Animal Model of Mitochondrial Dysfunction Generating Macrocytic Anemia and Myelodysplastic Bone Marrow Failure.

Studies on the etiology of MDS have been hampered by the lack of relevant animal model systems. Numerous murine models of MDS/MPD overlap syndromes have been described, but MDS/MPD overlap syndromes are now felt to be clinically distinct from MDS. Likewise, a number of murine models of myeloid leukemia have been developed in which dysplastic features arise during conversion to frank leukemia. Here we describe the hematopoietic phenotype of the PolgD257A mitochondrial mutator mouse and demonstrate that this model system fulfills the myeloid dysplasia criteria outlined in the Bethesda Proposals (Blood, 100, 238, 2002). The PolgD257A allele eliminates the proofreading activity of Polg, the sole mitochondrial DNA (mtDNA) polymerase, and PolgA/A mice rapidly accumulate mtDNA mutations. Starting at 9 months of age, PolgA/A mice display multiple features of accelerated aging and develop a progressive and ultimately lethal anemia (Science, 309, 481, 2005). In data obtained at time of necropsy for other studies, the hemoglobin of young PolgA/A mice is identical to that of controls but by 10 months of age begins to fall below 10 g/dL. Overall, while Polg+/+ mice maintain a hemoglobin of greater than 10 g/dL through 30 months of age, the hemoglobin of PolgA/A drops below 5 g/dL as the mice succumb to their anemia at 13–16 months of age. As the hemoglobin of aging PolgA/A mice falls the MCV demonstrates a striking increase, from 53.6 fL to 69.8 fL. In order to confirm these intial findings, complete blood counts have been assessed prospectively in mice of all three genotypes (Polg+/+, Polg+/A, and PolgA/A; 5 mice per genotype) every other month up to 8 months of age and monthly thereafter. Full data up to 11 months of age, as well as 12 month data from all 5 PolgA/A mice, will be presented. As expected, up to eight months of age no significant difference in hemoglobin levels are apparent, but by eight months PolgA/A mice demonstrate a significant increase in MCV (51.5 fL) compared to either Polg+/A (44.9, p=0.0015) or Polg+/+ (44.5, p=0.0010) littermate controls. The disparity in MCV between PolgA/A mice and the other genotypes is progressive, with the MCV at 9 months increasing to 53.4 fL in PolgA/A mice (N=5), compared to 44.8 (p<0.0001) in the available 9 month data from the other two genotypes (N=4). In order to determine if this macrocytic anemia is associated with defects in hematopoietic maturation, morphologic examinations were performed on the marrow of old (12–13 month old) mice of each genotype. Despite the profound anemia of PolgA/A mice, their marrow is hypercellular (95–100%) compared to littermate Polg+/+ mice (85–90%), displaying megakaryocyte dysplasia (paucity of cytoplasm and abnormal nuclear lobation) and an increase in immature MPO+ myeloid precursors. In sum, these feature fulfill the diagnostic criteria for a myeloid dysplasia without evidence of myeloproliferation. In order to determine if the macrocytic anemia with myelodysplastic bone marrow morphology seen in PolgA/A mice is the result of cell-intrinsic effects at the level of the hematopoietic stem cell, as opposed to systemic effects of mtDNA mutations, both competitive and non-competitive marrow transplants have been performed. Data at 16-weeks post-transplant will be presented.