Sharon Lin

Role of RhoA-Specific Guanine Exchange Factors in Regulation of Endomitosis in Megakaryocytes

Polyploidization can precede the development of aneuploidy in cancer. Polyploidization in megakaryocytes (Mks), in contrast, is a highly controlled developmental process critical for efficient platelet production via unknown mechanisms. Using primary cells, we demonstrate that the guanine exchange factors GEF-H1 and ECT2, which are often overexpressed in cancer and are essential for RhoA activation during cytokinesis, must be downregulated for Mk polyploidization. The first (2N-4N) endomitotic cycle requires GEF-H1 downregulation, whereas subsequent cycles (>4N) require ECT2 downregulation. Exogenous expression of both GEF-H1 and ECT2 prevents endomitosis, resulting in proliferation of 2N Mks. Furthermore, we have shown that the mechanism by which polyploidization is prevented in Mks lacking Mkl1, which is mutated in megakaryocytic leukemia, is via elevated GEF-H1 expression; shRNA-mediated GEF-H1 knockdown alone rescues this ploidy defect. These mechanistic insights enhance our understanding of normal versus malignant megakaryocytopoiesis, as well as aberrant mitosis in aneuploid cancers.

SENP1-mediated GATA1 deSUMOylation is critical for definitive erythropoiesis

Small ubiquitin-like modifier (SUMO) modification of proteins (SUMOylation) and deSUMOylation have emerged as important regulatory mechanisms for protein function. SENP1 (SUMO-specific protease) deconjugates SUMOs from modified proteins. We have created SENP1 knockout (KO) mice based on a Cre–loxP system. Global deletion of SENP1 (SENP1 KO) causes anemia and embryonic lethality between embryonic day 13.5 and postnatal day 1, correlating with erythropoiesis defects in the fetal liver. Bone marrow transplantation of SENP1 KO fetal liver cells to irradiated adult recipients confers erythropoiesis defects. Protein analyses show that the GATA1 and GATA1-dependent genes are down-regulated in fetal liver of SENP1 KO mice. This down-regulation correlates with accumulation of a SUMOylated form of GATA1. We further show that SENP1 can directly deSUMOylate GATA1, regulating GATA1-dependent gene expression and erythropoiesis by in vitro assays. Moreover, we demonstrate that GATA1 SUMOylation alters its DNA binding, reducing its recruitment to the GATA1-responsive gene promoter. Collectively, we conclude that SENP1 promotes GATA1 activation and subsequent erythropoiesis by deSUMOylating GATA1.

Targeted gene modification of hematopoietic progenitor cells in mice following systemic administration of a PNA-peptide conjugate.

Hematopoietic stem cell (HSC) gene therapy offers promise for the development of new treatments for a variety of hematologic disorders. However, efficient in vivo modification of HSCs has proved challenging, thus imposing constraints on the therapeutic potential of this approach. Herein, we provide a gene-targeting strategy that allows site-specific in vivo gene modification in the HSCs of mice. Through conjugation of a triplex-forming peptide nucleic acid (PNA) to the transport peptide, antennapedia (Antp), we achieved successful in vivo chromosomal genomic modification of hematopoietic progenitor cells, while still retaining intact differentiation capabilities. Following systemic administration of PNA-Antp conjugates, sequence-specific gene modification was observed in multiple somatic tissues as well as within multiple compartments of the hematopoietic system, including erythroid, myeloid, and lymphoid cell lineages. As a true functional measure of gene targeting in a long-term renewable HSC, we also demonstrate preserved genomic modification in the bone marrow and spleen of primary recipient mice following transplantation of bone marrow from PNA-Antp-treated donor mice. Our approach offers a minimally invasive alternative to ex vivo gene therapy, by eliminating the need for the complex steps of stem cell mobilization and harvesting, ex vivo manipulation, and transplantation of stem cells. Therefore, our approach may provide new options for individualized therapies in the treatment of monogenic hematologic diseases such as sickle cell anemia and thalassemia.

Serum response factor is an essential transcription factor in megakaryocytic maturation

Serum response factor (Srf) is a MADS-box transcription factor that is critical for muscle differentiation. Its function in hematopoiesis has not yet been revealed. Mkl1, a cofactor of Srf, is part of the t(1;22) translocation in acute megakaryoblastic leukemia, and plays a critical role in megakaryopoiesis. To test the role of Srf in megakaryocyte development, we crossed Pf4-Cre mice, which express Cre recombinase in cells committed to the megakaryocytic lineage, to Srf(F/F) mice in which functional Srf is no longer expressed after Cre-mediated excision. Pf4-Cre/Srf(F/F) knockout (KO) mice are born with normal Mendelian frequency, but have significant macrothrombocytopenia with approximately 50% reduction in platelet count. In contrast, the BM has increased number and percentage of CD41(+) megakaryocytes (WT: 0.41% ± 0.06%; KO: 1.92% ± 0.12%) with significantly reduced ploidy. KO mice show significantly increased megakaryocyte progenitors in the BM by FACS analysis and CFU-Mk. Megakaryocytes lacking Srf have abnormal stress fiber and demarcation membrane formation, and platelets lacking Srf have abnormal actin distribution. In vitro and in vivo assays reveal platelet function defects in KO mice. Critical actin cytoskeletal genes are down-regulated in KO megakaryocytes. Thus, Srf is required for normal megakaryocyte maturation and platelet production partly because of regulation of cytoskeletal genes.

