[Clinicopathologic analysis of micronodular thymoma with lymphoid stroma].

Objective: To investigate the clinicopathologic features of micronodular thymoma with lymphoid stroma(MNT). Methods: Five cases of MNT diagnosed from January 2007 to December 2016 in Henan Provincial People's Hospital were collected.Hematoxylin-Eosin staining and immunohistochemistry were used to evaluate the histological and immunophenotypic characteristics in 5 MNT cases. Epstein-Barr virus (EBV) status was detected by in situ hybridization for EBV-encoded small RNA (EBER). Polymerase chain reaction was used to detect the rearrangement of immunoglobulin genes. Results: Five cases were MNT, including 3 male and 2 female patients, mean aged 59 years (from 43 to 63 years). All patients had ananterior mediastinal mass, with no myasthenia gravis and autoimmune diseases, and underwent surgical resection.Half to ten years follow-up showed no recurrence.Grossly, the tumors were solid in 4 cases, and cystic and solid in 1 case; the border was clear. Histologically, the tumors presented as a distribution of micronodules separated by abundant lymphoid stroma with prominent germinal centers. The nodules were composed of neoplastic spindle, oval cells containing bland, oval nuclei.Immunohistochemical study showed strong positivity of the tumor cells for CKpan, CK19, CK5/6 and p63. Stains for EMA, CD117, calretinin, TTF1 were negative in the tumor cells.Scattered CD3, CD1a, and TdT positive immature T lymphocytes were noted in and around tumor nodules. Many lymphocytes in the stroma, including germinal centers, were positive for CD20.The bcl-2 was also detected in lymphocytes in the stroma, mantle and marginal zone of lymphoid follicles, and in part of tumor cells. Tumor cells and lymphocytes were negative for EBER. Immunoglobulin genes rearrangement analysis showed that B lymphocytes were polyclonal. Conclusions: MNT is a rare thymoma, which occurs in the elderly and has no obvious symptom. After complete resection, the prognosis is very good. The diagnosis should be based on a combination of clinicopathologic features, and other types of thymoma should be excluded.

V-AKT murine thymoma viral oncogene homolog/mammalian target of rapamycin activation induces a module of metabolic changes contributing to growth in insulin-induced hepatocarcinogenesis.

UNLABELLED: Mounting epidemiological evidence supports a role for insulin-signaling deregulation and diabetes mellitus in human hepatocarcinogenesis. However, the underlying molecular mechanisms remain unknown. To study the oncogenic effect of chronically elevated insulin on hepatocytes in the presence of mild hyperglycemia, we developed a model of pancreatic islet transplantation into the liver. In this model, islets of a donor rat are transplanted into the liver of a recipient diabetic rat, with resulting local hyperinsulinism that leads to the development of preneoplastic lesions and hepatocellular carcinoma (HCC). Here, we investigated the metabolic and growth properties of the v-akt murine thymoma viral oncogene homolog/mammalian target of rapamycin (AKT/mTOR) pathway, a major downstream effector of insulin signaling, in this model of insulin-induced hepatocarcinogenesis. We found that activation of insulin signaling triggers a strong induction of the AKT/mTOR cascade that is paralleled by increased synthesis of fatty acids, cholesterol, and triglycerides, induction of glycolysis, and decrease of fatty acid oxidation and gluconeogenesis in rat preneoplastic and neoplastic liver lesions, when compared with the healthy liver. AKT/mTOR metabolic effects on hepatocytes, after insulin stimulation, were found to be mTORC1 dependent and independent in human HCC cell lines. In these cells, suppression of lipogenesis, glycolysis, and the pentose phosphate pathway triggered a strong growth restraint, despite insulin administration. Noticeably, metabolic abnormalities and proliferation driven by insulin were effectively reverted using the dual PI3K/mTOR inhibitor, NVP-BEZ235, both in vitro and in vivo. CONCLUSIONS: The present results indicate that activation of the AKT/mTOR cascade by unconstrained insulin signaling induces a defined module of metabolic alterations in hepatocytes contributing to aberrant cell growth. Thus, inhibition of AKT/mTOR and related metabolic changes might represent a novel preventive and therapeutic approach to effectively inhibit insulin-induced hepatocarcinogenesis.

