BXH-2 Mice Research Articles

IRF-8 regulates expansion of myeloid-derived suppressor cells and Foxp3+ regulatory T cells and modulates Th2 immune responses to gastrointestinal nematode infection.

Interferon regulatory factor-8 (IRF-8) is critical for Th1 cell differentiation and negatively regulates myeloid cell development including myeloid-derived suppressor cells (MDSC). MDSC expand during infection with various pathogens including the gastrointestinal (GI) nematode Heligmosomoides polygyrus bakeri (Hpb). We investigated if IRF-8 contributes to Th2 immunity to Hpb infection. Irf8 expression was down-regulated in MDSC from Hpb-infected C57BL/6 (B6) mice. IRF-8 deficient Irf8-/- and BXH-2 mice had significantly higher adult worm burdens than B6 mice after primary or challenge Hpb infection. During primary infection, MDSC expanded to a significantly greater extent in mesenteric lymph nodes (MLN) and spleens of Irf8-/- and BXH-2 than B6 mice. CD4+GATA3+ T cells numbers were comparable in MLN of infected B6 and IRF-8 deficient mice, but MLN cells from infected IRF-8 deficient mice secreted significantly less parasite-specific IL-4 ex vivo. The numbers of alternatively activated macrophages in MLN and serum levels of Hpb-specific IgG1 and IgE were also significantly less in infected Irf8-/- than B6 mice. The frequencies of antigen-experienced CD4+CD11ahiCD49dhi cells that were CD44hiCD62L- were similar in MLN of infected Irf8-/- and B6 mice, but the proportions of CD4+GATA3+ and CD4+IL-4+ T cells were lower in infected Irf8-/- mice. CD11b+Gr1+ cells from naïve or infected Irf8-/- mice suppressed CD4+ T cell proliferation and parasite-specific IL-4 secretion in vitro albeit less efficiently than B6 mice. Surprisingly, there were significantly more CD4+ T cells in infected Irf8-/- mice, with a higher frequency of CD4+CD25+Foxp3+ T (Tregs) cells and significantly higher numbers of Tregs than B6 mice. In vivo depletion of MDSC and/or Tregs in Irf8-/- mice did not affect adult worm burdens, but Treg depletion resulted in higher egg production and enhanced parasite-specific IL-5, IL-13, and IL-6 secretion ex vivo. Our data thus provide a previously unrecognized role for IRF-8 in Th2 immunity to a GI nematode.

CD8α Dendritic Cells Drive Establishment of HSV-1 Latency

It is generally accepted that CD8 T cells play the key role to maintain HSV-1 latency in trigeminal ganglia of ocularly infected mice. Yet, comparably little is known about the role of innate immunity in establishment of viral latency. In the current study, we investigated whether CD8α DCs impact HSV-1 latency by examining latency in the trigeminal ganglia (TG) of wild-type (WT) C57BL/6 versus CD8α−/− (lack functional CD8 T cells and CD8α+ DCs), CD8β−/− (have functional CD8α+ T cells and CD8α+ DCs), and β2m−/− (lack functional CD8 T cells but have CD8α+ DCs) mice as well as BXH2 (have functional CD8 T cells but lack CD8α+ DCs) versus WT C3H (have functional CD8α T cells and CD8α+ DCs) mice. We also determined whether the phenotype of CD8α−/− and BXH2 mice could be restored to that of WT mice by adoptive transfer of WT CD8+ T cells or bone marrow (BM) derived CD8α+ DCs. Our results clearly demonstrate that CD8α DCs, rather than CD8 T cells, are responsible for enhanced viral latency and recurrences.

Irf8-Regulated Genomic Responses Drive Pathological Inflammation during Cerebral Malaria

