ALBERT

Description: Abstract 3186 Identification of new key players in erythropoiesis can lead to a better understanding of the etiology of anemia of unknown origin. Mouse models have significantly contributed to our understanding of normal erythropoiesis and the pathogenesis of erythroid disorders. Recently, we identified in the scat (severe combined anemia and thrombocytopenia) mouse model a missense mutation (G125V) in the Rasa3 gene, encoding a Ras GTPase activating protein (GAP). Homozygous scat mice present a cyclic phenotype with alternating episodes of crisis and remission. Crisis episodes are characterized by severe anemia and thrombocytopenia while, remarkably, the phenotype reverts to normal in the remission phase. Remissions are transient, however, and 94% of scat/scat mice die by P30 during a second crisis episode. We recently demonstrated a mechanism contributing to crisis episodes. The G125V mutation in Rasa3 is a loss of function mutation causing the protein to be mislocalized to the cytosol in scat reticulocytes. This results in loss of GAP activity and, as a consequence, increased levels of active GTP-bound Ras and a severe block in erythroid differentiation. Morpholino knockdowns of rasa3 in zebrafish result in profound anemia, confirming a conserved and non-redundant role for Rasa3 in vertebrate erythropoiesis. Here, we report that the cell cycle is affected in scat erythroid progenitors and extend studies to human primary erythroid cells. Using propidium iodine and flow cytometry, we found a significant increase in the G0/G1 phase (46.8% ± 3.1% in crisis vs 34.8% ± 2.5 in controls, × ± SD p

Description: Diamond Blackfan anemia (DBA) is a rare inherited bone marrow failure syndrome characterized by red blood cell hypoplasia, congenital anomalies and cancer predisposition. In addition, short stature and poor skeletal growth are found in a subset of DBA patients, suggesting similar developmental abnormalities in erythropoiesis and osteogenesis in that subset. Furthermore it has been shown recently that osteoblasts secrete erythropoietin, linking the marrow niche to the modulation of erythropoiesis. DBA has been shown in the majority of cases to result from haploinsufficiency of large or small ribosomal subunit proteins. The p53 pathway, known to be activated by abortive ribosome assembly, contributes to the erythroid failure of DBA. We studied two DBA genotypes in vitro using murine embryonic stem (ES) cell lines harboring gene trap mutations in ribosomal proteins RPS19 and RPL5, respectively. Both mutants had decreased embryoid body (EB) formation, decreased definitive erythroid colony formation and similar p53-dependent primitive erythroid differentiation defects (see Figure A). Cell cycle analyses were normal in the Rps19 mutant ES cells, but there was a significant G2/M arrest in the Rpl5 mutant ES cells, which was unaffected by p53 knockdown. In addition, the Rpl5 mutant cells had a more pronounced growth defect in culture compared to the Rps19 mutant cells (Figure B). ES cells were differentiated, in vitro, to osteoblasts using established culture conditions, and confirmed both by morphology and molecular characterization (e.g. RUNX2 and Osteopontin). Following 14 days of osteogenic differentiation, bone mineralization was confirmed via Alizarin Red staining. A marked reduction in Alizarin Red staining was seen in the Rpl5 mutant cells while there was only a slight diminution of staining in the Rps19 mutant ES cultures (see Figure C). Therefore the erythroid differentiation defect appears similar in both the Rps19 and Rpl5 mutant ES cells. However the Rpl5 mutant appears to have a more severe phenotype at the ES stage, as evidenced by a pronounced p53-independent G2/M arrest and slower growth rate and subsequently during osteogenic differentiation. These data suggest an explanation for the more severe non-erythroid phenotype seen in a subset of DBA patients. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 2242 Background: Shwachman-Diamond syndrome (SDS) is an autosomal recessive disorder characterized by pancreatic exocrine dysfunction, neurocognitive and skeletal abnormalities, and bone marrow failure. Mutations in SBDS have been shown to cause SDS. From experiments on its yeast ortholog (Haematologica 2010. 