Although tribes differ with regard to the use of alcohol and drugs, the U.S. Indian Health Service has cited alcohol, tobacco, and drug dependence as one of the most urgent health problems facing Native Americans (
1). Large-scale U.S. epidemiological studies demonstrate that compared with other U.S. ethnic groups, Native Americans have the highest rates of alcohol and other drug dependence (
2), and Native American adolescents have been reported to have the highest rates of substance use and substance-related disorders (
3). Lifetime rates of alcohol dependence in the small number of individual tribal groups studied have been reported to be in the range of 20%–70% (
4–
6), which is higher than the epidemiological rate of DSM-IV alcohol dependence of 13% in the U.S. general population (
7). The causes for higher rates of alcohol and drug dependence among Native Americans are thought to have both environmental and genetic determinants.
In this study, we present a review of the findings supporting a substantial genetic component contributing to the development of substance dependence in Native Americans. Such findings have the potential to yield important insights into the genetics of substance dependence given that genetic epidemiology studies conducted in well-defined populations, such as Native American tribes, can be particularly informative in view of the relative environmental and genetic homogeneity of some of these populations compared with larger, more stratified general population samples.
Candidate Gene Studies for Alcohol and Other Drug Dependence in Native Americans
The genes involved in alcohol metabolism represent obvious candidate genes for alcohol use disorders and thus have been the focus of much research in a number of different ethnic populations. The seven alcohol dehydrogenase genes (
ADH7,
ADH1C,
ADH1B,
ADH1A,
ADH6,
ADH4, and
ADH5) are located in a single cluster on chromosome 4q21–24, with each gene coding for a unique isozyme. The relationship between this chromosomal region and alcohol dependence has been reported in a number of linkage studies of diverse ethnic groups, including Native Americans (
20), and association studies have produced replicable evidence of association between polymorphisms in these genes and alcohol-related phenotypes in Native Americans. For example, two functional polymorphisms identified in the
ADH1B gene have been used to describe the presence of three alleles:
ADH1B*1,
ADH1B*2 (identified by rs1229984), and
ADH1B*3 (identified by rs2066702). The
ADH1B*2 and
ADH1B*3 alleles have demonstrated a protective relation with alcohol dependence and related phenotypes in Asian and Caucasian samples and in African American samples, respectively, and both alleles have been observed in the studied Native American populations. The
ADH1B*2 allele was observed in both the California and Southwest American Indian populations. The
ADH1B*3 allele, which has only been observed in the California Indian population, has been associated with reduced risk for alcohol dependence, reduced alcohol consumption, and reduced risk for alcohol withdrawal (
43,
44). However, studies of other Native American samples did not report the presence of the
ADH1B*3 allele (
45). Additionally, a polymorphism in the promoter region of
ADH4 (rs3762894) that has been shown to produce a more active version of the alcohol dehydrogenase enzyme (
46) has demonstrated a protective association with alcohol misuse phenotypes in multiple Caucasian and Native American populations. This polymorphism has shown evidence of association with reduced risk for alcohol withdrawal in the California Indian population (
43) and with reduced risk for alcohol dependence in the Southwest American and Plains Indian populations (
45). Although an initial study (
47) suggested a relationship between a functional polymorphism in
ADH1C (rs698), subsequent studies have not detected a relationship between alcohol misuse phenotypes and
ADH1C polymorphisms (
43,
45), including a proline-theonin substitution in codon 351 of
ADH1C (rs35719513) that has been observed almost exclusively in Native American populations (
48).
The two aldehyde dehydrogenase genes involved in alcohol metabolism are
ALDH1A, located on chromosome 9q21.13, and
ALDH2, located on chromosome 12q24.2. The ALDH2 enzyme is the primary enzyme responsible for acetaldehyde metabolism, and a mutation in
ALDH2 (commonly referred to as the
ALDH2*2 allele) produces a largely inactive aldehyde dehydrogenase enzyme that leads to elevated acetaldehyde levels when alcohol is consumed. The
ALDH2*2 allele has been shown to produce an aversive flushing reaction and an increased level of response to alcohol that is associated with lower rates of alcohol use and alcoholism in Japanese and Chinese samples, demonstrating its protective effect against the development of alcoholism (
46). Nonetheless, this allele does not appear to be present in Native American populations (
49).
