Association between a functional polymorphism at the DRD2 gene and the liability to substance abuse
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Letter to the Editor Association Between a Functional Polymorphism at the DRD2 Gene and the Liability to Substance Abuse To the Editor: In 1997, this journal published a paper by Goldman et al. presenting an analysis of a functional polymor- phism (ser/cys substitution) in the DRD2 gene in a Na- tive American population. No association was found for either substance abuse/dependence or alcohol depen- dence. Since this polymorphism causes considerable variation in the receptor function, the paper’s negative conclusion, if valid, is obviously a strong argument against any further DRD2-related substance abuse re- search. It should be noted, however, that the Native Ameri- can population where the Goldman et al. data were obtained had an extremely high frequency of alcohol dependence (155 out of 186 males and 121 out of 251 females in the sample) and substance abuse/ dependence (73 females and 97 males). The effect of the DRD2 gene would be difficult to detect in such a popu- lation if it was not a major contributor to liability varia- tion, considering the powerful background influence of other genetic and/or environmental factors that re- sulted in the high prevalence of the disorder. Indeed, the comparison of genotypic distributions presented in the paper did not reveal differences between substance abusers and nonaffected individuals ( 2 4 3.19, P 4 0.203). Since, however, it is the cys allele that impairs the DRD2 function, and this effect, if any, is expected to be most strongly expressed in the cys/cys genotype, it may be more informative to compare the frequencies of this particular genotype in affected and nonaffected sub- jects. In fact, since the hypothesized association is func- tional, i.e., mediated by the receptor phenotype (nota- bly, the alleles are named after their phenotypic ex- pressions), the actual relationship being tested is the one between the two phenotypes, receptor and liability. If the ser allele is dominant, then splitting the ser/- phenotypic class into two, according to the genotypes it contains, decreases the power of analysis. From Table III in the Goldman et al. report, the frequency of the cys/cys genotype in substance abusers is more than twice that in the nonaffected subjects (0.053 vs. 0.022). This difference is marginally significant ( 2 4 2.91; one- sided P 4 0.04; the use of one-sided criterion is justi- fied because the direction of expected difference is known). This is consistent with the association hypoth- esis and, due to the direct influence of the polymor- phism on the receptor function, with the causative re- lationship between the DRD2 gene and the risk for substance abuse in the sample studied. It is possible that the very high frequencies of the cys allele and the cys/cys genotype in this Native American population— reported to be five and 27 times higher, respectively, than in the Caucasian population—contribute to the differences in the alcoholism and SA prevalence be- tween these populations. The relationship could be more pronounced in a population with a lower frequency of the disorder. The fact that the sample (especially males) was almost uni- formly alcohol-dependent and, consequently, the se- vere truncation of the liability variance may have dra- matically diminished power to detect any contribution to the liability variation. It is thus not surprising that no association could be found for alcoholism. The asso- ciation is, however, found for substance abuse, for which the sex ratios in cases and controls did not differ and the frequency of the disorder was generally lower. One should also take into account that, at least in the substance abuse analysis, the control sample appar- ently contained alcoholics. This could further decrease the power of analysis. Furthermore, the female and male data presented in the paper were pooled, whereas the relationship may be moderated by sex (e.g., be stronger in females), as observed, for example, in the DRD5 study (Vanyukov et al., 1998). Combining males and females in the DRD2 analyses, in case of alcohol dependence, resulted in the comparison of the predomi- nantly male sample of affected individuals (155 males and 121 females) with the overwhelmingly female sample of nonaffected subjects (31 males and 130 fe- males). Separation by sex in the association analyses of substance use disorders is also necessary considering the differences in the prevalences of the disorder and in the composition of the phenotypic variance in males *Correspondence to: Michael M. Vanyukov, Ph.D., Center for Education and Drug Abuse Research, Departments of Psychiatry and Human Genetics, University of Pittsburgh, 3811 O’Hara Street, Pittsburgh, PA 15213. E-mail: [email protected] Received 9 November 1998; Accepted 9 November 1998 American Journal of Medical Genetics (Neuropsychiatric Genetics) 88:446–447 (1999) Published 1999 Wiley-Liss, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America.