95:57-64), SBDS has been implicated in maturation and function of the 60S ribosomal subunit. In particular, subunit maturation in the SDS yeast model was associated with delayed export and accumulation of 60S-like particles in the nucleoplasm. Methods and Results: To clarify its role in human cells, erythroleukemia TF-1 cells were transduced with lentiviral vectors expressing short hairpin RNA (shRNA) against SBDS. Immunoblot assays confirmed approximately 60% knockdown in individual TF-1 cell clones expressing different shRNAs. The growth and hematopoietic colony forming potential of TF-1 knockdown cells were markedly hindered when compared to cells stably transduced with shRNA against a scrambled SBDS sequence. Using Hoechst 33342/Pyronin Y staining and flow cytometry, we also found an increased percentage of knockdown cells retained at the G0/G1 cell cycle phase. To address whether near-complete knockdown of SBDS affected ribosome synthesis as it does in yeast cells, we silenced SBDS in A549 cells. Our data revealed a reduction in polysomes but in contrast to what was observed in yeast, there was no evidence of half-mer polysomes indicative of decreased 60S subunits participating in translation. The absence of half-mers is not unusual in mammalian systems, so to better analyze the effect of SBDS on 60S subunit maturation subunit localization was assessed by co-transfection with a vector expressing a fusion between human RPL29 and enhanced GFP. Preliminary studies indicated a higher percentage of SBDS-depleted cells with nuclear localization of 60S subunits, when compared with normal controls (Fig. 1). Conclusions: Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 2237 Background: Diamond Blackfan anemia (DBA) is one of the rare inherited bone marrow failure syndromes (IBMFS), characterized by erythroid hypoplasia, congenital anomalies and a cancer predisposition. DBA is caused by ribosomal protein haploinsufficiency, which somehow triggers apoptosis of erythroid precursors, possibly through activation of p53. Some DBA patients show a response to steroid treatment, while others remain transfusion-dependent. While the mechanism of action of steroids in DBA is unclear, recent work has suggested that p53 may antagonize glucocorticoid-induced proliferation of normal erythroid progenitors1. Objective: Our goal was to create a murine embryonic stem (ES) cell model of DBA with a mutation in Rps19 to study the pathophysiology of DBA and to test glucocorticoid responsiveness. Methods: The Rps19-mutated murine embryonic stem cell line, S17-10H1 was created using a gene trap strategy. The ES cells were induced to undergo primary differentiation into embryoid bodies (EBs). Day 9–11 EBs, representing definitive hematopoiesis, were re-plated with hematopoietic cytokines (stem cell factor, interleukin (IL)-3, IL-6 and erythropoietin) in methylcellulose, and secondary differentiated colonies were scored on day 10. S17-10H1 cells were transfected by electroporation with a plasmid vector expressing either wild-type Rps19 cDNA or an empty vector control. Protein and mRNA levels of the tumor suppressor p53 were measured at the ES cell stage. Secondary differentiated hematopoietic colonies were grown with and without glucocorticoids (either dexamethasone at 1nM, 10nM, 100nM, 1μM or hydrocortisone at 50nM, 2.5μM concentration). Results: Western blot analyses confirmed S17-10H1 Rps19-haploinsufficiency. The mutant cell line had reduced EB formation (hematopoietic and non-hematopoietic) following primary differentiation. Significant defects in both erythroid (BFU-E) and myeloid (CFU-GM) formation were found following secondary hematopoietic differentiation of day 9–11 EBs. These defects were specific to Rps19 haploinsufficiency since all defects were rescued by stable transfection of the mutant cell lines with an Rps19-expressing correction vector. However, glucocorticoid treatment was unable to rescue defects in secondary differentiation (hematopoietic colony formation). Finally, we also examined baseline levels of the tumor suppressor p53, which may be induced following abortive ribosome assembly resulting from ribosomal protein