The present study sought to evaluate the evidence for association between the 7 ADH
genes located on chromosome 4q and alcohol dependence as well as phenotypes associated with long-term alcohol misuse. Two SNPs, rs2066702 located in exon 9 of ADH1B
and rs3762894 located at the 5' end of ADH4
, yielded significant evidence for an association with withdrawal symptoms. For both SNPs, the minor allele (C allele of rs3762894; T allele of rs2066702) showed a protective relation to the development of withdrawal symptoms. This result was further supported by an analysis of haplotypes constructed from these two SNPs that suggested haplotypes containing either of the minor alleles was protective against alcohol withdrawal relative to the ancestral haplotype (i.e., a haplotype consisting of the major allele at both SNPs). Only nominally significant evidence of an association with alcohol dependence or the severe use phenotype was found with the tested polymorphisms. Though previous studies conducted in this population reported evidence of linkage between the region and the severe use phenotype (Ehlers et al., 2004b
) and associations between SNPs located in ADH1B
and DSM-III-R defined alcohol dependence diagnoses (Wall et al., 2003
), the present study included an expanded sample using a distinct analytic approach, which may have led to difficulties in reproducing the previous findings.
Both of the SNPs associated with withdrawal symptoms have shown previous evidence of association with alcohol dependence and related phenotypes. Specifically, rs2066702, which has been described in the literature as identifying the ADH1B*3
allele, has been associated with alcohol dependence in samples of African descent (Edenberg et al., 2006
; Ehlers et al., 2001a
; Luo et al., 2006
) and an earlier study of this Native American population that used a subset of the participants described in the present study (Wall et al., 2003
). Similarly, rs3762894 has been associated with alcohol dependence in several recent studies of ADH4
polymorphisms (Edenberg et al., 2006
; MacGregor et al., 2009
). Both SNPs have also been shown to affect the kinetic properties of ADH with the minor alleles of each SNP producing more active ADH isozymes than the major alleles (Birley et al., 2009
; Edenberg, 2007
; Thomasson et al., 1995
). It has been suggested that these more active isozymes lead to a more rapid buildup of acetaldehyde, thus leading to a stronger response to alcohol and possibly more severe withdrawal symptoms. Nonetheless, there have been negative results reported for these SNPs (e.g., Kuo et al., 2008
), thus posing the question of why the associations have been observed in some studies but not others.
Genetically complex disorders like alcohol dependence are likely influenced by a number of genes each exerting only a small effect on the broad clinical phenotype (Lander and Schork, 1994
). These small effect sizes can complicate the search for susceptibility loci given that normal sampling variability will produce both positive and negative results when studies are insufficiently powered due to small sample sizes as is frequently the case. Nonetheless, such genes might be detected if they have a larger effect on a more narrowly defined phenotype. For example, withdrawal symptoms, the presence of which can indicate alcohol dependence with a "physiological component" as defined by DSM-IV (American Psychiatric Association 1994
), appear to have particular clinical relevance and may identify an important subpopulation of alcohol dependent individuals with a more severe clinical course (Langenbucher et al., 2000
; Schuckit et al., 1998
). It is possible that genes involved in alcohol metabolism may play an important role in the etiology of the disorder within this subpopulation relative to individuals diagnosed with alcohol dependence without a "physiological component." The results of the present study as well as those of a previous linkage scan for alcohol dependence and related phenotypes (Ehlers et al., 2004b
) are consistent with these conclusions in demonstrating that genetic linkage and association can be detected for withdrawal symptoms even when such associations are not observed for the broader alcohol dependence diagnosis.
The results of the present study also provide evidence that the observed associations between the ADH4 and ADH1B SNPs and alcohol dependence phenotypes previously reported in African, Asian, and Caucasian populations can be extended to Native American populations. This is particularly relevant for Native American populations given that the increased prevalence of alcohol dependence among Native Americans relative to Caucasians has unfortunately resulted in many negative stereotypes of Native Americans. Among these is the common belief that Native Americans may be more genetically susceptible to developing alcohol dependence due to unique differences in the metabolism of alcohol. The results of the present study suggest that in relation to the ADH genes located on chromosome 4q, the evidence for association is consistent with that observed in other ethnic groups, thus casting doubt on such theories.
The results of the present study should be interpreted in the context of several limitations. First, the findings may not generalize to other Native Americans or represent all Native Americans in this population. Second, the study gathered clinical data using retrospective methods; therefore, more information is needed using longitudinal techniques. Third, the association between ADH variants and alcohol-related phenotypes in this Native American population may not extend to other large population samples due to differences in genetic and environmental variables. Despite these limitations, this report represents an important step in an ongoing investigation to determine risk and protective factors associated with the development of substance use disorders in this high risk and understudied ethnic group.
In summary, the present study examined evidence for association between the cluster of ADH genes located on chromosome 4q and alcohol dependence and related phenotypes. Only nominal evidence for association between SNPs in these gene and alcohol dependence was observed, but the presence of withdrawal symptoms was significantly related to a SNP at the 5' end of ADH4 and a coding SNP in exon 9 of ADH1B with a known functional impact on ADH activity. Evidence of association between these two genes and alcohol misuse phenotypes has been reported in previous studies of ethnic groups including Caucasians, individuals of African descent, and Native Americans.