Recombinant Human Hemoglobins Designed for Gene Therapy of Sickle Cell Disease

Two human hemoglobins designed to inhibit the polymerization of sickle hemoglobin (Hb S; α2βS 2) have been produced. Mutations that disrupt the ability of Hb S to form polymers were introduced into the normal human β-globin gene by site-specific mutagenesis. These mutations affect the axial and lateral contacts in the sickle fiber. The recombinant hemoglobin designated anti-sickling hemoglobin 1 (Hb AS1) contains the mutations β 22 glutamic acid to alanine and β 80 asparagine to lysine. Hb AS2 has the same β 22 glutamic acid to alanine mutation combined with β 87 threonine to glutamine. Human α- and βAS-globin genes were separately fused downstream of β-globin locus control region sequences and these constructs were coinjected into fertilized mouse eggs. Transgenic mouse lines that synthesize high levels of each anti-sickling hemoglobin were established and anti-sickling hemoglobins were purified from hemolysates and characterized. Both AS hemoglobins bind oxygen cooperatively and the oxygen affinities of these molecules are in the normal range. Delay time experiments demonstrate that Hb AS2 is a potent inhibitor of Hb S polymerization; therefore, locus control region βAS2-globin gene constructs may be suitable for future gene therapy of sickle cell disease.