Viral infection in thymoma and thymic tumors with autoimmune diseases.

A thymoma is a type of thymic tumor which is rarely malignant that is frequently reported in adult patients. A number of thymoma-related immune disorders are observed including autoimmune diseases, which suggests a strong connection between thymoma development and immunological mechanisms. Characterized by association with humoral and cellular immunodeficiency, thymoma patients are susceptible to opportunistic infections by environmental factors. Recent reports have suggested that viral infection may play a role in the etiological mechanisms of thymoma development associated with dysregulated immunity. In this review, we summarize the case reports and studies related to viral infection, such as CMV, EBV and HSV, that probably play a part in the pathogenesis of thymoma and related diseases. Furthermore, we demonstrate the underlying mechanisms by which viruses may induce the occurrence of thymoma with autoimmune diseases. Lastly, we discuss the potential application of antiviral therapy in the treatment of thymic diseases.

A Novel Retrovirus (Gunnison's Prairie Dog Retrovirus) Associated With Thymic Lymphoma in Gunnison's Prairie Dogs in Colorado, USA.

As part of research and wildlife disease surveillance efforts, we performed necropsy examinations of 125 free-ranging (n = 114) and captive (n = 11) prairie dogs in Colorado from 2009 to 2017. From these cases, we identified three cases of thymic lymphoma in free-ranging Gunnison's prairie dogs (Cynomys gunnisoni), and we identified a novel retroviral sequence associated with these tumors. The viral sequence is 7700 nucleotides in length and exhibits a genetic organization that is consistent with the characteristics of a type D betaretrovirus. The proposed name of this virus is Gunnison's prairie dog retrovirus (GPDRV). We screened all 125 prairie dogs for the presence of GPDRV using PCR with envelope-specific primers and DNA extracted from spleen samples. Samples were from Gunnison's prairie dogs (n = 59), black-tailed prairie dogs (Cynomys ludovicianus) (n = 40), and white-tailed prairie dogs (Cynomys leucurus) (n = 26). We identified GPDRV in a total of 7/125 (5.6%) samples including all three of the prairie dogs with thymic lymphoma, as well as spleen from an additional four Gunnison's prairie dogs with no tumors recognized at necropsy. None of the GPDRV-negative Gunnison's prairie dogs had thymic lymphomas. We also identified a related, apparently endogenous retroviral sequence in all prairie dog samples. These results suggest that GPDRV infection may lead to development of thymic lymphoma in Gunnison's prairie dogs.

Detection of human parvovirus B19 infection in the thymus of patients with thymic hyperplasia-associated myasthenia gravis.

OBJECTIVE: To investigate the association between myasthenia gravis (MG) and human parvovirus B19 (B19V) infection in the thymus. METHODS: The presence of human B19V DNA and protein was assessed in 138 samples-including 68 thymic hyperplasias (39 with MG), 58 thymomas (23 with MG), and 12 normal thymus tissues-using a nested polymerase chain reaction, immunohistochemistry, laser capture microdissection, and sequencing in a double-blinded manner. RESULTS: B19V DNA was detected mainly in thymic hyperplasia, and the positivity rate (41.18%, 28/68) was significantly higher than that in thymoma (3.45%, 2/58) (p <0.001) but not that in normal thymic tissues. Correspondingly, the positivity rate in thymic hyperplasia with MG (30.77%, 12/39) was significantly higher than that in thymoma with MG (4.35%, 1/23) (p=0.021). However, it was higher in thymic hyperplasia without MG (55.17%, 16/29) than in thymic hyperplasia with MG (30.77%, 12/39) (p=0.043). Cells in thymic hyperplasia positive for B19V VP1/VP2 protein (63.24%, 43/68) were identified mainly in ectopic germinal centres and thymic corpuscle epithelial cells, but were rare in thymomas (1.72%, 1/58) (p <0.001). Moreover, the positivity rate was significantly higher in thymic hyperplasia with MG (74.36%, 29/39) than in thymic hyperplasia without MG (48.28%, 14/29) (p=0.027). CONCLUSIONS: To our knowledge, the present study is the first to show that human B19V infection is closely associated with thymic hyperplasia and thymic-hyperplasia-associated MG, but is not related to thymoma or thymoma-associated MG. The findings reveal a previously unrecognized aetiopathogenic mechanism of thymic-hyperplasia-associated MG, evoking numerous questions that require further investigation.

