Lack of CD54 expression and mutation of p53 gene relate to the prognosis of childhood Burkitt's lymphoma

Therapeutic implications of intratumor heterogeneity for TP53 mutational status in Burkitt lymphoma

Therapeutic implications of intra-tumor heterogeneity are still undefined. In this study we report a genetic and functional analysis aimed at defining the mechanisms of chemoresistance in a 43-year old woman affected by stage IVB Burkitt lymphoma with bulky abdominal masses and peritoneal effusion. The patient, despite a transient initial response to chemotherapy with reduction of the bulky masses, rapidly progressed and died of her disease. Targeted TP53 sequencing found that the bulky mass was wild-type whereas peritoneal fluid cells harbored a R282W mutation. Functional studies on TP53 mutant cells demonstrated an impaired p53-mediated response, resistance to ex vivo doxorubicin administration, overexpression of DNA damage response (DDR) activation markers and high sensitivity to pharmacologic DDR inhibition. These findings suggest that intra-tumor heterogeneity for TP53 mutational status may occur in MYC-driven cancers, and that DDR inhibitors could be effective in targeting hidden TP53 mutant clones in tumors characterized by genomic instability and prone to intra-tumor heterogeneity.

Rho GTPases and regulation of hematopoietic stem cell localization

Bone marrow engraftment in the context of hematopoietic stem cell and progenitor (HSC/P) transplantation is based on the ability of intravenously administered cells to lodge in the medullary cavity and be retained in the appropriate marrow space, a process referred to as homing. It is likely that homing is a multistep process, encompassing a sequence of highly regulated events that mimic the migration of leukocytes to inflammatory sites. In leukocyte biology, this process includes an initial phase of tethering and rolling of cells to the endothelium via E- and P-selectins, firm adhesion to the vessel wall via integrins that appear to be activated in an "inside-out" fashion, transendothelial migration, and chemotaxis through the extracellular matrix (ECM) to the inflammatory nidus. For HSC/P, the cells appear to migrate to the endosteal space of the bone marrow. A second phase of engraftment involves the subsequent interaction of specific HSC/P surface receptors, such as alpha(4)beta(1) integrin receptors with vascular cell-cell adhesion molecule-1 and fibronectin in the ECM, and interactions with growth factors that are soluble, membrane, or matrix bound. We have utilized knockout and conditional knockout mouse lines generated by gene targeting to study the role of Rac1 and Rac2 in blood cell development and function. We have determined that Rac is activated via stimulation of CXCR4 by SDF-1, by adhesion via beta(1) integrins, and via stimulation of c-kit by the stem cell factor-all of which involved in stem cell engraftment. Thus Rac proteins are key molecular switches of HSC/P engraftment and marrow retention. We have defined Rac proteins as key regulators of HSC/P cell function and delineated key unique and overlapping functions of these two highly related GTPases in a variety of primary hematopoietic cell lineages in vitro and in vivo. Further, we have begun to define the mechanisms by which each GTPase leads to specific functions in these cells. These studies have led to important new understanding of stem cell bone marrow retention and trafficking in the peripheral circulation and to the development of a novel small molecule inhibitor that can modulate stem cell functions, including adhesion, mobilization, and proliferation. This chapter describes the biochemical footprint of stem cell engraftment and marrow retention related to Rho GTPases. In addition, it reviews abnormalities of Rho GTPases implicated in human immunohematopoietic diseases and in leukemia/lymphoma.

p53 activates ICAM-1 (CD54) expression in an NF-kappaB-independent manner

Intercellular adhesion molecule-1 (ICAM-1) is a crucial receptor in the cell-cell interaction, a process central to the reaction to all forms of injury. Its expression is upregulated in response to a variety of inflammatory/immune mediators, including cellular stresses. The NF-kappaB signalling pathway is known to be important for activation of ICAM-1 transcription. Here we demonstrate that ICAM-1 induction represents a new cellular response to p53 activation and that NF-kappaB inhibition does not prevent the effect of p53 on ICAM-1 expression after DNA damage. Induction of ICAM-1 is abolished after treatment with the specific p53 inhibitor pifithrin-alpha and is abrogated in p53-deficient cell lines. Furthermore, we map two functional p53-responsive elements to the introns of the ICAM-1 gene, and show that they confer inducibility to p53 in a fashion similar to other p53 target genes. These results support an NF-kappaB-independent role for p53 in ICAM-1 regulation that may link p53 to ICAM-1 function in various physiological and pathological settings.

