Chromatin insulators and position effects

Publisher Summary Most mechanisms of stable gene transfer in mammalian cells, including methods of physical transfection and virus transduction, require chromosomal integration. This allows for the stable and faithful transmission of the transferred gene to all of the progeny of the targeted cell. However, once integrated, expression of the transferred gene is subject to the influence of surrounding chromatin, a phenomenon known as “position effects.” Because chromosomal integration is generally random and the majority of the mammalian genome is often in the form of silent and condensed heterochromatin, position effects generally manifest as the partial or complete loss of expression. There are several potential approaches to overcoming the negative influence of position effects on the expression of randomly integrated genes. Such effects can be mitigated through the use of appropriate transcriptional promoters or other cis -regulatory elements such as enhancers, matrix-attachment regions, and locus control regions (LCRs). Another approach involves the use of homologous recombination to integrate the transferred gene into a specific site where the surrounding chromatin can support the desired pattern of expression. A growing body of literature has emerged on the use of an alternative class of cis -regulatory elements, known as “chromatin insulators,” as a means of overcoming the influence of position effects on the expression of genes transferred in mammalian cells. These are naturally occurring DNA elements found in a diverse range of species which function as boundary elements to separate differentially regulated chromosomal loci. This chapter discusses the problems of position effects as they apply to gene transfer and expression in mammalian cells, the properties of chromatin insulators which make them useful for abrogating these position effects, and several examples where chromatin insulators have been used successfully for this and other purposes.