Abstract B19: ARID1A is a 1p36 tumor suppressor that collaborates with N-Myc in neuroblastoma

Neuroblastoma (NB) is an embryonal cancer of the sympathetic nervous system (SNS) that causes 15% of pediatric cancer deaths. A major barrier to advancing targeted therapy in NB is identifying the unknown oncogenes and tumor suppressors that are buried within large segmental chromosomal gains and losses that characterize high-risk NB. To assist in identifying them, we have established a novel system for studying NB based on the transformation of neural crest cells (NCCs), the progenitor cells of the SNS, isolated from mouse embryonic day 9.5 trunk neural tube explants. Based on pathology and gene expression analysis, we have demonstrated that the enforced expression of N-Myc in wild-type NCCs generates phenotypically and molecularly accurate NB that most closely models MYCN-amplified NB. Using this system we next addressed a longstanding problem in the NB field. Though 70% of MYCN-amplified NBs have 1p36 deletions, representing ~28% of high-risk NBs, the 1p36 tumor suppressor that collaborates with N-Myc is unknown. Using CRISPR/Cas9 technology in primary NCCs, we screened multiple 1p36 candidate tumor suppressors in a tumorsphere-forming assay. We found tumorspheres only formed when two chromatin-remodeling factors, CHD5 and ARID1A, were both lost, though their loss was not sufficient to generate tumors in vivo. Therefore, we next tested whether deletion of Chd5 and Arid1a, Arid1a alone, or deletion of the entire syntenic 1p36 region, as validated by FISH and karyotype analysis, is selected for during N-Myc driven transformation of primary NCCs. To achieve this, we started with low-efficiency deletion of each target in three independently derived NCC lines and then allowed clonal evolution during tumorigenesis to identify the favored deletion for N-Myc overexpressing NCCs. Tumors with CRISPR/Cas9 targeted 1p36 genes formed with a reduced latency of ~50 days over control (p<0.05), indicating that there was loss of an N-Myc collaborating tumor suppressor. From whole-exome sequencing we observed clonal selection from parental lines to tumors for both indel mutations in Arid1a and for larger deletions that encompassed Arid1a and surrounding genes. Arid1a mutations went from below level of detection in the parental NCC lines to >80% of the reads in the daughter tumors. We also saw selection for indel mutations and deletions of CHD5, but not in every tumor analyzed and no selection for loss of the entire 1p36 locus in any tumor. Using ex vivo adeno-cre mediated deletion of one or both floxed Arid1a alleles in NCCs, we found that loss of one Arid1a allele significantly (p=0.02) decreased latency by ~40 days over control in collaboration with N-Myc. Loss of both alleles conferred no advantage, indicating that when combined with N-Myc, ARID1A is an obligate haplo-insufficient tumor suppressor. In sum, these tumor studies support a model in which loss of one Arid1a, but not the entire syntenic 1p36 locus, is strongly selected for during N-Myc driven NB tumorigenesis, suggesting that Arid1a is the critical 1p36 tumor suppressor. The next step in this longstanding problem is to understand why loss of ARID1A collaborates with MYCN amplification and to exploit this discovery to improve treatment for MYCN amplified/1p36 deleted NBs.

Citation Format: Jesus Garcia-Lopez, Kirby Wallace, Joel Otero, Rachelle Olsen, David Finkelstein, Jerold Rehg, Yongdong Wang, Kevin Freeman. ARID1A is a 1p36 tumor suppressor that collaborates with N-Myc in neuroblastoma [abstract]. In: Proceedings of the AACR Special Conference: Pediatric Cancer Research: From Basic Science to the Clinic; 2017 Dec 3-6; Atlanta, Georgia. Philadelphia (PA): AACR; Cancer Res 2018;78(19 Suppl):Abstract nr B19.