Low prevalence of Merkel cell polyomavirus in human epithelial thymic tumors.

BACKGROUND: The etiology of thymic epithelial tumors is unknown. Murine polyomavirus strain PTA has been shown to induce thymomas in mice. Recently, using diverse molecular techniques, we reported the presence of human polyomavirus 7 (HPyV7) in thymic epithelial tumors. In the present study, we investigated the prevalence of Merkel cell polyomavirus (MCPyV) in thymic epithelial tumors. METHODS: Thirty-six thymomas were screened for MCPyV by PCR and subsequently tested by DNA and RNA in situ hybridization and immunohistochemistry. Twenty-six thymomas were diagnosed with myasthenia gravis (MG). RESULTS: MCPyV DNA was detected by PCR in 7 (19.4%) of the 36 thymic epithelial tumors and in six of these, the presence of MCPyV was confirmed by fluorescence situ hybridization. Of these, 3 (28.6%) revealed weak MCPyV LT-antigen protein expression. In addition, one of the MCPyV positive thymomas tested positive for MCPyV LT RNA with RNAscope. Of interest, two out of the three thymomas that previously tested positive for MCPyV by immunohistochemistry also tested positive for HPyV7. One of the 11 MG-negative and 2 of the 25 MG-positive were positive for MCPyV. CONCLUSIONS: MCPyV DNA and MCPyV protein expression can be detected in human epithelial thymoma; however, to a far lesser extent than HPyV7. Our data strongly indicate that because of its infrequent detection and weak expression, MCPyV is unlikely to play an important role in the etiopathogenesis of human thymomas.

Type AB thymoma accompanied by pure red cell aplasia and Good syndrome with CMV infection of tumor cells.

Reported herein is a case of type AB thymoma accompanied by pure red cell aplasia (PRCA) and Good syndrome. The patient was a 55-year-old woman who was found to be anemic and to have an abnormal shadow at the left pulmonary hilus on routine medical examination. Bone marrow aspiration was performed and she was diagnosed as having PRCA. She also had hypogammaglobulinemia. The anemia was temporarily cured with oral prednisolone and cyclosporin A, and the patient underwent total thymectomy. The tumor was type AB thymoma, composed mainly of type A components; widespread spindle cell components showed slight to moderate infiltration of immature T-cells. In addition, CMV inclusion bodies were scattered throughout this tumor. The CMV-infected cells were tumor cells, because they were positive for pancytokeratin and negative for CD34. Several cases of Good syndrome with widespread CMV infection have been reported, but it is extremely rare for CMV-inclusion bodies to be found in tumor cells. No thymoma cases involving CMV infection of tumor cells have been reported.

The clinical characteristics and therapy response of patients with acquired pure red cell aplasia.

OBJECTIVE: To summarize the clinical characteristics of acquired pure red cell aplasia (PRCA) patients diagnosed in our hospital in the last 10 years. METHOD: The clinical features, immune state and treatment response of acquired PRCA patients diagnosed in our hospital from January 2007 to January 2017 were retrospectively analyzed. RESULTS: The results showed that thymoma (13.21%) and parvovirus B19 (11.32%) were the most common causes for secondary PRCA. Ferritin (Fer) levels and erythropoietin (EPO) levels were increased in PRCA patients. The total CR and PR rate of immunosuppressive therapy in our studies was 68.29% and 12.20%, respectively. Patients with EPO level >400 U/L and Fer level >200 ng/ml had significantly lower CR rate than others. The patients with EPO level >400 U/L also had longer hemoglobin recovery time than patients with EPO level ≤400 U/L. Patients treated with corticosteroids (CS) + cyclosporine A (CsA) had lower relapse rate compared to the CS group (29.17% vs. 80.00%, P < .05). CONCLUSION: Our data showed that patients with PRCA had high EPO and Fer levels. Thymoma and viral infections are the most common causes for secondary PRCA. The CS+ CsA group had lower relapse rate than CS group although response rate was similar. Increased EPO and Fer levels might be the negative factors for prognosis of acquired PRCA.

