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Experimental Biology and Medicine 226:753-757 (2001)
© 2001 Society for Experimental Biology and Medicine

ORIGINAL ARTICLE

Telomerase Activity in Kaposi's Sarcoma, Squamous Cell Carcinoma, and Basal Cell Carcinoma

Ziping Chen*, Kathleen J. Smith{ddagger}, Henry G. Skelton, III§, Terry L. Barrett||, Herbert T. Greenway, Jr. and Shyh-Ching Lo{dagger},1

* American Registry of Pathology and
{dagger} Armed Forces Institute of Pathology, Washington, District of Columbia 20306;
{ddagger} National Naval Medical Center, Bethesda, Maryland 20889;
§ Laboratory Corporation of America, Herndon, Virginia 20171;
|| Johns Hopkins Medical Center, Baltimore, Maryland 21287;
Scripps Clinic, La Jolla, California 92037


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Patients with acquired immune deficiency syndrome (AIDS) often develop Kaposi's sarcoma (KS), an unusual skin tumor. The malignant nature of KS has long been disputed. Telomerase activity that maintains telomere length and ensures chromosomal stability, a frequently appearing marker in human malignancies, has been proposed to play a critical role in supporting continued cell growth, hence formation of tumors. We examined telomerase activity in tissue extracts from 22 KS, 10 squamous cell carcinoma (SCC), and 22 basal cell carcinoma (BCC) using the telomeric repeat amplification protocol (TRAP). All of the tumor tissues were previously cryopreserved at -80°C. In this study, all tumor samples tested were positive for telomerase activity. Consistent with the presence of the enzyme activity, the skin tumors had relatively long telomeres. Inhibitors in the tissue extracts of some samples needed to be diluted or extracted by phenol before the enzyme activity was detected in the TRAP assay. All KS as well as two other skin carcinoma samples revealed positive telomerase activity. Our finding supports telomerase's role in tumor cell immortality and suggests the true neoplastic nature of KS.

Key Words: telomerase • Kaposi's sarcoma • malignancy


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Telomeres, specialized structures found at the ends of eukaryotic chromosomes, appear to have important functions in chromosome protection, positioning, and replication (1, 2). Consisting of hundreds of copies of a repeat nucleotide sequence 5'-TTAGGG-3' in vertebrates (3), telomeres may prevent the activation of DNA-damage check points and may counter the loss of terminal DNA segments that occurs during DNA replication (1, 4, 5). With each cycle of DNA replication, one end of the linear chromosome produces a short 8- to 12-bp gap generated by the removal of the RNA primer. Due to the inability of conventional DNA polymerase to fully replicate the gap just created, each chromosome end will shorten by 50 to 200 nucleotides with each cell division (6). Such shortening of telomeres provides a means of counting cell division and acts as a biological clock that associates decreasing telomere length with cell aging and senescence (7).

Telomerase, a ribonucleoprotein, adds telomeric repeats to the chromosome ends to compensate for the loss that occurs with each cell division. Maintenance of a constant telomere length ensures chromosome stability, prevents cell aging, and may confer cellular immortality. Studies of human tumor and human tumor cell lines indicate that telomerase activity may play a critical role in tumor cell growth by sustaining cellular immortality (8). In human cancer, telomerase activity was first demonstrated in ovarian carcinoma tissue (9) and has subsequently been detected in most samples from a wide range of primary human malignancies. In one study (10), cell extracts from 90 out of 101 distinct tumors representing 12 cancer types and from 98 out of 100 independent immortalized cell lines were positive for telomerase activity. In comparison, no enzyme activity could be detected in benign tumors, somatic tissues, and proliferating mortal cell lines. Because of the high prevalence in a broad range of human malignancies, telomerase activity could serve as a potential malignant marker in human tumors. Although a broad range of malignancies was investigated, telomerase activity has not been reported in Kaposi's sarcoma (KS).

