Research Component 6:
PI: Kari Buck
Chromosome 1 QTG Role in Ethanol Withdrawal and Preference and Impulsivity
Co-I: Robert Hitzemann, John Belknap, Tamara Phillips, John Crabbe
We have fine-mapped a chromosome 1 quantitative trait loci (QTL) and have identified high-quality quantitative trait gene (QTG) candidates to underlie its effect on withdrawal. During the next 5 years, Component #6 proposes to continue to focus on a chromosome 1 QTL.
Our analyses identify with high certainty (LOD>7.6, p<2x10-9) QTLs on distal chromosome 1 that affect withdrawal after acute and chronic ethanol exposure in mice. Detection and fine-mapping using a panel of interval-specific congenic strains delineated two QTLs that affect withdrawal from ethanol and/or other drugs within the starting QTL interval. The more proximal QTL affects ethanol withdrawal, but not pentobarbital or zolpidem withdrawal, while the more distal QTL affects pentobarbital, ethanol, and zolpidem withdrawal.
QTLs are also detected on distal mouse chromosome 1 for ethanol preference, conditioned taste aversion, and sensitivity to its activating and hypothermic actions, as well as coincident, reciprocally-acting QTLs for ethanol consumption and acute withdrawal. This convergence of QTLs makes it tempting to speculate that the gene(s) underlying one or both withdrawal QTLs on chromosome 1 may have pleiotropic effects on a variety of ethanol responses in mice, including preference, and that the human homologs of the underlying QTGs may contribute to alcoholism risk, making the genes compelling translational targets.
Component #6 has and will continue to focus on the more distal Chr 1 QTL. Using robust behavioral models of withdrawal, positional cloning, expression and sequence analyses, we have already identified high-quality QTG candidates to underlie this QTL’s effect on withdrawal; and for a particularly promising candidate, Kcnj9, we developed a novel knockout (KO) model.
The human relevance of mouse QTL data depends on using robust animal models and on the high degree of homology between the human and mouse genomes. Thus, QTLs mapped in mice can predict their location in the human genome. Moreover, QTL mapping is likely to identify genes for regulatory or rate-limiting proteins that are important therapeutic targets. Of relevance to this proposal, the human homolog of Kcnj9 (and the other potential candidates in the QTL interval) maps to human chromosome 1q23.2, where multiple human studies detect the influence of a QTL(s) associated with alcohol dependence (reviewed by Ehlers et al 2010). Thus, KCNJ9 is a viable candidate for loci detected in human studies of alcohol dependence.
We view the ultimate goal of QTL research in mice as the identification of genetic targets (for risk and/or treatment) in humans. In some cases the QTG will be the same in mouse and man. In other cases QTL research will identify a relevant gene network. Analyses of proven QTGs as well as associated gene networks will be more powerful than either alone toward advancing network and mechanism discovery. We propose a strategy that can provide the interlocking levels of proof to move from QTG to network to mechanism. A comprehensive understanding of genetic variation, both in humans and animal models, is crucial to establish relationships between genotype and biological function.
Overview of Experimental Approach
The general strategy of this Component embodies the latter stages of a behavior→QTL→gene→network→mechanism approach. The overall goal of this renewal is to determine whether the QTG(s) on distal chromosome 1 influences ethanol responses beyond ethanol withdrawal convulsions, to directly test whether Kcnj9 is a causal QTG, and to progress in elucidating the mechanism by which a chromosome 1 QTG(s) affects ethanol withdrawal. Accordingly, the proposed studies use a robust mouse models to assess behavior. Aim 1 will test the hypothesis that Kcnj9 (and/or a linked gene) influences behaviors genetically correlated with acute ethanol withdrawal. Aim 2 will develop a genetic animal model that can establish with certainty that Kcnj9 is responsible for the QTL phenotypic effect on ethanol withdrawal. Aim 3 seeks to elucidate the network and mechanism by which the QTG may influence behavior and response to ethanol.
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