16.2: Genetic Screens
Genetic screens are tools used to identify genes and mutations responsible for phenotypes of interest. Genetic screens help identify individuals or a group of people at risk of developing genetic diseases and help them with early intervention, targeted therapy, and reproductive options.
Forward genetic screens
Forward or “classical” genetic screens involve creating random mutations in an organism’s DNA using radiation, mutagens, or insertion of additional bases, which result in visible changes in the phenotype. The mutant is inbred to obtain progenies that are homozygous for the mutation. The mutation and its associated phenotype are identified, and gene locus on the chromosome are mapped.
Reverse genetic screens
Reverse genetic screens involve disruption of known genes, followed by screening for mutant phenotypes resulting from these manipulations. Expression screening is a type of reverse genetic screen that involves vector libraries containing the protein-coding sequences of various genes extracted from the genome of an organism or environmental samples. These screens help in the identification of novel proteins. For example, in zebrafish, using reverse genetic screening, the genes involved in early development are identified.
Genetic screens have several applications, such as identifying protein interactions, characterizing gene-drug interactions, and understanding the cause of diseases. For example, a mutant yeast library generated using large-scale random mutagenesis with transposon insertions can be grown in the presence of a drug. The effect of the drug on each mutant can be analyzed using PCR followed by microarrays or sequencing analysis. Similarly, screens can be used for the analysis of genes and molecular networks disrupted in human disease. For example, genes involved in neurodegenerative diseases can be identified by culturing neurons in the presence of RNA-encoding viruses to knock down the expression of different target genes. Cells can then be immunostained and analyzed to detect physical abnormalities and identify genes responsible.