Eradication of established tumors by vaccination with recombinant Bordetella pertussis adenylate cyclase carrying the human papillomavirus 16 E7 oncoprotein.

High-risk human papillomaviruses (HPV) such as HPV16 are associated with the development of cervical cancer. The HPV16-E6 and HPV16-E7 oncoproteins are expressed throughout the replicative cycle of the virus and are necessary for the onset and maintenance of malignant transformation. Both these tumor-specific antigens are considered as potential targets for specific CTL-mediated immunotherapy. The adenylate cyclase (CyaA) of Bordetella pertussis is able to target dendritic cells through specific interaction with the alpha(M)beta(2) integrin. It has been previously shown that this bacterial protein could be used to deliver CD4(+) and CD8(+) T cell epitopes to the MHC class II and class I presentation pathways to trigger specific Th and CTL responses in vivo, providing protection against subsequent viral or tumoral challenge. Here, we constructed recombinant CyaA containing either the full sequence or various subfragments from the HPV16-E7 protein. We show that, when injected to C57BL/6 mice in absence of any adjuvant, these HPV16-recombinant CyaAs are able to induce specific Th1 and CTL responses. Furthermore, when injected into mice grafted with HPV16-E7-expressing tumor cells (TC-1), one of these recombinant proteins was able to trigger complete tumor regression in 100% of the animals tested. This therapeutic efficacy compared favorably to that of strongly adjuvanted peptide and was marginally affected by prior immunity to CyaA protein. This study represents the first in vivo demonstration of the antitumoral therapeutic activity of recombinant CyaA proteins carrying human tumor-associated antigens and paves the way for the testing of this vector in clinical trials.

A full-length Notch1 allele is dispensable for transformation associated with a provirally activated truncated Notch1 allele in Moloney MuLV-infected MMTV(D)/myc transgenic mice.

The Notch1 gene was previously found to be targetted by provirus insertion in a high proportion of T-cell lymphomas arising in Moloney MuLV-inoculated MMTV(D)/myc transgenic mice. Proviral activation of Notch1 was associated with overexpression of truncated Notch1, deleted of the sequences coding for the extracellular domain. The high levels of truncated Notch1 RNA and proteins in these tumors are thought to be involved in the oncogenic transformation. However, in addition to these truncated RNA and proteins, high level expression of full-length Notch1 RNA and proteins was also observed in several tumors, suggesting that they could also contribute to the transformation process. To test this hypothesis, we used a genetic approach and studied MMTV(D)/myc transgenic mice in which one of the Notch1 alleles was mutated by targeted mutagenesis (Notch1+/- mice). Heterozygote (Notch1+/-) and wild-type (Notch1+/+) transgenic mice were inoculated with Moloney MuLV and the frequency of Notch1 rearrangements was compared between both groups. Notch1 was rearranged at similar frequencies in both groups, indicating that the full-length Notch1 allele is dispensable in tumors harboring an activated Notch1 allele.

Cooperation of Myb and Myc proteins in T cell lymphomagenesis.

The v-Myb oncogene causes late onset T cell lymphomas when expressed in the T cell lineage of transgenic mice. In order to define the cellular mutations cooperating with s-Myb to cause lymphomas, we have infected v-Myb transgenic mice with Moloney murine leukemia virus (M-MuLV). Tumor formation is significantly accelerated from a mean age of onset of 60 weeks in uninfected vMyb transgenics to 13 weeks in infected vMyb transgenics. We studied the loci into which the M-MuLV had inserted, and found that in 73% of animals, either the c-myc or the N-myc genes had been disrupted and deregulated. Therefore, v-myb and c-myb can cooperate to induce T cell lymphomas.

