Loss of p53 function leads to metastasis in ras+myc-initiated mouse prostate cancer

To study the interactions between dominantly acting oncogenes and tumor suppressor genes we used p53 'knockout' mouse urogenital sinus tissue for retroviral transduction of ras and myc in the mouse prostate reconstitution (MPR) model system. Epithelial hyperplasia was observed in all wild-type p53 MPRs with one small focal cancer and no evidence of metastasis. Prostatic cancer was found in 100% of the heterozygous and homozygous p53 mutant MPRs with metastatic deposits in 95% of the mice. The pattern of metastasis was remarkably similar to that in human prostate cancer with gross metastatic deposits in the lung, lymph nodes, bone and liver of many animals. Progression of carcinomas in the ras+myc-initiated heterozygous p53 mutant MPRs was invariably associated with either complete loss, partial deletion or loss of expression of the wild-type p53 allele. Southern blotting analysis of proviral-cellular DNA junction fragments in primary carcinomas and cell lines derived from metastatic deposits revealed that metastases do not necessarily seed out from the most abundant clone in the primary carcinoma.

Mice deficient in both p53 and Rb develop tumors primarily of endocrine origin

To examine whether a cooperative role exists between inherited Rb and p53 deficiency in tumorigenesis, crosses were made between p53- and Rb-deficient mice and were monitored for subsequent tumor incidence and spectrum. Parental mice containing either Rb or p53 mutant alleles showed a predisposition for pituitary adenomas or lymphomas and sarcomas, respectively. Mice heterozygous for both Rb and p53 mutant alleles developed tumors of endocrine origin (medullary thyroid carcinomas, pancreatic islet cell carcinomas, and pituitary adenomas) in addition to lymphomas and sarcomas. Except for pituitary adenomas, these endocrine tumors were rarely seen in the parental p53 or Rb mutant mice. Mice deficient for both Rb and p53 showed a faster rate of tumor development than mice deficient only in Rb or p53. These results indicate that p53 and Rb do cooperate in the acceleration of tumorigenesis and in the development of endocrine tumor types.

A mutant p53 transgene accelerates tumour development in heterozygous but not nullizygous p53-deficient mice

To test the behaviour of a mutant form of p53 in the presence and absence of wild-type p53 in vivo, we mated p53-deficient mice containing a p53 null allele to transgenic mice containing multiple copies of a mutant p53 gene (Val 135). Animals hemizygous for the endogenous wild-type p53 gene with the mutant transgene exhibited accelerated tumour development and an altered tumour spectrum compared to their non-transgenic counterparts. In contrast, transgenic and non-transgenic animals nullizygous for endogenous p53 developed tumours at the same rate. Thus, the mutant Val-135 p53 allele may act in vivo in a dominant negative manner in the presence of wild-type p53 but does not display gain of function activity in the absence of wild-type p53.

Susceptibility to radiation-carcinogenesis and accumulation of chromosomal breakage in p53 deficient mice

The p53 tumour suppressor gene is an important participant in the cellular response to ionizing radiation and other DNA damaging agents. Cells which lack p53 are unable to arrest cell cycle or enter into apoptotic cell death following irradiation. Moreover, these p53 deficient cells exhibit an increased resistance to DNA damaging agents, including radiation. The significance of this radiation-resistance and its relationship to the role that p53 plays in tumour suppression and the cellular radiation response has not yet been determined. In this report we have analyzed p53 deficient mice, expressing either a mutant p53 transgene or having a targeted p53 null allele, in order to investigate the role that p53 plays in governing susceptibility to radiation-carcinogenesis and in controlling the in vivo accumulation of chromosomal abnormalities. We show that wild-type p53 plays a critical role in controlling susceptibility to gamma-radiation-induced tumorigenesis, and sarcomas and lymphomas rapidly appear in irradiated p53 transgenic mice. Moreover, this susceptibility to radiation-carcinogenesis is associated with a two-fold increase in the in vivo accumulation of radiation-induced double stranded chromosomal breaks relative to that observed in wild-type animal. Taken together, these observations suggest that p53 acts to suppress tumour formation in vivo by preventing the accumulation of cells that have sustained radiation-induced DNA damage.