Interferon Regulatory Factor 8 (IRF8) is required for development, maturation and expression of anti-microbial defenses of myeloid cells. BXH2 mice harbor a severely hypomorphic allele at Irf8 (Irf8R294C) that causes susceptibility to infection with intracellular pathogens including Mycobacterium tuberculosis. We report that BXH2 are completely resistant to the development of cerebral malaria (ECM) following Plasmodium berghei ANKA infection. Comparative transcriptional profiling of brain RNA as well as chromatin immunoprecipitation and high-throughput sequencing (ChIP-seq) was used to identify IRF8-regulated genes whose expression is associated with pathological acute neuroinflammation. Genes increased by infection were strongly enriched for IRF8 binding sites, suggesting that IRF8 acts as a transcriptional activator in inflammatory programs. These lists were enriched for myeloid-specific pathways, including interferon responses, antigen presentation and Th1 polarizing cytokines. We show that inactivation of several of these downstream target genes (including the Irf8 transcription partner Irf1) confers protection against ECM. ECM-resistance in Irf8 and Irf1 mutants is associated with impaired myeloid and lymphoid cells function, including production of IL12p40 and IFNγ. We note strong overlap between genes bound and regulated by IRF8 during ECM and genes regulated in the lungs of M. tuberculosis infected mice. This IRF8-dependent network contains several genes recently identified as risk factors in acute and chronic human inflammatory conditions. We report a common core of IRF8-bound genes forming a critical inflammatory host-response network.

PML-RAR co-operates with Sox4 in acute myeloid leukemia development in mice

Acute promyelocytic leukemia is characterized by a chromosomal translocation involving the retinoic acid receptor alpha gene. To identify co-operating pathways to leukemogenesis, we crossed MRP8-PML/RARA transgenic mice with BXH-2 mice which harbor an endogenous murine leukemia virus that causes acute myeloid leukemia. Approximately half of the leukemias that arose in this cross showed features of acute promyelocytic leukemia. We identified 22 proviral insertion sites in acute promyelocytic-like leukemias and focused our analysis on insertion at Sox4, a HMG box transcription factor. Using a transplant model, co-operation between PML-RARα and Sox4 was confirmed with increased penetrance and reduced latency of disease. Interestingly, karyotypic analysis revealed cytogenetic changes suggesting that the factors combined to initiate but not complete leukemic transformation. The cooperation between these transcription factors is consistent with the paradigm of multiple routes to the disease and reinforces the concept that transcription factor networks are important therapeutic targets in myeloid leukemias.

Enhanced expression of p210BCR/ABL and aberrant expression of Zfp423/ZNF423 induce blast crisis of chronic myelogenous leukemia

AbstractChronic myelogenous leukemia (CML) is a hematopoietic disorder originating from p210BCR/ABL-transformed stem cells, which begins as indolent chronic phase (CP) but progresses into fatal blast crisis (BC). To investigate molecular mechanism(s) underlying disease evolution, CML-exhibiting p210BCR/ABL transgenic mice were crossed with BXH2 mice that transmit a replication-competent retrovirus. Whereas nontransgenic mice in the BXH2 background exclusively developed acute myeloid leukemia, p210BCR/ABL transgenic littermates developed nonmyeloid leukemias, in which inverse polymerase chain reaction detected 2 common viral integration sites (CISs). Interestingly, one CIS was transgene’s own promoter, which up-regulated p210BCR/ABL expression. The other was the 5′ noncoding region of a transcription factor, Zfp423, which induced aberrant Zfp423 expression. The cooperative activities of Zfp423 and p210BCR/ABL were demonstrated as follows: (1) introduction of Zfp423 in p210BCR/ABL transgenic bone marrow (BM) cells increased colony-forming ability, (2) suppression of ZNF423 (human homologue of Zfp423) in ZNF423-expressing, p210BCR/ABL-positive hematopoietic cells retarded cell growth, (3) mice that received a transplant of BM cells transduced with Zfp423 and p210BCR/ABL developed acute leukemia, and (4) expression of ZNF423 was found in human BCR/ABL-positive cell lines and CML BC samples. These results demonstrate that enhanced expression of p210BCR/ABL and deregulated expression of Zfp423/ZNF423 contribute to CML BC.