Recurring proviral integration suggests a role for proto-oncogene activation in thymomas induced with Mo-MuLV-rescued BCR/ABL virus.

Intrathymic injection of Moloney murine leukemia virus (Mo-MuLV)-pseudotyped bcr-abl retrovirus (bcr-abl/M) causes thymic lymphoma but only after a prolonged latent period similar to that seen after intrathymic injection of Mo-MuLV alone. Since thymomas induced by Mo-MuLV show recurring proviral integration near certain cellular proto-oncogenes, it was reasoned that if the pathogenesis of bcr-abl/M thymomas is affected by viral integration, then it may be possible to detect proviral insertion near common Mo-MuLV integration sites in bcr-abl-induced thymomas. A panel of thymomas induced by intrathymic injection of Mo-MuLV, Abelson murine leukemia virus (A-MuLV), or the bcr-abl/M virus was analyzed for proviral integration near c-myc, N-myc, Pim-1, and Mlvi-1 loci that are frequently occupied by provirus in Mo-MuLV-induced T cell lymphomas, and for integration near Ahi-1 that is often occupied in A-MuLV/M-induced pre-B cell lymphoma. As expected, thymomas induced with Mo-MuLV showed frequent rearrangements in these loci while thymomas induced with A-MuLV/M (which does not require Mo-MuLV) did not. The bcr-abl/M-induced tumors also showed recurring proviral integration near c-myc, Pim-1 and Mlvi-1, albeit at a lower frequency than seen in the Mo-MuLV tumors. Unexpectedly, four independent thymomas that were clearly of T cell origin demonstrated proviral integration within the Ahi-1 region which was previously thought to only occur in A-MuLV/M induced pre-B cell lymphoma. These observations suggest that recurring proviral insertion in c-myc, Pim-1, Mlvi-1, and Ahi-1 may provide a selective advantage for bcr-abl/M transformed T lymphoid cells. This model may provide a tool for identifying cellular genes that can cooperate with bcr-abl in lymphoid transformation.

Human herpes virus 8 (Kaposi's sarcoma herpes virus) and malignant lymphoproliferations in France: a molecular study of 250 cases including two AIDS-associated body cavity based lymphomas.

The new human herpes virus 8 (HHV8) was recently detected in cases of body cavity based lymphoma (BCBL), a rare B cell lymphoma, mostly AIDS-associated. We investigated for HHV8 DNA sequences a series of 250 B or T cell lymphoproliferative malignancies, as seen in France, including 126 leukemias and 124 lymphomas (232 non-AIDS-associated and 18 AIDS-associated tumors). HHV8 sequences were detected in only three patients. The first two were homosexual males, HIV-infected since 1985 who suffered from a BCBL initially characterized in one case by a pleural lymphomatous effusion and a peritoneal one in the other case. A high level of HHV8 copies was detected in the tumoral cells of these two BCBL. In contrast, in the third positive patient who had an AIDS-associated immunoblastic lymphoma, the HHV8 sequences level was quite low. In the two BCBL patients, the HHV8-infected clonal B cells had a large immunoblastic feature with an indeterminate phenotype and were also infected by Epstein-Barr virus. In one BCBL case, a semiquantitative PCR analysis revealed that the HHV8 sequences were much more abundant in the effusion tumor cells than in the cutaneous Kaposi's biopsy while no HHV8 sequence was detectable in the peripheral blood lymphocytes. This study reports HHV8-associated BCBL in European AIDS patients and confirms that HHV8 is present at a high copy number in the tumoral B cells of this malignancy. Furthermore, HHV8 does not seem to play a pathogenic role in any of the other T or B malignant lymphoid neoplasias studied so far. This study also stresses the necessity for quantification studies in interpretation of a positive PCR analysis for HHV8 sequences, especially in patients at risk for HIV infection or Kaposi's sarcoma.