haploinsufficiency. Although there was no difference in transcription levels of p53 mRNA between mutant and control cells as determined by quantitative PCR, baseline levels of p53 protein were significantly increased in the mutant ES cells as compared to controls. Conclusions: (1) In our model system, Rps19 mutation was associated with a relatively early defect in hematopoietic progenitors, since both erythroid and myeloid (or mixed) colony formation was diminished. (2) Hematopoietic defects in our murine ES cell model of DBA were rescued by Rps19 gene transfer but not by glucocorticoid therapy. (3) We found significant accumulation of baseline p53 protein (but not mRNA) levels in mutant ES cells, suggesting that Rps19 haploinsufficiency leads to inhibition of the p53-degradation pathway. (4) Taken together, these results suggest that p53 may antagonize glucocorticoid-induced proliferation in our Rps19-mutant ES cell model. Experiments to test this hypothesis are in progress. Reference: Ganguli G, Back J, Sengupta S, Wasylyk B. The p53 tumour suppressor inhibits glucocorticoid-induced proliferation of erythroid progenitors. EMBO Rep. 2002;3:569-574. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 1270 Shwachman Diamond syndrome (SDS) is a rare autosomal recessive bone marrow failure syndrome mainly characterized by neutropenia, exocrine pancreatic insufficiency and an increased risk of myelodysplastic syndrome and leukemia. The phenotype in patients is variable for unclear reasons, but approximately 90% of patients have biallelic mutations in the SBDS gene. At least one action of the SBDS protein is to couple with the GTPase ELF1 to facilitate release of the eIF6 protein from the 60S ribosome subunit, thus enabling joining of the 60S and 40S ribosome subunits, a function that has prompted many to consider SDS a “ribosomopathy”. We created a cellular model of SDS using TF-1 erythroleukemia cells transduced with lentiviral vectors containing two different shRNAs against SBDS or a scrambled sequence. Clones were grown under puromycin selection and a clone from each shRNA was selected. In each clone, knockdown of SBDS was verified at the protein level by western blot, and expression levels of SBDS were less than 1%. Both clones underwent differentiation to either myeloid or erythroid colonies by culturing in GM-CSF or erythropoietin, respectively. The 2–12 clone had a significant decrease in the number and size of both myeloid and erythroid colonies (see Table) when compared with the scrambled shRNA control. In contrast, the 1–7 clone had the same number of myeloid and erythroid colonies as the control. Previous work by other investigators in SDS yeast models revealed that missense mutations in the anti-association factor, Tif6 suppress the slow growth phenotype of SDS-mutant yeast cells. In exploring the molecular basis for the difference in phenotype observed in our TF-1 cells, we therefore focused on eIF6, the human ortholog of Tif6. The 2–12 clone had similar expression of the eIF6 protein when compared to the scrambled control. However, the 1–7 clone had a significantly decreased amount of eIF6 protein compared to the control. DNA sequencing did not reveal any mutations in the eIF6 gene, and quantitative RT-PCR showed similar levels of eIF6 mRNA transcripts, suggesting that the differences in eIF6 protein levels may be due to post-translational modifications. Pressato and colleagues (Br J Haematol 157:503, 2012) have recently speculated that the relatively benign course of SDS patients with a deletion of chromosome 20q may be due to loss of the eIF6 protein (whose gene is located on 20q). Our findings add to the hypothesis that antagonizing eIF6 may modify or rescue the SDS phenotype, possibly by reducing the requirement of SBDS in giving rise to 60S subunits lacking eIF6. Scramble colonies +/− SE 2–12 colonies +/−SE 1–7 colonies +/− SE Myeloid 131+/−4.4 112+/−3.5 p

