The TM6SF2 Variants, Novel Genetic Predictors for Nonalcoholic Steatohepatitis

Abstract

Nonalcoholic fatty liver disease (NAFLD) is a spectrum of liver disease ranging from benign steatosis (hepatocellular triglyceride [TG] accumulation of >5% of liver weight) through nonalcoholic steatohepatitis (NASH, which is steatosis combined with inflammation and ballooning degeneration) to fibrosis and ultimately cirrhosis and hepatocellular carcinoma (HCC), in the absence of excessive alcohol consumption (Gastroenterology 2007;132:2191–2207; Nat Rev Gastroenterol Hepatol 2013;10:645–655). NAFLD is linked with the features of metabolic syndrome such as obesity, insulin resistance, type 2 diabetes mellitus and dyslipidemia. Recently, 3 independent groups have reported that the single nucleotide polymorphisms (SNPs) related to transmembrane 6 superfamily member 2 (TM6SF2) are associated with NAFLD development. The first paper reported by Kozlitina et al. identified the association between hepatic TG content determined by proton magnetic resonance spectroscopy (1H-MRS) and the TM6SF2 variants (rs58542926) through exome-wide association study in a multiethnic, multi-ancestry, population-based cohort derived from the Dallas Heart Study (DHS; total 2,736 participants; 1,324 African Americans, 882 European Americans, 467 Hispanic, and 63 other ethnicities). This TM6SF2 variant is an adenine-to-guanine transversion at nucleotide position 499, resulting in the replacement of the 167th glutamate by lysine (c.499A>G; p.Glu167Lys). The Glu167 of TM6SF2 is highly conserved among mammals. This variant is more common in people of European-American (7.2%) than of Hispanic (4.7%) or African-American (3.4%) descent. The investigators determined the association of the TM6SF2 variant with elevations in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) as a surrogate for NASH through association studies using 3 cohorts (DHS, the Dallas Biobank and the Copenhagen study). The TM6SF2 variant encoding p.Glu167Lys correlated with the increases in serum ALT and AST levels and the decreases in plasma levels of TG and low-density lipoprotein (LDL) cholesterols, but there was no association with the levels of plasma high-density lipoprotein (HDL) cholesterols. The elevated hepatic TG content together with the reduced plasma TG and cholesterol levels suggest TM6SF2 variant to be associated with the secretion of the very-low-density lipoprotein (VLDL). The researchers then performed functional analysis for the TM6SF2 in mouse liver by silencing Tm6sf2 using adeno-associated viral vector that selectively infects the liver. Silencing Tm6sf2 in the liver caused 3-fold increases in hepatic TG levels and decreases in plasma levels of TG, both LDL and HDL cholesterols and TG content of VLDL. Consistently, the VLDL secretion rates were suppressed in Tm6sf2-silenced mice, whereas ALT levels were unchanged. The investigators also tested the effects of a high sucrose diet that enhances hepatic TG synthesis. TG and cholesterols were increased in the liver and decreased in plasma in Tm6sf2-silenced mice fed a high sucrose diet. Their results demonstrated that TM6SF2 regulates hepatic TG secretion; thus, functional impairment of TM6SF2 would increase hepatic TG content, thereby promoting NAFLD. The second study reported by Mahdessian et al used expression quantitative trait locus analysis and showed that the rs10401969 SNP was associated with plasma lipid levels. Using 206 human liver biopsy specimens, the authors reported that the rs10401969 SNP was associated with decreased hepatic mRNA levels of TM6SF2. Furthermore, they found that a positive correlation between hepatic TM6SF2 mRNA and plasma TG levels, but did not find any correlation between hepatic TM6SF2 and plasma LDL and HDL cholesterols. The study then examined the subcellular localization and function of TM6SF2. TM6SF2 is mainly localized in the endoplasmic reticulum and endoplasmic reticulum–Golgi intermediate compartment in human hepatoma cells. The TM6SF2 silencing in hepatoma cell lines reduced the expression of TG synthesis-related genes (ACSS2, DGAT1, DGAT2) and the secretion of TG-rich lipoprotein. The TM6SF2 silencing increased the size and the number of lipid droplets whereas overexpression of full-length TM6SF2 showed a decrease in number and size of lipid droplets. Consistent with the first study (Nat Genet 2014;46:352–356), the current study also demonstrates TM6SF2 to regulate hepatic lipoprotein secretion. Moreover, TM6SF2 was found to influence hepatic TG content through gene regulation of TG synthesis. The third study reported by Liu et al analyzed the relationship between the TM6SF2 rs58542926 SNP and NASH-associated fibrosis/cirrhosis and HCC. Utilizing a (n = 349) “discovery” and (n = 725) “validation” cohort of patients with biopsy-proven NAFLD, the authors found that the TM6SF2 rs58542926 SNP was associated with both the severity of NASH (necroinflammation and ballooning hepatocytes) and advanced liver fibrosis (advanced fibrosis [F2–F4] versus mild fibrosis [F0–F1]) in patients with NAFLD. Although combining the 2 cohorts showed that homozygous individuals have an increased incidence of NAFLD-related HCC, these findings were no longer significant after the adjustment of age, gender, BMI, and type 2 diabetes mellitus to the regression models. Taken together, these 3 studies provide evidence that the TM6SF2 variant is associated with the development of NAFLD/NASH through regulation of lipid metabolism.