**3. Genetic variants affecting alcohol metabolism**

Genes encoding for alcohol metabolizing enzymes are supposed to have major influence on development of alcoholism. There are multiple ADH and ALDH enzymes encoded by different genes. Some of these genes have been reported to occur in several variants or alleles. The enzymes encoded by different alleles can differ in the rate at which they metabolize ethanol (Edenburg, 2007).

#### **3.1 Genetic variants of alcohol dehydrogenase**

Researchers have studied the genetic variants of ADH1B and ADH1C genes that result in the production of enzymes with different kinetic properties and have been implicated in the susceptibility to develop alcoholism. These genetic variants or SNPs and their effects have been widely studied in different populations and three different alleles have been reported which alter the amino acid sequence of the encoded beta subunit. ADH1B\*1 allele, (reference allele) encodes for β1 subunit that has arginine at positions 48 and 370. ADH1B\*1 is the predominant allele in most populations. ADH1B\*2 encodes for β2 subunit with histidine at position 48 and is commonly found in Asians. ADH1B\*3 encodes for β3 subunit that has cysteine at position 370 and is prevalent in people of African descent. In β2 and β3 subunits,

Pharmacogenetics – A Treatment Strategy for Alcoholism 237

enzyme in the liver. People with an ALDH2\*2 allele show an alcohol flush reaction even when they consume alcohol in relatively small amounts (Harada et al., 1981). The presence of even a single ALDH2\*2 allele has been shown to be strongly protective against alcohol dependence. The protective effect of ALDH2\*2 is the most widely reproduced association of a specific gene with alcoholism (Chen et al., 1999; Hurley et al., 2002; Luczak et al., 2006;

Another enzyme, microsomal ethanol oxidizing enzyme involved in alcohol metabolism is encoded by CYP2E1 gene. CYP2E1 is induced (increase in activity up to 10 fold) by chronic alcohol drinking and may contribute to development of metabolic tolerance in alcoholics. Studies have revealed that polymorphism in CYP2E1 (CYP2E1\*1D) has been found to be significantly associated with alcohol dependence in Canadian native Indians (Howard et al., 2003; Itoga et al., 1999). Another rare mutant named as c2 allele (CYP2E1\*5B) in CYP2E1 gene has been found to be associated with higher transcriptional activity leading to elevated level of the enzyme as compared to wild type c1 allele i.e. CYP2E1\*5A (Hayashi et al., 1991).

Studies have revealed that subjects with positive family history of alcoholism have a higher mean activity of catalase as compared to control subjects (Koechling and Amit et al., 1992). A significant positive correlation was observed in brain and blood catalase activity in rats (Amit and Aragon., 1988). Koechling and Amit (1992) have reported a significant coorelation between blood catalase activity with alcohol consumption. However, there are no studies on

The neuropharmacological actions of alcohol such as cognitive impairment and other behavioral changes are mediated via their interaction with brain neurotransmitters. Neurotransmitters are the chemicals involved in communication of neurons in brain and may be inhibitory or excitatory depending upon their mechanism of action. Although alcohol does not have any specific target neurotransmitter, it acts on multiple

Chronic alcohol consumption may cause cognitive impairment, tolerance and physical dependence due to changes in neurotransmitter system in brain. The neuropharmacological changes caused by chronic alcoholism involve monoamine oxidase, neurotransmitter amino acids and calcium ion channels and some other pathways leading to neuroadaptations and development of tolerance (Zaleski et al., 2004). The complex mechanism of action involving neurochemical changes explains why even moderate doses of alcohol may lead the subject to develop psychiatric complications and alcohol dependence. The addictive and alcohol seeking behavior can be explained by understanding the neurotransmitter involved in the

neurotransmitter systems (Deitrich and Erwin, 1996; Tabakoff and Hoffman, 1992).

Few of the neurotransmitters involved in alcohol dependence are as follows:

**3.3 Genetic variants of microsomal ethanol oxidizing system** 

the association of genetic polymorphism of catalase with alcoholism.

**4. Neuropharmacological aspects of chronic alcoholism** 

Thomasson et al., 1991).

**3.4 Genetic variants of catalase** 

processes (Vengeliene et al., 2006).

amino acid substitutions occur at an amino acid which contacts with coenzyme nicotineamide dinucleotide (required for ethanol oxidation) (Hurley et al., 2002). The substitution results in enzymes, which have 70- to 80- fold higher turnover rate than the β1 subunit. This is because the coenzyme is released more rapidly at the end of reaction.