C/EBPε directs granulocytic-vs-monocytic lineage determination and confers chemotactic function via Hlx

OBJECTIVE Mutations in the CCAAT enhancer binding protein epsilon (C/EBPepsilon) gene have been identified in the cells of patients with neutrophil specific granule deficiency, a rare congenital disorder marked by recurrent bacterial infections. Their neutrophils, in addition to lacking specific granules required for normal respiratory burst activity, also lack normal phagocytosis and chemotaxis. Although the specific granule deficiency phenotype has been replicated in C/EBPepsilon(-/-) (knockout [KO]) mice, the mechanisms by which C/EBPepsilon mutations act to decrease neutrophil function are not entirely clear. MATERIALS AND METHODS In order to determine the role of C/EBPepsilon in neutrophil differentiation and migration, we generated immortalized progenitor cell lines from C/EBPepsilon KO and wild-type mice and performed expression and flow cytometric analysis and functional studies. RESULTS Expression of lineage-specific cell surface antigens on our in vitro differentiated cell lines revealed persistent expression of monocytic markers on KO granulocytes. We verified this in primary murine peripheral blood and bone marrow cells. In addition, KO bone marrow had an increase in immature myeloid precursors at the common myeloid progenitor and granulocyte/monocyte progenitor levels, suggesting a critical role for C/EBPepsilon not only in granulocyte maturation beyond the promyelocyte stage, but also in the monocyte/granulocyte lineage decision. We found that restoration of Hlx (H2.0-like homeo box 1) expression, which was decreased in C/EBPepsilon KO cells, rescued chemotaxis, but not the other defects of C/EBPepsilon KO neutrophils. CONCLUSIONS We show two new regulatory functions of C/EBPepsilon in myelopoiesis: in the absence of C/EBPepsilon, there is not only incomplete differentiation of granulocytes, but myelopoiesis is disrupted with the appearance of an intermediate cell type with monocyte and granulocyte features, and the neutrophils have abnormal chemotaxis. Restoration of expression of Hlx provides partial recovery of function; it has no effect on neutrophil maturation, but can completely ameliorate the chemotaxis defect in C/EBPepsilon KO cells.

Deletion of mecom in mouse results in early-onset spinal deformity and osteopenia

Recent studies have indicated a role for a MECOM allele in susceptibility to osteoporotic fractures in humans. We have generated a mutation in Mecom in mouse (termed ME(m1)) via lacZ knock-in into the upstream transcription start site for the gene, resulting in disruption of Mds1 and Mds1-Evi1 transcripts, but not of Evi1 transcripts. We demonstrate that ME(m1/m1) mice have severe kyphoscoliosis that is reminiscent of human congenital or primary kyphoscoliosis. ME(m1/m1) mice appear normal at birth, but by 2weeks, they exhibit a slight lumbar lordosis and narrowed intervertebral space. This progresses to severe lordosis with disc collapse and synostosis, together with kyphoscoliosis. Bone formation and strength testing show that ME(m1/m1) mice have normal bone formation and composition but are osteopenic. While endochondral bone development is normal, it is markedly dysplastic in its organization. Electron micrographs of the 1week postnatal intervertebral discs reveals marked disarray of collagen fibers, consistent with an inherent weakness in the non-osseous connective tissue associated with the spine. These findings indicate that lack of ME leads to a complex defect in both osseous and non-osseous musculoskeletal tissues, including a marked vertebral osteopenia, degeneration of the IVD, and disarray of connective tissues, which is likely due to an inherent inability to establish and/or maintain components of these tissues.

F-MuLV acceleration of myelomonocytic tumorigenesis in SV40 large T antigen transgenic mice is accompanied by retroviral insertion at Fli1 and a novel locus, Fim4

We describe here the development of a murine system for the identification of genes involved in myelomonocytic neoplasms. Transgenic C57BL/6J mice expressing SV40 early region under a myelomonocytic promoter develop histiocytic sarcomas with a latency of 167 days. We used retroviral proviral tagging to accelerate tumorigenesis and to uncover genetic changes that contribute to tumor development. Infection of transgenic mice with Friend murine leukemia virus (F-MuLV) shortened the latency of morbidity to 103 days (P< 0.001); this was associated with clonal proviral integrations in tumor DNA. As expected for F-MuLV, proviral insertions occurred at Fli1 in both transgenic and nontransgenic tumors. Four insertions were found at a novel locus, termed Fim4, on chromosome 6. This region is syntenic to human 7q32, a region that is commonly deleted in human myelodysplastic syndrome and acute myeloid leukemia. A murine BAC containing Fim4 was sequenced and analyzed, and while there was significant human–mouse homology in the area of the insertions, no candidate gene has been identified. Thus we have established a system to identify genes involved in myelomonocytic tumors, and have used it to identify Fim4, a new common site of proviral insertion. Study of this locus may provide insight into genes involved in AML-associated 7q32 deletions in humans.