Deletion of the Prion Protein Does not Affect Erythropoiesis in a Mouse Model of Beta-Thalassemia

Abstract 3213

The cellular prion protein (PrPc) is a highly conserved GPI-linked cell surface sialoglycoprotein which plays a key role in the pathogenesis of neurodegenerative prion diseases. Although PrPc is conserved across species, its normal function is not clearly understood. PrPc tissue distribution varies by cell type or by level of expression in different species. Circulating red blood cells (RBCs) in humans and mice have similar levels of PrPc and reports in the literature suggest that it has a role in survival of cells under stress conditions. We previously reported that an absence of PrPc in PrPc knock out (Prnp-/-) mice negatively influenced their response to acute anemia induced with phenylhydrazine (PHZ) (Zivny et al. 2008. Blood Cells, Molecules and Diseases. 40: 302–307). Prpn-/- mice displayed a reduction of erythroid cells and erythropoietin production suggesting the importance of PrPc expression for stress erythropoiesis. In order to further explore this observation, we mated chronically anemic β-thalassemic (th3/+) male mice (generously donated by Dr. M Sadelein, NY) with female Prnp-/- mice. Th3/+ mice have a defect in the β chain of hemoglobin and have lower hematocrits (HCT). Their offspring, heterozygous for PrPc (Prnp+/−), were genotyped and anemic th3/+ Prnp+/− males were mated with non-anemic (WT) Prnp+/− females to produce mice for the study. The PrPc genotype of two cohorts of offspring combined (n=127 total), Prnp+/+ (18%), Prpn+/− (56%) and Prnp-/- (26%), did not show significant deviation from a Mendelian distribution. Similarly, the proportion of Prnp-/- genotype among anemic th3/+ offspring (29.5%) was slightly higher than the proportion of Prnp+/+ genotype (18.1%), suggesting that PrPc deletion does not lead to higher in utero mortality of th3/+ mice. Evaluation of the microhematocrit at 12 weeks of age demonstrated decreased HCT levels of th3/+ mice (37.4±5.0% and 35.1±2.9%) in comparison to their WT siblings (51.6±2.5% and 49.1±3.5%). No significant differences were detected among different Prnp genotypes in both th3/+ and WT mice. In order to test if PrPc expression in these mice is important for the recovery from acute anemia, the th3/+ and the WT siblings of all PrPc genotypes (n=6 per group) were injected with PHZ (80 mg/kg). The following parameters were measured over a 7 day period using standard methods: HCT levels, plasma erythropoietin (EPO) levels, peripheral reticulocyte count, percentage of cells in the spleen expressing CD71, and the weight of animals and their spleens at the end of the study. On day 7 after induction of acute anemic stress, plasma EPO levels were higher in th3/+ animals compared to WT animals, 1125±48.4 pg/mL and 338±104.5 pg/mL respectively. The percent of erythroid precursor cells (CD71+) were lower in th3/+ (10.0±0.7%) than in WT (21.8±2.8%) splenocytes but the spleen sizes were larger in the th3/+ mice. Hematocrits were not different with statistical significance between th3/+ (28.8±0.6%) and WT (31.5±0.75%) animals on day 7, but circulating reticulocyte counts were higher in th3/+ (54.2±4.9% vs. 40.9±3.8%) animals. The PrPc genotype (Prnp+/+, Prnp+/− and Prnp-/-) did not affect the results in either the th3/+ or WT animals. Th3/+ animals were chronically anemic and had inefficient erythropoiesis which produced a lower percentage of CD71+ cells, in spite of higher levels of plasma EPO in response to similar levels of hypoxic stress as in the WT mice. However, the overall RBC production of th3/+ mice in response to acute hypoxia was maintained by a large spleen size. The prion protein deletion did not affect the percent of RBC precursors or the final RBC output. Thus, prion protein expression does not appear to influence RBC production in a chronically anemic mouse model. The findings and conclusions in this abstract have not been formally disseminated by the Food and Drug Administration and should not be construed to represent any Agency determination or policy. (GAUK86408, GACR 310/08/0878)