Prox1-Heterozygosis Sensitizes the Pancreas to Oncogenic Kras-Induced Neoplastic Transformation

The current paradigm of pancreatic neoplastic transformation proposes an initial step whereby acinar cells convert into acinar-to-ductal metaplasias, followed by progression of these lesions into neoplasias under sustained oncogenic activity and inflammation. Understanding the molecular mechanisms driving these processes is crucial to the early diagnostic and prevention of pancreatic cancer. Emerging evidence indicates that transcription factors that control exocrine pancreatic development could have either, protective or facilitating roles in the formation of preneoplasias and neoplasias in the pancreas. We previously identified that the homeodomain transcription factor Prox1 is a novel regulator of mouse exocrine pancreas development. Here we investigated whether Prox1 function participates in early neoplastic transformation using in vivo, in vitro and in silico approaches. We found that Prox1 expression is transiently re-activated in acinar cells undergoing dedifferentiation and acinar-to-ductal metaplastic conversion. In contrast, Prox1 expression is largely absent in neoplasias and tumors in the pancreas of mice and humans. We also uncovered that Prox1-heterozygosis markedly increases the formation of acinar-to-ductal-metaplasias and early neoplasias, and enhances features associated with inflammation, in mouse pancreatic tissues expressing oncogenic Kras. Furthermore, we discovered that Prox1-heterozygosis increases tissue damage and delays recovery from inflammation in pancreata of mice injected with caerulein. These results are the first demonstration that Prox1 activity protects pancreatic cells from acute tissue damage and early neoplastic transformation. Additional data in our study indicate that this novel role of Prox1 involves suppression of pathways associated with inflammatory responses and cell invasiveness.

Abstract A04: Modeling pediatric malignancies by transforming primary neural crest cells

Neural crest cells (NCCs) are a highly migratory multipotent cell population first specified during neural tube closure at the neural plate border. Soon after neural tube closure NCCs undergo epithelial to mesenchymal transitioning (EMT) to allow migration throughout the body plan where they generate diverse cell types including neurons, Schwann cells, melanocytes, and osteoblasts. NCCs have been proposed to be the embryonic precursor cell population to multiple pediatric malignancies including neuroblastoma, peripheral primitive neuroectodermal tumors (pPNET), malignant peripheral nerve sheath tumors (MPNST), cranial-facial osteosarcoma as well as adult malignancies like melanoma. Despite this very little work has been done with primary NCCs in pediatric cancer research.

The most common of these pediatric cancers is neuroblastoma, which arises in the sympathoadrenal lineage of trunk NCCs. To establish a system for studying the earliest events in neuroblastoma oncogenesis we developed an assay based on the oncogenic transformation of primary NCCs. Isolation of primary trunk NCCs from day 9.5 embryos resulted in a population that was >95% positive for the NCC markers Sox10, p75 and Ascl1 (MASH1) as assessed by immunofluorescence. These isolated NCCs were also capable of generating Tuj1, Map2, and tyrosine hydroxylase (TH) positive neurons when placed in neurogenic media.

We next wanted to test the transformation of primary NCCs. Since a prominent category of high-risk neuroblastoma is N-Myc amplification, we first determined if N-Myc overexpression was sufficient to transform wild-type NCCs in our assay. For this and all other tumor experiments, NCCs were retrovirally infected within 48 hours of isolation and 10,000 cells were injected the next day subcutaneously into the flanks of mice. N-Myc alone generated a single tumor in 1 out of 16 mice with the resulting tumor being uniformly positive for the neuronal markers synaptophsyon, MAP2 and tyrosine hydroxylase (TH). This tumor also contained neuropil and ganglion cells and was characterized as neuroblastoma, differentiating subtype, stroma poor. To determine if loss of p53 could increase the efficiency of transformation, NCCs from p53-compromised mice were infected with N-Myc. This led to 100% penetrance with 19 out of 19 mice generating tumors and with equivalent frequency in nude or syngeneic C57BL/6 mice showing this approach can be used in immune competent mice. The resulting tumors from this genetic combination were characterized as primitive neuroectodermal tumors with divergent differentiation. Regions of these tumors that were diffusely positive for neuronal markers appeared neuroblastoma-like or PNET-like. Osteosarcoma was also a prominent feature in every tumor with loss of p53. Additionally we observed yet to be defined tumor types that were negative by IHC for neuronal markers, Schwannian markers and melanoma markers. Generally, in mice with the fastest tumor onset micrometastases to the lung and liver were observed, while slower onset correlated with macroscopic metastases to the same organs. Injection of cell lines derived from primary tumors lead to aggressive metastatic spread to lung and liver.