Transcript of Association between a functional polymorphism at the DRD2 gene and the liability to substance abuse
Letter to the Editor
Association Between a Functional Polymorphismat the DRD2 Gene and the Liability toSubstance Abuse†
To the Editor:
In 1997, this journal published a paper by Goldmanet al. presenting an analysis of a functional polymor-phism (ser/cys substitution) in the DRD2 gene in a Na-tive American population. No association was found foreither substance abuse/dependence or alcohol depen-dence. Since this polymorphism causes considerablevariation in the receptor function, the paper’s negativeconclusion, if valid, is obviously a strong argumentagainst any further DRD2-related substance abuse re-search.
It should be noted, however, that the Native Ameri-can population where the Goldman et al. data wereobtained had an extremely high frequency of alcoholdependence (155 out of 186 males and 121 out of 251females in the sample) and substance abuse/dependence (73 females and 97 males). The effect of theDRD2 gene would be difficult to detect in such a popu-lation if it was not a major contributor to liability varia-tion, considering the powerful background influence ofother genetic and/or environmental factors that re-sulted in the high prevalence of the disorder. Indeed,the comparison of genotypic distributions presented inthe paper did not reveal differences between substanceabusers and nonaffected individuals (2 4 3.19, P 40.203).
Since, however, it is the cys allele that impairs theDRD2 function, and this effect, if any, is expected to bemost strongly expressed in the cys/cys genotype, it maybe more informative to compare the frequencies of thisparticular genotype in affected and nonaffected sub-jects. In fact, since the hypothesized association is func-tional, i.e., mediated by the receptor phenotype (nota-bly, the alleles are named after their phenotypic ex-pressions), the actual relationship being tested is theone between the two phenotypes, receptor and liability.If the ser allele is dominant, then splitting the ser/-phenotypic class into two, according to the genotypes it
contains, decreases the power of analysis. From TableIII in the Goldman et al. report, the frequency of thecys/cys genotype in substance abusers is more thantwice that in the nonaffected subjects (0.053 vs. 0.022).This difference is marginally significant (2 4 2.91; one-sided P 4 0.04; the use of one-sided criterion is justi-fied because the direction of expected difference isknown). This is consistent with the association hypoth-esis and, due to the direct influence of the polymor-phism on the receptor function, with the causative re-lationship between the DRD2 gene and the risk forsubstance abuse in the sample studied. It is possiblethat the very high frequencies of the cys allele and thecys/cys genotype in this Native American population—reported to be five and 27 times higher, respectively,than in the Caucasian population—contribute to thedifferences in the alcoholism and SA prevalence be-tween these populations.
The relationship could be more pronounced in apopulation with a lower frequency of the disorder. Thefact that the sample (especially males) was almost uni-formly alcohol-dependent and, consequently, the se-vere truncation of the liability variance may have dra-matically diminished power to detect any contributionto the liability variation. It is thus not surprising thatno association could be found for alcoholism. The asso-ciation is, however, found for substance abuse, forwhich the sex ratios in cases and controls did not differand the frequency of the disorder was generally lower.One should also take into account that, at least in thesubstance abuse analysis, the control sample appar-ently contained alcoholics. This could further decreasethe power of analysis. Furthermore, the female andmale data presented in the paper were pooled, whereasthe relationship may be moderated by sex (e.g., bestronger in females), as observed, for example, in theDRD5 study (Vanyukov et al., 1998). Combining malesand females in the DRD2 analyses, in case of alcoholdependence, resulted in the comparison of the predomi-nantly male sample of affected individuals (155 malesand 121 females) with the overwhelmingly femalesample of nonaffected subjects (31 males and 130 fe-males). Separation by sex in the association analyses ofsubstance use disorders is also necessary consideringthe differences in the prevalences of the disorder and inthe composition of the phenotypic variance in males
*Correspondence to: Michael M. Vanyukov, Ph.D., Center forEducation and Drug Abuse Research, Departments of Psychiatryand Human Genetics, University of Pittsburgh, 3811 O’HaraStreet, Pittsburgh, PA 15213. E-mail: [email protected]
Received 9 November 1998; Accepted 9 November 1998
American Journal of Medical Genetics (Neuropsychiatric Genetics) 88:446–447 (1999)
Published 1999 Wiley-Liss, Inc. †This article is a USGovernment work and, as such, is in the public domain in theUnited States of America.
and females [e.g., Pickens et al., 1991]. In summary,the data used to infer the paper’s definitive conclusionof “[n]o role for presence or homozygosity of the func-tionally impaired D2 receptor allele . . . in . . . sub-stance abuse” may, in fact, support the opposite view.