Disseminated and Rapidly Fatal Tuberculosis in Mice Bearing a Defective Allele at IFN Regulatory Factor 8

The interferon regulatory factor (IRF) family member IRF-8 participates in IFN-gamma-dependent transcriptional activation of genes containing in their promoter regions IFN-stimulated response element or IFN-gamma activation site elements. To test the role of IRF-8 in host defenses against tuberculosis, BXH-2 mice, which bear a defective IRF-8(R294C) allele, were challenged with low doses of virulent Mycobacterium tuberculosis via the i.v. and aerosol routes. BXH-2 mice were found to be extremely susceptible to M. tuberculosis, as demonstrated by rapid and uncontrolled microbial replication in spleen, liver, and lungs leading to very early death. The BXH-2 defect was expressed very early (10 days postinfection) as uncontrolled intracellular pathogen replication in NOS2-expressing lung macrophages, impaired granuloma formation, rapid dissemination of the infection to distant sites, and rapid necrosis of infected tissues. There was complete absence of IL-12p40 induction, severely reduced IFN-gamma production, and impaired T cell priming in the lungs of infected BXH-2, highlighting the critical role of IRF-8 in this process. Collectively, these results identify IRF-8 as a critical regulator of host defenses against tuberculosis.

The BXH2 mutation in IRF8 differentially impairs dendritic cell subset development in the mouse

Among dendritic cell (DC) subsets, CD8alpha(+) DCs and plasmacytoid DCs (pDCs) produce high levels of IL12 and type I interferons (IFNs), respectively, and confer early innate immunity. Development of CD8alpha(+) DCs and pDCs requires the interferon regulatory factor 8 (IRF8). Recently, a spontaneous point mutation was identified in the Irf8/Icsbp gene in the BXH2 mouse, which exhibits an immunodeficient phenotype similar to the IRF8 knockout (KO) mouse. We show that this mutation, designated IRF8(R294C), abolishes the development of CD8alpha(+) DCs without impairing pDC development, and eliminates production of IL12p40, while retaining that of type I IFNs. Electrophoretic mobility shift and chromatin immunoprecipitation assays indicated that IRF8(R294C) failed to interact with partner transcription factors and did not bind certain promoters that require partner interactions. Together, this work indicates that IRF8-partner interactions play different roles in CD8alpha(+) DCs and pDCs, revealing a mechanistic separation that underlies development of these DC subsets.

B Cell Lineage-Specific Deletion of Icsbp/IRF8 Reveals Roles for IRF8 in the Regulation of Marginal Zone and Follicular B Cell Development.

Interferon regulatory factor 8 (IRF8), also known as interferon consensus sequence-binding protein (ICSBP), is a transcription factor that expresses in T cells, B cells and macrophages and plays a role in myeloid development. Targeted deletion of IRF8 in mice (IRF8−/−) induced progressive increase in the numbers of granulocytes in various lymphoid organs and development of a syndrome similar to human chronic myelogenous leukemia. In addition to defective development of macrophages and dendritic cells, B cell development was also impaired in IRF8−/− mice. This includes decreased numbers of early B cells, expanded marginal zone (MZ) B cells and diminished follicular (OF) B2 cells. Because abnormal myeloid cells could alter microenvironment required for normal B cell development, we have generated IRF8 conditional knockout mice to specifically investigate the function of IRF8 in B lineage cells. Mice were engineered to have exon 2, encoding the DNA binding domain of IRF8, flanked by loxP sites (designated IRF8f/+). These mice were then crossed with the CD19Cre strain in which the expression of Cre-recombinase is controlled by the endogenous CD19 locus. Homozygous mice (designated (IRF8f/f x Cre)F1) underwent germline excision of IRF8 in CD19+ B lineage cells. As a result, there was no detectable mRNA and protein of IRF8 in their splenic B cells. Flow cytometry analysis revealed expanded MZ B cells and reduced OF B2 cells in the spleen of (IRF8f/f x Cre)F1 mice. Interestingly, the expression level of CD23 on OF B cells was significantly decreased in (IRF8f/f x Cre)F1 mice, indicating that IRF8 is required for maintaining a normal OF phenotype. In the peritoneum of (IRF8f/f x Cre)F1 mice, while the numbers of B1a and B2 cells were slightly decreased, the number of B1b cells was slightly increased. Furthermore, BXH2 mice carrying a mutation (C915T) in the Icsbp1 gene exhibited similar expansion of MZ B cells and low expression of CD23 in OF B cells. Taken together, these analyses indicate that IRF8 is required for development of normal MZ and B2 cells.

