The recent buzz surrounding the drinking habits of Drosophila, from dealing with rejection to killing parasites by self-medicating, got me thinking: How might fruit flies be used for research in understanding the cause of alcohol use disorder (AUD)? AUD, which encompasses both alcohol abuse and dependency, affects millions of Americans and is a complex disorder influenced by a myriad of environmental and genetic factors. Not surprisingly, efforts to identify the genes responsible for AUD in human and animal models are fraught with limitations. Human studies comparing the genetic variability between affected and unaffected individuals have identified many potential genes responsible for AUD, but only with weak statistical support, thus leaving scientists to ponder which genes are bona fide candidates. On the other hand, more feasible experiments performed in animals and cell culture have identified genes involved in ethanol response, but whether these genes are relevant to a complex, human disorder such as AUD is unknown.
In a study published last December in G3: Genes, Genomes, Genetics, biologists from UCSD and UCSF used a multispecies approach to work around these obstacles to identify and test potential genes involved in alcohol-induced behavioral responses (Josyln et al., 2011). Behavioral changes in response to ethanol can be indicative of later AUD development. To prune the list of potential genes from the entire genome, the researchers analyzed available mouse genetic data to identify regions of the mouse genome associated with ethanol-induced ataxia (loss of coordination). These genomic regions represent clusters of genes that could influence AUD. Despite differences in overall structure and number of chromosomes, many of these gene clusters have been maintained in the genomes of many organisms–this is known as synteny. The researchers exploited this fact to cross-reference the mouse-identified regions to the corresponding sections in the human chromosomes, allowing them to focus on smaller regions rather than the daunting task of searching across the entire human genome.
Corresponding genomic regions between mouse and human genomes are color-coded.
Next, an alcohol challenge study was performed in human subjects. Study participants were given ethanol to consume and the researchers then measured how much ethanol affected the participants inclination to sway to the left and right (another measure of coordination similar to ataxia). Using DNA collected from participants, a genetic association analysis of the human genome corresponding to the regions of the mouse genome associated with ethanol-induced ataxia was performed to identify genes linked to ethanol-induced body sway. The logic behind genetic association is that differences in DNA sequence (genetic variability) amongst individuals underly the variability in human traits (phenotypes, i.e. hair color, eye color, and in this case disease). Thus the researchers looked for a change in DNA sequence (genetic variant) that was common to individuals with the most pronounced ethanol-induced body sway (phenotype). This analysis revealed glypican 5 (GPC5) as a candidate gene involved in ethanol response in humans. GPC5 belongs to a class of genes that encode cell surface proteins, known as glypicans, that act like cellular antennas to receive protein-encoded messages from other cells (see figure below). These protein messages can induce specific responses in the cell, such as turning on the expression of specific genes or alter cellular metabolism. This form of cellular communication is known as signal transduction and is vital for the proper development of an organism.
Adapted from U. Häcker et al., 2005
To test whether glypicans can affect ethanol response the researchers turned to Drosophila. The researchers found that mutations in the equivalent (homologous) Drosophila glypican genes, dally and dally-like (dlp), affected fruit fly behaviors sensitive to ethanol exposure. Normally, when exposed to ethanol vapor, fruit flies are initially startled and display elevated locomotor activity that becomes increasingly uncoordinated until the flies eventually becomes sedated. Mutations in both dally and dlp affected ethanol-induced locomoter activity and decreased the time it took for the fruit flies to become sedated. Interestingly, wildtype Drosophila become tolerant to ethanol since a second exposure to ethanol vapor takes a longer time to induce sedation. Only the mutation in dally displayed an inability to develop this second exposure tolerance suggesting that dally and dlp have different roles in developing tolerance to alcohol. These results confirmed that glypicans influenced alcohol-induced behaviors in Drosophila.
The authors of the study reasoned that the convergence of data obtained from their mouse, human and fly studies ”provides strong support to the hypothesis that GPC5 is involved in cellular and organismal ethanol response and the etiology of alcohol use disorders in humans.” In further support of their hypothesis, they point to other research indicating that the signal transduction pathways regulated by glypicans are also involved in ethanol response. While the role of GPC5 in the etiology of AUD requires further study, this research provides an example of a powerful, combinatorial genetic strategy that may prove useful in identifying causative genes in the context of other complex, multifactorial diseases such as cancer, metabolic syndrome, or heart disease.
* April is Alcohol Awareness Month
Joslyn, G., Wolf, F., Brush, G., Wu, L., Schuckit, M., White, R., & Hall, I. (2011). Glypican Gene GPC5 Participates in the Behavioral Response to Ethanol: Evidence from Humans, Mice, and Fruit Flies G3: Genes|Genomes|Genetics, 1 (7), 627-635 DOI: 10.1534/g3.111.000976