Neuroscience perspectivesFlanking gene and genetic background problems in genetically manipulated mice
Section snippets
Complications in induced-mutation experiments
An apt illustration of how the neglect of classic genetics can lead to problems has been the development and use of so-called gene knockout techniques, whereby homologous recombination is used to block the function of a known gene (Wynshaw-Boris et al 1999). Two common but fundamentally different genetic processes might complicate the interpretation of the results of such an induced-mutation experiment: linkage and epistatic interaction. The first might lead to false results, when an effect of
Linkage and flanking genes
Typically, a null mutation is induced in an ES cell, which then is injected into a blastocyst to generate a chimeric animal. Almost invariably, the ES cells are derived from a different inbred strain than the blastocyst. The chimera is then mated and, if the mutated ES cells have passed into the chimera's germline, a mutant line is established. In subsequent experiments, homozygous null mutants (if they are viable) are compared with homozygous wild-type animals to establish the function of the
Experimental solutions to the flanking allele problem
Recently, we proposed several relatively simple experimental solutions to the flanking gene problem (Wolfer et al 2002). Some of the proposed solutions were approximate, representing the best attainable in most situations (i.e., those situations in which the original chimera does not exist any more and has not been backcrossed to the ES cell donor). For an unbiased test of flanking gene effects, one needs a 129 co-isogenic strain as well as a C57BL/6J congenic strain for the null mutation under
The general genetic background
As explained above, genetic background effects concern the phenomenon that the expression of a mutation depends on the genotype at other loci, regardless of whether they are linked or localized on completely different chromosomes. In other words, these effects are due to epistatic interactions. Several examples are known from the classic behavior genetics literature (Ehrman and Parsons, 1981, Fuller and Thompson, 1978). An early example from the literature on targeted mutations was the
Conclusions
Often, new techniques are greeted with considerable enthusiasm—and even hype—in the research community, only to become part of the standard toolbox of the researcher once not only their advantages but also their drawbacks become more firmly established. The induction of targeted mutations has not been an exception (Gerlai 2001). Initially, it was greeted as a technique that would allow researchers to establish rapidly the functions of any gene of interest and advance our understanding of
Acknowledgements
Many of the ideas expressed above were developed over the years in stimulating discussions with my colleagues and friends Robert Gerlai (Indianapolis, Indiana), Hans-Peter Lipp, and David Wolfer (Zurich, Switzerland). I also thank the anonymous referees for their numerous valuable suggestions regarding the manuscript; any errors or omissions, however, are completely my responsibility.
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