Pathogenesis of KS remains uncertain. For decades, the varied histopathology and unusual presentation of KS have engendered debate over its neoplastic and histogenic nature. Several characteristics of KS appear to set it apart from typical neoplastic processes. Cytogenetic abnormalities are rarely observed in KS (11, 12), and spontaneous regression of a tumor may occur in a setting where lesions at other sites are progressing (13). Vascular proliferation is a prominent feature of KS, but it is also seen early in its precursor lesion (14). These observations led many to hypothesize that KS may more accurately reflect a reactive process with prominent, yet reversible, vascular hyperplasia rather than a true neoplastic process (15). KS is the most commonly occurring tumor in patients with acquired immune deficiency syndrome (AIDS) or with human immunodeficiency virus (HIV) infection. To better assess the malignant nature of this unusual tumor, we sought to determine if telomerase activity occurred frequently in KS. In this study we also examined telomerase activity in other skin cancers, particularly squamous cell carcinoma (SCC) and basal cell carcinoma (BCC).


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Tumor Specimens and Patients.
The fresh tissue biopsies of 54 skin tumors (22 KS, 10 SCC, and 22 BCC) obtained from clinics were stored immediately at -80°C, and some were snap-frozen in optimal cutting temperature (OCT) embedding compound and then transferred to a -80°C freezer. KS biopsies were from male patients ages 29 to 54 with HIV infection and CD4 counts less than 200 for at least 1 year. All biopsy specimens were obtained from either plaque or nodular KS lesions from the lower extremities. All but two of the patients were on prophylaxis for opportunistic infections. However, none of the patients was on nucleoside analogue reverse transcriptase (RT) inhibitors, non-nucleoside reverse transcriptase (NRT) inhibitors, or HIV-1 proteinase inhibitors (PI), although some had previously been on RT. All tumors of SCC and BCC were from chronically sun-exposed skin with moderate to severe solar elastosis histopathologically. Tumors of SCC were either well-differentiated or moderately well-differentiated tumors with no evidence of metastasis spread from the head and neck and distal upper extremities. The tumors of BCC were all nodular BCC biopsied from the head, neck, and trunk.

Cells and Cell Cultures.
Cells from the primary culture of human skin fibroblasts and the suspension culture of Epstein-Barr virus (EBV)-immortalized human B lymphocytes (clone 1656) were used as negative controls for the telomerase TRAP assay in this study. Transformed human embryonal kidney cell line 293 (ATCC CRL 1573, American Type Culture Collection, Rockville, MD) and human cervical squamous carcinoma cell line SiHa (ATCC HTB 35) were used as the positive controls. All cells were grown in cultures using RPMI-1640 medium supplemented with 10% heat-inactivated fetal bovine serum (FBS, Gibco-BRL, Gaithersburg, MD) and were kept in a 5% CO2 incubator at 37°C. Cells were harvested from the logarithmically growing cultures, pelleted by centrifugation at 2000 rpm for 20 min, and kept in -80°C freezer.

Extract Preparation.
Individual tissues were washed once with 0.5 ml of cold lysis extraction buffer [10 mM Tris-HCl, pH 7.5, 1 mM MgCl2, 1 mM EGTA, 0.1 mM phenylmethylsulfonyl fluoride, 5 mM ß-mercaptoethanol, and 0.5% 3-[(3-Cholamidopropyl)dimethylammonio]-1-propane-sulfonate (CHAPS)] on ice to remove traces of blood and embedding compound. Each tissue (50–150 mg) was minced with sterile surgical scissors and then homogenized using motor-driven disposable pestles in 200 µl of lysis extraction buffer on ice. The samples were incubated on ice for 30 min and were centrifuged at 13,000g for 30 min at 4°C. The supernatants were collected and immediately stored at -80°C. Tissue culture cell extracts were similarly prepared using 108 cell pellets. The protein concentrations of the extracts were between 0.5 to 1.6 µg/µl.

Telomerase Assay.
Telomerase activity was determined using the TRAP assay developed by others (10) with some modification. Telomerase-mediated extension of the TS (5`-AATCCGTCGAGCAGAGTT-3`) primer was carried out in a 50-µl reaction mixture containing 1 or 2 µl of cell extract, 1x reaction buffer (20 mM Tris-HCl, pH 8.3, 68 mM KCl, 1.5 mM MgCl2, 1 mM EGTA, and 0.005% Tween 20), 0.1 µg of TS primer, 1 µg of T4 gene 32 protein, 50 µM of each deoxynucleotide triphosphate, and 2 units of Taq DNA polymerase at 23°C for 60 min. After extension, 2 µl of solution containing 0.1 µg of CX [5`-(CCCTTA)3CCCTAA-3`] primer and 0.4 µl of {alpha}-32P-dCTP (10 µCi/µl, 3000 Ci/mmol) was added to the reaction and subsequent PCR was carried out with 30 cycles at 94°C for 30 sec, 50°C for 30 sec, and 72°C for 90 sec. One-half of the volume of the PCR reaction was analyzed on 10% nondenaturing polyacrylamide gels in 0.5x Tris-borate-EDTA buffer at 150 V for 2 hr. Then gels were dried and exposed to film.

