1: Carcinogenesis. 2004 Aug;25(8):1305-13. Epub 2004 Jan 23. Recombination at chromosomal sequences involved in leukaemogenic rearrangements is differentially regulated by p53. Boehden GS, Restle A, Marschalek R, Stocking C, Wiesmuller L. Universitatsfrauenklinik, Prittwitzstrasse 43, D-89075 Ulm, Johann Wolfgang Goethe Universitat, Biozentrum, N230, R303, Marie-Curie-Strasse 9, D-60439 Frankfurt/Main, Germany. Chromosomal translocations and retroviral integration events at breakpoint cluster regions (bcrs) have been associated with leukaemias. To directly compare the effect of different cis-regulatory sequences on recombination, we adapted our SV40 based model system to the analysis of correspondingly selected bcrs from the TAL1, LMO2, retinoic acid receptor alpha (RARalpha) and MLL genes. We show that a 399 bp fragment from the MLL bcr is sufficient to cause a 3-4-fold stimulation of spontaneously occurring DNA exchange and to respond to etoposide by up to 10-fold further elevated frequencies, i.e. to mimic the fragility of the 8.3 kb bcr during chemotherapy. To analyse the regulatory role of p53 in recombination involving leukaemia-related sequences, we stably expressed wtp53 and a transactivation negative mutant. Consistent with the proposed role of p53 as a suppressor of error-prone recombination, both p53 proteins down-regulated recombination with most of the sequences tested, even with the MLL bcr after etoposide treatment. Surprisingly, however, p53 stimulated recombination, in constructs carrying the RARalpha bcr fragment. This is the first study, which provides evidence for a stimulatory role of p53 in homologous recombination. Our data further indicate that inhibition of topoisomerase I can mimic the effects of p53 on stimulating recombination on the RARalpha bcr. Thus, these data also firstly describe a biological role of the biochemical interactions between p53 and topoisomerase I that may have implications for a gain-of-function phenotype of certain p53 mutants in genetic destabilization. PMID: 14742315 [PubMed - indexed for MEDLINE] --------------------------------------------------------------- 2: Oncogene. 1999 Apr 8;18(14):2373-9. Effects of chromosomal integration site upon p53 interactions with DNA consensus sequence homologies. Cook JL, Zhang Z, Alam J, Re RN. Division of Research, Alton Ochsner Medical Foundation, New Orleans, Louisiana 70121, USA. In the present study, we report that, despite the presence of one perfect p53 consensus sequence homology (designated SCL CS) and four half-sites within the 3'-untranslated region of the stem cell leukemia (SCL) gene, the native endogenous gene is not regulated by p53. We employ a tet-repressible system to show that, under conditions in which the WAF1 mRNA steady-state level is upregulated fourfold by p53, the SCL mRNA level is not altered. In a previous report, we demonstrated that p53 interactions with the SCL CS can upregulate downstream reporter gene activity 43-fold in transient reporter assays. This disparity prompted us to explore the differences between p53 regulation of SCL CS activity in organized (chromosomally integrated) and disorganized (non-replicating episomal plasmid) chromatin. We show that p53 can increase (between 3-80-fold), decrease (between 5-33-fold) or have no effect upon transactivation of an SCL CS/reporter fusion gene depending upon chromosomal integration site. Most studies used to characterize p53 binding sites employ transient transfection assays. Our results suggest that characterization of consensus sequence homologies by assay of transiently transfected cells may be inaccurate. PMID: 10327058 [PubMed - indexed for MEDLINE] --------------------------------------------------------------- 3: Cancer Detect Prev. 1998;22(5):405-15. Expression of lymphomagenic oncogenes in T-cell lymphomas of HPV 16 transgenic mice. Yang JT, Liu CZ, Domer P, Iannaccone P. Department of Pediatrics and the Children's Memorial Institute for Education and Research, Northwestern University Medical School, Chicago, IL 60614, USA. We have previously established that a dimer repeat of the complete HPV 16 genome is sufficient to cause multiple organ malignancies, either carcinomas or T-cell lymphomas, in transgenic mice. Here, we report the expression of oncogenes supporting the notion that these tumors arose via multiple oncogenic pathways. In these mice, the transgenic HPV 16 genome cosegregated with the tumor phenotype. E6/E7 expression was observed in both carcinomas and T-cell lymphomas, while E2 expression was observed only in T-cell lymphomas. Some of the T-cell lymphomas revealed E2 expression alone, implying that oncogenic pathways of HPV other than the one involving E6/E7 existed in these transgenic mice. To establish that this is the