Generation of Transgenic Mice Expressing Human Hemoglobin E.

Abstract

HbE (β26 Glu → Lys) is the most common pathogenic Hb variant in the world and is found with the greatest frequency in Southeast Asia. The HbE mutation at codon 26 of the βE-globin leads to an alternative splicing site, making this globin both a structural as well as a thalassemic mutation; furthermore, βE globin chains are unstable. HbE trait is asymptomatic and the HbEE genotype has only mild clinical manifestations. On the other hand, HbE/thalassemia presents a panoply of phenotypes from very severe to a mild beta/+ thalassemia. To date, there is no animal model of HbE disease. An animal model of this disorder could lead to better understanding of the effects of modifier genes and some of the pathogenic features of this disease. In addition, an animal model can facilitate the development of gene therapy. We report here the creation of four lines of transgenic mice that are PCR positive for HbE: two with founders that express high levels of HbE (32% human α, 26% human βE), one with low expression of HbE (9% human α, 8% βE), and one without detectable expression, but PCR positive. Knockouts (KOs) for mouse α and β globins have been bred into the founders. To date, the most severe mouse generated has a full KO of mouse α and a partial KO of mouse β. RBCs from all of the partial KOs generated to date have normal MCHC and minimally elevated reticulocyte counts (less than 1% greater than the C57Bl background). We previously demonstrated that HbEE RBCs are microcytic, but have normal MCHC. We found that HbE mice with full α-KO and partial β-KO, are microcytic (MCH 10.9 vs 14.2 for C57Bl and MCV 35.4 vs 47.5 for C57Bl), but have normal MCHC (MCHC 30.0 vs 30.5 for C57Bl). Smears from these mice exhibit numerous target cells that are not necessarily associated with low MCHC. As reported for human red cells containing HbE, elevated levels of hemoglobin oxidation products were detected in founder mice expressing HbE, but not in negative litter mates, and were found, at an even higher level, in partial KO mice expressing HbE. The next step will be to generate a mouse similar to the HbE/thalassemia phenotype by generating mice expressing exclusively HbE from either the lines expressing high levels of HbE or the line expressing low levels of HbE. These models will help elucidate several of the challenging features of the phenotypes described in humans and allow for the development of the potential cure or amelioration of HbE/thalassemia.