Alcohol Sensitivity in Drosophila: Translational Potential of Systems Genetics

Abstract

Identification of risk alleles for human behavioral disorders through genomewide association studies (GWAS) has been hampered by a daunting multiple testing problem. This problem can be circumvented for some phenotypes by combining genomewide studies in model organisms with subsequent candidate gene association analyses in human populations. Here, we characterized genetic networks that underlie the response to ethanol exposure in Drosophila melanogaster by measuring ethanol knockdown time in 40 wildderived inbred Drosophila lines. We associated phenotypic variation in ethanol responses with genomewide variation in gene expression and identified modules of correlated transcripts associated with a first and second exposure to ethanol vapors as well as the induction of tolerance. We validated the computational networks and assessed their robustness by transposon-mediated disruption of focal genes within modules in a laboratory inbred strain, followed by measurements of transcript abundance of connected genes within the module. Many genes within the modules have human orthologs, which provides a stepping stone for the identification of candidate genes associated with alcohol drinking behavior in human populations. We demonstrated the potential of this translational approach by identifying seven intronic single nucleotide polymorphisms of the Malic Enzyme 1 (ME1) gene that are associated with cocktail drinking in 1687 individuals of the Framingham Offspring cohort, implicating that variation in levels of cytoplasmic malic enzyme may contribute to variation in alcohol consumption.

The Effects of Weak Genetic Perturbations on the Transcriptome of the Wing Imaginal Disc and Its Association With Wing Shape in Drosophila melanogaster

“…Instead of examining individual transcripts for associations with target phenotypes (Passador-Gurgel et al 2007), we provide evidence that association between modules and target phenotypes should be evaluated . Dpp pathway components, or wing margin determinants, collectively associate with aspects of wing shape, mirroring a recent report of association of modules of various classes of genes with behavioral attributes in adult flies Harbison et al 2009;Morozova et al 2009). The same objective, unsupervised MMC approach adopted by these authors did not reveal enrichment for gene ontology classes in our data set and revealed only weak association between shape and expression.…”

Transcriptional Networks for Alcohol Sensitivity in Drosophila melanogaster

“…The key to solving this challenge is the realization that new mutations and segregating variants do not affect organismal phenotypes directly, but do so via complex interacting networks of transcriptional, protein, metabolic, and other molecular endophenotypes (Anholt et al 2003;Sieberts and Schadt 2007;Chen et al 2008;Emilsson et al 2008;Keller et al 2008;Ayroles et al 2009;Cookson et al 2009;Dobrin et al 2009;Edwards et al 2009a;Harbison et al 2009;Morozova et al 2009;Schadt 2009;Schadt et al 2009;Winrow et al 2009). If we know the network elements (genes) and connections (cis-and trans-regulation) associated with any complex phenotype, we can begin to make predictions about the effects of genetic perturbations on the organismal trait and whole network responses to such perturbations.…”

“…However, the expense of whole-genome expression analysis limits the number of mutations studied to only a few of the large number of mutations affecting each trait, and the pleiotropic effects of each mutation on multiple traits confounds our ability to infer which of many coregulated transcripts are associated with the focal trait. The second approach utilizes populations of lines suitable for linkage or association mapping of complex traits in which genome-wide expression and organismal phenotypes have been jointly assessed (Passador-Gurgel et al 2007;Ayroles et al 2009;Harbison et al 2009;Morozova et al 2009). Quantitative trait transcripts associated with the trait can then be grouped into correlated transcriptional networks associated with the trait.…”

“…We chose alcohol sensitivity as an example of a complex trait because alcohol is a well-defined, ecologically relevant chemical cue for localization of decomposing fruit on which flies feed, and understanding genetic networks associated with alcohol sensitivity in flies is likely to show evolutionary conservation with relevance to human alcohol consumption and addiction (Park et al 2000;Rothenfluh and Heberlein 2002;Guarnieri and Heberlein 2003;Mackay and Anholt 2006;Li et al 2008;Morozova et al 2009). Furthermore, alcohol knockdown time can be readily and accurately measured in an ''inebriometer'' (Weber 1988).…”

“…When exposed to alcohol, flies initially become hyperactive, but soon lose postural control and fall through the inebriometer at a rate that is correlated with their sensitivity. Following recovery, a second exposure reveals the development of tolerance reflected in a longer knockdown time (Moore et al 1998;Scholz et al 2000;Singh and Heberlein 2000;Cowmeadow et al 2005Cowmeadow et al , 2006Scholz et al 2005;Wen et al 2005;Morozova et al 2006Morozova et al , 2009Kong et al 2009). …”