Alcohol dehydrogenase polymorphisms and predisposition to alcoholic cirrhosis

Overview of the role of alcohol dehydrogenase and aldehyde dehydrogenase and their variants in the genesis of alcohol-related pathology

Alcohol dehydrogenase (ADH) and mitochondrial aldehyde dehydrogenase (ALDH2) are responsible for metabolizing the bulk of ethanol consumed as part of the diet and their activities contribute to the rate of ethanol elimination from the blood. They are expressed at highest levels in liver, but at lower levels in many tissues. This pathway probably evolved as a detoxification mechanism for environmental alcohols. However, with the consumption of large amounts of ethanol, the oxidation of ethanol can become a major energy source and, particularly in the liver, interferes with the metabolism of other nutrients. Polymorphic variants of the genes for these enzymes encode enzymes with altered kinetic properties. The pathophysiological effects of these variants may be mediated by accumulation of acetaldehyde; high-activity ADH variants are predicted to increase the rate of acetaldehyde generation, while the low-activity ALDH2 variant is associated with an inability to metabolize this compound. The effects of acetaldehyde may be expressed either in the cells generating it, or by delivery of acetaldehyde to various tissues by the bloodstream or even saliva. Inheritance of the high-activity ADH β2, encoded by theADH2*2gene, and the inactiveALDH2*2gene product have been conclusively associated with reduced risk of alcoholism. This association is influenced by gene–environment interactions, such as religion and national origin. The variants have also been studied for association with alcoholic liver disease, cancer, fetal alcohol syndrome, CVD, gout, asthma and clearance of xenobiotics. The strongest correlations found to date have been those between theALDH2*2allele and cancers of the oro-pharynx and oesophagus. It will be important to replicate other interesting associations between these variants and other cancers and heart disease, and to determine the biochemical mechanisms underlying the associations.

CD14 promoter polymorphism in Chinese alcoholic patients with cirrhosis of liver and acute pancreatitis

AIM: To investigate the relationship between genetic polymorphism of the CD14 promoter and the occurrence of alcoholic cirrhosis and alcoholic pancreatitis, and to challenge the conclusion made earlier that the patients with acute alcoholic pancreatitis and patients with alcoholic cirrhosis of liver are two different subpopulations.

METHODS: Using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method, we determined the polymorphism of CD14 gene and aldehyde dehydrogenase gene 2 (ALDH 2) in 335 alcoholic patients with different organ complications i.e., cirrhosis of liver (n = 100), acute pancreatitis (n = 100), esophageal cancer (n = 82) and avascular necrosis of hip joint (AVN) (n = 53) and 194 non-alcoholic controls in a Chinese group.

RESULTS: The results showed that the carriage of T allele was not different among alcoholic patients with cirrhosis of liver, alcoholic patients with other complication and non-alcoholic controls. On the other hand, the carriage of the C allele was significantly more prevalent for alcoholic pancreatitis than for esophageal cancer (0.79 vs 0.60, P<0.001), alcoholic AVN (0.79vs 0.65, P<0.025) and non-alcoholic controls (0.79 vs 0.68, P<0.025). Furthermore, when only subjects with ALDH2 1-1 genotype were examined, the C allele frequency was significantly more prevalent for alcoholic pancreatitis than for alcoholic liver cirrhosis (0.82 vs 0.69, P<0.025), esophageal cancer (0.82 vs 0.61, P<0.01), alcoholic AVN (0.82 vs 0.64, P<0.01) and non-alcoholic controls (0.82 vs 0.69, P<0.05).

CONCLUSION: The C allele may be associated with some mechanism, which is important in the pathogenesis of alcoholic pancreatitis, and that alcoholic patients with acute pancreatitis and cirrhosis of liver are probably two different subpopulations.

Are genetic polymorphisms of tumour necrosis factor alpha, interleukin-10, CD14 endotoxin receptor or manganese superoxide dismutase associated with alcoholic liver disease?

BACKGROUND

Four polymorphisms have been described associated with either increased risk for alcoholic liver disease (ALD) or more serious histological lesions: tumour necrosis factor alpha (TNF-alpha) G(-238)A, interleukin-10 (IL-10) C(-627)A, promoter of CD14 endotoxin receptor gene C(-159)T and manganese superoxide dismutase (MnSOD) C(-1183)T/valine --> alanine.

