Despite TM6SF2 being located in the endoplasmic reticulum and Golgi complex, the lipids that accumulate in its absence reside in lipid droplets.

Pnpla3I148M knockin mice accumulate PNPLA3 on lipid droplets and develop hepatic steatosis.

UNLABELLED: A sequence polymorphism (rs738409, I148M) in patatin-like phospholipid domain containing protein 3 (PNPLA3) is strongly associated with nonalcoholic fatty liver disease (NAFLD), but the mechanistic basis for this association remains enigmatic. Neither ablation nor overexpression of wild-type PNPLA3 affects liver fat content in mice, whereas hepatic overexpression of the human 148M transgene causes steatosis. To determine whether the 148M allele causes fat accumulation in the liver when expressed at physiological levels, we introduced a methionine codon at position 148 of the mouse Pnpla3 gene. Knockin mice had normal levels of hepatic fat on a chow diet, but when challenged with a high-sucrose diet their liver fat levels increased 2 to 3-fold compared to wild-type littermates without any associated changes in glucose homeostasis. The increased liver fat in the knockin mice was accompanied by a 40-fold increase in PNPLA3 on hepatic lipid droplets, with no increase in hepatic PNPLA3 messenger RNA (mRNA). Similar results were obtained when the catalytic dyad of PNPLA3 was inactivated by substituting the catalytic serine with alanine (S47A). CONCLUSION: These data provide the first direct evidence that physiological expression of PNPLA3 148M variant causes NAFLD, and that the accumulation of catalytically inactive PNPLA3 on the surfaces of lipid droplets is associated with the accumulation of TG in the liver.

Hepatic TM6SF2 Is Required for Lipidation of VLDL in a Pre-Golgi Compartment in Mice and Rats.

BACKGROUND & AIMS: Substitution of lysine for glutamic acid at residu 167 in Transmembrane 6 superfamily member 2 (TM6SF2) is associated with fatty liver disease and reduced plasma lipid levels. Tm6sf2-/- mice replicate the human phenotype but were not suitable for detailed mechanistic studies. As an alternative model, we generated Tm6sf2-/- rats to determine the subcellular location and function of TM6SF2. METHODS: Two lines of Tm6sf2-/- rats were established using gene editing. Lipids from tissues and from newly secreted very low density lipoproteins (VLDLs) were quantified using enzymatic assays and mass spectrometry. Neutral lipids were visualized in tissue sections using Oil Red O staining. The rate of dietary triglyceride (TG) absorption and hepatic VLDL-TG secretion were compared in Tm6sf2-/- mice and in their wild-type littermates. The intracellular location of TM6SF2 was determined by cell fractionation. Finally, TM6SF2 was immunoprecipitated from liver and enterocytes to identify interacting proteins. RESULTS: Tm6sf2-/- rats had a 6-fold higher mean hepatic TG content (56.1 ± 28.9 9 vs 9.8 ± 3.9 mg/g; P < .0001) and lower plasma cholesterol levels (99.0 ± 10.5 vs 110.6 ± 14.0 mg/dL; P = .0294) than their wild-type littermates. Rates of appearance of dietary and hepatic TG into blood were reduced significantly in Tm6sf2-/- rats (P < .001 and P < .01, respectively). Lipid content of newly secreted VLDLs isolated from perfused livers was reduced by 53% (TG) and 62% (cholesterol) (P = .005 and P = .01, respectively) in Tm6sf2-/- mice. TM6SF2 was present predominantly in the smooth endoplasmic reticulum and endoplasmic reticulum-Golgi intermediate compartments, but not in Golgi. Both apolipoprotein B-48 and acyl-CoA synthetase long chain family member 5 physically interacted with TM6SF2. CONCLUSIONS: TM6SF2 acts in the smooth endoplasmic reticulum to promote bulk lipidation of apolipoprotein B-containing lipoproteins, thus preventing fatty liver disease.

Exome-wide association study identifies a TM6SF2 variant that confers susceptibility to nonalcoholic fatty liver disease.

Nonalcoholic fatty liver disease (NAFLD) is the most common form of liver disease. To elucidate the molecular basis of NAFLD, we performed an exome-wide association study of liver fat content. Three variants were associated with higher liver fat levels at the exome-wide significance level of 3.6 × 10(-7): two in PNPLA3, an established locus for NAFLD, and one (encoding p.Glu167Lys) in TM6SF2, a gene of unknown function. The TM6SF2 variant encoding p.Glu167Lys was also associated with higher circulating levels of alanine transaminase, a marker of liver injury, and with lower levels of low-density lipoprotein-cholesterol (LDL-C), triglycerides and alkaline phosphatase in 3 independent populations (n > 80,000). When recombinant protein was expressed in cultured hepatocytes, 50% less Glu167Lys TM6SF2 protein was produced relative to wild-type TM6SF2. Adeno-associated virus-mediated short hairpin RNA knockdown of Tm6sf2 in mice increased liver triglyceride content by threefold and decreased very-low-density lipoprotein (VLDL) secretion by 50%. Taken together, these data indicate that TM6SF2 activity is required for normal VLDL secretion and that impaired TM6SF2 function causally contributes to NAFLD.

Hepatitis B and C virus variants in long-term immunosuppressed renal transplant patients in Latvia.

The incidence of genome variants of hepatitis B and hepatitis C viruses among 38 long-term (2-15 years) immunosuppressed patients after renal transplantation and 10 patients undergoing dialysis was investigated. Twelve patients had only HBV infection, 9 had only HCV infection and 14 were co-infected. Regions corresponding to the HBV X/EnII/BCP, preC/C, preS/S and to the HCV core were sequenced for molecular characterization of the HBV and HCV genomes. Fifty-seven percent of HBV DNA isolates belonged to genotype D and 42% to genotype A, whereas 77% of HCV RNA isolates belonged to genotype 1b and only 17% to genotype 3a. One sample (6%) was of genotype 2c. Detailed analysis of the above-mentioned HBV genome regions revealed the presence of nucleotide point mutations, which, in some cases, resulted in amino acid substitutions. The clinical significance of such mutations is discussed.