For ADH1C gene, there are 3 alleles ADH1C\*1 encoding γ1 subunit with arginine at position 272 and isoleucine at position at position 350. ADH1C\*2 encodes the γ2 subunit which has glutamine (Gln) at position 272 and a valine (Val) at position 350. These two SNPs occur together (i.e., are in very high linkage disequilibrium). It has been found that the ADH with two γ1 subunits (i.e., the γ1γ1 homodimeric enzyme) has a turnover rate that is about 70 percent higher than that of the γ2γ2 enzyme (Edenberg, 2007). ADH1C\*Thr352 encodes for a subunit with threonine at position 352 and has been found in Native Americans (Osier et al., 2002). However, the studies on this protein are still lacking. Researchers have identified the differences in the rate of metabolism of ethanol in liver on the basis of difference in amino acid sequence resulting in difference in kinetic properties of encoded enzyme. If a person carries two copies of reference allele i.e. ADH1B\*1 and ADH1C\*1 alleles (homozygous for ADH1B\*1and ADH1C\*1) the enzyme (together α, β, γ subunits) together accounts for liver's 70% ethanol oxidizing capacity, additionally π ADH accounts for 30% (Hurley et al., 2002). ADH1B\*1 allele has been reported to reduce the occurrence of alcohol abuse and alcoholism in Asians, in Whites and in Jewish populations where this allele has a relatively high prevalence (Carr et al., 2002; Neumark et al., 1998). ADH1B\*2 has been found to occur in a higher frequency in nonalcoholics and in moderate drinkers relative to heavy drinkers. As far as ADH1B\*2 is concerned, this allele has been found to be associated with lower rates of heavy drinking and alcohol dependence in Native Americans (Quertemont, 2007).

A meta-analysis conducted by Whitfield has concluded that ADH1B\*1 allele is associated with a threefold increase in risk of alcoholism in comparison with ADH1B\*2 allele.ADH1B\*2 allele encodes for an enzyme with a faster ethanol oxidation rate (Whitfield, 1997). It has been assumed that this allele protects against alcoholism and alcohol abuse because of the unpleasant effects associated with acetaldehyde accumulation (Yin, 1994). The frequency of ADH1C\*1 allele has been reported to about 50% in European population and up to 90% in some Asian and African populations (Goedde et al., 1992; Osier et al., 2002). This allele has also been shown to provide a protection against alcohol abuse and alcoholism since a higher frequency of this allele has been found in nonalcoholics especially from Asian population.

Gene-gene interactions have also been found to play an intricate role in development of alcoholism. Oseir et al. (2004) found that there is potential epistatic interaction between ADH1B and ADH7 which leads to protective effect against alcoholism among Han Chinese population (Oseir et al., 2004).

#### **3.2 Genetic variants of aldehyde dehydrogenase (ALDH)**

The best known variation of alcohol metabolizing enzymes has been associated with ALDH2 gene. A variant of this gene known as ALDH2\*2 allele leads to the substitution of lysine to glutamine at 504 position (Chou et al., 1999). This substitution results in the production of a nearly inactive ALDH2 enzyme which no longer oxidizes acetaldehyde to acetate. Studies have demonstrated that this variant is dominant because people who are heterozygous (ALDH2\*1 and ALDH2\*2) have almost no detectable activity of ALDH2

amino acid substitutions occur at an amino acid which contacts with coenzyme nicotineamide dinucleotide (required for ethanol oxidation) (Hurley et al., 2002). The substitution results in enzymes, which have 70- to 80- fold higher turnover rate than the β1