METHODS

We sought confirmatory evidence, through individual and simultaneous analysis of the four aforementioned polymorphisms, in 176 heavy drinkers: ALD (n = 100) if histology-compatible or clinical evidence of hepatic decompensation; and no evidence of liver disease (NLD) (n = 76) if normal liver tests on two occasions or normal liver histology (steatosis alone).

RESULTS

Patients with ALD were older (53+/-10 vs. 48+/-10 years, P<0.05), with a similar sex distribution. TNF-alpha G(-238)A showed no difference in heterozygous GA-genotype prevalence (ALD, 9.0%/NLD, 7.9%). IL-10 C(-627)A showed no difference between groups, either homozygote AA (8.0% vs. 10.5%) or heterozygote CA (34.0% vs. 39.5%). CD14 promoter C(-159)T showed no difference between groups in T-allele frequency, either homozygote TT (27% vs. 21%) or heterozygote CT (49% vs. 50%). Alanine MnSOD allele carriers showed no difference between groups in either the heterozygote (55.0% vs. 49.3%) or homozygote (22% vs. 25%). No difference was observed in the probability of having simultaneously two, three or four of the implicated polymorphisms: respectively, 43%, 33% and 0% in ALD, and 43%, 24% and 5% in NLD (not significant).

CONCLUSIONS

No association was found between the previously implicated polymorphisms of TNF-alpha, IL-10, CD14 and MnSOD, either individually or simultaneously, and the presence of established ALD.

The epidemiology of fatty liver

There are insufficient data available on the epidemiology of fatty liver to design a complete and correct view of the prevalence, incidence and natural history of this disorder. This article, mainly based on the revision of recently published papers in this field, attempts to give an overview of the current findings on the epidemiology of fatty liver worldwide. The possible factors involved in the development of fat accumulation in the liver, and their potential role in the progression of the disorder will be also addressed.

Risk factors for alcoholic liver disease

Abstract Alcoholic liver disease (ALD) is still a frequent disorder, even though its incidence appears to be decreasing. In spite of intense investigation, the precise mechanisms leading to ALD are still imprecisely known. This is due in part to the lack of a reliable animal model; in part to the difficulty of obtaining clinical data of adequate sample size and derived from unblased populations and finally from the lack of uniformity of the criteria used to define ALD. This paper will review what is known of the various pieces of this puzzle, with particular emphasis not only on the total amount of alcohol consumed, but also on drinking patterns and type of alcoholic beverage ingested. The other potential factors such as age, gender, genetic background, nutritional status, occupational hazards and viral diseases (especially HCV infection) will be touched upon.

Regulation of the mammalian alcohol dehydrogenase genes

This review focuses on the regulation of the mammalian medium-chain alcohol dehydrogenase (ADH) genes. This family of genes encodes enzymes involved in the reversible oxidation of alcohols to aldehydes. Interest in these enzymes is increased because of their role in the metabolism of beverage alcohol as well as retinol, and their influence on the risk for alcoholism. There are six known classes ADH genes that evolved from a common ancestor. ADH genes differ in their patterns of expression: most are expressed in overlapping tissue-specific patterns, but class III ADH genes are expressed ubiquitously. All have proximal promoters with multiple cis-acting elements. These elements, and the transcription factors that can interact with them, are being defined. Subtle differences in sequence can affect affinity for these factors, and thereby influence the expression of the genes. This provides an interesting system in which to examine the evolution of tissue specificity. Among transcription factors that are important in multiple members of this gene family are the C/EBPs, Sp1,USF, and AP1, HNF-1, CTF/NF-1, glucocorticoid, and retinoic acid receptors, and several as-yet unidentified negative elements, are important in at least one of the genes. There is evidence that cis-acting elements located far from the proximal promoter are necessary for proper expression. Three of the genes have upstream AUGs in the 5' nontranslated regions of their mRNA, unusual for mammalian genes. The upstream AUGs have been shown to significantly affect expression of the human ADH5 gene.

