Anemia is the predominant clinical manifestation of myelodysplastic syndromes (MDS). Loss or deletion of chromosome 7 is commonly seen in MDS and leads to a poor prognosis. However, the identity of functionally relevant, dysplasia-causing, genes on 7q remains unclear. Dedicator of cytokinesis 4 (DOCK4) is a GTPase exchange factor, and its gene maps to the commonly deleted 7q region. We demonstrate that DOCK4 is underexpressed in MDS bone marrow samples and that the reduced expression is associated with decreased overall survival in patients. We show that depletion of DOCK4 levels leads to erythroid cells with dysplastic morphology both in vivo and in vitro. We established a novel single-cell assay to quantify disrupted F-actin filament network in erythroblasts and demonstrate that reduced expression of DOCK4 leads to disruption of the actin filaments, resulting in erythroid dysplasia that phenocopies the red blood cell (RBC) defects seen in samples from MDS patients. Reexpression of DOCK4 in −7q MDS patient erythroblasts resulted in significant erythropoietic improvements. Mechanisms underlying F-actin disruption revealed that DOCK4 knockdown reduces ras-related C3 botulinum toxin substrate 1 (RAC1) GTPase activation, leading to increased phosphorylation of the actin-stabilizing protein ADDUCIN in MDS samples. These data identify DOCK4 as a putative 7q gene whose reduced expression can lead to erythroid dysplasia.yelodysplastic syndromes (MDS) are a group of clonal hematopoietic disorders that are characterized by cytopenias caused by ineffective hematopoiesis (1-3). Even though MDS may transform to acute leukemia in one-third of patients who have MDS, cytopenias drive morbidity for most patients (4). Most of the morbidity experienced by these patients is due to low red blood cell (RBC) counts; therefore, studies on the molecular pathogenesis of dysplastic erythropoiesis are critically needed. Cytogenetic studies have shown that stem and progenitor cells in MDS contain deletions in chromosomes 5, 7, 20, and others (5). Deletions of the chromosomal 7q region are seen in 10% of cases and are associated with significantly worse survival (6). These genomic deletions are usually large, and even though some candidate pathogenic genes have been postulated (7), it is not clear which of the deleted genes contribute to the pathogenesis of ineffective erythropoiesis and dysplasia in MDS.In a previous study (8), we had observed that numerous 7q genes, including dedicator of cytokinesis 4 (DOCK4), were deleted or epigenetically silenced in MDS, thus prompting an examination of its role in erythropoiesis in the present study. DOCK4 belongs to a large family of proteins (CED5/DOCK180/MYOBLAST CITY class) that is well conserved in mammals (9-11). They are large proteins (>200 kb) that act as signaling intermediates and provide docking sites for many other signaling molecules. One of the welldescribed functions of DOCK4 is its ability to activate GTPases, such as ras-related C3 botulinum toxin substrate 1 (RAC1) and RAP1, in man...