TEMPORALLY AND SPATIALLY REGULATED SOMATIC MUTAGENESIS IN MICE

In mice transgenesis through oocyte injection or DNA recombination in embryonal stem (ES) cells allows mutations to be introduced into the g ermline. However, the earliest phenotype of the introduced mutation ca n eclipse later effects. We show in mice that site-specific genomic re combination can be induced ina selected cell type, B lymphocytes,,at a chosen time. This precision of somatic mutagenesis was accomplished b y limiting expression of a Ore recombinase-estrogen receptor fusion pr otein to B lymphocytes by use of tissue-specific elements in the promo ter of the transgene employed. The expressed fusion protein remained i nactive until derepressed by systemic administration of an exogenous l igand for the estrogen receptor, 4-OH-tamoxifen. Upon derepression the Ore recombinase enzyme deleted specific DNA segments, flanked by loxP sites, in B lymphocytes only. The efficiency of recombination in cell s expressing the fusion protein could be varied from low levels to >80 %, depending on the dose of ligand administered. Our work presents a p aradigm applicable to other uses of site-specific recombination in som atic mutagenesis where both temporal and spatial regulation are desire d.