YEAST AS A MODEL IN HIGH-THROUGHPUT SCREENING OF SMALL-MOLECULE LIBRARIES - Diversity-Oriented Synthesis - page 461

FIGURE 14.3 Schematic representation of the classical genetic approaches. (a) Forward
genetics select the gene whose mutation induces a selected phenotype. (b) Reverse genetics
investigate the function of a gene by observing the effects of its mutation.
genes often induce cell death, thus limiting the functional and mutational study to
nonessential genes. Actually, the effects induced by nonlethal mutations could also
be overlooked by classical genetic approaches, because of the ability of the mutant
organism to compensate for the loss of the gene function prompted by the mutation. A
further limitation of the genetic approach is that most mutations are not conditional;
thus, they cannot be turned on or off at will. Even if the mutation is designed to be
conditional (i.e., when the mutated allele is under an inducible promoter), the expres-
sion of the allele entails a stimulus that itself induces a cell stress, thus concealing
the effect of the mutation.
A relatively new approach to the dissection of biological functions is based on RNA
interference (RNA i ). By using small RNA molecules able to bind specific m RNAs
selectively, expression of the protein coded by the targeted m RNA can be inhibited.
In this way, several disadvantages of the classical genetic approaches are bypassed.
Nevertheless, the target inhibition cannot be modulated, as the effects are indeed
“off-target effects.” Although there are several limitations in using classical genetics
approaches, their application to the model organism S. cerevisiae made it possible
to learn fundamental lessons in cell biology that could be generalized to mammalian
cells. One of the most representative and fruitful efforts in their direction was that
of Leland H. Hartwell. Thanks to his studies based on S. cerevisiae , which rewarded
him with the Nobel Prize in Medicine in 2001, the cell cycle and the principal actors
involved in its control and progression were deeply understood. More than 100 genes
were identified as being involved in cell cycle control and determining the pathway
of cell cycle regulation events [35]. The pivotal role of the class of CDC (cell division
control) genes in S. cerevisiae cell cycle was then confirmed in mammalian cells by
two others scientists, who shared the Nobel prize in 2001 with L. H. Hartwell. One
of them, Paul Nurse, discovered the first human equivalent of yeast's CDC genes: the
cyclin-dependent kinase 1 (the CDK1 gene) [36].