CXCR4 Promotes Neuroblastoma Growth and Therapeutic Resistance through miR-15a/16-1-Mediated ERK and BCL2/Cyclin D1 Pathways

CXCR4 expression in neuroblastoma tumors correlates with disease severity. In this study, we describe mechanisms by which CXCR4 signaling controls neuroblastoma tumor growth and response to therapy. We found that overexpression of CXCR4 or stimulation with CXCL12 supports neuroblastoma tumorigenesis. Moreover, CXCR4 inhibition with the high-affinity CXCR4 antagonist BL-8040 prevented tumor growth and reduced survival of tumor cells. These effects were mediated by the upregulation of miR-15a/16-1, which resulted in downregulation of their target genes BCL-2 and cyclin D1, as well as inhibition of ERK. Overexpression of miR-15a/16-1 in cells increased cell death, whereas antagomirs to miR-15a/16-1 abolished the proapoptotic effects of BL-8040. CXCR4 overexpression also increased miR-15a/16-1, shifting their oncogenic dependency from the BCL-2 to the ERK signaling pathway. Overall, our results demonstrate the therapeutic potential of CXCR4 inhibition in neuroblastoma treatment and provide a rationale to test combination therapies employing CXCR4 and BCL-2 inhibitors to increase the efficacy of these agents.Significance: These results provide a mechanistic rationale for combination therapy of CXCR4 and BCL-2 inhibitors to treat a common and commonly aggressive pediatric cancer.Cancer Res; 78(6); 1471-83. ©2017 AACR.

Development of Novel Promiscuous Anti-Chemokine Peptibodies for Treating Autoimmunity and Inflammation

Chemokines and their receptors play critical roles in the progression of autoimmunity and inflammation. Typically, multiple chemokines are involved in the development of these pathologies. Indeed, targeting single chemokines or chemokine receptors has failed to achieve significant clinical benefits in treating autoimmunity and inflammation. Moreover, the binding of host atypical chemokine receptors to multiple chemokines as well as the binding of chemokine-binding proteins secreted by various pathogens can serve as a strategy for controlling inflammation. In this work, promiscuous chemokine-binding peptides that could bind and inhibit multiple inflammatory chemokines, such as CCL2, CCL5, and CXCL9/10/11, were selected from phage display libraries. These peptides were cloned into human mutated immunoglobulin Fc-protein fusions (peptibodies). The peptibodies BKT120Fc and BKT130Fc inhibited the ability of inflammatory chemokines to induce the adhesion and migration of immune cells. Furthermore, BKT120Fc and BKT130Fc also showed a significant inhibition of disease progression in a variety of animal models for autoimmunity and inflammation. Developing a novel class of antagonists that can control the courses of diseases by selectively blocking multiple chemokines could be a novel way of generating effective therapeutics.

Effect of BL-8040, high-affinity CXCR4 antagonist, on T-cell infiltration, tumor growth, and synergy with immunomodulatory agents

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Background: Cancer cells affect their micro-environment by recruiting immune cells that support tumor growth, metastasis and inhibition of anti-tumor effector T and NK cell recruitment. In this study, we investigated the role of BL-8040, a CXCR4 antagonist in cancer immunotherapy and its ability to modulate the immunosuppressive tumor micro-environment. Methods: The effect of BL8040 on tumor micro-environment was tested in 3 different cancer mouse models: lung cancer, pancreatic cancer and melanoma. The mobilization of immune cells to the periphery in response to BL8040 was tested, as well as the accumulation of immune cells both within and surrounding the tumor in the pancreatic cancer mouse model. Results: BL8040 was found to be a potent and robust mobilizer of immune cells. Immunophenotyping of the mobilized cells revealed that the mobilization of CD4 and CD8 T lymphocytes, as well as of dendritic cells (DC), was significantly increased in the cancer-bearing mice compared to their naïve counterparts. Importantly, a significant mobilization of effector CD8 T cells and activated CD8 T cells in the cancer-bearing mice was also detected following BL8040 treatment. Concomitantly, in the pancreatic cancer mouse model, treatment with BL8040 increased CD8 T cell accumulation within the tumor and inhibited tumor growth. Conclusions: The immune cell population that is mobilized in response to BL8040 treatment is different in cancer mouse models and naïve mice. The ability of BL8040 to induce mobilization of leukocytes, cytotoxic and activated CD8 T cells and DCs is affected by the presence of a tumor. In our models of pancreatic cancer, mobilization of immune cells from the bone marrow into the circulation and their accumulation within the tumor and tumor microenvironment resulted in inhibition of tumor growth. These results indicate that BL8040 may affect the tumor microenvironment and therefore can potentially synergize with immunomodulatory agents. In vivo pre-clinical studies as well as clinical studies are currently ongoing for testing the combination of BL8040 with immunomodulatory agents in different cancer models.