Absence of latent Epstein-Barr virus in thymic epithelial tumors as demonstrated by Epstein-Barr-encoded RNA(EBER) in situ hybridization.

BACKGROUND: Several studies have established that Epstein-Barr virus (EBV) is associated with lympho-proliferative disorders such as Burkitt's lymphoma and Hodgkin's disease. EBV is also present in undifferentiated nasopharyngeal carcinomas and in tumors of similar morphology (lymphoepithelioma-like carcinomas) arising in a variety of organs, predominantly in stomach, salivary gland and thymus. As reports of EBV-positive thymic epithelial tumors (TET) have been divergent and as different methods have been used to detect EBV, the aim of this study was to investigate the possible role of EBV in TET of Danish patients. MATERIAL AND METHODS: Archival material of 157 cases of TET (105 thymomas and 52 thymic carcinomas, including 4 lymphoepithelioma-like thymic carcinomas (LELTC)) was analyzed for EBV by applying a sensitive and specific method for detecting latently EBV-infected cells (in situ hybridization for EBV-encoded RNA (EBER)). RESULTS: All investigated cases were EBER negative. CONCLUSIONS: EBV does not seem to be implicated in the pathogenesis of TET. However, a review of the literature showed that 28% of LELTC were EBER ISH positive. As they occurred in young people (mean 18 years), at an age when the patients were susceptible to infection by EBV, it is suggested that EBV merely acts as an innocent bystander.

Telomerase activity in thymomas and mammary gland adenocarcinomas induced by polyoma virus in AKR mice.

Telomerase is an enzyme that stabilizes telomere length in transformed cells and tumors. Its role in tumor development is far from clear. In this paper, a new experimental model to study telomerase activity during tumorigenesis is presented. After infection with Polyoma virus, AKR mice developed thymomas and mammary gland adenocarcinomas. Polyoma antigens were observed by the peroxidase-antiperoxidase technique on tissue sections, and by Western blot on tumor extracts. The TRAP assay was performed to detect telomerase activity. It was not present in normal mammary gland, but it was positive in mammary gland adenocarcinomas. A different pattern was seen in thymic tissues: normal thymus had higher telomerase activity than thymomas. The incubation of thymoma extracts with normal thymus extracts decreased telomerase activity in the latter. These results demonstrate two different patterns of telomerase activity in tumors induced by Polyoma virus, and suggest the presence of telomerase inhibitory factors in thymomas.

Myasthenia gravis accompanied by thymomas not related to foamy virus genome in Belarusian's patients.

The spectrum and features of neurological disorders have been changed due to the Chernobyl catastrophe in the Republic of Belarus. More recently neurologists in Belarus have noted a significant increase in the frequency of myasthenia gravis (MG) with concomitant rise in the thymomas. There is some evidence suggesting that retroviruses play a key role in the development and pathogenesis of autoimmune diseases. This study analyzed thymomas from 45 MG patients from the Republic of Belarus by using PCR and primers for two regions of FV--gag and bel-2 genes. The results showed that none of the varied thymuses from the 45 MG patients contained FV genome. No relationship can be confirmed between FV and this disease and the results suggest that no pathological link between FV and MG exists.

Epstein-Barr virus--positive undifferentiated thymic carcinoma in a 12-year-old white girl.

Thymic epithelial malignant diseases are extremely rare in children. The authors report a 12-year-old white girl admitted for a polymetastatic tumor of the anterior mediastinum. Tumor proliferation was typical of an undifferentiated thymic carcinoma. A close link between Epstein-Barr virus (EBV) and the tumor was established by a high titer of anti-VCA IgA and the presence of EBV RNA and DNA in the tumor. In addition, monoclonal viral episomes were present in tumor cells, indicating that EBV infection was an early event in the oncogenic process. The patient died despite resection, irradiation, and chemotherapy.

Detection of Epstein-Barr virus genome within thymic epithelial tumours in Taiwanese patients by nested PCR, PCR in situ hybridization, and RNA in situ hybridization.