Icsbp1/IRF-8 Is Required for Innate and Adaptive Immune Responses against Intracellular Pathogens

The chronic myeloid leukemia syndrome of the BXH-2 mouse strain (Mus musculus) is caused by a recessive mutation (R294C) in the transcriptional regulator Icsbp1/IRF-8. In trans activation assays using an IL-12p40 gene reporter construct introduced in RAW 264.7 mouse macrophages, we show that the Icsbp1(C294) isoform behaves as a partial loss-of-function. The Icsbp1(C294) hypomorph allele appears to have a threshold effect on IL-12 production, with pleiotropic consequences on resistance to different types of infections in vivo. Despite the presence of a resistance Nramp1(G169) allele, BXH-2 mice (Icsbp1(C294)) show impaired control of Mycobacterium bovis (bacille Calmette-Guérin) multiplication both early and late during infection, with uncontrolled replication linked to inability to form granulomas in infected liver and spleen. Studies in informative (BXH-2 x BALB/cJ)F(2) mice show that homozygosity for Icsbp1(C294) causes susceptibility to Salmonella enterica serovar Typhimurium to a level comparable to that seen for mice lacking functional Nramp1 or TLR4. Finally, impaired Icsbp1(C294) function is associated with the following: 1) increased replication of the Plasmodium chabaudi AS malarial parasite during the first burst of blood parasitemia, and 2) recurring waves of high blood parasitemia late during infection. These results show that Icsbp1 is required for orchestrating early innate responses and also long-term immune protection against unrelated intracellular pathogens.

Bcl11a Causes Leukemia/Lymphoma in Cooperation with Nf1 Inactivation.

Loss of the NF1 (Neurofibromatosis Type 1) tumor suppressor gene is associated with chronic and acute forms of myeloid diseases including juvenile myelomonocytic leukemia (JMML). In the BXH-2 strain of mice, acute myeloid leukemia (AML) is caused by chronic infection by a murine leukemia virus (MuLV). We have reported the identification of 37 common proviral insertion sites (CIS), loci containing candidate genes whose mutation could cooperate with inactivation of Nf1 to result in BXH-2 AML. Among these candidate genes, Bcl11a, was shown to be expressed in JMML and CMML leukemia samples, and to confer a growth advantage when expressed in Nf1-deficient myeloblast cells. In this report, we have first tried to determine whether dysregulation of Bcl11a affects cellular behavior other than promoting cell growth. Using a granulocyte-colony stimulating factor induced myeloid terminal differentiation of 32D cell culture model, we found no difference in the percentage of differentiated cells between Bcl11a-transduced cells and empty vector-transduced cells, indicating overexpression of Bcl11a may have no influence on myeloid differentiation. Moreover, no effect of Bcl11a on 32D survival after IL-3 withdrawal was detected. To investigate the potential role of Bcl11a in the development of leukemia with Nf1 loss, a bone marrow transplantation assay was then performed. Bone marrow cells were collected from the Nf1(+/+) or Nf1(Flox/Fcr) donor mice carrying pIpC-induced expression of Cre, and transduced with Bcl11a-expressing retroviral construct or empty vector, before transplantation via the tail vein into the lethally irradiated recipient BoyJ mice. Successful bone marrow engraftment, pIpC-induced deletion of Nf1 and delivery of Bcl11a were confirmed by PCR on peripheral blood cells. Interestingly, 5 months after pIpC induction, 10 out of 11 Nf1(Flox/Fcr);Cre+;Bcl11a-transduced bone marrow recipient mice became moribund, whereas no or only 1 out of 10 or 11 recipient mice in each control groups died for unknown reason. The Nf1(Flox/Fcr);Cre+;Bcl11a-transduced mice display myeloid blast-like cells in their peripheral blood, enlarged lymph node and liver, and splenomegaly. Spleen touchpreps also showed excessively infiltration of blast-like cells in their spleens. Unexpectedly, two out of ten moribund recipient mice also developed markedly enlarged thymi. These tumors were demonstrated to be derived from transplanted donor cells by immunophenotyping for the presence of the differential cell surface marker CD45.2, expressed on the donor cells. The nature and cell lineage origin of these tumors are under study. These data show that BXH-2 mouse is a powerful genetic system to dissect Nf1-cooperating genetic events in leukemogenesis, and suggest that overexpression of Bcl11a can induce leukemia/lymphoma with Nf1 gene inactivation.

C-Myb Is an Essential Downstream Target for Homeobox Mediated Transformation of Hematopoietic Cells.