Evidence that p53 and bcl-2 are regulators of a common cell death pathway important for in vivo lymphomagenesis

Multistep lymphomagenesis involves the deregulation of oncogenes and inactivation of tumor suppressor genes resulting in altered rates of proliferation as well as apoptotic cell death in tumor cells. The contribution of bcl-2 and p53 to the regulation of cell death during lymphomagenesis is assessed using bcl-2-1g, p53 'knock-out' (p53 KO), and p53 KO/bcl-2 hybrid mice. PCR-SSCP and DNA sequence analysis demonstrated that p53 somatic mutations are uncommon in lymphomas arising in bcl-2-Ig transgenic mice. Reduction in tumor latency was not observed in p53 KO/bcl-2 hybrid mice compared to p53 KO mice. Furthermore, overexpressed bcl-2 suppressed wild-type p53 associated apoptosis following gamma-radiation. These findings indicate that bcl-2 and p53 serve a suppressor and effector function, respectively, of a common cell death pathway. These findings also suggest that p53 somatic mutations provide no selective advantage during in vivo multistep lymphomagenesis in the context of bcl-2 gene deregulation.

Basic fibroblast growth factor and fibroblast growth factor receptor I are implicated in the growth of human astrocytomas

Malignant astrocytomas are highly invasive, vascular neoplasms that comprise the majority of nervous system tumors in humans. A strong association has previously been made between malignancy in human astrocytic tumors and increased expression of certain fibroblast growth factor (FGF) family members, including basic and acidic FGF. The influence of endogenous basic FGF on glioblastoma cell growth in vitro was evaluated using basic FGF-specific antisense oligonucleotides. These studies indicated that human glioblastoma cell growth in vitro, can be inhibited by suppressing basic FGF expression. Human astrocytomas also exhibited changes in FGF receptor (FGFR) expression during the course of their progression from a benign to a malignant phenotype. FGFR2 (bek) expression was abundant in normal white matter and in all low grade astrocytomas, but was not observed in glioblastomas. Conversely, FGFR1 (flg) expression was absent or barely detectable in normal white matter, but was significantly elevated in glioblastomas. Glioblastomas also expressed an alternatively spliced form of FGFR1 containing two immunoglobulin-like disulfide loops (FGFR1 beta), whereas normal human adult and fetal brain expressed a form of the receptor containing three immunoglobulin-like disulfide loops (FGFR1 alpha). Intermediate grades of astrocytic tumors exhibited a gradual loss of FGFR2 and a shift in expression from FGFR1 alpha to FGFR1 beta as they progressed from a benign to a malignant phenotype. The underlying cytogenetic changes that contribute to these alterations are not entirely understood, but abnormalities in the p53 tumor suppressor gene may influence expression of bFGF as well as the FGFR. These results suggest that alterations in FGFR signal transduction pathways may play a critical role in the malignant progression of astrocytic tumors.

Complementation of human immunodeficiency virus type 1 vif mutants in some CD4+ T-cell lines

The viral infectivity factor gene, vif of human immunodeficiency virus type 1 (HIV-1), is required for full infectivity in most T-cell lines. The replication kinetics exhibited by these mutants has been shown to be cell type-dependent. In H9 cells as well as primary lymphocytes, vif mutants are incapable of establishing infection. This has led to classification of these cell types as non-permissive for vif mutant replication. The T-cell lines Sup T1 and C8166 are able to replicate the vif mutant virus, leading to their classification as permissive for vif mutant replication. In this study, four cell lines (Sup T1, C8166, Molt 4 Clone 8, and A3.01) were tested for their ability to replicate vif mutant virus derived from two different strains of HIV-1 (HXB2 and NL4-3) that had been passaged on various cell lines. Although the kinetics of initial infection was delayed in all cells, by the second passage of vif mutant virus on Sup T1 or Molt 4 cells the kinetics of replication were identical to wild type virus. In contrast, mutant virus displayed delayed replication kinetics in C8166 and A3.01 cells in both initial and subsequent passages. In addition, the levels of viral DNA in infected Sup T1 cells were similar for delta vif and wild type virus, but in C8166 cells delta vif virus DNA levels were reduced compared to wild type virus. These results argue that in Sup T1 and Molt 4 cells there is a factor present that is able to complement the defect in vif mutant viruses which is absent or inefficient in its activity in C8166 and A3.01 cells.

