Children with Down syndrome have a 10-20-fold elevated risk of developing leukemia, particularly acute megakaryoblastic leukemia (AMKL). While a subset of pediatric AMKLs is associated with the 1;22 translocation and expression of a mutant fusion protein, the genetic alterations that promote Down syndrome-related AMKL (DS-AMKL) have remained elusive. Here we show that leukemic cells from every individual with DS-AMKL that we examined contain mutations in GATA1, encoding the essential hematopoietic transcription factor GATA1 (GATA binding protein 1 or globin transcription factor 1). Each mutation results in the introduction of a premature stop codon in the gene sequence that encodes the amino-terminal activation domain. These mutations prevent synthesis of full-length GATA1, but not synthesis of a shorter variant that is initiated downstream. We show that the shorter GATA1 protein, which lacks the N-terminal activation domain, binds DNA and interacts with its essential cofactor Friend of GATA1 (FOG1; encoded by ZFPM1) to the same extent as does full-length GATA1, but has a reduced transactivation potential. Although some reports suggest that the activation domain is dispensable in cell-culture models of hematopoiesis, one study has shown that it is required for normal development in vivo. Together, these findings indicate that loss of wildtype GATA1 constitutes one step in the pathogenesis of AMKL in Down syndrome.

Room temperature negative differential resistance (NDR) has been measured through individual organic molecules on degenerately doped Si(100) surfaces using ultrahigh vacuum scanning tunneling microscopy (STM). For styrene molecules on n-type Si(100), NDR is observed only for negative sample bias because positive sample bias leads to electron stimulated desorption. By replacing styrene with a saturated organic molecule (2,2,6,6-tetramethyl-1-piperidinyloxy), electron stimulated desorption is not observed at either bias polarity. In this case, NDR is observed only for negative sample bias on n-type Si(100) and for positive sample bias on p-type Si(100). This unique behavior is consistent with a resonant tunneling mechanism via molecular orbitals and opens new possibilities for silicon-based molecular electronic devices and chemical identification with STM at the single-molecule level.

Individual and concerted actions of ER and PR highlight the prognostic and therapeutic value of PR in ER+/PR+ breast cancers.

As many as 10% of infants with Down syndrome (DS) present with transient myeloproliferative disorder (TMD) at or shortly after birth. TMD is characterized by an abundance of blasts within the peripheral blood and liver, and notably undergoes spontaneous remission in the majority of cases. TMD may be a precursor to acute megakaryoblastic leukemia (AMKL), with an estimated 30% of TMD patients developing AMKL within 3 years. We recently reported that mutations in the transcription factor GATA1 are associated with DS-AMKL. To determine whether the acquisition of GATA1 mutations is a late event restricted to acute leukemia, we analyzed GATA1 in DNA from TMD patients. Here we report that GATA1 is mutated in the TMD blasts from every infant examined. These results demonstrate that GATA1 is likely to play a critical role in the etiology of TMD, and mutagenesis of GATA1 represents a very early event in DS myeloid leukemogenesis. IntroductionChildren with Down syndrome (DS) have a 10-to 20-fold increased risk of developing leukemia, in particular acute megakaryoblastic leukemia (AMKL). 1 Children with DS are also predisposed to a related myeloid disorder, termed transient myeloproliferative disorder (TMD). 2 As many as 10% of infants with DS develop TMD, in which immature megakaryoblasts accumulate in the peripheral blood and liver. TMD spontaneously resolves in most cases, without therapeutic intervention. However, severe and sometimes fatal forms of TMD do occur, with hepatic fibrosis and liver dysfunction. On the basis of the liver infiltration and the spontaneous remission, it has been speculated that TMD may arise from fetal liver hematopoietic progenitors. 2 Of note, approximately 30% of infants with DS and TMD develop AMKL within 3 years. TMD blasts are morphologically indistinguishable from those observed in AMKL, contributing to the hypothesis that the second disease is derived from the first. 1,3 It is likely that AMKL results from the acquisition of additional genetic mutations following remission of TMD.We recently reported that mutations in the essential X-linked hematopoietic transcription factor gene GATA1 are tightly associated with AMKL in Down syndrome. 4 We detected mutations in GATA1 in 6 out of 6 DS-AMKL samples, but did not find mutations in GATA1 in leukemic cells of patients with DS who had other types of acute leukemia, or in other patients with AMKL who did not have DS. Furthermore, we did not detect GATA1 mutations in DNAs from more than 75 other patients with acute leukemia or from 21 healthy individuals. Finally, we established that these mutations are somatically acquired, as remission samples from patients did not harbor GATA1 mutations. On the basis of these observations, we hypothesized that disruption of normal GATA-1 function is an essential step in the initiation or progression of megakaryoblastic leukemia in DS.To determine whether GATA1 mutations represent a late event that contributes to the acute phase of DS myeloid leukemia, we assayed DNA samples from the peripheral blood of infa...

