Association between heroin dependence and prodynorphin gene polymorphisms

Abstract

Dynorphin peptides and k-opioid receptor are important in the rewarding effects of drugs of abuse such as heroin. This study examined potential association between heroin dependence and four single nucleotide polymorphisms (SNPs) of prodynorphin (PDYN) gene (rs35286281 in promoter region and rs1022563, rs2235749, rs910080 in 3′UTR). Participants included 304 heroin-dependent subjects and 300 healthy controls. Genotype, allele frequencies and difference between groups were analyzed by HaploView 4.0 and SPSS 11.5 software. The analysis indicated a significant higher frequency of the PDYN 68bp VNTR (rs35286281) H allele in heroin-dependent subjects than in controls (p = 0.002 after Bonferroni correction). Strong linkage disequilibrium was observed between rs1022563, rs2235749 and rs910080 polymorphism (D′ > 0.9). Significantly more TCT haplotypes were found in heroin-dependent patients than in the controls (p = 0.006 after Bonferroni correction). We found significant pointwise correlation of these three variants (rs1022563, rs2235749 and rs910080) with heroin dependence. These findings support the important role of PDYN polymorphism in heroin dependence, and may guide future studies to identify genetic risk factors for heroin dependence.

Introduction

Heroin addiction is a chronic relapsing condition characterized by compulsive drug seeking, drug abuse, tolerance and physical dependence. An adoption study indicated a genetic component to substance dependence, especially opioid dependence [2]. Twin studies have suggested that genetic factors account for 30–60% of the overall variance in the risk of developing drug addiction [17], [31], [33]. Genetic component of opioid addition is even greater [31].

Prodynorphin (PDYN) is an opioid peptide precursor protein that gives rise to α- and β-neoendorphins, dynorphin A- and dynorphin B-related peptides [14]. These products of the PDYN genes have been shown to be able to inhibit neurotransmission by acting through kappa opioid receptors, and are implicated in reward, mood regulation, stress responses, and motor functions [7], [13], [16]. The dynorphins/kappa opioid receptors (Dyn/KOP) system plays a crucial role in addiction. Dysregulation of the Dyn/KOP system is induced by repeated drug abuse and involves the mesolimbic reward system. Thus, the dopaminergic pathway of the ventral tegmental area to the nucleus accumbens is seen as the main site of Dyn action in addiction. The importance of the Dyn/KOP systems is discussed not only with regard to habit learning and establishment, but also with regard to the reinstatement of addiction [28].

Several animal studies have shown that chronic administration of drugs of abuse, such as morphine [33], cocaine [1], [11], [18], [19], [26], [27], [37], amphetamine [32], nicotine and ethanol [9], [10], [20], [23], altered the activity of PDYN in the brain. Drugs of abuse can increase prodynorphin expression in clinical models. Postmortem studies of human cocaine addicts demonstrated an increase in both mRNA and protein levels of dynorphin compared to controls [8], [15]. Furthermore, polymorphisms in prodynorphin gene have been associated with opioid dependence in females [5]. In a human study, evidence suggested that dynorphin administered intravenously produced decreased withdrawal symptoms in heroin addicts [34]. It had been found that genetic variability of endogenous dynorphin opioid systems, which mediate dysphoria, may be related to opioid abuse, and thus has a strong heritability [12], [30], [31].

Human PDYN gene contains four exons and three introns. Zimprich et al. identified a functional polymorphism in PDYN gene promoter region with one to four repeats of a 68-bp element containing one binding site per repeat for transcription factor AP-1 (c-Fos/c-Jun) [39]. In vitro evidence has implied that this variable nucleotide tandem repeat (68-bp VNTR) polymorphism was able to induce transcriptional activation with four or three, but not less, copies of the repeats [38]. Such allelic variations could influence gene expression and contribute to individual psychophysiological variability [3], [6]. However, studies on genetic association have failed to produce consistent results with regards to potential involvement of PDYN 68bp VNTR polymorphism in cocaine, methamphetamine and heroin dependence [3], [6], [23], [25], [32], [39].

In addition to 68-bp VNTR polymorphism, the most well-studied one, other polymorphisms of PDYN genes in 3′UTR had also been associated with decreased PDYN mRNA expression in human brain tissues [37], suggesting that these PDYN gene variants might influence gene expression and thus PDYN function in human. Recent findings suggest that two SNPs (rs1022563 and rs1997794) in 3′UTR of this gene were associated with increased risk of developing opioid dependence in females [5]. In genotype and allelic tests, Yuferov et al. found experiment-wise significant association between three SNPs (rs910080, rs910079 and rs2235749) in 3′UTR and both cocaine dependence and cocaine/alcohol codependence in Caucasians, but not in African Americans [37]. However, genetic association studies have not been repeated in heroin dependence.

In view of the crucial role of the PDYN in addiction disorders, as well as the controversy in genetic association studies, we conducted a study with relatively large sample size and same ethnic origin to verify the putative association between PDYN polymorphisms and heroin dependence.