Gene-targeting and the p53 tumor-suppressor gene

Gene-targeting techniques are now frequently applied to embryonic stem (ES) cells to introduce mutations of endogenous genes in mice. Modifications introduced into tumor-suppressor genes by this technology have produced mice and cell lines with unique tumorigenic and growth characteristics, respectively. A number of strategies have been developed to enhance the efficiency of homologous recombination between targeting vectors and endogenous genes. This review describes recent advances in the techniques used to construct mice with a variety of genetic alterations. In addition, an application of gene-targeting is illustrated in the study of a class of genes with tumor-suppressor function. Recent findings from experiments using gene targeted mice to study the p53 tumor-suppressor gene are discussed and the potential of gene-targeting for the discovery and study of novel tumor-suppressor genes are explored.

Infrequent p53 mutations in 7,12-dimethylbenz[a]anthracene-induced mammary tumors in BALB/c and p53 hemizygous mice

We conducted experiments to determine if p53 alterations, which are frequent in human breast cancers, were also common in murine mammary tumors. In 13 mammary tumors from 7,12-dimethylbenz[a]anthracene (DMBA)-treated BALB/c mice were immunohistochemically analyzed for overexpression of p53; p53 protein was not detectable. Three of the tumors were established as cell lines in vitro. p53 protein was rarely detected at passage 4 in these lines but was overexpressed by passage 8 in two of them. The p53 nucleotide sequence was shown to be wild type in one primary mammary tumor and in the two p53-overexpressing cell lines. One cell line that overexpressed p53 in vitro was implanted into BALB/c mice. The resulting tumors retained the wild-type p53 nucleotide sequence but no longer expressed detectable levels of p53 protein, suggesting that the overexpression of wild-type p53 was related to in vitro culture conditions. The effect of DMBA on mammary-tumor development was also tested in mice rendered hemizygous for p53. These mice and wild-type littermate controls had no differences in susceptibility to induction of mammary tumors by oral administration of DMBA. Furthermore, Southern blot hybridization detected no gross alterations in the wild-type p53 allele in mammary tumors from the p53-deficient mice. Point mutation of the wild-type p53 allele was also infrequent in the DMBA-induced mammary tumors from hemizygous p53 mice; it occurred in only one of seven tumors. Thus, the p53 gene is apparently not a primary target for genetic alterations in DMBA-induced mammary tumors. Next, we examined mammary tumors derived from D1 and D2 transplantable hyperplastic alveolar nodule (HAN) outgrowths, which rapidly form tumors containing Ha-ras mutations after DMBA treatment. As ras and p53 mutants can cooperate in transformation, we examined whether D1 and D2 HAN outgrowths have p53 mutations. Unlike in the DMBA-induced primary mammary tumors, nuclear p53 accumulation was observed frequently (10 of 14) in tumors that arose from D1 and D2 HAN outgrowths. Direct sequencing of the entire coding region of the p53 cDNA from six D1 and D2 tumors confirmed that the sequence was wild type. Although wild-type p53 was retained in both DMBA-induced mammary tumors and mammary tumors derived from D1 and D2 preneoplastic outgrowths, wild-type p53 overexpression was detected only in D1 and D2 tumors. Therefore, D1 and D2 tumors appear to arise by a pathway in which p53 expression is altered, whereas DMBA induction affects a different pathway that does not require such alteration.