Abstract 3556: CXCR4 controls BCL-2 expression and function by regulating miR-15a/16-1 expression in tumor cells

Background: CXCR4 is overexpressed in the majority of tumor cells and its degree of expression often correlates with disease severity. Binding of CXCL12 to the CXCR4 receptor activates signaling pathways which are crucial for the interaction of hematopoietic cells with the microenvironment and cell survival. Signaling activated through CXCR4 was shown to be detrimental by increasing survival of tumor cells and promoting resistance to therapy in many types of cancer. CXCR4-antagonists, such as BL-8040, currently in phase II trials, were shown to selectively inhibit tumor cell growth and to induce apoptosis in vitro and in vivo. However, the molecular mechanism by which CXCR4 overexpression triggers tumor cell survival and its inhibition leads to cell death is not fully understood.

Objective: To study the mechanism by which the CXCR4 pathway controls malignant cell survival and death through regulating miR-15a/16-1 expression.

Method: We assessed the effect of CXCR4 overexpression, its activation and inhibition, on the expression of miR-15a/16-1 and their target genes, BCL-2, MCL-1 and cyclin D1, in a variety of tumor cells in vitro and in vivo.

Results:

We found that overexpression of CXCR4 in tumor cells or stimulation of cells with its ligand, CXCL12, lead to up-regulation of miR-15a/16-1, resulting in down-regulation of their target genes BCL-2, MCL-1 and cyclin D1. Furthermore, overexpression of CXCR4 in these cells increases tumorgenesis and shifts their oncogenic dependency from the BCL-2 to the CXCR4/ERK signaling pathway. Antagonists of CXCR4 such as BL-8040 were shown to induce apoptotic cell death of malignant cells. BL-8040 was found to increase the expression of miR-15a/16-1 and reduce the expression of BCL2, MCL1 and cyclin D1. Importantly, CXCR4 inhibition using BL-8040 induced apoptosis in vitro and in vivo in AML and neuroblastoma tumors. This was mediated by inhibition of survival signals by ERK and down-regulation of BCL-2 expression. In support of these results overexpression of miR-15a/16-1 in AML and neuroblastoma cells was shown to induce their apoptosis.

Conclusions:

Our results demonstrate, for the first time to the best of our knowledge, that CXCR4 signaling regulates the expression of miR-15a/16-1 and their target genes. Our results suggest that overexpression of CXCR4 may override the survival dependency of tumor cells on BCL-2, MCL-1, and cyclin D1 leading to resistance of tumor cells to inhibition of these pathways. Furthermore, these results indicate that ligands of CXCR4 may tip the balance toward cell death by down- regulating survival signals through miR-15a/16-1 suppression of BCL2, MCL1 and cyclin D1 expression.

Citation Format: Shiri Klein Silberman, Michal Abraham, Baruch Bulvik, Hanna Wald, Orly Eizenberg, Dvorah Olam, Lola Weiss, Katia Beider, Ori Wald, Shlomo Bulvik, Abraham Avigdor, Ohad Benjamini, Eithan Galun, Arnon Nagler, Yaron Pereg, Amnon Peled. CXCR4 controls BCL-2 expression and function by regulating miR-15a/16-1 expression in tumor cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3556.

Combination of imatinib with CXCR4 antagonist BKT140 overcomes the protective effect of stroma and targets CML in vitro and in vivo