For ADH1C gene, there are 3 alleles ADH1C\*1 encoding γ1 subunit with arginine at position 272 and isoleucine at position at position 350. ADH1C\*2 encodes the γ2 subunit which has glutamine (Gln) at position 272 and a valine (Val) at position 350. These two SNPs occur together (i.e., are in very high linkage disequilibrium). It has been found that the ADH with two γ1 subunits (i.e., the γ1γ1 homodimeric enzyme) has a turnover rate that is about 70 percent higher than that of the γ2γ2 enzyme (Edenberg, 2007). ADH1C\*Thr352 encodes for a subunit with threonine at position 352 and has been found in Native Americans (Osier et al., 2002). However, the studies on this protein are still lacking. Researchers have identified the differences in the rate of metabolism of ethanol in liver on the basis of difference in amino acid sequence resulting in difference in kinetic properties of encoded enzyme. If a person carries two copies of reference allele i.e. ADH1B\*1 and ADH1C\*1 alleles (homozygous for ADH1B\*1and ADH1C\*1) the enzyme (together α, β, γ subunits) together accounts for liver's 70% ethanol oxidizing capacity, additionally π ADH accounts for 30% (Hurley et al., 2002). ADH1B\*1 allele has been reported to reduce the occurrence of alcohol abuse and alcoholism in Asians, in Whites and in Jewish populations where this allele has a relatively high prevalence (Carr et al., 2002; Neumark et al., 1998). ADH1B\*2 has been found to occur in a higher frequency in nonalcoholics and in moderate drinkers relative to heavy drinkers. As far as ADH1B\*2 is concerned, this allele has been found to be associated with lower rates of heavy

subunit. This is because the coenzyme is released more rapidly at the end of reaction.

drinking and alcohol dependence in Native Americans (Quertemont, 2007).

**3.2 Genetic variants of aldehyde dehydrogenase (ALDH)** 

population (Oseir et al., 2004).

A meta-analysis conducted by Whitfield has concluded that ADH1B\*1 allele is associated with a threefold increase in risk of alcoholism in comparison with ADH1B\*2 allele.ADH1B\*2 allele encodes for an enzyme with a faster ethanol oxidation rate (Whitfield, 1997). It has been assumed that this allele protects against alcoholism and alcohol abuse because of the unpleasant effects associated with acetaldehyde accumulation (Yin, 1994). The frequency of ADH1C\*1 allele has been reported to about 50% in European population and up to 90% in some Asian and African populations (Goedde et al., 1992; Osier et al., 2002). This allele has also been shown to provide a protection against alcohol abuse and alcoholism since a higher frequency of this allele has been found in nonalcoholics especially from Asian population. Gene-gene interactions have also been found to play an intricate role in development of alcoholism. Oseir et al. (2004) found that there is potential epistatic interaction between ADH1B and ADH7 which leads to protective effect against alcoholism among Han Chinese

The best known variation of alcohol metabolizing enzymes has been associated with ALDH2 gene. A variant of this gene known as ALDH2\*2 allele leads to the substitution of lysine to glutamine at 504 position (Chou et al., 1999). This substitution results in the production of a nearly inactive ALDH2 enzyme which no longer oxidizes acetaldehyde to acetate. Studies have demonstrated that this variant is dominant because people who are heterozygous (ALDH2\*1 and ALDH2\*2) have almost no detectable activity of ALDH2 enzyme in the liver. People with an ALDH2\*2 allele show an alcohol flush reaction even when they consume alcohol in relatively small amounts (Harada et al., 1981). The presence of even a single ALDH2\*2 allele has been shown to be strongly protective against alcohol dependence. The protective effect of ALDH2\*2 is the most widely reproduced association of a specific gene with alcoholism (Chen et al., 1999; Hurley et al., 2002; Luczak et al., 2006; Thomasson et al., 1991).

#### **3.3 Genetic variants of microsomal ethanol oxidizing system**

Another enzyme, microsomal ethanol oxidizing enzyme involved in alcohol metabolism is encoded by CYP2E1 gene. CYP2E1 is induced (increase in activity up to 10 fold) by chronic alcohol drinking and may contribute to development of metabolic tolerance in alcoholics. Studies have revealed that polymorphism in CYP2E1 (CYP2E1\*1D) has been found to be significantly associated with alcohol dependence in Canadian native Indians (Howard et al., 2003; Itoga et al., 1999). Another rare mutant named as c2 allele (CYP2E1\*5B) in CYP2E1 gene has been found to be associated with higher transcriptional activity leading to elevated level of the enzyme as compared to wild type c1 allele i.e. CYP2E1\*5A (Hayashi et al., 1991).

#### **3.4 Genetic variants of catalase**

Studies have revealed that subjects with positive family history of alcoholism have a higher mean activity of catalase as compared to control subjects (Koechling and Amit et al., 1992). A significant positive correlation was observed in brain and blood catalase activity in rats (Amit and Aragon., 1988). Koechling and Amit (1992) have reported a significant coorelation between blood catalase activity with alcohol consumption. However, there are no studies on the association of genetic polymorphism of catalase with alcoholism.