Transgenic animals in toxicology

Recent advances have been made in the characterization of a number of transgenic animal models. These animal models have provided a powerful toxicological tool for studying in vivo chemical effects and have increased our understanding of the role of specific genetic alterations as predisposing factors for chemical carcinogenesis. The goal of this symposium was to introduce the development of transgenic animals and the utilization of transgenics in toxicology research focusing on understanding tissue-specific mutation, chemical effects, and cancer. The production of transgenic animals, including gene insertions and gene knockouts, and the utilization of transgenic technology for studying multistage carcinogenesis and tumor suppressor genes are described. Data on the application and implications of transgenics as a genetic endpoint are also discussed. The use of transgenic animals in toxicology should improve our understanding of the role of specific genetic alterations in the carcinogenic process and lead to improved estimations of human health risks.

Spontaneous and carcinogen-induced tumorigenesis in p53-deficient mice

Using gene targeting techniques, mice that have been generated with two germ-line p53 null alleles (homozygotes) develop normally but are highly susceptible to early onset spontaneous tumours. Here, we show that mice with a single null p53 allele (heterozygotes) produced in the same way are also susceptible to spontaneous tumours, but with a delayed onset compared to homozygotes. The most frequent tumour type in homozygotes was malignant lymphoma; in heterozygotes, osteosarcomas and soft tissue sarcomas predominated. Heterozygous mice treated with a liver carcinogen, dimethylnitrosamine, showed a decreased survival time in comparison to treated wild type mice, suggesting that the p53-deficient mice may be useful for some in vivo carcinogenesis assays.

Mice with reduced levels of p53 protein exhibit the testicular giant-cell degenerative syndrome

Transgenic mice which carry hybrid p53 promoter-chloramphenicol acetyltransferase (CAT) transgenes were found to express CAT enzymatic activity predominantly in the testes. Endogenous levels of p53 mRNA and protein were lower than in the nontransgenic control mice. The various p53 promoter-CAT transgenic mice exhibited in their testes multinucleated giant cells, a degenerative syndrome resulting presumably from the inability of the tetraploid primary spermatocytes to complete meiotic division. The giant-cell degenerative syndrome was also observed in some genetic strains of homozygous p53 null mice. In view of the hypothesis that p53 plays a role in DNA repair mechanisms, it is tempting to speculate that the physiological function of p53 that is specifically expressed in the meiotic pachytene phase of spermatogenesis is to allow adequate time for the DNA reshuffling and repair events which occur at this phase to be properly completed. Primary spermatocytes which have reduced p53 levels are probably impaired with respect to DNA repair, thus leading to the development of genetically defective giant cells that do not mature.

Reduction of p53 gene dosage does not increase initiation or promotion but enhances malignant progression of chemically induced skin tumors

The availability of p53 knockout mice generated by gene targeting has enabled us to investigate the functional role of the p53 tumor suppressor gene in initiation, promotion, and progression of carcinogenesis in vivo, using mouse skin as a model system. The number, size, and growth rate of benign papillomas were not increased in the p53 heterozygous mice in comparison with wild type. The p53 null mice showed a reduced yield of papillomas, but these underwent much more rapid malignant progression, with some poorly differentiated carcinomas developing after only 10 weeks of promotion. Progression rate was also greater in heterozygous than in wild-type mice and was associated with loss of the remaining wild-type allele. Most tumors from all groups had activating mutations in the H-ras gene. Absence of p53, therefore, does not augment the frequency of initiation or the rate of promotion but greatly enhances malignant progression.

In vitro growth characteristics of embryo fibroblasts isolated from p53-deficient mice

Fibroblast cultures were derived from mouse embryos containing either one (p53+/-) or two (p53-/-) inactivated p53 alleles and compared to normal embryo fibroblasts for a number of growth parameters. Early passage p53-deficient embryo fibroblasts (p53-/-) divided faster than normal embryo fibroblasts, achieved higher confluent densities, and had a higher fraction of division-competent cells under conditions of low cell density. Flow cytometry studies of early passage embryo fibroblasts showed that the percent of p53-deficient cells in G0/G1 was lower than in normal cells, consistent with the argument that p53 mediates a G1 block. When p53-deficient and normal cells were passaged for long periods of time, the homozygote (p53-/-) fibroblasts grew at a high rate for over 50 passages and never entered a non-growing senescent phase characteristic of the heterozygote (p53+/-) and normal (p53+/+) cells. The p53-deficient fibroblasts were genetically unstable during passaging, with the p53-/- cells showing a high degree of aneuploidy and the p53+/- cells displaying a moderate level of chromosomal abnormalities by passage 25. Surprisingly, the heterozygote cells lost their single wild type allele very early during culturing and in spite of this loss most heterozygote lines entered into senescence. We conclude that the loss of p53 by itself is insufficient to confer immortality on a cell, but does confer a growth advantage. Taken together, the findings confirm that the absence of p53 promotes genomic instability, which in turn may result in genetic alterations which directly produce immortality.

