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S Bacchetti

Publications and source records attributed to S Bacchetti.

At least 37 records · Page 2Linked to original sources
PubMed1998

Reconstitution of wild-type or mutant telomerase activity in telomerase-negative immortal human cells.

Telomere shortening in human somatic cells and telomere maintenance in most human immortal cell lines and tumours correlate respectively with the absence and presence of telomerase, the enzyme that synthesizes telomeric DNA de novo . However, approximately 30% of in vitro immortalized human cell lines do not express this enzyme and maintain telomeres by an alternative pathway (ALT) that may also operate in some tumours. Human telomerase is a reverse transcriptase comprising minimally an RNA subunit (hTER) and a catalytic protein moiety (hTERT). Normal somatic cells retain expression of hTER but not of hTERT, and can be converted to a telomerase-positive phenotype by ectopic expression of the catalytic protein. We similarly have restored enzymatic activity to those ALT cell lines that retain hTER expression. We also report that in those ALT cells that are hTER negative, reintroduction of both hTER and hTERT is necessary and sufficient for conversion to telomerase positivity. Moreover, transfection of these cells with hTERT in conjunction with hTERs with a mutated template results in the expression of an enzyme with altered specificity. Reconstitution of telomerase activity in ALT cells, particularly an activity capable of synthesizing mutant telomeric DNA, may be exploited for the study of the ALT mechanism and its interaction with the telomerase-dependent pathway, and for assessing the effects of mutant telomeres on cell viability.

Cell Line, Transformed↗
PubMed1998

Melanoma of the choroid above the optic disc: considerations concerning a clinical case.

The authors report the case of a patient suffering from melanoma of the choroid above the optic disc. Given the good level of visual acuity and the location and extent of the damage, he was given a radiotherapy treatment with proton beams. At an interval of 6 months from this treatment, the neoformation appeared limited, and no vascular changes due to irradiation were observed.

Choroid Neoplasms↗
PubMed1997

hEST2, the putative human telomerase catalytic subunit gene, is up-regulated in tumor cells and during immortalization.

Telomerase, the ribonucleoprotein enzyme that elongates telomeres, is repressed in normal human somatic cells but is reactivated during tumor progression. We report the cloning of a human gene, hEST2, that shares significant sequence similarity with the telomerase catalytic subunit genes of lower eukaryotes. hEST2 is expressed at high levels in primary tumors, cancer cell lines, and telomerase-positive tissues but is undetectable in telomerase-negative cell lines and differentiated telomerase-negative tissues. Moreover, the message is up-regulated concomitant with the activation of telomerase during the immortalization of cultured cells and down-regulated during in vitro cellular differentiation. Taken together, these observations suggest that the induction of hEST2 mRNA expression is required for the telomerase activation that occurs during cellular immortalization and tumor progression.

Amino Acid Sequence↗
PubMed1997

Telomeres in the haemopoietic system.

The limited life span of most blood cells requires the continuous production of cells, which in adults exceeds 10(12) cells/day. This impressive production of cells (approximately 4 x 10(16) cells over a lifetime) is achieved by the proliferation and differentiation of committed progenitor cells, which themselves are derived from a population of pluripotent stem cells with self-renewal potential. Paradoxically, the large majority of stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 55 divisions can yield 4 x 10(16) cells. First, it was shown that 'candidate' stem cells purified from fetal and adult tissue showed dramatic functional differences in turn-over time and the ability to produce cells with stem cell properties, Second, these functional differences were found to correlate with a measurable loss of telomere repeats despite the presence of low but readily detectable levels of telomerase in all purified cell fractions. In order to address questions about the role of telomeres in normal and malignant haemopoiesis, we developed a quantitative fluorescence in situ hybridization technique. Here we review the characteristics of this novel tool to assess the number of telomere repeats at the end of individual chromosomes and provide an overview of recent observations.

B-Lymphocytes↗
PubMed1997

Induction of fragility at the human RNU2 locus by cytosine arabinoside is dependent upon a transcriptionally competent U2 small nuclear RNA gene and the expression of p53.

Chromosomal fragile sites are regions that are intrinsically unstable and are susceptible to experimentally induced damage. In most cases, the target and mechanism of induction of fragility are unknown. Using ectopic integration of engineered DNA arrays to create "new" fragile sites, we and others have previously shown that the transcriptionally competent U2 gene is necessary and sufficient for induction of fragility at the RNU2 locus upon infection of human cells with Adenovirus 12. In the present study we have investigated the response of the RNU2 locus to cytosine arabinoside (araC), an inhibitor of DNA polymerases and a common inducer of fragile sites. We demonstrate that the RNU2 locus is sensitive to the drug and that araC-induced fragility is dependent upon a functional U2 gene and on the expression of the cellular p53 protein. Our results identify a novel DNA structure associated with fragile sites and suggest a role for transcription and repair processes in RNU2 fragility.

