Abstract B27: Novel insights into the pathogenesis of myelodysplastic syndrome using Crebbp+/- mice
Authors: Zhou Ting , Cheng Ziming , Walter Christi , Rebel Vivienne I. 
Published: May 2015

Myelodysplastic syndrome (MDS) is a hematopoietic stem cell (HSC) disease, characterized by ineffective hematopoiesis, dysplasia in one or more hematopoietic lineages and increased risk of leukemic transformation. Although predominantly found in the elderly, MDS also occurs in children, particularly in those with hereditary defects in genes important for DNA repair and in those after treatment with DNA damaging alkylating agents and topoisomerase II inhibitors. Childhood MDS often progresses rapidly to acute myeloid leukemia (AML), at which point there are no good therapeutic options and thus the prognosis of this disease is extremely poor. Acquired and inherited perturbations in CREB binding protein (Crebbp) have been associated with MDS and AML in children. Mice, engineered to carry only one allele of the Crebbp gene are defective in DNA repair and all eventually will develop MDS, with some progressing to AML. Using this mouse model, we initiated a line of research aimed at improving our understanding of the cellular and molecular pathogenesis of MDS and its progression to AML.

First, in a bone marrow transplantation model, we found that the MDS-initiating cells are confined to the Lin-;Sca-1+;c-Kit++ (LSK) compartment, a subpopulation of cells enriched for HSCs. Leukemic transformation however, does not occur in LSK cells, but in its progeny. Second, we found that clinical MDS in this Crebbp+/- mouse model is associated with a markedly increased number of mutations in all hematopoietic subpopulations tested, including LSKs. Moreover, Crebbp heterozygosity not only alters the number of mutations but also the mutation spectra, suggesting that genomic instability is part of the etiology of MDS in Crebbp+/- mice. This was determined by crossing Crebbp+/- mice to mice transgenic for the mutation reporter gene, LacI, generating Crebbp+/-,LacI+/- mice and Crebbp+/-,LacI+/- control mice. From both sets of mice, we harvested hematopoietic cells at different ages, i.e., at E14.5 (when no overt hematopoietic abnormalities are present in Crebbp+/- mice), at 6 months of age (pre-MDS stage) and 12 months (clinical MDS). Mutagenesis measurements were made at the whole tissue level and in FACS-purified bone marrow subpopulations, including LSKs and myeloid cells of increasing differentiation (namely, common myeloid progenitors (CMPs), granulocyte-monocyte progenitors (GMPs), immature (Gr1lo) and mature myeloid cells (Gr1+)). Suspected mutations were confirmed by sequencing. This study provided us with additional important insights into MDS pathogenesis: 1) In control mice, the mutant frequency in hematopoietic cells isolated from 6 month-old mice is significantly higher than that in E14.5 hematopoietic cells. 2) 6 month-old LSKs from control mice show a significantly higher mutation frequency than their CMP progeny cells. These two observations fit with the notion that MDS is HSC disease (these data show HSCs simply have a higher chance to acquire the first essential mutation event for developing MDS) and the elderly (with each cell devisions, the chances increase of having acquired a potential disease-promoting mutation). 3) 6 month-old (pre-MDS) Crebbp+/- LSKs harbor significantly fewer mutations than the control, which possibly reflects the stage of increased apoptosis (as previously published).

In summary, these data suggest that MDS and its progression to AML is not simply a process confined to the HSC compartment. Unraveling the intricate details of this disease may provide essential cues for the development of novel strategies for the treatment of this dreadful disease.

Citation Format: Ting Zhou, Ziming Cheng, Christi Walter, Vivienne I. Rebel. Novel insights into the pathogenesis of myelodysplastic syndrome using Crebbp+/- mice. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr B27.