Functional role of CXCR4 in chronic myelogenous leukemia (CML) progression was evaluated. Elevated CXCR4 significantly increased the in vitro survival and proliferation in response to CXCL12. CXCR4 stimulation resulted in activation of extracellular signal-regulated kinase (Erk)-1/2, Akt, S6K, STAT3, and STAT5 prosurvival signaling pathways. In accordance, we found that in vitro treatment with CXCR4 antagonist BKT140 directly inhibited the cell growth and induced cell death of CML cells. Combination of BKT140 with suboptimal concentrations of imatinib significantly increased the anti-CML effect. BKT140 induced apoptotic cell death, decreasing the levels of HSP70 and HSP90 chaperones and antiapoptotic proteins BCL-2 and BCL-XL, subsequently promoting the release of mitochondrial factors cytochrome c and SMAC/Diablo. Bone marrow (BM) stromal cells (BMSC) markedly increased the proliferation of CML cells and protected them from imatinib-induced apoptosis. Furthermore, BMSCs elevated proto-oncogene BCL6 expression in the CML cells in response to imatinib treatment, suggesting the possible role of BCL6 in stroma-mediated TKI resistance. BKT140 reversed the protective effect of the stroma, effectively promoted apoptosis, and decreased BCL6 levels in CML cells cocultured with BMSCs. BKT140 administration in vivo effectively reduced the growth of subcutaneous K562-produced xenografts. Moreover, the combination of BKT140 with low-dose imatinib markedly inhibited tumor growth, achieving 95% suppression. Taken together, our data indicate the importance of CXCR4/CXCL12 axis in CML growth and CML-BM stroma interaction. CXCR4 inhibition with BKT140 antagonist efficiently cooperated with imatinib in vitro and in vivo. These results provide the rational basis for CXCR4-targeted therapy in combination with TKI to override drug resistance and suppress residual disease.

The high-affinity CXCR4 antagonist BKT140 is safe and induces a robust mobilization of human CD34+ cells in patients with multiple myeloma

PURPOSE

CXCR4 plays an important role in the retention of stem cells within the bone marrow. BKT140 (4F-benzoyl-TN14003) is a 14-residue bio stable synthetic peptide, which binds CXCR4 with a greater affinity compared with plerixafor (4 vs. 84 nmol/L). Studies in mice demonstrated the efficient and superior mobilization and transplantation of stem cells collected with GCSF-BKT140, compared with those obtained when using stem cells obtained with each one of these mobilizing agent alone. These results have served as a platform for the present clinical phase I study.

EXPERIMENTAL DESIGN

Eighteen patients with multiple myeloma who were preparing for their first autologous stem cell transplantation were included. Patients received a standard multiple myeloma mobilization regimen, consisting of 3 to 4 g/m(2) cyclophosphamide (day 0), followed by granulocyte colony-stimulating factor (G-CSF) at 5 μg/kg/d starting on day 5 and administered between 8 and 10 pm until the end of stem cell collection. A single injection of BKT140 (0.006, 0.03, 0.1, 0.3, and 0.9 mg/kg) was administered subcutaneously on day 10 in the early morning, followed by G-CSF 12 hours later.

RESULTS

BKT140 was well tolerated at all concentrations, and none of the patients developed grade 3 and 4 toxicity. A single administration of BKT140 at the highest dose, 0.9 mg/kg, resulted in a robust mobilization and collection of CD34(+) cells (20.6 ± 6.9 × 10(6)/kg), which were obtained through a single apheresis. All transplanted patients received ∼5.3 × 10(6) CD34(+) cells/kg, which rapidly engrafted (n = 17). The median time to neutrophil and platelet recovery was 12 and 14 days, respectively, at the highest dose (0.9 mg/kg).

CONCLUSIONS

When combined with G-CSF, BKT140 is a safe and efficient stem cell mobilizer that enabled the collection of a high number of CD34(+) cells in 1 and 2 aphaeresis procedures, resulting in successful engraftment.

Sequential administration of the high affinity CXCR4 antagonist BKT140 promotes megakaryopoiesis and platelet production

Platelets are the terminal differentiation product of megakaryocytes (MKs). Cytokines, such as thrombopoietin (TPO), are known to influence different steps in MK development; however, the complex differentiation and platelet localization processes are not fully understood. MKs express the receptor CXCR4 and have been shown to migrate in response to CXCL12 and to increase their platelet production. In this study, we studied the role of CXCR4 in platelet production with the high affinity CXCR4 antagonist, BKT140. Single and sequential administration of BKT140 significantly increased the number of MKs and haematopoietic progenitors (HPCs) within the bone marrow (BM). Increased megakaryopoiesis was associated with increased platelet production. Single and sequential administration of BKT140 also increased the number of HPCs in the blood. In a model of 5-fluorouracil-induced thrombocytopenia, BKT140 significantly reduced the severity and duration of thrombocytopenia and cytopenia when administered before and after chemotherapy. Our results demonstrated that the CXCR4 antagonist, BKT140, mediated unique beneficial effects by stimulating megakaryopoiesis and platelet production. These results provide evidence for the possible therapeutic use of BKT140 for modulating platelet numbers in thrombocytopenic conditions.

