Identification of the causative gene for inherited sideroblastic anemia and analysis of sideroblastis induced in vitro

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alas2 encodes the erythroid specific 8-aniinoleyuIinate synthase (ALAS2), the first enzyme in heme biosynthesis in erythroid cells. In vivo in mice, alas2-null primitive erythroblasts showed a maturation arrest as well as massive cytoplasmic iron accumulation. However, the effect of ALAS2 deficiency, on definitive erythropoiesis still remains unclear, because alas2-null mice died in utero by embryonic day 11.5 before definitive erythropoiesis developed. To clarity the effect of heme deficiency on differentiation of definitive erythroid cells, ES cells lacking the alas2 gene were induced to differentiate specifically into erythroid lineage in vitro, and phenotypes of definitive erythroblasts were examined. In comparison to red cell pellets of Wild-type erythroblasts, alas2-null definitive erythroblasts were totally white due to a marked deficiency of heme, although the morphology was similar to wild-type erythroblasts. Consistent with their similar morphology, levels of erythroid-specific gene expression such as GATA-1, NF-E2 andTER119 in alas2-null definitive erythroblasts were also similar to those of wild-type cells, indicating that both alas2-null and wild-type erythroblasts developed to the stage of mature definitive erythroblasts. In contrast to their similar morphology, however, iron content in alas2-null definitive erythroblasts was twice more than that of wild-type, cells. Consistent with the aberrant increase in iron content, alas2-null definitive erythroblasts were more oxidized compared with wild-type erythroblasts as judged by increased formation of peroxidized metabolites. These findings suggest that alas2 deficiency does not influence erythroid differentiation per se, but induces aberrant iron accumulation and oxidized condition in definitive erythroblasts.