Ultrahigh vacuum scanning tunneling microscopy is employed for the nanofabrication and characterization of atomically registered heteromolecular organosilicon nanostructures at room temperature. In the first fabrication step, feedback controlled lithography (FCL) is used to pattern individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radical molecules at opposite ends of the same dimer row on the Si(100)-2×1:H surface. In atomic registration with the first pattern, FCL is subsequently applied for the removal of a single hydrogen atom. The resulting dangling bond templates the spontaneous growth of a styrene chain that is oriented along the underlying dimer row. The styrene chain growth is bounded by the originally patterned TEMPO molecules, thus resulting in a heteromolecular organosilicon nanostructure. The demonstration of multistep FCL suggests that this approach can be widely used for fundamental studies and fabricating prototype devices that require atomically registered organic molecules mounted on silicon surfaces.

The ligand-binding domains (LBDs) of the thyroid/retinoid receptor gene subfamily contain a series of heptad motifs important for dimeric interactions. This subfamily includes thyroid hormone receptors (T3Rs), all-trans retinoic acid (RA) receptors (RARs), 9-cis RA receptors (RARs and retinoid X receptors [RXRs]), the 1,25-dihydroxyvitamin D 3 receptor (VDR), and the receptors that modulate the peroxisomal ␤-oxidation pathway (PPARs). These receptors bind to their DNA response elements in vitro as heterodimers with the RXRs. Unliganded receptors in vivo, in particular the T3Rs, can mediate gene silencing and ligand converts these receptors into a transcriptionally active form. The in vivo interactions of these receptors with RXR were studied by using a GAL4-RXR chimera containing the yeast GAL4 DNA-binding domain and the LBD of RXR␤. GAL4-RXR activates transcription from GAL4 response elements in the presence of 9-cis RA. Unliganded T3R, which does not bind or activate GAL4 elements, represses the activation of GAL4-RXR by 9-cis RA in HeLa cells. However, addition of T3 alone leads to transcriptional activation. These findings suggest that T3R can repress or activate transcription while tethered to the LBD of GAL4-RXR and that heterodimerization can occur in vivo without stabilization by hormone response elements. Similar ligand-dependent activation was observed in HeLa cells expressing RAR, VDR, or PPAR and in GH4C1 cells from endogenous receptors. Replacement of the last 17 amino acids of the LBD of RXR␤ with the 90-amino-acid transactivating domain of the herpes simplex virus VP16 protein leads to a GAL4 constitutive activator that is repressed by wild-type T3R but not by a ninth heptad mutant that does not form heterodimers. This finding suggests that the ninth heptad of T3R is important for gene silencing and that the LBD of RXR does not exhibit silencing activity. This conclusion was verified with GAL4-LBD chimeras and with wild-type receptors in assays using appropriate response elements. These studies indicate that the LBD has diverse functional roles in gene regulation.

No abstract

The ultrahigh vacuum scanning tunnelling microscope was used to probe charge transport through two different organic monolayers adsorbed on the Si(100) substrate at room temperature. I–V measurements were taken on monolayers of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and cyclopentene for degenerately doped n-type and p-type substrates. Initial I–V measurements for transport through the TEMPO monolayer exhibited a suppression of negative differential resistance (NDR) relative to previously reported charge transport through isolated molecules. I–V measurements were also performed on isolated cyclopentene molecules and cyclopentene monolayers. Similarly to TEMPO monolayers, the cyclopentene monolayers exhibited attenuated NDR behaviour relative to isolated molecules. The observed NDR suppression suggests that the high packing density of organic monolayers influences charge transport through molecule–semiconductor junctions.