Genetic polymorphisms in non-alcoholic fatty liver disease: Clues to pathogenesis and disease progression

Abstract

The spectrum of non-alcoholic fatty liver disease (NAFLD) ranges from simple steatosis through steatohepatitis to advanced fibrosis and cirrhosis. Although the reason why only a minority of patients develop progressive forms of disease still remains largely unclear, recent research has identified genetic factors as a possible basis for this variation in disease presentation. Most of the studies have been focused on finding associations between advanced disease forms and selected single nucleotide polymorphisms in genes encoding various proteins involved in disease pathogenesis. Although there are many limitations regarding the study design and interpretation of published data, further carefully planned studies together with implementation of new genetic technologies will likely bring new insights into disease pathogenesis and potential benefits to the management of patients with NAFLD.

Key Words: Genetics; Liver fibrosis; Non-alcoholic fatty liver disease; Non-alcoholic steatohepatitis; Single nucleotide polymorphisms

INTRODUCTION

Non-alcoholic fatty liver disease (NAFLD) has emerged as the most common form of chronic liver disease. The spectrum of NAFLD ranges from simple steatosis through steatohepatitis (NASH) to advanced fibrosis and cirrhosis, and the minority of patients progress to end-stage liver disease requiring liver transplantation or develop hepatocellular carcinoma[1]. However, the vast majority of patients only have simple steatosis with a benign long-term prognosis. It has been observed that even when considering patients with similar environmental and metabolic NAFLD risk factors (diet, exercise, obesity and insulin resistance being the most important factors), they still differ largely in terms of disease phenotype and degree of progression[2]. This led to the research focus more recently being placed on genetic factors that may possibly have a role in NAFLD etiology, and genetic variability is now implied to be one of the most important determinants of disease phenotype and progression in individual patients.

GENETIC INFLUENCES IN NAFLD

Possible genetic risk for advanced NAFLD was initially suggested in studies which showed coexistence of NASH and/or cryptogenic cirrhosis within several kindreds, and it was not invariably associated with similar major metabolic risk factors[3,4]. Further evidence comes from reports of ethnic differences in the prevalence of steatosis, NASH and cryptogenic cirrhosis. The prevalence of all forms of NAFLD was shown to be highest in Hispanic and lowest in African American populations, and this variability did not always correlate with differences in the prevalence of major risk factors[5-7]. Furthermore, it was reported that Asian patients with NAFLD had a significantly lower body mass index (BMI) than all other racial groups[8].

As most of the common diseases today, NAFLD is considered to be a genetically complex disorder. In complex diseases, several or many different genes interact with environmental factors in determining disease presence or its phenotype, and individual genes only have a small effect on disease risk and can therefore be very difficult to identify. Methods for detecting genes in complex disorders have included family-based linkage studies, hypothesis-based candidate gene allele association studies, genome-wide single nucleotide polymorphism (SNP) scanning and, recently, microarray and proteomic studies. Almost all of the data available on genes associated with NAFLD has so far come from the candidate gene association studies, where candidate genes are usually selected on the basis of their suggested role in disease pathogenesis, and the frequency of one or more known SNPs within or close to those genes is compared in cases and controls, in the search for a positive or negative association with the disease. Genes that are candidates for study in NAFLD have included genes influencing insulin resistance, fatty acid metabolism, oxidative stress, immune regulation and fibrosis development.

GENETIC POLYMORPHISMS

Peroxisome proliferator-activated receptor γ coactivator 1α (PPARGC1A)

PPARGC1A has been involved with different metabolic pathways, such as regulation of gene expression in glucose and lipid metabolism and transcriptional control of cellular metabolism, mainly through control of mitochondrial function and biogenesis[9,10]. Several studies have shown that PPARGC1A regulates several key hepatic gluconeogenic genes, is directly involved in the homeostatic control of systemic energy metabolism, and PPARGC1A Gly482Ser polymorphism has also been associated with the development of insulin resistance, obesity and diabetes[11-14]. PPARGC1A knockout mice are prone to develop hepatic steatosis due to a combination of reduced mitochondrial respiratory capacity and an increased expression of lipogenic genes[15]. Yoneda et al[16] therefore examined 15 SNPs in the PPARGC1A gene and found that the rs2290602 polymorphism was significantly associated with NAFLD (more closely with NASH than with simple steatosis), and the frequency of the T allele (allele with rs2290602 polymorphism) was significantly higher in the NASH patients than in the control subjects. They also found that intrahepatic PPARGC1A mRNA expression was significantly lower in the TT genotype group than in the GG or GT group. On the other hand, Hui et al[17] did not find any association between the Gly482Ser variant and NAFLD in Chinese Han people. However, they have reported a correlation between C161T PPAR-γ gene SNP, consequent lower plasma levels of adiponectin and increased susceptibility to NAFLD.

