Chapter 12 - Making and Using Genetically Modified Organisms

Abstract

A genetically modified organism is any organism whose genome has been modified using genetic engineering techniques. If the genomic modifications are in germ cells (sperm or eggs), then the altered DNA is heritable and passed to all future offspring. Using modern techniques such as CRISPR/Cas9, it is possible to remove genetic sequences, edit and replace genetic sequences, or add foreign genetic sequences to the genomes of virtually any model organism. Genetically modified organisms have proven extremely useful for studying the genetic basis of behavior and other neural phenotypes. For example, genetic knockout animals allow scientists to test the hypothesis that a gene is necessary for a certain biological process by removing the gene from the genome; alternatively, an investigator can overexpress a gene and determine the effects of a gain of function of that gene. Genetically modified organisms have also proven extremely useful for targeting specific cell types with useful genetically encoded tools. By using genetic regulatory elements found only in specific populations of neurons, it is possible to express a transgene useful for studying that population's structure, projection patterns, and activity patterns, as well as to perturb the population's neural activity. The purpose of this chapter is to survey the rationales for engineering a genetically modified organism and explain the process of removing or inserting genetic material in model organisms. We will also survey several commonly used transgenes and describe how these transgenes can be targeted to cell types to study discrete populations of neurons. Finally, we describe the process of targeting mRNA transcripts rather than genes themselves.