Though subcutaneous tumors rarely metastasize this was a salient feature of our neural crest derived cancers, which recapitulates the highly metastatic nature of human NCC derived cancers like neuroblastoma and melanoma. N-Myc in a wild type background caused a single phenotypically accurate neuroblastoma while loss of p53 caused an increase in cancer divergence underscoring the capacity of NCCs to generate a diverse array of cancer types including osteosarcoma. From this study we believe primary NCCs can be used as a platform for identifying initiating cancer events in neuroblastoma and possibly other pediatric malignancies.

Citation Format: Rachelle R. Olsen, Otero Joel, Kirby Wallace, Jerold Rehg, Kevin W. Freeman. Modeling pediatric malignancies by transforming primary neural crest cells. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr A04.

Abstract 2977: Myc and Miz1 in medulloblastoma

Medulloblastoma (MB) is the most common malignant pediatric brain tumor, which arises in the cerebellum. Human MBs are classified into four subgroups: Wingless (WNT), Sonic Hedgehog (SHH), Group 3 (G3) and Group 4 (G4). In 2012, we developed a mouse model of the most aggressive and least curable G3 MB, by overexpressing MYC but not MYCN, in Trp53-null cerebellar granule neuron progenitors (GNPs), which were implanted into the cortices of recipient CD-1 nu/nu mice. Remarkably, enforced expression of MYCN in Trp53-null GNPs induced a SHH MB. This was unexpected since MYC and MYCN bind to the same E-box DNA binding sequences. We hypothesized that the difference between MYC- and MYCN-induced MBs might be due to their interaction with different partners in GNPs. Human G3 MBs are distinguished by MYC overexpression. The MYC proto-oncogene encodes a bHLH/leucine-zipper transcription factor that forms heterodimers with MAX to drive transcription. However, MYC can repress transcription when the MYC/MAX heterodimer is recruited to core promoter sequences by MIZ1, a member of the POZ-domain/zinc-finger transcription factor. MIZ1, ubiquitously expressed during embryonic development, can either be a transcription activator or repressor depending on its binding partners.

To investigate whether the MYC/MIZ1 complex contributed to G3 MBs, we used a mutant of MYC which no longer binds to MIZ1 in which a Valine is substituted for an Aspartic acid amino acid at position 394 (MYCV394D). We enforced the expression of MYC or MYCV394D in GNPs purified from seven-day-old Trp53-null mouse pups using retroviral transduction, and implanted transduced GNPs into the cerebral cortices of recipient mice. MYCV394D-induced tumors developed later than MYC-induced G3 MBs. Histopathological analysis revealed that MYC-induced tumors exhibited a large cell/anaplastic (LCA) phenotype and were classified as G3 MBs; however, MYCV394D-induced tumors were described as unclassified primitive neuroectodermal tumors. In vitro, MYCV394D-induced tumors formed tumorspheres that were highly apoptotic and less proliferative compared to MYC tumorspheres. Global gene expression of MYCV394D tumors was distinct from murine G3 and SHH subgroup MBs. Particularly, genes needed for neuronal differentiation and which are strongly repressed by MYC in G3 MBs, were no longer repressed by the MYCV394D mutant, indicating that the MYC/MIZ1 complex was responsible for their repression.

