Haploinsufficiency of Ribosomal Protein S6 In Mice Mimics Bone Marrow Failure Syndromes In Humans

Abstract 194

Haploinsufficiency of ribosomal proteins has been implicated in the pathogenesis of Diamond Blackfan anemia and 5q- myelodysplastic syndrome, two diseases that share phenotypic features including anemia and thrombocytosis. The molecular and cellular basis of the specific cytopenias observed in affected individuals has remained enigmatic. Here, we examined the effect of bone marrow-specific hemizygosity for a ribosomal protein, Rps6, in the mouse, and find that the model recapitulates many aspects of human bone marrow failure syndromes. Rps6^lox/+;Tg.MxCre animals exhibit a robust macrocytic anemia that worsens over time with a 36% reduction in red blood cell count and a 25% increase in mean corpuscular volume (MCV) at 16-weeks post induction of Cre recombinase. Mutant mice also exhibit marked leukopenia and a progressive thrombocytosis. Morphologic examination of Rps6^lox/+;Tg.MxCre/+ bone marrow reveals a prominent megakaryocytosis (2.7-fold increase) composed of small, dysplastic megakaryocytes and a 50% reduction in cellularity in compared to control animals. There is also a significant increase in the number of immature erythroid precursors (basophilic erythroblasts, P=0.004), and a significant decrease in mature erythroid cells (poly- or orthochromatic erythroblasts, P=0.027). We also measured HSC and committed progenitors in Rps6^lox/+; Tg.MxCre/+ mutant and nonmutant (Tg.MxCre/+) controls. Flow cytometric evaluation revealed that mutant animals exhibited a significant decrease in HSCs (P=0.027), accompanied by significant increases of most downstream committed progenitor populations (MPP, P=0.002; PreMegE, P=0.004; Pre CFU-E, P=8.5×10⁻⁶; MpK, P=0.00007) (Pronk et al., Cell Stem Cell 2007). Mutant animals also exhibited a significant (P=0.017) reduction of more mature erythroid progenitor cells (CFU-E/ProEry). Based on prior work implicating p53 in the pathogenesis of bone marrow failure, Rps6^lox/+;Tg.MxCre mice were crossed with p53 knockout (Trp53^ko/ko) animals and evaluated. In Rps6^lox/+;Tg.MxCre/+ animals, deficiency for p53 completely mitigated the hematopoietic alterations in the peripheral blood and the bone marrow. Together, these data indicate that Rps6 haploinsufficiency mimics many aspects of human bone marrow failure syndromes and does so through a p53-dependent pathway. This mouse model will serve as a useful tool to investigate the molecular and cellular mechanisms underlying bone marrow failure syndromes mediated by RPS haploinsufficiency.