AlcoholismDon't Drink and Fly!
Drosophila melanogaster, commonly known as fruit flies, tend to dine on rotting or fermenting fruit which release significant amounts of alcohol (up to 4%). In the wild, fruit flies are resistant to the toxic effects of alcohol, but in the lab mutant strains of flies have been created that are alcohol sensitive. When sensitive flies are exposed to alcohol vapors (alcohol in gas form), their behavior can be remarkably similar to some humans! The flies become hyperactive, uncoordinated, sedated, and pass out (see video of "Barflies" from Science News.)
To model human alcoholism behaviors in fruit flies, scientists study genes associated with alcohol sensitivity and tolerance. To sort the ‘sober’ from the intoxicated flies, scientists use a device known as the inebriometer. Because alcohol-sensitive flies become uncoordinated and sedated more rapidly than flies that are resistant, the sensitive flies fall to the bottom of the inebriometer column at a faster rate. Sorting flies on the basis of their alcohol-responsive behavior allows researchers to isolate alleles (different copies of the genes) to study alcoholism.
Many alcoholism-related genes have been identified. Two important ones are alcohol dehydrogenase (ADH) and the acetylaldehyde dehydrogenase (ALDH) genes. In humans and flies, the ADH enzyme converts alcohol into aldehyde which is subsequently converted by ALDH to citric acid which is then used to generate energy (in the form of ATP).
When flies or humans are first exposed to alcohol, they become hyperactive. Repetitive or increased alcohol exposure leads to sedation/intoxication. Flies and humans with initially higher tolerance for alcohol (i.e., that can intake excessive alcohol before feeling the effects) tend to overconsume. In humans, certain ALD and ALDH alleles or single nucleotide polymorphisms (SNPs), which are DNA regions that vary by a single nucleotide between individuals, are associated with alcohol intake. For example, the ADH1B, ADH1C, and ALDH2 alleles appear to be protective against alcoholism in some human populations.
To look for naturally occurring alcohol-related genes, scientists used genetically identical flies that were bred by mating wild-caught fruit flies to their siblings over many-generation (inbred fly strains). There are more than 580 alleles of the ADH gene in Drosophila inbred stains. After selecting inbred flies that were either acutely sensitive to alcohol or extremely tolerant/resistant to alcohol using the inebriometer, scientists could look at which genes were used. They did this by identifying the RNA, which we call gene transcripts, from each type of alcohol-responsive fly. The alleles have altered responses to alcohol compared to wild-type (normal) flies. For example, flies with the null allele, Adhn1, which do not have any ADH protein (enzyme), do not act hyperactive upon initial alcohol exposure, and tend to become uncoordinated or lose locomotor activity more rapidly.
However, alcoholism is not a single-gene disease; rather, it involves multiple genes. Scientists have found that gene transcripts occurred at different levels in various inbred fly strains that were either alcohol sensitive or alcohol tolerant. Thus, whole networks of genes including genes involved in olfactory memory (memories induced by odors or scents) and lipid biosynthesis(making certain essential fats) have been discovered as alcohol-responsive genes.
The researchers can select each of these hundreds of genes and look for human orthologs (similar genes that share an evolutionary history) and determine whether those same genes are important for human alcohol consumption.
In a recent report, the orthologous gene coded for the Malic Enzyme 1 protein (ME1) in humans (known as the Men gene in flies). ME1/Men functions in the oxidation-reduction pathway found in mitochondria and its function is to metabolize malate, which is an intermediate in the citric acid cycle used to generate cellular energy including the energy molecules, ATP and NADH. The researchers found single-nucleotide polymorphisms (SNPs) in the Drosophila Men gene associated with high alcohol tolerance affected the amount of the gene transcript produced in response to alcohol exposure – in other words, the differences were in the gene’s regulatory region. They then tested if they could find similar gene variations in a human population. Indeed, they found seven SNPs in the human ME1 gene that corresponded with higher alcohol intake in humans. This type of research is referred to as a translational study and is a fantastic start to identifying the multitude of genes that are involved in a disease that effect about 76.3 million people worldwide. Once such SNPs are identified, one could test individuals for their predisposition toward alcoholism and this testing may eventually lead to medical treatments. Ultimately, Drosophila research could unveil potential drug targets to cure human alcoholism.
The modENCODE website reports the type and quantity of gene transcripts that result from the process of transcription from many genome-wide experiments during the developmental stages of they fly. It might be interesting to see when during development the ADH, ALDH, and ME1/Men gene transcripts first occur – are they on all the time? Does chromatin state affect their expression?
- Explain how changing a single nucleotide (a single-nucleotide polymorphism, or SNP) in the coding sequence of the alcohol dehydrogenase gene would increase or decrease someone’s predisposition towards alcoholism. (Hint: think of the Central Dogma of Molecular Biology.)
- For the Malic Enzyme 1 gene, the SNPs are in the introns. The polymorphisms do not affect the protein-coding sequence -- how might the SNPs at the DNA level affect transcription of the gene? (Hint: think of gene transcript amounts.) What consequence (phenotype or trait) would this have for a fly or a human?
- Often epigenetic effects (changes in the shape of DNA-bound proteins) can affect gene expression. During development, modification of histones occurs to alter the chromatin state (e.g., active or silent), and alcohol disrupts histone modification. Alcohol exposure during development can cause Fetal Alcohol Syndrome (FAS), which is a major cause of mental retardation in humans has also been studied in flies. Thinking about the all the genes in the genome and the importance of expressing them at the right time and in the right cell, how would altering the chromatin state by alcohol affect the fly or human embryo? Specifically, think about brain and body growth.
- Alcoholism is a multi-gene disease. Why is it so important to use model organisms to study this human disease?
Lesson & Laboratory Plan
- Access to Excellence Lab Lesson Plan and another lesson plan for alcohol tolerance.
- High school level laboratory kit using Adh alleles from Carolina Biological.
- Science News video of intoxicated fly
- Science Central interview with Ulricke Heberlein and with video footage of ‘drunk’ or ‘inebriated’ fruit flies falling in the inebriometer.
Science Daily Links
- Drunken Flies Help Scientist Find Potential Drug Targets for Alcoholism
- Bar Flies: Fruit Flies Help Unravel the Genetics of Alcohol Sensitivity
Fetal Alcohol Syndrome
- Alcohol in pregnancy; Drinking alcohol during pregnancy; Alcohol-related birth defects; Fetal alcohol effects
NIH National Institute on Alcohol Abuse and Alcoholism
- Search NIAAA for alcoholism related information
- PDF explaining the ADH & ALDH alleles associated with reduced rates of alcohol dependence
Fly allele links:
Students are invited to explore alleles, phenotypes, gene transcript expression data, etc. of the fly genes discussed in the alcoholism vignette at the data repository for Drosophila known as FlyBase. Under the External Crossreferences & Linkout section, one can find the linkout for modMine which interfaces with data generated by the modENCODE project: clicking on the FB# under modMine will take you the gene specific site.
Research Article (open access):
SNP research article looking at Malic Enzyme humans correlated to flies: Alcohol Sensitivity in Drosophila: Translational Potential of Systems Genetics.