Differential regulatory and compensatory responses in hematopoiesis/erythropoiesis in alpha- and beta-globin hemizygous mice

Characterization of hematopoiesis/erythropoiesis in thalassemias from multipotent primitive cells to mature erythrocytes is of fundamental importance and clinical relevance. We investigated this process in alpha- and beta-globin hemizygous mice, lacking the two adult tandemly organized genes from either the alpha- or beta-globin locus. Although both mice backcrossed on a homogeneous background exhibited similar reduced red blood cell (RBC) survival, beta-globin hemizygous mice had less severe reticulocyte loss and globin chain imbalance, suggesting an apparently milder thalassemia than for alpha-globin hemizygous mice. In contrast, however, beta-globin hemizygous mice displayed a more marked perturbation of hematologic parameters. Quantification of erythroid precursor subpopulations in marrow and spleen of beta-globin hemizygous mice showed more severely impaired maturation from the basophilic to orthochromatophilic erythroblasts and substantial loss of these late precursors probably as a consequence of a greater susceptibility to an excess of free alpha-chain than beta-chain. Hence, only erythroid precursors exhibiting stochastically moderate chain imbalance would escape death and mature to reticulocyte/RBC stage, leading to survival and minimal loss of reticulocytes in the beta-globin hemizygous mice. Furthermore, in response to the ineffective erythropoiesis in beta-globin hemizygous mice, a dynamic compensatory hematopoiesis was observed at earlier differentiation stage as evidenced by a significant increase of erythroid progenitors (erythroid colony-forming units approximately 100-fold) as well as of multipotent primitive cells (day 12 spleen colony-forming units approximately 7-fold). This early compensatory mechanism was less pronounced in alpha-globin hemizygous mice. The expansion of multipotent primitive and potentially stem cell populations, taken together with ineffective erythropoiesis and increased reticulocyte/RBC destruction could confer major cumulative advantage for gene targeting/bone marrow transplantation. Therefore, this study not only corroborated the strong potential effectiveness of transplantation for thalassemic hematopoietic therapy but also demonstrated the existence of a differential regulatory response for alpha- and beta-thalassemia.     

Differential expression of alpha- and beta-globin genes during differentiation of cultured erythroleukemic cells.

Murine erythroleukemic cells induced to differentiate in vitro with dimethylsulfoxide provide a model for events involved in the regulated expression of the globin genes. Here we examine alpha- and beta-globin gene expression in such cells which contain no detectable globin RNA prior to induction. To quantitate alpha- and beta-globin RNAs in cellular RNA samples by molecular hybridization techniques, highly radioactive complementary DNAs were synthesized using mouse alpha- and beta-globin RNAs purified by formamide gel electrophoresis. Maximally induced erythroleukemic cells and mouse reticulocytes contain nearly equal relative amounts of alpha- and beta-globin RNA. During the period in which globin RNA accumulates in differentiating erythroleukemic cells, however, alpha- and beta-globin RNAs are not present in equivalent amounts. alphaRNA is present in substantial excess (alpha/beta ratio 3.7) early in induction, and the alpha/beta RNA ratio progressively approaches 1 as differentiation proceeds further. These observations directly suggest that the alpha- and beta-globin genes are differentially expressed during cellular differentiation and raise questions as to how relative expression of globin genes is controlled during normal development.