Disruption of gastrulation and heparan sulfate biosynthesis in EXT1-deficient mice

Mutations in the EXT1 gene are responsible for human hereditary multiple ex ostosis type 1. The Drosophila EXT1 homologue, tout-velu, regulates Hedgeho g diffusion and signaling, which play an important role in tissue patternin g during both invertebrate and vertebrate development. The EXT1 protein is also required for the biosynthesis of heparan sulfate glycosaminoglycans th at bind Hedgehog. In this study, we generated EXT1-deficient mice by gene t argeting. EXT1 homozygous mutants fail to gastrulate and generally lack org anized mesoderm and extraembryonic tissues, resulting in smaller embryos co mpared to normal littermates. RT-PCR analysis of markers for visceral endod erm and mesoderm development indicates the delayed and abnormal development of both of these tissues. Immunohistochemical staining revealed a visceral endoderm pattern of Indian hedgehog (Ihh) in wild-type E6.5 embryos. Howev er, in both EXT1-deficient embryos and wild-type embryos treated with hepar itinase I, Ihh failed to associate with the cells. The effect of the EXT1 d eletion on heparan sulfate formation was tested by HPLC and cellular glycos yltransferase activity assays. Heparan sulfate synthesis was abolished in E XT1 -/- ES cells and decreased to less than 50% in +/- cell lines. These re sults indicate that EXT1 is essential for both gastrulation and heparan sul fate biosynthesis in early embryonic development. (C) 2000 Academic Press.