Targeting the CD20 and CXCR4 pathways in non-hodgkin lymphoma with rituximab and high-affinity CXCR4 antagonist BKT140

PURPOSE

Chemokine axis CXCR4/CXCL12 is critically involved in the survival and trafficking of normal and malignant B lymphocytes. Here, we investigated the effect of high-affinity CXCR4 antagonist BKT140 on lymphoma cell growth and rituximab-induced cytotoxicity in vitro and in vivo.

EXPERIMENTAL DESIGN

In vitro efficacy of BKT140 alone or in combination with rituximab was determined in non-Hodgkin lymphoma (NHL) cell lines and primary samples from bone marrow aspirates of patients with NHL. In vivo efficacy was evaluated in xenograft models of localized and disseminated NHL with bone marrow involvement.

RESULTS

Antagonizing CXCR4 with BKT140 resulted in significant inhibition of CD20+ lymphoma cell growth and in the induction of cell death, respectively. Combination of BKT140 with rituximab significantly enhanced the apoptosis against the lymphoma cells in a dose-dependent manner. Moreover, rituximab induced CXCR4 expression in lymphoma cell lines and primary lymphoma cells, suggesting the possible interaction between CD20 and CXCR4 pathways in NHL. Primary bone marrow stromal cells (BMSC) further increased CXCR4 expression and protected NHL cells from rituximab-induced apoptosis, whereas BKT140 abrogated this protective effect. Furthermore, BKT140 showed efficient antilymphoma activity in vivo in the xenograft model of disseminated NHL with bone marrow involvement. BKT140 treatment inhibited the local tumor progression and significantly reduced the number of NHL cells in the bone marrow. Combined treatment of BKT140 with rituximab further decreased the number of viable lymphoma cells in the bone marrow, achieving 93% reduction.

CONCLUSIONS

These findings suggest the possible role of CXCR4 in NHL progression and response to rituximab and provide the scientific basis for the development of novel CXCR4-targeted therapies for refractory NHL.

CXCR4 antagonist 4F-benzoyl-TN14003 inhibits leukemia and multiple myeloma tumor growth

OBJECTIVE

The chemokine receptor CXCR4 and its ligand CXCL12 are involved in the progression and dissemination of a diverse number of solid and hematological malignancies. Binding CXCL12 to CXCR4 activates a variety of intracellular signal transduction pathways that regulate cell chemotaxis, adhesion, survival, proliferation, and apoptosis.

MATERIALS AND METHODS

Here, we demonstrate that the CXCR4 antagonist, 4F-benzoyl-TN14003 (BKT140), but not AMD3100, exhibits a CXCR4-dependent preferential cytotoxicity toward malignant cells of hematopoietic origin. BKT140 significantly and preferentially stimulated multiple myeloma apoptotic cell death. BKT140 treatment induced morphological changes, phosphatidylserine externalization, decreased mitochondrial membrane potential, caspase-3 activation, sub-G1 arrest, and DNA double-stranded breaks.

RESULTS

In vivo, subcutaneous injections of BKT140 significantly reduced, in a dose-dependent manner, the growth of human acute myeloid leukemia and multiple myeloma xenografts. Tumors from animals treated with BKT140 were smaller in size and weights, had larger necrotic areas and high apoptotic scores.

CONCLUSIONS

Taken together, these results suggest a potential therapeutic use for BKT140 in multiple myeloma and leukemia patients.

A peptide derived from activity-dependent neuroprotective protein (ADNP) ameliorates injury response in closed head injury in mice

Brain injury induces disruption of the blood-brain barrier, edema, and release of autodestructive factors that produce delayed neuronal damage. NAPSVIPQ (NAP), a femtomolar-acting peptide, is shown to be neuroprotective in a mouse model of closed head injury. NAP injection after injury reduced mortality and facilitated neurobehavioral recovery (P < 0.005). Edema was reduced by 70% in the NAP-treated mice (P < 0.01). Furthermore, in vivo magnetic resonance imaging demonstrated significant brain-tissue recovery in the NAP-treated animals. NAP treatment decreased tumor necrosis factor-alpha levels in the injured brain and was shown to protect pheochromocytoma (PC12 cells) against tumor necrosis factor-alpha-induced toxicity. Thus, NAP provides significant amelioration from the complex array of injuries elicited by head trauma.