Antimetabolites, Antineoplastic↗
PubMed1997

A survey of telomerase activity in human cancer.

Research on the association of the ribonucleoprotein enzyme, telomerase, with human cancer has expanded rapidly in recent years. Essentially all major types of cancer have been screened and the presence of telomerase activity has been detected in the vast majority of cases. In this article we provide a summary, in table form, of the current data.

Humans↗
PubMed1997

The telomere lengthening mechanism in telomerase-negative immortal human cells does not involve the telomerase RNA subunit.

According to the telomere hypothesis of senescence, the progressive shortening of telomeres that occurs upon division of normal somatic cells eventually leads to cellular senescence. The immortalisation of human cells is associated with the acquisition of a telomere maintenance mechanism which is usually dependent upon expression of the enzyme telomerase. About one third of in vitro immortalised human cell lines, however, have no detectable telomerase but contain telomeres that are abnormally long. The nature of the alternative telomere maintenance mechanism (referred to as ALT, for Alternative Lengthening of Telomeres) that must exist in these telomerase-negative cells has not been elucidated. It has previously been shown that abnormal lengthening of yeast telomeres may occur due to mutations in the yeast telomerase RNA gene. That this is not the mechanism of the abnormally long telomeres in ALT cell lines was demonstrated by the finding that seven of seven ALT lines have wild-type human telomerase RNA (hTR) sequence, including a novel polymorphism that is present in 30% of normal individuals. We found that two ALT cell lines have no detectable expression of the hTR gene. This shows that the ALT mechanism in these cell lines is not dependent on hTR. Expression of exogenous hTR via infection of these cells with a recombinant hTR-adenovirus vector did not result in telomerase activity, indicating that their lack of telomerase activity is not due to absence of hTR expression. We conclude that the ALT mechanism is not dependent on the expression of hTR, and does not involve mutations in the hTR sequence.

Base Sequence↗
PubMed1997

Reprogramming of telomerase by expression of mutant telomerase RNA template in human cells leads to altered telomeres that correlate with reduced cell viability.

Telomerase synthesizes telomeric DNA by copying the template sequence of its own RNA component. In Tetrahymena thermophila and yeast (G. Yu, J. D. Bradley, L. D. Attardi, and E. H. Blackburn, Nature 344:126-131, 1990; M. McEachern and E. H. Blackburn, Nature 376:403-409, 1995), mutations in the template domain of this RNA result in synthesis of mutant telomeres and in impaired cell growth and survival. We have investigated whether mutant telomerase affects the proliferative potential and viability of immortal human cells. Plasmids encoding mutant or wild-type template RNAs (hTRs) of human telomerase and the neomycin resistance gene were transfected into human cells to generate stable transformants. Expression of mutant hTR resulted in the appearance of mutant telomerase activity and in the synthesis of mutant telomeres. Transformed cells were not visibly affected in their growth and viability when grown as mass populations. However, a reduction in plating efficiency and growth rate and an increase in the number of senescent cells were detected in populations with mutant telomeres by colony-forming assays. These results suggest that the presence of mutant telomerase, even if coexpressed with the wild-type enzyme, can be deleterious to cells, likely as a result of the impaired function of hybrid telomeres.

Cell Survival↗
PubMed1996

Development of retinoblastoma in the absence of telomerase activity.