Epstein-Barr virus (EBV) is known to be associated with a variety of tumours, including Burkitt's lymphoma, nasopharyngeal carcinoma, and some carcinomas of other organs with similar lymphoepithelioma-like features. The association between EBV and thymic epithelial tumours is inconclusive, as reports in this regard are not entirely consistent and the methods employed are of different sensitivity and specificity. This study examined 78 thymomas and 21 thymic carcinomas in Taiwanese patients, to detect the viral genome at both DNA and RNA levels. The tissue blocks were first screened by nested polymerase chain reaction (PCR) targeting on the first tandem internal repeats. The positive cases were further submitted for viral localization by in situ PCR insitu hybridization (ISH) and Epstein-Barr-encoded RNA-1 (EBER-1) ISH. None of the thymomas showed a detectable EBV genome. Eight thymic carcinomas were positive for EBV by nested PCR, of which six displayed nuclear signals within the tumour cells by in situ PCR ISH and/or RNA ISH, one displayed signals within the lymphocytes, and one showed no discernible in situ signals. Most of them exhibited a lymphoepithelioma-like morphology. These results show that nested PCR is a sensitive method for screening the EBV genome in thymic epithelial tumours. In situ PCR ISH is reliable for localization of the virus, in addition to EBER-1 RNA ISH. Thymomas are not related to EBV, even in this endemic area. Thymic carcinomas, especially the lymphoepithelioma-like thymic carcinomas, are more often associated with the virus.

Methylnitrosourea induction of thymomas in AKR mice requires one or two "hits" only.

Induction of thymomas by methylnitrosourea in many strains of mice requires 3 "hits". AKR mice develop thymomas spontaneously late in life, probably because of their large load of viral leukemia oncogenes. It was expected therefore, and so found, that methylnitrosourea induces thymomas in AKR mice with only 1 or 2 "hits". The viral oncogene therefore appears to function as a dominant "hit" gene cooperating with the chemical carcinogen.

Duplication of U3 sequences in the long terminal repeat of mink cell focus-inducing viruses generates redundancies of transcription factor binding sites important for the induction of thymomas.

The ability of mink cell focus-inducing (MCF) viruses to induce thymomas is determined, in part, by transcriptional enhancers in the U3 region of their long terminal repeats (LTRs). To elucidate sequence motifs important for enhancer function in vivo, we injected newborn mice with MCF 1dr (supF), a weakly pathogenic, molecularly tagged (supF) MCF virus containing only one copy of a sequence that is present as two copies (known as the directly repeated [DR] sequence) in the U3 region of MCF 247 and analyzed LTRs from supF-tagged proviruses in two resulting thymomas. Tagged proviruses integrated upstream and in the reverse transcriptional orientation relative to c-myc provided the focus of our studies. These proviruses are thought to contribute to thymoma induction by enhancer-mediated deregulation of c-myc expression. The U3 region in a tagged LTR in one thymoma was cloned and sequenced. Relative to MCF 1dr (supF), the cloned U3 region contained an insertion of 140 bp derived predominantly from the DR sequence of the injected virus. The inserted sequence contains predicted binding sites for transcription factors known to regulate the U3 regions of various murine leukemia viruses. Similar constellations of binding sites were duplicated in two proviral LTRs integrated upstream from c-myc in a second thymoma. We replaced the U3 sequences in an infectious molecular clone of MCF 247 with the cloned proviral U3 sequences from the first thymoma and generated an infectious chimeric virus, MCF ProEn. When injected into neonatal AKR mice, MCF ProEn was more pathogenic than the parental virus, MCF 1dr (supF), as evidenced by the more rapid onset and higher incidence of thymomas. Molecular analyses of the resultant thymomas indicated that the U3 region of MCF ProEn was genetically stable. These data suggest that the arrangement and/or redundancy of transcription factor binding sites generated by specific U3 sequence duplications are important to the biological events mediated by MCF proviruses integrated near c-myc that contribute to transformation.