There are large individual differences in drug abuse vulnerabilities among humans that are likely to display genetic as well as environmental components. During this FY, these investigators continued to explore possible roles of allelic variants at candidate gene loci in possibly contributing to human individual differences in drug abuse vulnerability, and begun genome scanning approaches to identifying previously-unanticipated genes conferring drug abuse vulnerabilities. This work revealed patterns of linkage disequilibrium at the dopamine transporter and D2 dopamine receptor gene loci and with flanking gene markers that allows improved interpretation of initially-positive association findings in substance abuse, the paranoia associated with cocaine use, and in attention deficit/hyperactivity disorder, which is a significant drug-abuse comorbidity. It sharpened attention to features of a previously- reported association between novelty-seeking and DRD4 polymorphisms. It identified a COMT functional variant highly associated with substance abuse vulnerability. It revealed no association with candidate markers at the DRD3 locus. These workers aided studies of the diagnostic criteria for drug abuse that appear to display substantial genetic and those that display substantial environmental influences, and established affected-sib pair strategies that would allow approaches to positional identification of alleles that enhance drug abuse vulnerability in man. We have generated results from a continuing study to analyze genes involved in dopamine function as candidates genes for substance abuse related behavior. In addition to a highly significant difference between substance abusers exhibiting high quantities of drug use and those with low drug use at the Catechol-O-Methyltransferase (COMT) locus, we find that a difference also exists between the substance abusers and control individuals for alleles of the dopamine receptor D4. Alleles at the D4 gene encode receptors that show differences in their ability to couple to the cAMP second messenger system, and thus provide a second functional polymorphism in the dopaminergic genes. We have found a significant increase in the frequency of the long forms of the D4 VNTR in substance abusers compared to controls. Further analysis showed that the interaction of the COMT and D4 alleles that is at least additive, suggesting that individuals with high COMT and the long form of the D4 receptor may be at greater risk of substance abuse. An additional collaboration with Dr. Paul Costa, next door at the Gerontology Research Center, National Institute on Aging investigated the possibility of an association of the D4 gene and novelty seeking. We have failed to replicate the findings of an association that was first suggested by reports from Israel and NIH. This work compliments our studies of the underlying genetics of the inter-individual differences in susceptibility to substance abuse.
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