Analysis of Terminal Restrictive Fragments.
Genomic DNAs were prepared from frozen tumor tissues. Briefly, 100 to 200 mg of tissue was minced and homogenized in 1 ml of lysis buffer (10 mM Tris, pH 8.0, 0.15 M NaCl, 25 mM EDTA, pH 8.0, 0.5% sodium dodecyl sulfate, and 0.2 mg/ml of proteinase K) and was then incubated overnight at 55°C. Samples were extracted once with phenol and once with phenol:chloroform:isoamyl alcohol (25:24:1), and then precipitated with ethanol. DNA pellets were resuspended in TE buffer (10 mM Tris, pH 8.0, 1 mM EDTA, pH 8.0) and digested with MseI plus MnlI to release telomere fragments. Approximately 1 µg of digested DNA per lane was loaded onto an 0.8% agarose gel. Separated DNA was electrically transferred to a nylon membrane and the membrane was hybridized with the 32P-labeled oligonucleotide (TTAGGG)3. Telomere length was estimated from migration of DNA size markers run parallel with the samples. Mean telomere lengths were determined by analysis of autoradiographs. Briefly, the autoradiographs were scanned and the peak optical density reading was calculated with UN-SCAN-IT (Silk Scientific Corporation, Orem, UT)


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
We included immortalized tissue culture cells (transformed human embryonal kidney cell line 293 and human cervical SCC line SiHa) as positive controls, and primary culture cells (human skin fibroblasts and EBV-immortalized human B lymphocytes) were used as negative controls for telomerase activity in all of our assays. In this study, initially 9 out of 22 KS, 14 out of 22 BCC, and 5 out of 10 SCC tissue extracts tested positive for telomerase activity. Figure 1 Go shows the characteristic ladder pattern of positive telomerase activity in KS, BCC, and SCC tissue extracts in the TRAP assay. Inhibitors that could interfere with the TRAP assay are often present in tissue extracts (16). To minimize the potential inhibitory effect in the tissue extracts, we re-examined the enzyme activity after the tissue extracts were further diluted to lower the concentration of potential inhibitor(s) in the preparations. A simple dilution of the tissue extract preparations allowed telomerase activity to be detected in an additional 10 KS, 6 BCC, and 3 SCC samples (Table IGo).



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  		Figure 1. Representative TRAP assay for telomerase activity in extracts of immortalized cell lines (293 and SiHa cells), primary human cell cultures (lymphocytes and skin fibroblasts), and tumors. Positive controls: lane 1, 293 cells; lane 2, SiHa human cervical carcinoma cells; negative controls: lane 3, human lymphocytes, lane 4, human skin fibroblasts; BCC: lanes 5 and 6; SCC: lanes 7 and 8; KS: lanes 9 through 15.

 

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  		Table I. Telomerase Positive Tissue Extracts after Dilution or Phenol Extraction
 
To remove potential inhibitors from the preparations of tissue extracts, we extracted the reaction mixtures once with phenol and once with phenol/chloroform, and then precipitated them with alcohol after telomerase-mediated extension (17). The pellets were then suspended to continue PCR in the TRAP assay. After additional phenol/chloroform treatment, the remaining tissue samples of 3 KS, 2 BCC, and 2 SCC also tested positive for the enzyme activity. Thus, all 54 tissue samples from 22 KS, 22 BCC, and 10 SCC were positive for the telomerase activity (Table IGo).