Description: Abstract 728 Background: Diamond Blackfan anemia (DBA) is a rare inherited bone marrow failure syndrome characterized by red blood cell hypoplasia, congenital anomalies and cancer predisposition. The disease has been shown to result from haploinsufficiency of large or small ribosomal subunit proteins. The p53 pathway, known to be activated by abortive ribosome assembly, may play a role in the pathogenesis of DBA. Previously, we described murine embryonic stem (ES) cell models of DBA and reported hematopoietic and erythroid defects common to Rps19- and Rpl5-deficient cell lines, as well as a primitive erythropoiesis defect unique to an Rpl5-deficient cell line [Blood 116(21), 877, 2010]. Methods: We studied the effects of p53 knockdown on hematopoiesis in our Rps19- and Rpl5-mutant murine ES cell lines created by gene trap technology. Small interfering RNA (siRNA) targeting p53 was transfected into mutant cell lines at the ES cell stage. A non-targeting siRNA served as a negative control. After 24 hours, cells were plated into methylcellulose medium with fetal bovine serum and stem cell factor (SCF) to generate embryoid bodies (EBs). On day 7, EBs were fed with medium containing SCF, interleukin-3 (IL-3), IL-6 and erythropoietin (epo). EBs were scored on day 12 for total quantity and hematopoietic percentage. For secondary differentiation into primitive erythroid colonies, day 5 EBs were disrupted, and individual cells were suspended in a methylcellulose medium containing fetal bovine plasma-derived serum and epo. Primitive erythroid colonies were counted on day 7 of culture. Definitive hematopoiesis assays were performed by disruption of day 7 EBs, followed by suspension of cells in methylcellulose medium containing SCF, IL-3, IL-6 and epo. Definitive hematopoietic colonies were counted on day 10. In an independent set of experiments, we created an isogenic pair of wild-type and mutant DBA ES cells by electroporation of another Rps19- mutant line with a plasmid vector expressing wild-type Rps19 cDNA (wild-type) or an empty vector (mutant). Results: By immunoblot assays, we detected an increased amount of p53 protein in our Rps19-and Rpl5- mutant cell lines. Following p53 siRNA transfection, we confirmed 82–95% reduction in p53 expression by quantitative PCR, whereas ES cells transfected with non-targeting siRNA did not alter p53 expression. For both Rps19- and Rpl5- mutants, previously shown to have EB formation defects in comparison to parental controls, p53 knockdown significantly improved EB formation, especially hematopoietic-type EBs, compared to mutants treated with non-targeting siRNA. In addition, p53 knockdown in both mutants reversed the definitive hematopoiesis defect by increasing the ratio of erythroid colony to myeloid colony formation. Furthermore, p53 siRNA transfection of the Rpl5- mutant rescued the primitive erythropoiesis defect previously shown by us. To further explore the mechanistic basis of our findings, we additionally tested the effects of Rpl11 knockdown in our DBA models. The presence of free RPL11 secondary to abortive ribosome assembly has been hypothesized to be responsible for increased p53 in DBA by binding to and inhibiting the p53 inhibitor HDM2 (Mdm2 in mice). Transfection of Rpl11 siRNA into both Rps19- and Rpl5-mutant cell lines at the ES cell stage led to a marked reduction in EB formation, compared to cells transfected with non-targeting siRNA. Finally, we also extended our analysis to an isogenic pair of Rps19- wild-type and mutant cells. In the mutant line, we confirmed a 5–8 fold rescue of EB formation with siRNA targeting p53 when compared to the non-targeting siRNA. In order to clarify the role of two major downstream effectors of p53, siRNA targeting either Bax or p21 was transfected into the mutant cell line. Surprisingly, neither siRNA was able to rescue the EB formation defect of the mutant cells. Conclusions: (1) Knockdown of p53 markedly improves erythroid defects of Rps19- and Rpl5-deficient murine ES cell models of DBA, while inhibition of the upstream target Rpl11 causes significant toxicity to cells already haploinsufficient for Rps19 or Rpl5. (2) Knockdown of either Bax or p21 does not recapitulate knockdown of p53, suggesting that neither plays a significant individual role in downstream signaling from p53 in this model. (3) Further exploration of the p53 pathway may provide insights into the pathogenesis of DBA and identify new targets for therapy. Disclosures: No relevant conflicts of interest to declare.