BACKGROUND: The length and stability of telomeres (essential functional structures at the end of eukaryotic chromosomes) have been implicated in the control of cell lifespan. Most somatic cells lack telomerase, the enzyme that synthesizes telomeric DNA, and their telomeres shorten with cell division. Cells immortalized in vitro, on the other hand, express telomerase and maintain their telomeres. Telomerase activity has also been detected in the large majority of tumors from a variety of cancers. These observations have suggested that telomere maintenance is required for unlimited cell proliferation and that telomerase is a marker for cell immortality in vitro and in vivo. PURPOSE: We investigated whether telomerase is activated during the development of retinoblastoma. This is a childhood eye cancer associated with a limited number of mutations in an embryonic tissue and thus likely to develop in cells that have long telomeres. The ease of detection of retinoblastoma makes it possible to screen relatively small tumors before extensive proliferation of the malignant cells. METHODS: We measured telomerase activity in 34 samples of retinoblastoma, four retinoblastoma-derived cell lines, and six cell lines derived from other cancers. Only three of the cell lines from other cancers expressed the retinoblastoma protein. Telomerase activity was assayed by a polymerase chain reaction protocol in extracts prepared from tumors or cell lines. The level of enzyme activity in cell extracts was quantified at several protein concentrations and expressed relative to that in a positive control, after normalization for the amount of protein. Telomere length was measured by Southern blot hybridization of genomic DNA with a telomere-specific probe. Average values of telomere length in telomerase-positive and telomerase-negative tumors and in cell lines were compared by two-sided, two-sample Student's t test. RESULTS: No telomerase activity was detected in 17 (50%) of 34 retinoblastomas. Assays of cell lines derived from other cancers revealed no association between the presence or the level of the enzyme activity and the expression of the retinoblastoma protein. Telomeres were significantly longer in telomerase-negative tumors than in telomerase-positive tumors (P = .0008). CONCLUSIONS: Our results indicate that retinoblastoma can develop when telomeres are still relatively long and in the absence of telomerase. Telomerase activity associated with short telomeres is, however, observed in 50% of the retinoblastomas and in retinoblastoma-derived cell lines. IMPLICATIONS: Telomerase may not be a marker for acquisition of the malignant phenotype in the case of tumors that are derived from cells with long telomeres and that are associated with a low number of mutations.

Base Sequence↗
PubMed1996

Human telomerase RNA and telomerase activity in immortal cell lines and tumor tissues.

Telomerase activity has been detected in many human immortal cells lines and in tumor tissues, whereas it is generally absent from primary cell strains and from many tumor adjacent tissue samples. With the recently cloned human telomerase RNA (hTR), we used Northern analysis to follow the levels of hTR in primary, precrisis, and immortalized cells. It was surprising that the amount of hTR was high in cell strains that lacked telomerase activity, and the levels did not parallel the increases in telomerase activity, which accompanies immortalization. In addition, although the hTR levels were somewhat higher in tumor samples compared to nontumor tissues, the level of hTR in a variety of different human tumors did not predict the level of telomerase activity in the tumor. Thus, whereas hTR was detected in all samples that have telomerase activity, the presence of the RNA was not a good predictor of the presence or amount of telomerase activity.

B-Lymphocytes↗
PubMed1996

Telomeres and telomerase in normal and malignant haematologic cells.

Telomeres, the ends of eukaryotic chromosomes are structural and functional units composed of proteins and repetitive DNA sequences. Telomeres protect the ends of chromosomes from DNA loss caused by incomplete replication of 3' ends. The obligatory loss of terminal sequence with each cell division leads to telomere shortening, and is counteracted in germline cells by an enzymatic activity termed telomerase that resynthesizes telomeric DNA de novo. Telomere length and telomerase activity have been measured by several groups in both normal and malignant blood and marrow cells. Telomere length decreases with age in normal blood and bone marrow, despite the presence of a detectable telomerase activity. In most hematologic malignancies telomere length is short and telomerase activity is enhanced, compatible with the late activation of the enzyme in tumour development. The implications of these findings for tumour pathogenesis, diagnosis, and treatment are discussed.

Biomarkers, Tumor↗
PubMed1996

Telomere maintenance in tumour cells.

Telomere attrition may regulate the proliferative life span of somatic cells and contribute to the genetic instability of tumour cells, and telomere maintenance is required for cell survival. The ample, but mainly correlative, evidence in support of these hypotheses is now beginning to be complemented by experimental results. There have also been unexpected findings that attest to the complexity of the biological processes and systems under study. Mammalian telomere biology has definitely entered into an exciting phase. Given the increasing pace of research on this topic, it seems most likely that a not too distant future will provide us with answers to many of the unresolved issues mentioned in this article.

Cell Division↗
PubMed1995

Telomerase activity in normal and malignant murine tissues.

Telomere shortening may contribute to the limited lifespan of somatic cells and telomerase, the enzyme that elongates telomeric DNA and maintains telomere length, may be essential for unlimited cell proliferation in vivo and in vitro. Telomerase is not expressed in most human somatic cells but is a nearly ubiquitous tumour marker, being activated in malignant cells from many cancers. Inhibition of telomerase may lead to telomere shortening and eventually limit the proliferative capacity of malignant cells and hence be of therapeutic value. With the intent of characterizing an animal model for inhibition studies, we investigated telomerase activity during mammary tumorigenesis in transgenic mice overexpressing the neu gene. We detected activity in primary mammary tumours and lung metastases but also in normal mammary glands and other organs. Activity was elevated in tumors versus normal tissues and was enhanced by short-term culturing of normal cells. Telomerase activity was also present in somatic tissues from the non-transgenic parental strain and the outbred Mus spretus strain. As we recently detected telomerase activity in normal human hemopoietic tissues, mouse models of tumorigenesis may provide useful experimental systems for assessing the outcome of in vivo inhibition of telomerase in both malignant and normal cells.