For skin tumors with sufficient tissue for further study (6 KS, 7 BCC, and 6 SCC), genomic DNAs were prepared for the telomere length examination. The terminal restrictive fragments (TRF) of the telomere DNA were revealed by hybridization with a probe of telomere-specific sequence. The representative result is shown in Figure 2Go. The TRF ranged between 5 and 15 KB. The mean TRF lengths for KS, BCC, and SCC were determined to be 12.1, 12.6, and 10.4 kb, respectively. No significant difference existed between KS, BCC, and SCC.



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  		Figure 2. Length of terminal restrict fragments of representative KS samples (lanes 1–5), BCC (lanes 6–10), and SCC (lanes 11–15).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
It is of vast biomedical interest that a high proportion (about 80%–90%) of tumors in animals and humans have been found to contain telomerase activity (18, 19). Although the mechanisms of activation of telomerase in tumors are still not fully understood, many scientists believe that activation of the enzyme is required for uncontrolled continuous cell proliferation, the hallmark of tumors. Thus, positive telomerase activity could be an important tumor marker. A new cancer paradigm is raised based on telomerase activation and oncogenesis. Some scientists have also proposed a new approach to treating a wide range of tumors, using an anti-telomerase strategy (19, 20). In the present study, our finding that 100% of 22 KS, 10 SCC, and 22 BCC samples tested contain telomerase activity further supports the possible role of telomerase in tumor development.

As previously described by other laboratories, inhibitors that interfere with the assay for telomerase activity are often present in tissue extracts. Fortunately, these inhibitory effects either can be minimized by a serial dilution or can be removed by applying an additional step of phenol/chloroform extraction. After dilution or successful removal of tissue inhibitor(s), all 22 BCC and 10 SCC were positive for the telomerase enzyme activity. The length of telomeres of skin tumors was also found to be rather long (12.1, 12.6, and 10.4 kb for KS, BCC, and SCC, respectively). However, many studies have indicated that there is apparently no significant relationship between telomerase status and the size of telomere lengths (21–23).

Our finding is consistent with a recent report that found telomerase activity in high frequency in BCC (24). However, Parris et al. (21) reported a lower prevalence of telomerase activity in SCC. They claimed that large parts of telomerase-negative SCC did not result from inactivation of protein samples and inhibitors. It is not clear why they detected telomerase activity in only 25% of SCC. Our study and others (24, 25) found a high frequency of positive telomerase in SCC. It was somewhat surprising to find that all 22 KS tumors tested also had telomerase activity. The fact that none of the biopsied patients were on RT inhibitors may have some significance, since these drugs can inhibit the enzymatic component of the telomerase (26). The widespread use of RT inhibitors even prior to institution of PIs may, in part, explain the decreasing incidence of KS that occurred earlier in the HIV- 1 epidemic, more so than the use of PIs (27). The addition of PI, with its improvement in the immunologic status of the patient, appears to have further decreased the incidence of KS (28).

In this context, it is important to note that the telomerase model in oncogenesis has become more complex. Many recent studies have shown that detection of the enzyme activity could simply be correlated to the growth index of cells. For example, low levels of enzyme activity in mouse mammary tissue and skin samples became greatly elevated when the cells were grown in short-term culture (29). Normal skin samples have very low levels of activity. On the other hand, when skin layers are dissected, a high level of telomerase activity is detected in the proliferating basal cells, whereas the quiescent dermis is telomerase negative (30). Thus, this enzyme activity is likely to be growth regulated in various human tissues in vivo. However, two telomerase-negative normal human cell types, retinal pigment epithelial cells and foreskin fibroblasts, were recently transfected with vector encoding the telomerase catalytic subunit (31). In contrast to telomerase-negative control clones that showed telomere shortening and senescence, telomerase-expressing clones had elongated telomeres and they divided vigorously. These telomerase-expressing clones had a normal karyotype and had already exceeded their normal life span by at least 20 doublings (31). The result has established a causal relationship between positive telomerase activity and prolonged cellular senescence. More recent studies have further implicated telomerase's role in cell immortalization, but not necessarily in malignant transformation (32, 33).

Our finding of relatively long telomeres and prominent telomerase activity at high frequency in KS would indicate that KS cells are immortalized, not the result of reactive hyperplasia of various endothelial or mesenchymal cells. Tumors of BCC and SCC are evidently also immortalized with upregulated telomerase and long telomeres. Upregulation of telomerase in chronically sun-exposed skin appears to be an early event. It occurs even prior to UVB-induced signature P53 mutation (25, 34). Thus, upregulation of telomerase may be found early at high frequency in UVB-induced tumors such as BCC and SCC.


