Expression of normal and truncated forms of human endoglin

Endoglin is a transmembrane glycoprotein 633 residues in length expressed a t the surface of endothelial cells as a disulphide-linked homodimer; the sp ecific cysteine residues involved in endoglin dimerization are unknown. Mut ations in the coding region of the endoglin gene are responsible for heredi tary haemorrhagic telangiectasia type 1 (HHT1), a dominantly inherited vasc ular disorder. Many of these mutations, if translated, would lead to trunca ted forms of the protein. It is therefore of interest to assess the protein expression of different truncated forms of endoglin. Infections in vitro o r in vivo with recombinant vaccinia virus, as well as transient transfectio ns with expression vectors, were used to express normal and truncated forms of endoglin. Truncated mutants could be classified into three different gr oups: (1) those that did not produce stable transcripts; (2) those that pro duced stable transcripts but did not secrete the protein; and (3) those tha t secreted a soluble dimeric protein. This is the first time that a recombi nant truncated form of endoglin has been found to be expressed in a soluble form. Because a chimaeric construct encoding the N-terminal sequence of pl atelet/endothelial cell adhesion molecule (PECAM-1) antigen fused to residu es Ile(281)-Ala(658) of endoglin also yielded a dimeric surface protein, th ese results suggest that cysteine residues contained within the fragment Cy s(330)-Cys(412) are involved in disulphide bond formation. Infection with v accinia recombinants encoding an HHT1 mutation did not affect the expressio n of the normal endoglin, and did not reveal an association of the recombin ant soluble form with the transmembrane endoglin, supporting a haplo-insuff iciency model for HHT1.