Overexpression of Hox genes is frequently associated with leukemogenic transformation of hematopoietic cells. In particular the abdominal-type HoxA genes together with their homeodomain containing dimerization partners of the TALE (three amino acid loop extension) family are well documented oncogenes in various hematologic malignancies. Paradigmatic examples are Hoxa9 and Meis1 that are coactivated by retroviral insertions in the BXH2 mouse leukemia model. In addition Hoxa9 is a fusion partner of NUP98 in acute myeloid leukemias with a t(7;11) and Meis1 as well as Hoxa9 are critical targets of MLL (mixed lineage leukemia) fusion proteins. Despite their important role in leukemogenesis little is known how they exert their transforming function. Here we used a novel system of transient transformation by an inducible oncoprotein to generate populations of primary hematopoietic cells that overexpress Hoxa9 in combination with Meis1. The RNA inventory of these cells was compared to control cells by Affymetrix array analysis. Interestingly the RNA of c-Myb (myeloblastosis leukemia virus oncogene) and of a known c-myb downstream target (Gstm1) was amongst the top ten genes upregulated by Hoxa9/Meis1. c-Myb overexpression was verified by Northern blot and quantitative RT-PCR. c-Myb levels also responded to the introduction of MLL-ENL, an oncoprotein that upregulates Hoxa9 and Meis1. Small interfering RNAs directed against c-Myb suppressed transformation by MLL-ENL but had no effect on transformation of hematopoietic precursors by E2A-HLF an oncogene that does not work by increasing Hoxa9/Meis1 levels. The anti c-Myb siRNA effect was abrogated by coexpression of a c-Myb derivative with a mutated siRNA target site. The coexpression of a dominant negative c-Myb mutant had a similar but weaker effect on MLL-ENL mediated transformation. Ectopic expression of c-Myb in hematopoietic precursor cells induced a mild differentiation block. However, c-Myb alone was not able to give a positive readout in a serial replating assay under conditions where the combined overexpression of Hoxa9 together with Meis1 was clearly transforming. In summary the results suggest that c-Myb is essential but not sufficient for the oncogenic activity of Hoxa9/Meis1.

Genetic Dissection of Leukemogenesis-Related Genes/Pathways Cooperating with AML1/ETO.

The AML1/ETO fusion gene is caused by chromosomal translocation t(8;21), which is among the most frequent chromosomal rearrangement seen in acute myeloid leukemia (AML). AML1/ETO functions in a dominant negative fashion to directly repress AML1-mediated transcriptional activation, and block the differentiation of myeloid progenitors. Notably, although it is oncogenic, AML1/ETO is not by itself sufficient to cause fully malignant AML, but can do so when there are concurrent oncogenic mutations (Higuchi 2002). This suggests that AML1/ETO requires cooperation with additional genetic alterations that are as yet largely unknown. The BXH-2 mouse strain is characterized by high frequency of spontaneous leukemia caused via insertional mutagenesis by a B-ecotropic murine leukemia virus. The proviral insertion site can then be used as a molecular tag, proviral insertion tag (PIT), to identify mutations/dysregulations of genes involved in leukemogenesis. Indeed, the BXH-2 strain has been utilized as a powerful forward genetic system to identify a number of leukemia-associated genes by our lab and collaborators. In this study, we have attempted to identify AML-related genes and pathways that could cooperate with AML1/ETO expression in development of AML. We have introduced the conditional AML1/ETO allele and Mx1-Cre, whose expression is induced by pIpC, into BXH-2 strain background to generate AEB mice after 4~6 consecutive backcrossing, and monitored daily these mice for sign of disease. Notably, the latency of leukemia in pIpC-injected AEB mice (mostly 5~8 months) is significantly shorter than that in saline-injected AEB control mice (mostly over one year). Immunophenotyping the tumor cells showed positivity for myeloid cell surface markers. Furthermore, leukemogenicity was tested and confirmed by transplanting the tumor cells generated from enlarged lymph nodes into B6C3F1 recipients. These data suggest that these mice developed a fatal, aggressive myeloid leukemia. Southern blot showed, that in these leukemic cells, the recombined allele of AML1/ETO was formed by Cre induction. Induced expression of AML1/ETO was not only detected in peripheral white blood cells but also in tumors. Importantly, these AML1/ETO AML also harbor clonal proviral insertions, indicating that AML1/ETO-initiated leukemia is dramatically accelerated by chronic retroviral infection in our model. To identify AML1/ETO-cooperative leukemia genes, we have isolated several PITs from AML1/ETO-accelerated leukemias. These PITs have also been mapped onto mouse genome using Celera and Ensembl mouse genome databases, and been found to target new candidate genes. Interestingly, we have discovered a new common PIT, which have not been reported in previous work which has identified over 2000 PITs. This common PIT targets a G-protein coupled receptor-like gene, a member of the GPCR family. We believe that these studies will provide important information for better understanding the mechanisms of development of leukemia with t(8;21), and reveal promising candidate genes or pathways serving as the potential therapeutic targets for improving AML treatment.