Retroviral vector-mediated transduction of K-ras antisense RNA into human lung cancer cells inhibits expression of the malignant phenotype

A retroviral vector system was developed to transduce a K-ras antisense construct efficiently into human cancer cells. A 2-kb fragment of K-ras gene DNA in antisense orientation was linked to a beta-actin promoter and inserted into retroviral vector LNSX in two different orientations. The constructs were transfected into amphotropic packaging cell line GP+envAm12 followed by alternating transduction between the ecotropic packaging cell line psi-2 and GP+envAm12. Titers up to 9.7 x 10(7) colony-forming units (cfu)/ml were achieved without detectable replication-competent virus. The human large cell lung carcinoma cell line H460a, which has a homozygous codon 61 K-ras mutation, was transduced with an efficiency of 95% after five to seven repeated transductions. DNA polymerase chain reaction (PCR) and genomic DNA Southern blot analysis showed that the retroviral construct was integrated into the genome of H460a cells. K-ras antisense RNA expression was detected in the cells by Northern analysis, slot blot hybridization, and reverse transcriptase-PCR. Translation of the mutated K-ras p21 protein RNA was specifically inhibited, whereas expression of other p21 species was unchanged. Proliferation of H460a cells was suppressed 10-fold following transduction by the antisense construct. Colony formation in soft agarose and tumorigenicity in an orthotopic lung cancer model in nu/nu mice were dramatically reduced in H460a cells expressing antisense K-ras. We conclude that an antisense construct for K-ras can be expressed effectively in a retroviral vector that can efficiently transduce human cancer cells.

Genetic background alters the spectrum of tumors that develop in p53-deficient mice

Using gene targeting in embryonic stem cells, we have generated mice with one or two null p53 germ line alleles. Mice with both p53 alleles inactivated are developmentally normal but highly susceptible to the early development of spontaneous tumors. Initial studies were performed in mice with a mixed inbred genetic background (75% C57BL/6 and 25% 129/Sv) (Donehower et al., Nature (London) 356, 215-221, 1992). To study the effect of genetic background on tumorigenesis in p53-deficient mice, we have put the p53 null allele into a pure 129/Sv background and monitored tumor development. 129/Sv mice with two p53 null alleles developed tumors sooner than the mixed genetic background p53-deficient animals. The most frequently observed tumor in p53 null mice of both genetic backgrounds was a malignant lymphoma. Because the 129/Sv strain has a low incidence of lymphoma, the frequent occurrence of lymphomas in all p53 null mice suggests that this particular tumor type may be a direct result of p53 loss and not a result of a particular genetic background. In addition to malignant lymphomas, the 129/Sv p53-deficient mice showed an increased incidence of aggressive teratocarcinomas (8 of 18 tumor-bearing males), a tumor type rare in virtually all inbred mice except for 129 strains. Thus, it appears that loss of p53 may accelerate a prior tumor predisposition and that genetic background can play a role in mediating both the rate and spectrum of tumor development in these mice.