Microsomal triglyceride transfer protein (MTTP)

A higher incidence of -493G/T polymorphism in the MTTP gene promoter has been reported in patients with NAFLD; GG homozygosity was associated with more severe liver histology and has been considered as a risk factor for NAFLD[18]. Gambino et al[19] suggested that NASH patients with GG homozygosity have more atherogenic postprandial lipoprotein profiles and lipoprotein metabolism, which leads to increased peroxidative liver injury.

Leptin

Leptin is an adipocytokine whose main role is regulation of food intake. It probably has an important role in the pathogenesis of NAFLD; leptin-deficient ob/ob mice develop steatohepatitis when fed with a methionine-choline-deficient diet[20]. Common variants in the human leptin receptor (LEPR) gene have been associated with traits of metabolic syndrome such as obesity, insulin resistance, type 2 diabetes mellitus and altered lipid metabolism, and possibly with NAFLD[21-23]. The LEPR 3057 variant may link obesity to NAFLD in Chinese patients with type 2 diabetes mellitus through interference with leptin receptor signaling and regulation of lipid metabolism and insulin sensitivity[24].

Adiponectin

Adiponectin, an adipocyte-derived cytokine has an important role in mobilization, transport and muscle oxidation of free fatty acids leading to improvements in lipid profiles and insulin sensitivity[25,26]. High levels of tumor necrosis factor-α (TNF-α) mRNA in adipose tissue and high plasma TNF-α concentrations were detected in adiponectin-knockout mice, resulting in severe diet-induced insulin resistance[27]. Musso et al[28] reported that the adiponectin SNPs 45TT and 276GT/TT were more prevalent in Italian NAFLD patients than in the general population; these polymorphisms independently predicted the severity of liver disease in NASH and exhibited a blunted postprandial adiponectin response and higher postprandial triglyceride levels.

Hepatic lipase

Zhan et al[29] investigated the prevalence of the hepatic lipase gene promoter polymorphism at position -514 in Chinese patients with NAFLD. They reported a higher frequency of the CC genotype and C allele in the NAFLD group and both the CC genotype and CT genotypes were associated with higher relative risk for development of NAFLD[29].

Phosphatidylethanolamine N-methyltransferase (PEMT)

Phosphatidylcholine is required for hepatic formation and secretion of very low density lipoproteins, and it has been shown that a choline-deficient diet leads to accumulation of fat droplets in hepatocyte cytosol and the development of fatty liver[30]. PEMT catalyzes de novo synthesis of phosphatidylcholine and is responsible for approximately 30% of phosphatidylcholine formed in liver, the rest of it being synthesized by another pathway from dietary choline. Song et al[31] showed that SNP (G to A substitution in exon 8) that leads to Val to Met substitution at residue 175 of PEMT is associated with significantly diminished activity of the enzyme, and determined the frequency of this polymorphism in NAFLD patients and controls. The loss of function AA genotype (Met/Met) occurred significantly more frequently in NAFLD patients than in control subjects, which led to the conclusion that genetically inherited low PEMT activity is an important risk factor for developing NAFLD. This was further proven in a Japanese study published by Dong et al[32]. Although the polymorphism is much rarer in the Japanese population than in Caucasians, the frequency of A allele was significantly higher in NASH patients compared with controls. NASH patients who were carriers of the Val175Met variant had significantly lower BMI and were more frequently non-obese than NASH patients who were wild-type homozygotes, further proving the role of this polymorphism as an independent risk factor for NAFLD development.

Methylenetetrahydrofolate reductase (MTHFR)

Sazci et al[33] investigated whether the C677T and A1298C polymorphisms of the MTHFR gene which lead to hyperhomocysteinemia and development of liver steatosis were associated with NASH. They found that the MTHFR 1298C allele was associated with increased risk for NASH in patients of both genders, C1298C genotype and C677C/C1298C compound genotype in female and C677C/A1298C compound genotype in male NASH patients.