Haploinsufficiency of Runx1/AML1 promotes myeloid features and leukaemogenesis in BXH2 mice

Haploinsufficiency of RUNX1/AML1 is associated with familial platelet disorder with a predisposition to acute myeloid leukaemia (FPD/AML), but the causal relationship remains to be addressed experimentally. Mice heterozygous for the Runx1 null mutation, Runx1+/-, are considered to be genetically comparable with human FPD/AML patients but do not develop spontaneous leukaemia. To induce additional genetic alterations, retroviral insertional mutagenesis was employed with the use of BXH2 mice, which develop myeloid leukaemia because of the random integration of retrovirus present in the mouse. Heterozygous disruption of Runx1 in BXH2 mice resulted in a shortening of the latency period of leukaemia. In addition, BXH2-Runx1+/- mice exhibited more marked myeloid features than control mice. Moreover, the c-Kit gene, mutated in human RUNX leukaemias, was recurrently activated in BXH2-Runx1+/- mice, and a colony-forming assay revealed synergism between the Runx1+/- status and c-KIT overexpression. In conclusion, the BXH2-Runx1+/- system is a promising mouse model to investigate the mechanism of leukaemogenesis in FPD/AML.

Increased dosage of Runx1/AML1 acts as a positive modulator of myeloid leukemogenesis in BXH2 mice

The RUNX1/AML1 gene on chromosome 21 is most frequently inactivated in human leukemias. In addition, an increased dose of RUNX1 is suggested as a basis for several kinds of leukemias. Amplifications of chromosome 21 or the RUNX1 gene are shown to be associated with leukemias with lymphoid lineage, whereas its involvement in myeloid lineage remains unclear. In this study, we generated GATA-1 promoter-driven Runx1 transgenic (Tg) mice, which showed a transient mild increase of megakaryocyte marker-positive myeloid cells but no spontaneous leukemia. These mice were then crossed with BXH2 mice, which have a replication-competent retrovirus in the mouse and develop myeloid leukemia due to insertional mutagenesis by random integration of the virus. Overexpressed Runx1 transgene in BXH2 mice resulted in shortening of the latency of leukemia with increased frequency of megakaryoblastic leukemia, suggesting that increased Runx1 dosage is leukemogenic in myeloid lineage. Identifications of retroviral integration sites revealed the genetic alterations that may cooperate with Runx1 overdose in myeloid leukemogenesis. This mouse model may be useful for analysing the pathogenesis of myeloid leukemias with RUNX1 overdose, especially to examine whether an extra-copy of RUNX1 by trisomy 21 is causally related to Down's syndrome-related acute megakaryoblastic leukemia (DS-AMKL).

A mutation in the Icsbp1 gene causes susceptibility to infection and a chronic myeloid leukemia–like syndrome in BXH-2 mice