Description: Severe sepsis is a leading cause of death and disability. Anemia in sepsis survivors affects close to 100% of patients after the third day of in-hospital stay, regardless of blood levels on admission. Circulating levels of Erythropoietin (Epo) are low; paradoxically, administration of recombinant Epo is ineffective, and related to increased morbidity. During sepsis, bone Marrow is hypoproliferative. While transfusions can improve outcome in the short term, its use increases the risk of infection and mortality without any sustained beneficial effect. The pathogenesis of anemia during sepsis is unclear. High mobility group box 1 (HMGB1), a cytokine that is a critical mediator of sepsis, is released into circulation a few days after sepsis onset, remaining increased for 8 weeks after severe sepsis. HMGB1 levels are increased for at least 8 weeks in murine models of sepsis survival. To induce severe sepsis, cecal ligation and puncture (CLP) was performed in BALB/c mice. Three days after CLP, mice developed persistent anemia, represented by a significant reduction in hematocrit (Sham=49.8±3.2 vs. CLP=29.7±6.7%; p≤0.001), hemoglobin (16.7±1.2 vs. 9.9±2.4mg/dL; p≤0.001), and red blood cells mass (10.2±0.7 vs. 5.4±1.7 x106/µL; p≤0.001). Anemia persisted for at least 25 days after CLP. In CLP survivors, reticulocyte counts were erratic, and insufficient to the degree and duration of anemia (8.2±0.8 vs. 6.6±2.1%; p=ns). Analysis of terminal erythroid differentiation using CD44 and Ter119 or CD44 and FSC as markers demonstrated a significant decrease in all erythroid progenitors, from proerythroblast to orthochromatic erythroblast. Concomitantly, mice surviving CLP developed splenomegaly. Splenic architecture was disrupted after CLP, with expansion of the red pulp, characteristic of stress erythropoiesis. Analysis of terminal erythroid differentiation demonstrated an increase in the quantity of erythroid progenitors. An anti-HMGB1 mAb (2G7) was administered after CLP. Strikingly, 2G7-treated septic mice were significantly protected from developing anemia, and had levels of hemoglobin and hematocrit similar to sham-operated mice. These results highlight a critical role for HMGB1 as key modulator of stress erythropoiesis in a murine model of sepsis survivors. To get further insight into the function of HMGB1 and translate our findings to the pathophysiology of human erythropoiesis, we used CD34+ cells derived from cord blood. Cord blood-derived CD34+ cells were incubated in MethoCult in the presence or not of HMGB1. HMGB1 induced a dose dependent decrease in CFU-E. In murine sepsis, there is a stepwise elevation of different redox forms of HMGB1, with an early increase in all-thiol (inflammatory), followed by a partially oxidized before a fully oxidized (with no known inflammatory activity) appears. At day 7, all-thiol HMGB1 reduced significantly the number of CFU-E, while the fully oxidized had no significant effect. At day 14, the number of BFU-E was reduced in the presence of HMGB1, and further decreased with all-thiol HMGB1. In conclusion, our findings suggest that CLP is a reproducible model to study anemia of sepsis. In mice surviving sepsis, stress erythropoiesis is consistently found. Administration of anti-HMGB1 monoclonal antibody reverses anemia of murine sepsis, demonstrating that HMGB1 can be a potential target in the anemia of sepsis survivors. Translating the findings to the human system, we found that HMGB1 impairs differentiation of CD34+ cells towards the BFU-E and CFU-E stages in colony formation assays, implying that HMGB1 might play a role early during differentiation. The redox status of HMGB1 is critical for its biological function, since its effects are not retrieved when HMGB1 is fully oxidized. Disclosures: No relevant conflicts of interest to declare.

Description: Fetal hemoglobin (HbF) is a known modifier of sickle cell disease (SCD) severity. KLF-1 is a regulator of the globin switch. It does so by increasing beta-globin production and up-regulating BCL11A, a repressor of HbF synthesis. Pomalidomide, a second generation immunomodulatory drug (IMiD), regulates HbF and F-cell production during erythropoiesis in human CD34+ cells. The mechanism by which pomalidomide enhances F-cell production is not well understood. In this study, CD34+ cells were obtained after purification of peripheral blood and positive selection and cultured using a three-phase in vitro liquid culture system which recapitulates erythropoiesis, including terminal differentiation and enucleation, in the presence of no drug, pomalidomide, hydroxyurea, or dimethyl sulfoxide (DMSO; vehicle control). Erythroid differentiation was assessed morphologically and by flow cytometry using the transferrin receptor and glycophorin A as markers of erythroid maturation. Flow cytometry was used to quantify F-cells. RT-qPCR was used to quantify mRNA expression of BCL11A, KLF-1, and gamma-globin. Western blot was used to measure the total expression levels of BCL11A. In this culture system pomalidomide increased F-cells more than hydroxyurea in both SCD and normal control erythroid cultures. There was a significant decrease in BCL11A expression levels, a repressor of HbF synthesis, with pomalidomide but not with hydroxyurea. This decrease was seen in both SCD and normal samples. KLF-1 was not affected by pomalidomide. These findings suggest a very different mechanism of action for pomalidomide versus hydroxyurea in increasing F-cell production. Pomalidomide appears to target the erythroid specific BCL11A but not the more pleiotropic transcription regulator KLF-1. Since the F-cell production was augmented in the presence of pomalidomide in controls as well as SCD erythroid cultures this study suggests a role for pomalidomide in the pharmacologic augmentation of fetal hemoglobin levels, perhaps in addition to hydroxyurea, not only in SCD but in any beta-hemoglobinopathy. Disclosures: Chan: BioTheryX Inc: Employment.