The inheritance of alcoholic liver disease

Available evidence supports the concept that alcoholic liver disease (ALD) is a multifactorial disease with a heritable component. A number of polymorphic genes with small and additive effects will thus encode susceptibilty to this 'polygenic' disease. Molecular genetic studies of ALD are in their infancy, and methods available for the genetic dissection of complex traits are discussed. Some candidate genes have been identified, and studies have been undertaken to test for association between specific alleles and ALD susceptibility. There is evidence to support a role for alleles of two genes encoding enzymes involved in the oxidative metabolism of alcohol (acetaldehyde dehydrogenase2*2 and cytochrome P4502E1 c2 allele) in susceptibility to ALD. More recently, attention has focused on cytokines, and there are now data showing association of specific alleles of both tumour necrosis factor alpha and interleukin-10 with predisposition to ALD. These candidate genes need to be subjected to rigorous evaluation in different populations. Such research should help to define more precisely the molecular mechanisms underlying the development of ALD in a sub-population (< 20%) of alcoholics, thereby improving the hepatologist's ability to develop rational treatments.

The influence of alcoholism and cirrhosis on benzodiazepine receptor function

In a previous study we reported that the affinity of the platelet benzodiazepine receptor was greater in alcoholic cirrhotic patients compared with normal controls and that there were detectable ligands for the neuronal benzodiazepine receptor in plasma from both alcoholic and nonalcoholic cirrhotic patients. The aim of the present study was to assess the separate contributions of alcoholism and cirrhosis to the presence of ligands in plasma for the neuronal and peripheral benzodiazepine receptors and to changes in peripheral benzodiazepine receptor binding in platelets. These parameters were measured in 10 alcoholic cirrhotics, 9 nonalcoholic cirrhotics, 7 alcoholics with a normal liver function, and 15 nonalcoholic subjects and normal liver function. Both groups of alcoholics had been abstinent for several months and the nonalcoholic groups had abstained for 24 h before the study. The concentration of ligands for the peripheral benzodiazepine receptor were significantly higher in both cirrhotic groups compared with the other two groups, suggesting that cirrhosis was responsible for this accumulation. Furthermore, the cirrhotic patients with detectable concentrations of these ligands had significantly poorer episodic memory than those without ligands. However, the presence of ligands for the peripheral benzodiazepine receptor did not correlate with the change in receptor affinity, which was increased in the alcoholic cirrhotic group compared with all other groups. Neither cirrhosis nor alcoholism altered the peripheral benzodiazepine receptor number. The cirrhotic patients with detectable ligands for the neuronal benzodiazepine receptor showed psychomotor slowing and executive dysfunction. The results suggest that the ligands for the peripheral benzodiazepine receptor may contribute to some of the cognitive deficits seen in hepatic encephalopathy, but are not responsible for the receptor affinity change seen in the alcoholic cirrhotics. This affinity change is not solely due to the effects of alcohol and could possibly serve as a marker for those at risk for developing alcoholic cirrhosis.

Genetic aspects and risk factors in alcoholism and alcoholic liver disease

There is a great deal of evidence for genetic predisposition to alcoholism; considerably less is known regarding predisposition to alcoholic liver disease. The specific genes involved in either disorder are not well understood, although the enzymes of alcohol metabolism appear to play some role. It will be interesting to determine whether genetic factors that alter the expression of these enzymes, in addition to altering the kinetics of the enzymes, could modify responses to drinking. Work in the next few years will include determination of which responses to alcohol are indeed genetically influenced in twin studies, testing additional candidate genes for alcohol-related traits in populations and families, as well as the application of genomic mapping methodologies to alcoholic pedigrees. The latter strategy will be integrated into the larger number of studies that will grow from the Human Genome Project. Animal studies with selectively bred lines of rodents that differ in voluntary alcohol consumption will lead the way to define the neuronal and behavioral substrates responsible for differences in alcohol-drinking behavior. The use of the quantitative trait locus (QTL) mapping approach in F2 intercross between two inbred strains of rodents with opposite alcohol-response characteristics and in recombinant inbred strains derived from F2 intercross already has and will continue to help identify chromosomal locations of genes relevant to voluntary alcohol consumption. Perhaps in the future selective breeding of rodents and QTL mapping strategies can also be used to determine the biology and genetics of alcohol-induced liver injury.