Model Organisms in Drug Discovery

ear both dominant markers and recessive mutations that allow them to be
followed easily during stock maintenance and genetic crosses. Balancers are of
tremendous utility for the isolation of mutations and for the maintenance of
mutant stocks, as well as for genetic experiments. Few other multicellular
experimental organisms have balancer chromosomes.
Mutagenesis
Random mutagenesis
RESEARCH TOOLS IN DROSOPHILA STUDIES 103
Random mutagenesis has been the cornerstone of Drosophila forward genetics
(see Section 4.1; using Drosophila for drug target identification and validation).
There are three major methods to induce mutations in flies: chemical mutagens,
radiation and transposons (Ashburner, 1989; Greenspan, 1997; Roberts, 1998).
The most widely used chemical mutagen in Drosophila today is
ethylmethanesulfonate (EMS), which is normally used to make point
mutations although not all mutations derived from an EMS mutagenesis
experiment are point mutations and chromosomal aberrations may occur.
Because EMS only affects one strand of the DNA helix, an induced mutation
may be reverted or fixed after an additional round of DNA replication. Very
often EMS induces missense mutations and there is a 5–10% chance that the
isolated mutations are conditional mutations. By combining EMS and the
FLP/FRT mitotic recombination system (see later section on analytical tools),
high-throughput F1 genetic screens can be performed easily.
X-ray radiation from X-ray tube and g-ray radiation from 60 Co or 137 Cs are
normally used to induce chromosome/DNA aberrations such as deletions,
inversions, duplications and translocations.
The advantage of chemical- and radiation-based mutagenesis is their
relative non-selectivity on DNA sequence context, therefore the induced
mutations are more randomly distributed in the genome than other methods.
If the aim of a genetic screen is to discover most of the genes that affect the
assay phenotype, these types of mutagens, especially chemical mutagens,
should be used. However, their major disadvantage is that the induced
mutations need to be mapped before knowing which genes are mutated.
Mapping mutations is a very time-consuming process, even with genome-wide
SNP information (Hoskins et al., 2001).
Engineered transposons, such as the P element, hobo element and PiggyBac
element (Handler and Harrell, 1999; Horn and Wimmer, 2000; Horn et al.,
2002; Thibault, 2002), are used as mutagens for insertional mutagenesis. The
great advantage of using transposons as mutagens is that they also serve as
DNA ‘tags’ in the mutated gene. By using the inverted polymerase chain
reaction (PCR) method (Takagi et al., 1992), the location of mutations in the