Animals↗
PubMed1995

Telomere elongation in immortal human cells without detectable telomerase activity.

Immortalization of human cells is often associated with reactivation of telomerase, a ribonucleoprotein enzyme that adds TTAGGG repeats onto telomeres and compensates for their shortening. We examined whether telomerase activation is necessary for immortalization. All normal human fibroblasts tested were negative for telomerase activity. Thirteen out of 13 DNA tumor virus-transformed cell cultures were also negative in the pre-crisis (i.e. non-immortalized) stage. Of 35 immortalized cell lines, 20 had telomerase activity as expected, but 15 had no detectable telomerase. The 15 telomerase-negative immortalized cell lines all had very long and heterogeneous telomeres of up to 50 kb. Hybrids between telomerase-negative and telomerase-positive cells senesced. Two senescent hybrids demonstrated telomerase activity, indicating that activation of telomerase is not sufficient for immortalization. Some hybrid clones subsequently recommenced proliferation and became immortalized either with or without telomerase activity. Those without telomerase activity also had very long and heterogeneous telomeres. Taken together, these data suggest that the presence of lengthened or stabilized telomeres is necessary for immortalization, and that this may be achieved either by the reactivation of telomerase or by a novel and as yet unidentified mechanism.

Base Sequence↗
PubMed1995

Telomerase activity associated with acquisition of malignancy in human colorectal cancer.

Shortening of telomeres may contribute to the control of the proliferative capacity of normal cells, and telomerase, the enzyme that elongates telomeric DNA, may be essential for unlimited cell proliferation. We have shown previously that telomerase activity is present in human cells immortalized in vitro and in metastatic ovarian carcinoma cells but is undetectable in normal cultured cells or normal tissues. We have determined the temporal pattern of telomerase activity during colorectal carcinogenesis in man. We report that telomerase activity is associated with acquisition of malignancy as it is detectable in colorectal carcinoma but not in adenomatous polyps. Mutations leading to reactivation or upregulation of the enzyme may represent an additional required event in the multistep development of colorectal cancer.

Adenomatous Polyps↗
PubMed1995

Telomerase activity in normal leukocytes and in hematologic malignancies.

Telomeres are essential for function and stability of eukaryotic chromosomes. In the absence of telomerase, the enzyme that synthesizes telomeric DNA, telomeres shorten with cell division, a process thought to contribute to cell senescence and the proliferative crisis of transformed cells. We reported telomere stabilization concomitant with detection of telomerase activity in cells immortalized in vitro and in ovarian carcinoma cells, and suggested that telomerase is essential for unlimited cell proliferation. We have now examined the temporal pattern of telomerase expression in selected hematologic malignancies. We found that, unlike other somatic tissues, peripheral, cord blood, and bone marrow leukocytes from normal donors expressed low levels of telomerase activity. In leukocytes from chronic lymphocytic leukemia (CLL) patients, activity was lower than in controls in early disease, and comparable with controls in late disease. Relative to bone marrow, telomerase activity was enhanced in myelodysplastic syndrome (MDS) and more significantly so in acute myeloid leukemia (AML). Regardless of telomerase levels, telomeres shortened with progression of the diseases. Our results suggest that early CLL and MDS cells lack an efficient mechanism of telomere maintenance and that telomerase is activated late in the progression of these cancers, presumably when critical telomere loss generates selective pressure for cell immortality.

Acute Disease↗
PubMed1995

High frequency recombination between loxP sites in human chromosomes mediated by an adenovirus vector expressing Cre recombinase.

An adenovirus vector (AdCre1) expressing Cre recombinase has been used to induce recombination between loxP sites in human chromosomes. G418 resistant cells with one loxP site, generated by transfection with a plasmid containing loXp between the SV40 promoter and the G418 resistance (neo) gene, were infected with AdCre1 and transfected with a plasmid containing loxP adjacent to a promoterless hisD gene. This resulted in integration of hisD downstream of the SV40 promoter with gain of histidinol and loss of G418 resistance. Since AdCre1 is non-replicating and Cre expression transient, histidinol resistant cells containing the hisD gene flanked by loxP sites were stable. Reinfection of these cells with AdCre1 induced excision of hisD in over 90% of infected cells. This high efficiency of site-specific recombination suggests that AdCre1 may be exploited for temporal and tissue-specific regulation of gene expression and for chromosome engineering in vitro and in animals.

Adenoviridae↗