   Acknowledgments
 
The authors thank Dr. Douglas J. Wear and Susan Ditty for their critical review of this manuscript.


   Footnotes
 
The study was supported in part by the American Registry of Pathology.

1 To whom requests for reprints should be addressed at Department of Infectious and Parasitic Disease Pathology, Armed Forces Institute of Pathology, Building 54, Room 4091, 14th and Alaska Avenues, NW, Washington, DC 20306–6000. E-mail: los{at}afip.osd.mil Back


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Blackburn EH. Structure and function of telomeres. Nature 350:569–573, 1991.[Medline]
  2. Wright WE, Shay JW. Telomere positional effects and the regulation of cellular senescence. Trends Genet 8:193–197, 1992.[Medline]
  3. Allshire RC, Dempster M, Hastie ND. Human telomeres contain at least three types of G-rich repeat distributed non-randomly. Nucleic Acids Res 17:4611–4627, 1989.[Abstract/Free Full Text]
  4. Rhyu MS. Telomeres, telomerase, and immortality. J Natl Cancer Inst 87:884–894, 1995.[Abstract/Free Full Text]
  5. Harley CB. Telomere loss: Mitotic clock or genetic time bomb? Mutat Res 256:271–282, 1991.[Medline]
  6. Allsopp RC, Chang E, Kashefi-Aazam M, Rogaev EI, Piatyszek MA, Shay JW, Harley CB. Telomere shortening is associated with cell division in vitro and in vivo. Exp Cell Res 220:194–200, 1995.[Medline]
  7. Harley CB, Vaziri H, Counter CM, Allsopp RC. The telomere hypothesis of cellular aging. Exp Gerontol 27:375–382, 1992.[Medline]
  8. Shay JW, Wright WE. Telomerase activity in human cancer. Curr Opin Oncol 8:66–71, 1996.[Medline]
  9. Counter CM, Hirte HW, Bacchetti S, Harley CB. Telomerase activity in human ovarian carcinoma. Proc Natl Acad Sci U S A 91:2900–2904, 1994.[Abstract/Free Full Text]
  10. Kim NW, Piatyszek MA, Prowse KR, Harley CB, West MD, Ho PL, Coviello GM, Wright WE, Weinrich SL, Shay JW. Specific association of human telomerase activity with immortal cells and cancer. Science 266:2011–2015, 1994.[Abstract/Free Full Text]
  11. Delli BP, Donti E, Knowles DM II, Friedman-Kien A, Luciw PA, Dina D, Dalla-Favera R, Basilico C. Presence of chromosomal abnormalities and lack of AIDS retrovirus DNA sequences in AIDS-associated Kaposi's sarcoma. Cancer Res 46:6333–6338, 1986.[Medline]
  12. Sanchez M, Ames E, Erhardt K, Auer GU. Analysis of DNA distribution in Kaposi's sarcoma in patients with and without the acquired immune deficiency syndrome. Anal Quant Cytol Histol 10:16–20, 1988.[Medline]
  13. Real F, Krown SE. Spontaneous regression of Kaposi's sarcoma in patients with AIDS. N Engl J Med 313:1659, 1985.[Medline]
  14. Friedman-Kien AE, Laubenstein LJ, Rubinstein P, Buimovici-Klein E, Marmor M, Stahl R, Spigland I, Kim KS, Zolla-Pazner S. Disseminated Kaposi's sarcoma syndrome in homosexual men. Ann Intern Med 96:693–700, 1982.
  15. Gottlieb GL, Ackerman AB. Kaposi's sarcoma: an extensively disseminated form in young homosexual men. Hum Pathol 13:882–892, 1982.[Medline]
  16. Piatyszek MA, Kim NW, Weinrich SL, Weinrich SL, Hiyama K, Hiyama E, Wright WE, Shay JW. Detection of telomerase activity in human cells and tumors by a telomeric repeat amplification protocol (TRAP). Methods Cell Sci 17:1–15, 1995.
  17. Removing PCR inhibitors. In: TRAPEZE Telomerase Detection Kit Manual, Oncor, 3:25, 1998.