Studies on a type D retrovirus isolated from an AIDS patient lymphoma

The development of AIDS-related lymphomas (ARL) has been on the rise in recent years. During an analysis of ARL from AIDS patients, one individual developed atypical syncytial variants of high-grade Burkitt's-type B-cell lymphomas, which prompted further study. However, the search for a HIV-1 retrovirus, which we hypothesized was infecting these cells, led to the subsequent discovery of a type D retrovirus in two early-passage lymphoma cell lines derived from this patient. Nucleotide and amino acid sequence analysis, as well as immunologic reactivity, indicated that the virus was closely related to Mason-Pfizer monkey virus (MPMV) or simian retrovirus type 1 (SRV-1). MPMV and SRV-1 are immunosuppressive type D retroviruses that cause an AIDS-like syndrome in rhesus macaques. Amplification of DNA from the patient's diagnostic bone marrow biopsy specimen by the polymerase chain reaction generated MPMV-specific fragments indicative of infection by a retrovirus similar to MPMV. Additionally, the patient's serum contained antibodies that recognized type D retroviral env proteins (gp20 and gp70) and gag proteins (p27 and p14) as assayed by immunoblot and radioimmunoprecipitation techniques. Although there have been reports of human cell lines infected with type D retroviruses and of type D reactive human sera, this is the first report of a type D retrovirus infection in a human confirmed by virus isolation, serum immunoreactivity, and viral DNA identification in tumor tissue.

Mice deficient for p53 are developmentally normal but susceptible to spontaneous tumours

Mutations in the p53 tumour-suppressor gene are the most frequently observed genetic lesions in human cancers. To investigate the role of the p53 gene in mammalian development and tumorigenesis, a null mutation was introduced into the gene by homologous recombination in murine embryonic stem cells. Mice homozygous for the null allele appear normal but are prone to the spontaneous development of a variety of neoplasms by 6 months of age. These observations indicate that a normal p53 gene is dispensable for embryonic development, that its absence predisposes the animal to neoplastic disease, and that an oncogenic mutant form of p53 is not obligatory for the genesis of many types of tumours.

Isolation of a type D retrovirus from B-cell lymphomas of a patient with AIDS

An atypical syncytial variant of a high-grade Burkitt's-type B-cell lymphoma from a patient with AIDS who was seropositive for human immunodeficiency virus type 1 was studied. A productive type D retrovirus infection was identified in early-passage cell lines derived from two lymphomas from this patient. Nucleotide and amino acid sequence analysis as well as immunologic reactivity indicated that the isolated virus was highly related to Mason-Pfizer monkey virus (MPMV). MPMV is an immunosuppressive type D retrovirus that causes an AIDS-like syndrome in rhesus macaques. Amplification of DNA from the patient's diagnostic bone marrow biopsy specimen by polymerase chain reaction generated the appropriate MPMV-specific fragments and indicated that the patient was infected with the MPMV-like retrovirus. In addition, the patient's serum contained antibodies which recognized type D viral env proteins (gp70 and gp20) and gag proteins (p27 and p14). Although there have been reports of human cell lines infected with type D retroviruses and of type D-reactive human sera, this is the first evidence of a type D retrovirus infection in a human confirmed by virus isolation, serum reactivity, and viral DNA identification in tumor tissue.

Detection of a human intracisternal A-type retroviral particle antigenically related to HIV

Sjögren's syndrome is an autoimmune disease that is characterized by dryness of the mouth and eyes. The loss of salivary and lacrimal gland function is accompanied by lymphocytic infiltration. Because similar symptoms and glandular pathology are observed in certain persons infected with human immunodeficiency virus (HIV), a search was initiated for a possible retroviral etiology in this syndrome. A human intracisternal A-type retroviral particle that is antigenically related to HIV was detected in lymphoblastoid cells exposed to homogenates of salivary tissue from patients with Sjögren's syndrome. Comparison of this retroviral particle to HIV indicates that they are distinguishable by several ultrastructural, physical, and enzymatic criteria.

The use of primers from highly conserved pol regions to identify uncharacterized retroviruses by the polymerase chain reaction

Two degenerate oligonucleotide primers derived from regions of pol conserved among retroviruses have been synthesized. Polymerase chain reactions utilizing these primers amplify a 135-bp pol fragment in every retrovirus DNA tested to date. The polymerase chain reaction has been linked to a reverse transcriptase step so that a pol-specific DNA fragment can be obtained from a moderate amount of a purified retrovirus or viral RNA. The identity of an unknown retrovirus can be determined by sequencing of the amplified fragment following molecular cloning. This procedure was tested on an unidentified (non-HIV) retrovirus expressed by a B-cell lymphoma line obtained from an AIDS patient. Our PCR assay identified the retrovirus as being highly similar to Mason-Pfizer monkey virus (MPMV) and simian retrovirus 1, which are closely related immunosuppressive type D viruses that cause simian AIDS.

