S.B. Prusiner proposed that the infectious agent of scraple, the prion, is PrPSc, a modified form of the normal host protein PrPC. Prn-p0/0 mice devoid of PrPC showed normal development and behavior. When inoculated with mouse scrapie prions, they remained free of scrapie symptoms for at least 13 months while wild-type controls all died within 6 months. Surprisingly, heterozygous Prn-p0/+ mice also showed enhanced resistance to scrapie. After introduction of Syrian hamster PrP transgenes, Prn-p0/0 mice became highly susceptible to hamster but not to mouse prions. These experiments show that PrPC, possibly at close to normal levels, is required for the usual susceptibility to scrapie and that lack of homology between incoming prions and the host's PrP genes retards disease.

PrPC is a host protein anchored to the outer surface of neurons and to a lesser extent of lymphocytes and other cells. The transmissible agent (prion) responsible for scrapie is believed to be a modified form of PrPC. Mice homozygous for disrupted PrP genes have been generated. Surprisingly, they develop and behave normally for at least seven months, and no immunological defects are apparent. It is now feasible to determine whether mice devoid of PrPC can propagate prions and are susceptible to scrapie pathogenesis.

Primary human lymphedema (Milroy's disease), characterized by a chronic and disfiguring swelling of the extremities, is associated with heterozygous inactivating missense mutations of the gene encoding vascular endothelial growth factor C͞D receptor (VEGFR-3). Here, we describe a mouse model and a possible treatment for primary lymphedema. Like the human patients, the lymphedema (Chy) mice have an inactivating Vegfr3 mutation in their germ line, and swelling of the limbs because of hypoplastic cutaneous, but not visceral, lymphatic vessels. Neuropilin (NRP)-2 bound VEGF-C and was expressed in the visceral, but not in the cutaneous, lymphatic endothelia, suggesting that it may participate in the pathogenesis of lymphedema. By using virus-mediated VEGF-C gene therapy, we were able to generate functional lymphatic vessels in the lymphedema mice. Our results suggest that growth factor gene therapy is applicable to human lymphedema and provide a paradigm for other diseases associated with mutant receptors.