Model Organisms in Drug Discovery

170 MECHANISM OF ACTION IN MODEL ORGANISMS
further supported by the finding that BMS AG6B gamma secretase inhibited
mammalian Notch processing in vitro (data not shown).
It is interesting that the genetic interactions with the two presenilins in
worms differed. The finding that hop-1 mutants were more sensitive than sel-
12 mutants to the effects of the drugs suggests that because the genes act
redundantly the drug might inhibit sel-12 or sel-12-dependent pathways more
strongly. Presenilin sel-12 is more homologous to human presenilin (50%
identical to PS1 versus 33% identical for hop-1). Alternatively, differences in
response to the two worm presenilins could be explained by different
contributions of the two proteins to signaling in the affected tissues, supported
by the observation that more severe phenotypes are observed in the absence of
maternal sel-12 than in the absence of maternal hop-1 (Westlund et al., 1999).
In any case, the differential effect is an example of the high level of specificity
that can be achieved by phenotyping and genetic analysis.
Not only do these experiments highlight the power of model organism
genetics for target identification and analysis of disease genes, but they also
point to the utility of compounds as pathway probes and screening tools.
Because a compound effect on flies and worms is dose dependent, it can be
used to generate an ‘allelic series’ of the inhibited genes for phenotyping and
screening. Compound administration can be timed to mimic a temperaturesensitive
mutant, which might avoid undesired lethality of a complete loss-offunction
mutant. Compounds such as gamma secretase inhibitors are effective
sensitizers of pathways and can be used as the entry point for genetic screens,
not only for target identification but to generate pathway information. For
example, it would be possible to screen for mutants that, say, enhanced the
phenotype of BMS AG6B-treated worms such that they became glp-like
sterile. In fact, a screen for enhancers of the sel-12 mutant – similar in
concept – identified two new regulators of presenilin signaling (Francis et al.,
2002).
6.6 New chemical genetic strategies: genome-wide cell-based
genetic screens
The examples used above combine chemical-induced phenotypes and genetic
mutagenesis screens to reveal the molecular basis of chemical action. Reverse
genetic approaches utilizing RNA interference (RNAi) technology are
becoming increasingly popular. The RNAi introduces double-stranded
RNA (dsRNA) into a system (C. elegans, Drosophila and cell culture) as a
post-transcriptional method of gene knock-down (Fraser et al., 2000).The
significance of this approach can be seen in C. elegans, where RNAi to every
gene on chromosome I was systematically tested for gene function. Using