Reversible conversion of immortal human cells from telomerase-positive to telomerase-negative cells

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 CI 6 from telomerase-positive (Tel(+)) to telomerasenegative (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 (113) lysine (K) 9 methylation to each of H3-K4 methylnation, H3-K9 acetylation, and H3-KI4 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.