Norbert Perrimon, Ph.D.
Harvard Medical School
Dept. of Genetics/HHMI
200 Longwood Avenue
Boston, MA 02115

Drosophila melanogaster has been a favored tool for genetic studies for over 90 years and is an excellent model system to identify genes involved in developmental and cellular processes. The contribution made by studies in Drosophila are numerous, and many important discoveries were made first in this organism. In particular, Drosophila is a model of choice for developmental studies, signal transduction, cell biology and immunity. The Drosophila genome is about 180 Mb in size and a third of which is centric heterochromatin. Completion of the sequence of the Drosophila euchromatic regions provides us with an unprecedented resource, as we can now fully evaluate the degree of conservation of this organism with others (Adams, M. et al. 2000. Science 287: 2185-2195). The relevance of Drosophila to humans is perhaps best illustrated by the realization that more than 60% of the genes identified in human diseases (177 out of 289) have counterparts in Drosophila (Rubin G.M. et al. 2000. Science 287: 2204-2215).

The initial analysis of the Drosophila genome by the Berkeley Drosophila Genome Project (BDGP) and Celera has led to the annotation of 13,600 genes. Interestingly, the current literature discusses only approximately 20% of these genes, and only half of these have been characterized genetically. Thus, it is clear that a wealth of information remains to be mined from this model organism. Although conventional genetic approaches will clearly continue to provide valuable information, new powerful methods that can systematically and quickly analyze the functions of all ~14,000 predicted genes in specific assays are needed.

The recent advent of RNA-mediated interference (RNAi) in Drosophila cell cultures provides a direct and powerful method (Clemens et al., 2000. PNAS USA 12: 6499-6503.) to test the effects of specific gene disruption, yielding efficient phenocopy of loss-of-function mutations. The availability of gene-specific sequences from the complete genome in combination with RNAi application in cell cultures allows the remarkable opportunity to design a functional genomic approach to many cell biological and signal transduction processes. Towards this goal we have generated a set of 21,000 dsRNAs that cover every annotated gene in the Drosophila genome (in collaboration with Dr. Renato Paro's group), and have developed high-throughput RNAi screens using Drosophila cells cultured in 384-well plates. The use of model organisms to elucidate gene functions and molecular pathways has relied on genetic epistasis tests and modifier screens. Our current work demonstrates that the powerful classical genetic tools, such as loss of function analyses and genetic modifier screens, can be conducted in cell culture now as previously done in vivo.

Such a powerful, rapid approach can be applied to further illuminate and clarify the molecular pathways that relate to Tuberous Sclerosis Complex (TSC). TSC, an autosomal dominant disorder linked to mutations in the Tsc1 and Tsc2 genes, culminates in the formation of benign tumors termed hamartomas. One of the hallmarks of the cells constituting this tumor type is their increased cell size. Consistently, targeted deletion of Tsc1 or Tsc2 results in elevated levels of S6 kinase activity. This supports the current model of S6 kinase activity being repressed by the proteins of the Tsc1/Tsc2 complex. Therefore, understanding the molecular networks modifying the phosphorylation and activity of S6 kinase might open novel opportunities to regulate S6 kinase activity and revert the pathology of Tsc1/Tsc2 mutant patients. Our group will conduct a number of full genome RNAi screens to identify new components of the Tsc/S6-kinase pathway, as well better understand its structure.

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