The
ALDH1A1 gene appears to play a lesser role in acetaldehyde metabolism relative to
ALDH2, but a growing number of studies suggest that this gene contains one or more polymorphisms that influence alcohol-related phenotypes in Native American populations. One of the earliest reported results involved a 17-base-pair deletion in the promoter region of
ALDH1A1, commonly referred to as the
ALDH1A1*2 allele. Similar to reports of
ALDH2 in Far East Asian populations, this allele has been associated with a reduced risk of alcohol dependence, reduced alcohol consumption, and reduced risk of cigarette smoking (
50). Additional polymorphisms in
ALDH1A1 have shown relations with alcohol dependence in the Southwest American and Plains Indian populations (
45), as well as in Caucasian populations (
51). Thus,
ALDH1A1 represents an interesting candidate gene with respect to alcoholism in several populations, including Native Americans.
Investigations of candidate genes other than those coding for alcohol metabolizing enzymes in Native American populations have thus far included genes involved in drug-reward pathways, serotonergic genes, and positional candidates based on previous linkage studies. For example, polymorphisms in the
CNR1 gene, which encodes for the cannabinoid receptor type 1, have been related to a number of alcohol, cannabis, and other substance use phenotypes in multiple populations, including the COGA sample (
52). In the California Indian sample,
CNR1 polymorphisms were associated with a trait measure of impulsivity (
53). Trait impulsivity is hypothesized to underlie the lack of behavioral control associated with substance use disorders, and thus this finding suggests that
CNR1 may act as a general risk factor for alcohol and drug misuse.
In the Southwest American and Plains Indian populations, associations between alcohol-related phenotypes and polymorphisms in several GABA receptor genes have been tested. A region of chromosome 4p containing
GABRA2,
GABRB1, and the GABA
G1 receptor gene (
GABRG1) has been identified as a susceptibility locus in previous linkage scans of alcohol dependence and quantitative EEG traits in the COGA sample (
34). Additionally, polymorphisms in
GABRA2 and
GABRG1 have shown evidence of association with alcohol use phenotypes (
40). In the Plains Indian sample,
GABRA2 and
GABRG1 polymorphisms have yielded evidence of association with alcohol use diagnoses (
54,
55). In the Southwest American Indian sample, a second GABA receptor gene cluster located on chromosome 5q34, containing the GABA
1A (
GABRA1), GABA
6A (
GABRA6), GABA
B2 (
GABRB2), and GABA
G2 (
GABRG2) receptor genes, has also yielded evidence of association with alcohol dependence, with evidence suggesting that the association is due to a causal variant in
GABRA6 (
56). Studies of other ethnic groups, including Caucasian (
57) and Asian (
58) populations, have reported similar associations with
GABRA6 polymorphisms.
Other candidate genes that have shown evidence of association with alcohol and other drug misuse phenotypes in Native Americans include the
OPRM1 gene, which encodes for the mu opioid receptor and is the primary site of action for opioids such as morphine and heroin (
59), the serotonin 1B receptor gene (
HTR1B) (
60), the catechol
O-methyltransferase gene (
COMT), which encodes for an enzyme involved in synaptic dopamine metabolism, and the alpha-synuclein gene (
SNCA), which encodes for a protein involved in dopamine neurotransmission. These and additional candidate gene studies of alcohol and other substance misuse phenotypes in Native American samples are summarized in
Table 2, which highlights that with a few exceptions, these genes have been investigated in only a single study. Additionally, each of these genes has been studied in relation to alcohol and drug-related phenotypes in other ethnic groups, yielding a mix of both positive and negative results (
21,
61). Thus, given the low replication rate that has been noted for candidate gene studies of complex traits in general and for the relationships between the described candidate genes and alcohol and substance use phenotypes specifically, it is important to note the preliminary nature of these findings and the need for additional studies in larger Native American samples.
Candidate gene studies of some substance-related endophenotypes have also been conducted. For example, a study of the Plains Indian sample suggested that EEG alpha power, which was related to comorbid alcohol dependence and antisocial personality disorder, demonstrated a significant relation with polymorphisms in the serotonin 3B receptor gene (
HTR3B) (
62). EEG alpha power also demonstrated a relation with the same
COMT polymorphism that was associated with alcoholism and smoking among women in the Plains Indian sample (
63). Thus, these studies provide further support for the use of EEG measures as endophenotypes for alcohol and other substance use disorders.