Expression of GTP-dependent and GTP-independent tissue-type transglutaminase in cytokine-treated rat brain astrocytes

Tissue-type transglutaminases (TGases) were recently shown to exert dual enzymatic activities; they catalyze the posttranslational modification of proteins by transamidation, and they also act as guanosine triphosphatase (GTPase). Here we show that a tissue-type TGase is expressed in rat brain astrocytes in vitro, and is induced by the inflammation-associated cytokines interleukin-1beta and to a lesser extent by tumor necrosis factor-alpha. Induction is accompanied by overexpression and appearance of an additional shorter clone, which does not contain the long 3'-untranslated region and encodes for a novel TGase enzyme whose C terminus lacks a site that affects the enzyme's interaction with guanosine triphosphate (GTP). Expression of two clones revealed that the long form is inhibited noncompetitively by GTP, but the short form significantly less so. The different affinities for GTP may account for the difference in physiological function between these two enzymes.

p53 plays a regulatory role in differentiation and apoptosis of central nervous system-associated cells

This study demonstrated the involvement of the tumor suppressor protein p53 in differentiation and programmed cell death of neurons and oligodendrocytes, two cell types that leave the mitotic cycle early in development and undergo massive-scale cell death as the nervous system matures. We found that primary cultures of rat oligodendrocytes and neurons, as well as of the neuronal PC12 pheochromocytoma cell line, constitutively express the p53 protein. At critical points in the maturation of these cells in vitro, the subcellular localization of p53 changes: during differentiation it appears mainly in the nucleus, whereas in mature differentiated cells it is present mainly in the cytoplasm. These subcellular changes were correlated with changes in levels of immunoprecipitated p53. Infection of cells with a recombinant retrovirus encoding a C-terminal p53 miniprotein (p53 DD), previously shown to act as a dominant negative inhibitor of endogenous wild-type p53 activity, inhibited the differentiation of oligodendrocytes and of PC12 cells and protected neurons from spontaneous apoptotic death. These findings suggest that p53, upon receiving appropriate signals, is recruited into the nucleus, where it plays a regulatory role in directing primary neurons', oligodendrocytes, and PC12 cells toward either differentiation or apoptosis in vitro.

Interleukin-2 transcripts in human and rodent brains: possible expression by astrocytes

Cytokines have been suggested to be involved in the cross talk between the immune and the nervous systems, under normal and pathological conditions. For example, the cytokine interleukin-2 was suggested to be involved in response to CNS trauma and spontaneous regeneration. Here, we examined whether mammalian CNS has an intrinsic potential to produce interleukin-2 and, if so, what its cellular origin is. mRNA sequences encoding for interleukin-2 were detected in brains of humans and rodents. Northern blot analysis revealed the presence of several interleukin-2 transcripts of different sizes in the brain, all recognized by lymphocyte-derived interleukin-2 cDNA probes. One of the transcripts, a high molecular weight form of approximately 5 kb, appeared to be unique to the brain. Reverse transcription and amplification by PCR of human fetal brain mRNA revealed one cDNA product that, upon sequence analysis, showed a high degree of homology with the human lymphocyte-derived interleukin-2 coding sequence. To identify the possible cellular source of the interleukin-2 transcripts within the mammalian brain, we similarly analyzed mRNA of rat brain cells in culture. Northern blot analysis revealed that astrocytes contain transcripts that hybridize with interleukin-2 cDNA probe. These findings point to the astrocytes as a possible source of brain interleukin-2.

Direct involvement of p53 in programmed cell death of oligodendrocytes

A covalent dimer of interleukin (IL)-2, produced in vitro by the action of a nerve-derived transglutaminase, has been shown previously to be cytotoxic to mature rat brain oligodendrocytes. Here we report that this cytotoxic effect operates via programmed cell death (apoptosis) and that the p53 tumor suppressor gene is involved directly in the process. The apoptotic death of mature rat brain oligodendrocytes in culture following treatment with dimeric IL-2 was demonstrated by chromatin condensation and internucleosomal DNA fragmentation. The peak of apoptosis was observed 16-24 h after treatment, while the commitment to death was already observed after 3-4 h. An involvement of p53 in this process was indicated by the shift in location of constitutively expressed endogenous p53 from the cytoplasm to the nucleus, as early as 15 min after exposure to dimeric IL-2. Moreover, infection with a recombinant retrovirus encoding a C-terminal p53 miniprotein, shown previously to act as a dominant negative inhibitor of endogenous wild-type p53 activity, protected these cells from apoptosis.

