Elevated P21 (CDKN1a) Mediates Apoptosis of Beta-Thalassemic Erythroid Cells in Mice but Its Ablation Doesn't Improve Erythroid Maturation

Beta-thalassemias are caused by mutations in the β-globin gene leading to anemia. In β-thalassemia, excessive accumulation of unpaired α globin chains in erythroblasts, triggers redox-mediated reactions, which is associated with increased production of immature erythroid precursors that fail to mature. This impaired maturation is in part due to increased apoptosis of late maturing erythroblasts in β-thalassemic patients that aggravates anemia despite enhanced erythropoiesis leading to what is called ineffective erythropoiesis and ultimately resulting in extramedullary expansion of hematopoiesis. The mechanism of apoptosis in beta-thalassemia remains poorly understood.

To investigate this, we examined the status of mediators of stress response during erythroid cell maturation ofHbb^th3/+ (th3^/+) mice, a model that mimics the beta-thalassemia intermedia phenotype in humans. We found that both Foxo3 and p53 were prematurely activated in th3^/+ beta-thalassemic erythroblasts as compared to wild type controls. We crossed Hbb^th3/+ (th3^/+) and Foxo3^-/- mice and found that red blood cell (RBC) count and hemoglobin content were improved (by 1g/L, n=10), and erythroblast apoptosis was decreased to similar levels as in the WT during erythroblast maturation of double mutant mice. However, loss of Foxo3 did not ameliorate the splenomegaly of th3^/+mice. We also found that p53 direct target, p21 the cyclin-dependent kinase inhibitor was greatly upregulated in th3^/+erythroblasts as well as in beta-thalassemic patients' erythroblasts. To address the contribution of p21, we crossed p21^-/- and Th3/⁺. It showed a significant decrease of apoptosis in CD45- TER119+ erythroblasts both in the bone marrow and spleen of double mutant mice (30% and 23% reduction respectively, n=6 mice each genotype). Although, as in beta-thalassemic patients, serum erythropoietin (Epo) was elevated in the peripheral blood of th3^/+mice, the double mutant mice had significantly lower level of Epo than th3^/+ (45% reduction, n=3 mice per genotype).In p21^-/-th3^/+, CD45- TER119+ cells also showed lesser ROS accumulation(12% less, n=3 per genotypes). However, to our surprise, the deletion of p21 on beta-thalassemic background did not have any effect on splenomegaly (n=6 mice each genotype), complete blood count, hemoglobin, RBC production or bone marrow erythroid cell maturation (n=12 mice each genotype). To further examine the underlying mechanism, we analyzed cell cycle in double mutant p21^-/-th3^/+ erythroblast at distinct stages of maturation identified by CD45, TER119, CD44 and cell size (n=3 mice per genotype) using ki67 staining at distinct stages of maturation. We found p21^-/-th3^/+erythroblasts proliferate much less than their th3^/+ counterparts (basophilic erythroblasts G2 14% less, polychromatic erythroblasts 20% less, p<0.05 n=3 mice per genotype). This may partially explain lack of improvement of RBC production and anemia despite enhanced erythroblast survival. ROS levels were also reduced in double mutant p21^-/-th3^/+ erythroblasts as compared to controls. Next we investigated the status of p53 and Foxo3 in double mutant p21^-/-th3^/+ erythroblasts as compared to controls. We confirmed as we had observed earlier that nuclear p53 and Foxo3 expression were greater in th3^/+ primitive erythroid (TER119^-/low, c-KIT⁺, CD71^Hi) cells than in wild type (n=3 mice per genotype) controls. Strikingly, the double mutant p21^-/-th3^/+ erythroblasts exhibited the greatest nuclear Foxo3 in all four groups, while nuclear p53 was dramatically reduced by over 80% (n=2 mice each genotype. Each mouse taking >=30 cells to calculate nuclear MFI) even as compared to wild type. These combined studies suggest that ameliorating apoptosis may not improve anemia in beta-thalassemia.