REFERENCESGoldman D, Urbanek M, Guenther D, Robin R, Long JC. 1997. Linkage
and association of a functional variant [Ser311Cys] and DRD2 markersto alcoholism, substance abuse, and schizophrenia in SouthwesternAmerican Indians. Am J Med Genet 74:386–394.
Pickens RW, Svikis DS, McGue M, Lykken DT, Heston LL, Clayton PJ.1991. Heterogeneity in the inheritance of alcoholism: a study of maleand female twins. Arch Gen Psychiat 48:19–28.
Vanyukov MM, Moss HB, Gioio AE, Hughes HB, Kaplan BB, Tarter RE.1998. An association between a microsatellite polymorphism at theDRD5 gene and the liability to substance abuse: pilot study. BehavGenet 28:75–82.
Michael M. Vanyukov*Center for Education and Drug Abuse ResearchDepartments of Psychiatry and Human GeneticsUniversity of PittsburghPittsburgh, Pennsylvania
Response
To the Editor:
Vanyukov reanalyzes our data on the relationship ofa functional DRD2 receptor polymorphism to alcohol-ism, substance abuse and schizophrenia, focusing onthe substance abuse result and on one of three DRD2markers we considered [Goldman et al., 1997]. Thatmarker is a Ser311Cys polymorphism occurring in thethird intracellular loop of the receptor—the G-proteinbinding domain. The Cys311 allele impairs the abilityof the receptor to lower intracellular cAMP levels afterit is stimulated by dopamine [Cravchik et al., 1996].The Cys311 variant is not rare in Caucasians (allelefrequency 3%) but is relatively common (allele fre-quency 16%) in a Southwestern Indian population. Theabundance of the Cys311 allele in this SouthwesternIndian tribe had encouraged us to perform sib pairlinkage and association studies in a large family-basedsample of 459 subjects (373 sib-pairs), which included15 Cys311/Cys311 homozygotes. In this psychiatricallyinterviewed sample, which was ascertained solely onthe basis of family structure rather than diagnosis,there were a substantial number of subjects with alco-holism (276) and substance abuse (170). Only five ofthe subjects had schizophrenia.
Vanyukov has grouped the Ser311Cys genotypes inone of the three possible ways in which two genotypescan be compared with a third. Under a scenario inwhich Ser311 acts dominantly, he has lumped togetherindividuals with Ser/Ser and Ser/Cys genotypes. How-ever, without evidence for a dominant action of Ser311,this post hoc arrangement of genotypes is arbitrary.For example, there would be no advantage of the [Ser/Ser + Ser/Cys] arrangement over the grouping [Ser/Cys+ Cys/Cys]. It should also be noted that at other genes(for example, the DRD3 receptor gene in schizophre-nia), investigators have lumped together both homozy-gous genotypes and compared them with the heterozy-gous genotype. No data have been reported to supporta dominant action of either the Ser311 or Cys311 allele[Cravchik et al., JBC, 1996]. The author of that reportinforms us that no supplementary biochemical data tosupport the proposed dominant action of Ser311 areavailable beyond that which was published. Specifi-cally, the Cys311 and Ser311 were never cotransfectedto create a cell line equivalent to the heterozygous Ser/Cys genotype [A. Cravchik, personal communication].
In addition to sib-pair analyses, we used model-freeassociation approaches to evaluate the effect of theDRD2 Ser311Cys polymorphism, as well as two otherDRD2 markers [Goldman et al., 1997]. For the associa-tion analyses we used a chi-square statistic with onedegree of freedom for the allele frequency comparisonand two degrees of freedom for the analysis of the dis-tribution of phenotypes across the three available geno-types. We suggest that the Ser311Cys DRD2 genotype/phenotype analyses involve two degrees of freedom, orif different genotype arrangements are to be used ar-bitrarily and post hoc, correction for the three possiblearrangements of the data.
REFERENCES
Cravchik A, Sibley DR, Gejman PV. 1996. Functional analysis of the hu-man dopamine receptor missense variants. J Biol Chem 271:26013–26017.
Goldman D, Urbanek M, Guenther D, Robin R, Long J. 1997. Linkage andassociation of a functional DRD2 variant [Ser311Cys] and DRD2 mark-ers to alcoholism, substance abuse and schizophrenia in SouthwesternAmerican Indians. Am J Med Genet (Neuropsychiat Genet) 74:386–394.
D. GoldmanM. UrbanekD. GuentherR. RobinJ. LongLaboratory of Neurogenetics, NIAAA, NIHRockville, Maryland
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