Aberrant Expression of DOCK4 Leads to Disruption of the F-Actin Skeleton and Altered Membrane Stability in MDS Erythroblasts and Mature Erythrocytes

Abstract 924

Myelodysplastic syndromes (MDS) are a group of clonal hematopoietic disorders that are commonly characterized by anemia due to ineffective hematopoiesis. Even though a third of patients with MDS may transform to acute leukemias, cytopenias drive morbidity for most patients. Anemia remains a major cause of morbidity from fatigue. Most of the morbidity experienced by such patients is due to low red blood counts and therefore studies on the molecular pathogenesis of dysplastic erythropoiesis leading to anemia are critically needed. We have identified DOCK4, an important cofactor for various GTPases located on the chromosome 7q segment as a novel silenced gene in MDS and show that it's down regulation leads to disruption of normal erythropoiesis.

In an attempt to uncover genes aberrantly expressed in MDS, we initially performed an integrative genomic analysis of primary hematopoietic cells from MDS patients. These studies revealed that DOCK4 is significantly under-expressed and hypermethylated in MDS stem and progenitor cells. Immunohistochemcial analysis revealed significantly reduced levels of DOCK4 in MDS erythroblasts. We then evaluated DOCK4 expression in a large published cohort of MDS gene expression datasets (N=183) and found that DOCK4 expression was strikingly reduced in the subset of 55 MDS patients with refractory anemia (RA; P value = 0.006). The RA subset of patients only has isolated anemia as the clinical presentation and has no apparent abnormalities in white cells or platelets. This association strongly alludes to a role of DOCK4 down-regulation in the erythroid dysplasia and anemia seen in this disease. Inorder to elucidate the functional implications of aberrant DOCK4 expression during erythropoiesis we used a dynamic model of human erythropoiesis to determine normal expression pattern during terminal differentiation and the impact of silencing DOCK4 expression on healthy primary erythroblasts. These studies revealed that DOCK4 is highly expressed during late stages of normal erythropoiesis and knockdown of DOCK4 in primary erythroblasts disrupted the F-actin skeleton. We then examined F-actin skeletal disruption in CD34+ derived erythroblasts from MDS patients. In order to quantify the extent of actin filament disruption directly in patient derived erythroblasts we first developed an assay based on multispectral flow cytometry (ImageStream™). This assay not only allowed us to visualize individually F-actin-stained cells but also allowed us to determine the percentages of cells in a given sample that contained shorter fragmented F-actin. These experiments revealed that approximately 85% of the cells in MDS patients with -7q deletion and/or hypermethylated promoter region in the DOCK4 gene contained shorter disrupted actin compared to the healthy controls that showed only 10% of the cells with disrupted F-actin. The level of F-actin disruption in healthy samples treated with cytochalasin D, an inhibitor of actin polymerization was 90%. We then examined the membrane stability of -7q MDS erythrocytes by performing osmotic fragility assays and found that these patients possessed erythrocytes that were more fragile compared to healthy erythrocytes. Based on these results we conclude that DOCK4 is an important signaling intermediate that is instrumental in maintaining erythroblast membrane homeostasis and silencing of DOCK4 in MDS contributes to anemia.