TNF-α

TNF-α has long been proven to be one of the key cytokines in the development of all chronic liver diseases. In NAFLD, it has been shown that it may cause hepatocyte injury and apoptosis, neutrophil chemotaxis, and hepatic stellate cell activation, as well as contribute to systemic and hepatic insulin resistance[34-36]. Crespo et al[37] found that obese patients with NASH compared to those without NASH have significantly increased liver expression of TNF-α and its receptor p55, as well as increased expression of TNF-α in adipose tissue. Valenti et al[38] investigated the relationship between insulin resistance, occurrence of NAFLD and -238 and -308 TNF-α promoter polymorphisms known to be associated with an increased release of this cytokine. The prevalence of the 238 TNF-α polymorphism was higher in subjects with NAFLD than controls, and patients with these polymorphisms had higher insulin resistance indices. Tokushige et al[39] determined the prevalence of several TNF-α promoter region polymorphisms (positions -1031, -863, -857, -308 and -238) in a group of Japanese NAFLD patients and control subjects. There were no significant differences in the allele frequencies of any of the six polymorphisms among the group of patients with NAFLD and the control group, including the -238 polymorphism which was previously reported to be associated with NAFLD in Italian patients, but this polymorphism was much less frequent in the Japanese population[38]. However, the frequency of the -1031C polymorphism was significantly higher in the NASH group compared to the simple steatosis group, as was the frequency of the -863A polymorphism. The frequency of other polymorphisms did not differ significantly between the two groups. These two polymorphisms were also associated with higher levels of insulin resistance measured by HOMA-IR.

Transforming growth factor-β1 (TGF-β1) and angiotensin II

TGF-β1 and angiotensin II are two molecules that have been extensively studied in models of liver fibrogenesis. TGF-β1 has a major role in development of liver fibrosis by activation of hepatic stellate cells and stimulation of production of extracellular matrix proteins[40]. Besides its well-known effects in the cardiovascular and renal systems, angiotensin II also has an established role in liver fibrogenesis, and based on those observations, studies with angiotensin II receptor antagonists have been performed in patients with NASH[41,42]. There have been several suggestions that profibrotic effects of angiotensin II in heart and kidney are mediated by induction of transcription of TGF-β1[43,44]. Considering these data, and based on their previous study in hepatitis C patients, Dixon et al[45] investigated the relationship between the presence of advanced fibrosis and angiotensinogen G-6A polymorphism or TGF-β1 Pro25Arg polymorphism in a group of severely obese patients. There was no correlation between either high angiotensin or TGF-β1 producing genotypes alone and hepatic fibrosis. However, patients who inherited both high angiotensin and TGF-β1 producing polymorphisms had a higher risk of advanced fibrosis. These data also support the hypothesis that angiotensin II stimulated TGF-β1 production promotes hepatic fibrosis.

A comprehensive list of the above-mentioned polymorphism studies is shown in Table 1.

Table 1 Studies of genetic polymorphisms in non-alcoholic fatty liver disease (NAFLD) included.

Gene	Polymorphism	Ref.	No. of patients with NAFLD included in the study
Peroxisome proliferator-activated receptor γ coactivator 1α (PPARGC1A)	rs2290602	Yoneda et al[16], 2008	115
	Gly482Ser	Hui et al[17], 2008	96
Microsomal triglyceride transfer protein (MTTP)	-493G/T	Namikawa et al[18], 2004	63
		Gambino et al[19], 2007	29
Human leptin receptor	G3057A	Lu et al[24], 2009	104
Adiponectin	45G/T and 276G/T	Musso et al[28], 2008	70
Hepatic lipase	-514C/T	Zhan et al[29], 2008	106
Phosphatidylethanolamine N-methyltransferase (PEMT)	Val175Met	Song et al[31], 2005	28
		Dong et al[32], 2007	107
Methylenetetrahydrofolate reductase (MTHFR)	C677T and A1298C	Sazci et al[33], 2008	57
Tumor necrosis factor-α (TNF-α)	-238 and -308	Valenti et al[38], 2002	99
	-1031, -863, -857, -308 and -238	Tokushige et al[39], 2007	102
Angiotensinogen	G-6A	Dixon et al[45], 2003	105
Transforming growth factor-β1 (TGF-β1)	Pro25Arg

CONCLUSION

While all this and other evidence clearly indicates that genetic factors have a key role in determining susceptibility to advanced forms of NAFLD and its progression, the majority of studies mentioned here had small sample sizes and therefore limited statistical power, which makes it rather difficult to draw definitive conclusions. However, we believe that the development and wider availability of high throughput genetic technologies together with careful design and performance of large multicenter studies with adequate statistical power will soon provide new insights in this vast and very interesting area. Further study and new data on genetic effects have many potential benefits - advancement in understanding the pathogenesis of NAFLD, identification of new potential treatment targets, and, eventually, categorization of patients with respect to disease prognosis, leading to a change in management approach in specific subgroups of patients. Despite the currently limited data on genetic influences in NAFLD and all the difficulties in studying them, we believe that most of the variability in NAFLD presentation will eventually be attributed to and explained by variations in SNP frequencies and their effects on the function of factors involved in the pathogenesis of the disease.