To analyze whether the MYC/MIZ1 complex was present on chromatin in G3 MBs, we performed ChIP-Seq for MYC and MIZ1. In contrast to Trp53-null GNPs, where MIZ1 only binds 140 promoters, 11.549 promoters were occupied by MIZ1 in G3 MBs, 9.353 of which were co-occupied by MYC (81%). Because as many of the joined binding sites neither contained a canonical E-box sequence nor a bona fide MIZ1 binding motif, we speculate that cooperative binding of oncogenic MYC with MIZ1 is necessary for binding.

Our data suggest that the MYC-MIZ1 interaction is critical for the development of G3 MBs.

Citation Format: BaoHan T. Vo, Elmar Wolf, Daisuke Kawauchi, Jerold Rehg, David Finkelstein, Brian Murphy, Martin Eilers, Martine F. Roussel. Myc and Miz1 in medulloblastoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2977. doi:10.1158/1538-7445.AM2014-2977

Abstract 1434: A mouse model of the most aggressive subgroup of human medulloblastoma

Human medulloblastomas are molecularly classified into four major subgroups. Among them, the MYC-subgroup, the most aggressive and the least curable is characterized by amplification or overexpression of the c-MYC gene. While mouse models for the human SHH-subgroup have advanced the development of targeted treatments for this subgroup, the absence of a preclinical model for the MYC-subgroup has severely limited our understanding of its biology and treatment. We have now generated the first mouse model of MYC-subgroup medulloblastoma that faithfully recapitulate the transcriptome, histopathology, and clinical behavior of the human disease. This mouse model was generated by orthotopic transplantation of percoll-purified Myc-transduced cerebellar cells from postnatal day 6-7 Trp53-deficient mice into the cortices or the cerebellum of immunocompromised recipient animals. While >2 x 105 cells from the mouse SHH-tumors are required to induce a tumor, 100 MYC-tumor cells were sufficient, suggesting that most MYC-tumor cells are tumor initiating cells. MYC-tumors can be passaged indefinitely as neurospheres that express markers associated with stem cells, including Prom1, Lgr5, Oct4, Nanog, and Sox2 and induce secondary medulloblastomas with similar characteristics as the primary tumors, after orthotopic transplant into the cortices of recipient mice. This novel medulloblastoma model should significantly advance our efforts to develop new therapeutic modalities and to determine the origin of this deadly childhood cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1434. doi:1538-7445.AM2012-1434

A mouse model of the most aggressive subgroup of human medulloblastoma

Medulloblastomas that display a large cell/anaplastic morphology and overexpress the cellular c-MYC gene are highly aggressive and carry a very poor prognosis. This so-called MYC-subgroup differs in its histopathology, gene expression profile, and clinical behavior from other forms of medulloblastoma. We generated a mouse model of MYC-subgroup medulloblastoma by transducing Trp53-null cerebellar progenitor cells with Myc. The cardinal features of these mouse medulloblastomas closely mimic those of human MYC-subgroup tumors and significantly differ from mouse models of the Sonic-Hedgehog- and WNT-disease subgroups. This mouse model should significantly accelerate understanding and treatment of the most aggressive form of medulloblastoma and infers distinct roles for MYC and MYCN in tumorigenesis.

Fatal outcome of pandemic H1N1 2009 influenza virus infection is associated with immunopathology and impaired lung repair, not enhanced viral burden, in pregnant mice

Pandemic A (H1N1) 2009 influenza virus (pH1N1) infection in pregnant women can be severe. The mechanisms that affect infection outcome in this population are not well understood. To address this, pregnant and nonpregnant BALB/c mice were inoculated with the wild-type pH1N1 strain A/California/04/09. To determine whether innate immune responses are associated with severe infection, we measured the innate cells trafficking into the lungs of pregnant versus nonpregnant animals. Increased infiltration of pulmonary neutrophils and macrophages strongly correlated with an elevated mortality in pregnant mice. In agreement with this, the product of nitric oxide (nitrite) and several cytokines associated with recruitment and/or function of these cells were increased in the lungs of pregnant animals. Surprisingly, increased mortality in pregnant mice was not associated with higher virus load because equivalent virus titers and immunohistochemical staining were observed in the nasal cavities or lungs of all mice. To determine whether exacerbated inflammatory responses and elevated cellularity resulted in lung injury, epithelial regeneration was measured. The lungs of pregnant mice exhibited reduced epithelial regeneration, suggesting impaired lung repair. Despite these immunologic alterations, pregnant animals demonstrated equivalent percentages of pulmonary influenza virus-specific CD8(+) T lymphocytes, although they displayed elevated levels of T-regulator lymphocytes (Tregs) in the lung. Also, pregnant mice mounted equal antibody titers in response to virus or immunization with a monovalent inactivated pH1N1 A/California/07/09 vaccine. Therefore, immunopathology likely caused by elevated cellular recruitment is an implicated mechanism of severe pH1N1 infection in pregnant mice.