case, expression of genes downstream from E6/E7 and oncogenes involved in T-cell lymphoma formation were analyzed. p53 mutations were observed in two of five tumors that lacked E6 expression. High levels of c-myc gene expression were observed in five of six tumors with E7 expression, suggesting that a pathway involving E7, inactivation of Rb, and activation of c-myc is important in tumorigenesis of HPV 16 in these transgenic animals. High levels of expression of the c-Pim gene were also noted in two of three c-myc-expressing T-cell lymphomas, suggesting cooperation between these two proto-oncogenes. Activation of Hox-11, Tal2/SCL-2, and Rbtn1/Ttg1 expression, which are highly associated with human T-cell acute lymphoblastic leukemia (T-ALL), was observed in three of three T-cell lymphomas with E2 expression but not E6/E7 expression, showing that pathways to tumor formation not involving E6/E7 exist in these transgenic animals. At least two oncogenic pathways to tumors in HPV 16 transgenic mice exist, one involving E6/E7 and c-myc and the other involving E2 and lymphomagenic oncogenes. PMID: 9727621 [PubMed - indexed for MEDLINE] --------------------------------------------------------------- 4: Oncogene. 1997 Dec 11;15(24):2975-83. The CD2-scl transgene alters the phenotype and frequency of T-lymphomas in N-ras transgenic or p53 deficient mice. Curtis DJ, Robb L, Strasser A, Begley CG. The Walter and Eliza Hall Institute of Medical Research, Royal Melbourne Hospital, Victoria, Australia. Abnormal expression of SCL (TAL-1/TCL5) occurs in the majority of paediatric cases of acute T-cell lymphoblastic leukemia (T-ALL). Unexpectedly however, transgenic mice carrying scl coupled to the human T-cell specific CD2 enhancer (CD2-scl) did not spontaneously develop T-cell lymphomas despite high levels of scl expression in their thymocytes. Analogous to other transgenic models of lymphomagenesis, it is likely that additional genetic abnormalities are required to cooperate with scl to trigger lymphomagenesis. Two possible candidates are the p53 and N-ras genes which are mutated in some cases of T-ALL, particularly in relapsed disease. Therefore, we examined lymphomagenesis in the progeny of CD2-scl mice crossed with N-ras transgenic mice or p53 deficient. Surprisingly, the frequency of lymphomas in the p53 nullizygous or N-ras transgenic mice was not enhanced by expression of the scl transgene. In fact, expression of scl in both genetic backgrounds paradoxically reduced the frequency of thymic lymphomas and, at least in the p53 nullizygous mice, shifted the pattern of organ involvement to the peripheral lymphoid organs. In contrast, CD2-scl transgene expression accelerated lymphomagenesis in p53 heterozygous mice. These data suggest that the collaborative effects of scl with N-ras or p53 vary according to the developmental stage of the T-cell. PMID: 9416841 [PubMed - indexed for MEDLINE] --------------------------------------------------------------- 5: Cancer Res. 1996 Nov 15;56(22):5113-9. T-cell-directed TAL-1 expression induces T-cell malignancies in transgenic mice. Condorelli GL, Facchiano F, Valtieri M, Proietti E, Vitelli L, Lulli V, Huebner K, Peschle C, Croce CM. Kimmel Cancer Center, Jefferson Medical College, Philadelphia, Pennsylvania 19107, USA. The TAL-1 gene specifies for a basic domain-helix-loop-helix protein, which is involved in the control of normal hematopoiesis. In human pathology, the TAL-1 gene product is expressed in a high percentage of T-cell acute lymphoblastic leukemias in the pediatric age range; however, it has not been established whether the expression has a causal role in oncogenesis. In this report, we describe the phenotype of mouse transgenic lines obtained by inducing tal-1 protein expression in lymphoid tissues using the LCK promoter. The survival rate of tal-1 transgenic animals was much lower as compared with control mice. Histopathological analysis revealed lymphomas of T-cell type, often comprising a minor B-cell component. Some mice showed marked splenic lymphocyte depletion. Primary lymphocyte cultures showed partial independence from exogenous growth stimuli and increased resistance to low-serum apoptosis. To further unravel the tal-1 oncogenic potential, a strain of tal-1 transgenic mice was crossbred with p53-/- mice; the survival rate in these animals was reduced by more than one-half when compared with that of tal-1 mice, and histopathological analysis revealed exclusively T-cell lymphomas. These data indicate that TAL-1, expressed in T cells, is per se a potent oncogene, which may exert a key leukemogenetic role in the majority of T-cell acute lymphoblastic leukemias. PMID: 8912842 [PubMed - indexed for MEDLINE] ---------------------------------------------------------------