Conditional mutagenesis of the genome using site-specific DNA recombination

INTRODUCTIONAltering the genome of intact cells and organisms by site-specific DNA recombination has become an important gene-transfer methodology. DNA modifications produced by gene transfer and homologous recombination are typically static once integrated among target cell chromosomes. In contrast, the inclusion of exogenous recombinase target sequences within transferred DNA segments allows subsequent modifications to previously altered genomic structure that increase the utility of gene transfer and enhance experimental design. Creating tissue- and cell-type-specific genetic lesions in animal models, indelibly marking progenitors for cell fate mapping, inducing large-scale chromosomal rearrangements, and complementing gene defects in studies of phenotypic maintenance and reversion are all possible by directing recombinase expression using gene transfer among experimentally modified genomes. Moreover, this approach is effective in providing controlled data establishing genotype-phenotype relationships and allows for the excision of introduced marker genes that can affect neighboring chromatin structure and function. Although early work involved the yeast Flp recombinase, most studies in mammalian systems have used the Cre recombinase derived from bacteriophage P1. Both enzymes are members of the integrase family of recombinases but bind to distinct DNA target signals. Cre recombinase operates on the 34-bp loxP sequence and, like Flp, performs conservative recombination involving DNA segments positioned among these target sites.