Tumor suppressor genes in normal and malignant hematopoiesis

Over the last decade, a growing number of tumor suppressor genes have been discovered to play a role in tumorigenesis. Mutations of p53 have been found in hematological malignant diseases, but the frequency of these alterations is much lower than in solid tumors. These mutations occur especially as hematopoietic abnormalities become more malignant such as going from the chronic phase to the blast crisis of chronic myeloid leukemia. A broad spectrum of tumor suppressor gene alterations do occur in hematological malignancies, especially structural alterations of p15(INK4A), p15(INK4B) and p14(ARF) in acute lymphoblastic leukemia as well as methylation of these genes in several myeloproliferative disorders. Tumor suppressor genes are altered via different mechanisms, including deletions and point mutations, which may result in an inactive or dominant negative protein. Methylation of the promoter of the tumor suppressor gene can blunt its expression. Chimeric proteins formed by chromosomal translocations (i.e. AML1-ETO, PML-RARalpha, PLZF-RARalpha) can produce a dominant negative transcription factor that can decrease expression of tumor suppressor genes. This review provides an overview of the current knowledge about the involvement of tumor suppressor genes in hematopoietic malignancies including those involved in cell cycle control, apoptosis and transcriptional control.

The role of p53 and cell death by apoptosis and necrosis in 4-hydroperoxycyclophosphamide-induced limb malformations

The exposure of embryonic murine limbs in vitro to an activated analog of cyclophosphamide, 4-hydroperoxycyclophosphamide (4OOH-CPA), induced limb malformations and apoptosis. The purpose of this study was to investigate the role of the tumor suppressor/cell cycle checkpoint gene, p53, and of cell cycle arrest in the response of the limbs to cyclophosphamide. Limbs, excised on day 12 of gestation from wild-type, heterozygous or homozygous p53-knockout transgenic murine embryos, were treated with vehicle (water) or 4OOH-CPA (0.3, 1.0 or 3.0 microgram/ml) and cultured for 6 days. Exposure of wild-type (+/+) limbs to 4OOH-CPA resulted in limb malformations, and reduced limb areas and developmental scores. The homozygous (−/−) limbs were dramatically more sensitive to the effects of 4OOH-CPA, as assessed by limb morphology, area and score. Heterozygous limbs exposed to the drug were intermediate for each parameter. Apoptosis, as assessed by the formation of a DNA ladder, was increased in drug-exposed wild-type limbs, but not in the drug-exposed homozygous limbs. Light and electron microscopy examination of the limbs revealed that drug treatment of wild-type limbs induced the morphological changes typical of apoptosis, particularly in the interdigital regions. In contrast, there was no evidence of apoptosis in homozygous limbs exposed to 4-OOH-CPA; morphological characteristics of necrosis such as cell membrane breakdown, mitochondrial swelling and cellular disintegration were evident throughout these limbs. Heterozygous limbs had cells dying with the characteristics of both apoptosis and necrosis. Fragments of poly(ADP-ribose) polymerase characteristic of necrosis predominated in the drug-treated heterozygous and homozygous limbs. 4-OOH-CPA-treatment of limbs from wild-type embryos led to arrest of the cell cycle at the G1/S phase. No cell cycle arrest was observed after drug treatment of homozygous limbs, in which populations of cells in S and G2/M phases, as well as a population of sub G1 cells, were found. Thus, the presence of p53 and of p53-dependent apoptosis protect organogenesis-stage limbs from insult with a teratogen. The absence of p53 may decrease DNA repair capacity and contribute to the accumulation of DNA damage in limb cells and their daughter cells; the failure of apoptosis to eliminate cells with DNA damage may result in increased cell death by necrosis and major limb malformations.