Identification of a conserved sequence in the non-coding regions of many human genes

We have analyzed a sequence of approximately 70 base pairs (bp) that shows a high degree of similarity to sequences present in the non-coding regions of a number of human and other mammalian genes. The sequence was discovered in a fragment of human genomic DNA adjacent to an integrated hepatitis B virus genome in cells derived from human hepatocellular carcinoma tissue. When one of the viral flanking sequences was compared to nucleotide sequences in GenBank, more than thirty human genes were identified that contained a similar sequence in their non-coding regions. The sequence element was usually found once or twice in a gene, either in an intron or in the 5' or 3' flanking regions. It did not share any similarities with known short interspersed nucleotide elements (SINEs) or presently known gene regulatory elements. This element was highly conserved at the same position within the corresponding human and mouse genes for myoglobin and N-myc, indicating evolutionary conservation and possible functional importance. Preliminary DNase I footprinting data suggested that the element or its adjacent sequences may bind nuclear factors to generate specific DNase I hypersensitive sites. The size, structure, and evolutionary conservation of this sequence indicates that it is distinct from other types of short interspersed repetitive elements. It is possible that the element may have a cis-acting functional role in the genome.

Structural analysis of a hepatitis B virus genome integrated into chromosome 17p of a human hepatocellular carcinoma

Hepatitis B virus (HBV) is clearly a factor in the development of hepatocellular carcinoma, but its mechanism of action remains obscure. One possibility is that the HBV integration event alters the expression of a nearby growth-regulatory cellular gene. A 9-kilobase (kb) DNA fragment containing an HBV insert plus flanking cellular sequences was cloned from a hepatoma specimen from Shanghai, People's Republic of China. Restriction mapping of the insert revealed a large inverted repeat structure consisting of both viral sequences (encompassing all of the core and pre-S regions and portions of the X and S genes) and at least 3 kb of unique cellular sequences. The virus-cell junction mapped 11 nucleotides from the DR1 region, in a position within the HBV X gene and included in the cohesive overlap region. A probe generated from 1.0 kb of the flanking cellular DNA mapped the viral insert to chromosome 17 in the region designated 17p11.2-17p12, which is near the human proto-oncogene p53. Sequence data from a portion of the flanking cellular DNA revealed a stretch of approximately 70 base pairs that showed highly significant homology with a conserved region of a number of functional mammalian DNAs, including the human autonomously replicating sequence 1 (ARS1).

Analysis of mutant Moloney murine leukemia viruses containing linker insertion mutations in the 3' region of pol

Twelve linker insertion mutations have been constructed in the 3' part of the pol gene of Moloney murine leukemia virus. This region of the Moloney murine leukemia virus genome encodes IN or p46pol, which is required for integration of the retroviral DNA into the host cell chromosome. Viral proteins synthesized by these mutants were used to pseudotype a neo-containing retroviral vector. Ten of twelve linker insertion mutant pseudotypes were unable to generate stable proviruses in infected mouse cells, as measured by the formation of G418-resistant colonies. Two mutants mapping at the 3' terminus of the IN-encoding region were competent for the formation of stable vector proviruses (hundreds of G418-resistant colonies per mutant pseudotype-infected plate). Representative linker insertion mutants were also tested for the ability to synthesize viral unintegrated DNA in newly infected cells. All assayed mutants were capable of synthesizing all normal forms of viral unintegrated DNA. The structure of integrated vector proviruses generated by defective and nondefective linker insertion mutants was also analyzed. All replication-competent mutants generated normal proviruses, while the few obtainable proviruses generated by replication-defective mutants were sometimes aberrant in structure. These results argue strongly (and confirm previous data) that the IN-encoding region of pol does not play a significant role in DNA synthesis, but is absolutely required for the formation of normal proviral DNA.