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

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Abstract
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.