Addiction is a chronic, often relapsing illness characterized by persistent and compulsive use of drugs. This maladaptive behavior ultimately leads to physical and psychological dependence. Changes in brain function result from continued exposure, and these lead not only to increased tolerance to the drug but also to physiological changes that can extend for long periods of time after drug use ceases. Both genetic and environmental factors contribute to all aspects of addiction, including initiating use of addictive substances, the vulnerability to become addicted to the drug, the successful discontinuation of drug use. The genetic contribution to addictions, including nicotine dependence, alcoholism and addiction to drugs such as cocaine, has been well established in studies of cohorts of twins. Linkage mapping and association mapping, including recent genome wide association studies, have identified susceptibility loci for addiction related phenotypes. These and the studies of twins suggest that both common and substance specific genetic factors can contribute to risk for addiction. Recent genetics studies support the involvement of a number of loci encoding nicotinic acetylcholine receptors (nAChRs) in addiction genetics. nAChR are ligand gated ion channels, composed of various combinations of five subunit proteins, encoded by 11 different genes, the cholinergic nicotinic receptor subunit (CHRN) genes in mammals. These receptors are also the physiological targets of nicotine activating the mesolimbic dominergic reward and pleasure pathways. In its most simplistic form, treatment of nicotine dependence (ND) simply supplies this stimulus in the absence of the other components of tobacco smoke. Recent genetic data suggest that the nACh receptors may play a more complex role in the psychopathology of addiction, and that development of novel therapeutic agents directed at these receptors could be useful for the treatment of other addiction disorders as well as ND. Variants in the nACHRs are reported to increase risk of development of dependence on cocaine and alcohol. However, it is not clear how the variants in these genes alter the vulnerability to addiction, especially as the majority of the variants do not alter the amino acid composition of the encoded receptor. This has proven an obstacle for exploitation of the results from genetics studies directed at the identification of new therapeutic targets;nor has it been possible to use the presence of these variants as predictors for improved treatment and/or prevention. In this application we propose to develop a series of reagents that will address this limitation and accelerate research on addiction in many different venues. We propose to generate mouse lines that express human nAChR genes. These animals will be developed using a novel series of vectors that facilitate the excision and replacement of mouse loci with their human ortholog. Because these lines will express humanized nAChRs, they will provide an important tool for preclinical evaluation of therapeutic agents targeting this gene family. In the case of loci for which disease association has been demonstrated, the mouse gene can be replaced with various human haplotypes, either those believed to be protective or those associated with increased vulnerability to addiction. The phenotype of the mouse lines carrying the different human alleles can be compared, evaluating the impact of these SNPs on gene expression, gene-environment interactions, chromatin architecture, and behavior in paradigms that model addiction.
Cigarette smoking is a major public health problem, contributing to approximately five million deaths world wild (1). Approximately 50% of the individuals that smoke have developed a dependency on nicotine, the addictive component of tobacco. Nicotine acts by stimulating acetylcholine receptors (nAChRs), activating a number of pathways in the brain, including reward pathways. In this project we propose to generate mouse lines expressing the human nicotine receptors for two purposes. These lines will provide a preclinical model for testing new drugs for the treatment of addictions. In addition, these mouse lines provide a platform for determining whether differences noted between individuals in the structure of the genes encoding the nicotine receptors are predictive of vulnerability for developing an addiction. 1. World Health Organization: The Facts About Smoking and Health, May 30, 2006. ://www.wpro.who.int/media_center/fact_sheets/fs_20060530.htm