Description: Abstract LBA-2 Background: 5q- myelodysplastic syndrome is a rare, acquired macrocytic anemia with a female predominance. The bone marrow is characterized by a paucity of erythroid precursors with relatively normal leukocyte and platelet counts and no excess blasts. The mean age at diagnosis is approximately 70 years. The phenotype of 5q deletion has been shown to result from haploinsufficiency of the RPS14 gene. Historically red blood cell transfusions have been the primary treatment; however lenalidomide has recently been effective in ameliorating the anemia with a response rate of 67%. DBA is a rare heritable red cell aplasia which usually presents in infancy. It too is characterized by a bone marrow deficient in erythroid precursors. Mutations or deletions in eleven ribosomal protein (RP) genes, resulting in protein haploinsufficiency, have been reported in 50–60% of patients. To date RPS14 mutations have not been identified in DBA patients. Array Comparative Genomic Hybridization (CGH) has been used to identify large deletions in patients with DBA, but a more sensitive approach was hypothesized to identify additional deletions. Purpose: To address the question of whether chromosomal deletions could be the underlying defect in patients with DBA who did not have mutations in the known RP genes, Single Nucleotide Polymorphism (SNP) genotyping array hybridization was utilized. Methods: Seventy-five patient samples from the DBA Registry (DBAR) underwent resequencing of 80 RP genes. Approximately 40% of the patients had no identifiable mutation. High resolution SNP array genotyping analysis was done on 23 probands who did not have a mutation detected by resequencing. Results: An acquired internal deletion on chromosome 5q involving RPS14 was identified in one of 23 patients with presumed DBA. The patient presented with anemia at 5 10/12 years of age. The hemoglobin was 8.4 g/dl, MCV 108.2 fL, and reticulocyte count 0.4%. The erythrocyte adenosine deaminase (eADA) activity, elevated in 85% of DBA patients, was normal. The bone marrow showed decreased cellularity and megaloblastoid changes in the erythroid series. There were adequate numbers of megakaryocytes with no hypolobulation. Cytogenetics performed at diagnosis in 1991 appeared normal. The patient had no significant family history or congenital anomalies. A diagnosis of non-classical DBA was made. The patient failed a trial of corticosteroids and had remained transfusion-dependent for 19 years. No RP gene mutation was identified by sequencing. SNP array genotyping analysis identified mosaicism in two discrete regions covering ∼17.7 Mb on 5q-, with an estimated 63.7% monosomy and 36.3% disomy in this region. The major region extends from 141.1M to 157.2M (hg18), including all of the 5q- syndrome commonly deleted region (CDR) at 5q33, though it excludes the 5q31 CDR, miR146a, as well as Cdc25C and PPP2Acα, factors for which haploinsufficient expression has previously been suggested to be important in response to lenalidomide. SNP array genotyping from purified populations indicated that lymphocytes were 〉95% normal, while the myeloid cells were 〉95% 5q-. CD34+ cells showed a marked decrease in both myeloid and erythroid colony formation. Patient fibroblasts were normal and neither of the parents have 5q abnormalities by SNP analysis. Although the deletion was not identified in 1991, the 46,XX,der(5)del(5)(q15q22)del(5)(q32q33) deletion was detected on high resolution karyotyping in a post-SNP array genotyping marrow sample. Haploinsufficiency of RPS14 was confirmed by quantitative RT-PCR. After a trial of lenalidomide, complicated by Grade 4 neutropenia and Grade 3 thrombocytopenia, the patient has a reticulocyte count of 7.4% (from a previous baseline of

