The expression of the imprinted gene Ipl is restricted to extra-embryonic tissues and embryonic lateral mesoderm during early mouse development

Genes with restricted expression within the developing embryo represent valuable tools as they allow distinct tissue types to be distinguished and studied. In order to identify genes that are expressed within a particular germ layer, a differential screen was performed using germ layer-specific cDNA libraries derived from gastrulation stage mouse embryos. The gene expression profiles of the germ layers were compared following the hybridisation of some 20,000 cDNA clones with probes derived from germ layer-specific Ectoderm, Mesoderm and Endoderm libraries. A cDNA clone (5005) was identified that hybridised with the Mesoderm-derived probe but not Ectoderm or Endoderm. 5005 derives from IpI/Tssc3/BWR1C an imprinted gene which in human maps to chromosome 11p15.5. This region has been associated with Beckwith-Weidemann Syndrome, Wilms' tumour and ovarian, breast and lung cancer. In the gastrulating mouse embryo, wholemount RNA in situ hybridisation revealed that IpI expression is restricted not only to the mesodermal germ layer, but specifically to lateral mesoderm and the most posterior extent of the primitive streak from which lateral and extra-embryonic mesoderm is derived. Moreover, IpI is expressed in extra-embryonic tissues prior to gastrulation and afterwards in extra-embryonic mesoderm, ectoderm and endoderm. This expression profile indicates that IpI is a good molecular marker for embryonic mesoderm and extra-embryonic tissues. In addition heterotopic grafting studies indicate that nascent mesoderm, which expresses IpI, is restricted in its potential and therefore may be committed to its fate.