Associations among types of impulsivity, substance use problems and Neurexin-3 polymorphisms

Abstract

Background

Some of the genetic vulnerability for addiction may be mediated by impulsivity. This study investigated relationships among impulsivity, substance use problems and six neurexin-3 (NRXN3) polymorphisms. Neurexins (NRXNs) are presynaptic transmembrane proteins that play a role in the development and function of synapses.

Methods

Impulsivity was assessed with the Barratt Impulsiveness Scale Version 11 (BIS-11), the Boredom Proneness Scale (BPS) and the TIME paradigm; alcohol problems with the Michigan Alcoholism Screening Test (MAST); drug problems with the Drug Abuse Screening Test (DAST-20); and regular tobacco use with a single question. Participants (n = 439 Caucasians, 64.7% female) donated buccal cells for genotyping. Six NRXN3 polymorphisms were genotyped: rs983795, rs11624704, rs917906, rs1004212, rs10146997 and rs8019381. A dual luciferase assay was conducted to determine whether allelic variation at rs917906 regulated gene expression.

Results

In general, impulsivity was significantly higher in those who regularly used tobacco and/or had alcohol or drug problems. In men, there were modest associations between rs11624704 and attentional impulsivity (p = 0.005) and between rs1004212 and alcohol problems (p = 0.009). In women, there were weak associations between rs10146997 and TIME estimation (p = 0.03); and between rs1004212 and drug problems (p = 0.03). The dual luciferase assay indicated that C and T alleles of rs917906 did not differentially regulate gene expression in vitro.

Conclusions

Associations between impulsivity, substance use problems and polymorphisms in NRXN3 may be gender specific. Impulsivity is associated with substance use problems and may provide a useful intermediate phenotype for addiction.

Introduction

There is abundant evidence that an individual's vulnerability to addiction is influenced by their genetic makeup but identifying the genes that contribute to that risk has been and continues to be a vexing task (Li and Burmeister, 2009). For a given substance use disorder, there is an expectation that there are genes that affect addiction vulnerability to that particular substance, and that there are also genes that influence traits that increase vulnerability to addiction, in general. For example, polymorphisms in genes that code for enzymes involved in alcohol metabolism (i.e., alcohol dehydrogenase [ADH] and aldehyde dehydrogenase [ALDH]) have been convincingly shown to influence risk for alcohol dependence, and a gene that codes for an enzyme involved in catecholamine metabolism (i.e., catechol-o-methyl transferase [COMT]) appears to influence addiction risk through its influence on behavioral traits such as impulsivity and anxiety (Ducci and Goldman, 2008).

Impulsivity is an interesting trait in the context of addiction vulnerability. The general construct of “impulsivity” represents several independent facets such as response inhibition, resistance to delay of reinforcement, timing, behavioral switching, motor impulsivity, cognitive impulsivity, preparation, execution outcome, premature responding and lack of persistence (Evenden, 1999). Individuals with elevated levels of impulsivity are at increased risk for problems with alcohol (Dick et al., 2009, Lejuez et al., 2010), stimulants (Ersche et al., 2010), and nicotine (Doran et al., 2009, Spillane et al., 2010). In animal models, individual differences in different facets of impulsivity predict drug self-administration and exposure to drugs and increase impulsivity (see Winstanley et al., 2010 for a review). Slow developing behavioral control in children is associated with increased risk for adolescent substance use (Wong et al., 2006) and gender appears to modify the association between different types of impulsivity and alcohol problems (Stoltenberg et al., 2008). The relations among different facets of impulsivity and aspects of substance use or problems are not yet fully characterized, but there is growing appreciation of their complexity (Lejuez et al., 2010). There is some emerging evidence that individual differences in certain facets of impulsivity are influenced by genes in neurotransmitter systems (e.g., Stoltenberg et al., 2006, Walderhaug et al., 2010), but little is known about the underlying genetic architecture of impulsivity.

Recent empirical evidence suggests that the gene that codes for Neurexin-3 (NRXN3) may be a good candidate for general addiction vulnerability. Certain alleles of three single nucleotide polymorphisms (SNPs) within the fifth splice site of the NRXN3 gene were more common in alcohol dependent subjects than in matched controls (Hishimoto et al., 2007). A genome wide association study found suggestive evidence that a NRXN3 SNP (rs2221299) was associated with nicotine dependence (Bierut et al., 2007). Another NRXN3 SNP (rs1004212) was associated with the amount of nicotine consumption in schizophrenia patients (Novak et al., 2009). A genome-wide linkage study indicated an area on chromosome 14q, on which the NRXN3 gene is located, was linked to opioid dependence (Lachman et al., 2007).

Neurexins (NRXNs) are presynaptic transmembrane proteins that function as cell adhesion molecules, binding with neuroligins to stabilize the synapse (Hata and Südhof, 1995). There is growing evidence that neurexins are key elements properly functioning synapses and that NRXN dysfunction may play a role in diseases with a cognitive component (Südhof, 2008). The genes that code for NRXNs are large, contain numerous polymorphisms and are subject to alternative splicing. Regulatory region and splice site variants are likely to have a substantial impact on NRXN expression. NRXN proteins are encoded by three separate, unlinked genes: NRXN1 (2p16.3), NRXN2 (11q13), and NRXN3 (14q31). Each of the three NRXN genes has two promoters from which a longer alpha and shorter beta NRXNs are transcribed (Rowen et al., 2002). The alpha promoters are located at the 5′ end of the genes, while the beta promoters are located between exons 17 and 18 (Rowen et al., 2002). Each of these NRXN genes also has multiple alternative splice sites and thousands of possible isoforms (Tabuchi and Südhof, 2002). The NRXN3 gene is one of the largest genes in the human genome containing 1,826,818 base pairs (Rowen et al., 2002).

Lines of mice with the α-NRXN gene knocked out showed that α-NRXN is responsible for the coupling of Ca²⁺-channels to synaptic vesicles in preparation for exocytosis, and that they are essential for normal neurotransmitter release (Missler et al., 2003). These α-NRXN knockout mice had low survival rates, and those that did survive had decreased neurotransmitter release at both inhibitory (gamma-amino butyric acid; GABA) and excitatory (glutamate) synapses. GABA is the brain's major inhibitory neurotransmitter, and alcohol had been shown to mimic its effects on the GABA_A receptor (Lovinger and Homanics, 2007). During development, α-NRXNs on pre-synaptic neurons promote post-synaptic specialization of GABAergic neurons by clustering GABA_A receptors (Kang et al., 2007). The involvement of NRXNs in normal neurotransmitter release and synaptic integrity suggests that variation in their genes may have widespread and substantial effects on key behavioral phenotypes such as impulsivity. To the best of our knowledge, there have been no studies to date to examine potential associations between NRXN3 polymorphisms and impulsivity.

This study was designed to investigate potential associations among types of impulsivity, substance use problems, and genetic polymorphisms in NRXN3. Our hypothesis is in line with the notion that impulsivity is a key construct in the pathways from genes to risky behaviors, which can lead to behavioral disorders such as addiction.