Abstract 3444: Enforced expression of MycN and C-Myc induces different medulloblastoma subtypes

Medulloblastoma (MB), a tumor of the cerebellum, is the most common malignant brain tumor of childhood. MBs are classified histologically and molecularly into four distinct major subgroups, two of which correspond to the deregulation of major developmental pathways, Sonic Hedgehog (SHH) and WNT. Among the two other subgroups, one correlates with high levels of C-MYC expression. Using orthotopic transplantation in the cerebellum or cortex of naïve recipient mice, we showed that enforced expression of MycN in cerebellar granule neuron progenitors (CGNPs) isolated from the cerebellum of postnatal (P) day 6-7 Cdkn2c-/-, Trp53-/-, Atoh1-GFP mice induced MBs with a SHH “signature”. Interestingly, overexpression of C-Myc in the same CGNPs induced MBs with Large Cell Anaplastic (LCA) features and mRNA and microRNA profiles distinct from the SHH subgroup. In vitro analysis revealed that tumor cells were resistant to Shh antagonists, such as cyclopamine and BMP4. Ongoing experiments are focusing on understanding the differences between MycN and C-Myc function in CGNPs.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3444. doi:10.1158/1538-7445.AM2011-3444

Fatal outcome of pandemic H1N1 2009 influenza infection is associated with neutrophil and macrophage lung infiltration but not enhanced viral burden in pregnant mice. (111.3)

Pandemic H1N1 2009 Influenza A virus (pH1N1) infection in pregnant or postpartum women is quite severe. The immunological mechanisms that affect infection outcome in these populations are not well understood. To address this, pregnant Balb/c and non-pregnant mice were inoculated with a wild-type pH1N1 virus. We determined that early cytokine responses were impaired in pregnant mouse lungs. Moreover, pulmonary infiltration of neutrophils and macrophages strongly correlated with an increase in mortality in pregnant mice. Increased mortality in pregnant animals was not associated with higher virus load because equivalent viral burden (virus titers and immunohistochemical staining) was observed in the nasal cavities and lower respiratory tracts of all mice. Pregnancy altered innate responses despite the presence of similar viral burden. Also, pregnant mice demonstrated equivalent influenza-specific activation of pulmonary CD8+ T lymphocytes however; they displayed elevated T-regulator lymphocyte (Tregs) responses in the lung and mediastinal lymph nodes. Despite these immunologic alterations, pregnant animals mounted equivalent hemagglutinin inhibition and IgG subclass titers in response to virus or immunization with monovalent inactivated pH1N1 vaccine. Therefore, the severity of pH1N1 in pregnant mice is associated with pulmonary neutrophil and macrophage invasion, dysregulated cytokine responses but not increased viral load.

