The NCCN Guidelines for Acute Lymphoblastic Leukemia (ALL) focus on the classification of ALL subtypes based on immunophenotype and cytogenetic/molecular markers; risk assessment and stratification for risk-adapted therapy; treatment strategies for Philadelphia chromosome (Ph)-positive and Ph-negative ALL for both adolescent and young adult and adult patients; and supportive care considerations. Given the complexity of ALL treatment regimens and the required supportive care measures, the NCCN ALL Panel recommends that patients be treated at a specialized cancer center with expertise in the management of ALL This portion of the Guidelines focuses on the management of Ph-positive and Ph-negative ALL in adolescents and young adults, and management in relapsed settings.

Megakaryoblastic leukemia 1 (MKL1), identified as part of the t(1;22) translocation specific to acute megakaryoblastic leukemia, is highly expressed in differentiated muscle cells and promotes muscle differentiation by activating serum response factor (SRF). Here we show that Mkl1 expression is up-regulated during murine megakaryocytic differentiation and that enforced overexpression of MKL1 enhances megakaryocytic differentiation.When the human erythroleukemia (HEL) cell line is induced to differentiate with 12-O-tetradecanoylphorbol 13-acetate, overexpression of MKL1 results in an increased number of megakaryocytes with a concurrent increase in ploidy. MKL1 overexpression also promotes megakaryocytic differentiation of primary human CD34 ؉ cells cultured in the presence of thrombopoietin. The effect of MKL1 is abrogated when SRF is knocked down, suggesting that MKL1 acts through SRF. Consistent with these findings in human cells, knockout of Mkl1 in mice leads to reduced platelet counts in peripheral blood, and reduced ploidy in bone marrow megakaryocytes. In conclusion, MKL1 promotes physiologic maturation of human and murine megakaryocytes.

Lymphatic vessels (LVs) are important structures for antigen presentation, for lipid metabolism, and as conduits for tumor metastases, but they have been difficult to visualize in vivo. Prox1 is a transcription factor that is necessary for lymphangiogenesis in ontogeny and the maintenance of LVs. To visualize LVs in the lymph node of a living mouse in real time, we made the ProxTom transgenic mouse in a C57BL/6 background using red fluorescent LVs that are suitable for in vivo imaging. The ProxTom transgene contained all Prox1 regulatory sequences and was faithfully expressed in LVs coincident with endogenous Prox1 expression. The progenies of a ProxTom × Hec6stGFP cross were imaged using two-photon laser scanning microscopy, allowing the simultaneous visualization of LVs and high endothelial venules in a lymph node of a living mouse for the first time. We confirmed the expression of Prox1 in the adult liver, lens, and dentate gyrus. These intensely fluorescent mice revealed the expression of Prox1 in three novel sites: the neuroendocrine cells of the adrenal medulla, megakaryocytes, and platelets. The novel sites identified herein suggest previously unknown roles for Prox1. The faithful expression of the fluorescent reporter in ProxTom LVs indicates that these mice have potential utility in the study of diseases as diverse as lymphedema, filariasis, transplant rejection, obesity, and tumor metastasis.

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IntroductionSerum response factor (Srf), a nearly ubiquitous transcription factor that is expressed in all hematopoietic cell types, is a founding member of the MCM1-agamous-deficiens-Srf (MADS) domain-containing family of transcription factors, and binds to so called CArG sites (CC/AT-rich/GG, with CCTTATATGG emerging as a major consensus sequence). 1 More than 200 CArG boxes control expression of more than 150 Srf target genes, including genes of the cytoskeleton as well as several immediateearly genes, for example the proto-oncogene c-fos, 2 and also bcl2, 3 whereby Srf participates in apoptosis, cell growth, differentiation, and cell-cycle regulation. Srf is a downstream target of many pathways; for example, the mitogen-activated protein kinase pathway that acts through the ternary complex factors (TCF) as well as Rho signaling, 4 which promotes actin polymerization. Srf plays an important role in the regulation of smooth, skeletal, and cardiac muscle genes 5-8 during development and in adult life including aging. 9,10 Srf can be activated to promote transcription in response to extracellular signals that induce its association with specific cofactors. The 2 families of Srf cofactors are the TCF proteins (Elk1, SAP1, SAP2), 11 and the myocardin family of proteins that includes myocardin, megakaryoblastic leukemia 1 (Mkl1), and Mkl2. 1,12,13 In some cell types, TCF proteins and Mkl1 compete for binding to Srf to activate or inhibit distinct transcription targets. 14,15 In Drosophila, Srf interaction with Mkl (MAL-D) promotes cytoskeletal strength during cellular migration. 16,17 In murine embryonic stem cells, Srf is crucial for actin cytoskeletal organization and focal adhesion assembly. 18 Murine embryos lacking Srf fail to form mesoderm and thus die early in development. 19 Prior work in our laboratory has focused on acute megakaryoblastic leukemia with the t(1;22) translocation, involving fusion between an RNA binding motif protein 15 (RBM15) and Mkl1. Mkl1 is known to act as a cofactor for Srf-mediated gene activation in muscle differentiation. 12,20 We have demonstrated a physiologic role for Mkl1 in megakaryopoiesis. 21 Mkl1 expression increases with normal megakaryocyte (Mk) differentiation, and promotes Mk polyploidization. Mkl1 knockout (KO) mice have normal hematopoietic stem cells and megakaryocyte-erythroid progenitors (Pre-Meg-E). However, there is a dramatic increase in the number of CD41 ϩ megakaryocytes with most of these having low (2N) ploidy. There is a significant decrease in the number of polyploid megakaryocytes and thrombocytopenia. Mkl1 requires Srf to enhance polyploidization during megakaryocytic differentiation. Based on these findings, we have examined the effects of the Mkl1 cofactor Srf in megakaryocyte development.We show, that Srf deletion specifically in the megakaryocytic lineage leads to macrothrombocytopenia, whereas bone marrow (BM) and spleen show significant accumulation of abnormal megakaryocytes. Examination of candidate Srf target genes reveals that several actin cyt...