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Reversible Conversion of Immortal Human Cells from Telomerase-Positive to Telomerase-Negative Cells

¹ Department of Pharmacology, Nippon Dental University, School of Dentistry at Tokyo, Tokyo, Japan and ² Laboratory of Biosystems and Cancer, National Cancer Institute, NIH, Bethesda, Maryland

Requests for reprints: J. Carl Barrett, Center for Cancer Research, National Cancer Institute, Building 31, Room 3A11, 31 Center Drive, MSC-2440, Bethesda, MD 20892-2240. Phone: 301-496-4345; Fax: 301-496-0775; E-mail: barrett{at}mail.nih.gov.

Immortal cell lines and tumors maintain their telomeres via the telomerase pathway or via a telomerase-independent pathway, referred to as alternative lengthening of telomeres (ALT). Here, we show the reversible conversion of the human papillomavirus type 16 E6-induced immortal human fibroblasts E6 Cl 6 from telomerase-positive (Tel⁺) to telomerase-negative (Tel^–) cells. Tel⁺ cells converted spontaneously to Tel^– cells that reverted to Tel⁺ cells following treatment with trichostatin A (TSA) and/or 5-aza-2'-deoxycytidine (5-AZC), which induced the reversion from complete to partial methylation of the CpG islands of the human telomerase reverse transcriptase (hTERT) promoter in Tel^– E6 Cl 6 cells. Tel^– E6 Cl 6 cells lacked the phenotypes characteristic of ALT cell lines such as very long and heterogenous telomeres and ALT-associated promyelocytic leukemia nuclear bodies (APB) but grew for >240 population doublings (PD) after they became telomerase negative. The ratios of histone H3 (H3) lysine (K) 9 methylation to each of H3-K4 methylation, H3-K9 acetylation, and H3-K14 acetylation of the chromatin containing the hTERT promoter in Tel^– E6 Cl 6 cells and ALT cell lines were greater than those in Tel⁺ cells and decreased following treatment with TSA and/or 5-AZC, inversely corresponding to telomerase activity. Our findings suggest the possibility that human tumors may be able to reversibly interconvert their telomere maintenance phenotypes by chromatin structure-mediated regulation of hTERT expression.

Key Words: immortal human cells • telomerase • alternative lengthening of telomeres • DNA methylation • chromatin structure

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