Designing Transgenes for Optimal Expression

In theory, designing a DNA construct to be used for transgene purposes, for standard pronuclear microinjection, would seem a rather easy task. The combination of a given promoter and some regulatory elements of choice, driving the expression of the construct to the desired tissue, with a suitable coding region of the gene of interest, and finishing the construct with an adequate transcription terminator would appear to be a straightforward process. However, chromosomal position effects, variegated expression, non-expressing transgenic mouse lines or those displaying ectopic and unexpected patterns of transgene expression are not uncommon. Therefore, great care should be invested in the design of the transgene, with optimal transgene expression the goal. With very few exceptions, there is no reliable catalogue of plasmid-based promoters that one could refer to when looking for robust tissue-specific transgene expression. Instead, BAC- and YAC-based transgenes have proven to produce optimal results, thus suggesting that genomic-type constructs may be more reliable as promoters than standard plasmid-type constructs. This and other observations will be discussed in this chapter. Three golden rules must be applied when designing a transgene (1) transgenes should not contain vector sequences; (2) transgenes should not contain DNA sequences derived from prokaryotic genomes; and, most importantly, (3) the more a transgene resembles the corresponding endogenous locus, the better it will behave in terms of expression levels and pattern. These very basic rules should be taken into account when preparing a DNA construct to be used as transgene, enabling easy removal of vector and prokaryotic sequences that are no longer required (and will normally have a detrimental effect upon transgene expression) and allowing the inclusion of genomic sequences that are fundamental for the faithful regulation of the locus.