BXH-2 mice develop a fatal myeloid leukemia by a two-step mutagenic process. First, a BXH-2–specific recessive mutation causes a myeloproliferative syndrome. Second, retroviral insertions alter oncogenes or tumor suppressors, resulting in clonal expansion of leukemic cells. We have identified a recessive locus on chromosome 8 (Myls) that is responsible for myeloproliferation in BXH-2. This Myls interval has been narrowed down to 2 Mb and found to contain several positional candidates, including the interferon consensus sequence–binding protein 1 gene (Icsbp, also known as interferon regulatory factor 8 [IRF8]). We show that BXH-2 mice carry a mutation (915 C to T) resulting in an arginine-to-cysteine substitution at position 294 within the predicted IRF association domain of the protein. Although expression of Icsbp1 mRNA transcripts is normal in BXH-2 splenocytes, these cells are unable to produce interleukin 12 and interferon-γ in response to activating stimuli, confirming that R294C behaves as a loss-of-function mutation. Myeloproliferation in BXH-2 mice is concomitant to increased susceptibility to Mycobacterium bovis (BCG) despite the presence of resistance alleles at the Nramp1 locus. These results suggest a two-step model for chronic myeloid leukemia in BXH-2, in which inactivation of Icsbp1 predisposes to myeloproliferation and immunodeficiency. This event is required for retroviral replication, and subsequent insertional mutagenesis that causes leukemia in BXH-2 mice.

Two strikes lead to leukemia

![Graphic][1] A mutation in IRF-8 causes myeloproliferation in the spleen which predisposes BXH-2 mice to leukemia.New genetic mapping studies explain why a certain mouse strain is predisposed to developing fatal leukemia. Turcotte et al., reporting on [page 881][2], identify a

Identification of cooperative genes for NUP98-HOXA9 in myeloid leukemogenesis using a mouse model

AbstractThe chromosomal translocation t(7; 11)(p15;p15), observed in human myeloid leukemia, results in a NUP98 and HOXA9 gene fusion. We generated a transgenic mouse line that specifically expressed the chimeric NUP98-HOXA9 gene in the myeloid lineage. While only 20% of the transgenic mice progressed to leukemia after a latency period, myeloid progenitor cells from nonleukemic transgenic mice still exhibited increased proliferative potential. This suggested that the NUP98-HOXA9 fusion induced a preleukemic phase, and other factors were required for complete leukemogenesis. NUP98-HOXA9 expression promoted the onset of retrovirus-induced BXH2 myeloid leukemia. This phenomenon was used to identify cooperative disease genes as common integration sites (CISs). Meis1, a known HOX cofactor, was identified as a CIS with a higher integration frequency in transgenic than in wild-type BXH2 mice. By the same means we identified further 4 candidate cooperative genes, Dnalc4, Fcgr2b, Fcrl, and Con1. These genes cooperated with NUP98-HOXA9 in transforming NIH 3T3 cells. The system described here is a powerful tool to identify cooperative oncogenes and will assist in the clarification of the multistep process of carcinogenesis.

Genetic Dissection of Cooperating Mutations in BXH-2 Acute Myeloid Leukemia with and without Nf1 Gene Mutation.

Loss of the NF1 (Neurofibromatosis Type 1) gene, a tumor suppressor, can cause myeloid diseases juvenile myelomonocytic leukemia (JMML), monosomy 7 syndrome (Mo7), and acute myeloid leukemia (AML). However, using knockout mice, it has been shown that loss of Nf1 expression in hematopoietic cells, by itself, does not lead to aggressive leukemia resulting instead in a relatively indolent myeloproliferative disease. Murine Leukemia Virus (MuLV) insertional mutagenesis in BXH-2 mice provides a model to dissect genetic alterations in AML. We have profiled proviral insertions in BXH-2 AML which do or do not have corresponding loss-of-function of Nf1. 197 PIS (68 from 25 Nf1-wild type AML and 129 from 55 Nf1-defective AML) were isolated. Nf1-defective AML were obtained from BXH-2 AMLs with proviral insertions into the endogenous Nf1 gene and AML that developed in leukemia-prone, heterozygous Nf1+/− BXH-2 mice. These latter AMLs develop faster than wild-type BXH-2 AMLs and show Nf1 gene LOH or proviral insertion into the wild-type Nf1 allele. These analyses led to 37 common proviral insertion sites (CIS), 13 of which have not been reported previously. Several of the CIS (including Lmo2, Cmyb, Meis1, Bcl11a, Spred2, Def8, Edg3, Hoxa9, and a novel Krab domain-zinc finger gene) were found repeatedly among the Nf1-defective group of AML. Expression of most could be detected in human JMML and CMML by RT-PCR, including BCL11A. Importantly, among the CIS we detected, PIS targeting Bcl11a were significantly enriched (p < 0.05) in Nf1-defective leukemia. Retroviral expression vectors for Bcl11a have been constructed and transduced into an immortalized Nf1-/- null myeloblast cell line. Growth assays show that the cumulative cell number of FACS-sorted Bcl11a-Nf1-/- cells increase by ~2.5 fold that of controls. BXH-2 provides a powerful genetic system to dissect Nf1-cooperating genetic events in tumorigenesis. Mutations at several novel common integration sites could be involved in development or progression of leukemia with NF1 gene inactivation. This work was supported by the National Cancer Institute (U01-CA84221-05) and the American Cancer Society (RPG LIB-106632) to DAL and by National Cancer Institute (R01 CA92095) and U.S. Dept. of Defense (DAMD17-97-1-7339) to MRW.

