Enhancer trap integrations in mouse embryonic stem cells give rise to staining patterns in chimaeric embryos with a high frequency and detect endogenous genes.

We have generated mouse embryonic stem cell lines that carry lacZ enhancer trap constructs integrated in their genome. Fifty-nine cell lines were analysed for lacZ expression in undifferentiated stem cells and at day 7.5, 8.5 and 12.5 of development in chimaeric embryos obtained after blastocyst injection. In 13 cell lines the lacZ reporter gene was expressed in undifferentiated stem cells ('blue', lines) as monitored by beta-galactosidase activity; 46 cell lines did not show detectable beta-galactosidase activity ('white', lines). In chimaeric embryos one-third of the analysed 59 embryonic stem cell lines gave rise to a variety of patterns. Six out of the 13 'blue' lines and 14 out of the 46 'white' lines showed spatially and temporally regulated patterns of beta-galactosidase expression and were additionally analysed on day 9.5. The majority of patterns showed staining exclusively or predominantly in structures of the developing nervous system, three patterns were observed only or predominantly in non-neuronal structures and five patterns were found exclusively in extraembryonic tissues. The analysis of DNA from cell lines that gave rise to staining patterns in chimaeric embryos showed that in 11 out of 15 cases simple integrations had occurred at a single site while in the remaining four cell lines multiple copies had integrated either at a single or at multiple sites. Flanking sequences from five reporter gene integrations have been cloned. At present, three integration sites have been analysed further and in all three cases we have identified transcribed sequences in the flanking DNA and isolated corresponding cDNA clones. The expression patterns of two of these genes were analysed by RNA in situ hybridisation. In both cases, expression of the endogenous genes was more widespread than the corresponding beta-galactosidase staining, suggesting that the reporter gene responded to only a subset of the regulatory elements of the endogenous gene. Our results demonstrate that enhancer trap integrations in embryonic stem cells can be used to efficiently identify transcriptional activation patterns during mouse embryogenesis and to isolate endogenous genes expressed in spatially and temporally regulated patterns.