Model Organisms in Drug Discovery

242 CHEMICAL MUTAGENESIS IN THE MOUSE
power of this approach, e.g. dissection of the ras signalling pathway (Gaul et
al., 1993). Secondly, a screen for medically relevant functions in mammals can
be designed to improve a disease phenotype caused by the primary genetic
defect, thus identifying ‘health genes’ rather than ‘disease genes’. It can be
assumed that players identified in such a design are prime candidates for
pharmacological intervention.
An illustration of this approach is identification of the locus Mom (modifier
of min), which influences the phenotype of the mutant line Min (Moser et al.,
1990); Min carries a mutation in the mouse homolog of the human familial
polyposis gene (Apc) and suffers from multiple intestinal polyps; Mom
strongly modifies the extent and progression of this polyposis. The Mom locus
encodes the secretory phospholipase Pla2g2aI, which could be shown in
transgenic rescue experiments to provide at least one component of the
modifier function (Cormier et al., 1997; 2000). The Mom locus is an allele that
occurred spontaneously, but similar approaches are under investigation in
several laboratories using ENU-induced modifiers.
9.5 The art of screen design: phenotyping
Whatever the genetic design of a screen, the right phenotyping protocol is a
prerequisite for finding informative mutants that will lead to the
identification of novel molecular pathways. The art of designing and
implementing a successful screen lies in the choice of the appropriate target
phenotype, combined with the establishment of scalable combinations of
primary and secondary assays to detect this phenotype sensitively and
specifically.
An excellent example is isolation of the mutant clock, which led to
identification of the first gene affecting circadian rhythm in mammals
(Vitaterna et al., 1994). The assay employed – measurement of the circadian
activity using a computer-monitored running wheel – is straightforward,
scalable and very specific, although great care had to be taken to establish
normal ranges and baselines. In contrast, the measurement of body weight
would not be sufficient to identify specifically the lean animals with reduced
body fat. Many animals initially would score positively, thus obscuring the
desired mutants, because there are many reasons for a mouse to have lower
body weight than normal, e.g. non-genetic runts and growth retardation
secondary to many other genetic defects.
Thus, a typical screening protocol employs several levels of activities. The
primary screen should employ simple parameters and assays that have a high
sensitivity for rapid and efficient enrichment of candidate mutants with altered
physiology in the areas of interest. Each ‘hit’ in these crude but sensitive
primary assays has to be followed up with more elaborate assays of higher