Functional plasticity of regulatory T cells (168.7)

Regulatory T cells (Tregs) play a pivotal role in maintaining immune homeostasis, preventing autoimmunity, controlling exaggerated immune responses against infections and suppressing anti-tumor immunity. Tregs utilize multiple mechanisms to mediate their function. However, the number of mechanisms critical to exert maximal suppressive activity is unclear. It is conceivable that suppressive mechanisms could be tightly regulated by cross-regulatory networks. It has been shown that interleukin 10 (IL10) and IL35 have critical role in Treg function. Therefore, we assessed the functional capacity of Tregs lacking the ability to secrete IL10 and IL35. Surprisingly, our study revealed that Tregs lacking both IL10 and IL35 (Ebi3-/-Il10-/- or Il12a-/-Il10-/-) were fully functional and were able to suppress Tconv cells both in vitro and in vivo . Absence of IL10 and IL35 was compensated by an increase in cathepsin E (CTSE), influencing the release of soluble TRAIL and thereby resulting in suppression of target cells in a TRAIL-dependent manner. Inhibiting TRAIL activity disrupted the function of Ebi3-/-Il10-/- Tregs. The loss of suppressive function was maximal in Tregs lacking IL10, IL35 and TRAIL (Ebi3-/-Il10-/-Tnfsf10-/-Tregs). These data suggests the existence of cross-regulatory networks which controls the utilization of suppressive factors, thereby maintaining Treg functional plasticity.

Two tumor suppressors, p27Kip1 and patched-1, collaborate to prevent medulloblastoma

Two cyclin-dependent kinase inhibitors, p18(Ink4c) and p27(Kip1), are required for proper cerebellar development. Loss of either of these proteins conferred a proliferative advantage to granule neuron progenitors, although inactivation of Kip1 exerted a greater effect. Mice heterozygous for Patched-1 (Ptc1+/-) that are either heterozygous or nullizygous for Kip1 developed medulloblastoma rapidly and with high penetrance. All tumors from Ptc1+/-;Kip1+/- or Ptc1+/-;Kip1-/- mice failed to express the wild-type Ptc1 allele, consistent with its role as a canonical "two-hit" tumor suppressor. In contrast, expression of the wild-type p27(Kip1) protein was invariably maintained in medulloblastomas arising in Ptc1+/-;Kip1+/- mice, indicating that Kip1 is haploinsufficient for tumor suppression. Although medulloblastomas occurring in Ptc1+/- mice were histopathologically heterogeneous and contained intermixed regions of both rapidly proliferating and nondividing more differentiated cells, tumors that also lacked Kip1 were uniformly less differentiated, more highly proliferative, and invasive. Molecular analysis showed that the latter medulloblastomas exhibited constitutive activation of the Sonic hedgehog signaling pathway without loss of functional p53. Apart from gains or losses of single chromosomes, with gain of chromosome 6 being the most frequent, no other chromosomal anomalies were identified by spectral karyotyping, and half of the medulloblastomas so examined retained a normal karyotype. In this respect, this mouse medulloblastoma model recapitulates the vast majority of human medulloblastomas that do not sustain TP53 mutations and are not aneuploid.

MN1-TEL, the product of the t(12;22) in human myeloid leukemia, immortalizes murine myeloid cells and causes myeloid malignancy in mice

MN1-TEL is the product of the recurrent t(12;22)(p12;q11) associated with human myeloid malignancies. MN1-TEL functions as an activated transcription factor, exhibiting weak transforming activity in NIH3T3 fibroblasts that depends on the presence of a functional TEL DNA-binding domain, the N-terminal transactivating sequences of MN1 and C-terminal sequences of MN1. We determined the transforming activity of MN1-TEL in mouse bone marrow (BM) by using retroviral transfer. MN1-TEL-transduced BM showed increased self-renewal capacity of primitive progenitors in vitro, and prolonged in vitro culture of MN1-TEL-expressing BM produced immortalized myeloid, interleukin (IL)-3/stem cell factor-dependent cell lines with a primitive morphology. Transplantation of such cell lines into lethally irradiated mice rescued them from irradiation-induced death and resulted in the contribution of MN1-TEL-expressing cells to all hematopoietic lineages, underscoring the primitive nature of these cells and their capacity to differentiate in vivo. Three months after transplantation, all mice succumbed to promonocytic leukemia. Transplantation of freshly MN1-TEL-transduced BM into lethally irradiated mice also caused acute myeloid leukemia within 3 months of transplantation. We infer that MN1-TEL is a hematopoietic oncogene that stimulates the growth of hematopoietic cells, but depends on secondary mutations to cause leukemia in mice.