  18. de Lange T. Activation of telomerase in a human tumor. Proc Natl Acad Sci U S A 91:2882–2885, 1994.[Free Full Text]
  19. Greider CW, Blackburn EH. Telomeres, telomerase, and cancer. Scientific American 274 (2):92–97, 1996.[Medline]
  20. Travis J. Missing enzyme incites cancer debate. Sci News 152:228, 1997.
  21. Parris CN, Jezzard S, Silver A, MacKie R, McGregor JM, Newbold RF. Telomerase activity in melanoma and non-melanoma skin cancer. Br J Cancer 79:47, 1999[Medline]
  22. Bolzan AD, Paez GL, Bianchi MS, Bianchi NO. Analysis of telomeric repeats and telomerase activity in human colon carcinoma cells with gene amplification. Cancer Genet Cytogenet 120:166, 2000[Medline]
  23. Chong EYY, Pang LCS, Ko CW, Poon WS, Ng HK. Telomere length and telomerase catalytic subunit expression in non-astrocytiv gliomas. Pathol Res Pract 196:691, 2000[Medline]
  24. Taylor RS, Ramirez RD, Ogoshi M, Chaffins M, Piatyszek MA, Shay JW. Detection of telomerase activity in malignant and non-malignant skin conditions. J Invest Dermatol 106:759–765, 1996.[Medline]
  25. Ueda M, Ouhtit A, Bito T, Nakazawa K, Lubbe J, Ichihashi M, Yamasaki H, Nakazawa H. Evidence of UV-associated activation of telomerase in human skin. Cancer Res 57:370–374, 1997.[Abstract/Free Full Text]
  26. Gomez DE, Olivero OA, Tejera AM. Irreversible telomere shortening by 3`-azido-2`,3`-dideoxythymidine (AZT) treatment. Biochem Biophys Res Commun 246:107–110, 1998.[Medline]
  27. Rabkin CS, Goedert JJ, Biggar RJ, Yellin F, Blattner WA. Kaposi's sarcoma in three HIV-1 infected cohorts. J Acquir Immun Defic Syndr 3:S38–S43, 1990.
  28. Cattelan AM, Calabro ML, Aversa SM, Zanchetta M, Meneghett F, De Rossi A, Chieco-Bianchi L. Regression of AIDS-related Kaposi's sarcoma following antiretroviral therapy with protease inhibitors: Biological correlates of clinical outcome. Eur J Cancer 35:1809–1815, 1999.
  29. Chadeneau C, Siegh P, Harley CB, Muller WJ, Bacchetti S. Telomerase activity in normal and malignant murine tissues. Oncogene 11:893–898, 1995.[Medline]
  30. Harle-Bachor C, Boukamp P. Telomerase activity in the regenerative basal layer of epidermis in human skin and immortal and carcinoma-derived skin keratinocytes. Proc Natl Acad Sci U S A 93:6476–6481, 1996.[Abstract/Free Full Text]
  31. Bodnar AG, Duellett, M, Frolkis M, Holt SE, Chiu CP, Morin GB, Harley CB, Shay JW, Lichtsteiner S, Wright WE. Extension of life-span by introduction of telomerase into normal human cells. Science 279:349–352, 1998.[Abstract/Free Full Text]
  32. Jiang, XR, Jimenez G, Chang E, Frokis M, Kusler B, Sage M, Beeche M, Bodnar AG, Wahl GM, Tlsty TD, Chiu CP. Telomerase expression in human somatic cells does not induce changes associated with a transformed phenotype. Nat Genet 21:111–114, 1999.[Medline]
  33. Morales, CP, Holt SE, Duellette M, Kaur KJ, Yan Y, Wilson KS, White MA, Wright WE, Shay JW. Absence of cancer-associated changes in human fibroblasts immortalized with telomerase. Nat Genet 21:115–118, 1999.[Medline]
  34. Parkinson EK, Newbold RF, Keith WN. The genetic basis of human keratinocyte immortalization in squamous cell carcinoma development: The role of telomerase reactivation. Eur J Can 33:727–734, 1997.
Received for publication December 1, 2000. Accepted for publication March 19, 2001.




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