Sequencing the human genome has done more than give researchers a powerful tool for unlocking the secrets of our genetic heritage. It has also ‘jump started’ many other areas of vital research by spawning a technique called gene array technology. Research presented in the June issue of Alcoholism: Clinical & Experimental Research (ACER) uses this technology to analyze gene expression in mice with different alcohol sensitivities, hoping to use the findings to better understand genetic predisposition to human alcoholism.
"We are just scratching the surface of the power of gene array technology," said James M. Sikela, associate professor of pharmacology and a member of the Human Medical Genetics Program at the University of Colorado Health Sciences Center, "in order to find genes and pathways that are involved in alcohol action." Sikela, whose lab was involved in the initial work on the genome project, is also the senior author of the ACER study. "Our research is an example of taking new approaches that have come out of the human genome project and applying them to the study of alcoholism so that new molecular processes can be identified and we can better understand how alcohol works and alcoholism develops."
All of our cells have exactly the same deoxyribonucleic acid (DNA), which means they all have the same genes. Certain cells appear to work differently with the same genes (giving each of us, for example, unique eyes, skin, hair, etc.) because only some genes are activated or "expressed" in each cell. This is called gene expression. The sequence of events is for DNA, or genes, to make ribonucleic acid (RNA), also called a "message," which is then used to make proteins. These proteins determine the appearance and function of each cell and, in turn, the proteins’ existence depends on gene expression. Thus, gene expression is a normal function of all cells and is well regulated to avoid mistakes. Drugs like alcohol, however, can change gene expression and thereby disturb normal functions of the cell and tissue.
This study examined the genes of two strains of mice bred for a differential response to a specific measure of sensitivity to alcohol: inbred long-sleep (ILS) and inbred short-sleep (ISS) mice. ILS mice are genetically more sensitive than ISS mice to many effects of alcohol. ISS mice will, for example, recover or get up quicker than ILS mice will after being given a sedating alcohol injection. Some researchers believe that, for both rodents and humans, low response to alcohol is a very good predictor of future alcohol dependence.
"This mouse model has similarities to a human pattern of alcohol response or alcohol sensitivity," said Sikela. "An individual who has a low initial response to alcohol, meaning they don’t feel much of an effect from alcohol, can drink quite a bit before they feel anything. These individuals may tend to be very much like the ISS mice in the sense that they may be at higher risk for developing alcohol problems."
Alcohol’s primary target is the central nervous system, where it influences neurotransmission to produce intoxication and, with chronic abuse, tolerance, dependence and neurotoxicity. Knowing that the ILS and ISS mice have such a markedly different response to alcohol, researchers wanted to identify those genes that are differentially expressed (turned on, off, up or down) in the ILS and ISS brains. They used data from two different gene arrays – an array is a small glass microscope slide that has thousands of different DNA samples attached to the glass – by Genome Systems, Inc. (providing more than 18,000 mouse genes) and Incyte Pharmaceuticals, Inc. (providing nearly 9,000 genes). They found that 41 genes – 18 of them ILS, and 23 of them ISS – displayed significant differences in expression.
"It remains to be seen which, if any, of the genes identified in this study are relevant to the two strains’ different sensitivities to alcohol," said Kari Buck, assistant professor of behavioral neuroscience at the Oregon Health Sciences University and the Portland Alcohol Research Center. "But homology with the human genome suggests that for each of the genes that are relevant to the two mouse strains’ differential alcohol responses, there will almost certainly be a human version. Thus, identification of genes that influence alcohol sensitivity in mice may help to elucidate genes that confer low alcohol sensitivity and higher risk for alcoholism in humans."
"The fact that these gene levels are different," added Sikela, "means that it’s very plausible that at least some of the genes themselves are involved in pathways in the brain that are involved in the effects of alcohol. The next step is to find out what these genes are, and what kind of pathways they may be involved in, because some of the pathways may already have been found. The GABA (gamma-aminobutyric acid ) system, for example. But many of the genes we’re finding are kind of new candidates that haven’t been seen before, and could identify new pathways that haven’t been appreciated or discovered before in terms of being important to alcohol."
Funding for this Addiction Science Made Easy project is provided by the Addiction Technology Transfer Center National Office, under the cooperative agreement from the Center for Substance Abuse Treatment of SAMHSA.
Articles were written based on the following published research:
Xu, Y., Ehringer, M., Yang, F., & Sikela, J.M. (2001, May). Comparison of global brian gene expression profiles between ILS and ISS mice in high density gene array hybridization. Alcoholism: Clinical & Experimental Research, 25(6), 810-818.