Prospective isolation of murine hematopoietic stem cells by expression of an Abcg2/GFP allele

Stem cells from a variety of tissues can be identified by a side population (SP) phenotype based on Hoechst 33342 dye efflux. The Abcg2 transporter is expressed in hematopoietic stem cells (HSCs) and confers this dye efflux activity. To further explore the relationship among Abcg2 expression, the SP phenotype, and HSC activity, we have generated mice in which a green fluorescent protein (GFP) reporter gene was inserted into the Abcg2 locus. In these mice, the majority of bone marrow (BM) cells that expressed the Abcg2/ GFP allele were Ter119(+) erythroid cells. The Abcg2/GFP allele was also expressed in approximately 10% of lineage-negative (Lin(-)) and in 91% of SP cells using stringent conditions for the SP assay. Flow cytometric sorting was used to isolate various Abcg2/GFP(+) BM cell populations that were then tested for HSC activity in transplant assays. There was significant enrichment for HSCs in sorted Lin(-)/ GFP(+) cells, with a calculated HSC frequency of approximately one in 75. There was no HSC activity detected in Lin(-)/GFP(+) cells. Altogether, these results show that Abcg2 is expressed on essentially all murine BM HSCs and can be used as a prospective marker for HSC enrichment.

Global transcriptional coactivators CREB-binding protein and p300 are highly essential collectively but not individually in peripheral B cells

CREB-binding protein (CBP) and its para-log p300 are transcriptional coactivators that physically or functionally interact with over 320 mammalian and viral proteins, including 36 that are essential for B cells in mice. CBP and p300 are generally considered limiting for transcription, yet their roles in adult cell lineages are largely unknown since homozygous null mutations in either gene or compound heterozygosity cause early embryonic lethality in mice. We tested the hypotheses that CBP and p300 are limiting and that each has unique properties in B cells, by using mice with Cre/LoxP conditional knockout alleles for CBP (CBP(flox)) and p300 (p300(flox)), which carry CD19(Cre) that initiates floxed gene recombination at the pro-B-cell stage. CD19(Cre)-mediated loss of CBP or p300 led to surprisingly modest deficits in B-cell numbers, whereas inactivation of both genes was not tolerated by peripheral B cells. There was a moderate decrease in B-cell receptor (BCR)-responsive gene expression in CBP or p300 homozygous null B cells, suggesting that CBP and p300 are essential for this signaling pathway that is crucial for B-cell homeostasis. These results indicate that individually CBP and p300 are partially limiting beyond the pro-B-cell stage and that other coactivators in B cells cannot replace their combined loss.

Two transactivation mechanisms cooperate for the bulk of HIF-1-responsive gene expression

The C-terminal activation domain (C-TAD) of the hypoxia-inducible transcription factors HIF-1alpha and HIF-2alpha binds the CH1 domains of the related transcriptional coactivators CREB-binding protein (CBP) and p300, an oxygen-regulated interaction thought to be highly essential for hypoxia-responsive transcription. The role of the CH1 domain in vivo is unknown, however. We created mutant mice bearing deletions in the CH1 domains (DeltaCH1) of CBP and p300 that abrogate their interactions with the C-TAD, revealing that the CH1 domains of CBP and p300 are genetically non-redundant and indispensable for C-TAD transactivation function. Surprisingly, the CH1 domain was only required for an average of approximately 35-50% of global HIF-1-responsive gene expression, whereas another HIF transactivation mechanism that is sensitive to the histone deacetylase inhibitor trichostatin A (TSA(S)) accounts for approximately 70%. Both pathways are required for greater than 90% of the response for some target genes. Our findings suggest that a novel functional interaction between the protein acetylases CBP and p300, and deacetylases, is essential for nearly all HIF-responsive transcription.