Linear dimeric interleukin-2 obtained by the use of a defective herpes simplex viral vector: conformation-activity relationship

An interleukin-2 dimer, produced enzymatically by a nerve-derived transglutaminase in vitro, is cytotoxic to oligodendrocytes, unlike the immune-derived monomeric interleukin-2. The object of this study was to establish a way to produce a dimer of interleukin-2 in quantities, by means of genetic engineering, and to confirm that the structure of the resulting molecule is critical for its function. A defective herpes simplex virus vector was utilized for overproduction of a dimeric interleukin-2. The resulting linear dimer, which is a translational product, differs from the enzymatically produced dimer, which is a posttranslational modification of interleukin-2. The linear dimer, while retaining the known interleukin-2 activity of monomeric interleukin-2 with respect to mitogenicity on T cells, was not cytotoxic to oligodendrocytes. This finding suggests that the lack of cytotoxicity of the linear dimeric interleukin-2 is not caused by a loss of activity during its preparation but is related to its conformational structure, which evidently does not meet the requirements for cytotoxicity. This study opens the way to the design at the transcriptional level of modified proteins and their efficient production, provided that the new transcript encodes for the desired modification in the protein at the appropriate sites.

Reduced levels of alpha 1 (I) collagen mRNA in cells immortalized by mutant p53 or transformed by ras

Genes whose expression patterns are altered in a cell line immortalized by mutant p53 were isolated by differential screening of a cDNA library. Levels of alpha 1 (I) collagen mRNA were reduced in the majority of immortalized cell lines which greatly overproduced the transfected mutant p53. This may reflect a co-selection during the establishment of the cell lines, rather than a direct effect of p53 on alpha 1 (I) collagen gene expression. On the other hand, a more direct relationship could be demonstrated between the expression of activated ras and a reduction in alpha 1 (I) collagen mRNA. Such reduction could partially account for the effects of ras on cell shape and cell proliferation.

Sequence homology within the morbilliviruses

Double-stranded cDNA synthesized from total polyadenylate-containing mRNA extracted from monkey kidney cells infected with canine distemper virus (CDV) was cloned into the PstI site of Escherichia coli plasmid pBR322. Clones containing CDV DNA were identified by hybridization to a CDV-specific 32P-labeled cDNA. A cDNA clone containing an insert 1,700 base pairs (CDV 364) has been identified as the reverse transcript of the mRNA coding for the nucleocapsid protein. The size of the mRNA species complementary to this insert is 1,850 nucleotides, as determined by the Northern technique. Hybridization experiments and heteroduplex mapping indicated homology between the central region of the CDV and measles virus nucleocapsid gene. The completion of the nucleotide sequence analysis of the measles virus gene allowed the reconstruction of the entire coding region of the measles virus gene and a comparison with the counterpart sequence of CDV. This comparison delineated three regions: (i) a region of high homology (nucleotides 501 to 1215), in which 77% of the nucleotides and 88% of the encoded amino acids are identical; (ii) a region of moderate homology at the 5' end of the message (nucleotides 1 to 500), in which 59% of the nucleotides and 66% of the encoded amino acids are identical; (iii) a region of little or no homology (nucleotides 1216 to 1625) near the 3' end of the message.

Cloning and characterization of DNA complementary to the canine distemper virus mRNA encoding matrix, phosphoprotein, and nucleocapsid protein

Double-stranded cDNA synthesized from total polyadenylate-containing mRNA, extracted from monkey kidney cells infected with canine distemper virus (CDV), has been cloned into the PstI site of Escherichia coli plasmid pBR322. Clones containing canine distemper virus DNA were identified by hybridization to a canine distemper virus-specific, 32P-labeled cDNA. Four specific clones containing different classes of sequences have been identified. The cloned plasmids contain inserts of 800 (clone 44-80), 960 (clone 74-16), 1,700 (clone 364), and 950 (clone 40-9) base pairs. The sizes of the mRNA species complementary to these inserts are 1,500, 1,850, 1,850 and 2,500 nucleotides, respectively, as determined by the Northern technique. Three of the cloned DNA fragments were further identified as the reverse transcripts of the mRNA coding for the matrix, phosphoprotein, and nucleocapsid protein of CDV.