Identification of Cooperating Genetic Alterations in RUNX Leukemia Using the Mouse Model for the Human Familial Leukemia, FPD/AML.

The RUNX1/AML1 gene is essential for hematopoiesis and is most frequently involved in human leukemias. Loss-of-function of RUNX1 is the common underlying mechanism of RUNX leukemias: those carrying t(8;21), inv(16) and RUNX1 point mutations. As the weakest mode, haploinsufficiency of RUNX1/AML1 causes familial platelet disorder with predisposition to acute myeloid leukemia (FPD/AML). Mice heterozygous for Runx1 null mutation, Runx1+/−, appear genetically equivalent to human FPD/AML patients, but showed no apparent defects and did not develop spontaneous leukemia, implying that additional genetic alterations are required for leukemia. To identify such abnormalities, retroviral insertional mutagenesis was employed by using BXH2 mice that have spontaneously transmittable ecotropic retrovirus. BXH2-Runx1+/− mice showed a shorter disease latency and more myeloid-specific phenotype than wild-type littermates. Therefore, BXH2-Runx1+/− mice appear to serve as the mouse model for FPD/AML to investigate myeloid leukemia. The genes selectively affected in BXH2-Runx1+/− mice by retroviral integration are likely to cooperate with Runx1+/−. c-Kit locus was involved twice in 24 BXH2-Runx1+/− but neither in 17 wild-type littermates nor in 135 inbred BXH2 mice reported in the RTCGD database, suggesting that c-KIT mutation cooperates with RUNX alteration in human leukemia. In fact, constitutively active mutants of c-KIT or functionally equivalent FLT3, were found in 10 out of 25 (40%) of human RUNX leukemias. Together, BXH2-Runx1+/− system appears useful to identify the genes cooperating with RUNX1 abnormalities.

Cooperative Pathways to Acute Myeloid Leukemia Include the Combining of Transcription Factor Alterations: PML-RARα Cooperates with SOX4.

One paradigm for transformation of normal myeloid cells into acute myeloid leukemia (AML) is the combining of transcription factor alteration with activation of signaling pathways, including cytokine receptor activation. To identify possible alternative pathways to leukemogenesis we crossed MRP8-PML/RARA transgenic mice with BXH-2 mice, which harbor an endogenous retrovirus that causes AML. Approximately half of the leukemias that arose in this cross showed features of acute promyelocytic leukemia (APL). We identified 22 insertions sites in 8 APL-like leukemias. Of these, 7 represented common insertion sites in the Mouse Retroviral Tagged Cancer Gene Database (RTCGD, http://rtcgd.ncifcrf.gov/mm4/index.html). We introduced into a retroviral vector cDNAs encoding 6 genes located at retroviral insertion sites identified in PML/RARA leukemias (Sox4, Meis1, Lck, Sfpi1, Nfil3, Cblb). PML/RARA transgenic bone marrow was transduced with these retroviruses and the marrow was used to reconstitute lethally irradiated recipient animals. Recipients of PML/RARα transgenic marrow transduced with a SOX4 retrovirus developed APL-like leukemias in 3 months. SOX4 is an HMG box transcription factor that exhibits sequence specific DNA binding and transactivation. It is the most common insertion site in RTCGD, and has been found to be overexpressed in small cell lung cancer and medulloblastoma, as well as other tumors. In light of other data indicating that combining transcription factor abnormalities can rapidly induce leukemia (e.g. Hoxa9 + Meis1), our finding that SOX4 + PML/RARα is potently leukemogenic supports the hypothesis that such cooperativity represents another important paradigm in myeloid leukemogenesis.