A transcription-factor-binding surface of coactivator p300 is required for haematopoiesis

The coactivators CBP (Cre-element binding protein (CREB)-binding protein) and its paralogue p300 are thought to supply adaptor molecule and protein acetyltransferase functions to many transcription factors that regulate gene expression. Normal development requires CBP and p300, and mutations in these genes are found in haematopoietic and epithelial tumours. It is unclear, however, which functions of CBP and p300 are essential in vivo. Here we show that the protein-binding KIX domains of CBP and p300 have nonredundant functions in mice. In mice homozygous for point mutations in the KIX domain of p300 designed to disrupt the binding surface for the transcription factors c-Myb and CREB, multilineage defects occur in haematopoiesis, including anaemia, B-cell deficiency, thymic hypoplasia, megakaryocytosis and thrombocytosis. By contrast, age-matched mice homozygous for identical mutations in the KIX domain of CBP are essentially normal. There is a synergistic genetic interaction between mutations in c-Myb and mutations in the KIX domain of p300, which suggests that the binding of c-Myb to this domain of p300 is crucial for the development and function of megakaryocytes. Thus, conserved domains in two highly related coactivators have contrasting roles in haematopoiesis.

The centrosomal protein TACC3 is essential for hematopoietic stem cell function and genetically interfaces with p53-regulated apoptosis

TACC3 is a centrosomal/mitotic spindle-associated protein that is highly expressed in a cell cycle-dependent manner in hematopoietic lineage cells. During embryonic development, TACC3 is expressed in a variety of tissues in addition to the hematopoietic lineages. TACC3 deficiency causes an embryonic lethality at mid- to late gestation involving several lineages of cells. Hematopoietic stem cells, while capable of terminal differentiation, are unable to be expanded in vitro or in vivo in reconstitution approaches. Although gross alterations in centrosome numbers and chromosomal segregation are not observed, TACC3 deficiency is associated with a high rate of apoptosis and expression of the p53 target gene, p21(Waf1/Cip1). Hematopoietic stem cell functions, as well as deficiencies in other cell lineages, can be rescued by combining the TACC3 deficiency with p53 deficiency. The results support the concept that TACC3 is a critical component of the centrosome/mitotic spindle apparatus and its absence triggers p53-mediated apoptosis.

Effectiveness of Rack Sanitation Procedures for Elimination of Bacteria from Automatic Watering Manifolds

An important responsibility of animal care programs is to protect research animals from exposure to potentially pathogenic microorganisms. To validate the need for steam sterilization of rodent automatic watering racks, we evaluate the post-sanitation microbial contamination of experimentally inoculated racks and of racks that had been used to house conventional mice. We tested three sanitation protocols: rack-washer sanitation without manifold flush, sanitation that included manifold flush, and sanitation that included manifold flush followed by autoclaving. Rack sanitation, with or without manifold flush, did not reliably eliminate microbial flora from the lixits or manifold drainage water. A total of 43% of all non-autoclaved racks were positive for bacterial contamination after sanitation, and racks that had been used for conventional animal housing were more frequently positive than were experimentally inoculated racks (79% vs 18%). These data indicate that steam sterilization is necessary for eliminating bacteria from automatic watering systems. These observations are particularly important in light of increasing numbers of immune-impaired rodents that may be inadvertently and unnecessarily exposed to microbial pathogens via the automatic watering system.

Acute hepatitis in an opossum (Didelphis virginiana) infected with Salmonella turnidorp

Salmonella turnidorp was isolated in pure culture from the liver of an opossum that died of acute hepatitis. Microscopically, there were random foci of hepatic coagulation necrosis and gram negative bacteria within hepatocytes. The Salmonella turnidorp isolate was aberrant in that it did not utilize citrate and did not agglutinate by a commercial Salmonella polyvalent antiserum. Additional Salmonella serotypes, including Salmonella mbandaka, Salmonella rubislaw, and Salmonella anatum were isolated from three of five healthy opossums caught in Texas.