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ABSTRACT The classical genetic approach for exploring biological pathways typically begins by identifying mutations that cause a phenotype of interest. Overexpression or misexpression of a wild-type gene product, however, can also cause mutant phenotypes, providing geneticists with an alternative yet powerful tool to identify pathway components that might remain undetected using traditional loss-of-function analysis. This review describes the history of overexpression, the mechanisms that are responsible for overexpression phenotypes, tests that begin to distinguish between those mechanisms, the varied ways in which overexpression is used, the methods and reagents available in several organisms, and the relevance of overexpression to human disease.
Moderation is a fatal thing. Nothing succeeds like excess (Oscar Wilde).
Too much of a good thing is wonderful (Mae West).
WHEN the preceding viewpoints on the benefits of excess were conveyed by Oscar Wilde and Mae West, it is safe to assume that they were not commenting on genetic methodology after scanning the latest scientific literature, but their sentiments nonetheless ring true with geneticists who have used gene overexpression as part of their research modus operandi. An impressive variety of molecular mechanisms ensures that genes are expressed at the appropriate level and under the appropriate conditions. It is obvious that a reduction of expression below some critical threshold for any given gene will result in a mutant phenotype, since such a defect essentially mimics either a partial or complete loss of function of the target gene. It is not necessarily intuitive, however, that increased expression of a wild-type gene can also be disruptive to a cell or organism, but phenotypes caused by overexpression abound (Figure 1). Serving as dramatic examples, overexpression of HER2, MYC, REL, or AKT2 are often the driving force in a variety of human cancers (Shastry 1995), and naturally occurring overexpression due to gene amplification results in drug-, insecticide-, and heavy metal-resistance (Stark and Wahl 1984). Because overexpression of wild-type genes can cause mutant phenotypes, it has been exploited by geneticists working in tractable genetic systems as a parallel approach to loss-of-function screens. This article reviews the history, applications, methods, mechanisms, and interpretation of overexpression phenotypes, focusing on its application in genetic screens but also providing examples of the utility of targeted overexpression. Many of the principles described here...