Description: Sickle cell disease (SCD) represents a major challenge in hematology, with approximately 100,000 Americans afflicted and the annual number of newborns with SCD set to rise over the next 40 years worldwide. Current treatment approaches rely on increasing levels of fetal hemoglobin (HbF) to prevent painful vaso-occlusive crises and hemolysis secondary to red cell sickling. Hydroxyurea remains the only pharmacologic intervention approved for SCD; however, it has limited efficacy and carries significant side effects such as myelosuppression. Thus, there is a critical need to develop drugs that enhance HbF production without similar dose limiting side effects. Second generation immunomodulatory drugs, such as pomalidomide, are a class of emerging HbF inducers both in vitro and in vivo. Recent work from our laboratory revealed that hydroxyurea and pomalidomide differentially regulate HbF production in CD34+ cells undergoing erythroid differentiation using a 3-phase culture system. Pomalidomide, but not hydroxyurea, was found to decrease BCL11A expression through a yet to be defined mechanism. In the present study, we sought to characterize erythropoiesis and the expression of key transcription factor networks in this 3-phase culture system to determine the mechanisms underlying pomalidomide’s effect. Following a four day expansion period, isolated CD34+ cells from the peripheral blood of SCD or normal individuals were differentiated along erythroid lineage in the presence of pomalidomide (1μM) or DMSO (vehicle) for 14 days. As an additional control, CD34+ cells were also treated with hydroxyurea (10μM). Proliferation and erythroid differentiation were assessed at 7, 11 and 14 days of culture. Although a 50% decrease in cell growth was noted in cells treated with hydroxyurea, no such decrement was found in control, DMSO and pomalidomide-treated cells. Moreover, pomalidomide produced a transient delay in erythroid differentiation between days 6 and 11 of culture phenotypically documented by flow cytometric analysis using glycophorin-A, α-4 integrin and band 3 as surface markers monitoring erythroid differentiation as well as morphologically by May-Grunwald Giemsa staining. In contrast, cells treated with hydroxyurea demonstrated accelerated differentiation, compared to the control cultures. However, by day 14 of culture, no significant difference was observed under any condition, suggesting that the delayed cells eventually finished terminal differentiation. In terms of HbF induction, we confirmed elevated production in the cultures with pomalidomide by measuring the number of F-cells by flow cytometry. We also evaluated the production of γ-globin chains by qRT-PCR and western blot at D4 and D11 and found a dramatic increase in the production of γ-globin, in both SCD and normal samples treated with pomalidomide. We posited that pomalidomide might foster changes in transcription factors known to play a role in both erythropoiesis and globin switching. To this end, we evaluated the expression kinetics of BCL11A, SOX6, KLF1, MI2β, GATA1 and FOG1 via qRT-PCR and western blot analyses. In DMSO-treated cultures the above transcription factors were maximally expressed between days 6-8, and their levels diminished during the remainder of the culture. Conversely, pomalidomide markedly decreased BCL11A, SOX6, KLF1 and MI2β between days 4 and 6 in cultures of both SCD and normal samples. In line with our results, MI2β acts as a positive regulator of BCL11A and KLF1, and previous studies have shown that its knock down in CD34 cells leads to decreased levels of BCL11A and KLF1. Western blot analyses confirmed the qRT-PCR data. Further, the divergent expression patterns correlated temporally with the differentiation delay suggesting that pomalidomide modulates expression of members of the BCL11Atranscription factor complex, thereby augmenting γ-globin production. Taken together, these data provide evidence that pomalidomide influences erythropoiesis by modulating transcription factor expression in CD34+ cells differentiated in the 3-phase culture system, leading to a decrease in BCL11A and activation of γ-globin production. Importantly, further exploration of these pathways that function to regulate erythropoiesis and promote HbF silencing, may help elucidate the mechanism of action of pomalidomide as well as identify additional druggable molecules. Disclosures No relevant conflicts of interest to declare.