A final area of study to be discussed in the context of candidate gene studies is gene-environment interaction studies. Several gene-environment interaction studies of substance use disorders have been conducted using Caucasian samples, but only one such study has been conducted in a Native American population. That study investigated whether a relationship between a functional polymorphism in the monoamine oxidase A gene (
MAOA) and alcoholism and antisocial personality disorder was moderated by childhood sexual abuse in the Southwest American Indian population (
36). The
MAOA gene has been previously implicated in antisocial personality disorder, and in one of the first gene-environment interaction studies conducted, the relationship between
MAOA and antisocial behavior was moderated by childhood maltreatment such that individuals possessing the high-risk genotype and were abused in childhood were more likely to exhibit antisocial behavior later in life compared with individuals in the other groups (
64). A similar interaction was observed in women from the Southwest American Indian population, in which those with the high-risk genotype were more likely to develop comorbid alcohol dependence and antisocial personality disorder but only if they were exposed to childhood sexual abuse. Although preliminary, that study highlights the potential effect of gene-by-environment interaction studies.
Discussion
We began this review with a summary of quantitative genetic studies establishing the heritability of substance misuse diagnoses, as well as the increased heritability of a more severe form of alcohol and drug dependence characterized by symptoms of increased tolerance and withdrawal. We then presented evidence from linkage analyses and candidate gene studies suggesting relationships between specific genes and genomic regions and substance use diagnoses. The reviewed linkage analyses suggest that the genes influencing risk for substance dependence and related phenotypes, such as BMI, drug sensitivity or tolerance, EEG patterns, and antisocial personality traits, are many and reside on several chromosomal regions (
Table 1). It appears that these regions are not unique to Native Americans, since similar findings have been reported in studies of other ethnic (primarily Caucasian) groups. Some overlap in the gene locations for substance dependence and BMI has been found, suggesting the possibility of a common genetic substrate for disorders of consumption.
Our review of candidate gene studies revealed a number of polymorphisms that have been found to be associated with substance dependence phenotypes in Native Americans. The strongest results were reported from studies investigating the genes that code for alcohol metabolizing enzymes, including variants in
ADH1B (rs1229984 and rs2066702) and
ADH4 (rs3762849). Notably, these results were not always consistent across tribal groups. Some of these differences may be the result of population differences, such as the observed association of the
ADH1B*3 allele (rs2066702) in the California Indian sample and the absence of this allele in the other tribal groups examined. Others, such as the
ADH1B*2 allele (rs1229984) and rs3762849 in
ADH4, were more likely the result of inadequate sample sizes given that the direction of effect was consistent across studies. Larger studies including a greater number of tribal groups are needed to definitively test these conclusions. Thus far, the reviewed studies suggest that some Native American tribes appear to lack protective variants in alcohol metabolizing enzyme genes (the
ALDH2*2 and
ADH1B*3 alleles) that are seen in East Asian and some African populations, but they provide little overall support for the theory that Native American groups have an “unusual” metabolism of alcohol. Additional genes that code for neurotransmitter receptors and neuromodulators, including
OPRM1,
CRN1,
COMT,
GABRA2,
MAOA, and
HTR3-B, have shown preliminary evidence for association with substance use phenotypes in some tribal groups (
Table 2). However, these associations have also been reported for other ethnic groups and also provide little evidence to support a genetic association specific to a Native American tribal group or to the Native American population as a whole.
Taken together, the results of genetics studies conducted to date suggest that the genetic influences contributing to substance use, abuse, and dependence in Native American populations are likely similar in kind and in magnitude to the genetic influences contributing to the liability for these phenotypes in other ethnic groups. One previous study demonstrated that a correlation exists between degree of Native American ancestry and substance dependence phenotypes (
20), but it remains to be seen whether this relationship is due to genetic or environmental factors. Nonetheless, this is an important issue deserving of further study because genetic methodology has demonstrated an advantage in examining Native populations, even when recent admixture between the population isolate and outside populations have occurred, if the phenotype of interest is correlated with degree of ancestry from the population isolate (
65). It is likely that more advanced genetic techniques, such as genome-wide association studies, sequencing strategies, and investigations of copy number variation, combined with admixture analyses, will shed further light on this issue. Additionally, a number of environmental factors could be targeted to potentially reduce rates of substance dependence. These include general economic and educational conditions, personal and historical trauma (
9,
10), and early age at onset of drinking (
11), as well as lack of contingency between access to basic life reinforcers (e.g., employment, housing, education, and health care) and sobriety (
12). Interventions that address underage drinking, such as motivational interviewing (
66), as well as tribal agreements to address social norms concerning drug and alcohol use and associated trauma, have the potential to substantially reduce substance use in these populations. Additional studies of the genetics of substance abuse in Native Americans are recommended, especially when key environmental variables are accounted for and gene-environment interplay can be assessed.