Inactivation of Mouse & - Globin Gene by Homologous Recombination : Mouse Model of Hemoglobin H Disease

HE HUMAN a-globin locus is located on chromosome 16 and is arranged in the order of ($(aa.’ The common molecular mechanism giving rise to a-thalassemia is caused by deletion of the a-globin structural genes. Because the a-globin genes are duplicated, a-thalassemia could result in three phenotypes. When one of the four a-globin genes in the diploid genome is deleted (-a/aa), a clinically silent carrier state results. When two of the a-globin structural genes are deleted either in cis (--ha) or in trans (&-a), the phenotype of a-thalassemia trait ensues. Deletion of three of the a-globin genes (-a/--) causes hemoglobin H (HbH) disease, which manifests as a mild to moderate hemolytic anemia, and deletion of all four a-globin genes ( -4”) results in hydrops fetalis, which is usually incompatible with life.*.’ a-Thalassemia can also be caused by point mutations. Most point mutations that inactivate the a-globin gene affect the upstream a-globin locus in the human, as this is the dominant locus and, therefore, its mutations cause more a-globin deficiency and are more readily detected than mutations of the downstream ~ O C U S . * ~ ~ In the mouse, as in most mammals, the a-globin genes are also duplicated.6 In contrast to humans, where the upstream and downstream loci are named a2 and a1, respectively, the two corresponding mouse a-globin genes are called a 1 and a2. Hence, in this report we will refer to the two human a loci as 5’ and 3’ gene and use 5‘ or a l , and 3’ or a2, respectively, for the corresponding mouse gene to avoid confusion. In this study, we investigated the function of the a-globin gene by homologous recombination in mouse embryo-derived stem (ES) cells and generated mice with disruption of the 5’ a-globin gene (daa).’ By breeding T