Ineffective erythropoiesis in beta-thalassemia major is due to apoptosis at the polychromatophilic normoblast stage

Objective. beta -Thalassemia major is characterized by ineffective erythrop oiesis, although it is difficult to define the dynamics of this process fro m the static information revealed by analysis of bone marrow (BM) aspirates . We aimed to study the kinetics of sequential erythroid differentiation in beta -thalassemia major. Materials and Methods. We isolated the progenitor cells (CD34(+) and CD34()CD38(-) cells) from BM of thalassemia major patients and studied in vitro erythropoiesis. This is the first report of an in vitro study in human beta -thalassemia major from purified BM CD34(+) progenitor cells, using erythr oid culture conditions, which allow unilineage differentiation to mature en ucleated red blood cells. Results. In contrast to normal donors, a high proportion of BM CD34(+) and CD34(+)CD38(-) progenitors from beta -thalassemia major coexpressed the lat e erythroid lineage-specific protein glycophorin A and generated a higher p roportion of erythroid colonies. However, despite the marked increase in er ythroid clonogenicity of the progenitor population, erythroid cultures init iated from beta -thalassemia major BM CD34(+) cells expanded 10- to 20-fold less than from normal BM. There were less viable cells during differentiat ion, specifically after the polychromatophilic normoblast stage. There was a progressive increase in the apoptotic erythroid progeny with differentiat ion, and apoptosis occurred predominantly at the polychromatophilic normobl ast stage. Conclusions. In thalassemia major, BM progenitor cells show increased eryth roid clonogenicity, increased expression of late erythroid lineage-specific proteins, and accelerated erythroid differentiation. However, despite the apparent increased erythroid commitment, ineffective erythropoiesis occurs due to apoptosis at the polychromatophil stage. Identification of the diffe rentiation stage at which apoptosis occurs will permit further studies of t he underlying mechanisms and target therapeutic strategies to improve red c ell production. (C) 2000 International Society for Experimental Hematology. Published by Elsevier Science Inc.